Earth-Science Reviews 161 (2016) 1–15
Contents lists available at ScienceDirect
Earth-Science Reviews
journal homepage: www.elsevier.com/locate/earscirev
Invited review
Hydroclimatic variability on the Indian subcontinent in the past
millennium: Review and assessment
Yama Dixit ⁎, Sampat K. Tandon 1
Department of Earth Sciences, Indian Institute of Technology Kanpur, Kanpur 208016, India
a r t i c l e
i n f o
Article history:
Received 12 February 2016
Received in revised form 25 July 2016
Accepted 1 August 2016
Available online 6 August 2016
Keywords:
Indian Summer Monsoon
Little Ice Age (LIA)
Medieval Climate Anomaly (MCA)
Last millennium
Medieval Warm Period (MWP)
Indian subcontinent
Westerlies
a b s t r a c t
Paleoclimate records spanning the past millennium show manifestations of two distinct climate anomalies – the
Medieval Warm Period (MWP), also known as the Medieval Climate Anomaly (MCA) in the context of tropical
and sub-tropical regions, followed by the Little Ice Age (LIA). The occurrence of these warm and cool periods differs from region to region, in terms of timing, duration and magnitude of the temperature anomalies. PAGES 2K
consortium (2013) compiled global temperature estimates for the last two millennia; however, the Indian subcontinent remained under-represented in this work. A substantial body of evidence and insights exist, based on
traditional and novel proxy data as well as modeling, which has revealed intriguing new aspects of the reconstructed climate of the last millennium in the Indian subcontinent. Here, we present a synthesis of the past millennium hydroclimate variability in India inferred from proxy records from regions affected by the Indian
Summer Monsoon (ISM) and the Westerlies. The possible physical mechanisms linked to the moisture variations
during the past millennium are also discussed. The aim of this work is to improve our current understanding and
stress the gaps that exist in the knowledge of climate variability in the last millennia in one of the most populous
regions of the world. We find that although there were no globally synchronous warm or cold intervals that define a MCA or LIA on the Indian subcontinent, a pattern of generally coherent regional precipitation variation during MCA and LIA period can be observed. The reconstructions from the ISM regime show generally wet conditions
between 900 and 1350 CE (MCA), punctuated in some regions of India by megadroughts, which was followed by
relatively dry period during the global LIA interval from 1500 to 1800 CE, punctuated by ‘wet rain spells’ in the
Central Himalayas. In the Westerlies-dominated regions, very limited data restricts our understanding of rainfall
variability during the MCA period; however, the LIA period was characterized by increased precipitation in these
regions. The summer temperature reconstructions closely follow the ASIA 2K temperature reconstructions and
also the global temperature trends. On the contrary, spring temperatures in the western Himalayas show an opposite trend with a rapid cooling during the 20th century. Changes in local sea surface temperature (SST) fields
and other external boundary conditions like ENSO are suggested to play a major role for summer monsoon variations during the MCA-LIA period. Cooler oceans and continental temperatures are suggested to have pushed the
low pressure systems associated with the Westerlies further south to be carried by the southern winter jet stream
along the southern margin of the Tibetan Plateau bringing more precipitation in northern India during the LIA
period. For the 20th century decreasing trend of ISM, increased anthropogenic atmospheric aerosol loading
over south Asia is a possible causal factor.
© 2016 Elsevier B.V. All rights reserved.
Contents
1.
2.
3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Modern climate of the Indian subcontinent . . . . . . . . . . . . . . . . . . . . . . .
Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.
Hydroclimatic variability within various climate zones on multi-centennial timescale .
⁎ Corresponding author at: IFREMER, Laboratoire Environnements Sédimentaires, BP70, 29280 Plouzané, France.
E-mail address: yama.dixit@ifremer.fr (Y. Dixit).
1
Now at Department of Earth and Environmental Sciences, IISER Campus, Bhauri, Bhopal, India.
http://dx.doi.org/10.1016/j.earscirev.2016.08.001
0012-8252/© 2016 Elsevier B.V. All rights reserved.
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Y. Dixit, S.K. Tandon / Earth-Science Reviews 161 (2016) 1–15
4.1.1.
Variability within the Indian Summer Monsoon regime . . . . . . . .
4.1.2.
Variability within the Westerlies regime . . . . . . . . . . . . . . .
4.2.
Intra-archival coherence in speleothems in the ISM zone . . . . . . . . . . . .
4.3.
Temperature variation on the Indian subcontinent . . . . . . . . . . . . . . .
4.4.
Possible mechanisms for the hydroclimate variability during the past millennium.
5.
Conclusions and summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction
The mean global temperatures are widely recognized to be rising
since the mid-19th century (Crowley, 2000; Esper et al., 2002; Jones
and Mann, 2004; Mann et al., 1999; Moberg et al., 2005; Osborn and
Briffa, 2004). Our society is therefore increasingly interested in understanding the future climate variability and in determining whether
there are clues that can be drawn based on the patterns of the past climate changes. Anthropogenic global warming is a general term, based
on averages for the global temperature changes during the so-called
'Anthropocene' period. However, climate variations at continental to
sub-continental scale, are more relevant to the regional ecosystems
and societies than globally averaged conditions (Wanner et al., 2008).
In this context, studying the last millennium climate variability at regional scale can serve as a benchmark for comparisons with climate
model simulations at a similar scale aimed at understanding 20th century climate change; this may also help to distinguish between the natural
vs anthropogenic forcings.
Previous studies report centennial- to millennial-scale changes in
the ISM during the Holocene from the Arabian Sea (e.g. Gupta et al.,
2003; Staubwasser et al., 2003), the Arabian Peninsula (e.g. Fleitmann
et al., 2007; Fleitmann et al., 2003; Neff et al., 2001), and the Indian subcontinent (e.g. Berkelhammer et al., 2012; Dixit et al., 2015; Dixit et al.,
2014a; Dixit et al., 2014b; Dixit, 2013; Dutt et al., 2015; Nakamura et al.,
2015); relatively fewer records exist, for the past millennium. Globally,
paleoclimate records spanning the past millennium are characterized as
including some manifestation of a warm period from 900 to 1300 CE,
known as the Medieval Climate Anomaly (MCA) or the Medieval
Warm Period (MWP) followed by a cool Little Ice Age (LIA) from 1500
to 1850 CE (Graham et al., 2011). Previous reviews of climate variability
during the past millennium have shown these warm and cold periods
are quite conspicuous in the Northern latitudes, and also emphasized
the spatial and temporal heterogeneities that exist globally (Bradley et
al., 2003; Cronin et al., 2010; Grove, 2001; Ljungqvist et al., 2012;
Ljungqvist, 2009; Mann et al., 2009; Wanner et al., 2008). Grove
(2001) identified the LIA as a time interval from 1500 to 1850 CE that
was composed of several periods, each lasting several decades, with
higher glacial extents.
Recently, the PAGES 2K consortium used past temperatures for
seven continental-scale regions to understand the spatio-temporal
pattern in climate variability during the past two millennia and suggested generally colder conditions during the LIA than those prevalent during medieval times (PAGES 2K consortium, 2013). During
both intervals, distinct hydroclimatic anomalies have been identified
on regional and global scales (e.g. Cook et al., 2007; Feng and Hu,
2008; Graham et al., 2011; Ljungqvist et al., 2012; Newton et al.,
2006; Seager et al., 2007; Wanner et al., 2015 and references
therein). Statistical analysis also demonstrated that the MCA and
LIA intervals are not synchronous globally and that the ‘specific
timing of peak warm and cold intervals varies regionally, with
multi-decadal variability resulting in regionally specific temperature
departures from an underlying global cooling trend’ (PAGES 2K
consortium, 2013). However, the temperature variations on the Indian subcontinent during the past one to two millennia remained
under represented in the analysis. The temperature reconstructions
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used for Asia in the PAGES 2K study included four short-term tree
ring records from the Indian subcontinent since the beginning of
the 16th century to present. Therefore, a discontinuous picture of climate variability was taken from India, while the MCA and LIA signal
recorded in various other archives was poorly documented.
The MWP/MCA and LIA periods are defined in terms of temperature
anomalies observed in the Northern Hemisphere, however in the tropics where monsoon plays an important role in determining the regional climate, these temperature anomalies primarily impact monsoonal
dynamics and hence are expressed as hydroclimatic variability.
Trenberth et al. (2003) showed that an increase in temperature is associated with enhanced moisture holding capacity of the atmosphere that
consequently changes the amount and intensity of precipitation
(Trenberth et al., 2003). Furthermore, globally averaged land precipitation records show around 9 mm increase in precipitation during the
20th century (New et al., 2001). Therefore, increasing evidence of
20th century global warming implies severe changes in the hydrological
cycle over the monsoon Asia.
Recently, Chen et al. (2015) synthesized the most up-to-date records available from the Asian monsoon system to understand the
hydroclimatic variability in the past millennium and explored the
possible underlying physical mechanisms (Chen et al., 2015).
Rehfeld et al. (2013) used network analysis of paleoclimate records
from the Indian subcontinent and China to study the internal dynamics of the Asian monsoon system during the late Holocene focusing
on the MCA and LIA periods (Rehfeld et al., 2013). Twelve records
from various archives were used from the Indian subcontinent to illustrate a strong ISM influence on the East Asian Summer Monsoon
during the MCA and weak ISM circulation during the cold LIA. Following this, multi-proxy records and model simulations from the Indian subcontinent were used to understand the mechanisms
involved in shaping the climate of the Indian subcontinent
(Polanski et al., 2014). Polanski et al. (2014) demonstrated that the
western and Central Himalayas are influenced by the extra-tropical
Westerlies during winters while the eastern Himalayas have a strong
influence of the thermal gradient between the Bay of Bengal and the
Indian subcontinent; and Central India is primarily influenced by the
sea surface temperature anomalies in the northern Arabian Sea. This
study highlights the necessity of regional scale division of the Indian
subcontinent on the basis of the moisture-source systems while
studying the climate variability, a novel approach for understanding
the past millennium climate on the Indian subcontinent.
Here we present a detailed synthesis of the hydroclimate variability in the Indian subcontinent during the past millennia inferred
from available up-to-date comprehensive multi-proxy moisture/
precipitation records. The records are analyzed on the basis of the
primary source of rainfall in the region. We also discuss the intra-regional comparison of various proxy records that are available and
also make intra-archival comparison within the ISM dominated
zone. Also, in Section 4 an attempt is made to assess and add critical
new aspects about the temperature variability recorded in the Indian
subcontinent. Finally, we present a qualitative discussion on the possible cause-effect relation of the moisture patterns in the Indian subcontinent with the global climate variability and their possible
underlying physical mechanisms.
Y. Dixit, S.K. Tandon / Earth-Science Reviews 161 (2016) 1–15
2. Modern climate of the Indian subcontinent
The typical present-day Indian climate is characterized by warm and
wet summers with about 70% of annual rainfall in most part of India falling from June to September and the remainder during the mild and dry
winters. The present-day climate is strongly governed by the annual latitudinal migration of the Inter Tropical Convergence Zone (ITCZ) (Fig.
1). In spring the ITCZ moves northward across the Indian Ocean and
reaches its northernmost position in August, at the peak of the ISM.
From June to September, strong winds transport large quantities of
moisture from the Indian Ocean, which is then released as monsoon
precipitation over the Indian subcontinent (Fleitmann et al., 2007).
The southwest monsoon is therefore driven by the thermal contrast between the Indian subcontinent and the Northern Indian Ocean, bringing
rains from June to September. The ISM is categorized into Arabian sea
branch and Bay of Bengal branch based on the spread of moisture
laden winds coming on the subcontinent. The Arabian Sea branch enters
through the west coast of India and through the Narmada–Tapti gap.
These winds bring rainfall in the extensive areas of Central India before
eventually mixing with the Bay of Bengal vapor source after traveling
over the states of Maharashtra and Madhya Pradesh (Sengupta and
Sarkar, 2006). The Bay of Bengal branch of the ISM strikes the coast of
Myanmar and Bangladesh and is deflected to the Himalayas and northwest India after mingling with the Arabian Sea branch in North India.
In autumn, the ITCZ then retreats southward and reaches its southernmost position in January. The reversed pressure gradient during
the winter months causes the Northeast monsoon rain in southeast
India (Fig. 1). The precipitation in winter and spring months in northwest India is primarily brought by westerly disturbances originating
over the Mediterranean, Black Sea and North Atlantic area as an
extratropical frontal system moving eastward towards the Himalayan
region. These are common in winter and spring and sometimes before
the summer monsoon and bring rains in Himalayan region and northwest India (Das, 1972; Yadav, 2011a).
3
During winters, the upper Westerlies jet stream over Asia branches
into two currents, one north and the other south of the Tibetan plateau
due to the topographic barrier to airflow (Liang et al., 2015). The stronger southern branch over northern India corresponds to a strong latitudinal thermal gradient (from November to April). The winter
depressions are steered over northern India by the upper jet and the
lows penetrate across the Middle East from the Mediterranean and are
primary sources of rainfall for northern India and Pakistan, especially
as the evaporation is quite low during this period (Liang et al., 2015).
3. Methodology
The 57 records given in Table 1 are plotted according to the major
source of rainfall for individual sites during the MCA and LIA period.
The Indian subcontinent receives rainfall from three major rainfall systems – (i) the southwest monsoon or the ISM, (ii) the Westerliessourced rains and (iii) the northeast monsoon. We divided the records
on the basis of the major monsoon systems that bring rainfall on the Indian subcontinent, first the ISM and second the Westerlies system (Fig.
1). There are no proxy records from the northeast monsoon region
spanning the last millennium, except for a qualitative discussion on
the NE rainfall variability inferred from historical data of the last
60 years extrapolated to the late Holocene period (Gunnell et al.,
2007). They suggested that northeast monsoon and ISM rainfall varies
synchronously on seasonal to millennial timescales.
Based on the available records from the ISM and Westerlies region,
Figs. 2 and 3 are the relative wetness maps where wetness is classified
as dry, wet, moderately dry, moderately wet and drought (see figure
legends for colour scheme) following the original author's work. Fig. 2
shows the precipitation variability during the MCA and Fig. 3 documents the precipitation variability during the LIA period, in the ISM
and Westerlies zones. The circles represent the time span of 900 to
1350 CE and 1500 to 1850 CE for MCA and LIA period respectively.
The chronology followed in the pie charts is taken from the original
Fig. 1. Map of the Indian subcontinent showing the climate regimes mainly divided into the ISM dominated in pink, the westerlies in light orange, core westerlies in orange and northeast
monsoon in yellow. The key to the colour and symbol scheme is given in the figure legend. Black dotted line denotes the location of the ITCZ during summer monsoon period between June
to September and the blue arrows show the movement of southwest monsoon (redrawn from Pant and Rupa Kumar, 1997)). Bold dashed black arrows denote the westerlies, blue arrows
show the Arabian Sea and Bay of Bengal branch of the ISM and orange arrows show the northeast monsoon along the eastern coast of the peninsular India (redrawn from Gunnell et al.,
2007).
4
Y. Dixit, S.K. Tandon / Earth-Science Reviews 161 (2016) 1–15
Table 1
Proxy palaeo-precipitation and temperature records used in this study classified according
to the archive. For all the records, the age range, moisture and temperature classification
has been taken from the literature.
Location of proxies
Age range
Reference
Tree rings (22)
Western Himalayas
Last 775
Yadav et al., 2004
Western Himalayas
years
1730–1987
Singh and Yadav, 2005
Western Himalayas
CE
771–2003
Singh et al., 2006
Eastern Himalayas
CE
1852–2006
Yadava et al., 2015
Western Himalayas
CE
420–2003
Yadav et al., 2006
Western Himalayas
CE
940–2008
Yadav et al., 2011
Western Himalayas
CE
1168–1988
Yadav et al., 1999
Western Himalayas
CE
1698–1986
Yadav et al., 1997
Western Himalayas
CE
1450–2000
Borgaonkar et al., 2011
Western Himalayas
CE
1711–1985
Borgaonkar et al., 1996
Nepal Himalayas
CE
1440–1996
Cook et al., 2003
Western Himalayas
CE
1452–2004
Borgaonkar et al., 2002
Western Himalayas
CE
1295–2005
Singh and Yadav, 2013
Western Himalayas
CE
1455–2002
Singh and Yadav, 2014
Lesser Himalayas
CE
805–2002
Singh et al., 2004
Western Himalayas
CE
1310–2004
Singh et al., 2009a
Northern Pakistan, Karakoram
CE
950–1990
Treydte et al., 2006
Western Himalayas
CE
1410–2005
Yadav, 2011a
Western Himalayas
CE
16
Yadav, 2013
Western Himalayas
CE
1300–2000
Western Himalayas
CE
1460–2008
Western Himalayas
CE
1600–1985
Yadav, 2009
Yadav and Bhutiyani,
2013
Yadav and Singh, 2002
CE
675–1950
Lonar Lake, Madhya Pradesh
Madhya Pradesh, Central India
CE
Holocene
Last 3800
Badanital Lake, Central Himalayas
Thar Desert margin, Western India
Pookode Lake, Kerala
years
0–4221 BP
0–7000 BP
6240–565
Paradise lake, Northeast India
BP
Last 1800
years
Speleothems (14)
Kothinal cave, Central Himalayas
500–1800
Pokhara valley, Nepal
Dharamjali cave, Central Himalayas
CE
0–2300 BP
~200–2000
Chulerasim cave, Lesser Himalayas
CE
1590–2006
Dandak Cave, Central India
CE
600–1500
Location of proxies
Jhumar Cave, Central India; Wah-Shikar,
Northeastern India
Age range
Reference
CE
1000–2000
Kotlia et al., 2012
Sinha et al., 2011a,
2011b
Andaman Islands
CE
Last 4000
Gupteshwar cave, Central India
years
Last 3400
Sanji cave, Central Himalayas
years
Last ca. 4000
Dandak Cave, Central India
years
950–1300
Sahiya Cave, Central Himalayas
CE
1200–2000
Western Ghats, Penninsular India
CE
1666–1997
Yadava et al., 2004
Panigarh Cave, Hilamayan foothills
CE
1256–2000
Liang et al., 2015
Laskar et al., 2013
Yadava and Ramesh,
2006
Kotlia et al., 2014
Berkelhammer et al.,
2010
Sinha et al., 2015
CE
Marine (7)
Southeastern Arabian Sea
460–2600
Chauhan et al., 2010
Off Indus river, Arabian Sea
Off Orissa, Bay of Bengal
Off Oman margin, Arabian Sea
BP
0–5000 BP
Holocene
Last 2000
von Rad et al., 1999
Ponton et al., 2012
Gupta et al., 2003
Eastern Arabian Sea
years
800–1900
Agnihotri et al., 2002
Eastern Arabian Sea
Southeastern Arabian Sea
CE
Holocene
800–1600
Sarkar et al., 2000
Tiwari et al., 2005
CE
BCE–2006
Lacustrine (7)
Khedla Quila Lake, Madhya Pradesh
Table 1 (continued)
Quamar and Chauhan,
2014
Prasad et al., 2014a
Chauhan and Quamar,
2012
Kotlia and Joshi, 2013
Prasad et al., 1997
Veena et al., 2014
Shah and Chaudhary,
2007
Padmakumari et al.,
2010
Denniston et al., 2000
Sanwal et al., 2013
Kotlia et al., 2016,
Pollen (7)
Gangotri Glacier
Last 2000
Bhogdoi Swamp, Northeast India
Sitikher Bog, Himachal Pradesh
Kiktikah Swamp, Madhya Pradesh
years
0–3795 BP
0–2300 BP
0–1600 BP
Northwestern Himalayas
0–8300
Kumaon Peat, Central Himalayas
years
Last 3500
Mahanadi Floodplain, Odisha
years
0–5840 BP
Kar et al., 2002
Dixit and Bera, 2013
Chauhan et al., 2000
Chauhan and Quamar,
2012
Bhattacharyya et al.,
2011
Phadtare and Pant,
2006
Tripathi et al., 2014
published records. Using Fig. 2, we derived general patterns of summer
and Westerlies rainfall in India during the MCA and LIA period. Fig. 2
clearly demonstrates that the regions of the Indian subcontinent affected by the ISM were generally wet during the MCA. There is a paucity of
MCA records from the Westerlies dominated regions as compared to the
LIA, nonetheless the available records suggest that the Westerlies
brought less rain during the MCA (Fig. 2). During the LIA period, the
summer monsoon weakened as is observed from the preponderant orange colour in the pie-charts of Fig. 3 in the ISM regime, while increased
Westerlies rains characterized this period in the western Himalayas
(Fig. 3). There is also evidence of severe drought-like conditions in decadal-scale resolved speleothem records (Sinha et al., 2011a, 2011b)
from Central India as indicated by the red colour in the pie-charts.
The pie-chart in Figs. 2 and 3 present a general picture of the prevalent climatic conditions on the Indian landmass during the globally observed MCA and LIA periods. However, for detailed centennial to
decadal scale hydroclimatic variations recorded in various archives in
the ISM dominated regime, we used speleothem and marine archives
as these are relatively better chronologically constrained. The idea was
to understand the centennial to decadal scale intra-regional ISM variability, recorded in terrestrial and marine archives. For the Westerlies
regime, as the precipitation was primarily inferred from tree rings, we
used these records to evaluate the intra-regional variability. Palynological and other qualitative lake records with poor chronological control
are discussed in the text. An interesting aspect was to understand the
Y. Dixit, S.K. Tandon / Earth-Science Reviews 161 (2016) 1–15
5
Fig. 2. Wetness pie-chart map based on multi-proxy records during the MCA period in various climate zones of the Indian subcontinent. The pie-chart is divided into the period 900–
1350 CE. Wetness is classified as dry, wet, moderately dry, moderately wet and drought (see figure legend for colour scheme) following author's original work. Northern India marked
in rectangle is enlarged. Blue rectangle encloses a pie chart representative of published speleothem studies on Dandak Cave and Gupteshwar Cave (located 30 km from Dandak)
speleothem. The pie charts follow the age interval for MCA and LIA as given in the text.
intra-archival climate variability within the ISM regime. For this purpose, we also attempted to study the coherence within speleothem records from the ISM regime to decipher the intra-regional variability
recorded in a single archive that could possibly shed light on the local
physical mechanisms affecting the summer monsoon precipitation.
The MCA and LIA are defined as a temperature anomaly with relatively warm and cold periods respectively in the Northern Hemisphere
on the basis of reconstructed surface temperatures using the proxy network (Mann et al., 2009). Therefore, to evaluate the temperature variations during the MCA and LIA periods on the Indian subcontinent with
respect to the other Asian and global records, the seasonal temperature
reconstructions inferred from tree rings is also compared against the
ASIA 2K temperatures (PAGES 2K network) and the Northern Hemisphere temperatures (Jones and Mann, 2004; Mann et al., 2002).
4. Results and discussion
4.1. Hydroclimatic variability within various climate zones on multi-centennial timescale
4.1.1. Variability within the Indian Summer Monsoon regime
The monsoon rainfall variability during the past millennium in the
ISM-dominated regions on the Indian subcontinent is reasonably well
documented from north India (Himalayas), northeast India, northwest
Fig. 3. Wetness pie-chart map based on multi-proxy records during the LIA in various climate zones of the Indian subcontinent. The pie-chart is divided into the period 1500–1850 CE.
Wetness is classified as dry, wet, moderately dry, moderately wet and drought (see figure legend for colour scheme) following author's original work.
6
Y. Dixit, S.K. Tandon / Earth-Science Reviews 161 (2016) 1–15
India, central India and south India (Western Ghats) using various terrestrial and marine archives. For example, a palynological and geochemical approach is used in lacustrine sediment from central India
(Chauhan, 2015; Chauhan and Quamar, 2012; Prasad et al., 2014b),
NW India (Dixit et al., 2014a; Dixit et al., 2011; Prasad and Enzel,
2006) and NE India (Shah and Chaudhary, 2007), peninsular India
(Veena et al., 2014), speleothems from the Himalayas (Kotlia et al.,
2016; Kotlia et al., 2014; Kotlia et al., 2012; Liang et al., 2015; Sanwal
et al., 2013; Sinha et al., 2015), Central India (Sinha et al., 2011a,
2011b; Sinha et al., 2007) and South India (Yadava et al., 2004) and in
the marine sediments (Chauhan et al., 2010; Gupta et al., 2003;
Ponton et al., 2012; Tiwari et al., 2005). Most of the tree-rings based climate reconstructions are reported from the Lesser Himalayas (Yadav,
2013).
The tree ring-inferred precipitation records from the ISM-influenced
western and Central Himalayas are relatively few and there exists poor
calibration between tree ring and precipitation data owing to the strong
topographic forcing on the spatial variability in precipitation in the Himalayan region (Singh and Yadav, 2005).
Based on the available proxy records, speleothem and marine records are relatively better resolved in terms of the chronology as opposed to the lake records (Chauhan and Quamar, 2012; Prasad et al.,
Fig. 4. Hydroclimate variability in the ISM regions of the Indian subcontinent. H, M and P denote High (decadal-scale), Medium (centennial scale) and Poor (greater than centennial scale)
resolution respectively. Yellow and blue band represents MCA and LIA respectively. The vertical grey bars are the abrupt precipitation events observed in the presented records.
Speleothem records are plotted in orange from Central India (Sinha et al., 2011b), Western Ghats (Yadava et al., 2004), Lesser Himalayas (Sinha et al., 2015), North east India (Sinha et
al., 2011a), Central Himalayas (Kotlia et al., 2014), Lesser Himalayas (Kotlia et al., 2016; Kotlia et al., 2012), Kumaun lesser Himalayas (Sanwal et al., 2013). Marine records are in the
shades of blue from Arabian Sea (Tiwari et al., 2005), Bay of Bengal (Ponton et al., 2012), SE Arabian Sea (Chauhan et al., 2010), NW Arabian Sea (Gupta et al., 2003).
Y. Dixit, S.K. Tandon / Earth-Science Reviews 161 (2016) 1–15
2014b; Tripathi et al., 2014; Veena et al., 2014). The rainfall variability
inferred from both, speleothem and marine records from the Indian
sub-continent demonstrate spatially variable monsoon strength during
the last two centuries (Fig. 4). Although the available climate reconstructions are of different chronological resolution, there are general
trends that can be observed in the last millennium (Fig. 2). Generally,
the records from Central India and the Arabian Sea show decreased
monsoon precipitation at the beginning of MCA followed by strengthening of the monsoon. The decadal-scale monsoon rainfall in Central India
brought by the Arabian Sea branch of the ISM, during the MCA and LIA
periods is very well documented in the Dandak and Jhumar Cave
speleothem records. While Dandak Cave record is the best available record in terms of chronological control on decadal to centennial scale,
other records from the Arabian Sea have poor age control to compare
the monsoon variation at such short scale. For the Bay of Bengal branch
of ISM, both the marine record off the mouth of the Godavari river
(Ponton et al., 2012) and the Andaman speleothem (Laskar et al.,
2013) are chronologically too poorly resolved to be able to identify decadal-centennial scale climate variations. Also, the Chulerasim cave record from Lesser Himalayas that receives rainfall from both the
Arabian Sea and Bay of Bengal branches of the ISM was initially published with a poor chronology (Kotlia et al., 2012), and was revisited recently with a slightly improved age-control (Kotlia et al., 2016),
however it only spans the last 400 year period. Nonetheless, in this section we have attempted to compare, discuss and identify major climate
events during the past two millennia on the basis of available records.
The beginning of the MCA around 900 CE was characterized by weakened ISM precipitation, inferred on the basis of lower 18O in
speleothems from Central India (Sinha et al., 2011b; Sinha et al., 2007)
and also from the Lesser Himalayas (Kotlia et al., 2014; Sanwal et al.,
2013; Sinha et al., 2015) (Fig. 4). Evidence for decreased monsoon
strength around 900 CE has also been obtained from the Arabian Sea
(Chauhan et al., 2010; Gupta et al., 2003). In the NW Arabian Sea, the
lower abundance of foraminifera G. ruber off the Oman margin (Gupta
et al., 2003) and increased δ18O of foraminifera in SE Arabian Sea
(Chauhan et al., 2010) indicate weakened ISM in the beginning of the
MCA period. However, the monsoon variability recorded in the marine
sediments collected off the River Godavari mouth in Bay of Bengal
(Ponton et al., 2012) show an opposing trend. Ponton et al. (2012) reconstructed a Holocene monsoon variability record, with only two
AMS 14C dates employed for age determination of the past two
millennia; we believe that this inadequate chronological assessment
possibly contributed to the mismatch of the general trend during the
last millennium. In both terrestrial and marine records, the monsoon recovered after around 950 CE and the interval between 950 and 1250 CE
is observed as a generally strong monsoon period, especially in the Lesser Himalayas, Central India, and also in the Arabian Sea records. It is apparent from Fig. 4 that the monsoon weakened significantly during the
end of the MCA period around 1300 CE. Sinha et al. (2011a, 2011b)
showed that Central India was characterized by droughts on either
side of the period 950–1250 CE, and suggested that nearly every
major drought was independently corroborated by historical accounts
of famine in India (Sinha et al., 2007). The major drought between
1300 and 1350 CE was also observed in the Andamans (Laskar et al.,
2013) and marine records from the Arabian Sea (Gupta et al., 2003;
Tiwari et al., 2005). These major droughts are suggested to be of higher
intensity than any of the droughts documented in the instrumental record and also are termed as the ‘monsoon megadroughts or MMDs’
(Sinha et al., 2011a). However, such MMDs were not observed in the
studied records from the Lesser and Central Himalayas (Denniston et
al., 2000; Kotlia et al., 2014; Liang et al., 2015; Sanwal et al., 2013;
Sinha et al., 2015).
The ISM recovered in most parts of the subcontinent after 1400 CE,
which marks the transition from the MCA to LIA. The beginning of the
LIA period after 1500 CE, in the ISM-dominated regions was observed
as being characterized by generally average monsoon conditions in
7
both marine and speleothem records. At around 1600 CE, the summer
monsoon weakened drastically throughout the subcontinent, as recorded in the Lesser Himalayas, Central India, Arabian Sea and NE India except the Andaman Islands, where this period corresponds to increased
monsoon precipitation and could possibly be an artefact of an extremely
poorly resolved age model or the island setting of the site (Laskar et al.,
2013). Based on the tree ring-width based reconstructions (Cook et al.,
2007) and speleothems of Central India (Sinha et al., 2011b), it is evident that the episodic and widespread recurrence of monsoon
megadroughts continued throughout the LIA. Using speleothem-based
late Holocene reconstructions of monsoon variability from India
(Sinha et al., 2007) and China (Zhang et al., 2011), Sinha et al. (2011a,
2011b) suggested that there were at least five episodes of MMDs during
the LIA (1350–1850 CE). These MMDs occurred during the period of
generally reduced monsoon strength in Asia during LIA, between 1300
and 1700 CE (Fleitmann et al., 2007; Fleitmann et al., 2003; Hu et al.,
2008; Wang et al., 2005).
The Central Himalayas however, experienced relatively high precipitation during the LIA period (Denniston et al., 2000; Kotlia et al., 2014;
Liang et al., 2015; Sinha et al., 2015) as compared to the rest of the monsoon zone which was characterized by weaker monsoon and
megadroughts in the core monsoon zone in central India. Stalagmites
from Siddha Baba cave in the Pokhara Valley, central Nepal
(Denniston et al., 2000) was characterized by dense, optically clear calcite layers during the beginning of LIA period from 1550 to 1640 CE indicating a less-evaporative cave environment than previously and
cooler moist conditions. Liang et al. (2015) also show that similar concordant mineralogical changes from aragonite to calcite in Panigarh stalagmite at the beginning of the LIA points to a shift to a wetter climate in
the foothills of the Himalayas. The possible reason for the discordance in
the precipitation patterns between the Central Himalayas and the rest
of India during the LIA is that the megadroughts in the monsoon-dominated regions are the result of the ‘break-dominated’ monsoon cycle
when the ITCZ shifts to the south, which weakens the easterly jet and
consequently subsiding air is forced to rise by the Himalayas along a
break trough located above the foothills, which replaces the monsoon
trough. This circulation brought rain to the Central Himalayas and Brahmaputra valley as observed in the Panigarh Cave, Sahiya Cave in the
Central Himalayas and Siddhi Baba cave (Nepal) in the Lesser Himalayas
(Liang et al., 2015; Sinha et al., 2015) during the LIA period. Another
possible factor could be the moisture source for these regions, Central
Himalayas receives rainfall from both the Arabian Sea and the Bay of
Bengal branch of the ISM, while central India receives most of the rainfall from the Arabian Sea branch; therefore the asynchronous pattern of
precipitation could be a manifestation of the weakening of the Arabian
Sea branch as compared to the Bay of Bengal branch of the ISM.
The summer monsoon recovered after 1650 CE for a short period of
about 50 years followed by a decreasing trend of ISM monsoon observed after 1700 CE in the speleothem records from Himalayas, Western Ghats, and Central India. Another remarkable feature observed in
the reconstructions just at the end of the LIA period at 1850 CE, is a sudden decrease in the monsoon precipitation over the Indian subcontinent
(Laskar et al., 2013; Sinha et al., 2015; Sinha et al., 2011b; Yadav et al.,
2014) (Fig. 4). The beginning of 20th century was characterized by decreasing ISM precipitation (Fig. 4). Kumar et al. (2013) used high-resolution regional climate models to estimate the future temperature and
precipitation changes in the late 21st century. They demonstrated that
the projected rainfall changes show considerable spatial variability,
with an increase in precipitation only over the peninsular India and
coastal areas and, either no change or a decrease further inland.
4.1.2. Variability within the Westerlies regime
In contrast to the ISM records, only tree ring records, except one lacustrine (Leipe et al., 2013), exist from the Westerlies dominated regions in the Indian subcontinent and most records extend only to
1300 CE. Therefore, a data gap exists for the MCA period from the
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Y. Dixit, S.K. Tandon / Earth-Science Reviews 161 (2016) 1–15
Westerlies regime (Cook et al., 2003). The multi-proxy record based on
lacustrine sediments of Tso Moriri lake suggest a decreasing monsoon
trend from the beginning to the end of MCA at 1350 CE (Leipe et al.,
2013). The tree ring-based precipitation reconstructions present a consistent picture of rainfall variability caused by western disturbances
brought by the Westerlies. The beginning of LIA period after 1500 CE
shows an increasing rainfall trend from 1500 to 1600 CE in the western
and northwestern Himalayas. This period of increased rainfall brought
by the extra-tropical storms originating in the Mediterranean corresponds to a generally moderate summer monsoon rainfall in the Indian
subcontinent.
The western Himalayas experienced a series of drought like events
after 1750 CE. This is recorded in the tree ring based precipitation and
the Lake Tso-Moriri record as well. The increased Westerlies rainfall declined between 1750 and 1800 CE, which was the most prominent drying in the last 500 years (Yadav, 2011a). Following this drought, the
rainfall further decreased between 1850 and 1900 (Fig. 5). This pattern
of drought is observed in the western Himalayas and also in the
Karakoram ranges (Singh et al., 2009b; Singh et al., 2006; Treydte et
al., 2006; Yadav, 2011b). The Westerlies-brought precipitation increased significantly after 1950 CE in the western Himalayas (Fig. 5).
Treydte et al. (2006) used δ18O of tree-ring cellulose to demonstrate
Fig. 5. Hydroclimate variability in the Westerlies. Yellow and blue band represents MCA and LIA respectively. The vertical grey bars are the abrupt decline in precipitation observed in the
presented records. These records are for annual precipitation from Karakoram and Himalayas (Treydte et al., 2006) and spring rainfall from western and northwestern Himalayas as
follows: March–April–May–June precipitation in western Himalayas (Yadav et al., 2011), April to June precipitation in western Himalayas (Yadav, 2011a, 2011b), March–May
precipitation in western Himalayas (Singh et al., 2006), March–July precipitation in western Himalayas (Yadav, 2011a, 2011b), March to July precipitation in Northwestern Himalayas
(Singh et al., 2009b) and pollen based annual precipitation in the northwestern Himalayas (Leipe et al., 2013).
Y. Dixit, S.K. Tandon / Earth-Science Reviews 161 (2016) 1–15
the increasing rainfall brought by Westerlies since the mid-nineteenth
century. An opposite trend is observed after 1975 CE which points to declining rainfall to the present day.
4.2. Intra-archival coherence in speleothems in the ISM zone
Speleothems are considered to be the most reliable archive for past
climate reconstructions because they yield high-resolution climate records owing to their unique character to be accurately dated over decadal to centennial time scales. For this reason, we chose, based on our
literature survey, speleothem records from the summer monsoon dominated zone to make a comparison, and to test whether or not the
9
records are well correlated. The available speleothem records are primarily from three different regions: southern India (Yadava et al.,
2004); Central India (Berkelhammer et al., 2010; Sinha et al., 2011b;
Sinha et al., 2007; Yadava and Ramesh, 2006); north and northeast
India (Kotlia et al., 2014; Sinha et al., 2015). All the records, except the
Andaman record, use Stalage algorithm or Monte Carlo simulations, taking into account the associated errors of U-Th dating, to constrain the
age model, giving an advantage of one single method for chronology development and hence aiding in comparing the records in an objective
manner. On centennial time scale, the monsoon precipitation pattern
gives a coherent picture with generally decreasing monsoon rainfall
during the MCA to LIA transitional period, as demonstrated in the
Fig. 6. Intra-archival variability in speleothems from the Indian Summer monsoon regime. Yellow and blue band represents MCA and LIA respectively. The speleothems are from the
Andaman islands (Laskar et al., 2013), Central Himalayas (Kotlia et al., 2014), Lesser Himalayas (Sinha et al., 2015), Northeast India (Sinha et al., 2011a), Central Himalayas (Sanwal et
al., 2013), Lesser Himalayas (Kotlia et al., 2016; Kotlia et al., 2012), central India (Sinha et al., 2011b). The central Himalayas show increased precipitation as opposed to decreasing
rainfall in other records during the LIA period.
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records from the Himalaya and NE India (Fig. 6). The only exception to
the general decreasing monsoon precipitation is the Andaman record,
which has a very poor age-resolution (only six data points) during the
LIA period and therefore is not suitable for inferring a decadal-centennial scale trend.
The most prominent drought in the past few centuries was around
1600 CE when the summer monsoon weakened drastically throughout
the subcontinent, however the records from NE India, Central
Himalayas and Andamans exhibit opposing behaviour. NE India and
Andaman receive majority of its rainfall during summer primarily driven by the thermal gradient between the Bay of Bengal and the Indian
subcontinent (Polanski et al., 2014). Sinha et al. (2011a, 2011b) attribute this opposing behaviour to the local internal dynamics of the ISM
which has oscillating periods with a persistently “active dominated”
and a “break-dominated” regime which are manifestations of sub-seasonal fluctuations in the mean position of the continental tropical convergence zone (CTCZ) over the core monsoon zone (e.g., Goswami et
al., 2006; Lawrence and Webster, 2001). The switch between these regimes is suggested to occur abruptly (within decades) at a time (in
this case from ~ 1650–1700 CE); and therefore on a centennial scale
the active-break dipole mode, associated with intra-seasonal oscillations, is the dominant source of ISM precipitation variability as opposed
to subcontinent-scale monsoon variability (Sinha et al., 2011b).
It is also noteworthy that the Sainji Cave from the Central Himalayan
region recorded the highest moisture conditions during the LIA (Fig. 6)
in contrast to the weakened summer monsoon rainfall in other regions,
even though the region is believed to receive most of its rainfall from the
ISM (Sinha et al., 2015). Increased precipitation is also observed in the
Panigarh cave in Central Himalayas and Siddhi Baba cave in Nepal
(Denniston et al., 2000; Liang et al., 2015). These episodes of increased
rainfall during the LIA occurred at the time of ‘monsoon-break conditions’, when the monsoon trough is located close to the foot of the
Himalayas, which leads to a striking decrease in rainfall over most of
India but to an increase along the Himalayas and parts of northeast
India and in the extreme southeast of Peninsular India (Liang et al.,
2015). This possibly explains the opposing precipitation trends in the
NE India and Central Himalayas speleothem records around 1600 CE.
Kotlia et al. (2014) suggested that the Westerlies might have played
an important role in the late Holocene climate of the Central Himalayas,
which could possibly explain consistent high moisture conditions in the
face of decreasing summer monsoon during the LIA period. Liang et al.
(2015) suggested that the western depressions originating in the Mediterranean may have been pushed south during the LIA by the prevailing
colder temperatures and carried to the south of Tibetan Plateau by the
southern winter jet bringing high winter rainfall. Furthermore, recent
model simulations demonstrate that the western and Central Himalayas
are influenced by the extra-tropical Westerlies during winters (Polanski
et al., 2014). The highest precipitation in this region during 1450–
1700 CE (beginning of LIA) is suggested to have resulted from the stronger Westerlies as opposed to the weak ISM during this period (Kotlia et
al., 2014; Kotlia et al., 2012) (Fig. 6).
4.3. Temperature variation on the Indian subcontinent
We compiled the available temperature records from the subcontinent and compared them against the classic 'Hockey stick' Mann and
Jones record (Mann and Jones, 2003) of the Northern Hemisphere temperature changes (Fig. 7). PAGES 2K consortium recently used tree ringbased temperature reconstructions from Asia (primarily China and SE
Asia) standardized to have a mean of zero and unit variance ultimately
to obtain the ASIA 2K temperature anomaly record. Since, only one record from India was taken into the ASIA 2K reconstruction, we also compare all the other available tree-ring based temperature reconstructions
from the Indian subcontinent with the ASIA 2K records to understand
the temperature variability with respect to Asia, northern latitudes
and globally averaged conditions.
The majority of the temperature records for the past millennium in
India are obtained using the tree ring-width studies from the Western
Himalayas. We used both summer and spring temperature reconstructions for comparison with the Northern Hemisphere and ASIA 2K temperatures. The lack of availability of temperature proxy records for the
MCA period from the Indian subcontinent restricts our evaluation of
temperature changes during this period. The only available record is
from Western Himalayas, which yields a complete history of the summer temperature (May–June–July–August) fluctuations extending
back to 940 CE (Yadav et al., 2011). The summer temperature record
from Western Himalayas follows closely that of the ASIA 2K and Northern Hemisphere temperatures (Mann et al., 2002). Centennial-scale
variations in the record of the summer temperature reveal the presence
of a warm period encompassing 11th–15th centuries corresponding to
the warming observed in the Northern Hemisphere and ASIA 2K record.
A decreasing trend in mean summer temperature occurred since
1450 CE and continued up to 1850 CE. This also follows the cold period
LIA observed in the Northern Hemisphere (Fig. 7). The mean summer
temperature over the western Himalaya also shows a positive correlation with summer monsoon intensity over north central India (Yadav
et al., 2011). These inferences therefore clearly suggest that the summer
temperatures in the Western Himalayas followed closely the Northern
Hemisphere temperature variations and thereby demonstrate a possible teleconnection between the high and the lower latitudes.
Borgaonkar et al. (2011) used winter temperature reconstruction from
western Himalayan conifers, and showed that the winter temperatures
are positively correlated with the mean annual temperatures in the
western Himalayas. The LIA interval in this region was characterized
by cooling periods during 1453–1590 CE and 1780–1930 CE, while the
20th century warming is clearly observed as anomalous higher growth
in the high altitude tree-ring chronologies (Borgaonkar et al., 2011).
Furthermore, regional climate models demonstrate that the ensemblemean warming over India is 1.5 °C at the end of 2050, whereas it is
3.9 °C at the end of century with respect to 1970–1999 (Kumar et al.,
2013).
The spring (March–April–May) temperature reconstructed form the
Western Himalayas, however, suggests that relatively warm temperatures dominated this region during the spring in LIA. Cook et al.
(2003) used the tree-ring based winter temperature record to suggest
that the LIA period was characterized by periods of cooling. However,
most tree-ring data from the western Himalayan region do not record
the winter temperature and instead deal with summer and spring temperatures, making it difficult to detect similar LIA signals (Borgaonkar et
al., 2002). Nonetheless, based on the comprehensive set of proxy records presented here in Fig. 7, it is clear that the spring and summer
temperature changes in the western Himalayas differed significantly
during the MCA and LIA. While the summer temperatures in the Western Himalayas closely follow that of the mean annual temperatures in
the Northern Hemisphere, the spring temperatures show an opposite
trend during the LIA period.
Another notable feature of the temperature reconstructions is the
20th century temperature trends in spring and summer temperatures
of the Western Himalayas. The mean global temperatures in the instrumental records show a rise since the beginning of the 20th century.
High-resolution climate proxies from high-latitudes of the Northern
and Southern Hemispheres indicate unprecedented warming in the
20th century (Jones and Mann, 2004). This is also exhibited in the summer temperature reconstruction in the Western Himalayas (Fig. 7).
However, the spring temperature records show a decline since the beginning of the 20th century (Singh and Yadav, 2014; Yadav et al.,
1997). The cooling recorded during the latter part of the 20th century
is in agreement with the instrumental records (Singh and Yadav,
2014). Tree-ring based temperature reconstructions from other Asian
mountain regions like Nepal and central Asia (Cook et al., 2003;
Osborn and Briffa, 2004) also document cooling during the last decades
of the 20th century. Yadav et al. (2004) attributed this rapid decrease in
Y. Dixit, S.K. Tandon / Earth-Science Reviews 161 (2016) 1–15
11
Fig. 7. Tree rings based temperature reconstructions from the Indian subcontinent. The temperature records show spring and summer temperatures from March to May and May to
September respectively as published in the original work. MAT is the Mean Annual Temperature in ASIA 2K and Northern Hemisphere records. W denotes records from the westerlies
regions and ISM denotes records from the Indian summer monsoon regions. Bottom-most records are the Northern Hemisphere reconstructions (Mann and Jones, 2003) and Asian
temperature records (PAGES 2K consortium).
the temperature to the large-scale deforestation and land degradation
in this area. Decreasing spring temperature in the 20th century is widespread and observed throughout the Western Himalayas. This asynchronous variation in seasonal temperatures also point towards the
extreme seasonality occurring in the western Himalayas, which is suggested to be mainly caused by the human activities and therefore constitutes the human-induced rapid cooling of night temperatures
during spring in the western Himalayas (Yadav et al., 2004).
It is difficult to discuss fully the asynchronous variations in seasonal
temperatures during the past millennium based on the limited number
of currently available proxy records. Therefore, it is essential to have
more temperature reconstructions for the past millennium to develop
a robust understanding of the temperature variations from the Indian
subcontinent. Nevertheless, the existence of clear synchronicity between the summer temperature on the Indian subcontinent and high
latitude Northern Hemisphere temperature suggests the existence of a
link between the summer temperature and precipitation patterns in
the western Himalayas and the Northern Hemisphere.
4.4. Possible mechanisms for the hydroclimate variability during the past
millennium
The causal mechanisms of the ISM rainfall variability on decadal to
orbital timescales has long been of interest to climatologists (for example, Agnihotri et al., 2002; Fleitmann et al., 2007; Kumar et al., 1999; Liu
et al., 2013; Rind and Overpeck, 1993). On millennial to orbital
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timescales, both paleoclimate proxy research and climate modeling
have suggested that the precipitation in the tropical and subtropical
monsoon areas is directly proportional to latitudinal migration of the
ITCZ (Fleitmann et al., 2007; Schneider et al., 2014; Wang et al., 2005).
With southward migration of the ITCZ, the precipitation in the Northern
Hemisphere summer monsoon area decreases and vice versa, when the
ITCZ moves northwards, the moisture-laden southwest winds bring
rainfall on the Indian subcontinent. Sachs et al. (2009) used geochemical proxies from lake sediments in the Northern Line Islands, Galápagos
and Palau to show that the Pacific ITCZ was south of its modern position
for most of the past millennium, by as much as 500 km during the LIA
(Sachs et al., 2009). The possible cause of southward movement of the
ITCZ was linked to the colder Northern Hemisphere at that time
owing to enhanced high-latitude ice cover and a slowdown of the Atlantic meridional overturning circulation during that period (Haug et al.,
2001). Conversely, a northward migration of the ITCZ mean position is
usually driven by increased Northern Hemisphere insolation input relative to the Southern Hemisphere. In terms of the hydrologic impact,
palaeo-proxy evidence suggests that during the relatively cold LIA period, regions located at the northern limit of the ITCZ rain belt (including
the Indian subcontinent), became drier relative to both the warm MCA
and the most recent 150 years, pointing to a possible southward shift of
the ITCZ (Yan et al., 2015). This also corroborates our synthesis of the Indian records that the ISM weakened significantly during the LIA as compared to the MCA period (Figs. 2 and 3). Recently, climate model
simulations suggested that in the western Pacific, the ITCZ not only migrated in the general northsouth pattern but also contracted over decadal to centennial timescales in response to external forcing during
the LIA period (Yan et al., 2015).
Apart from the seasonal movement of the ITCZ, Gupta et al. (2003)
used Arabian Sea sediments to correlate the planktic foraminifera abundance to the cold events in the North Atlantic. Increased G. bulloides during the MCA period coincided with a minimum in hematite and
therefore warmer sea surface temperature at the Bermuda rise in the
North Atlantic. Conversely, a minimum in G. bulloides at approximately
300–400 year BP was correlated with a brief maximum in hematite and
cooling at the Bermuda rise during the LIA, suggesting a teleconnection
between the ISM variability and the North Atlantic climate. Warm Eurasian landmass during the MCA is suggested to contribute to increased
transport of warm water to the North Atlantic, in turn warming the
ocean and the adjacent landmass; and the intensification of the thermal
gradient between the Indian Ocean and Eurasian landmass resulting in
stronger ISM during MCA. Feng and Hu (2008) used both instrumentation data of the 20th century and proxy records of the last 2000 years to
suggest that the North Atlantic SST anomalies strongly affect the Tibetan
Plateau surface temperature, which in turn affects the thermal gradient
between the Eurasian landmass and the Indian Ocean thereby changing
the summer monsoon circulation and rainfall.
The North Atlantic Oscillation (NAO) and El Nino-Southern Oscillation (ENSO) are the two important modes of climate variability
known to have significant influence on climate over the Indian subcontinent, particularly in the western Himalayas. A significant positive correlation between the winter NAO and winter precipitation in the
Karakoram and a negative correlation between NAO and summer rainfall is demonstrated (Archer and Fowler, 2004; Bhutiyani et al., 2010).
This could possibly explain increased precipitation during the beginning
of the LIA period in the western Himalayas (Fig. 5). For example, over
the western Himalayan region much of the precipitation in the season
of reconstruction (March–July) is contributed by westerly disturbances
and the precipitation was in phase with the NAO (Singh et al., 2009b).
Yadav (2009) also suggested that a positive phase of NAO and the
warm phase of ENSO are correlated with high winter precipitation.
The mechanism proposed is that the western disturbances are intensified over NW India by the intensification of Asian westerly jet stream
over the Middle East during positive phase of NAO and intensification
of Asian jet to the lower latitudes during the warm phase of ENSO
(Yadav, 2009). Yadav et al. (2011) used the Himalayan cedar tree
width chronology to suggest that reduced precipitation in the western
Himalayas could be associated with the decreasing trend in the frequency of the westerly disturbances.
Recently, Sinha et al. (2015) used stable isotopes in speleothems
from Northern India to study the ISM oscillations over the past two
millennia. The current drying trend observed in the records studied by
them are not only comparable to our compilation of other ISM records,
but also lie under “the envelope of monsoon oscillatory variability”
(Sinha et al., 2015). The thermodynamic effect on ISM rainfall is important on longer timescales (Wang et al., 2013), while at shorter timescales the ISM is more strongly influenced by changes in local SST
fields and other external boundary conditions like ENSO
(Krishnamurthy and Goswami, 2000). For the ISM variability during
the MCA period, Berkelhammer et al. (2010) used δ18O of Dandak
cave speleothem and compared it to the solar flux variability (Kodera,
2004) to suggest that the ENSO-modulated solar forcing (Emile-Geay
et al., 2007) was the cause of strong solar–monsoon relationship during
this period. There is also evidence from the eastern equatorial Pacific
about the change of a strong zonal gradient and weak ENSO-like conditions to a weak gradient and amplified ENSO around ~1500-1650 which
coincided with deepest LIA cooling and also weakened ISM during this
period. This was likely caused by southward shift of the Intertropical
Convergence Zone or vice-versa (Rustic et al., 2015). Outside the MCA
period, an absence of coherent relationships indicates varying ENSO dynamics and/or changing frequency and amplitude of Indian Ocean Dipole events (Berkelhammer et al., 2012), which can modulate the
ENSO impact on monsoon circulation (Ashok et al., 2004). Furthermore,
model simulations suggest that the decreasing trend of ISM in the 20th
century could be because of increased anthropogenic large-scale atmospheric aerosol loading over south Asia including the Indian subcontinent, which possibly checked the summer precipitation increase over
continental India which is predicted to have occurred in response to increasing global temperatures (Bollasina et al., 2011; Ramanathan et al.,
2005). The summer monsoon weakening is also suggested to be a result
of slowdown of the tropical meridional overturning circulation, which
compensates for the aerosol-induced energy imbalance between the
Northern and Southern Hemispheres (Bollasina et al., 2011).
5. Conclusions and summary
The MCA and LIA periods are well represented in the ISM and Westerlies regimes; however, there exists heterogeneity in terms of the
timing and duration. There is a data gap from the NE monsoon region
and also for the MCA period from the Westerlies dominated regions,
which restricted our interpretation of the spatial pattern of the
hydroclimatic variability in the last millennium. On a coarse temporal
resolution there is a synchronicity - the ISM was strong during the
MCA period and relatively weak during the LIA, while Westerlies
brought less rain during the MCA period and the Westerlies rainfall
strengthened significantly in the western Himalayas during the LIA.
However, the data density is too low to study the decadal-scale temporal structure of the MCA and LIA periods on the Indian subcontinent. Increased summer monsoon precipitation during the MCA is linked to the
ENSO-modulated solar forcing (Berkelhammer et al., 2010; Emile-Geay
et al., 2007). Increased precipitation in the Westerlies region during LIA
period is linked with the intensified western disturbances over NW
India caused by the intensification of the Asian westerly jet stream
over the Middle East during positive phase of NAO and migration of
the Asian jet to the lower latitudes during the warm phase of ENSO. Furthermore, cooler ocean and continental temperatures are also instrumental in forcing the Westerlies associated low-pressure systems
originating in the eastern Mediterranean, further south to be carried
by the southern winter jet stream, which brings more rainfall in the
Northern and North-eastern India during the LIA.
Y. Dixit, S.K. Tandon / Earth-Science Reviews 161 (2016) 1–15
Centennial-scale summer temperature changes in the western
Himalayas generally follow those of the Northern Hemisphere regions,
whereas spring temperatures do not show any correlation. The pretwentieth-century long-term global cooling trend is clearly displayed
in the summer temperatures in the Westerlies dominated regions. The
twentieth century warming observed globally and in the ASIA 2K records is also observed in the summer temperatures of the regions influenced by the ISM and Westerlies and a cooling trend is observed in
spring temperatures across Northern India.
This proxy-data compilation study is useful for understanding the
hydroclimatic variability during the past millennium in the Indian subcontinent and will also be useful for comparisons with the model simulations of the future hydroclimate variability in this region. This work
also compliments the recent study by Chen et al. (2015) of the synthesis
of the most up-to-date proxy precipitation records during the past millennium from China. Together, Chen et al. (2015) and this study offer a
comprehensive overview of the hydroclimatic variability for the past
millennium from subtropical Asia. These studies can serve as a benchmark for comparison of the 20th century climate change for better prediction of future climate variability in South Asia, a region that is
inhabited by more than half of the world's population.
Acknowledgement
This work was carried out entirely at the IIT Kanpur. YD acknowledges the support of the Institute Postdoctoral fellowship for her stay
at IITK. SKT acknowledges the liberal support provided by the Institute
through the D. N. Wadia Chair Professorship, as well as to the MoES,
Government of India who financially supported this chair professorship.
The unqualified support and encouragement of Head of the Department
of Earth Sciences, IIT Kanpur, Professor Rajiv Sinha is duly acknowledged. The authors thank Kanchan Mishra, Dr. Swati Sinha and Hojung
Kim for their help with editing of maps and figures. We also acknowledge Dr. Jayender Singh, Wadia Institute of Himalayan Geology, Dehra
Dun for sharing data as well as assistance in collection of literature on
some tree ring records. Finally, we also thank the editor for a meticulous
check of the manuscript.
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