Sensitivity of Indian summer monsoon rainfall and its variability to Indo-Pacific SST
patterns: Some observational and GCM studies
K. Krishna Kumar
Indian Institute of Tropical Meteorology, Pune, India
(krishna@tropmet.res.in)
Research Proposal
Preamble
The region of South Asia, particularly the Indian sub-continent, has a
geographical location with unique physical, cultural and economic characteristics,
whose destiny is tied to the Indian summer monsoon - one of the dominant systems of
the global climate. Most parts of India receive a major proportion of their annual
rainfall during the summer (June to September) monsoon season - the southern
peninsular region receives rainfall during the winter (October to December) monsoon
season. Extreme departures from normal seasonal rainfall, such as large-scale
droughts and floods, seriously affect agricultural output and regional economies. In
view of this, forecasting the monsoon rainfall, at least a season in advance, assumes
profound importance for policy-making and planning of mitigatory efforts (see
Krishna Kumar et al. 1995 for a detailed review on monsoon forecasting). The
questions that naturally come up are (i) what drives the monsoon variability from year
to year and (ii) can the rainfall be predicted in advance?
This problem of understanding and forecasting the monsoon rainfall has
caught the attention of Indian meteorologists from the early nineteenth century. By
the early 1900s, investigators had identified the two large-scale forcings - the
Himalayan/Eurasian snow extent (Blanford, 1884) and the El Niño Southern
Oscillation (ENSO) cycle (Walker, 1918). [These two forcings are still thought to be
most important for prediction of monsoon anomalies]. The extent of snow is generally
believed to provide an indication of the pre monsoon thermal condition over the Asian
land mass. Warmer conditions are thought to aid the buildup of a strong land-sea
thermal gradient during the summer (see Kripalani and Kulkarni 1999 for details on
monsoon rainfall-snow connections). ENSO, the largest known climatic forcing of
interannual monsoon variability, acts through the east-west displacement of largescale heat sources in the tropical Pacific. Numerous studies (Sikka, 1980; Pant and
Parthasarathy, 1981; Rasmusson and Carpenter, 1983, Shukla, 1987, Krishna Kumar
et al. 1995 and Webster et al., 1998) have shown a significant out of phase
simultaneous association (i.e. Strong ENSO leads to a weaker monsoon) between the
monsoon rainfall over Indian and ENSO indices. Models, both empirical and GCMs
utilize these connections to forecast the monsoon rainfall. However, the association
with ENSO has been weakening recently (Krishna Kumar et al., 1999a). Infact, most
climate models and empirical models predicted a strong drought over India during
1997, the year of the strongest El Niño of the last century, while the monsoon turned
out to be normal in that year. Although, the inter-decadal changes in the strength of
relationship between monsoon rainfall and various ENSO related precursors/indices
(Krishna Kumar et al. 1999b) are generally known to people, this lack of impact of
1997 El Niño event has led us to examine the possible causes. In Krishna Kumar et
al., (1999a) it was shown that the strength of the relationship between the monsoon
rainfall and the ENSO has been considerably weakening in the recent decades. The
reasons for this are generally believed to be due to (1) a southeast ward shift in the
ENSO induced anomalous Walker circulation leading to reduced suppression of
convection over the Indian sub-continent, thus favoring normal monsoon conditions
and (2) increased surface temperatures over Eurasia in winter and spring as a
consequence of recent mid-latitude continental warming trend, resulting in enhanced
land-ocean thermal gradient conducive for a strong monsoon.
Further probe into the past events of El Niño and associated monsoon rainfall
anomalies in the historical records indicates that the impact of different El Niño
events on the monsoon rainfall is not quite proportionate to their strength. For
example, the strong El Niño events of 1982 and 1997 have produced monsoon
seasonal rainfall anomalies, on all-India scale, of -13% and +2% respectively, while a
relatively weak event of 1987 has produced -18% anomaly. Incidentally this year's
(2002) weak/moderate El Niño with SST patterns fairly similar to 1987 El Niño event
has produced -19% rainfall anomalies for the India as a whole. This is another
example of failed forecast as most centers failed to anticipate severe drought
conditions over India. The failure of monsoon forecasts in these two recent years,
1997 and 2002, since the beginning of operational dynamical seasonal climate
forecast era that began in mid-90s is throwing up a challenge for the climate
community to identify other climatic forcings, if any, on the monsoon and their
possible role in upsetting well known ENSO linkages.
Proposed Activities
1. Sensitivity Experiments:
It
is hypothesized that the pattern of warming in the tropical Pacific is crucial to the
monsoon teleconnection, than the magnitude of warming. Hints of this can be seen
in the current ENSO and the ENSO of 1997. The current ENSO has warming in
the central Pacific and resulted in a drought over India, while the 1997 ENSO had
strong eastern Pacific warming which resulted in a normal rainfall over India. To
further understand this, we propose to perform series of idealized sensitivity
experiments using GCMs and lower order models. Whereby, models are forced
with different warming patterns over the Pacific and their dynamical atmospheric
response studied over the Indian region. This can lead to potential understanding of
changing relationships between ENSO and rainfalls over other parts of the world.
The
possible impact of recent continental warming and Indian ocean warming on the
connection between the Indian monsoon and ENSO is of great interest. It is
thought that the recent warming trend (part of the global warming) could interfere
with the ENSO monsoon teleconnection. Recent modeling experiments with
GCMs with a simple mixed layer ocean model, doubling the CO2 concentration
with fixing ENSO related SST pattern in the ocean indicate that the impact of
ENSO on the Indian monsoon is found to be much less in presence of strong
continental warming, suggesting that the enhanced land-sea gradient and an
enhanced monsoon due to general warming could counter the negative impacts of
El Niño on the monsoon to a large extent and thereby possibly weakening the
connection between these two important global climate phenomena. We propose
to investigate this hypothesis via analysis of data and model outputs and also
performing model experiments as described above. Other important aspect that
can be examined using these GCM experiments is the likely affect of recent Indian
Ocean and the continental Eurasian land warming in modulating the well known
biennial oscillation in the tropical monsoon system.
Potential Collaborators: Dr. Martin Hoerling, CDC (already contacted),
Dr. Henry Diaz, CDC (being contacted),
Dr.G.A. Meehl, NCAR (being contacted),
Dr. Balaji Rajagopalan and Dr. Andrew Moore,
Univ. of Colorado, Boulder (already contacted).
2. Forecasting of intraseaonal modes of monsoon rainfall
It is very much essential to address the issue of intraseasonal monsoon rainfall
variability and its predictability in the context of resource management. Extensive
daily rainfall data from a number of stations over the Indian sub-continent is
available. We propose to performing a detailed analysis of intra-seasonal attributes
of monsoon e.g., wet spells, dry spells, transition probabilities from wet to dry
spells etc. and their connections to large scale climate forcings. This will have
immediate application for water and agricultural management.
Potential Collaborator:
Dr. Balaji Rajagopalan, UC
3. Improving Seasonal Forecast Skills using Empirical Methods:
Some of the scientists contacted have shown great interest in revisiting the
empirical monsoon prediction methods that are currently being used and to
propose effective alternatives (i.e. nonparametric methods) for improvising the
predictive skills. Also, attempts will be made to improve the skills of GCM
simulations of seasonal forecasts over the Indian region by using
statistical/dynamical downscaling techniques involving multi-member/multi-model
retrospective forecasts.
Potential Collaborators:
Dr. Martin Hoerling, CDC
Dr. Balaji Rajagopalan, UC
4. Monsoon Hydrological Cycle
Understanding and quantifying various components of the hydrological/water
cycle of Asian summer monsoon in general and the Indian monsoon in particular will
be of tremendous use in assessing the sources and sinks of large scale moisture fields
in the ocean-land-atmosphere system in the Indo-Pacific region. Efforts will be made
to undertake such a study during the proposed fellowship tenure involving observed
and model simulated (reanalysis) regional and global data sets.
Potential Collaborators:
Dr. Balaji Rajagopalan, UCB
Prof. Vijay Gupta, CIRES/UCB
Dr. Andrew Moore, PAOS, UCB
References:
Blanford, H.H., 1884: On the connection of Himalayan snowfall and seasons of
drought in India. Proc. R. Soc., London, 37, 467-487.
Krishna Kumar, K., Soman, M.K. and Rupa Kumar, K., 1995. Seasonal forecasting of
Indian summer monsoon rainfall, Weather, 50, 12, 449-467.
Krishna Kumar, K., Rajagopalan, B. and Cane, M.A., 1999a: On the Weakening
Relationship between the Indian Monsoon and ENSO, Science, 284, 2156-2159.
Krishna Kumar, K., Kleeman, R., Cane, M.A. and Rajagopalan, B., 1999b: Epochal
changes in Indian Monsoon-ENSO Precursors, Geophysical Research Letters,
26, 75-78.
Kripalani, R.H. and Kulkarni, A. 1999, Climatology and variability of historical
Soviet snow depth data: Some new perspectives in snow-Indian monsoon
teleconnections, Climate Dynamics, 15, 475-489.
Pant, G.B. and Parthasarathy, B., 1981. Some aspects of an association between the
southern oscillation and Indian summer monsoon. Arch. Meteorol. Geophys.
Biokl., Sr. B., 29, 245-251.
Rasmusson, E.M. and Carpenter, T.H., 1983. The relationship between eastern
equatorial Pacific sea surface temperature and rainfall over India and Sri Lanka,
Mon. Wea. Rev., 111, 517-528.
Shukla, J. 1987, Interannual variability of monsoons, in Monsoons, edited by J.S. Fein
and P.L. Stephens, 399-464, John Wiley and Sons.
Sikka, D.R., 1980. Some aspects of the large-scale fluctuations of summer monsoon
rainfall over India in relation to fluctuations in the planetary and regional scale
circulation parameters. Proc. Ind. Acad. Sci. (Earth. Planet. Sci.), 89, 179-195.
Walker,G.T., 1918: Correlation in seasonal variation of weather. Q. J. R. Meteorol.
Soc., 44, 223-224
Webster,P.J., V.O. Magana, T.N. Palmer, J. Shukla, R.A. Tomas, M. Yanai and T.
Yasunari, 1998: Monsoons: Processes, predictability, and the prospects fro
prediction. J. Geophys. Res., 103, 14,451-14,510.
(There exists a vast literature on monsoon variability and global teleconnections and the references
cited here are not intended to be very exhaustive)