Mechanisms and trends of the synoptic conditions that led to the 14 October 2014 snowstorm
in the Nepalese Himalayas
S.-Y. Simon Wang12, Boniface Fosu2, Robert R. Gillies12, and Pratibha M. Singh3
1. Utah Climate Center, Utah State University, Logan, UT, USA
2. Dept. Plants, Soils and Climate, Utah State University, Logan, UT, USA
3. Nepal Department of Hydrology and Meteorology, Kathmandu, Nepal
Corresponding author: simon.wang@usu.edu
1. Introduction
October 14, 2014 was a particularly dark day in the history of Mount Everest when an
unanticipated blizzard initiated avalanches that resulted in the death of 16 Nepalese Sherpas
("Sorrow on the Mountain" National Geographic. Chip Brown. Web. Nov. 2014). The blizzard
condition were connected with tropical cyclone (TC) Hudhud – a category-4 hurricane that
developed in the Bay of Bengal (BoB) – that, after making landfall in eastern India proceeded
northward towards Nepal. Elsewhere, TC Hudhud led to upwards of more than 43 deaths on
popular Himalayan trekking routes in addition to a swath of destruction in eastern India.
Although TC Hudhud had been visible on radar and satellite images for days, authorities in
Nepal did not issue any cautionary notices or warnings as to the likely development of any
extreme weather conditions (Lella Mani Paridyal, Chief Secretary; The Government of Nepal.
Reuters WWW, Oct 17, 2014).
Nepal is ranked the fourth most climate-vulnerable country in the world; it is prone to a wide
variety of weather related hazards that encompass droughts, floods and landslides (Wang et al.
2013, Gillies et al. 2013). Tropical cyclones of Hudhud’s magnitude are not unprecedented in the
BoB, and neither are snowstorms or blizzards in the Himalayas. However, sudden weather
1
changes resulted from Cyclone Hudhud brought about an extreme event that created heavy
snowfall and reportedly caused avalanches like the one on Mount Dhaulagiri. Hence, it is
imperative to investigate the specific meteorological processes that trigger such extreme events,
as well as the climatological processes that tend to enhance them; this with a view perhaps to
improving future predictability.
2. Data and methods
The National Center for Environmental Prediction (NCEP)/National Center for
Atmospheric Research Global Reanalysis (R1; Kalnay 1996), the NCEP/Department of Energy
Global Reanalysis II (R2; Kanamitsu et al. 2002), and the Global Forecast System (GFS) initial
analysis were applied in the analysis. We used R1 data to derive transient activity and variations
in the post-monsoon (September-October) upper tropospheric jet stream. Transient eddies
associated with synoptic-scale disturbances were extracted from daily meridional winds (𝒗′ )
bandpassed for 2-8 days averaged yearly from September 15 to October 31 (centered around the
𝟐
avalanche event). The transient eddy activity is measured by the variance of ̅̅̅̅
𝒗′ . Additional data
sets included NOAA’s CPC Morphing Technique (CMORPH) precipitation (Joyce et al. 2004). .
TC best track records were obtained from the Joint Typhoon Warning Center (JTWC).
Since TC wind strength weakens quickly after making landfall, the official tracks do not record
their residual disturbances inland. Therefore, we utilized 850-mb streamline and vorticity
analyses from the R2 data to document those residual disturbances; this by tracking from the
TCs’ last known position onwards until the relative vorticity in the rotation center became
smaller than 3×10-5s-1 (Chen et al. 2005). The result of the hybrid tracks (i.e. TCs + residual
disturbances) for the post-monsoon season (September-October) is illustrated in Fig. 1.
2
For the purposes of attribution, we analyzed the Coupled Model Intercomparison Project
(CMIP5) historical single-forcing experiments that were driven by (i) aerosol forcing only
(AERO), (ii) greenhouse gas forcing only (GHG), (iii) natural forcing only (NAT; including
volcanic and solar forcings) and (iv) all of these forcings combined (ALL) – these were
initialized from long stable preindustrial (1850) control settings up to 2005 (Taylor et al. 2012).
Output from four models were used in the analysis, each with 3 members, including (a) the
Community Earth System Model version 1 (CESM-1), (b) the Community Climate System
Model version 4 (CCSM4), (c) the Norwegian Earth System Model version 2 (NorESM2) and
(d) the Goddard Institute for Space Studies Model E version 2 (GISS-E2).
3. Results
a. Observed changes
Fig. 1 shows all tropical cyclone tracks from 1979 to present, with the tracks before 1997
(after 1996) marked as blue (red) while showing TC Hudhud as highlighted. Unlike most BoB
storms that dissipate quickly over land or travel northwestwards across India, Hudhud was the
only TC whose residual disturbance ever crossed the Indo-Gangetic Plains to reach as far north
as the Himalayas. As shown by the color differences of TC tracks in Fig. 1, post-1996 cyclones
apparently are more concentrated in the northern BoB than pre-1997 ones, suggesting an increase
in TC threat over the Gangetic Plains during fall. It was found that BoB cyclones have grown
more intense since 1979 due to increased sea surface temperatures (Balaguru et al. 2014). In
addition, it has been found that increasing anthropogenic aerosol loading has also deepened the
quasi-stationary trough in the BoB and this has resulted in a tendency for TCs to turn
northeastward (Wang et al. 2013). However, how could a TC’s remnant result in a blizzard?
3
The synoptic conditions leading up to the 14 October 2014 event are illustrated by dailymean precipitation and GFS wind vectors at the 250 mb (Fig. 2a) and 850 mb (Fig. 2b) levels.
Over the 12-13 October period, a short-wave trough, embedded in the jet stream over South
Asia, was moving eastward across the Himalayas. On 14 October, the short-wave trough
deepened and subsequently collided with a weakening TC Hudhud over northern India. The
encounter enhanced precipitation conditions through the coupling of the frontal and TC
precipitation regimes while the interaction of the two systems meant that the precipitation band
was redirected northeastwards towards the Nepalese Himalayas. From a synoptic point of view,
the coupling of this short-wave trough and a recurving TC is somewhat reminiscent of the
conditions that lead to Hurricane Sandy (Torn et al. 2012), which devastated the northeastern
U.S. in October 2012, albeit in very different in latitudes (i.e. TC Hudhud around 25°N; Sandy
around 45°N). The upper- and lower-level coupling also points to the importance of tropicalmidlatitude interactions that adds complexity to cyclone track and precipitation intensity (e.g.,
Kim et al. 2012, Riemer et al. 2014), a feature that is more common in the Northwestern Pacific
than the BoB.
In order to examine whether this event was random or perhaps may be recurrent, linear trends
of the transient activity during September-October were computed over the period of 1979-2014
at upper and lower levels, respectively 250 mb (Fig. 3a) and 850mb (Fig. 3b). Here the synoptic
̅̅̅̅
′𝟐 ) serves as an objective indicator for upper-level short waves and lowertransient activity (𝒗
level disturbance activity. As is indicated by an arrow in Fig. 3a, at the upper level there is a
universal increase in ̅̅̅̅
𝒗′ all the way from the Mediterranean Sea to northern India and Nepal.
𝟐
𝟐
The immediate cause of this increase in upper-level ̅̅̅̅
𝒗′ was examined from the latitudinal cross-
section of 250-mb zonal winds (Fig. 3c) averaged over the longitude zone from 68°E to 78°E,
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i.e. over and upstream of western Nepal. Here, the analysis used the R1 data in order to trace the
circulation change back to 1950. A southward migration of the jet is discernable and this is
further illustrated by the linear trend of ̅̅̅̅
𝒗′ (contours). In fact since 1950, an estimated 275-km
𝟐
southward shift of the jet stream is observed in the latitudinal zone of Nepal; this southward shift
of zonal wind supports the increase (or enhancement) of synoptic waves to the south of the
𝟐
climatological jet core around 40°N, accompanied by a decrease in ̅̅̅̅
𝒗′ to the north.
𝟐
At the lower level (i.e. at 850mb, Fig. 3b), increased ̅̅̅̅
𝒗′ is observed over the BoB and when
𝟐
compared with the TC tracks (Fig. 1), such an increase in ̅̅̅̅
𝒗′ is suggestive of more and/or
stronger disturbances affecting northern India; this is indicated by the arrow in Fig. 3b. These
synoptic pattern changes suggest that an equatorward shift of the upper jet has increased the
chance for propagating short waves to interact with additional, if not amplified, inland
disturbances that cross the Gangetic Plains. To further clarify this supposition, we computed the
2-day mean of the eddy streamfunction (ψE) at 250 mb and 850 mb for 13-14 October 2015 and
compared it with any given 2-day mean of ψE over the September-November period from 1979
to 2014. The domain for comparison at 250 mb was set at 60°-80°E, 20°-40°N to cover the
dimension of the short-wave trough, while the domain for 850 mb was set to 75°-90°E, 15°-35°N
encompassing the extent of TC Hudhud. The spatial correlation coefficient (ρ) was then
computed for each pressure level between the 13-14 October 2014 case and all of the 2-day
means. We set a high bar for such case identification: For any given 2-day period, ρ had to be at
least 0.95 at 250 mb and above 0.9 at 850 mb. In Fig. 3d, we show the occurrences for the
identified cases at 250 mb and 850 mb separately and collectively. The occurrence of short-wave
troughs similar to the October 2014 case has increased five-fold over the 36-year period, while
the occurrence of low-level disturbances similar to the remnant of TC Hudhud has increased
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two-fold. For the ‘coupled’ case in which ρ at both levels satisfies the criteria at the same time,
the occurrence has increased two-fold, though the small sample size may not warrant a long-term
trend (sensitivity test described in the caption). The results indicate that the chance for a
propagating short wave embedded in the jet stream to interact with a tropical disturbance moving
from the BoB has significantly increased in the recent decade.
b. Attribution
We looked into the mechanism leading to the observed equatorward shift of the South Asian
jet by analyzing the change in the meridional temperature gradients
𝝏𝑻
𝝏𝒚
. Fig. 4a shows the
𝝏𝑻
observed (R1) 250-mb 𝝏𝒚 and it depicts a clear equatorward shift in correspondence to the zonal
𝝏𝑻
wind shift shown in Fig. 3c. Noteworthy is the accelerated migration of 𝝏𝒚 after the 1980s, which
is most pronounced in the southern flank of the jet stream at the latitude zone of 25°-35°N.
Evidently, the jet stream maintenance related to thermal wind balance has undergone a persistent
modulation by tropospheric temperature change, which started to prevail since the 1980s.
𝝏𝑻
The CMIP5 ensembles of Historical Single Forcing Experiments enabled us to analyze the 𝝏𝒚
change the same way as Fig. 4a from the ALL (b), GHG (c), NAT (d) and AERO (e) forcing
𝝏𝑻
experiments. The ALL-forcing experiment depicts the 𝝏𝒚 pattern where the modeled change is in
good agreement with the observed, i.e. it demonstrates a noticeable equatorward shift within 25°35°N. Compared to other single-forcing experiments, the equatorward shifting
𝝏𝑻
𝝏𝒚
appears to
result primarily from the increase in GHG (Fig. 4c), which reveals a widespread southward shift
𝝏𝑻
all the way to 15°N (or rather, an expansion of 𝝏𝒚 from the jet core ~40°N). The NAT-forcing
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experiment (Fig. 4d) did not result in any shift in
𝝏𝑻
𝝏𝒚
north of 30°N, but exhibited a slight
poleward shift south of it. A possible secondary cause comes from anthropogenic aerosols
(AERO; Fig. 4e), which produced a mild shift confined to the south of 30°N. While the
𝝏𝑻
widespread GHG effect likely has caused 𝝏𝒚 of the entire jet stream to expand, the AERO effect
(to warm the upper troposphere) is regionally confined to the south of the Himalayas with a
particularly high concentration in the Gangetic Plains (Lodhi et al. 2013, Kastaoutis et al. 2014,
Wang et al. 2013); this latter phenomenon may be the reason why the AERO-forced shift in
𝝏𝑻
upper-level 𝝏𝒚 is only observed south of 30°N.
4. Conclusion
The occurrence of extreme weather/climate events is almost always multifarious and,
under a changing climate, involves both internal and external forcings. In the case of the 14
October 2014 blizzard that struck the Nepalese Himalayas, the unusual coupling of a short-wave
upper trough with a lower-level tropical disturbance, both of which were strong and moistureladen, enhanced lifting and instability resulting in a blizzard concomitant with heavy snowfall.
The climate diagnostics presented here point to an observed tendency for more and/or stronger
upper-level short waves propagating from the Mediterranean Sea towards northern India in the
post-monsoon season (September-October) that will all the time coincide more with an increase
in the low-level tropical disturbances from the BoB. The changing synoptic climatology within
and over both northern India and Nepal would seem to signal an increased possibility of similar
events occurring in the future.
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Attribution analyses, using the CMIP5 single-forcing experiments, indicate that increased
anthropogenic GHG and aerosols both have contributed to the equatorward shift of the thermal
gradient and the jet stream, a feature that arguably diverts more/stronger short waves towards
South Asia and towards Nepal. Additional analysis is required to provide an in-depth
mechanistic understanding of the reported changes in the tropospheric circulations. Nevertheless,
given other documented changes such as a warming SST enhancing TCs in the BoB (Knutson et
al. 2004, Balaguru et al. 2014), the northward tendency of TC tracks in the BoB (Wang et al.
2013a), and the increased baroclinicity in the weather regime of northern South Asia during the
pre- and post-monsoon seasons (Wang et al. 2011, 2013b), we have come to a provisional
conclusion that the possibility for blizzards on the scale of the 13-14 October 2014 event has
increased and is likely to keep increasing.
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..1979-1996
..1997-2014
Residual low
disturbances
TC best track
Fig. 1 Fall (September-October) tropical cyclone (TC) tracks in the BoB, over the period of 1979-2014. Dotted tracks
are based on JTWC and solid lines are residual disturbances defined by manual tracking (see text). TC Hudhud is
shown in thick orange. Pre-1997 TC are indicated by blue while post-1996’s TCs are marked in red.
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mmd-1
mmd-1
ms-1
ms-1
Fig. 2 Daily-mean GFS initial winds (vectors) at (a) 250 mb and (b) 850 mb
overlaid with CMORPH precipitation (shadings) from 12-15 October 2014),
indicating TC Hudhud’s stages from landfall to impacting Nepal and its
coupling with the eastward-moving upper trough.
13
a.
c. 250-mb zonal wind
250 mb
d.
upper level
m2s-2yr-1
Number
3
850 mb
coupled
2
ms-1
b.
lower level
1
0
1950
1953
1956
1959
1962
1965
1968
1971
1974
1977
1980
1983
1986
1989
1992
1995
1998
2001
2004
2007
2010
2013
m2s-2yr-1
Year
Fig. 3. Linear trend slope of transient activity
over the period of Sep 15 to Oct 31 computed for each year from 1979 to 2014 at (a) 250 mb and
(b) 850 mb. (c) The latitude-time cross-section of the Sep-Oct mean 250-mb zonal winds averaged over 68°E to 78°E (shadings), superimposed with
the linear trends (contours). (d) Histogram of the occurrences of upper- and lower-level synoptic pattern identified to be similar to that of the 13-14
October 2014 situation based on spatial correlation ρ(see text); the coupled cases are shown in green. The 20-year running means (one-sided) are
shown as thick lines. Of note is that by changing the criteria of ρ between 0.92-0.98 for 250 mb and 0.85-0.95 for 850 mb, the occurrences
changed correspondingly but the rate of increase did not.
14
(a) R1
(b) ALL
(c) GHG
(d) NAT
(e) AREO
K Km-1
K Km-1
Fig. 4 The latitude-time diagram for the meridional temperature gradients at 250 mb averaged
o
o
over longitudes 60 E-80 E, using (a) the R1 dataset, and the Historical experiments of (b) ALL,
(c) GHG, (d) NAT and (e) AERO forcings. The linear trends are overlaid as contours.
15