Transcript All-Nepal Summer Monsoon Rainfall Variability, its

Teleconnection of Tropical Pacific and
Indian Ocean Oscillation with
Monsoon Rainfall Variability over
Nepal
Lochan P. Devkota
&
Ujjwal Tiwari
8/8/2014
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What is the status of monsoon 2014?
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What DHM records?
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Weak monsoon in 2014 is due to El Nino

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El Niño is the collapse of the Walker Circulation
A temporary circulation known as El Niño-Southern
Oscillation (ENSO) is replacing it
SST in El Niño (left) and La Niña (right) phenomenon
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Monsoon is influenced by large scale
circulations:

Hadley Circulation
• Walker
circulation
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Indian Ocean Dipole (IOD)

Difference in SST between western Arabian Sea and eastern
Indian Ocean (south of Indonesia)

Positive IOD (PIOD): Cold SST anomaly over eastern Indian
Ocean

Negative IOD (NIOD): Opposite anomalous feature over same
region
PIOD (left) and NIOD (right):
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Study Area
Study area map
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Data used
SST, wind, mslp, specific humidity taken from ERA interim
reanalysis dataset produced by ECMWF
 Out going long-wave radiation (OLR) data obtained from
NOAA
 All-Nepal summer monsoon rainfall (ANSMR) data was
derived from APHRODITE

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Spatial and temporal variation of ANSMR

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Large spatial and temporal variation
Northwestern part receive least rainfall
Several high rainfall pockets
Precipitation (mm)
800
750
700
650
600
550
500
1978
Spatial variation of ANSMR (JA) averaged
over the period 1978-2007
1984
1990
1996
2002
Interannual variability of summer
monsoon (JA) rainfall over Nepal
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Spatial and temporal variation of ANSMR

ANSMR shows 9-11 years oscillatory characteristics

Shrestha et al. (2000) observed significant peaks at an
interval of 2.5 and 11 years

Mann Kendall trend test did not reveal any trend in ANSMR
for the period 1978-2007

Below normal rainfall for 1991-1993 coincides with
prolonged El Niño and 1991 Mount Pinatubo eruption

Above normal rainfall for the period 1998-2001 coincides
with elongated La Niña
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Data Analysis

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Intensity of El Niño/La Niña measured by Nino 3.4 index
Intensity of IOD measured by dipole mode index (DMI)
Strength of Walker circulation measured by low level wind at
850hPa averaged over equatorial Pacific (100S-100N, 1400E1200W)
Hadley circulation index was defined as the maximum
absolute value of zonal mean meridional mass stream function
in the 00 to 300N latitude zone
Nino 3.4 and walker circulation
index
Dipole mode index
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Linkage among large scale atmospheric circulations
and ANSMR

Negative association between Walker and Hadley
circulations (r= -0.589, p<0.01)

Bjerkines (1969) also found an inverse relationship
between these
two circulations
Walker Circulation
Hadley Circulation
ANSM Precipitation
4
2
1.5
3
1
2
0.5
1
0
0
-0.5
-1
-1
-2
-1.5
-2
-3
1980
1984
1988
1992
1996
2000
2004
ANSMR anomaly with Walker circulation index and Hadley circulation
index. ANSMR is plotted on secondary axis
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El Niño, IOD and ANSMR
Nino 3.4 exhibit large inter-annual variability
3
y = -0.0143x + 0.249
R² = 0.0204
2.5
2
1.5
SST Anomaly

1
0.5
0
-0.5
-1
-1.5
-2
1979
1983
1987
1991
1995
1999
2003
2007
2011
Pacific SST anomaly averaged over Nino3.4 region
ANSMR negatively correlated (r=-0.59, p<0.01)
with ENSO
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El Niño, IOD and ANSMR
Rainfall anomaly during El Niño years
Rainfall anomaly during La Niña
years
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El Niño, IOD and ANSMR
OLR anomaly during El Niño years

OLR anomaly during El Niña years
Less cloud during El Niño and more cloud cover in La
Niña
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El Niño, IOD and ANSMR
Rainfall anomaly for the year in which El
Niño and PIOD co-occurred
Rainfall anomaly for the year in which El
Niño occurred alone
Rainfall anomaly for the year in which La Rainfall anomaly for the year in which La
Niña occurred alone
Niña and NIOD co-occurred
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El Niño, IOD and ANSMR

IOD phenomenon countered the effect of El Niño for
ANSMR
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Moisture budget
Moisture flux divergence in highest/least rainfall years
Moisture flux divergence (10-5
Kgm-2S-1) due to anomalous wind
and average humidity in the
highest rainfall years.
Moisture flux divergence (10-5
Kgm-2S-1) due to anomalous wind
and average humidity in the least
rainfall years
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Analysis of moisture budget
Moisture flux divergence in El Niño/La Niña years
Figure 3.26: Moisture flux
divergence (10-5 Kgm-2S-1) due
to anomalous wind and average
humidity in El Niño years
Figure 3.27: Moisture flux
divergence (10-5 Kgm-2S-1) due
to anomalous wind and average
humidity in La Niña years.
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Conclusion
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Large spatial and temporal variations of ANSMR
9-11 years oscillatory characteristics
No trend as per Mann Kendall trend test
Negatively correlated with Nino 3.4 index
IOD countered the effect of ENSO
Driven by the interplay between Walker and Hadley
circulations
Moisture divergence flux revealed dipole characteristics
between western and eastern Indian ocean
Convergence around Arabian Sea and western part of
Bangladesh in highest ANSMR months
Convergence around eastern coast of India in least ANSMR
months
Similar trend observed during La Nina and El Nino years
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Thank you!!!!!!
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