Teleconnection of Tropical Pacific and Teleconnection of Tropical Pacific and Indian Ocean Oscillation with Indian Ocean Oscillation with

Monsoon Rainfall Variability over Monsoon Rainfall Variability over NepalNepal

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Lochan P. Devkota&

Ujjwal Tiwari

What is the status of monsoon 2014?What is the status of monsoon 2014?

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What DHM records?What DHM records?

Weak monsoon in 2014 is due to El NinoWeak monsoon in 2014 is due to El Nino

El Niño is the collapse of the Walker CirculationA temporary circulation known as El Niño-Southern

Oscillation (ENSO) is replacing it

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SST in El Niño (left) and La Niña (right) phenomenon

Monsoon is influenced by large scaleMonsoon is influenced by large scalecirculations:circulations:

Hadley Circulation

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• Walker circulation

Indian Ocean Dipole (IOD)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

8/8/2014 6PIOD (left) and NIOD (right):

Study AreaStudy Area

Study area map

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Data usedData 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 ANSMRSpatial and temporal variation of ANSMR

Large spatial and temporal variationNorthwestern part receive least rainfallSeveral high rainfall pockets

Spatial variation of ANSMR (JA) averaged over the period 1978-2007

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Interannual variability of summer

monsoon (JA) rainfall over Nepal

Spatial and temporal variation of ANSMRSpatial 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 AnalysisData Analysis 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, 1400E-1200W)

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 index8/8/2014 11

Linkage among large scale atmospheric circulations Linkage among large scale atmospheric circulations and ANSMRand 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

ANSMR anomaly with Walker circulation index and Hadley circulation index. ANSMR is plotted on secondary axis

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El Niño, IOD and ANSMREl Niño, IOD and ANSMR

Nino 3.4 exhibit large inter-annual variability

Pacific SST anomaly averaged over Nino3.4 region

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ANSMR negatively correlated (r=-0.59, p<0.01) with ENSO

El Niño, IOD and ANSMREl Niño, IOD and ANSMR

Rainfall anomaly during El Niño years Rainfall anomaly during La Niña years

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El Niño, IOD and ANSMREl Niño, IOD and ANSMR

Less cloud during El Niño and more cloud cover in La Niña

OLR anomaly during El Niño years OLR anomaly during El Niña years

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El Niño, IOD and ANSMR 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 Niña occurred alone

Rainfall anomaly for the year in which La Niña and NIOD co-occurred

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El Niño, IOD and ANSMREl Niño, IOD and ANSMR

IOD phenomenon countered the effect of El Niño for

ANSMR

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Moisture budgetMoisture 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 budgetAnalysis 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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ConclusionConclusion

Large spatial and temporal variations of ANSMR9-11 years oscillatory characteristics No trend as per Mann Kendall trend testNegatively correlated with Nino 3.4 index IOD countered the effect of ENSODriven by the interplay between Walker and Hadley

circulationsMoisture divergence flux revealed dipole characteristics

between western and eastern Indian oceanConvergence around Arabian Sea and western part of

Bangladesh in highest ANSMR monthsConvergence 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!!!!!!Thank you!!!!!!

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