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On the intra-seasonal to decadal climate variability over South-Asia
Stockholm University, Faculty of Science, Department of Meteorology . (Climate Dynamics)
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

South Asia, a land of contrasting landscapes, seasons and climates, is highly vulnerable to climate variability over intra-seasonal to decadal time scales. In winter, precipitation over the western parts of south Asia and fog over the Indo-Gangetic (IG) plains are the two major climatic features. During summer most of the region comes under the grip of monsoon.

Winter precipitation over the north-western parts of South Asia is associated with eastwards propagating ‘western disturbances’ originating mostly from Mediterranean. Both observations and regional climate-model simulations show that the winter precipitation increases/decreases during the positive/negative phases of the North Atlantic Oscillation (NAO) and the warm/cold phase of the El Niño-Southern Oscillation (ENSO). During these phases, the intensification of western disturbances results from the effect of an enhanced trough visible at sea-level as well as at higher altitudes over central Asia. The inter-annual variability of fog is coupled over IG plains with a significant trend in the fog frequencies, both in observations and ERA-Interim reanalysis data. This increase shows two distinct regime shifts in 1990 and 1998 with respect to mean and variance, this in contrast to a gradual increase of the humidity over the region.

The thermodynamic analysis of the intra-seasonal summer monsoon active phases (APs) over Pakistan revealed that a few days before AP, an upper-level warm anomaly appears over the northern Hindu Kush-Himalaya region and is reinforced by surface heating. The baroclinic height anomalies, with a low-level anticyclone located east of the warming, causes a moisture convergence, strong enough to overcome the preexisting stable atmospheric conditions. The extratropical dynamics also play an important role for the inter-annual variation of the South-Asian monsoon. It is found that the two leading modes between the upper-level circulation in the Atlantic/European region and monsoon rainfall are the Circumglobal Teleconnection (CGT) and the summer NAO. The positive phase of the CGT is related to a widespread increase of monsoon rainfall, and a positive summer NAO is related to a precipitation dipole with its positive anomaly over Pakistan.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University , 2011. , 30 p.
Keyword [en]
South Asia, monsoon, western disturbances, fog, climate variability, climate dynamics, teleconnections, ENSO, NAO, CGT
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-62988ISBN: 978-91-7447-375-9 (print)OAI: oai:DiVA.org:su-62988DiVA: diva2:446306
Public defence
2011-11-18, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 13:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Submitted. Available from: 2011-10-27 Created: 2011-10-06 Last updated: 2011-10-13Bibliographically approved
List of papers
1. Regional climate model simulation of winter climate over Central-Southwest Asia, with emphasis on NAO and ENSO effects
Open this publication in new window or tab >>Regional climate model simulation of winter climate over Central-Southwest Asia, with emphasis on NAO and ENSO effects
2010 (English)In: International Journal of Climatology, ISSN 0899-8418, E-ISSN 1097-0088, Vol. 30, no 2, 220-235 p.Article in journal (Refereed) Published
Abstract [en]

We present a regional climate modeling study of the winter climate of Central-Southwest Asia (CSWA), focusing on the mean model climatology of temperature and precipitation, the simulated storm characteristics and the effects of El-Nino Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) on storm activity and precipitation. Forty-three winter seasons are simulated with the model RegCM3 driven by ERA40 reanalysis fields over a region encompassing CSWA and the Mediterranean sector. The model shows a good performance in reproducing the observed mean surface climate of the region as well as the characteristics of the western disturbances affecting CSWA. Observations show that both the NAO and ENSO have a substantial influence oil the CSWA climate, particularly in the region of Northern Pakistan, Afghanistan and Tajikistan, where precipitation increases (decreases) during the positive (negative) NAO phase and warm (cold) ENSO phase. The model captures well the observed signals and a detailed storm analysis shows that they are mostly associated with an intensification of western disturbances originating in the Eastern Mediterranean and Middle East regions and moving eastward across a 500 hPa trough situated over the CSWA during the positive NAO and warm ENSO phases. The NAO and ENSO signals are reinforced by enhanced moisture sources from the Mediterranean, Caspian and Arabian Seas. Our study provides encouraging indications towards the use of the model RegCM3 for climate simulations over CSWA.

Keyword
NAO, ENSO, Precipitation variability, western disturbances, extratropical storms, cyclone tracking, regional climate model RegCM3
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-50060 (URN)10.1002/joc.1887 (DOI)000274601900006 ()
Note
authorCount :4Available from: 2010-12-30 Created: 2010-12-21 Last updated: 2017-12-11Bibliographically approved
2. Are intraseasonal summer rainfall events micro monsoon onsets over the western edge of the South-Asian monsoon?
Open this publication in new window or tab >>Are intraseasonal summer rainfall events micro monsoon onsets over the western edge of the South-Asian monsoon?
2010 (English)In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 98, no 2-4, 341-346 p.Article in journal (Refereed) Published
Abstract [en]

A thermodynamic structure leading the active phase (AP) of the Western Edge of the South-Asian Monsoon (WESAM) is investigated The APs seems to have significant contribution in the mean seasonal rainfall in the region A few days before APs the upper level warm anomaly appears over the north Hindu Kush-Himalaya (HKH) region and It is reinforced by surface heating yielding the column average warming The height anomalies are baroclinic with the low-level anticyclone being located at the east of warming The low-level anticyclone causes the moisture convergence at the core WESAM region As the region keeps warming, the height anomalies and associated low-level convergence become stronger The AP starts when the low-level moisture convergence is strong enough to overcome the preexisting stable atmospheric condition due to the upper level warming The proposed mechanism of APs has some resemblance with large scale south Asian monsoon onset, whereas conventional south Asian monsoon intraseasonal oscillations (ISOs) do not show clear relationship with APs of WESAM

Keyword
Onset, Pakistan, Intraseasonal variability, Monsoon, Active phases
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-51241 (URN)10.1016/j.atmosres.2010.07.006 (DOI)000284983400016 ()
Note
authorCount :4Available from: 2011-01-14 Created: 2011-01-10 Last updated: 2017-12-11Bibliographically approved
3. Extratropical Influences on the Inter-Annual Variability of South-Asian Monsoon
Open this publication in new window or tab >>Extratropical Influences on the Inter-Annual Variability of South-Asian Monsoon
2012 (English)In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 38, no 7-8, 1661-1674 p.Article in journal (Refereed) Published
Abstract [en]

The effects of extratropical dynamics on the interannual variations in South-Asian Monsoon (SAM) are examined. Based on NCEP/NCAR reanalysis and CRU precipitation data, a conditional maximum covariance analysis is performed on sea level pressure, 200 hPa geopotential heights and the SAM rainfall by removing the linear effects of El-Niño Southern Oscillation from the fields. It is found that two modes provide a strong connection between the upper-level circulation in the Atlantic/European region and SAM rainfall: the Circumglobal Teleconnection (CGT) and the Summer North Atlantic Oscillation (SNAO). The structures in the 200 hPa heights of both modes in the Atlantic region are similar in the Atlantic region, and their southeastward extension to South Asia (SA) also corresponds to upper-level ridges (in their positive phases) in slightly different positions. Nevertheless, the influence of both modes on SAM rainfall is distinct. Whereas a positive CGT is related to a widespread increase of rainfall in SAM, a positive SNAO is related to a precipitation dipole with its positive phase over Pakistan and the negative phase over northern India. The physical mechanisms for the influence of CGT and SNAO on SAM are related to the upper-level geopotential anomaly which affects the amplitude and position of the low-level convergence. The small displacements of the centers of these responses and the low level cold advection from the north east of SA in case of SNAO explain the differences in the corresponding SAM rainfall distributions. These findings are confirmed with the relatively high-resolution coupled climate model EC-Earth, which gives confidence in the physical basis and robustness of these extratropical variability modes and their influence on the South-Asian monsoon rainfall.

Keyword
Inter-annual variability, Monsoon, CGT, Summer NAO, ENSO
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-62377 (URN)10.1007/s00382-011-1059-4 (DOI)000302247000025 ()
Note

4

Available from: 2011-09-16 Created: 2011-09-16 Last updated: 2017-12-08Bibliographically approved
4. On the fog variability over south Asia
Open this publication in new window or tab >>On the fog variability over south Asia
(English)In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894Article in journal (Refereed) Submitted
Abstract [en]

An increasing trend in fog frequencies over south Asia during winter in the last few decades has resulted in large economical losses and has caused substantial difficulties in the daily lives of people. In order to better understand the fog phenomenon, we investigated the climatology, inter-annual variability and trends in the fog occurrence from 1976 to 2010 using observational data from 82 stations, well distributed over India and Pakistan. Fog blankets large area from Pakistan to Bangladesh across north India from west to east running almost parallel to south of the Himalayas. An EOF analysis revealed that the fog variability over the whole region is coupled and must therefore be governed by some large scale phenomenon on the inter-annual time scale. Significant trends were found in the fog frequencies and this increase is not gradual, as seen in the humidity, but shows two distinct regimes shifts in 1990 and 1998 with respect to both mean and variance. The fog is also detected in ERA-Interim 3 hourly, surface and model level forecast data when using the concept of “cross-over temperature” combined with boundary layer stability. This detected fog index is able to reproduce the regime shift around 1998 and shows that the method can be applied to detect fog over south Asia. The inter-annual variability seems to be associated with the wave train originating from north Atlantic in the upper atmosphere that causes higher pressure over the region, resulting in increased boundary layer stability and surface-near relative humidity. The trend and shifts in the fog occurrence seems to be associated with the gradual increasing trend in relative humidity from 1990 onwards.

Keyword
fog, south-Asia, interannual variability, trend, fog detection
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-63078 (URN)
Available from: 2011-10-10 Created: 2011-10-08 Last updated: 2017-12-08Bibliographically approved

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