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On the fog variability over south Asia
Stockholm University, Faculty of Science, Department of Meteorology . (Climate Dynamics)
Stockholm University, Faculty of Science, Department of Meteorology .
Stockholm University, Faculty of Science, Department of Meteorology .
(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 [en]
fog, south-Asia, interannual variability, trend, fog detection
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-63078OAI: oai:DiVA.org:su-63078DiVA: diva2:446668
Available from: 2011-10-10 Created: 2011-10-08 Last updated: 2017-12-08Bibliographically approved
In thesis
1. On the intra-seasonal to decadal climate variability over South-Asia
Open this publication in new window or tab >>On the intra-seasonal to decadal climate variability over South-Asia
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
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:nbn:se:su:diva-62988 (URN)978-91-7447-375-9 (ISBN)
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

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