Satyaban B. Ratna

Anatomy of Indian heatwaves

India suffers from major heatwaves during March-June. The rising trend of number of intense heatwaves in recent decades has been vaguely attributed to global warming. Since the heat waves have a serious effect on human mortality, root causes of these heatwaves need to be clarified. Based on the observed patterns and statistical analyses of the maximum temperature variability, we identified two types of heatwaves. The first-type of heatwave over the north-central India is found to be associated with blocking over the North Atlantic. The blocking over North Atlantic results in a cyclonic anomaly west of North Africa at upper levels. The stretching of vorticity generates a Rossby wave source of anomalous Rossby waves near the entrance of the African Jet. The resulting quasi-stationary Rossby wave-train along the Jet has a positive phase over Indian subcontinent causing anomalous sinking motion and thereby heatwave conditions over India. On the other hand, the second-type of heatwave over the coastal eastern India is found to be due to the anomalous Matsuno-Gill response to the anomalous cooling in the Pacific. The Matsuno-Gill response is such that it generates northwesterly anomalies over the landmass reducing the land-sea breeze, resulting in heatwaves.

Dynamical Downscaling of Austral Summer Climate Forecasts over Southern Africa Using a Regional Coupled Model

AbstractThe prediction skill of dynamical downscaling is evaluated for climate forecasts over southern Africa using the Advanced Research Weather Research and Forecasting (WRF) model. As a case study, forecasts for the December–February (DJF) season of 2011/12 are evaluated. Initial and boundary conditions for the WRF model were taken from the seasonal forecasts of the Scale Interaction Experiment-Frontier Research Center for Global Change (SINTEX-F) coupled general circulation model. In addition to sea surface temperature (SST) forecasts generated by nine-member ensemble forecasts of SINTEX-F, the WRF was also configured to use SST generated by a simple mixed layer Price–Weller–Pinkel ocean model coupled to the WRF model. Analysis of the ensemble mean shows that the uncoupled WRF model significantly increases the biases (errors) in precipitation forecasted by SINTEX-F. When coupled to a simple mixed layer ocean model, the WRF model improves the spatial distribution of precipitation over southern Africa t...

Moisture variability over the Indo-Pacific region and its influence on the Indian summer monsoon rainfall

The Indo-Pacific Ocean (i.e. region between 30°E and 150°E) has been experiencing a warming since the 1950s. At the same time, the large-scale summer monsoon rainfall over India and the moisture over the East Africa/Arabian Sea are both decreasing. In this study, we intend to investigate how the decrease of moisture over the East Africa/Arabian Sea is related to the Indo-Pacific Ocean warming and how this could affect the variability of the Indian summer monsoon rainfall. We performed the analysis for the period 1951–2012 based on observed precipitation, sea surface temperature and atmospheric reanalysis products and we verified the robustness of the result by comparing different datasets. The decreasing trend of moisture over the East Africa/Arabian Sea coincides with an increasing trend of moisture over the western Pacific region. This is accompanied by the strengthening (weakening) of the upward motion over the western Pacific (East Africa/Arabian Sea) that, consequently, contributes to modulate the western Pacific-Indian Ocean Walker circulation. At the same time, the low-level westerlies are weakening over the peninsular India, thus contributing to the reduction of moisture transport towards India. Therefore, rainfall has decreased over the Western Ghats and central-east India. Contrary to previous decades, since 2003 moisture over the East Africa/Arabian Sea started to increase and this is accompanied by the strengthening of convection due to increased warming of sea surface temperature over the western Arabian Sea. Despite this moisture increase over the Arabian Sea, we found that moisture transport is still weakening over the Indian landmass in the very recent decade and this has been contributing to the decreased precipitation over the northeast India and southern part of the Western Ghats.

ENSO’s far reaching connection to Indian cold waves

During boreal winters, cold waves over India are primarily due to transport of cold air from higher latitudes. However, the processes associated with these cold waves are not yet clearly understood. Here by diagnosing a suite of datasets, we explore the mechanisms leading to the development and maintenance of these cold waves. Two types of cold waves are identified based on observed minimum surface temperature and statistical analysis. The first type (TYPE1), also the dominant one, depicts colder than normal temperatures covering most parts of the country while the second type (TYPE2) is more regional, with significant cold temperatures only noticeable over northwest India. Quite interestingly the first (second) type is associated with La Niña (El Niño) like conditions, suggesting that both phases of ENSO provide a favorable background for the occurrence of cold waves over India. During TYPE1 cold wave events, a low-level cyclonic anomaly generated over the Indian region as an atmospheric response to the equatorial convective anomalies is seen advecting cold temperatures into India and maintaining the cold waves. In TYPE2 cold waves, a cyclonic anomaly generated over west India anomalously brings cold winds to northwest India causing cold waves only in those parts.