Earth and Climate Science

Dr. Pranab Deb
IIT Kharagpur

Rossby wave trains triggered by tropical convection strongly influence several aspects of global climate and weather, e.g., the North Atlantic Oscillation, quasi-stationary blocking events, cyclone frequency over the Northern Hemisphere, the South Asian monsoon, and sea ice cover and ice shelf melting over coastal Antarctica. Using satellite data and a 35‐year hindcast of the West Antarctic summer climate (using the Weather Research and Forecasting model), we found that El Niño events are associated with enhanced surface melting over Pine Island and Thwaites Glaciers. The projected future increase in El Niño episodes could therefore increase the risk of disintegration of West Antarctic ice shelves. The effect of Pacific warm events has been documented as far away as the Indian sector of Antarctica. Using daily gridded observational and reanalysis data, we demonstrate that a technique based on linear response theory realistically captures the evolution of 250-hPa geopotential height anomalies in the extratropics (in this example, Northern Hemisphere), using examples of step-like changes in precipitation over selected tropical areas during boreal winter. Application of this method to six Coupled Model Intercomparison Project Phase 5 (CMIP5) models, using the same tropical forcing, reveals a large intermodel spread in the linear response, associated with intermodel differences in Rossby wave guide structure. The technique is then applied to a projected tropics-wide precipitation change in the HadGEM2-ES model during 2025-2045 DJF, a period corresponding to a 2°C rise in the mean global temperature under the RCP8.5 scenario, and the response is found to depend on whether the mean state underlying the technique is calculated using observations, the present-day simulation, or the future projection; indeed, the bias in extratropical response to tropical precipitation because of errors in the basic state is much larger than the projected change in extratropical circulation itself. We therefore propose the linear step response method as a semi-empirical method of making near-term future projections of the extratropical circulation, which should assist in quantifying uncertainty in such projections.

Meeting ID: 927 8474 0177

Passcode: 408766