Characteristics of Bay of Bengal Monsoon Depressions in the 21st Century

Abstract

We show that 21st century increase in radiative forcing does not significantly impact the frequency of South Asian summer monsoon depressions (MDs) or their trajectories in the Coupled Model Intercomparison Project Phase 5 general circulation models (GCMs). A significant relationship exists between the climatological occurrences of MDs and the strength of the background upper (lower) tropospheric meridional (zonal) winds and tropospheric moisture in the core genesis region of MDs. Likewise, there is a strong relationship between the strength of the meridional tropospheric temperature gradient in the GCMs and the trajectories of MDs over land. While monsoon dynamics progressively weakens in the future, atmospheric moisture exhibits a strong increase, limiting the impact of changes in dynamics on the frequency of MDs. Moreover, the weakening of meridional tropospheric temperature gradient in the future is substantially weaker than its inherent underestimation in the GCMs. Our results also indicate that future increases in the extreme wet events are dominated by nondepression day occurrences, which may render the monsoon extremes less predictable in the future.

Regional Climate Response of Middle Eastern, African, and South Asian Monsoon Regions to Explosive Volcanism and ENSO Forcing

“…This suggests that climate forcings (such as volcanism and ENSO) induce hemispheric thermal gradient, especially the land-sea thermal gradient at surface to upper tropospheric levels. These findings are consistent with earlier studies showing the impact of external forcings on the ITCZ and monsoon system through changes in hemispheric thermal contrast on the surface and in the troposphere and associated changes of subtropical jet and Hadley circulation (Chou, 2003;Dogar, 2018;Rastogi et al, 2018;Turner & Annamalai, 2012). To better account for volcanic-induced direct radiative and circulation impacts over MEA and South Asian region both in the model and observations and to better elucidate the impacts of circulation changes, we decompose the effects of ENSO and volcanic aerosols in the following section.…”

“…Earlier studies have emphasized that stratospheric volcanic aerosols induce perturbations to the radiative forcing (Hansen et al, 1997;Ramaswamy et al, 2001) that result in significant global-and regional-scale changes in temperature and precipitation distribution (Dogar, Stenchikov, et al, 2017;Fischer et al, 2007).…”

Tracing Future Spring and Summer Drying in Southern Africa to Tropical Lows and the Congo Air Boundary

“…We apply the TRACK algorithm (Hodges 1994(Hodges , 1999. The application of TRACK to CMIP5 model experiments has previously been documented by Rastogi et al (2018) and Bengtsson et al (2007). identified southern African tropical lows using 6-hourly vertical mean vorticity averaged across pressure levels at 600, 700 and hPa.…”

Assessing Historical Variability of South Asian Monsoon Lows and Depressions With an Optimized Tracking Algorithm

“…The reduction in depression counts has been argued to be associated with a decrease in total summer rainfall in east central India, the region of highest LPS track density (Vishnu et al., 2016). A decrease in overall LPS activity, including that of both lows and depressions, has been projected for the coming century as global mean temperature increases and the large‐scale, seasonal mean monsoon circulation weakens (Rastogi et al., 2018; Sandeep et al., 2018). This projected decrease is accompanied by a poleward shift in the region of LPS genesis in next‐century simulations using one global climate model (Sandeep et al., 2018), but the connection of such greenhouse gas‐forced changes to past trends remains unclear, especially given the possible dominance of aerosol forcings in historical trends of mean monsoon strength (Bollasina & Nigam, 2009; Ramanathan et al., 2005).…”