Research

Projected expansion of Hadley circulation is linked to changes in the ocean variability

Atmospheric surface wind drives upper ocean circulations. Hence, a change in the near-surface atmospheric wind pattern causes a change in ocean circulation. There is evidence that climate change alters the spatial pattern and strength of the surface winds. In particular, easterly (east to west) prevailing winds in the subtropical regions are projected to expand poleward in accordance with the poleward expansion of the tropical atmospheric circulation, also known as the Hadley circulation. The poleward shift of the subtropical easterly wind belt also has potential implications for the subtropical sea surface temperature (SST) variations because two important drivers of the SST are related to the surface wind. The first one is the turbulent air-sea heat exchange, which is a combination of latent and sensible heat fluxes and is dependent upon available atmospheric moisture and near-surface wind speed. The second one is the advection of ocean temperatures by anomalous wind-driven ocean currents (also known as Ekman heat flux), which is related to the surface wind stress. In the subtropical regions (roughly in between 10-to-30-degree latitudes), these two fluxes typically oppose each other, acting to reduce the amplitude of SST variations. Meanwhile, in the midlatitudes (poleward of subtropics), the fluxes tend to reinforce each other to enhance SST variability. Since both observations and climate model simulations suggest that the human-induced forcing (e.g., greenhouse gas forcing) due to climate change has already expanded and will expand the subtropical easterly prevailing wind poleward in the future, we hypothesize that the change of the prevailing surface wind can potentially impact subtropical SST variability by modulating the relationship between Ekman and turbulent heat fluxes. Using a series of climate model simulations, we find that the subtropical regions where the anomalous turbulent and Ekman heat fluxes oppose each other are projected to expand towards the poles. This result is consistent with the projected poleward shift in the subtropical easterlies due to the Hadley expansion, which has potential implications for the change in subtropical SST variability.

Hemisphere-dependent response of the Hadley circulation strength to ocean and atmospheric forcings

The Hadley Circulation (HC) is a fundamental component of the atmospheric general circulation, which regulates the global weather and climate by redistributing heat, moisture, and energy from the tropics to higher latitudes. The variability of the Hadley circulation strength (HCS), crucial to tropical climate variability, is attributed to both oceanic and atmospheric forcings. El Niño-Southern Oscillation (ENSO), a leading mode of coupled variability, may be considered a dominant driver of the HCS variability through its influence on the meridional sea surface temperature gradient. Alternatively, variations in the extratropical eddy-driven upper tropospheric forcing also modulate the HCS through the meridional exchange of energy and momentum. However, the relative contributions of these oceanic and atmospheric forcings to the hemispheric HCS variability are not well understood. In particular, how much anomalous wind stress-driven ocean dynamics, including ENSO, impact HCS variability remains an open question. To address these gaps, we investigate the drivers of the interannual HCS variability using global coupled model experiments. We find that the anomalous wind stress-driven ocean circulation variability significantly amplifies HCS variability in the Southern Hemisphere (SH). ENSO is the leading modulator of the SH HCS variability, which offers the potential to improve the predictability of HC-related hydrological consequences. On the other hand, the Northern Hemisphere (NH) HCS variability is predominantly influenced by the eddy-driven internal atmospheric variability with little role for ocean dynamics. We hypothesize that the large eddy variability in the NH and concentrated ENSO-associated heating and precipitation in the SH lead to the hemisphere-dependent response of the interannual HCS variability.

The physical mechanism of the flood causing extreme monsoon rainfall events in Bangladesh and India

Bangladesh and Northeast India are home to some of the most densely populated areas in the world, where annual flash floods cause widespread destruction, resulting in loss of life and economic turmoil. Therefore, understanding the impact of climate change on extreme precipitation events and flash floods in these areas is paramount. This study provides insight into how climate change exacerbates the frequency and severity of flash floods, making it an important topic for research. The findings of this study reveal that climate change has quadrupled (e.g., increased four times) the likelihood of monsoon extreme precipitation events in these regions, leading to a significant increase in flash floods. The physical mechanism involves the warming of sea surface temperature in the Bay of Bengal region of the Indian Ocean to enhance the available moisture contents. This moisture, carried by low-level wind, converges over the region’s high terrain (e.g., Meghalaya area), culminating in heavy rainfall during extreme events across Bangladesh and Northeast India. In future warming climates, the enhancement of both low-level wind variability and available moisture content over the Bay of Bengal further increases moisture transport over the inland area. The results from our study suggest that more devastating flood-causing extreme rainfall events will become more frequent in the future. Therefore, the project makes a valuable contribution to the field of meteorology and climate science, potentially paving the way for the development of avenues for policymaking, disaster management, and adaptation strategies amidst the challenges posed by climate change.

News coverage:

The Daily Star: Rise in Extreme Rainfall Behind Flash Floods in Sylhet