The influence of direct radiative forcing versus indirect sea surface temperature warming on southern hemisphere subtropical anticyclones under global warming

Southern hemisphere subtropical anticyclones are projected to change in a warmer climate during both austral summer and winter. A recent study of CMIP 5 & 6 projections found a combination of local diabatic heating changes and static-stability-induced changes in baroclinic eddy growth as the dominant drivers. Yet the underlying mechanisms forcing these changes still remain uninvestigated. This study aims to enhance our mechanistic understanding of what drives these Southern Hemisphere anticyclones changes during both seasons. Using an AGCM, we decompose the response to CO2-induced warming into two components: (1) the fast atmospheric response to direct CO2 radiative forcing, and (2) the slow atmospheric response due to indirect sea surface temperature warming. Additionally, we isolate the influence of tropical diabatic heating with AGCM added heating experiments. As a complement to our numerical AGCM experiments, we analyze the Atmospheric and Cloud Feedback Model Intercomparison Project experiments. Results from sensitivity experiments show that slow subtropical sea surface temperature warming primarily forces the projected changes in subtropical anticyclones through baroclinicity change. Fast CO2 atmospheric radiative forcing on the other hand plays a secondary role, with the most notable exception being the South Atlantic subtropical anticyclone in austral winter, where it opposes the forcing by sea surface temperature changes resulting in a muted net response. Lastly, we find that tropical diabatic heating changes only significantly influence Southern Hemisphere subtropical anticyclone changes through tropospheric wind shear changes during austral winter.

Erratic Asian Summer Monsoon 2020: COVID-19 Lockdown Initiatives Possible Cause for These Episodes?

The summer (June through September) monsoon 2020 has been very erratic with episodes of heavy and devastating rains, landslides and catastrophic winds over South Asia (India, Pakistan, Nepal, Bangladesh), East Asia (China, Korea, and Japan), and Southeast Asia (Singapore, Thailand, Vietnam, Laos, Cambodia, Philippines, Indonesia). The withdrawal of the summer monsoon over India was delayed by two weeks. The monsoon season over East Asia has been the longest. China recorded a Dam burst in the 20th century. Furthermore, the Korean Peninsula has experienced back-to-back severe tropical cyclones. Could the lockdown activities initiate to control the COVID-19 spread a possible cause for these major episodes?The strict enforcement of the lockdown regulations has led to a considerable reduction of air pollutants – dust and aerosols throughout the world. A recent study based on satellites and merged products has documented a statistically significant mean reduction of about 20%, 8% and 50% in nitrogen dioxide, Aerosol Optical Depth (AOD) and PM2.5 concentrations, respectively over the megacities across the globe. Our analysis reveals a considerable reduction of about 20% in AOD over South as well as over East Asia, more-over East Asia than over South Asia. The reduced aerosols have impacted the strength of the incoming solar radiation as evidenced by enhanced warming, more-over the land than the oceans. The differential warming over the land and the ocean has resulted in the amplification of the meridional ocean-land thermal contrast and strengthening of the monsoon flow. These intense features have supported the surplus transport of moisture from the oceans towards the main lands. Some similarity between the anomalous rainfall pattern and the anomalous AOD pattern is discernable. In particular, the enhancement of rainfall, the reduction in AOD and the surface temperature warming match very well over two regions one over west-central India and the other over the Yangzte River Valley. Results further reveal that the heavy rains over the Yangzte River Valley could be associated with the preceding reduced aerosols, while the heavy rains over west-central India could be associated with reduced aerosols and also due to the surface temperature warming

Imprints of evaporation and vegetation type in diurnal temperature variations

<p>Diurnal temperature variations are strongly shaped by the absorption of solar radiation, but evaporation, or the latent heat flux, also plays an important role. Generally, evaporation cools, but its relation to diurnal temperature variations is unclear. This study investigates the diurnal response of surface and air temperatures to solar radiation and how evaporation and vegetation modify their response. We used the warming rate of temperature to absorbed solar radiation in the morning under clear-sky conditions and evaluated how the warming rates change for different evaporative fractions. Results for 51 FLUXNET sites show that air temperature carries very weak imprints of evaporation across all vegetation types. However, surface temperature warming rates of short vegetation decrease significantly by ~23 x 10<sup>-3</sup> K/W m<sup>-2</sup> from dry to wet conditions. Contrarily, warming rates of surface and air temperatures are similar at forest sites and carry literally no imprints of evaporation. We explain these contrasting patterns with a surface energy balance model. The model reveals a strong sensitivity of the warming rates to evaporative fraction and aerodynamic conductance. However, for the large aerodynamic conductance, the sensitivity to the evaporative fraction is strongly reduced. We then show that in addition to the higher aerodynamic conductance of forests, imprints of evaporation in the warming rate of surface temperature are reduced by 50% through an enhanced aerodynamic conductance under dry conditions. This contribution is comparatively weak (28%) for short vegetation. These findings have implications for the interpretation of land-atmosphere interactions and the roles of moisture limitation and vegetation on diurnal maximum temperatures, which is of key importance for ecological functioning. We conclude that surface temperature warming rate is a promising predictor of evaporation for short vegetation. These findings are in agreement with our previous study (Panwar et al., 2019) where the weaker response of air temperature to the evaporative fraction is explained by the larger growth of the boundary layer on drier days. In forests, however, the diurnal variation in temperatures is governed by their aerodynamic properties resulting in no imprint of evaporation in diurnal temperature variations.</p><p>Reference: Panwar, A., Kleidon, A. and Renner, M.: Do Surface and Air Temperatures Contain Similar Imprints of Evaporative Conditions?, Geophysical Research Letters, 46(7), 3802–3809, doi:10.1029/2019GL082248, 2019.</p>