Spatiotemporal Variability of the Tropical Dynamical Warm Pool and Its Influence on Continental Precipitation

Peter J. Webster has made several seminal contributions to the understanding of the dynamics of the tropical atmosphere in different spatial and temporal scales, always from a holistic perspective. In many cases Peter’s contributions have had direct societal impacts since his ideas have been the building blocks of applied tools that promote sustainability: food security, risk management and optimal decision making in economically competitive scenarios. One of the topics that has intrigued Peter since he was a graduate student is tropical convection across different spatio-temporal scales and its overall role in climate variability. The convective activity in the tropical regions plays a crucial role in the global weather and climate and in its response to changes in the magnitude of the external forcing factors. Twentieth century observations show that during the last 60 years the sea-surface temperature (SST) of the tropical oceans has increased by ~0.7°C and the area of SST>26.5 and 28°C (referred to as the oceanic warm pool –OWP-) traditionally associated with convective activity and storminess, by 15 and 50% respectively. Analysis of 20^th century observations and CMIP3/CMIP5 models show that the tropical OWP has increased in area by nearly 70% since 1910 and is projected to double during 21st century in association with greenhouse warming in a moderate CO₂ forcing scenario. However, during the same period the dynamical warm pool (DWP), defined as the area of positive atmospheric heating, has remained constant in area. The threshold SST (T_H), which demarks the region of net heating and cooling, has increased from 26.6°C in the 50s to 27.1°C in the last decade and it is projected to increase to ~28.5°C by 2100 in the CMIP3/CMIP5 simulations. The DWP area is projected to remain constant during the 21^st century. The area of the DWP remains constant while SSTs increase universally across the tropics as a result of a near exact matching between increases in convective heating within the DWP and in radiative cooling outside the DWP. Although the area of the DWP remains constant, the total tropical atmospheric heating has risen from 1950 to 2000 by about 10% and it is projected to increase by further 20% by 2100, suggesting an enhancement of the atmospheric circulation, with growth in convective heating within the warm pool, and an increase of subsiding air and stability outside the convective warm pool and vertical shear in the DWP boundaries. On the other hand, analysis of the variability of total convective heating within the warm pool shows a very significant relationship with slow-varying precipitation over continental regions as well as over the ocean, revealing a dipole-like behaviour in the tropics in the interannual time-scale: periods with positive (negative) rainfall anomalies over continents coincide with periods with negative (positive) anomalies over the ocean.