The Coupling of Convection, Large-Scale Atmospheric Dynamics, Surface Radiation, and Sea-Surface Temperature Hot Spots as Characterized by MODIS, TRMM, CERES, and ECMWF-Interim Reanalysis Data

Abstract:

Understanding ocean-atmosphere interactions over the tropics is paramount to understanding climate and even climate sensitivity, especially as they relate to Earth’s highest surface temperatures. We examine the coupling and feedbacks between very high SSTs, which we refer to as SST hot spots above 30˚C, and deep convection, precipitation, large-scale dynamics, and the surface energy budget. Using extensive time series and hovmoller analyses in areas most favorable for hot spot formation - just south of the equator in the central Pacific, where SSTs > 30˚C for up to two months/year in one 20˚ longitude by 10˚ latitude box, we show that maximum large-scale ascent, deep convective cloud fraction, rain rates, and upward latent heat fluxes are observed during the decay stage of hot spots, during maximum -∂SST/∂t. Considered as part of a predator-prey system, convection preys on the hot spots until depleted, after which convection draws down and SSTs eventually rise again, illustrating the strongly coupled nature of the ocean-atmosphere system.

The nature of the relationship of SST and deep convection varies both as a function of time scale and SST window. Climatologically, deep convection, rain rate, and upward ω₅₀₀ maximize at an SST of 30˚C over the tropics, approximately 1.5˚C above the SST peak. In contrast, (∂convection/∂SST)_max corresponds to the climatological SST peak. Interannually, both the SST peak and the convection peak SST are highest during the 2009/2010 El Nino. At the synoptic scale, deep convection positively correlates most strongly with SST when SST leads by 5-20 days, but mostly during 70-day time windows that contain SSTs>30˚C, suggesting the importance of hot spots on convection and large-scale dynamics at synoptic to submonthly timescales. Synoptic ∂SST/∂t and convection are most strongly negatively correlated at zero-day lag, but only weakly correlated at longer time scales, unlike SST_max and (deep convection)_max, which are more strongly correlated. Vertically, both ω and anvil CF tilt with time at the synoptic scale, with mid-level convection transitioning to deep convection with tops as cold as ~200 K in days. Our analysis demonstrates the coupling of SSTs and deep convection, large-scale dynamics, and the surface energy budget from the synoptic to annual scale continuum.