Are Tropical Ocean Mixed Layer Heat Flux Trends Indicative of Intensifying Hadley Circulation?

Abstract:

We investigate trends in the tropical (30^oS - 30^oN) ocean mixed layer heat budget concurrent with SST and near surface wind observations (1980 – 2012). Trends are reported at the 95 % significance level and calculated from mean annual anomalies fit to a lag-1 autocorrelation-corrected linear model. SST and near surface wind trends are analyzed in ocean observations (ICOADS) and various atmosphere reanalysis products. The ocean mixed layer heat budget trends are reconstructed using several ocean and atmosphere reanalysis products. Results show dominantly negative (out of the ocean) zonal mean mixed layer net surface heat flux trends of 3-6 Wm^-2 per 32 years, correlated in ocean and atmosphere reanalyses with enhanced surface latent heat flux. Trends of accelerating near surface u-wind component are correlated with enhanced mixed layer horizontal heat advection in some regions. In other regions, positive net surface heat flux trends are correlated with increased net shortwave radiation flux. Most tropical regions broadly follow one of these two cases, resulting in regionally positive and negative net surface heat flux trends. Our results are consistent with observations and projections from theories that predict intensification and widening of the Hadley circulation.