Observational Constraints on Atmospheric and Oceanic Cross-Equatorial Heat Transports: Revisiting the Precipitation Asymmetry Problem in Climate Models

Monday, 14 December 2015: 09:45
3006 (Moscone West)
Norman G Loeb1, Hailan Wang2, Anning Cheng3, Seiji Kato4, John Fasullo5, Kuan-Man Xu1 and Richard Philip Allan6, (1)NASA Langley Research Center, Hampton, VA, United States, (2)Science Systems and Applications, Inc. Hampton, Hampton, VA, United States, (3)EMC/NOAA Center for Weather and Climate Prediction, College Park, MD, United States, (4)NASA Langley Research Ctr, Hampton, VA, United States, (5)National Center for Atmospheric Research, Boulder, CO, United States, (6)University of Reading, READING, United Kingdom
Recent studies have shown strong linkages between hemispheric asymmetries in atmospheric and oceanic energy budgets, tropical precipitation and the mean position of the Intertropical Convergence Zone (ITCZ). The energetics framework has been used to explain why the mean position of the ITCZ is in the Northern Hemisphere and to study large-scale circulation and precipitation responses to changes in the hemispheric distribution of heating. Here, we expand upon these earlier studies by also considering estimates of hemispheric asymmetry in surface and atmospheric radiation budget derived from satellite observations, which enables a decomposition of cross-equatorial heat transport in terms of radiative and non-radiative (i.e., combined latent and sensible heat) components.

Satellite observations of top-of-atmosphere (TOA) and surface radiation budget from the Clouds and the Earth’s Radiation Budget (CERES) are combined with mass corrected vertically integrated atmospheric energy divergence from reanalysis to infer the regional distribution of the TOA, atmospheric and surface energy budget terms over the globe. Observed radiative and combined sensible and latent heat contributions to atmospheric and oceanic cross-equatorial heat transports are compared with simulations from 30 models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5).

Results show that most CMIP5 models that overestimate tropical precipitation in the SH have too much net downward surface radiation and combined latent and sensible heat flux in the SH relative to the NH. In addition, many of the models also underestimate atmospheric radiative cooling in the SH compared to the NH. Consequently, the models have excessive heating of the SH atmosphere and anomalous SH to NH cross-equatorial heat transport. The anomalous northward heat transport occurs via the upper branch of the northern Hadley Cell, while anomalous NH to SH moisture transport occurs in the lower branch of the northern Hadley cell, which supplies moisture to a SH ITCZ. The hemispheric bias in net surface radiative flux is due to too much longwave surface radiative cooling in the NH tropics in both clear and all-sky conditions and excessive shortwave surface radiation in the SH subtropics and extratropics due to an underestimation in reflection by clouds.