Observational constraints on atmospheric and oceanic cross-equatorial heat transports: revisiting the precipitation asymmetry problem in climate models

A number of recent studies have shown a strong link between hemispheric asymmetry in tropical precipitation and the atmospheric energy budget. The energetics framework has been used to explain why the mean position of the Intertropical Convergence Zone (ITCZ) is in the Northern Hemisphere (Frierson et al. 2013; Marshall et al. 2013), to demonstrate the large-scale circulation and precipitation responses to changes in the hemispheric distribution of heating due to various forcing mechanisms (Yoshimori and Broccoli 2008; Kang et al. 2008, 2009; Frierson and Hwang 2012; Hwang et al. 2013; Voigt et al. 2014; Haywood et al. 2015), and to evaluate the realism of climate models using observations (Hwang and Frierson 2013; Donohoe et al. 2013). Satellite observations indicate that there is a net gain of radiative energy at the top-of-atmosphere (TOA) in the Southern Hemisphere (SH) and a net loss in the Northern Hemisphere (NH). To compensate, the combined atmospheric and oceanic circulations transport energy across the equator from the SH to the NH (Frierson et al. 2013; Marshall et al. 2013). A common approach for inferring the oceanic contribution to cross-equatorial energy transport is to estimate the hemispheric mean surface energy budget in the SH and NH from the difference between satellite TOA net radiation and divergence of total atmospheric energy Abstract Satellite based top-of-atmosphere (TOA) and surface radiation budget observations are combined with mass corrected vertically integrated atmospheric energy divergence and tendency from reanalysis to infer the regional distribution of the TOA, atmospheric and surface energy budget terms over the globe. Hemispheric contrasts in the energy budget terms are used to determine the radiative and combined sensible and latent heat contributions to the cross-equatorial heat transports in the atmosphere (AHTEQ) and ocean (OHTEQ). The contrast in net atmospheric radiation implies an AHTEQ from the northern hemisphere (NH) to the southern hemisphere (SH) (0.75 PW), while the hemispheric difference in sensible and latent heat implies an AHTEQ in the opposite direction (0.51 PW), resulting in a net NH to SH AHTEQ (0.24 PW). At the surface, the hemispheric contrast in the radiative component (0.95 PW) dominates, implying a 0.44 PW SH to NH OHTEQ. Coupled model intercomparison project phase 5 (CMIP5) models with excessive net downward surface radiation and surface-to-atmosphere sensible and latent heat transport in the SH relative to the NH exhibit anomalous northward AHTEQ and overestimate SH tropical precipitation. 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