Deconstructing the Hadley cell heat transport



The mechanisms responsible for poleward atmospheric heat transport at low latitudes are examined in the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) dataset. Deep meridional overturning circulations are found in regions experiencing frequent convection (i.e. the Indo-Pacific ‘warm pool’) and it is suggested that these are the primary reason why atmospheric heat transport seems dominated by axisymmetric motions or ‘Hadley cell heat transport’. In contrast, the more complex distribution of meridional mass transport by the circulations over the ‘cold tongue’ regions (such as the eastern Atlantic area) and the presence of a pronounced minimum in moist static energy at mid-levels constrain these regions to contribute little to poleward heat transport year round. Regions experiencing frequent convection do not, however, account for all the annual mean poleward atmospheric heat transport. At the Equator, an annual net southward transport of heat of about 0.4 PW (1 PW = 1015 W) is found, but the deep overturning cells found in the most convectively active regions contribute only 25% of this. This small contribution of ∼0.1 PW is understood to reflect a seasonal cancellation between large (∼1 PW) northward and southward heat transports in boreal winter and summer respectively. During December-February, most of the cross-equatorial heat transport is attributable to the convective regions, whereas in June-August, only half is attributable to these regions. In this season, there is a significant amount of heat transport by the strong Somali jet associated with the Asian summer monsoon. Rather than attributing the net southward cross-equatorial heat transport to asymmetries in the climatology associated with the northerly position of the intertropical convection zones throughout the year over the eastern Pacific and Atlantic oceans, our diagnostics suggest that the Somali jet is a significant contributor to the annual net southward atmospheric heat transport at the Equator.