Hadley Circulation as a Modulator of the Extratropical Climate

Abstract

Studies based on GCM ensemble forecasts have shown that an intensification of the cross-equatorial Hadley circulation associated with a latitudinal displacement of the zonally averaged convective heating in the Tropics can lead to remote warming in the winter high latitudes. This work further investigates this tropical?extratropical connection in a perpetual winter experiment using an idealized GCM without orography to focus on the role of transient eddies, and tests against observations using a multiyear reanalysis produced by the Goddard Earth Observing System-Version 1 (GEOS-1) Data Assimilation System. The GCM results show that the intensification and poleward expansion of the cross-equatorial Hadley cell induced by a tropical heating shift can lead to westerly acceleration in the winter subtropics and enhanced vertical shear of the zonal wind in the subtropics and midlatitudes. The increased baroclinicity outside the Tropics is accompanied by reduced meridional temperature and potential vorticity (PV) gradients, consistent with enhanced PV mixing and increased poleward heat transport by baroclinic eddies. But if the changes in the Hadley cell are such that they produce a deceleration of the zonal wind in the winter subtropics, stronger temperature and PV gradients result in the winter extratropics. The midlatitude response to Hadley acceleration of the subtropical jet is dominated by enhanced power in low-frequency planetary-scale waves that peaks at zonal wavenumber 2 with a period of 40 days. The extent to which this tropical?extratropical connection may be present in nature is tested using the GEOS-1 reanalysis for five austral winters from 1985 to 1989. Results show that the year-to-year variation in the zonally averaged extratropical temperature gradient in austral winters is correlated with the variation in the acceleration of the subtropical zonal wind by the winter Hadley cell. The anomaly correlation coefficients range from 0.80 to 0.92, depending on the statistical test. The positive Hadley acceleration anomaly in the subtropics during the 1988 austral winter is accompanied by stronger than normal zonal wind shears in the subtropics and midlatitudes, a colder troposphere in the midlatitudes, and a warmer pole. The extratropical temperature anomalies are associated with a reduced PV gradient, and the midlatitude geopotential height anomaly shows a spectral peak at wavenumbers 2?3 with periods between 40 and 60 days, similar to the idealized GCM results. The implication of this study is that the Hadley circulation may play a role in modulating the temperature difference between middle and high latitudes by modifying the zonal wind shear in the subtropics and midlatitudes.