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The Hadley Cell (HC) plays a key role in the climate system. Generally defined as the zonal mean meridional mass circulation in the atmosphere bounded roughly by 30oS and 30oN, with warmer air rising in the tropics and colder air sinking in the subtropics, this circulation transports momentum flux to the subtropics and heat from the tropics to the subtropics and to high latitudes through extra-tropical baroclinic eddies broadly organized into storm tracks and stationary Rossby waves. Through anomalous upper tropospheric divergence, modifications in the HC are connected to changes in the generation of Rossby waves that propagate into the extra-tropics. Thus, tropical variability affects the extra-tropical atmospheric circulation. Changes in the location, intensity or seasonality of major climatological features of the general circulation, such as the HC or Rossby waves, can have important implications for regional climates by modifying patterns of temperature and precipitation. Several studies have identified that, in the late 1970s and early 1980s, a major shift occurred in the structure of large-scale circulation in both hemispheres, including a weakening of HC. Thus, in this paper we investigate how the changes in the HC, which occurred in the mid1970s, have affected the stationary Rossby wave energy propagation in the Southern Hemisphere (SH) extratropics. We have employed the CSIRO Mk-3L general circulation model in a simulation with observed sea surface temperatures (SSTs) and time evolving historical carbon dioxide concentrations. Our focus is on the performance of the model in simulating the climatology for the twenty year periods 1949-68 and 1975-94 and on comparisons with the corresponding observations. We find that the model is quite skilful in reproducing the broad features of the important inter-decadal changes that occurred in the mid-1970s. We have also analysed the Rossby Wave Source (RWS) and mid-latitude circulation responses at upper levels associated with tropical variability. The RWS forcing is found to be located directly over the high latitude downward branch of the local HC, which is associated with upper-level convergence zones. The vortex stretching term is the largest contribution to the total RWS and the term of absolute vorticity advection by the divergent flow is mainly associated with the anticyclonic anomaly sources in the subtropics. The results also show that the weakening of the HC, which occurred in the period 1975-1994, leads to higher pressure developing over southern Australia, especially over southwestern Australia, contributing to rainfall reductions in these regions.

Assessing the impacts of changes in the Hadley Circulation on stationary Rossby wave propagation