The seasonal mean flow and distribution of large‐scale weather systems in the southern hemisphere: The effects of moisture transports

Circulation statistics, based on seasonal mean fields calculated from archived analyses at ECMWF, are presented for the southern hemisphere. Particular emphasis is placed on the large‐scale distribution of the high frequency (periods 2–6 days) transient eddy activity during the period July–August 1982. This was found to be a maximum over the Atlantic and Indian Oceans, and a minimum over the Pacific. The relative distribution of the temperature and momentum fluxes suggests that the high frequency transients are baroclinic waves, which reach large amplitudes and then decay, mainly as a result of Rossby wave radiation. The location at which the transients began to grow cannot be related to the regional variations of the mean flow. In particular, it is poorly related to the distribution of low‐ and middle‐level baroclinicity defined on the basis of dry, linear theory. It is suggested that the entrainment of moisture into the low‐level westerlies over the Atlantic could account for the large increase of transient activity there. It is shown that the major low‐level flux of moisture from the tropics into the southern hemisphere mid‐latitudes occurred over South America. Since moisture data cannot be extracted reliably from the data archives, moisture fluxes are deduced indirectly. Use is made of a general circulation model of the southern hemisphere March flow to illustrate the entrainment of moisture into the baroclinic zone over the mid‐Atlantic.