The exchange of atmospheric mass between the northern and southern hemisphere occurs with considerable regularity on intraseasonal time-scales. Observational evidence from previous studies indicates that anomalous and persistent regional atmospheric mass distributions may often be related to interhemispheric atmospheric mass exchange.

In this study we examine an event where the northern hemisphere undergoes a loss of 1.80 hPa of hemispherically averaged dry air surface pressure over a 9 day period commencing on 3 March 1989. The value of 1.8 hPa represents greater than 60% of the mean annual cycle of northern hemisphere dry atmospheric mass. Similar to a 25 case composite of northern hemisphere cold season dry atmospheric mass fall events over a 30 year period from 1968 to 1997, this event is accompanied by a rapid buildup of the Siberian high and a subsequent pressure surge over Southeast Asia.

Using the European Centre for Medium Range Weather Forecasts (ECMWF) reanalysis and daily outgoing longwave radiation (OLR) data, we show that the northeasterly flow resulting from the pressure surge crosses the equator in broad zone extending from 100oE to 130oE. Longitude-time plots of OLR document a significant enhancement of convection within the monsoon trough of northern Australia commencing 7 March. The intense convection initiates only after the low-level northerly surge winds cross the equator and converge with the southeasterly winds from a southerly surge off the west coast of Australia.

The vertical structure of the zonal winds within the monsoon trough depict the initiation of an active phase of the Australian summer monsoon (ASM). A deep layer of westerly winds extends from the surface to 400 hPa overlain by easterly winds. Within the subtropics of the southern hemisphere (10oS) the low-level westerly wind burst occurs in response to the intensified zonal pressure gradient resulting from the atmospheric mass buildup in the region of convergence between the two hemispheric surges.

We show that the diabatic heating anomalies associated this onset of an active phase of the ASM act to redistribute dry atmospheric mass in the upper tropospheric divergent outflow. Three prominent anticyclonic circulations, stretching from the South Indian Ocean eastwards to the South Pacific, intensify beneath regions of upper tropospheric dry atmospheric mass convergence, originating from the monsoon convection outflow. These anticyclonic circulations are largely responsible for the dry atmospheric mass increase in the southern hemisphere.