Interactions between Tropical Convection and Mid-Latitude Waves that Lead to Climatological Localization of Tropical–Extratropical Moisture Transport: Some Insights from Aquaplanet Simulations

The major “diagonal” convection zones in the Southern Hemisphere that stand out in climatology are among the most active regions of tropical-extratropical interaction in the atmosphere. The northwest-southeast oriented patterns of convection seen in time averages are manifestations of high-frequency transient wave activity in the tropics and extratropics. Examination of ERA-Interim daily fields of total column water and potential vorticity in the Southern Hemisphere shows frequent events of Rossby waves traveling along the midlatitude jet, breaking deeply into the tropics, and drawing along-frontal filaments of moisture poleward. These filaments appear in animations of daily data to show no obvious preference for location, but an objective method of searching for filaments with a diagonal orientation shows a statistical preference for the regions of the South Pacific, South Atlantic, and South Indian Convergence Zones.

A number of hypotheses, including ones involving the distribution of land masses or diagonal SST patterns formed by atmosphere-ocean interactions, have been offered to explain the location and orientation of the major Southern Hemisphere convergence zones and associated moisture transport. In this this talk we focus on dynamics in the absence of land or active ocean processes, and present some results from simulations conducted with an aquaplanet version of the Community Earth System Model. We discuss how deep convection and related tropical disturbances excited in the model by a spatially localized tropical SST maximum interact with mid-latitude waves to statistically focus tropical-extratropical moisture transport in preferred regions, and we assess the extent to which moisture transport processes in the simple model simulations compare with what is seen in observations.