Polar-tropical interactions involving the Ross Sea sector of Antarctica (INVITED)

Diagnostic studies of the precipitation rate over Marie Byrd Land adjacent to the Ross Ice Shelf have shown a close, but bimodal relationship with the Southern Oscillation Index (SOI) from the early 1980s to today. The bimodal behavior of the time-average circulation over the Ross Sea is consistent with the precipitation variations over Marie Byrd Land, but the hemispheric-scale circulation is unimodal as shown by van Loon and Shea (1987). This means that the mid-tropospheric circumpolar vortex that is usually centered to the northeast of the Ross Ice Shelf is strongly but locally modulated by the El Niño-Southern Oscillation phenomenon. The Ross Sea sector is due south of the tropical Pacific Ocean sector where SST changes associated with ENSO are most pronounced. Candidate mechanisms responsible for this pronounced ENSO modulation of the coupled atmosphere-ocean-sea ice system are explored.

Sensitivity studies with global climate models (NCAR CCM2 and CCM3) have revealed a modeled teleconnection between Wilkes Land adjacent to the Ross Sea and the East Asian monsoon during the late austral winter/boreal summer. The surface pressure field shows that the centers of the teleconnection reside over southeastern Australia and Wilkes Land in the Southern Hemisphere and over the western North Pacific Ocean in the Northern Hemisphere. Positive pressure anomalies are found over Australia and the North Pacific, and negative anomalies over and adjacent to Wilkes Land. This teleconnection can be excited in the GCMs by a variety of Antarctic forcings (such as large sea ice anomalies and modified cloud-radiation interactions over Antarctica), suggesting that these longitudes are prime areas for equatorward propagation of atmospheric long waves generated over Antarctica. Observational studies with the NCEP/NCAR reanalysis have shown a similar teleconnection pattern in August that has variability on ENSO time scales, and is linked to the tropical intraseasonal oscillation. It is significant that the opposite phase of the teleconnection often includes troughing in the Tasman Sea. Van Loon and colleagues have shown that amplification of the Tasman trough in the austral winter often precedes development of El Niño events a several months later. This observation suggests that Antarctic forcing could play a role in the onset of El Niño conditions which subsequently influence the precipitation variations over Marie Byrd Land to the east of the Ross Sea.