Variations in the Southern Hemisphere Atmospheric Circulation During Extreme Events of Sea Ice Cover in the Ross Sea on Interannual Time Scale

Abstract:

Some studies about formation and propagation of atmospheric waves suggest that the Ross Sea may be a key area to analyze low frequency variability in the Southern Hemisphere (SH) atmospheric flow. The objective of this study is to investigate atmospheric circulation patterns during extreme events of sea ice in the Ross Sea in Antarctic on interannual time scale. During periods of minimum sea ice the SST anomalies showed a pattern very similar to the warm phase of ENSO, with an anomalous convective activity over the center-east Pacific warm pool. With this warming in the South Pacific, there is a weakening of western zonal flow across the troposphere. In this region the polar jet is weakened while the subtropical jet is strengthened and shifted to southeast between southern South America and the South Atlantic. This pattern favors a smaller cyclonic activity and a greater atmospheric blocking frequency. Thus, in the southern South Pacific there is an anomalous ridge at middle levels, while in the central South Pacific (around 50° S) there is an anomalous trough, which extends to the southern tip of South America and southern South Atlantic. The same pattern of anomalies is observed at low and high levels. This pattern is very similar to PSA1, which is the second EOF pattern of seasonal mean 500-hPa height anomalies for the SH. Thus, the poleward warm advection is weakened in the Bellingshausen Sea and western Weddell Sea, which results in cooling south of 30° S of the South America. The pattern of cold anomalies extending northward over the Andes mountains to near the equator is an indicative of cold outbreaks. While there is convection in the southern tip of South America, there is suppression in the northeastern of Brazil. On the other hand, during periods of maximum sea ice cover in the Ross Sea the interannual anomalies of atmospheric and surface fields are nearly in opposite phase to periods of minimum coverage.