Abstract
Observational records starting in the 1950s show West Antarctica is amongst the most rapidly warming regions on the planet. Together with increased intrusions of warm circumpolar deep water (CDW) onto the continental shelf due to local wind forcing (the primary mechanism in recent decades), this has resulted in enhanced surface and basal melting of floating ice shelves and an associated acceleration and thinning of West Antarctic outlet glaciers, increasing the rate of global sea level rise. In this study, it is shown that during the austral spring season, significant surface warming across West Antarctica has shifted westward to the Ross Ice Shelf in recent decades in response to enhanced cyclonic circulation over the Ross Sea. These circulation changes are caused by a Rossby wave train forced by increasing sea surface temperatures in the western tropical Pacific, which is tied to the springtime shift of the Interdecadal Pacific Oscillation (IPO) to its negative phase after 1992. While the local wind trends enhance warm air advection and surface warming across the Ross Ice Shelf, the strong easterly component of the wind trends reduces the likelihood for intrusions of CDW onto the continental shelf in this region. This suggests that during spring there are competing mechanisms of surface and basal melting of the Ross Ice Shelf, both of which are closely tied to natural tropical Pacific decadal variability. Moreover, that the projected transition of the IPO back to its positive phase in the coming decade, though likely to reduce surface warming on the Ross Ice Shelf, could increase the risk of disintegration of Ross Sea ice shelves due to increased intrusions of CDW and enhanced basal melting.
Highlights
During summer[24,25,26,27], and it is likely to become more frequent by the end of the century[31,32]
This study demonstrates strong covariability of spring circulation and temperature over the Ross Ice Shelf with remote forcing from the western tropical Pacific tied to natural variability in the springtime Interdecadal Pacific Oscillation (IPO)
Performing a sensitivity experiment with a climate model shows that positive sea surface temperature (SST) anomalies in the western tropical Pacific results in a Rossby wave train that reproduces the observed increase in cyclonic circulation in the Ross Sea and the associated westward shift of West Antarctic spring warming to the Ross Ice Shelf
Summary
Observational records starting in the 1950s show West Antarctica is amongst the most rapidly warming regions on the planet. The climatological difference in 500 hPa geopotential height between the perturbed and control experiments during spring (Fig. 5a) is broadly consistent with the circulation pattern evident in the reanalysis trends (Fig. 2b), depicting an anomalous wave train emanating from the western tropical Pacific region, which propagates southeastward into the Ross and Amundsen Seas. The new reanalysis trends show a more intense cyclonic circulation in the Ross Sea, an anti-cyclonic circulation in the Amundsen Sea, 0.2–0.4 °C decade−1 stronger warming over the eastern Ross Ice Shelf and 0.1–0.2 °C decade−1 stronger warming near McMurdo, and strengthened easterlies over the continental shelf break This provides additional evidence that the circulation seen in the simulation is consistent with variability in the IPO and not La Niña, and tropical Pacific decadal variability (not ENSO) is the primary driver of the recent spring changes in circulation, SAT, and local wind forcing of CDW across the Ross Ice Shelf
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