Baroclinic Ocean Response to Climate Forcing Regulates Decadal Variability of Ice‐Shelf Melting in the Amundsen Sea. Issue 24 (14th December 2022) - Silvano, Alessandro
Abstract: Warm ocean waters drive rapid ice‐shelf melting in the Amundsen Sea. The ocean heat transport toward the ice shelves is associated with the Amundsen Undercurrent, a near‐bottom current that flows eastward along the shelf break and transports warm waters onto the continental shelf via troughs. Here we use a regional ice‐ocean model to show that, on decadal time scales, the undercurrent's variability is baroclinic (depth‐dependent). Decadal ocean surface cooling in the tropical Pacific results in cyclonic wind anomalies over the Amundsen Sea. These wind anomalies drive a westward perturbation of the shelf‐break surface flow and an eastward anomaly (strengthening) of the undercurrent, leading to increased ice‐shelf melting. This contrasts with shorter time scales, for which surface current and undercurrent covary, a barotropic (depth‐independent) behavior previously assumed to apply at all time scales. This suggests that interior ocean processes mediate the decadal ice‐shelf response in the Amundsen Sea to climate forcing. Plain Language Summary: The West Antarctic Ice Sheet is losing mass, causing sea level rise. Most of this loss occurs in the Amundsen Sea Embayment, due to melting of coastal glaciers by warm ocean waters. These warm waters are transported toward the glaciers by the Amundsen Undercurrent, a near‐seafloor eastward‐flowing current located at the boundary between the deep ocean and the shallower seas around Antarctica. Changes in the undercurrent thus regulate the amount of heat available to melt the glaciers. Here, we use a model to assess the undercurrent's variability on time scales of decades, as decadal ocean forcing drives periods of enhanced ice‐sheet retreat. Contrary to previous work, our model shows that wind fluctuations, associated with surface temperature changes in the tropical Pacific, lead to changes in the interior ocean density field on decadal time scales. Decadal anomalous cyclonic atmospheric circulation over the Amundsen Sea, associated with cooling in the tropical Pacific, accelerates the near‐surface ocean flow westward, but also accelerates the eastward‐flowing undercurrent and enhances glacial melting. Our work suggests that previous assumptions about the decadal oceanic response of the Amundsen Sea to wind variability might need to be reconsidered, with implications for melting of West Antarctic glaciers. Key Points: Our modeled decadal strengthening of the Amundsen Undercurrent corresponds with increase in ice‐shelf basal melting Undercurrent variability on decadal time scales is predominantly baroclinic (i.e., anticorrelated with the surface flow variability) At decadal timescales, this model shows enhanced ice‐shelf melting under westward wind anomalies, a reverse relationship to previous studies