Anomalous upper-ocean characteristics in Marguerite Bay, western Antarctic Peninsula, in response to the 1997/8 ENSO

A year-round sequence of hydrographic casts is used to trace the seasonal and interannual evolution of the upper ocean waters in Marguerite Bay on the western Antarctic Peninsula (WAP) between 1998 and 2002. Data from winter 1998 are seen to be anomalous, since they show an unusually deep mixed layer that became progressively more saline until the onset of spring, eventually reaching salinities as high as 34. While local ocean temperatures are fixed by the freezing point of seawater, the remnant of the 1998 winter mixed layer (the Winter Water) was the deepest and most saline of the period of data. Winter 1998 was a time of unusually low sea ice concentrations in the region, anomalously high air temperatures, and a higher frequency of northerly wind episodes. These are known to be the local manifestations of an extreme in large-scale interannual climate variability, specifically the strong ENSO event of 1997/8 that was weakening during the austral winter of 1998. The extreme depth of the mixed layer in winter 1998 is a consequence of the enhanced frequency of northerly winds, which kept the sampling region relatively ice-free thereby exposing the upper water column to more wind-induced mixing and allowing greater ice production. The higher salinity of the WW produced is due to a combination of greater ice production and stronger excavation of the top layer of the underlying saline Circumpolar Deep Water. The low stratification created during winter 1998 persisted through the following austral spring. Since primary production depends in part on vertical density structure, such winters have the potential to influence the following season's phytoplankton bloom. Although it is not yet certain whether this will result in an overall increase or decrease in phytoplankton (e.g. due to competing needs for nutrients and light), the bloom following winter 1998 was observed to be relatively weak and delayed compared with the following years. Since the forcing for the mixed layer changes is variable on ENSO timescales, a modulation of primary production with the same periods is expected in response.