Oceanic Regime Shift to a Warmer Continental Shelf Adjacent to the Shackleton Ice Shelf, East Antarctica

The long-held view that the East Antarctic margin is isolated from warm offshore waters has been challenged by recent observations showing incursions of warm modified Circumpolar Deep Water (mCDW) reaching several East Antarctic ice shelves. However, large areas of the East Antarctic continental shelf remain poorly observed, making it challenging to determine if the supply of oceanic heat to the ice shelves is changing. Here, we use temperature and salinity profiles to the west of the Shackleton Ice Shelf (SIS; approximate to 100 degrees E) spanning 60 years to assess the variability of the water masses in the context of a changing climate. We document warming and freshening of shelf waters. Prior to 1996, cold mCDW water (theta < -1.6 degrees C) was found below the surface mixed layer and cold Dense Shelf Water (DSW) with a salinity of >34.5 dominated the water column. After 2010, warm mCDW (>= -1.0 degrees C) was widespread over the continental shelf and DSW with salinity over 34.5 was no longer present. The mixing ratio of glacial meltwater indicates that warm mCDW observed in 2011 caused basal melting of the SIS, possibly reducing the salinity of DSW. Increased access of warm waters to the continental shelf may have also occurred on the eastern side of the ice shelf, where glaciological evidence shows the grounding line has retreated. These observations suggest a shift occurred prior to 2011 that has increased the ocean heat supply to the continental shelf and to the SIS, increasing basal melt and reducing DSW formation.