Western Antarctic Peninsula physical oceanography and spatio–temporal variability

Abstract

Abstract This study focuses on 12 years of physical oceanography data, collected during the Palmer, Antarctica, Long-Term Ecological Research program (PAL LTER) over the continental margin of the western Antarctic Peninsula (WAP). The dataset offers the most long-lived consistent CTD-gridded observations of Antarctic waters collected anywhere in the Southern Ocean. The physical characteristics, water column structure and spatio–temporal variability of the various properties are examined for physically consistent and ecologically important patterns and modes of variability. Unique findings of note include: (1) The average annual ocean heat flux (to the atmosphere) over the continental shelf shows a decreasing trend through time averaging 0.6 W m−2 yr−1, with an annual average ocean heat flux of ∼19 W m−2. The ocean heat content over the shelf shows a linearly increasing trend of 2.6×107 J m−2yr−1, due predominantly to increased upwelling of warm Upper Circumpolar Deep Water (UCDW) onto the shelf with a small contribution due to a slight warming of UCDW (but over longer time scales (50 yr), the warming of UCDW dominates), (2) optimal multi-annual average vertical turbulent diffusivity coefficient (kz) is ∼8.5×10−5 m2s−1, determined by inversion considering warming of trapped remnant winter mixed layer water, (3) the water masses in the grid are well separated according to bathymetrically controlled features, dividing the sample domain into 3 sub-regions: slope, shelf and coastal waters; (4) the Antarctic Circumpolar Current (ACC) was always present along the shelf-break (consistent with the Orsi et al. [1995. On the meridional extent and fronts of the Antarctic Circumpolar Current. Deep-Sea Research I 42 (5), 641–673.] climatology) where UCDW shows its farthest southern extent and forms the Southern ACC Front (SACCF). The spatio–temporal variability of the delivery and distribution of ocean heat is dictated by the dynamics that are consistent with changes in the state of ENSO (La Nina drives enhanced upwelling in this region) and in the strength of the Southern Annular Mode (SAM; +SAM drives a local response similar to that of La Nina). The large 1997–1998 El Nino, followed by the transition to the strong La Nina of 1998–1999 (amplified by a large +SAM) introduced a regime shift on the shelf, resulting in the elimination of ∼0.5 m of sea ice melt (presumably from the loss of sea ice being grown). 2002 was an anomalous year coinciding with an extraordinary storm forcing driving a 4.5σ increase in the heat content on the shelf. These jumps coincide with considerable changes in sea ice distribution as well. Pure UCDW on the shelf is primarily restricted to the deep canyons, with occasional appearances on the shelf floor near the middle of the grid. Anomalies in summer sea surface temperatures reflect wind strength (stronger winds mixing more cold winter water to the surface, with cooler SST; light winds, the opposite).