Winter mesoscale circulation on the shelf slope region of the southern Drake Passage

An austral winter cruise in July–August 2006 was conducted to study the winter circulation and iron delivery processes in the Southern Drake Passage and Bransfield Strait. Results from current and hydrographic measurements revealed a circulation pattern similar to that of the austral summer season observed in previous studies: The Shackleton Transverse Ridge (STR) in the southern Drake Passage blocks a part of the eastward Antarctic Circumpolar Current (ACC) which forces the ACC to detour southward, produces a Taylor Column over the STR, and forms an ACC jet within the Shackleton Gap, a deep channel between the STR and the shelf of Elephant Island. Observations show that to the west of the STR, the Upper Circumpolar Deep Water (UCDW) intruded onto the shelf around the South Shetland Islands while to the east of the STR, shelf waters were transported off the northern shelf of Elephant Island. Along a similar west–east transect approximately 50 km off the shelf, the northward transport of shelf waters was approximately 2.4 and 1.2 Sv in the austral winter and summer, respectively. The waters around Elephant Island primarily consist of the UCDW that has been modified by local cooling and freshening, unmodified UCDW that has recently intruded onto the shelf, and Bransfield Current water that is a mixture of shelf and Bransfield Strait waters. Weddell Sea outflows were observed which affect the hydrography and circulation in the Bransfield Strait and indirectly affect the circulation patterns in the southern Drake Passage and around Elephant Island. Two coupled Fe enrichment and transport mechanisms are proposed which operate together to firstly enrich water with iron and then transfer this water offshelf. These are an initial intrusion of UCDW onto the northern shelf region of the South Shetland Islands which occurs due to Ekman pumping driven by the prevailing westerly wind in the region, and subsequently, an offshelf transport of shelf waters which occurs east of Elephant Island due to acquisition of positive vorticity by shelf waters from horizontal mixing with ACC waters being intruded onto the shelf.