The subantarctic and polar fronts in relation to deep water masses through the southwestern Atlantic

Abstract

The two principal fronts within the Antarctic Circumpolar Current (ACC), the Subantarctic Front (SAF) and the Polar Front (PF), are investigated with respect to their spatial patterns and relations to deep water masses from the northern Drake Passage to the western and southern Argentine Basin. Observations from four recent cruises in adjoining and overlapping regions are supplemented with satellite infrared imagery and trajectories from surface drifters. East of Drake Passage, the width of the Polar Frontal Zone (PFZ) increases rapidly when the SAF turns sharply northward as a part of the Falkland (Malvinas) Current, while the PF remains oriented mainly toward the northeast. Found within this region of the PFZ are mesoscale eddies, the upper layers of which consist of Subantarctic Mode Water from the Pacific. The contrasts in upper layer water properties that identify the SAF can be traced the entire length of the Falkland Current to the Brazil‐Falkland confluence zone and then southward to the southern Argentine Basin. There the SAF turns eastward and later, but perhaps not always, merges with the PF north of Ewing Bank to form a single, intense current core strongly influenced by bottom topography. At times this eastward current loops southward through a gap in the Falkland Ridge into the Georgia Basin, thereby supplying relatively warm and salty Subantarctic water to the Antarctic Zone. East of the gap, the two fronts become separated and are once again distinct features in the vicinity of the Islas Orcadas Rise. At depth, within Drake Passage and the western Scotia Sea, the SAF and PF are not lateral boundaries between distinct water masses, but are instead identified by enhanced vertical displacements of property isopleths. With the northward turn of the SAF east of Drake Passage, a thick layer of Circumpolar Deep Water (CDW) is advected over the Falkland Plateau into the Argentine Basin. There it is joined by waters entering the Argentine Basin via a deep spreading route through the Georgia Basin: denser CDW, deep water from the Weddell Sea, and episodically, deep water from the southeastern Pacific Ocean. Together, these waters form a northward flow off the Patagonian shelf that extends from the sea surface to the bottom, most of which turns back toward the south with the Falkland Current return. Also flowing south, along the seaward side of the Falkland Current return well away from the western boundary, is the main core of North Atlantic Deep Water (NADW). Detatched masses of NADW are observed within the CDW at the western boundary as well as within the ACC south of the SAF in the south central Argentine Basin.

Modification and pathways of Southern Ocean Deep Waters in the Scotia Sea

“…Silicate values, though lower by about 10 mmol kg À1 than in Drake Passage, reach a maximum of 125 mmol kg À1 at the bottom (Fig. 3d), which compares well with the SPDW silicate content observed by Peterson and Whitworth (1989) downstream of the Scotia Sea in the Falkland Escarpment. Although we do not suggest a link between the two sites (see Section 4.2.4), the existence of a previously unreported route for modified SPDW to escape the Scotia Sea following the SACCF through Georgia Passage is indicated by our data.…”

“…2c and d is the occurrence of a number of relatively warm and saline features with considerable fine structure in the proximity of the salinity maximum. These are attributable to entrainment of NADW, reported in the past at mid depths above the Falkland Escarpment (Peterson and Whitworth, 1989;Whitworth et al, 1991;Arhan et al, 1999). The spatial distribution of WSDW over the South Scotia Ridge is shown in Fig.…”

“…The presence of the low-oxygen CDW type at 591S is likely due to the shedding of an anticyclonic eddy from the SAF, as is suggested by the reversal in the slope of the isopycnals there. Southward export of northern waters across the SAF through episodic events is common in the northwestern Scotia Sea (Peterson and Whitworth, 1989).…”

“…Because it is formed downstream of the deep western boundary current of the South Pacific (Warren, 1981), SPDW was considered in the past to be of little global importance. This view has changed however since the detection of SPDW in the boundary current of the South Atlantic (Peterson and Whitworth, 1989;Arhan et al, 1999).…”

Kinematic elements of Antarctic Intermediate Water in the western South Atlantic

“…At greater depth the definition of Subantarctic Front can be approximated by the 4°C isotherm at 200 m depth (Peterson and Whitworth III, 1989). An indicator of the boundary of the Brazil Current Front is the 8°C isotherm at 500 m (Roden, 1986).…”