Pacific/Circumpolar and North Atlantic Deep Waters in the Argentine Basin: Lateral Stirring within the AMOC

Sawyer V.S. Brand1, Lynne D Talley2, Matthew R Mazloff2, Channing Prend3, Isabella Rosso4 and Paul Chamberlain5, (1)University of Rhode Island, United States, (2)Scripps Institution of Oceanography, UCSD, La Jolla, United States, (3)Columbia University of New York, Palisades, United States, (4)Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, United States, (5)University of California San Diego, La Jolla, United States
Abstract:
The meridional overturning circulation (MOC) in the South Atlantic is usually characterized in terms of southward transport of North Atlantic Deep Water (NADW) and northward transport of Antarctic Bottom Water and Antarctic Intermediate Water/thermocline water. However, throughout the Deep Water depth/density range there is horizontal circulation of multiple water masses that interleave and mix. In the Argentine Basin, high oxygen, warm, saline NADW is modified by lateral mixing with low oxygen, cool, fresh Upper Circumpolar Deep Water (UCDW) that is dominated by Pacific Deep Water (PDW) entering from Drake Passage, which can be observed with Southern Ocean Carbon and Climate Observation and Modeling (SOCCOM) biogeochemical Argo floats. Two floats in the northern Argentine Basin have recorded anomalously low oxygen in the UCDW’s oxygen minimum. One anomaly (33°S) averaged 15 micromol/kg lower than all other profiles from the same float (potential density range 27.5-27.7 kg/m3), while the other (39°S) averaged 25 micromol/kg lower than nearby profiles. Associated temperature/salinity anomalies were ~ 0.3°C and 0.05 psu lower on isopycnals than surrounding waters. Argo trajectory analysis suggests the source of Argentine Basin waters in this potential density range is mostly southeast Pacific waters, flowing through the Drake Passage. Therefore the anomalies are likely isolated eddies of nearly unadulterated PDW injected into the subtropical gyre at the Malvinas/Brazil Current confluence. T/S anomalies in this density range in the Argo data base in the Argentine Basin and T/S/O2 anomalies in the GLODAPv2 hydrographic data are investigated to show the historic distribution of anomalies and evidence of direct stirring. Mean circulation analyses (e.g. Reid, Progress in Oceanography 1994; Stramma and England, JGRO 1999) suggest a circuitous anticyclonic route for UCDW to the northern Argentine Basin, but these anomalies indicate a partial shortcut via eddies.
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