Atmospherically forced interannual variability of Subantarctic Mode Water volume and properties

Ivana Cerovecki, University of California San Diego, Scripps Institution of Oceanography, La Jolla, United States and Andrew Meijers, NERC British Antarctic Survey, Cambridge, United Kingdom
The dominant Subantarctic Mode Water (SAMW) formation regions are located in the southwestern Indian Ocean, southeastern Indian Ocean, Central Pacific and the southeast Pacific. In years with strong wintertime (July-September) Southern Annular Mode (SAM) and/or El Nino Southern Oscillation (ENSO), the volume of newly formed SAMW in the two sections of the Indian Ocean, as well as the two sections of the Pacific Ocean, tend to be out of phase. For positive phases of the SAM and ENSO greater volumes of SAMW are formed on the eastern side of each basin and less than normal in the west, and visa versa for negative phases. Deep wintertime mixed layers that are associated with SAMW formation show the same dipole pattern in each basin, as does the wintertime surface ocean heat loss. In years with a particularly strong El Nino or La Nina isopycnal heave is introduced in the SAMW density range south of New Zealand (around 180ºE), with its greatest amplitude around 27.0 kg m-3. These anomalies are advected eastward, impacting SAMW volume in the Central Pacific and later the southeast Pacific in subsequent years. SAMW formation is thus influenced not only by local formation driven by atmospheric forcing, but also by eastward propagating isopycnal heave that acts to precondition subsequent downstream SAMW formation. The interannual variability in formation rates also has a signal in SAMW temperature and SST. The strong interannual variability of SAMW volume and properties are super imposed on a longer term (quasi-decadal) variability possibly associated with changes in SAM and Interdecadal Pacific Oscillation.