FY 1997

Southwest Pacific Ocean water-mass changes between 1968/69 and 1990/91

Johnson, G.C., and A.H. Orsi

J. Climate, 10(2), 306–316, doi: 10.1175/1520-0442(1997)010<0306:SPOWMC>2 (1997)

Water-mass changes are estimated in the southwest Pacific Ocean by comparing a meridional hydrographic section along 170°W between 60°S and the equator occupied in 1968/69 during the Southern Cross cruise and again in 1990 during a NOAA Climate and Global Change cruise. Another comparison is made using a hydrographic section along 35°S between the date line and 169°W occupied in 1969 during USNS Eltanin cruise 40 and again in 1991 during a Mapkiwi cruise. The most robust change consists of cooling (and freshening) on isopycnals, with peak differences exceeding -1.0°C (-0.25 pss) at the base of the subtropical thermocline. The cooling and freshening starts above the stratification minimum of the Subantarctic Mode Water and persists to below the salinity minimum of the Antarctic Intermediate Water. Amplitudes are largest at 48°S, near where these water masses subduct, and decay toward 20°S, near the axis of the Subtropical Gyre. This change is likely the result of surface warming and/or freshening at high latitudes, where these water masses are formed before they ventilate the base of the subtropical thermocline. Isopycnals tend to deepen south of 35°S and to shoal more weakly from 35° to 20°S. These changes are consistent with a simple model response to high-latitude warming. Results from the section comparisons are put in a larger context by estimating interdecadal changes on isopycnals throughout the South Pacific Ocean. In addition, two changes consistent with a strengthened southern influence are found within the Lower Circumpolar Water from the Chatham Rise at 43°S to the Samoa Passage at 10°S. Cooling and freshening erode the top of the deep salinity maximum of the modified North Atlantic Deep Water. In the weakly stratified abyssal layer, the modified Antarctic Bottom water cools by about 0.025°C.