Changes in meridional fluxes and water properties in the Southern Hemisphere subtropical oceans between 1992/1995 and 2003/2004

Abstract

As part of the WOCE (World Ocean Circulation Experiment) Hydrographic Programme (WHP), nominally zonal lines across the subtropical gyres in the Southern Hemisphere oceans were occupied from 1992 to 1995: line P06 (nominally along 32.5°S in the Pacific Ocean), A10 (along 30°S in the Atlantic Ocean), and I03 and I04 (along 20°S in the Indian Ocean). These lines were revisited from 2003 to 2004 to examine changes in circulation and water properties over the 8- to 12-year interval between visits. A simple comparison of the two hydrographic sections would be affected by variations with time scales shorter than the interval between visits even in the deep ocean, so two ocean global circulation models, OFES and SODA, were employed to study meridional flux changes. A time series of the heat flux across the latitude circle 32.5°S from 1980 to 2005 shows moderate negative correlations (with the correlation coefficient r = −0.5 to −0.6) with the Southern Oscillation Index. A composite study shows that during El Niño years, the westerly jet just south of 32.5°S in the Pacific shifts northward, resulting in a northward Ekman temperature transport anomaly, and thus weaker southward temperature transport. The transport variations also show moderate negative correlations ( r = −0.4 to −0.6) with the Southern Annular Mode Index. A similar composite study shows that during positive Southern Annular Mode Index years, the Ekman pumping anomaly around 32.5°S spins down the wind-driven gyre over the South Indian and South Pacific oceans, causing a southward transport anomaly. The Atlantic transport is not sensitive to either index. An Ekman transport anomaly in the South Indian Ocean also contributes to the southward transport anomaly. The salt transports are correlated negatively with the Southern Oscillation Index which is similarly explained by the Ekman transport, but the relationship between the salt transport and the Southern Annular Mode Index remains unknown. The substantial seasonal or higher-frequency variations are filtered out by comparing the water properties on neutral density surfaces. Along the South Pacific P06 line, Subantarctic Mode Water on γ = 26.8 (the neutral density surface of 1026.8 kg m −3) becomes cooler and fresher by up to 0.2 °C and 0.04 psu (practical salinity unit). The property changes are largest in the Chile Basin and smallest in the Tasman Sea. In the deep Southwest Pacific Basin, a small but statistically significant freshening of 0.002 psu was found in the Circumpolar Deep Water ( γ = 28.11). Along the South Atlantic A10 line, Subantarctic Mode Water and Antarctic Intermediate Water were found warmer and saltier to the east of the Mid-Atlantic Ridge (e.g. by 0.2 °C and 0.05 psu on γ = 26.8; 0.1 °C and 0.02 psu on γ = 27.4). North Atlantic Deep Water and Antarctic Bottom Water were found warmer and saltier particularly in the Argentine Basin (e.g. by 0.02 °C and 0.004 psu on γ = 28.0; 0.01 °C and 0.002 psu on γ = 28.2). Along the south Indian I03/I04 lines, Subantarctic Mode Water was found warmer and saltier to the west of 95°E (e.g. by 0.2 °C and 0.05 psu on γ = 26.8) and cooler and fresher to the east (e.g. a difference of −0.2 °C and −0.05 psu on γ = 26.8).