Front (oceanography)

Abstract: The approximation of geostrophic balance across a front is studied. Making this approximation, an analytic approach is made to a frontogenesis model based on the classic horizontal deformation field. Kelvin's circulation theorem suggests the introduction of a new independent variable in the cross-front direction. The problem is solved exactly for a Boussinesq, uniform potential vorticity fluid. Non-Boussinesq, non-uniform potential vorticity, latent heat, and surface friction effects are all studied. Using a two-region fluid we model the effects of confluence near the tropopause. A similar approach is made to the appearance of fronts in the finite-amplitude development of the simplest Eady wave; this is also solved analytically. Based on the surface fronts produced by these models, we give a general model of a strong surface front. There is a tendency to form discontinuities in a finite time.

Abstract: All available meridional sections have been analyzed to investigate the evolution of main fronts between 0° and 150°E. The central South Atlantic is featured by the Subtropical Frontal Zone (STFZ), bordered by the North and South Subtropical Fronts (NSTF and SSTF, respectively), and by the Polar Frontal Zone (PFZ), bordered by the Subantarctic and Polar Fronts (SAF and PF, respectively). This structure becomes more complex in the African sector as the Agulhas Retroflection and the bottom topography force a more convoluted pattern. The Retroflection and associated Agulhas Front (AF) press the SSTF from 38° to 42°–43°S. Strong interactions of the AF, SSTF, and SAF with topography shift the fronts but do not obliterate them. The AF can be traced reliably up to 52°E, sometimes up to 75°E. The SAF is deflected from 45° to 43°S by the Mid-Ocean Ridge and converges with the SSTF north of the Prince Edward Islands to form a combined SSTF/SAF. This front intensifies east of 50°–52°E as a result of the confluence with the AF, and between 52° and 65°E a triple AF/SSTF/SAF (“the Crozet Front”) is observed. The PF continues along 49° and 50°S between the Crozet Plateau and the Ob-Lena (Conrad) Rise, passing north of Kerguelen, nearly joining the triple Crozet Front. Downstream of the Kerguelen-Amsterdam Passage, the canonical structure is being restored (SSTF, SAF, PF); however, the front parameters in the Australian sector are different from the African sector, largely because of strong air-sea interaction and cross-frontal exchanges in the Crozet-Kerguelen region. The SSTF, squeezed between the AF and SAF, loses characteristics to both. The SSTF/SAF interaction results in the Australian SAF being warmer and saltier downstream, while the SSTF becomes shallower and weaker. The Australian STF derives its characteristics mostly from the AF, thus bringing the modified Agulhas waters to the Pacific Ocean. The newly defined North Subtropical Front (NSTF) was distinguished in the Indian Ocean between 31° and 38°S. The front marks the southern boundary of the subtropical salty, warm water pool of the central South Indian Ocean. The NSTF location is coincident with the position of the wind convergence between westerlies and easterlies, suggesting the possible wind-driven frontogenesis.

Abstract: AbyssalglobalanddeepSouthern Oceantemperature trendsarequantifiedbetweenthe1990sand2000sto assesstheroleofrecentwarmingoftheseregionsin globalheatandsealevelbudgets.Theauthors1)compute warming rates with uncertainties along 28 full-depth, high-quality hydrographic sections that have been occupied two or more times between 1980 and 2010; 2) divide the global ocean into 32 basins, defined by the topography and climatological ocean bottom temperatures; and then 3) estimate temperature trends in the 24 sampled basins. The three southernmost basins show a strong statistically significant abyssal warming trend, with that warming signal weakening to the north in the central Pacific, western Atlantic, and eastern Indian Oceans. Eastern Atlantic and western Indian Ocean basins show statistically insignificant abyssal cooling trends. Excepting the Arctic Ocean and Nordic seas, the rate of abyssal (below 4000 m) global ocean heat content change in the 1990s and 2000s is equivalent to a heat flux of 0.027 (60.009) W m 22 applied over the entire surface of the earth. Deep (1000‐4000 m) warming south of the Subantarctic Front of the Antarctic Circumpolar Current adds 0.068 (60.062) W m 22 . The abyssal warming produces a 0.053 (60.017) mm yr 21 increase in global average sea level and the deep warming south of the Subantarctic Front adds another 0.093 (60.081)mm yr 21 . Thus, warmingin theseregions,ventilatedprimarilyby AntarcticBottomWater,accounts for a statistically significant fraction of the present global energy and sea level budgets.