Variability of dense water formation in the Ross Sea

Abstract

This paper presents results from a model study of the interannual variability of high salinity shelf water (HSSW) properties in the Ross Sea. Salinity and potential temperature of HSSW formed in the western Ross Sea show oscillatory behaviour at periods of 5-6 and 9 years superimposed on long-term fluctuations. While the shorter oscillations are induced by wind variability, variability on the scale of decades appears to be related to air temperature fluctuations. At least part of the strong decrease of HSSW salinities deduced from observations for the period 1963-2000 is shown to be an aliasing artefact due to an undersampling of the periodic signal. While sea ice formation is responsible for the yearly salinity increase that triggers the formation of HSSW, interannual variability of net freezing rates hardly affects changes in the properties of the resulting water mass. Instead, results from model experiments indicate that the interannual variability of dense water characteristics is predominantly controlled by variations in the shelf inflow through a sub-surface salinity and a deep temperature signal. The origin of the variability of inflow characteristics to the Ross Sea continental shelf can be traced into the Amundsen and Bellingshausen Seas. The temperature anomalies are induced at the continental shelf break in the western Bellingshausen Sea by fluctuations of the meridional transport of circumpolar deep water with the eastern cell of the Ross Gyre. In the Amundsen Sea, upwelling due to a persistently cyclonic wind field carries the signal into the surface mixed layer, leading to fluctuations of the vertical heat flux, anomalies of brine release near the sea ice edge, and consequently to a sub-surface salinity anomaly. With the westward flowing coastal current, both the sub-surface salinity and deep temperature signals are advected onto the Ross Sea continental shelf. Convection carries the signal of salinity variability into the deep ocean, where it interacts with modified circumpolar deep water upwelled onto the continental shelf as the second source water mass of HSSW. Sea ice formation on the Ross Sea continental shelf thus drives the vertical propagation of the signal rather than determining the signal itself.

An isopycnic ocean carbon cycle model

“…Major features of the horizontal circulation (Fig. 6b) are in general well-reproduced with an ACC strength of 150-160 Sv, a Weddell Gyre of 50 Sv (Beckmann et al, 1999), a Ross Gyre of 20-30 Sv (Assmann and Timmermann, 2005) and a North Atlantic Sub-tropical and Sub-polar Gyres of 30-40 Sv.…”

Ocean/ice shelf interaction in the southern Weddell Sea: results of a regional numerical helium/neon simulation

“…9). In the Ross Sea, ISW rich water follows the circulation of the Ross Gyre (e.g., Assmann and Timmermann (2005)) as far as Cap Adare (western tip) where the signal splits into two branches (Fig. 9).…”

On the outflow of dense water from the Weddell and Ross Seas in OCCAM model

“…For example, a long-term freshening of shelf water masses of both the Weddell and Ross Seas has been observed ; Jacobs and Giulivi, 2010; Hellmer et al, 2011), but the causes of this freshening are still under debate (e.g. Assmann and Timmermann, 2005;Rignot and Jacobs, 2002). Hellmer et al (2009) showed that decadal variability of the salinity of the southwestern Weddell Sea shelf water can be related to the Southern Annular Mode, which impacts both the ocean dynamics and sea ice processes in the Southern Ocean (Hall and Visbeck, 2002) and, consequently, the AABW contribution to the deep ocean (Kerr et al, 2009a).…”