Variability of dense water formation in the Ross Sea

The paper presents results from a model study of the interannual variability of High Salinity Shelf Water (HSSW) properties in the Ross Sea.Salinity, potential temperature and volume 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 High Salinity Shelf Water, 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.Upwelling in the centre of this gyre carries the signal into the surface layer where it causes anomalies of brine release near the sea ice edge in the Amundsen Sea, which results in 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

Multiple sea-ice states and abrupt MOC transitions in a general circulation ocean model

Sea ice has been suggested, based on simple models, to play an important role in past glacial–interglacial oscillations via the so-called “sea-ice switch” mechanism. An important requirement for this mechanism is that multiple sea-ice extents exist under the same land ice configuration. This hypothesis of multiple sea-ice extents is tested with a state-of-the-art ocean general circulation model coupled to an atmospheric energy–moisture-balance model. The model includes a dynamic-thermodynamic sea-ice module, has a realistic ocean configuration and bathymetry, and is forced by annual mean forcing. Several runs with two different land ice distributions represent present-day and cold-climate conditions. In each case the ocean model is initiated with both ice-free and fully ice-covered states. We find that the present-day runs converge approximately to the same sea-ice state for the northern hemisphere while for the southern hemisphere a difference in sea-ice extent of about three degrees in latitude between the different runs is observed. The cold climate runs lead to meridional sea-ice extents that are different by up to four degrees in latitude in both hemispheres. While approaching the final states, the model exhibits abrupt transitions from extended sea-ice states and weak meridional overturning circulation, to less extended sea ice and stronger meridional overturning circulation, and vice versa. These transitions are linked to temperature changes in the North Atlantic high-latitude deep water. Such abrupt changes may be associated with Dansgaard–Oeschger events, as proposed by previous studies. Although multiple sea ice states have been observed, the difference between these states is not large enough to provide a strong support for the sea-ice-switch mechanism

Statistical optimization with nonlinear constraints and parameter identification for the cutoff height of pressure ridges

Sociedade Brasileira de Matemática Aplicada e Computacional.To determinate the cutoff height of pressure ridges on the sea ice surface, a statistical optimal model with nonlinear constraints is established. The performance criterion consists of two parts: the deviation between the numerical results and the measured data of the ridge height; the deviation between the numerical results and the measured data of the ridge spacing. The cutoff height is taken as the identified parameter. Then, properties of the statistical optimal model and the existence of the optimal parameter are discussed, and the optimal cutoff height is identified via an optimal algorithm. Finally, the obtained optimal cutoff height is applied to separate pressure ridges from other sea ice surface undulations in northwestern Weddell Sea of Antarctic, and the correlation between the ridge height and frequency is demonstrated. Numerical results show that our model and method are both effective than the previous works

Sea Ice Modelling

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