Temperature Minimum Layer Research Articles

A two-dimensional model for the thermohaline circulation under an ice shelf

The production of Antarctic Bottom Water is influenced by Ice Shelf Water which is formed due to the modification of shelf water masses under huge ice shelves. The coupling of inflow conditions, thermohaline processes at the ice shelf base and the sub-ice shelf circulation is studied with a two-dimensional thermohaline circulation model which has been developed for a section perpendicular to the ice shelf edge. Different boundary conditions appropriate to the Filchner Ice Shelf regime are considered. The model results indicate that, in general, shelf water is transported toward the grounding line, where at the ice shelf base melting occurs with a maximum rate of 1.5 my−1. Accumulation of ice takes place at the end of the melting zone close to the ice shelf edge with a rate on the order of 0.1 my−1. The location of this accumulation zone determines whether or not the density increase by salt rejection causes an upper circulation cell and the separation of the modified water mass from the ice shelf base at mid-range depth. At the ice shelf edge the simulated temperature, salinity, helium and δ18O values for the temperature minimum layer are typical for Ice Shelf Water. However the sub-ice shelf circulation is highly variable as well as sensitive to changes in boundary conditions. Moderate changes in the characteristics of the inflowing water or in sea-floor topography may double the intensity of the circulation. Non-linear processes in the accumulation zone cause variabilities which can be described by an ice shelf edge oscillator influencing the entire circulation regime.

Sea ice, winter convection, and the temperature minimum layer in the Southern Ocean

The structure of the near surface waters in the Southern Ocean, poleward of the Antarctic Polar Front but away from continental margins, is investigated with a three‐dimensional time‐dependent numerical model which resolves the annual sea ice cycle. The growth and decay of the ice field is predicted, using one of Semtner's (1976) thermodynamic ice models, in terms of specified atmospheric data and computed thermohaline characteristics of the ocean layers. The ice field is found to be sensitive to the lateral advection of heat by the oceanic circulation as well as vertical heat transports due to deep winter convection. The model treats the temperature minimum layer in the Southern Ocean as the remnant of a deep winter mixed layer which becomes capped by surface heating and precipitation in summer. The predicted thermohaline characteristics of the temperature minimum layer and the surface mixed layer are in good agreement with observations. Finally, the annual air‐sea heat exchange predicted by the model is discussed. The model's predicted area averaged heat loss experienced by the ocean south of the Atlantic Polar Front is much smaller than was previously estimated. This is attributed to errors in the southern region of the model domain. It is suggested that models, which include the interactions between sea ice and surface, deep and bottom waters along the continental margins, are needed to investigate this region.

Climatic characteristics of the Antarctic Polar Front zone

The thermal structures for the upper 500 m presented in four long meridional sections across the subantarctic and antarctic regimes of the Southern Ocean display a number of similar features as well as some differences. The similarities are associated with the latitudinal variation of depth and temperature of the temperature minimum (T min) layer and the common occurrence of much vertical relief of isotherms north of the polar front, which is believed to reflect the presence of transient features, sucfy as meanders and eddies. The differences seem to be related to the details of the thermal structure of the polar front, the continuity of the T min layer at its northern extent, and the presence of strong thermal fronts north of the polar front. Neither local sea‐air flux of heat nor the wind‐induced Ekman drift is consistent with the presence of the polar front. Hence the front is not locally generated. The sea‐air heat flux in the vicinity of the front reverses the large‐scale trend of increased heat loss of the ocean as latitude increases: the heat flux is from ocean to atmosphere to the immediate north of the polar front and from atmosphere to ocean to the immediate south. Northward Ekman drift of surface waters is near the circumpolar maximum at the polar front, which means that the front divides the convergence region to the north from the divergence region to the south. The Antarctic Polar Front is quite similar to the Subantarctic Front of the North Pacific in nearly all respects.