Paola Picco

An investigation of thermohaline circulation in Terra Nova Bay polynya

This paper addresses the study of the thermohaline circulation and convective processes induced by the presence of a coastal polynya in Terra Nova Bay (Ross Sea, Antarctica). These processes are analysed by means of oceanographic observations and numerical simulations. Time series of hydrological parameters and currents show the High Salinity Shelf Water (HSSW) formation with the mixing of the water column down to great depths, and a seasonal cycle of kinetic energy. A two-dimensional time dependent model for the numerical simulation of the effects of polynya on the circulation has been implemented. This model allows estimation of the contribution of thermohaline forcing on the circulation pattern, the amount of ice production consistent with the observed salinity distribution and annual cycle of currents and the amount of HSSW exported. The thermohaline forcing due to the polynya is able to induce strong horizontal currents up to 40 cm s−1 near the polynya area, as well as vertical movements of the order of 1 cm s−1. An annual ice production, ranging from 45 m to 90 m, mixes the water column at great depths, up to 750 m, and increases the salinity of the surface incoming waters up to 34.82. The estimated net north-eastward transport of HSSW ranges from 0.34 Sv to 1.23 Sv, depending on ice production.

Upper ocean layer dynamics and response to atmospheric forcing in the Terra Nova Bay polynya, Antarctica

Abstract A one-year time series of Acoustic Doppler Current Profiler (ADCP) data was collected in Terra Nova Bay (TNB) polynya (Ross Sea, Antarctica) during 2000. Together with Automatic Weather Station (AWS) Eneide meteorological data and Special Sensor Microwave Imager (SSM/I) ice concentration data, ADCP data were analysed to investigate upper layer dynamics and variability due to atmospheric forcing. Empirical Orthogonal Function (EOF) analysis was performed to separate the surface variability caused by local forcing from the large-scale circulation component. In particular, the first mode represented the barotropic circulation while the second the stronger surface currents. The decrease in shelf water density from melting sea ice resulted in an off-shore density gradient producing a southern shift in the circulation. This result proved to be consistent with the in situ data acquired during February–April at 120 m depth. The observed variability of the surface currents was assessed with respect to the thermal wind equation and the steady Ekman model. Strong katabatic winds shifted the surface currents eastward with respect to the general north-eastern circulation. The wind stress acted as a relevant forcing for the surface large-scale circulation in TNB, but had negligible effects on the vertically integrated transport.

Monitoring polynyas with Ocean Acoustic Tomography: a feasibility study in Terra Nova Bay

This study looks at the feasibility of using Ocean Acoustic Tomography for long-term monitoring of polynyas using both observations in Terra Nova Bay polynya (Ross Sea) and simulations with a range dependent, multi-layered adiabatic normal mode acoustic propagation model. The summer sound speed profile is characterized by surface values of around 1450 m s−1, a minimum of 1441 m s−1 around 50 m depth and a linear increase with a 0.016 s−1 slope. Thus, the sound propagation is apparently ducted in the near surface layer and is refracted upward below it. During winter, due to water cooling and mixing processes, the subsurface minimum disappears, the surface sound speed is about 1440 m s−1 and no near surface layer ducted propagation occurs. Because of the specificity of the Terra Nova Bay seasonal sound speed profile and to cope with both deep and shelf water applicability, the feasibility study of acoustic inversion was undertaken using normal mode Match Field Tomography instead of the more classical travel-time inversion. The results from simulations demonstrate that ocean acoustic tomography is able to reproduce quite well the vertical sound speed profile, in particular the temporal evolution of summer stratification and winter mixing processes, thus providing information on the upper layer, where direct measurements are not possible.

On the Ekman Equations for Ocean Currents Driven by a Stochastic Wind

The basic assumptions for the depth-dependent Ekman equations are presented. An analysis of three wind stress time series, from different geographical locations, is performed: The results lead to interpret the wind stress as a stochastic process, with components fluctuating with deterministic frequencies. The stochastic equations coupling wind stress and ocean currents are formulated; their solutions are stochastically bounded. Results of numerical simulations are given to show the main behaviors of the system.