No abstract

An array of five moorings was deployed from February 2009 to February 2010 across the Antarctic shelf and slope in the southeastern Weddell Sea (~18°W). Observations demonstrate the key processes responsible for variability in water masses and transport in the region. Rapid fluctuations in temperature and salinity throughout the year are linked with variability in wind stress over the array. This causes the deepening or shoaling of the pycnocline, past the depth of the moorings. In the upper 500 m, the seasonal cycle in salinity shows freshening in autumn, with the strongest freshening at the shallowest mooring (~250 m), furthest on‐shelf. The sea ice concentration over the array exceeds 90% during this period and contributes a positive salt flux into the ocean during autumn. Freshening begins during strong along‐shore (easterly) winds in late April 2009. This demonstrates that variations in Ekman transport and wind‐driven mixing play a key role in determining the salinity of shelf waters around Antarctica. Transport of the Antarctic Slope Current also shows a seasonal cycle with a maximum during late April. Model simulations show the importance of along‐shore advection, as the arrival of a fresh anomaly from upstream determines the timing of the salinity minimum at the array. These processes are likely to be important for other regions around the Antarctic continent.

The Antarctic Slope Front presents a dynamical barrier between the cold Antarctic shelf waters in contact with ice shelves and the warmer subsurface waters offshore. Two hydrographic sections with full‐depth current measurements were undertaken in January and February 2009 across the slope and shelf in the southeastern Weddell Sea. Southwestward surface‐intensified currents of ∼30 cm s−1, and northeastward undercurrents of 6–9 cm s−1, were in thermal‐wind balance with the sloping isopycnals across the front, which migrated offshore by 30 km in the time interval between the two sections. A mid‐depth undercurrent on February 23 was associated with a 130‐m uplift of the main pycnocline, bringing Warm Deep Water closer to the shelf break. This vertical displacement, comparable to that caused by seasonal variations in wind speed, implies that undercurrents may affect the exchanges between coastal and deep waters near the Antarctic continental margins.

Observations of semidiurnal surface currents in the Kauai Channel, Hawaii, are interpreted in the light of the interaction of internal tides with energetic surface-intensified mesoscale currents. The impacts on internal tide propagation of a cyclone of 55-km diameter and ;100-m vertical decay scale, as well as of vorticity waves of ;100-km wavelength and 100-200-m vertical decay scales, are investigated using 3D ray tracing. The Doppler-shifted intrinsic frequency is assumed to satisfy the classic hydrostatic internal wave dispersion relation, using the local buoyancy frequency associated with the background currents through thermal-wind or gradient-wind balance. The M 2 internal tide rays with initial horizontal wavelength of 50 km and vertical wavelength of O(1000 m) are propagated from possible generation locations at critical topographic slopes through idealized mesoscale currents approximating the observed currents. Despite the lack of scale separation between the internal waves and background state, which is required by the ray-tracing approximation, the results are qualitatively consistent with observations: the cyclone causes the energy of internal tide rays propagating through its core to increase near the surface (up to a factor of 15), with surfacing time delayed by up to 5 h (;1508 phase lag), and the vorticity waves enhance or reduce the energy near the surface, depending on their phase. These examples illustrate the fact that, even close to their generation location, semidiurnal internal tides can become incoherent with astronomical forcing because of the presence of mesoscale variability. Internal tide energy is mainly affected by refraction through the inhomogeneous buoyancy frequency field, with Doppler shifting playing a secondary but not negligible role, inducing energy transfers between the internal tides and background currents. Furthermore, the vertical wavelength can be reduced by a factor of 6 near the surface in the presence of the cyclone, which, combined with the energy amplification, leads to increased vertical shear within the internal tide rays, with implications for internal wave-induced mixing in the ocean.

High‐resolution (2 km and hourly) observations of surface currents from High‐Frequency Radars are analyzed in terms of sea level anomalies (SLA) and compared with data from two satellite altimeter ground tracks. Purpose is to investigate whether ocean submesoscale processes can be observed with satellite altimetry. Our results highlight two major problems that must be overcome before being able to resolve submesoscale processes with altimetry: (i) signal contamination from high‐frequency motions and in particular from incoherent internal tides (near‐inertial oscillations have no effect on SLA), and (ii) measurement noise which prevents the computation of accurate cross‐track currents on scales (10 km). The latter may be overcome by future satellite altimeter missions, but the former will require taking into account the effect of mesoscale variability on internal tide propagation in regions where internal tides are significant.