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.

Two winter monsoon surge events (northerly and easterly) of January 2005 are captured in a one-way coupled atmosphere (8 km resolution) and ocean (3 km resolution) simulation of the Philippines region. Intensified wind jets and wakes in the lee of Mindoro and Luzon Islands induce the generation and migration of a pair of counter-rotating oceanic eddies in the model, with propagation direction related to the orientation of the winds during each of the surges. Features shared by the eddies include size (100–200 km), depth (~300 m) and propagation speed (0.1–0.15 m s-1 for cyclones). Mean wintertime model wind stress positive (negative) curl coincides with the climatological cyclone (anticyclone) distribution from a prior 8-year altimetry-based census of eddies in the southeast quadrant of the South China Sea during the winter monsoon. Moreover, the simulation results agree with contemporaneous satellite and historical in situ data characterizing regional oceanic eddy and atmospheric surface jet properties

The evolution of a submesoscale anticyclonic vortex was observed by high-frequency Doppler radio current meters and satellite radiometers. The vortex formed between two large cyclones to the southwest of Oahu, Hawaii. The radius of the core was ∼15 km; the azimuthal velocity reached 35 cm s−1; and the surface vorticity remained below −f for 9 days, reaching an extremum of −1.7f. The flow was ageostrophic near the center and around the periphery of the vortex. The initial growth may have been driven by negative wind stress curl in the lee of Oahu. The vortex was prone to inertial, symmetric, and anticyclonic ageostrophic instabilities, but the temporal evolution of radial profiles of vorticity was inconsistent with angular momentum redistribution by inertial instability. A tongue of surface water 0.7°C warmer became entrained northward between the vortex and the colder cyclone to the west. As the vortex strengthened, a 0.14°C km−1 front formed along the eastern flank of the tongue. The sea surface temperature gradient remained weaker on the western flank. The flow was anticyclonic (−0.4f ) and divergent (0.1f ) on the warm side of the front but cyclonic (0.6f ) and convergent (−0.2f ) on the cold side. This suggests ageostrophic cross-frontal circulations maintaining alongfront thermal wind balance in the presence of large-scale strain σ. Surface divergence δ was proportional to vorticity ζ during the 3-day frontogenesis: δ ∼ −(σ/f )ζ. This is consistent with a semigeostrophic model of a front confined to a surface layer of zero potential vorticity.

The monitoring of turbidity currents enables accurate internal structure and timing of these flows to be understood. Without monitoring, triggers of turbidity currents often remain hypothetical and are inferred from sedimentary structures of deposits and their age. In this study, the bottom currents within 20 m of the seabed in one of the Pointe-des-Monts (Gulf of St. Lawrence, eastern Canada) submarine canyons were monitored for two consecutive years using Acoustic Doppler Current Profilers. In addition, multibeam bathymetric surveys were carried out during deployment of the Acoustic Doppler Current Profilers and recovery operations. These new surveys, along with previous multibeam surveys carried out over the last decade, revealed that crescentic bedforms have migrated upslope by about 20 to 40 m since 2007, despite the limited supply of sediment on the shelf or river inflow in the region. During the winter of 2017, two turbidity currents with velocities reaching 0Á5 m sec À1 and 2Á0 m sec À1 , respectively, were recorded and were responsible for the rapid (<1 min) upstream migration of crescentic bedforms measured between the autumn surveys of 2016 and 2017. The 200 kg (in water) mooring was also displaced 10 m down-canyon, up the stoss side of a bedform, suggesting that a dense basal layer could be driving the flow during the first minute of the event. Two other weaker turbidity currents with speeds <0Á5 m sec À1 occurred, but did not lead to any significant change on the seabed. These four turbidity currents coincided with strong and sustained wind speed >60 km h À1 and higher than normal wave heights. Repeat seabed mapping suggests that the turbidity currents cannot be attributed to a canyon-wall slope failure. Rather, sustained windstorms triggered turbidity currents either by remobilizing limited volumes of sediment on the shelf or by resuspending sediment in the canyon head. Turbidity currents can thus be triggered when the sediment volume available is limited, likely by eroding and incorporating canyon thalweg sediment in the flow, thereby igniting the flow. This process appears to be particularly important for the generation 1045 of turbidity currents capable of eroding the lee side of upslope migrating bedforms in sediment-starved environments and might have wider implications for the activity of submarine canyons worldwide. In addition, this study suggests that a large external trigger (in this case storms) is required to initiate turbidity currents in sediment-starved environments, which contrasts with supply-dominated environments where turbidity currents are sometimes recorded without a clear triggering mechanism.