Abstract. Coastal polynyas in the Ross Sea are important source regions of high salinity shelf water (HSSW) – the precursor of Antarctic Bottom Water that supplies the lower limb of the thermohaline circulation. Here, the response of sea ice production and HSSW formation to synoptic- and meso-scale cyclones were investigated for the Ross Ice Shelf Polynya (RISP) using a coupled ocean-sea ice-ice shelf model targeted on the Ross Sea. When synoptic-scale cyclones prevailed over RISP, sea ice production (SIP) increased rapidly by 20–30 % over the entire RISP. During the passage of mesoscale cyclones, SIP increased by about 2 times over the western RISP but decreased over the eastern RISP, resulting respectively from enhancement in the offshore and onshore winds. HSSW formation mainly occurred in the western RISP and was enhanced responding to the SIP increase under both types of cyclones. Promoted HSSW formation could persist for 12–48 hours after the decay of the cyclones. The HSSW export across the Drygalski Trough was negatively correlated with the meridional wind speed, while the export across the Glomar Challenger Trough was positively correlated with the meridional wind. Such correlations are mainly controlled by variations in geostrophic ocean currents that result from sea surface elevation change.
The wind dependence of sea-ice motion was studied on the basis of ice velocity and windobservations, and weather model output. The study area was a transition zone between openwater and the ice-covered ocean in the northern Baltic Sea. In the centre of the basin the seaicemotion was highly wind-dependent and the linear relationship between the wind and thedrift velocities explained 80% of the drift's variance. On the contrary, the wind-drift dependencewas low near the coast. The wind-drift coherence was significant over a broader frequencyrange in the central part of the basin than for the coastal drift. The ice motion was simulatedby a numerical model forced with five types of wind stress and with two types of current data, and the outcome was compared with the observed buoy drift. The wind and the wind-inducedsurface current were the main factors driving the ice in the basin's centre, while internal icestresses were of importance in the shear zone near the fast ice edge. The best wind forcing wasachieved by applying a method dependent on atmospheric stability and ice conditions. Theaverage air–ice drag coefficient was 1.4×10-3 with the standard deviation of 0.2×10-3. Theimprovement brought about by using an accurate wind stress was comparable with thatachieved by raising the model grid resolution from 18 km to 5 km.
Abstract Atmospheric and oceanic conditions associated with southern Australian heat waves are examined using phase 5 of the Coupled Model Intercomparison Project (CMIP5) models. Accompanying work analyzing modeled heat wave statistics for Australia finds substantial increases in the frequency, duration, and temperature of heat waves by the end of the twenty-first century. This study assesses the ability of CMIP5 models to simulate the synoptic and oceanic conditions associated with southern Australian heat waves, and examines how the classical atmospheric setup associated with heat waves is projected to change in response to mean-state warming. To achieve this, near-surface temperature, mean sea level pressure, and sea surface temperature (SST) from the historical and high-emission simulations are analyzed. CMIP5 models are found to represent the synoptic setup associated with heat waves well, despite showing greater variation in simulating SST anomalies. The models project a weakening of the pressure couplet associated with future southern Australian heat waves, suggesting that even a non-classical synoptic setup is able to generate more frequent heat waves in a warmer world. A future poleward shift and strengthening of heat wave–inducing anticyclones is confirmed using a tracking scheme applied to model projections. Model consensus implies that while anticyclones associated with the hottest future southern Australian heat waves will be more intense and originate farther poleward, a greater proportion of heat waves occur in association with a weaker synoptic setup that, when combined with warmer mean-state temperatures, gives rise to more future heat waves.
[1] An Antarctic cyclone climatology was created based on simulations of the Antarctic Mesoscale Prediction System (AMPS) and the University of Melbourne Cyclone (UM) detection and tracking algorithm for the period 2001–2009. Over 17,000 cyclone tracks were included in the climatology, and 20% of these cyclones were mesoscale in terms of their size. Mesoscale systems were common south of the Antarctic Circumpolar Trough (ACT) over the coastal oceans of the Indian Ocean sector and in the Ross Sea, while large synoptic systems occurred most frequently in the ACT. A novel technique was applied to study the relationships between cyclone characteristics and surface properties over the Southern Ocean and the coastal areas of Antarctica. Our comprehensive study has revealed that up to half of the cyclones simulated by AMPS correlated with the surface latent heat flux, sensible heat flux, or temperature gradient. These cyclones either modified the surface or were being modified by it. In the former category, 85% of the systems were synoptic cyclones associated predominantly with baroclinicity and advection, and the atmospheric boundary layer had a little influence in the generation and development of cyclones. On the contrary, in the latter category, 36% of the systems were mesoscale cyclones gaining energy from instabilities in the boundary layer associated with strong turbulent fluxes.
Abstract Strong offshore wind events (SOWEs) occur frequently near the Antarctic coast during austral winter. These wind events are typically associated with passage of synoptic- or meso-scale cyclones, which interact with the katabatic wind field and affect sea ice and oceanic processes in coastal polynyas. Based on numerical simulations from the coupled Finite Element Sea-ice Ocean Model (FESOM) driven by the CORE-II forcing, two coastal polynyas along the East Antarctica coast––the Prydz Bay Polynya and the Shackleton Polynya are selected to examine the response of sea ice and oceanic properties to SOWEs. In these polynyas, the southern or western flanks of cyclones play a crucial role in increasing the offshore winds depending on the local topography. Case studies for both polynyas show that during SOWEs, when the wind speed is 2–3 times higher than normal values, the offshore component of sea ice velocity can increase by 3–4 times. Sea ice concentration can decrease by 20–40%, and sea ice production can increase up to two to four folds. SOWEs increase surface salinity variability and mixed layer depth, and such effects may persist for 5–10 days. Formation of high salinity shelf water (HSSW) is detected in the coastal regions from surface to 800 m after 10–15 days of the SOWEs, while the HSSW features in deep layers exhibit weak response on the synoptic time scale. HSSW formation averaged over winter is notably greater in years with longer duration of SOWEs.
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