Antarctic Bottom Water Formation and Deep-Water Chlorofluorocarbon Distributions in a Global Ocean Climate Model (original) (raw)
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Antarctic Bottom Water variability in a coupled climate model
2008
The natural variability of the Weddell Sea variety of Antarctic Bottom Water (AABW) is examined in a long-term integration of a coupled climate model. Examination of passive tracer concentrations suggests that the model AABW is predominantly sourced in the Weddell Sea. The maximum rate of the Atlantic sector Antarctic overturning (atl) is shown to effectively represent the outflow of Weddell Sea deep and bottom waters and the compensating inflow of Warm Deep Water (WDW). The variability of atl is found to be driven by surface density variability, which is in turn controlled by sea surface salinity (SSS). This suggests that SSS is a better proxy than SST for post-Holocene paleoclimate reconstructions of the AABW overturning rate. Heat-salt budget and composite analyses reveal that during years of high Weddell Sea salinity, there is an increased removal of summertime sea ice by enhanced wind-driven ice drift, resulting in increased solar radiation absorbed into the ocean. The larger ice-free region in summer then leads to enhanced air-sea heat loss, more rapid ice growth, and therefore greater brine rejection during winter. Together with a negative feedback mechanism involving anomalous WDW inflow and sea ice melting, this results in positively correlated-S anomalies that in turn drive anomalous convection, impacting AABW variability. Analysis of the propagation of-S anomalies is conducted along an isopycnal surface marking the separation boundary between AABW and the overlying Circumpolar Deep Water. Empirical orthogonal function analyses reveal propagation of-S anomalies from the Weddell Sea into the Atlantic interior with the dominant modes characterized by fluctuations on interannual to centennial time scales. Although salinity variability is dominated by along-isopycnal propagation, variability is dominated by isopycnal heaving, which implies propagation of density anomalies with the speed of baroclinic waves.
Thermohaline variability and Antarctic bottom water formation at the Ross Sea shelf break
Deep Sea Research Part I: Oceanographic Research Papers, 2011
We use hydrological and current meter data collected in the Ross Sea, Antarctica between 1995 and 2006 to describe the spatial and temporal variability of water masses involved in the production of Antarctic Bottom Water (AABW). Data were collected in two regions of known outflows of dense shelf water in this region; the Drygalski Trough (DT) and the Glomar-Challenger Trough (GCT). Dense shelf water just inshore of the shelf break is dominated by High Salinity Shelf Water (HSSW) in the DT and Ice Shelf Water (ISW) in the GCT. The HSSW in the northern DT freshened by $ 0.06 in 11 y, while the ISW in the northern GCT freshened by $ 0.04 in 8 y and warmed by $ 0.04 1C in 11 y, dominated by a rapid warming during austral summer 2001/02. The Antarctic Slope Front separating the warm Circumpolar Deep Water (CDW) from the shelf waters is more stable near GCT than near DT, with CDW and mixing products being found on the outer DT shelf but not on the outer GCT shelf. The different source waters and mixing processes at the two sites lead to production of AABW with different thermohaline characteristics in the central and western Ross Sea. Multi-year time series of hydrography and currents at long-term moorings within 100 km of the shelf break in both troughs confirm the interannual signals in the dense shelf water and reveal the seasonal cycle of water mass properties. Near the DT the HSSW salinities experienced maxima in March/April and minima in September/October. The ISW in the GCT is warmest in March/April and coolest between August and October. Mooring data also demonstrate significant high-frequency variability associated with tides and other processes. Wavelet analysis of near-bottom moored sensors sampling the dense water cascade over the continental slope west of the GCT shows intermittent energetic pulses of cold, dense water with periods from $ 32 h to $ 5 days.
Journal of Geophysical Research, 2007
The formation of deep and bottom waters along Antarctica's perimeter is determined by ocean interaction with the atmosphere, sea ice, ice shelves, and bottom topography. It initiates a chain of processes that contribute to the ventilation of the global abyss. To identify the formation sites and investigate the combined effects of the local forcing mechanisms on water mass transformation and spreading in the Southern Ocean, chlorofluorocarbon (CFC) simulations with the regional ocean circulation model (BRIOS-1) were performed. The model uses terrainfollowing vertical coordinates to better represent both near-bottom and mixed layer processes, and includes an explicit formulation of the ice shelf-ocean interaction. In agreement with observations, the results show the main deep and bottom water formations sites to be located in the Ross Sea, Prydz Bay, and southwestern Weddell Sea. The Ross Sea ventilates the South East Pacific and Australian Antarctic Basins. Both Ross Sea and Prydz Bay ventilate via the Antarctic Coastal Current the Weddell-Enderby Basin. The latter signal is overprinted by sources in the Weddell Sea which ventilate the South Scotia Sea and also the Weddell-Enderby Basin. Despite the general agreement between observed and simulated quantities like bottom layer CFC distribution and inventories along the Greenwich Meridian, the model tends to underestimate the ventilation of the abyssal ocean like other models with coarse resolution.
Deep Sea Research Part I: Oceanographic Research Papers, 2010
The knowledge of chlorofluorocarbon (CFC11, CFC12) concentrations in ocean 9 surface waters is a prerequisite for deriving formation rates of, and water mass ages in, deep and bottom waters on the basis of CFC data. In the Antarctic coastal region, surface-layer data are sparse in time and space, primarily due to the limited 12 accessibility of the region. To help filling this gap, we carried out CFC simulations using a regional ocean general circulation model (OGCM) for the Southern Ocean, which includes the ocean-ice shelf interaction. The simulated surface layer 15 saturations, i.e. the actual surface concentrations relative to solubility-equilibrium values, are verified against available observations. The CFC input fluxes driven by concentration gradients between atmosphere and ocean are controlled mainly 18 by the sea ice cover, sea surface temperature, and salinity. However, no uniform explanation exists for the controlling mechanisms. Here, we present simulated long-term trends and seasonal variations of surface-layer saturation at Southern 21 Ocean deep and bottom water formation sites and other key regions, and we discuss differences between these regions. The amplitudes of the seasonal saturation cycle vary from 22% to 66% and their long-term trends range from 0.1%/year to 24 0.9%/year. The seasonal saturation maximum lags the ice cover minimum by two months. We show that ignoring the trends and using instead the saturations actually observed can lead to systematic errors in deduced inventory-based formation 27 rates by up to 10% and suggest an erroneous decline with time.
Quantifying Antarctic deep waters in SODA reanalysis product
The Antarctic intermediate and deep water masses present in the Atlantic sector of the Southern Ocean were quantified through the inverse method known as Optimum Multiparameter (OMP) analysis. The method was applied to the Simple Ocean Data Assimilation (SODA) product, which assimilates real observed ocean data into a hydrodynamic model. Results here show that the SODA dataset is able to capture reasonably well the intermediate and deep water mass (i.e. Warm Deep Water, Weddell Sea Deep Water, Weddell Sea Bottom Water, and Circumpolar Deep Water) regional distribution and contribution to the total mixture in the Weddell Sea and Weddell-Scotia Confluence. Those regions are, respectively, the main Antarctic Bottom Water source and export areas to the global ocean. We infer some aspects of the ocean circulation from the water mass distribution obtained. However, some efforts are still needed to better represent the deep salinity in these areas. The weak representation of this hydrographic parameter could be associated with the model's lack of important cryospheric processes directly involved with bottom water formation.
Sensitivity of Antarctic Bottom Water to Changes in Surface Buoyancy Fluxes
Journal of Climate, 2016
The influence of freshwater and heat flux changes on Antarctic Bottom Water (AABW) properties are investigated within a realistic bathymetry coupled ocean–ice sector model of the Atlantic Ocean. The model simulations are conducted at eddy-permitting resolution where dense shelf water production dominates over open ocean convection in forming AABW. Freshwater and heat flux perturbations are applied independently and have contradictory surface responses, with increased upper-ocean temperature and reduced ice formation under heating and the opposite under increased freshwater fluxes. AABW transport into the abyssal ocean reduces under both flux changes, with the reduction in transport being proportional to the net buoyancy flux anomaly south of 60°S. Through inclusion of shelf-sourced AABW, a process absent from most current generation climate models, cooling and freshening of dense source water is facilitated via reduced on-shelf/off-shelf exchange flow. Such cooling is propagated to ...
Deep Sea Research Part I: Oceanographic Research Papers, 2005
Temperature, salinity and chlorofluorocarbons (CFCs) 11, 12 and 113 were measured on a line of stations along the front of the Ross Ice Shelf in the austral summers of 1984, 1994 and 2000. Water mass distributions were similar each year but with high variability in the cross-sectional areas. CFC concentrations increased and salinity decreased with time throughout the water column. CFC saturation levels in the shelf and surface waters also increased with time and ranged from 43% to 90%. The undersaturation was due to inflow of low-CFC modified Circumpolar Deep Water, gas exchange limited by sea ice cover and isolation of water from the atmosphere beneath the ice shelf. The residence time of dense shelf waters resulting from sea ice formation is less well constrained by the chemical data than is the strong flow into the Ross Ice Shelf cavity. Shelf waters are transformed over about 3.5 years, by net basal melting of the ice shelf, into fresher Ice Shelf Water (ISW), which emerges as a large plume near the central ice front at temperatures below the sea surface freezing point. We estimate an average ISW production rate of 0.86 Sv and an average net basal melt rate of 60 km 3 /year for the Ross Ice Shelf exceeding a 300 m draft (75% of the ice cavity) during recent decades from box and stream tube models fit to all of the CFC and salinity data. Model fits to the individual data sets suggest ISW production and net basal melt rate variability due to interannual changes on a shorter time scale than our observations. ISW production based on the CFC budget is better constrained than net basal melting based on thermohaline data, with a heat budget yielding a rate of only 20 km 3 /yr. Reconciling differences between apparent freshwater and temperature changes under the ice shelf involves considerations of mixing, freezing and the flow of meltwater across the ice shelf grounding line. r