Stephanie M. Downes

The GFDL CM3 Coupled Climate Model: Characteristics of the Ocean and Sea Ice Simulations

AbstractThis paper documents time mean simulation characteristics from the ocean and sea ice components in a new coupled climate model developed at the NOAA Geophysical Fluid Dynamics Laboratory (GFDL). The GFDL Climate Model version 3 (CM3) is formulated with effectively the same ocean and sea ice components as the earlier CM2.1 yet with extensive developments made to the atmosphere and land model components. Both CM2.1 and CM3 show stable mean climate indices, such as large-scale circulation and sea surface temperatures (SSTs). There are notable improvements in the CM3 climate simulation relative to CM2.1, including a modified SST bias pattern and reduced biases in the Arctic sea ice cover. The authors anticipate SST differences between CM2.1 and CM3 in lower latitudes through analysis of the atmospheric fluxes at the ocean surface in corresponding Atmospheric Model Intercomparison Project (AMIP) simulations. In contrast, SST changes in the high latitudes are dominated by ocean and sea ice effects absen...

Impacts of wind stress on the Antarctic Circumpolar Current fronts and associated subduction

[1] Recent studies suggest that the overturning circulation in the Antarctic Circumpolar Current (ACC) region shows a weak sensitivity to overlying wind stress changes, due to balancing of changes in the eddy‐induced and Eulerian mean transports. Using an eddy‐permitting coupled climate model, we analyze the response of the ACC transport, and associated water mass subduction rates, in response to an idealized poleward shift and intensification of the westerlies. As in previous studies, we find a small increase in the net ACC transport, and a poleward shift in the mean position of the ACC flow. However, the ACC is restructured, with the Subantarctic Front (SAF) and Polar Front (PF) branches shifting poleward by between 0.9° and 2.5° of latitude, resulting in a weaker ACC flow in both the SAF and PF zones. The wind stress anomaly drives a stronger northward Ekman transport of cool surface waters, deepening the winter mixed layer and causing a 12.7 Sv increase in the subduction of Subantarctic Mode Water (SAMW) north of the SAF zone and a 6.5 Sv increase in the subduction of Antarctic Intermediate Water (AAIW) within the SAF and PF zones. Our results suggest that changes in the wind stress restructure the Southern Ocean large‐scale circulation, including the flow of the ACC in its primary jets, and that this affects the formation rates of SAMW and AAIW in this complex region. Citation: Downes, S. M., A. S. Budnick, J. L. Sarmiento, and R. Farneti (2011), Impacts of wind stress on the Antarctic Circumpolar Current fronts and associated subduction, Geophys. Res. Lett., 38, L11605, doi:10.1029/ 2011GL047668.

Water Mass Exchange in the Southern Ocean in Coupled Climate Models

AbstractThe authors estimate water mass transformation rates resulting from surface buoyancy fluxes and interior diapycnal fluxes in the region south of 30°S in the Estimating the Circulation and Climate of the Ocean (ECCO) model-based state estimation and three free-running coupled climate models. The meridional transport of deep and intermediate waters across 30°S agrees well between models and observationally based estimates in the Atlantic Ocean but not in the Indian and Pacific, where the model-based estimates are much smaller. Associated with this, in the models about half the southward-flowing deep water is converted into lighter waters and half is converted to denser bottom waters, whereas the observationally based estimates convert most of the inflowing deep water to bottom waters. In the models, both Antarctic Intermediate Water (AAIW) and Antarctic Bottom Water (AABW) are formed primarily via an interior diapycnal transformation rather than being transformed at the surface via heat or freshwate...

Southern Ocean Circulation and Eddy Compensation in CMIP5 Models

Thirteen state-of-the-art climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are used to evaluate the response of the Antarctic Circumpolar Current (ACC) transport and Southern Ocean meridional overturning circulation to surface wind stress and buoyancy changes. Understanding how these flows—fundamental players in the global distribution of heat, gases, and nutrients— respond to climate change is currently a widely debated issue among oceanographers. Here, the authors analyze the circulation responses of these coarse-resolution coupled models to surface fluxes. Under a future CMIP5 climate pathway where the equivalent atmospheric CO2 reaches 1370ppm by 2100, the models robustly project reduced Southern Ocean density in the upper 2000m accompanied by strengthened stratification. Despite an overall increase in overlying wind stress (;20%), the projected ACC transports lie within 615% of their historical state, and no significant relationship with changes in the magnitude or position of the wind stress is identified. The models indicate that a weakening of ACC transport at the end of the twenty-first century is correlated with a strong increase in the surface heat and freshwater fluxes in the ACC region. In contrast, the surface heat gain across the ACC region and the wind-driven surface transports are significantly correlated with an increased upper and decreased lower Eulerian-mean meridional overturning circulation. The change in the eddy-induced overturning in both the depth and density spaces is quantified, and it is found that the CMIP5 models project partial eddy compensation of the upper and lower overturning cells.

Impacts of Climate Change on the Subduction of Mode and Intermediate Water Masses in the Southern Ocean

Abstract Changes in the temperature, salinity, and subduction of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) between the 1950s and 2090s are diagnosed using the CSIRO Mark version 3.5 (Mk3.5) climate system model Caps under a CO2 forcing that reaches 860 ppm by the year 2100. These Southern Ocean upper-limb water masses ventilate the ocean interior, and changes in their properties have been related to climate change in numerous studies. Over time, the authors follow the low potential vorticity and salinity minimum layers describing SAMW and AAIW and find that the water column in the 2090s shifts to lighter densities by approximately 0.2 kg m−3. The model projects a reduction in the SAMW and AAIW annual mean subduction rates as a result of a combination of a shallower mixed layer, increased potential vorticity at the base of the mixed layer, and a net buoyancy gain. There is little change in the projected total volume of SAMW transported into the ocean interior via the subduction...

Changes in the Subduction of Southern Ocean Water Masses at the End of the Twenty-First Century in Eight IPCC Models

Abstract A multimodel comparison method is used to assess the sensitivity of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) formation to climate change. For the Intergovernmental Panel on Climate Change A2 emissions scenario (where atmospheric CO2 is 860 ppm at 2100), the models show cooling and freshening on density surfaces less than about 27.4 kg m−3, a pattern that has been observed in the late twentieth century. SAMW (defined by the low potential vorticity layer) and AAIW (defined by the salinity minimum layer) warm and freshen as they shift to lighter density classes. Heat and freshwater fluxes at the ocean surface dominate the projected buoyancy gain at outcrop regions of SAMW and AAIW, whereas the net increase in the Ekman flux of heat and freshwater contributes to a lesser extent. This buoyancy gain, combined with shoaling of the winter mixed layer, reduces the volume of SAMW subducted into the ocean interior by a mean of 8 Sv (12%), and the subduction of AAIW decreases by...

Controls on circulation, cross‐shelf exchange, and dense water formation in an Antarctic polynya

Circulation on the Antarctic continental shelf influences cross-shelf exchange, Antarctic Bottom Water formation, and ocean heat flux to floating ice shelves. The physical processes driving the shelf circulation and its seasonal and interannual variability remain poorly understood. We use a unique time series of repeat hydrographic observations from the Adelie Land continental shelf and a box inverse model to explore the relationship between surface forcing, shelf circulation, cross-shelf exchange, and dense water formation. A wind-driven northwestward coastal current, set up by onshore Ekman transport, dominates the summer circulation. During winter, strong buoyancy loss creates dense shelf water. This dense water flows off the shelf, with a compensating on-shelf flow that is an order of magnitude larger in winter than in summer. The results demonstrate the importance of winter buoyancy loss in driving the shelf circulation and cross-shelf exchange, as well as dense water mass formation.

Role of the Seasonal Cycle in the Subduction Rates of Upper-Southern Ocean Waters

AbstractA kinematic approach is used to diagnose the subduction rates of upper–Southern Ocean waters across seasonally migrating density outcrops at the base of the mixed layer. From an Eulerian viewpoint, the term representing the temporal change in the mixed layer depth (which is labeled as the temporal induction in this study; i.e., Stemp = ∂h/∂t where h is the mixed layer thickness, and t is time) vanishes over several annual cycles. Following seasonally migrating density outcrops, however, the temporal induction is attributed partly to the temporal change in the mixed layer thickness averaged over a density outcrop following its seasonally varying position and partly to the lateral movement of the outcrop position intersecting the sloping mixed layer base. Neither the temporal induction following an outcrop nor its integral over the outcrop area vanishes over several annual cycles. Instead, the seasonal eddy subduction, which arises primarily because of the subannual correlations between the seasonal...

The Energetics of Southern Ocean Upwelling

AbstractThe ocean’s meridional overturning circulation is closed by the upwelling of dense, carbon-rich waters to the surface of the Southern Ocean. It has been proposed that upwelling in this region is driven by strong westerly winds, implying that the intensification of Southern Ocean winds in recent decades may have enhanced the rate of upwelling, potentially affecting the global overturning circulation. However, there is no consensus on the sensitivity of upwelling to winds or on the nature of the connection between Southern Ocean processes and the global overturning circulation. In this study, the sensitivity of the overturning circulation to changes in Southern Ocean westerly wind stress is investigated using an eddy-permitting ocean–sea ice model. In addition to a suite of standard circulation metrics, an energy analysis is used to aid dynamical interpretation of the model response. Increased Southern Ocean wind stress enhances the upper cell of the overturning circulation through creation of avail...

Sensitivity of Antarctic Bottom Water to Changes in Surface Buoyancy Fluxes

AbstractThe 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 the abyssal ocea...