Projected Changes to the Southern Hemisphere Ocean and Sea Ice in the IPCC AR4 Climate Models

Abstract

Fidelity and projected changes in the climate models, used for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4), are assessed with regard to the Southern Hemisphere extratropical ocean and sea ice systems. While individual models span different physical parameterizations and resolutions, a major component of intermodel variability results from surface wind differences. Projected changes to the surface wind field are also central in modifying future extratropical circulation and internal properties. A robust southward shift of the circumpolar current and subtropical gyres is projected, with a strong spinup of the Atlantic gyre. An associated increase in the core strength of the circumpolar circulation is evident; however, this does not translate into robust increases in Drake Passage transport. While an overarching oceanic warming is projected, the circulation-driven poleward shift of the temperature field explains much of the midlatitude warming pattern. The effect of this shift is less clear for salinity, where, instead, surface freshwater forcing dominates. Surface warming and high-latitude freshwater increases drive intensified stratification, and a shoaling and southward shift of the deep mixed layers. Despite large intermodel differences, there is also a robust weakening in bottom water formation and its northward outflow. At the same time the wind intensification invigorates the upwelling of deep water, transporting warm, salty water southward and upward, with major implications for sequestration and outgassing of CO 2 . A robust decrease is projected for both the sea ice concentration and the seasonal cycling of ice volume, potentially altering the salt and heat budget at high latitudes.

Ocean circulation response to anthropogenic aerosol and greenhouse gas forcing in the CSIRO-Mk3.6 climate model

“…This is a plausible explanation for the opposing responses of the DPT in the GHG-and AAinduced cases, because AAs tend to mitigate the enhanced tropical tropospheric warming caused by increasing GHGs . Note that Sen Gupta et al (2009) found that, even though the 21st century projections show an trends in zonal wind stress, averaged across the equatorial Pacific, resemble a weakening of the Walker circulation; this is contrary to the hypothesis from Ming and Ramaswamy (2011), who argued that AAs should strengthen the Pacific Walker circulation, by offsetting the expected GHG-induced weakening. However, the prevailing view that the Walker circulation weakens in response to increasing GHGs has recently been challenged by Meng et al (2012); these authors argue that increasing GHGs tend to drive an enhanced equatorial SST gradient between the Maritime Continent and the eastern Pacific, and this in turn strengthens the Walker circulation.…”

“…Further, DPT trends in GHG do not account for the effects of drift. The change in the DPT is much stronger in the 21st century; however, as noted above, this is model-dependent (Sen Gupta et al 2009). …”

Description and basic evaluation of Beijing Normal University Earth System Model (BNU-ESM) version 1

“…5b). Gupta et al (2009) noted that relatively small errors in the position of the ACC lead to more obvious biases in the SST. Over continents, the temperature biases are likely consistent with cloud fraction and TOA shortwave cloud forcing (SWCF) biases (Figs.…”

Advection in polar and sub-polar environments: Impacts on high latitude marine ecosystems

“…In the Southern Ocean, there will also be increased warming of surface waters under climate change, with the greatest warming predicted to occur between 40°and 60°S, owing mostly to a southward translation of warmer subtropical waters (Sen Gupta et al, 2009). Recent CMIP5 projections (Meijers et al, 2012) show no consistent trend in the Antarctic Circumpolar Current among models.…”

“…Eventually, the advection of heat from lower latitudes (Sen Gupta et al, 2009) will result in thinning of sea ice and earlier melt back, although such exchanges are at present limited. Atmospheric warming will result in ice melt, and thus an increase stratification, as well as provide increased irradiance to the water column.…”