Southern high-latitude ocean climate drift in a coupled model

Climate drift in coupled models affects the response of the coupled system to an external forcing. In most existing coupled models that employ Flux ad justments, the southern high latitudes, in particular, are still affected b y some climate drift. In the CSIRO coupled model, within 100 years followin g coupling, the Antarctic Circumpolar Current (ACC) intensifies by about 30 Sv (Sv equivalent to 10(6) m(3) s(-1)). This happens despite the use of nu x adjustments. Many other model fields such as sea ice, surface albedo, and heat fluxes of the coupled system also experience drift from the precouple d spinup states. It is therefore important to study the processes that give rise to these drifts. The primary cause of drift in the CSIRO model is due to changes in the patt ern of convection in the Southern Ocean relative to the spinup steady state . Upon coupling, the pattern of convection alters systematically regardless of surface boundary conditions. Consequently, overturning at shallow to in termediate depths (from the surface to about 2000 m) weakens, while that be low these depths intensifies. The decline of overturning at shallow to inte rmediate depths leads to reduced surface temperatures because a lesser amou nt of warm subsurface water is mixed up into the colder surface mixed layer . The cooler surface temperature leads to an initial increase in sea ice, w hich is exacerbated by a significant albedo-temperature-sea ice feedback. T he resulting increase in sea ice formation at the higher southern latitudes leads to increased brine rejection and a general increase in salinity thro ughout much of the high-latitude wafer column. This increase in salinity in tensifies deep convection and bottom water formation, driving a stronger AC C. Several additional experiments are performed to trace various oceanic and o cean-atmosphere feedbacks that give the drift its character. It is demonstr ated that the feedbacks significant to the drift in the present model are t he positive albedo-temperature-sea ice feedback and a negative feedback bet ween sea ice and overturning. The role of these two feedbacks in the interc onnection between the drifts in various model fields is discussed.