What induced the trend shift of mixed-layer depths in the Antarctic Circumpolar Current region in the mid-1980s?

An obvious trend shift in the annual mean and winter mixed layer depth(MLD)in the Antarctic Circumpolar Current(ACC)region was detected during the 1960–2021 period.Shallowing trends stopped in mid-1980s,followed by a period of weak trends.The MLD deepening trend difference between the two periods were mainly distributed in the western areas in the Drake Passage,the areas north to Victoria Land and Wilkes Land,and the central parts of the South Indian sector.The newly formed ocean current shear due to the meridional shift of the ACC flow axis between the two periods is the dominant driver for the MLD trends shift distributed in the western areas in the Drake Passage and the central parts of the South Indian sector.The saltier trends in the regions north to Victoria Land and Wilkes Land could be responsible for the strengthening mixing processes in this region.

Improving the estimate of wind energy input into the Ekman layer within the Antarctic Circumpolar Current

Based on the data and method offered by Liu et al. （2009）, the direct wind and Stokes drift-induced energy inputs into the Ekman layer within the Antarctic Circumpolar Current （ACC） area are reestimated since the results of the former have been proved to be underestimated. And the result shows that the total rate of energy input into the Ekman-Stokes layer within the ACC area is 852.41 GW, including 649.75 GW of direct wind energy input （76%） and 202.66 GW of Stoke drift-induced energy input （24%）. Total increased energy input, due to wave-induced Coriolis-Stokes forcing added to the classical Ekman model, is 52.05 GW, accounting for 6.5% of the wind energy input into the classical Ekman layer. The long-term variability of direct wind and Stokes drift-induced energy inputs into the Ekman layer within the ACC is also investigated, and the result shows that the Stokes drift hinders the decadal increasing trend of direct wind energy input. Meanwhile, there is a period of 4-5 a in the energy spectrums, as same as the Antarctic circumpolar wave.

Fronts,baroclinic transport,and mesoscale variability of the Antarctic Circumpolar Current in the southeast Indian Ocean

Fronts, baroclinic transport, and mesoscale variability of the Antarctic Circumpolar Current （ACC） along 115°E are examined on the basis of CTD data from two hydrographic cruises occupied in 1995 as a part of the World Ocean Circulation Experiment （WOCE cruise I9S） and in 2004 as a part of CLIVAR/CO2 repeat hydrography program. The integrated baroclinic transport across I9S section is （97.2×106±2.2×106） m3/s relative to the deepest common level （DCL）. The net transport at the north end of I9S, determined by the south Australian circulation system, is about 16.5× 106 m3/s westward. Relying on a consistent set of water mass criteria and transport maxima, the ACC baroclinic transport, （117×106±6.7×106） m3/s to the east, is carried along three fronts： the Subantarctic Front （SAF） at a mean latitude of 44°-49°S carries （50.6×106=t=13.4×106） m3/s; the Polar Front （PF）, with the northern branch （PF-N） at 50.5°S and the southern branch （PF- S） at 58°S, carries （51.3×106±8.7×106） m3/s; finally, the southern ACC front （SACCF） and the southern boundary of the ACC （SB） consist of three cores between 59°S and 65°S that combined carry （15.2× 106±1.8× 106） m3/s. Mesoscale eddy features are identifiable in the CTD sections and tracked in concurrent maps of altimetric sea level anomalies （SLA） between 44°-48°S and 53°-57°S. Because of the remarkable mesoseale eddy features within the SAF observed in both the tracks of the cruises, the eastward transport of the SAF occurs at two latitude bands separating by 1°. Both the CTD and the altimetric data suggest that the mesoscale variability is concentrated around the Antarctic Polar Frontal Zone （APFZ） and causes the ACC fronts to merge, diverge, and to fluctuate in intensity and position along their paths.

An analytical diagnostic model of the Antarctic Circumpolar Current

Based on a linear quasi-geostrophic vorticity equation, we have set up an analytical diagnostic mode for the Antarctic Circumpolar Current. The results show that the balance among wind stress,ocean horizontal viscous stress and the β-effect is the basic balance of the Antarctic Circumpolar Current, and the center velocity of the jet, coming from the Drake Passage,will decrease due to the ocean dynamic adjustment, and the width in the north-south direction will decreasegradually. The jet coming from the Drake Passage moves about 4°northward because of the inducement of winddrivencurrents. The clockwise vorticity appearing in the south-west corner corresponds to the Weddell Gyre.The characters of the Antarctic Circumpolar Current described by the model are congruous with observations basically.

Two equilibria of the Antarctic Circumpolar Current and its associated Meridional Overturning Circulation

The Antarctic Circumpolar Current (ACC) and its associated Meridional Overturning Circulation (MOC) is investigated through a nonlinear inertia theory model, which consists of two layers--an upper Ekman layer driven mainly by sea surface wind stress and a lower thermocline controlled by ideal fluid nonlinear equations which can be solved by identifying the form of the arbitrary functions. The results show that the thermocline has a two-equilibrium solution though given the same Ekman layer condition. Compared to the first equilibrium, the second one has a heavier intensity and deeper circulation, which seems more consistent with the existing data.

Fronts and surface zonal geostrophic current along 115°E in the southern Indian Ocean

Altimeter and in situ data are used to estimate the mean surface zonal geostrophic current in the section along 115°E in the southern Indian Ocean,and the variation of strong currents in relation to the major fronts is studied.The results show that,in average,the flow in the core of Antarctic Circumpolar Current（ACC） along the section is composed of two parts,one corresponds to the jet of Subantarctic Front（SAF） and the other is the flow in the Polar Front Zone（PFZ）,with a westward flow between them.The mean surface zonal geostrophic current corresponding to the SAF is up to 49 cm · s^-1 at 46°S,which is the maximal velocity in the section.The eastward flow in the PFZ has a width of about 4.3 degrees in latitudes.The mean surface zonal geostrophic current corresponding to the Southern Antarctic Circumpolar Current Front（SACCF） is located at 59.7 °S with velocity less than 20 cm · s^-1.The location of zonal geostrophic jet corresponding to the SAF is quite stable during the study period.In contrast,the eastward jets in the PFZ exhibit various patterns,i.e.,the primary Polar Front（PF1） shows its strong meridional shift and the secondary Polar Front（PF2） does not always coincide with jet.The surface zonal geostrophic current corresponding to SAF has the significant periods of annual,semi-annual and four-month.The geostrophic current of the PFZ also shows significant periods of semi-annual and four-month,but is out of phase with the periods of the SAF,which results in no notable semi-annual and fourmonth periods in the surface zonal geostrophic current in the core of the ACC.In terms of annual cycle,the mean surface zonal geostrophic current in the core of the ACC shows its maximal velocity in June.

The Role of Southern High Latitude Wind Stress in Global Climate

In this paper, the role of westerly winds at southern high latitudes in global climate is investigated in a fully coupled ocean-atmosphere general circulation model. In the model, the wind stress south of 40°S is turned off with ocean and atmosphere fully coupled both locally and elsewhere. The coupled model explicitly demonstrates that a shutdown of southern high latitude wind stress induces a general cooling over the Antarctic Circumpolar Current （ACC） region, with surface Ekman flow and vertical mixing playing competitive roles. This cooling leads to an equatorward expansion of sea ice and triggers an equivalent barotropic response in the atmosphere to accelerate westerly anomalies. The shutdown of southern high latitude wind stress also significantly reduces global meridional overturning circulation （MOC）. The Antarctic MOC （AnMOC） nearly disappears while the Atlantic MOC （AMOC） is weakened by 50%, suggesting a strong control of the southern high latitude winds over the thermohaline circulation （THC）. In spite of a substantial weakening of the AMOC, the interhemispheric SST seesaw appears to be not significant due to an equatorward extension of the southern extratropical cooling through coupled wind-evaporation-SST （WES） feedback. In addition, it is found that the weakening of Atlantic MOC by as much as 50% is capable of cooling the time mean subpolar Atlantic temperature by only about 1°C.