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  • Author or Editor: Annie P. S. Wong x
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Annie P. S. Wong
and
Gregory C. Johnson

Abstract

The structure, formation, and destruction of South Pacific Eastern Subtropical Mode Water (SPESTMW) are analyzed. Geographic extent and water properties are discussed by using high-quality CTD sections collected between 1991 and 1996. Defined as having a planetary potential vorticity magnitude of less than 3 × 10−10 m−1 s−1, SPESMTW has a volume of about 1.1 × 1015 m3, estimated from CTD data. The ventilation of this mode water is described by using data from a high-resolution XBT section in concert with 30-month time series from profiling CTD floats, some of the first Argo deployments. Published subduction rates allow a mode-water formation rate estimate of 8.7 × 106 m3 s−1. Combining this estimate with the volume yields a residence time of about 4 years. The density-compensating covarying patterns of late winter surface temperature and salinity in the ventilation region of SPESTMW are shown to contribute to the strength of the mode water. However, while the destabilizing salinity gradient in SPESTMW contributes to its formation, it may also hasten its destruction by leaving it susceptible to double-diffusive convective mixing. SPESTMW spreads northwestward from its ventilation region within the subtropical gyre, eventually joining the South Equatorial Current. It is speculated that the proximity of the SPESTMW ventilation region to the Tropics, where winds and sea surface temperatures vary significantly, coupled with a direct interior circulation pathway to the equator, may allow SPESTMW to effect modulation of ENSO dynamics.

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Annie P. S. Wong
and
Stephen C. Riser

Abstract

Multiyear under-ice temperature and salinity data collected by profiling floats are used to study the upper ocean near the Wilkes Land coast of Antarctica. The study region is in the seasonal sea ice zone near the southern terminus of the Antarctic Circumpolar Current. The profiling floats were equipped with an ice-avoidance algorithm and had a survival rate of 74% after 2.5 yr in the ocean. The data show that, in this part of Antarctica, the rate of sea ice decay exceeds the rate of sea ice growth. During the sea ice growth period, the water column is weakly stratified because of brine rejection and is only marginally stable. The average winter mixed layer temperature is about 0.12°C above the surface freezing point, providing evidence of entrainment of warmer water from the permanent pycnocline. The average mixed layer salinity increases by 0.127 from June to October. A one-dimensional model is used to quantify evolution of the winter mixed layer under a sea ice cover. The local winter entrainment rate is estimated to be 49 ± 11 m over 5 months, supplying a heat flux of 34 ± 8 W m−2 to the base of the mixed layer in winter. Model output gives a thermodynamic sea ice growth of 28 ± 15 cm over the same period. The winter ocean–atmosphere heat loss through leads and sea ice is estimated to be 14–25 W m−2 in this area, which is broadly in line with other winter observations from the East Antarctic region.

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Earle A. Wilson
,
Stephen C. Riser
,
Ethan C. Campbell
, and
Annie P. S. Wong

Abstract

In this study, under-ice ocean data from profiling floats, instrumented seals, and shipboard casts are used to assess wintertime upper-ocean stability and heat availability in the sea ice–covered Southern Ocean. This analysis reveals that the southern Weddell Sea, which features a weak upper-ocean stratification and relatively strong thermocline, is preconditioned for exceptionally high rates of winter ventilation. This preconditioning also facilitates a strong negative feedback to winter ice growth. Idealized experiments with a 1D ice–ocean model show that the entrainment of heat into the mixed layer of this region can maintain a near-constant ice thickness over much of winter. However, this quasi-equilibrium is attained when the pycnocline is thin and supports a large temperature gradient. We find that the surface stress imparted by a powerful storm may upset this balance and lead to substantial ice melt. This response can be greatly amplified when coincident with anomalous thermocline shoaling. In more strongly stratified regions, such as near the sea ice edge of the major gyres, winter ice growth is weakly limited by the entrainment of heat into the mixed layer. Thus, the thermodynamic coupling between winter sea ice growth and ocean ventilation has significant regional variability. This regionality will influence the response of the Southern Ocean ice–ocean system to future changes in ocean stratification and surface forcing.

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