Browsing by Subject "eddy"
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Item A Climatology of the Arctic on Mid-Tropospheric Temperature Regulation(2014-06-24) Anthony, Jeremy PatrickThe Arctic is a unique and complex environment. Many factors play a role in determining the long-term climate of the Arctic, including mesoscale weather systems and many complex ice-albedo feedback mechanisms. Previous studies determined using real observations that although 500 hPa temperatures reach -45? by mid-November, temperatures very rarely drop below, despite a total loss of incoming solar radiation. This temperature value at 500 hPa follows moist-adiabatically to the surface value of approximately -2?, which is the freezing point of high-latitude sea water. Sea ice data was examined using satellite and model data to paint a picture of the environment during three distinct periods in history: the last glacial maximum (twenty-one thousand years ago), the mid-Holocene (six-thousand years ago), and the 20th century. Areal September minimum sea ice extent has reached record lows annually since 2007. Vertical temperature profiles created with CCSM4 model data during these three eras show the atmosphere becoming more moist-adiabatic at high latitudes over time. Saturation potential vorticity allows us to assess the convective environment, and it shows that the Arctic is becoming more tropical over time. Analysis of areal extent where temperatures fall below -45? at 500 hPa shows this to be an extremely rare occurrence, and temperatures never fall below -47.5?, except for very rare occurrences during the last glacial maximum. Transient eddy heat flux is increasing at higher latitudes, and the warm half of a cyclone contains convection (as seen in saturation potential vorticity). In this paper, we present several lines of evidence supporting a role for intermittent convection in maintaining mid-tropospheric temperatures across climate states of the past twenty-one thousand years.Item Antarctic Circumpolar Current System and its Response to Atmospheric Variability(2012-08-16) Kim, Yong Sun 1976-The Antarctic Circumpolar Current (ACC) is well known for its multiple bands with large meridional property gradients in the upper waters, each associated with a deep-reaching current core. A revised nineteen-year time series (1992?2011) of altimeter data from the CNES/CLS AVISO is analyzed to identify and trace the spatial distribution of ACC fronts. Specific contours of sea surface height (SSH) are selected within narrow continuous bands of relative maxima SSH slope in the Southwest Atlantic Ocean sector, where they closely follow the distribution of ACC fronts derived from inspection of concurrent high-resolution profile data at hydrographic stations. When applied to the full circumpolar belt, the frontal distribution derived from these new altimeter-based indicators also agrees well with the traces of current jets and in-situ dynamic height fields calculated from concurrent Argo profile data. The temporal variability of ACC fronts is analyzed in relation to dominant modes of atmospheric forcing variability in the Southern Ocean. All three ACC fronts have experienced large seasonal to decadal variability throughout the satellite altimetry era. The general seasonal tendency for each of these jets, with respect to long-term mean positions, is to be located farther to the south during the austral summer and to north in the winter. Circumpolar-mean annual frontal locations show a consistent linear trend of southward migration. However, the estimated decadal variability of the frontal distributions is highly localized, and due to selective response mechanisms to atmospheric variability. A persistent poleward drift of ACC fronts is observed in the Indian sector consistent with increasing sea surface temperature trends. In contrast, a vacillation in the meridional location of ACC fronts is observed in the Pacific sector in association to minor sea surface cooling trends. Therefore, unlike in the Indian sector, the regional Pacific Ocean response is significantly sensitive to dominant atmospheric forcing indices. Mesoscale eddies derived from instabilities at strong current cores are successfully identified with specific SSH gradient criteria. The new estimates of rings population in the Southern Ocean are tightly linked to interannual to decadal atmospheric variability. Increased number of mesoscale eddies correlate with positive SAM forcing about two years earlier, or negative ENSO forcing two to three months earlier. These cross-correlations might explain a prominent peak in rings abundance estimated during 2000 and 2001, and the short-lived maximum that appeared in 2010. There are no persistent trends in the estimated sea surface slope across Drake Passage, and therefore neither in the transport of the ACC. High cross-correlation between the abundance of mesoscale eddies and atmospheric forcing suggests that the overall ACC system is in an eddy-saturated state. However, Drake Passage positive sea level slope anomalies were two-year lagged with negative SAM forcing and with positive ENSO events. These regional responses are characteristic of eastward-propagating signals from a buoyancy-dominated Pacific sector of the Southern Ocean.