Comiso
Josefino C.
Comiso
Josefino C.
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ArticleStorm-driven mixing and potential impact on the Arctic Ocean(American Geophysical Union, 2004-04-09) Yang, Jiayan ; Comiso, Josefino C. ; Walsh, David ; Krishfield, Richard A. ; Honjo, SusumuObservations of the ocean, atmosphere, and ice made by Ice-Ocean Environmental Buoys indicate that mixing events reaching the depth of the halocline have occurred in various regions in the Arctic Ocean. Our analysis suggests that these mixing events were mechanically forced by intense storms moving across the buoy sites. In this study, we analyzed these mixing events in the context of storm developments that occurred in the Beaufort Sea and in the general area just north of Fram Strait, two areas with quite different hydrographic structures. The Beaufort Sea is strongly influenced by inflow of Pacific water through Bering Strait, while the area north of Fram Strait is directly affected by the inflow of warm and salty North Atlantic water. Our analyses of the basin-wide evolution of the surface pressure and geostrophic wind fields indicate that the characteristics of the storms could be very different. The buoy-observed mixing occurred only in the spring and winter seasons when the stratification was relatively weak. This indicates the importance of stratification, although the mixing itself was mechanically driven. We also analyze the distribution of storms, both the long-term climatology and the patterns for each year in the past 2 decades. The frequency of storms is also shown to be correlated (but not strongly) to Arctic Oscillation indices. This study indicates that the formation of new ice that leads to brine rejection is unlikely the mechanism that results in the type of mixing that could overturn the halocline. On the other hand, synoptic-scale storms can force mixing deep enough to the halocline and thermocline layer. Despite a very stable stratification associated with the Arctic halocline, the warm subsurface thermocline water is not always insulated from the mixed layer.
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ArticleDetection of change in the Arctic using satellite and in situ data(American Geophysical Union, 2003-12-24) Comiso, Josefino C. ; Yang, Jiayan ; Honjo, Susumu ; Krishfield, Richard A.The decade of the 1990s was the warmest decade of the last century, while the year 1998 was the warmest year ever observed by modern techniques, with 9 out of 12 months of the year being the warmest months. Satellite ice cover and surface temperature data, European Centre for Medium-Range Weather Forecasts (wind), and ocean hydrographic data are examined to gain insights into this warming phenomenon. Areas of ice-free water in both western and eastern regions of the Arctic are found to have followed a cyclical pattern with approximately decadal period but with a lag of about 3 years between the eastern and western regions. The pattern was interrupted by unusually large anomalies in 1993 and 1998 in the western region and in 1995 in the eastern region. The area of open water in 1998 was the largest ever observed in the western region and occurred concurrently with large surface temperature anomalies in the area and adjacent regions. This also occurred at a time when the atmospheric circulation changed from predominantly cyclonic in 1996 to anticyclonic in 1997 and 1998. Detailed hydrographic measurements over the same general area in April 1996 and April 1997 indicate a warming and significant freshening in the top layer of the ocean, suggesting increases in ice melt and/or river runoff. Continuous ocean temperature and salinity data from ocean buoys at depths of 8, 45, and 75 m confirm these results and show large interannual changes during the 1996–1998 period. Surface temperature data show a general warming in the region that is highly correlated with observed decline in summer sea ice, while hydrographic data suggest that in 1997 and 1998, the upper part of the ocean was unusually fresh and warm compared to available data between 1956 and 1996.
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ArticleAn unexpected seasonal variability of salinity in the Beaufort Sea upper layer in 1996–1998(American Geophysical Union, 2007-05-17) Yang, Jiayan ; Comiso, Josefino C.The salinity in the upper Beaufort Sea from the mixed layer to the thermocline layer was observed by drifting buoys from 1996 to 1998. The salinity in this depth range was lower in winter and higher in the summer, the exact opposite from what one would expect from the seasonal cycle of the freshwater flux associated with the ice melting/freezing and river runoff. In this study, we calculated the daily Ekman transport and upwelling velocity in the Beaufort Sea, using both satellite and buoy data. In fall and winter months, the offshore transport of low-salinity water from the coastal area toward the interior where the buoys were located was observed to be strong. This horizontal Ekman transport led to the freshening of the surface Ekman layer in the buoy location. The convergence of the Ekman transport resulted in a strong downwelling in the offshore regions, and so the halocline and thermocline were pushed downward. The downwelling then results in the freshening of the subsurface salinity as observed by buoys. Other processes, such as lateral advection, may have also played a role in the subsurface freshening. The lack of in situ observations needed to estimate the salinity gradient makes it difficult to assess more accurately the contribution from lateral advection. A scaling analysis using the salinity climatology suggests that the lateral salinity advection, though considerably smaller than the vertical one, may not be negligible.