92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Sunday, 22 January 2012
Increasing Dimethyl Sulfide Flux Over the Southern Ocean and off of the Antarctic Coast in Response to the Shifting and Intensifying Westerly Winds: A Possible Positive Feedback
Hall E (New Orleans Convention Center )
Stephanie N. Nance, New Mexico Tech, Socorro, NM; and O. W. Wingenter, O. Oluwasseun, S. M. Elliott, and N. J. Blake

The westerly winds in the Southern Hemisphere (SH) are an important factor determining the state of the climate. Several factors, some not identified previously, may be contributing to the increasing temperature difference between the southern mid- and high-latitudes. Global warming and loss of Antarctic ozone have been previously identified as causing strengthening of the westerly wind speeds and also shifting them 34 degrees southward. The alignment of the westerlies with the Antarctic Circumpolar Current is critical to the amount of ocean meridional overturning circulation and in determining the Earth's climate mode, i.e. glacial versus interglacial. Satellite measurements show an increase in reflectivity, most of which is likely as a result of increasing cloud albedo, around Antarctica which may be intensifying the westerlies.

The changes in the westerlies may be resulting in an increase in sea-to-air flux of dimethyl sulfide (DMS) and particle formation, which could lead to brighter clouds, increased cloud lifetime and coverage resulting in a positive feedback on the westerlies. We compare the differences in DMS flux between the decades 2000-2009, 1980-1989 and 1950-1959 over the Southern Ocean and globally based on scalar wind speeds and sea surface temperatures (SST) derived from NCEP/NOAA reanalysis. The DMS sea water concentrations employed for all three decades were obtained from a recently published database, as no comprehensive time series is available. Thus, our calculations assume no change in DMS concentrations over the decades examined.

Flux estimates between 2000-2009 versus 1980-1989 show minimum between 40-45 degrees south latitude with the latest decade having about 3-4 percent less DMS flux. This is a result of the westerlies having shifted southward over this time period. Further southward the increase in DMS flux increases sharply with a broad maxima between 55-60 degrees S of approximately 25 percent coinciding primarily with increased scalar wind speeds for the month of January. At 65 degrees S another minima occurs because of minima in DMS concentrations and wind speeds. However, the estimated flux is still 10 percent higher compared to the 1980s and begins to rise to about 30 percent greater flux approaching the Antarctic coast because of the increasing DMS concentration gradient and intensifying wind speeds. From 50-60 degrees S the absolute average increase in DMS flux was about 3 micromolar per meter squared. The contribution of the SST was assessed by comparing flux calculations for the 2000s and 1980s using their respective SSTs and by comparing them using just the SSTs from the 1980s. The difference in the contribution of flux in the SH was less than 1 percent. Therefore, changes in decadal DMS flux are driven primarily by changes in wind speed.

DMS flux estimates comparing 2000-2009 vs 1950-1959 show even more pronounced changes in DMS flux. We will discuss the monthly results and present yearly and global estimates of the impact of changing winds and SSTs on DMS flux.

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