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Isaac Held's Blog

15. Fluctuations and responses

From Son et al 2010, based on the CCMVal ensemble of models, the decorrelation time of the Southern annular mode (SAM)  plotted against the simulated latitude of the surface westerlies.  Also included is an estimate from NCEP-NCAR reanalysis.

A series of studies over the past decade, starting with Thompson and Solomon 2002, have built a very strong case that the ozone hole in the Southern Hemisphere (SH) stratosphere has caused a poleward shift in the SH surface westerlies and associated eddy fields, especially during the southern summer. The poleward shift is often described as a trend towards a more positive phase of the Southern Annular Mode (SAM). The SAM is a mode of atmospheric internal variability characterized by north-south shifts in the surface westerlies.

The mechanism by which the ozone hole causes this poleward shift is a hot topic in dynamical meteorology.  Not only is this response to the ozone hole important in itself,  but related mechanisms likely govern the effects on the troposphere of stratospheric perturbations due to volcanic eruptions, the solar cycle, and internal variability.   The starting point is the cooling of the lower stratosphere in the vicinity of the ozone hole, due to loss of UV absorption, thereby changing the north-south temperature gradient and associated wind fields in the lower stratosphere.  But there are a lot of competing ideas about how altered lower stratospheric winds and temperatures in turn affect the fluxes of angular momentum that maintain the surface westerlies.  (Some of my own lectures on the basic dynamics controlling the surface westerlies, including the key role of transport of angular momentum associated with the midlatitude storm tracks, can be found here.) GCMs consistently simulate a poleward shift in response to the ozone hole but of varying magnitude. They also consistently simulate a poleward shift due to increasing CO2.

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14. Surface salinity trends

From Durack and Wijffels, 2010:  A) Climatological surface salinity (0.5 pss contour), averaged over 1950-2000; B) the linear trend over these 50 years (pss/50 years) ; and C) the NOCS Southampton estimate of net climatological freshwater flux from ocean to atmosphere (m/yr).

In post #13, I discussed the argument that warmer temperatures => more water vapor in the atmosphere => more transport of water away from regions from which the atmosphere habitually extracts water, and  more transport to regions into which the atmosphere habitually adds water. The consequence is the expectation that “the wet get wetter and the dry get drier” if by wet/dry we mean regions with precipitation (P) greater than/less than evaporation (E).  In that post, I effectively ignored the presence of land.  Land introduces a variety of complications that make this kind of argument more difficult, most obviously because of the constraint that P must be greater than E on the time scales of interest  (ie. changes in water storage on land can be ignored and runoff must be positive).    I am going to continue to ignore the existence of land (and glaciers) in this post!

What is the evidence for trends in P or E or P-E  over the oceans?  Trends in the ocean salinity field promise to provide a test of our understanding — it is also helpful that the oceans provide a low-pass filter to noisy precipitation signals.

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