Annular and Dipole Modes

 

The primary modes of climate variability in extratropical regions are the North Atlantic Oscillation (NAO), the North Pacific Oscillation (NPO), and the Northern and Southern Annular Modes (NAM and SAM).  These modes are of considerable research interest, as they provide useful aggregate information on past trends in regional climate.   

 

Northern Modes

 

The NAO is an important mode of climate variability for the Northern Hemisphere, specifically for weather and climate in North America and Europe.  The NPO has a similarly important role in the North Pacific and surrounding continents.  Both of these modes are regional expressions of the NAM, which is an annular variation of sea level pressure in the Arctic.  Climate models simulate the general characteristics of the NAO and NPO, but the extent to which these modes respond to a warming climate is debated.  Long-term variability in the NAO has been underestimated by climate models, possibly due to missing or poorly understood process representation in the models.  It appears that future projections of NAO variability are sensitive to how well the climate models resolve processes in the stratosphere, including stratosphere-troposphere interactions.  Figure 14.16 summarizes multi-model simulations of the NAO and NAM indices for wintertime through 2100.  Both indices show small positive linear trends.     

 

 

 

 

 

 

 

 

 

 

 

 

    

 

 

 

 

 

 

 

 

 

 

 

 

Southern Modes

 

The Southern Annular Mode (SAM) is the dominant mode of climate variability for extratropical regions in the Southern Hemisphere, particularly for Antarctica, Australia, South Africa, and southern South America.  The SAM is well simulated by climate models.  Changes in the SAM index are generally attributed to ozone depletion (or recovery) and increases in greenhouse gas concentrations.  CMIP5 model projections indicate that the observed positive trend in the SAM index over the past several decades will weaken as the stratospheric ozone recovers in the high latitudes of the Southern Hemisphere (Fig. 14.16c). 

 

Summary

 

From the IPCC report: “Future boreal wintertime NAO is very likely to exhibit large natural variations and trend of similar magnitude to that observed in the past; is very likely to be differ quantitatively from individual climate model projections; is likely to become slightly more positive (on average) due to increases in GHGs. The austral summer/autumn positive trend in SAM is likely to weaken considerably as ozone depletion recovers through to the mid-21st century. There is medium confidence from recent studies that projected changes in NAO and SAM are sensitive to boundary processes, which are not yet well represented in many climate models currently used for projections, for example, stratosphere-troposphere interaction, ozone chemistry, solar forcing and atmospheric response to Arctic sea ice loss. There is low confidence in projections of other modes such as the NPO due to the small number of modelling studies.”