Rossby Wave Breaking and Nonlinear Reflection


 



Planetary wave breaking (PWB), or Rossby wave breaking, occurs when large amplitude quasi-stationary waves impinge upon regions of weak zonal flow (usually in the subtropics).  In doing so potential vorticity contours as irreversibly deformed about the subtropical tropopause permitting for the large scale mixing of air between the subtropics and extratropics. Occurrences of PWB as they impinge upon the weak winds of the subtropics is akin to ocean waves breaking as they impinge upon the shallow waters near the shoreline.  I recently found observational evidence for PWB to produce nonlinear reflection polewards.  In other words as a large amplitude equatorward propagating wave breaks in the subtropics it reflects some of the incipient wave energy back into the midlatitudes downstream of the break. 




I.  Dr. Gudrun Magnusdottir, and I have found evidence for nonlinear reflection following PWB.  This entails that wave activity is reversed and a reflected wave train emerges poleward and downstream of the breaking region in the days following the break. 

Abatzoglou, J.T., Magnusdottir, G. 2004. Nonlinear planetary wave reflection in the troposphere. GRL, Vol. 31, No. 9., doi:10.1029/2004GL019495




II.  A full 46 year climatology of PWB (MATLAB binary w/dates, location of breaking events) was performed by looking for breaking on multiple isentropic surfaces which intersect the subtropical tropopause.  The seasonal cycle of the "surf zone" is intricately linked to the subtropical jet.  Connections are also found to the monsoonal circulation over Asia.  We report on links to several climate modes (East Asian Summer Monsoon, ENSO, NAO).  Finally we discuss in more detail nonlinear reflection and the impact that the background flow has in permitting for reflection to occur. 


Abatzoglou, J.T.; Magnusdottir, G. 2006, Planetary Wave Breaking and Nonlinear Reflection: Seasonal Cycle and Interannual Variability, Journal of Climate, Vol. 19, No. 23, pp 6139-6152





III.  A very interesting connection between reflective and non-reflective events and the intraseasonal NAO was found by analyzing breaking events during DJFM over the Atlantic sector.  It turns out that non-reflective events lead to a profound increase in the intraseasonal NAO, while reflective events lead to its demise and reversal.  Eddy-momentum flux arguments are key in fostering this relationship. 

 

Abatzoglou, J.T.; Magnusdottir, G. 2006, Opposing Effects of Reflective and Non-reflective Planetary Wave Breaking on the NAO, Journal of Atmospheric Sciences, Vol. 63, No. 12, pp. 3448-3457




IV.  Going back to the original observations of planetary wave breaking by McIntyre and Palmer (1983) we implement our PWB diagnostic techniques to the three-dimensional stratospheric polar vortex.  Wave breaking in the stratosphere is observed to occur either in the upper-stratosphere (above 5hPa) or in the middle stratosphere (near 30hPa).  The impacts of PWB on the mean flow can be rather impressive.  For reflective events, wave activity is focused poleward leading to a wave driving at high latitudes that propagates downward into the upper-troposphere over the period of a few weeks - potentially impacting the NAM/AO.

 

Abatzoglou, J.T.; Magnusdottir, G. 2007, Wave breaking along the stratospheric polar vortex as seen in ERA-40 data, Geophysical Research Letters, Vol. 34, No. 8, L08812, doi:10.1029/2007FL029509


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