Polar-Tropical Interactions

Footprints of the Tropical Pacific Variability in the Decadal-Scale Variations of the Arctic Water Cycle

This NASA-funded project studies a "storm track" bridge that connects sea surface temperature (SST) variability in the tropical Pacific and decadal-scale variations in the Arctic energy and water cycle. We have shown that the two types of El Nino (i.e., central Pacific warming (CPW) versus eastern Pacific warming (EPW)) affects extratropical planetary wave and the strength of the polar stratospheric vortex in opposite directions (Hegyi and Deng 2011). This difference, plus the decadal-scale shift in preferred El Nino mode, translates into decadal-scale fluctuations in the Arctic Oscillation (AO), location of the North Atlantic storm track, and the associated poleward moisture transport by synoptic-scale disturbances, ultimately contributing to the long-term changes in the cool-season Arctic precipitation. The transient response of polar stratospheric vortex to tropical SST forcing shows great sensitivity to the initial vortex strength as demonstrated by a series of idealized-forcing experiments conducted with the NCAR WACCM model (Hegyi et al. 2012).                                      


                         Tropical Pacific sea surface temperature (SST, in K) regressed onto the total Arctic precipitation in boreal cool-season (October-March). (Hegyi and Deng 2011)



(a) Difference of the composite EP flux vectors between a 5-year period (1990-1994, CPW-Pacific) of above-normal Arctic precipitation and a 5-year period (1997-2001, EPW-Pacific) of below-normal Arctic precipitation.  (b) The composite difference of the power spectrum of the cool-season 500-mb geopotential height (in m, averaged over 45°N-90°N) between the two 5-year periods defined in (a), shown as a function of zonal wavenumber. (Hegyi and Deng 2011)



                                      Transient response of the polar stratospheric vortex to CPW and EPW SST forcing with different initial vortex strength. (Hegyi et al. 2012)