Lessened response of boreal winter stratospheric polar vortex to El Nio in recent decades

The decadal change of El Nio teleconnection and the corresponding response of the boreal winter stratospheric polar vortex are investigated through the composite analysis of El Nio events during the two periods 1958-78 and 1979-2015. It is found that, during the period 1958-78, El Nio generates an anomalous Aleutian low in the mid troposphere extending from northeastern Eurasia to the northeastern Pacific with the most significant center in the northwestern Pacific. The anomalous Aleutian low results in a marked increase in planetary wavenumber 1 but a weak decrease in wavenumber 2 from the upper troposphere to lower stratosphere. The Eliassen-Palm (EP) flux of planetary waves converges at the high latitudes in the stratosphere and brings about a significantly weakened polar vortex. In contrast, during the period 1979-2015, the wintertime El Nio-related Aleutian low shifts eastward into the northeastern Pacific. This variation in tropospheric El Nio teleconnection leads to a dramatic decrease in planetary wavenumber 2 but a relatively weak increase in wavenumber 1. Furthermore, the magnitude of the decrease of wavenumber-2 EP flux is comparable to the increase of wavenumber-1 EP flux in the stratosphere. Consequently, the stratospheric response lessens dramatically, showing a less disturbed and slightly enhanced polar vortex. The lessened stratospheric response is quite obvious in the stratosphere below 10 hPa regardless of the long-term trend being removed or not, indicating a dominant role of El Nio in the wintertime variability of lower polar stratosphere.