Longer duration of the weak stratospheric vortex during extreme El Niño events linked to spring Eurasian coldness

Abstract

The stratosphere is a key link between El Niño and Eurasian surface climate in winter. Instead of the amplitude of the stratospheric response discussed by many previous studies, we focus on the persistence of wintertime extreme El Niño impacts through the stratospheric pathway to midlatitude Eurasia in spring. A novel approach is used by running WACCM4 with El Niño forcing imposed only during winter to isolate the stratospheric role in the following spring. We show that a descending signal with a strong deceleration of zonal wind throughout the atmosphere could reach the surface and persist in spring during extreme El Niño events, whereas during moderate El Niño events the signal is generally confined to the stratosphere. Two different mechanisms are involved in maintaining the tropospheric signal of extreme El Niño. Under the westerly regime, the planetary Rossby waves from the wave centers over East Asia and the North Pacific transmit the signals from the stratosphere to the troposphere; and under the easterly regime, strong tropospheric eddy feedback over the midlatitude Atlantic favors the persistence of tropospheric responses during extreme El Niño events. The prolonged stratospheric pathway of extreme El Niño events amplifies and extends the equatorward shift of the midlatitude jet, allowing southward intrusions of cold polar air into Eurasia. Such circulation changes induced by a prolonged stratospheric pathway can cause springtime coldness and daily cold extremes in midlatitude Eurasia.

Publication
Journal of Geophysical Research: Atmospheres, 125(16):e2019JD032331

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