Synoptic-Scale Forcing Caused Stratospheric Sudden Warming of 2006
NAVAL RESEARCH LAB WASHINGTON DC E O HULBURT CENTER FOR SPACE RESEARCH
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Major stratospheric sudden warmings SSWs result from large changes in the mid-winter stratospheric circulation in response to upward propagating energy from the troposphere. The relevant tropospheric forcing systems are often large-scale, blocking-ridge, weather patterns. However, our work shows an example in which a smaller scale, non-blocking, upper tropospheric disturbance provided forcing for the major SSW of January 2006. We examined the major SSW of January 2006 using meteorological fields from GEOS-4 Goddard Earth Observing System analyses. Early January 2006 found the stratospheric polar vortex weaker than normal and displaced off the pole because of earlier minor warming events. On 16-17 January, a tropospheric weather system over the North Atlantic amplified at upper levels as it moved under the lower stratospheric vortex. The resulting upper tropospheric ascent produced cold temperatures at approx. 100 hPa as the altitude of the 360 K potential temperature surface rose from typical midlatitude values of 12.25 km to over 14.5 km at this time. As this large amplitude, upper-tropospheric perturbation moved eastward at -12 m S-1, directly disturbing the lower stratospheric flow, it launched large-amplitude Rossby waves into the stratosphere. In the mid-stratosphere -30 km altitude, -10 hPa or -1100 K this upward-propagating wave energy turned equatorward, producing subtropical wave breaking by 18 January. Finally, the advection of tropical midstratospheric air to the pole associated with the wave breaking led to the major SSW on 22 January. Forecast studies using the NOGAPS-ALPHA Navy Operational Global Atmospheric Prediction System-Advanced Level Physics High Altitude model confirm that correctly modeling the intensifying upper-level weather system in the North Atlantic was necessary to obtain the major SSW.