Baroclinic conditions and anomalous temperature excursions in the Arctic winter middle atmosphere.

摘要

The Arctic stratosphere and mesosphere are dynamically altered throughout the winter months by planetary waves that propagate up from the troposphere and disturb the winter circumpolar vortex in the stratosphere. When planetary waves break, the polar vortex is either severely distorted and/or splits into two cyclonic lobes. During these approximately week-long sudden stratospheric warming (SSW) events, the stratopause moves from altitudes near 50 km to near 30 km and has a region of intense warming on the order of 50 K. Mesospheric cooling by approximately 30 K occurs during SSW's. There are more regular occurrences of wave breaking and temperature excursions in the upper stratosphere and mesosphere that do not extend into the lower stratosphere; hereafter called stratopause warmings and mesosphere coolings (SWMC). We investigate these occurrences and their associated baroclinic conditions that can result in dramatic (50K) changes in upper stratosphere and mesosphere temperatures. Criteria were developed to identify 29 events in 18 years (1991-2009) of assimilated analyses from the United Kingdom Meteorological Office (MetO). The magnitude of the rapid heating and cooling cannot be explained by advection or radiative effects. The results suggest that a strong vertical circulation generates the required adiabatic heating and cooling. A Q-vector analysis technique is applied at stratopause altitudes during the SWMC events to qualitatively determine regions of air ascent and descent. Temperature measurements from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument are examined between 50 and 110 km during 3 SWMC events. Examining the events in SABER shows two mesopauses, one near 75 km and one near 100 km.