80 Synoptic Formation of Double Tropopauses

Tuesday, 27 June 2017
Salon A-E (Marriott Portland Downtown Waterfront)
Chengji Liu, Colorado State University, Fort Collins, CO; and E. A. Barnes

Double tropopauses are ubiquitous in the midlatitude winter hemisphere and represent the vertical stacking of two stable tropopause layers separated by a less stable layer. We demonstrate that they often occur during Rossby wave breaking and act to increase the stratosphere-to-troposphere exchange of mass. We further investigate the adiabatic formation of double tropopauses and propose two mechanisms by which they can occur. The first mechanism operates at the tropopause break in the subtropics where the higher tropical tropopause is on the one side of the break and the lower extratropical tropopause is on the other side. The double tropopauses are then formed by differential meridional advection of the higher and lower tropopauses on the two sides of the tropopause break. We show that anticyclonic wave breaking forms double tropopauses mainly by providing stronger poleward advection of the higher tropopause in its poleward lobe. Cyclonic wave breaking mainly forms double tropopauses by providing stronger equatorward advection of the lower tropopause in its equatorward lobe. We demonstrate in the observations that about half of the double tropopauses in the northern hemisphere winter can be directly attributed to such differential advection. For the second mechanism, adiabatic destabilization of the air above the tropopause contributes to the formation of double tropopause. In this case, a tropopause inversion layer (TIL) is necessary for this destabilation to results in a double tropopause. The mechanisms above can be used to understand the occurrence of double tropopauses, which contribute significantly to the stratosphere-troposphere exchange given their ubiquitousness.

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