Stratospheric warming analysis using the PV-theta view

Abstract

Sudden Stratospheric Warmings (SSWs) are events in which the middle stratosphere, where it is usually very cold, warms up by 30 to even 50 degrees Celcius. SSWs lead to a displacement or split of the polar vortex, a low pressure system above the Poles which is created every autumn and disappears every spring by the changing temperature differences between the equator and the poles. In many papers and literature, these stratospheric events are related to drastic changes in the general circulation in the troposphere. Increase of surface pressure at northern latitudes (negative AO and NAO) and weakening, meandering and southward displacement of the jet stream are frequently observed after a SSW.

There are accepted theories on the cause of SSWs. However, there is still much unclear how these disturbances in the stratosphere alter the general circulation in the troposphere. An SSW is defined as the reversal of the zonal mean zonal wind at a height of 10 hPa at a latitude of 60N. One of the shortcomings of the current definition of the SSW is that it does not take the vertical extent of the event into account since it is only restricted to a height of 10 hPa. In response to the Arctic air outbreak of 2018 in Europe, KNMI published a report where an improved definition with additional requirements regarding the depth of a SSW was proposed: the Deep Stratospheric Warming (DSW). A DSW is de?ned as a reversal of the zonal mean zonal wind in a layer between 10-100 hPa for at least 5 days, averaged between 60-70N. On at least 2 days, the layer thickness must be minimal 80 hPa. Since DSWs cover a much deeper layer of the stratosphere, there is a bigger chance that they influence the general tropospheric circulation significantly.

The downward propagation of perturbations coming from an SSW are investigated in this thesis. Five cases of SSWs, of which 4 cases are also DSWs, are thoroughly analyzed in order to provide an explanation for the downward effects which possibly affects the tropospheric circulation. In order to do that, the PV-theta view is used. In this view, the potential temperature is used as the vertical coordinate and the potential vorticity is the most important metric.

The disturbances caused by an SSW lead to the creation of PV anomalies. The atmosphere adjusts itself to these PV anomalies, which is actually adjustment to the thermal wind principle. By investigating the fluxes of potential vorticity in the form of isentropic mass and PVS (vorticity) flux, it turns out that before and during a SSW, the poleward mass circulation and equatorward PVS circulation in the stratosphere enhances generally. An increased poleward mass flux causes the pressure increases at northern latitudes. An increased equatorward PVS flux acts as a zonal force on the zonal wind, which decelerates the jet. The PV anomalies have a vertical reach in which they can influence the air masses: the Rossby scale height. The stratospheric PV anomalies have a Rossby height which is large enough to have influence on the troposphere.