The role of wavenumber one and two in the development of sudden stratospheric warmings

Abstract

In this thesis, we investigate the effects of planetary waves one and two on the polar stratosphere during boreal winter. We use MERRA reanalysis data and the FMS shallow-water model to compare and contrast their propagation into the stratosphere, their interactions within the stratosphere, and their effects on the polar vortex. The results have implications for the predictability of sudden stratospheric warmings (SSWs), theories on the developments of vortex splits and the role of zonal winds in the tropics. In Chapter 2, we use correlations and regressions to demonstrate that the tropopause affects wavenumber one amplitudes more than wavenumber two. Thus, the statistical predictability of SSWs, based on synoptic events in the mid-troposphere (e.g. blockings), is limited. Composites of extreme heat fluxes reveal that they are likely caused by linear interference of the climatology and anomalies. The phases of anomalous planetary waves align with the climatology only during the largest heat fluxes. In Chapter 3, the effect of wave-wave interactions within the stratosphere is quantified by analyzing eddy energy budgets. The energy transfer from wavenumber one toward wavenumber two plays a key role in the vortex split in January 2013 and several other SSWs. This mechanism might explain the growth of wavenumber two in the stratosphere in nonresonant conditions. However, wave-wave interactions are small in averages over all splits since 1979 suggesting that different processes can lead to vortex splits and that the common SSW definitions do not capture the timing of planetary wave growth. In Chapter 4, we employ a shallow-water model to isolate the effects of wave one and two on the polar vortex over a large range of forcing amplitudes and vortex strengths. We are able to simulate SSW splits, which are unequivocally caused by wave-wave interactions. Furthermore, the initial response of the polar vortex depends strongly on the wavenumber of the forcing.