Abstract
Abstract. The midwinter 2015/2016 was characterized by an unusually strong polar night jet (PNJ) and extraordinarily large stationary planetary wave (SPW) amplitudes in the subtropical mesosphere. The aim of this study is, therefore, to find the origin of these mesospheric SPWs in the midwinter 2015/2016 study period. The study duration is split into two periods: the first period runs from late December 2015 until early January 2016 (Period I), and the second period from early January until mid-January 2016 (Period II). While the SPW 1 dominates in the subtropical mesosphere in Period I, it is the SPW 2 that dominates in Period II. There are three possibilities explaining how SPWs can occur in the mesosphere: (1) they propagate upward from the stratosphere, (2) they are generated in situ by longitudinally variable gravity wave (GW) drag, or (3) they are generated in situ by barotropic and/or baroclinic instabilities. Using global satellite observations from the Microwave Limb Sounder (MLS) and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) the origin of the mesospheric SPWs is investigated for both time periods. We find that due to the strong PNJ the SPWs were not able to propagate upward into the mesosphere northward of 50∘ N but were deflected upward and equatorward into the subtropical mesosphere. We show that the SPWs observed in the subtropical mesosphere are the same SPWs as in the mid-latitudinal stratosphere. Simultaneously, we find evidence that the mesospheric SPWs in polar latitudes were generated in situ by longitudinally variable GW drag and that there is a mixture of in situ generation by longitudinally variable GW drag and by instabilities at mid-latitudes. Our results, based on observations, show that the abovementioned three mechanisms can act at the same time which confirms earlier model studies. Additionally, the possible contribution from, or impact of, unusually strong SPWs in the subtropical mesosphere to the disruption of the quasi-biennial oscillation (QBO) in the same winter is discussed.
Highlights
The Arctic winter 2015/2016 was extraordinary in many respects regarding the middle atmosphere
While the stationary planetary wave (SPW) 1 dominates in the subtropical mesosphere in Period I, it is the SPW 2 that dominates in Period II
This paper investigates the origin of mesospheric SPWs in a case study during one month in the Arctic midwinter 2015/2016 where an unusually strong SPW 1 and 2 amplitude was observed in the subtropical mesosphere
Summary
The Arctic winter 2015/2016 was extraordinary in many respects regarding the middle atmosphere. A significant disruption of the quasibiennial oscillation (QBO) occurred, beginning at the end of December 2015 and fully completing by mid-April 2016 (Osprey et al, 2016; Newman et al, 2016; Coy et al, 2017) This particular winter was characterized by one of the strongest El Niño events on record, with a strong polar stratospheric signature (Palmeiro et al, 2017). In addition to these global anomalies, there was a regional reversal in zonal wind from eastward to westward in the mesosphere over an altitude range of ∼ 10 km beginning at the end of December 2015 and lasting for four weeks, which was not connected to a sudden stratospheric warming (SSW) (Stober et al, 2017). The main aim of this paper is, to find the origin of the significantly enhanced SPW amplitude during this time period in the subtropical mesosphere
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