Abstract
Abstract. When orographic gravity waves (OGWs) break, they dissipate their momentum and energy and thereby influence the thermal and dynamical structure of the atmosphere. This OGW forcing mainly takes place in the middle atmosphere. It is zonally asymmetric and strongly intermittent. So-called “OGW hotspot regions” have been shown to exert a large impact on the total wave forcing, in particular in the lower stratosphere (LS). Motivated by this we investigate the asymmetrical distribution of the three-dimensional OGW drag (OGWD) for selected hotspot regions in the specified dynamics simulation of the chemistry-climate model CMAM (Canadian Middle Atmosphere Model) for the period 1979–2010. As an evaluation, we first compare zonal mean OGW fluxes and GW drag (GWD) of the model simulation with observations and reanalyses in the Northern Hemisphere. We find an overestimation of GW momentum fluxes and GWD in the model's LS, presumably attributable to the GW parameterizations which are tuned to correctly represent the dynamics of the Southern Hemisphere. In the following, we define three hotspot regions which are of particular interest for OGW studies, namely the Himalayas, the Rocky Mountains and East Asia. The GW drags in these hotspot regions emerge as strongly intermittent, a result that can also quantitatively be corroborated with observational studies. Moreover, a peak-detection algorithm is applied to capture the intermittent and zonally asymmetric character of OGWs breaking in the LS and to assess composites for the three hotspot regions. This shows that LS peak OGW events can have opposing effects on the upper stratosphere and mesosphere depending on the hotspot region. Our analysis constitutes a new method for studying the intermittency of OGWs, thereby facilitating a new possibility to assess the effect of particular OGW hotspot regions on middle atmospheric dynamics.
Highlights
Internal gravity waves (GWs) are a naturally occurring and ubiquitous phenomenon with large impact on the atmosphere’s thermal and dynamical structure (Andrews and McIntyre, 1987; Fritts and Alexander, 2003)
-called “orographic gravity waves (OGWs) hotspot regions” have been shown to exert a large impact on the total wave forcing, in particular in the lower stratosphere (LS). Motivated by this we investigate the asymmetrical distribution of the three-dimensional OGW drag (OGWD) for selected hotspot regions in the specified dynamics simulation of the chemistry-climate model CMAM (Canadian Middle Atmosphere Model) for the period 1979– 2010
In February, the modeled fluxes drop to much lower amplitudes at 30 km, which largely improves the agreement with the observations
Summary
Internal gravity waves (GWs) are a naturally occurring and ubiquitous phenomenon with large impact on the atmosphere’s thermal and dynamical structure (Andrews and McIntyre, 1987; Fritts and Alexander, 2003). OGW drag (OGWD) hotspots are associated with well-known topographic structures such as the Andes and the Antarctic Peninsula in the Southern Hemisphere (SH) and the Rocky Mountains, the Scandinavian range and the Himalayas in the Northern Hemisphere (NH). These structures produce zonally asymmetric and interannually variable torques, which significantly contribute to the total drag, emerging already as low as in the lower stratosphere (LS; Šácha et al, 2019). We present first results of a new method for studying the impact of spatiotemporally intermittent OGWD in Sect.
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