1668 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 70 A Modeling Study of Stratospheric Waves over the Southern Andes and Drake Passage QINGFANG JIANG,JAMES D. DOYLE, AND ALEX REINECKE Naval Research Laboratory, Monterey, California RONALD B. SMITH Department of Geology, Yale University, New Haven, Connecticut STEPHEN D. ECKERMANN Naval Research Laboratory, Washington, D.C. (Manuscript received 26 June 2012, in final form 20 November 2012) ABSTRACT Large-amplitude stratospheric gravity waves over the southern Andes and Drake Passage, as observed by the Atmospheric Infrared Sounder (AIRS) on 8–9 August 2010, are modeled and studied using a deep (0–70 km) version of the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) model. The simulated tropospheric waves are generated by flow over the high central Andes ridge and the Patagonian peaks in the southern Andes. Some waves emanating from Patagonia propagate southeastward across Drake Passage into the stratosphere over a horizontal distance of more than 1000 km. The wave momentum flux is characterized by a tropospheric maximum over Patagonia that splits into two comparable maxima in the stratosphere: one located directly over the terrain and the other tilting southward with altitude. Using spatial ray-tracing techniques and flow conditions derived from the numerical simulation, the authors find that waves that originate from the high ridge in the Central Andes are absorbed by a critical level in the lower stratosphere. The three-dimensional waves originating from Patagonia could be separated into three families—namely, a northeast-propagating family, which is absorbed by a critical level between 15 and 20 km; a localized family, which breaks down in the stratosphere and lower mesosphere directly above Patagonia; and a southeast-propagating family, which forms the observed linear stratospheric wave patterns oriented across Drake Passage. The southward group propagation, assisted by lateral wave refraction due to persistent meridional shear of the zonal winds, leads to stratospheric wave breaking and drag near 608S, well south of the parent orography. 1. Introduction Accurate parameterizations require in turn a funda- mental understanding of gravity wave dynamics, from Gravity waves entering into the stratosphere and me- generation at the source to propagation and dissipation sosphere play an important role in driving the general elsewhere in the atmosphere. Major tropospheric grav- circulation, enhancing vertical mixing, and contributing ity wave sources identified by previous studies include to polar stratospheric cloud formation, which has further mountains, convection in tropical areas, lower-tropospheric implications for ozone depletion over polar regions (e.g., frontal activity, and upper-tropospheric unbalanced jet Carslaw et al. 1998). Owing to finite computing resources, streams, each of which has been the subject of extensive global climate and weather models cannot run at spatial studies [see review by Fritts and Alexander (2003)]. Par- resolutions needed to resolve gravity waves, so their ticularly, our knowledge of gravity waves generated by effects must be parameterized (e.g., Kim et al. 2003). flow over mountains has been significantly advanced through several large field campaigns conducted over major Corresponding author address: Qingfang Jiang, Naval Research barriers such as the Rocky Mountains [Wave Momentum Laboratory, 7 Grace Hopper Ave., Monterey, CA 93940-5502. Flux Experiment (WAMFLEX); Lilly and Kennedy 1973], E-mail: [email protected] the European Alps [Mesoscale Alpine Programme (MAP); DOI: 10.1175/JAS-D-12-0180.1 Ó 2013 American Meteorological Society Unauthenticated | Downloaded 10/07/21 05:33 PM UTC JUNE 2013 J I A N G E T A L . 1669 Smith et al. 2007], and the Sierra Nevada range [Sierra The remainder of this paper is organized as follows. Waves Project (SWP) 1954 and Terrain-Induced Rotor The wave event and wave properties deduced from Experiment (T-REX); Grubisic et al. 2008]. Mountain satellite and radiosonde observations and the scientific waves over high-latitude southern regions, such as the issues to be pursued by this study are set forth in section southern Andes, have received much less attention. 2. Section 3 contains a description of the model config- Over the past decade, the advent of high resolution uration and an overview of the synoptic conditions satellite sensors has provided new insights into the during this wave event. The simulated wave character- global distribution of long-wavelength gravity wave ac- istics including spatial and temporal variations, wave tivity in the stratosphere (Wu et al. 2006). Observations spectra, and wave momentum fluxes are presented in from both limb and nadir sensors have revealed a strik- section 4. Wave dynamics is further examined in section ing maximum in stratospheric gravity wave variances 5 through ray-tracing calculations. The results and con- over the southern tip of the Andes, the Antarctic Pen- clusions are summarized in section 6. insula, and Drake Passage (Eckermann and Preusse 1999; McLandress et al. 2000; Jiang et al. 2002; Wu 2004; 2. The 8–10 August 2010 wave event Wu and Eckermann 2008; Alexander et al. 2008; Yan et al. 2010). A number of studies have demonstrated that The Atmospheric Infrared Sounder (AIRS) on the the high orography of the Andes and Antarctic Penin- National Aeronautics and Space Administration Aqua sula generates many of the intense stratospheric gravity satellite acquires radiances by scanning the atmosphere waves observed here, some of which appear to propa- symmetrically about nadir in a repeating cycle aligned gate downstream and meridionally to produce activity orthogonal to the orbit vector (Aumann et al. 2003). A over Drake Passage (e.g., Preusse et al. 2002; Jiang et al. number of previous studies have demonstrated that 2002; Alexander and Teitelbaum 2007; Baumgaertner gravity waves with long vertical wavelengths and hori- and McDonald 2007; Plougonven et al. 2008; Yamashita zontal wavelengths .40 km can be resolved as a two- et al. 2009; Shutts and Vosper 2011). However, others dimensional perturbation structure in swath radiance have questioned the presumed dominance of orographic imagery of certain stratospheric channels uncontaminated forcing in generating enhanced wave activity in this re- by high-tropospheric cloud (e.g., Wu et al. 2006; Alexander gion (e.g., de la Torre and Alexander 2005). For example, and Barnet, 2007; Eckermann et al. 2007). Here we use some studies have identified tropospheric convection and a channel-averaged AIRS radiance product registered jet stream instabilities as the sources of waves observed in at a series of heights from 100 to 2 hPa and summarized this same region (e.g., Yoshiki and Sato 2000; Yoshiki in Table A2 of Gong et al. (2012). We remove large- et al. 2004; de la Torre et al. 2006; Spiga et al. 2008; Hei scale backgrounds to isolate gravity wave perturbations et al. 2008; Llamedo et al. 2009). Still other studies have using techniques described by Eckermann and Wu argued that instabilities in the stratospheric vortex jet (2012). generate upward- and downward-propagating gravity Figure 1 shows the resulting brightness temperature waves that also contribute to this enhanced local wave (radiance) perturbations at several pressure levels on 8–9 activity (Sato and Yoshiki 2008; Moffat-Griffin et al. August 2010 within a focused region over the southern 2011). Clearly the origins and dynamics of the rich and Andes and Antarctic Peninsula. Linear wave patterns highly energetic gravity wave fields encountered in this are evident in the swath imagery over the southern tip of remote region of the planet require further research to the Andes and Drake Passage with at least three pairs of better understand and parameterize. wave crests and troughs discernible. Note that, while The research in this paper is also motivated by satellite these observed radiance perturbations are directly re- observations of stratospheric gravity waves in this region, lated to actual gravity-wave-induced temperature struc- which frequently reveal linear wave patterns over the ture in terms of the two-dimensional phase structure, the southern Andes and Drake Passage. The objectives of this brightness temperature amplitudes are a lower bound, study are twofold. First, we want to explore the capability and likely a considerable underestimate, of the actual of the Coupled Ocean–Atmosphere Mesoscale Prediction temperature amplitudes of these waves, owing to the System (COAMPS) in simulating stratospheric gravity vertical averaging effect of the broad nadir weighting waves by extending its model top from the previous functions that yield these channel radiances (Alexander capability at the 30-km level (i.e., lower stratosphere) to and Barnet 2007). The waves evident in Fig. 1 share the ;70 km MSL (i.e., lower mesosphere). Second, we want to following properties: 1) they extend across Drake Pas- advance our understanding of characteristics and dy- sage with phase lines oriented northwest–southeast; 2) namics of gravity waves over the southern Andes and Drake while the wave crests (troughs) vary in length at differ- Passage through examination of the simulated waves. ent levels, their northern edges are always anchored Unauthenticated | Downloaded 10/07/21 05:33 PM UTC 1670 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 70 FIG. 1. Small horizontal-scale brightness temperature anomalies (K) extracted from multichannel AIRS radiances peaking at the indicated altitudes of (from bottom to top) 80, 30, 7, and 2.5 hPa [see Table A2 and accompanying discussion of Gong et al. (2012) for details] from the ascending and descending overpasses of the southern Andes region on 8 and 9 Aug 2010. These perturbations were isolated using the algorithms described by Eckermann and Wu (2012). A 3 3 3 point smoothing of radiance anomalies in neighboring footprints was applied in these plots to suppress channel noise and accentuate the geophysical wave perturbations. Data from overpasses on ascending and descending orbits (different local times) are plotted in separate panels. above the southern Andes; 3) the horizontal wave- peaks higher than 3 km MSL.