1. Introduction
During austral winter, strong winds at the edge of the stratospheric polar vortex refract gravity waves to long vertical wavelengths, making them easier for satellite remote sensors to detect. These satellite observations reveal maxima or “hot spots” of elevated gravity wave variance. The most prominent peak occurs over the southern Andes, Drake Passage, and Antarctic Peninsula and results primarily from strong wave generation by flow over orography (Eckermann and Preusse 1999; McLandress et al. 2000; Preusse et al. 2002; Jiang et al. 2002; Alexander and Teitelbaum 2007; Baumgaertner and McDonald 2007; Shutts and Vosper 2011; Jiang et al. 2013). There are additional hot spots in orographic gravity wave activity over the mountainous regions of Antarctica, Australia, Africa, and New Zealand, and also above tiny mountainous islands scattered around the Southern Ocean (e.g., Wu et al. 2006; Alexander et al. 2009; Eckermann and Wu 2012; Alexander and Grimsel 2013). Observations during austral winter have also found occasional evidence of stratospheric gravity waves generated by nonorographic tropospheric sources, such as the jet stream (e.g., Guest et al. 2000; Yoshiki et al. 2004; Hei et al. 2008), yet there is no observational evidence that these sources produce any similar climatological enhancements in stratospheric gravity wave variances. This may be because nonorographic sources are too irregularly distributed in time and space to produce any similar type of hot spot in satellite radiance observations.
In this study, we take a closer look at the background levels of nonorographic gravity wave activity upon which hot spots in orographic gravity wave activity are superimposed in satellite observations. In particular, during the austral winter, satellite observations show a broad belt of enhanced stratospheric gravity wave variance at 50°–60°S that extends eastward from the southern Andes at 70°–80°W to southeast of Australia near 150°E (e.g., Wu et al. 2006; Wu and Eckermann 2008; Yan et al. 2010). Since this feature lies over the Southern Ocean and is located away from significant terrain and landmasses, it cannot be explained by orographic forcing alone, even though some remote downstream wave penetration from the Andes may be evident (e.g., Preusse et al. 2002; Shutts and Vosper 2011). To our knowledge, this feature has not been studied in any depth and remains unexplained.
Most satellite-based gravity wave observations of this feature are for specific periods or years, making it more difficult to achieve of robust climatology of the wave field. Accordingly, we examined stratospheric gravity waves in 9 years of data from the Atmospheric Infrared Sounder (AIRS) on the National Aeronautics and Space Administration (NASA)’s Aqua satellite, using the noise-reduced AIRS stratospheric radiance products and extraction algorithms for gravity waves described by Eckermann and Wu (2012) and references therein. Figure 1 shows the root-mean-square (rms) radiance perturbation amplitudes due to gravity waves at four stratospheric altitudes, averaged throughout the austral winter (June–August) for the years 2003–11. In addition to reproducing the well-known hot spots in orographic gravity wave activity cited above, in the middle stratosphere, we see a broad band of statistically significant elevated wave variances extending from the southern Andes to south of New Zealand, with variances here clearly and systematically enhanced with respect to the much weaker variances over the Pacific sector (150°E–80°W). This band spirals slightly poleward, from 55° to 62°S (Figs. 1e,f).
Root-mean-square AIRS brightness temperature amplitudes (K) due to imaged gravity wave perturbations at altitudes of (a) 80, (b) 20, (c) 10, and (d) 7 hPa. (e) Variation of amplitude with longitude and (f) variation of the latitude of maximum amplitude with longitude. These maps were derived using the reduced-noise AIRS radiance products and analysis methods described in section 2 of Eckermann and Wu (2012), with mean variances computed within 1° × 0.5° longitude–latitude grid boxes for each day of austral winter (June–August) for the years 2003–11 inclusive.
Citation: Journal of the Atmospheric Sciences 71, 5; 10.1175/JAS-D-13-0332.1
This variance band coincides with the strongest winds at the edge of the polar vortex that refract waves to long vertical wavelengths and allow satellite remote sensors to resolve a greater fraction of the total gravity wave energy spectrum (e.g., McLandress et al. 2000). Therefore, the zonal asymmetry of this elevated band of variances in Fig. 1 might too be explained if stratospheric winds were systematically stronger over the Atlantic and Indian Ocean regions relative to those over the Pacific. This possibility is investigated in the appendix, where it is shown that zonal asymmetries in stratospheric vortex winds are small and too weak to explain the observations. This symmetry also precludes an explanation based on zonally asymmetric local generation from dynamical imbalances within the stratospheric vortex jet (Sato and Yoshiki 2008). Thus, other explanations must be sought.
There are two clear features of the elevated variance band in Fig. 1 that any theory must explain: (i) variances are systematically larger over the southern Atlantic and southern Indian Ocean regions and systematically much weaker over the southern Pacific, and (ii) the central latitude of the band gradually moves poleward, starting at 50°S just east of the Andes and slowly migrating to 60°S to the south of Australia and New Zealand (Fig. 1f). The purpose of this paper is to provide evidence that this band of elevated variance in Fig. 1 represents an extended hot spot of nonorographic gravity wave activity produced by enhanced baroclinic wave activity and tropospheric jet structures in this region. An examination of tropospheric sources of stratospheric wave activity in Fig. 1, with a focus on possible nonorographic sources, is given in section 2. A discussion of the main results is given in section 3, and a summary is given in section 4.
2. Tropospheric sources of stratospheric waves
We use the Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data (Dee et al. 2011) from 1999 to 2009. We use 6-hourly fields interpolated to a 1° grid at 36 reference pressure levels from 1 to 1000 hPa. These ERA data resolve gravity wave horizontal wavelengths greater than 200 km and vertical wavelengths greater than approximately 1–3 km.
We begin by using ERA data to compare near-surface flow impinging on Andes orography with stratospheric gravity wave activity in the analyses above the Andes to test the hypothesis that the stratospheric gravity wave hot spot here is orographic in origin (e.g., Jiang et al. 2002; Jiang et al. 2013). Averages in a boxed region around the southern Andes (40°–60°S, 60°–80°W) were calculated for both the 700-hPa zonal wind and the absolute value of the divergence at 5 hPa. The former is a proxy for strong cross-flow impinging on the Andes, resulting in large-amplitude gravity wave production (Doyle et al. 2000, 2011). The latter is a proxy for gravity wave activity in the stratosphere (approximately 30 km MSL). In Fig. 2a, a time series of both quantities is shown in the austral winter for 1999. Peaks in stratospheric gravity wave activity tend to correspond to maxima in the 700-hPa zonal velocity in the southern Andes region, particularly in the months of June and July. This correlation is further confirmed by the scatterplots from 1999 to 2009 for June–September in Figs. 2b–e. In each month, there is a moderate positive correlation between the average 700-hPa velocity and the average absolute value of divergence at 5 hPa in the boxed region around the southern Andes. The correlation appears more nonlinear in June and July, where strong gravity wave activity is excited above a 700-hPa zonal velocity threshold of approximately 10 m s−1.
Stratospheric gravity wave activity over the southern Andes via orographic wave launching. (a) Time series of the absolute value of divergence and 700-hPa zonal winds in austral winter of 1999. Scatterplots for (b) June, (c) July, (d) August, and (e) September 1999–2009, with different symbols representing different years.
Citation: Journal of the Atmospheric Sciences 71, 5; 10.1175/JAS-D-13-0332.1
In Fig. 3, the average Eady growth rate at p = 525 hPa is given in the Southern Hemisphere in austral winter (June–September). The growth rate is computed at each synoptic time (i.e., 0000, 0600, 1200, and 1800 UTC) for the years 1999–2009 and then averaged over the entire time period. High σE values typically correspond to regions favorable to baroclinic wave activity (i.e., cyclones, jets, and frontal systems). There are two zonally elongated strips of σE. One is centered at approximately 30°S and extends from southern Africa through the southern Pacific Ocean, with a maximum in Australia; it is strongly correlated with the peak winds and shear of the subtropical winter jet. The next branch is centered at 50°S, extends from the southern Andes to south of Australia, and spirals slightly poleward toward Antarctica; it correlates with the peak winds and shear of the midlatitude spiral jet (Williams et al. 2007; Barnes and Hartmann 2010). These two branches correspond well with previous studies of winter cyclonic track density in the Southern Hemisphere (Trenberth 1991; Hoskins and Hodges 2005). There is no significant interannual or intraseasonal variability in these peak climatological σE features of Fig. 3. However, the maximum in the midlatitude branch is slightly higher in September than in the other months, while the maximum in the subtropical branch is highest in midwinter (July and August). There also exist peaks of σE over the Southern Andes and the mountainous regions of Antarctica, which may be indicative of terrain-induced baroclinic wave development (Hoskins and Hodges 2005).
1999–2009 average maximum Eady growth rate (day−1) at p = 525 hPa using the ERA-Interim data for (a) June, (b) July, (c) August, and (d) September. Latitudinal and longitudinal grid increments are 15°.
Citation: Journal of the Atmospheric Sciences 71, 5; 10.1175/JAS-D-13-0332.1
In Fig. 4, the average absolute value of divergence at p = 5 hPa due to resolved gravity waves in the analysis is shown for the same period and using the same averaging. Under each plot, the latitude of the peak value is also plotted versus longitude. The most striking feature in these plots is the maximum over the southern Andes due to deeply propagating orographic waves. The area of large divergence extends eastward from the southern tip of South America to the south of Australia, and spirals slightly toward the South Pole. This spiral is clearly evident in latitude plots, as the peak value moves from approximately 55° to 62°S and from 60°W to 180°, broadly consistent with Fig. 1f (AIRS). Some intraseasonal variability of the spiral exists.
As in Fig. 3, but for the average absolute value of the divergence (×10−5 s−1) at p = 5 hPa. Under each contour plot, the latitude of the peak value is shown vs longitude.
Citation: Journal of the Atmospheric Sciences 71, 5; 10.1175/JAS-D-13-0332.1
It is evident that the morphology of the midlatitude maxima in σE at p = 525 hPa closely corresponds with both the observed patterns of stratospheric wave activity in Fig. 1 and the analyzed stratospheric wave activity from the ERA-Interim data in Fig. 4 (i.e., there is consistency in the patterns in all three figures). This is further illustrated in Fig. 5, where the correlation between σE at p = 525 hPa and absolute value of the divergence at p = 5 hPa is shown for July in the latitude belt of the enhanced stratospheric wave activity region of Fig. 1. Here the 1999–2009 average values of each parameter are given. Higher (lower) values of the maximum Eady growth rate typically are correlated with higher (lower) values of the absolute value of the divergence. While the correspondence is evident in the large scale, there are a few discrepancies worth noting. First, the poleward spiral of the Eady growth rate is more significant than the stratospheric wave activity in Figs. 1 and 4, and second, the divergence patterns in Fig. 4 vary seasonally more than the Eady growth rate (Fig. 3). The subtropical branch of σE that extends in Fig. 3 from Madagascar east through Australia (at approximately 30°S) does not have an accompanying maximum in stratospheric gravity wave activity in Figs. 1 and 4. This is likely because of much weaker subtropical stratospheric westerlies (Eckermann and Wu 2012, their Fig. 2) that lead either to critical-level wave absorption or very short vertical wavelengths that satellite sensors and models cannot explicitly resolve.
Correlation of the 1999–2009 July average p = 5 hPa absolute value of the divergence with the p = 525 hPa value of the Eady growth rate over 50°–60°S, 0°–90°W.
Citation: Journal of the Atmospheric Sciences 71, 5; 10.1175/JAS-D-13-0332.1
3. Discussion
We have used maximum Eady growth rates in Fig. 3 to diagnose preferred locations for baroclinic wave growth and then, based on models of gravity wave generation from imbalance with rapidly evolving synoptic systems (e.g., O’Sullivan and Dunkerton 1995), we have used those diagnostics as a proxy for associated increases in nonorographic gravity wave generation. This could be viewed as favoring one specific nonorographic source over the many other mechanisms that have been proposed, including frontogenesis, vertical shear instability, and convection (Fritts and Alexander 2003). Here we will argue that these processes too are likely enhanced over the Atlantic and Indian Ocean sectors and suppressed over the Pacific sector, consistent with the observations in Figs. 1, 3, and 4.
Through Eq. (1), there is a strong coupling between elevated baroclinic wave growth and largest local vertical shears that can conceivably trigger Kelvin–Helmholtz instabilities that radiate gravity waves (e.g., Scinocca and Ford 2000). Indeed, the elevated Eady growth rates in Fig. 3 closely follow the climatology of the upper-tropospheric jets. In the Southern Hemisphere during austral winter, a double jet structure occurs, characterized by a strong subtropical jet that peaks over the Pacific, and a secondary midlatitude jet that peaks near 50°S over the Indian Ocean (Hoskins and Hodges 2005; Williams et al. 2007). As this midlatitude jet intensifies over the Atlantic and Indian Ocean regions, it slowly spirals poleward from 50° to 60°S, progressively weakens, and then dissipates to the south of New Zealand. This midlatitude spiral jet is driven by eddy feedbacks from anticyclonic wave-breaking events that peak in the southern Indian Ocean owing to a conducive local environment regulated by the stable waveguide provided by a strong subtropical jet (Williams et al. 2007; Barnes and Hartmann 2010). As a result, midlatitude storm tracks in austral winter that likely control most convective gravity wave generation also exhibit the same poleward spiraling structure through the Atlantic and Indian Ocean sectors, with storm tracks in the Pacific sector mostly absent at midlatitudes and confined to the subtropics or coastal Antarctica (Hoskins and Hodges 2005). In addition, frontal intensities during austral winter peak within a similar midlatitude band centered in the southern Indian Ocean region, with weakest intensities located in the Pacific sector (Simmonds et al. 2012, their Figs. 6b and 6d). Indeed, Barnes and Hartmann (2010) correlate all of this enhanced baroclinic weather activity in the southern Indian Ocean with a regional amplification of the southern annular mode through eddy feedbacks, likening it to the regional amplification of the northern annular mode and associated baroclinic activity in the North Atlantic. In short, the results of Fig. 3 suggest that all the major regions of midlatitude weather systems in austral winter that are potential nonorographic sources of gravity waves for the stratosphere are enhanced climatologically over the southern Atlantic and southern Indian Ocean regions and suppressed over the southern Pacific, just as observed and analyzed in Figs. 1 and 4, respectively.
Thus, the band of elevated stratospheric gravity wave variance over the Atlantic and Indian Ocean sectors apparent in Figs. 1 and 4 appears to be predominantly due to the stratospheric gravity waves resulting from enhanced tropospheric generation of nonorographic sources associated with midlatitude weather systems in these regions. This includes the poleward spiral of peak stratospheric wave variances from 50°S in the South Atlantic to 60°S south of New Zealand evident in Figs. 1 and 4, which follows a similar spiraling of the midlatitude jet, baroclinic growth rates, and major storm tracks (see Fig. 3). Likewise, the suppressed stratospheric wave variances over the Pacific appear to be the same stratospheric manifestation of reduced nonorographic wave generation from weaker synoptic-scale and mesoscale forcing mechanisms in this midlatitude region of the planet during austral winter. The AIRS instrument’s observing characteristics provide an additional small contribution to this spiraled pattern of radiance perturbations (see appendix).
4. Summary
A possible explanation has been provided for the observed belt of enhanced stratospheric gravity wave activity in austral winter, which extends from the southern Andes eastward to the south of Australia, migrating poleward slightly toward Antarctica. Using multiple years of both the ERA-Interim data and AIRS satellite observations, we have provided evidence that suggests that this belt of activity is a robust climatological feature due to nonorographic tropospheric gravity wave sources: spontaneous emission from jets in rapidly evolving baroclinic systems, frontogenesis, and convection, as evinced from the strong correlation with the midtropospheric maximum Eady growth rate in the same region. An observational campaign, the Deep-Propagating Gravity-Wave Experiment (DEEPWAVE), will be conducted during austral winter from June to August 2014 to better understand and characterize these various sources of gravity waves.
Acknowledgments
We gratefully acknowledge support from the Chief of Naval Research through PE-0601153N, as well as NASA through NNH09ZDA001N-TERRAQUA (The Science of Terra and Aqua), Grant NNH11AQ99.
APPENDIX
Contribution of AIRS Observational Characteristics to Zonal Asymmetries
(a) Vertical weighting function for AIRS channel 79 and (b) the corresponding fraction of wave temperature amplitude retrieved vs vertical wavelength from observations in this channel. (c) Peak zonal winds between 40° and 75°S at 10 (black) and 500 hPa (gray) as function of longitude, averaged over June–August for years 2003–11 from MERRA. (d) Peak 10-hPa AIRS gravity wave radiance perturbations (black, reproduced from Fig. 1e) and model prediction of effect of the 10-hPa wind variations in (c) on net gravity wave detectability in this AIRS channel based on the λz-dependent sensitivities in (b). See text for model details.
Citation: Journal of the Atmospheric Sciences 71, 5; 10.1175/JAS-D-13-0332.1
The weak zonal asymmetry in 10-hPa winds in Fig. A1c yields corresponding variations in λz via Eq. (A2), and hence in ε via Eq. (A1), which could produce some of the longitudinal asymmetry evident in the AIRS 10-hPa gravity wave amplitude in Fig. 1e and reproduced as the black curve in Fig. A1d.
Since we do not know the distribution of nonorographic gravity wave c and ϕ values in the stratosphere, nor their wave amplitudes, we chose a uniform distribution of c values from 0 to 100 m s−1 and ϕ values from 0° to 360°. For every wave within these c and ϕ ranges, we computed λz from Eq. (A2) using 10-hPa U values from Fig. A1c and N = 0.02 s−1, then ε(λz) from Eq. (A1). We then computed rms ε values for the wave spectrum at each longitude. The result is plotted as the gray curve in Fig. A1d, after scaling to a representative rms AIRS radiance.
While the model curve reproduces the phase of the observed longitudinal variability in AIRS rms radiance, it underestimates the amplitude of the variation by approximately a factor of 3. Thus, while an AIRS observing effect due to longitudinal variations in peak 10-hPa winds can explain some of the observed variability, it does not explain all of it. Indeed, the 500-hPa winds, plotted in gray in Fig. A1c, show a much larger variation with longitude with the same phase that is consistent with the larger variation in the AIRS 10-hPa gravity wave amplitudes in Fig. A1d, further supporting arguments in the main text that most of the observed variation is due to tropospheric-source asymmetries associated with varying wind speeds and shear in the troposphere, as encapsulated by the maximum Eady growth-rate diagnostic σE.
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