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Seasonal Variations of Subtropical Precipitation Associated with the Southern Annular Mode

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Seasonal Variations of Subtropical Precipitation Associated with the Southern Annular Mode 3446 JOURNAL OF CLIMATE VOLUME 27 Seasonal Variations of Subtropical Precipitation Associated with the Southern Annular Mode HARRY H. HENDON,EUN-PA LIM, AND HANH NGUYEN Centre for Australian Weather and Climate Research, Bureau of Meteorology, Melbourne, Australia (Manuscript received 10 September 2013, in final form 20 January 2014) ABSTRACT Seasonal variations of subtropical precipitation anomalies associated with the southern annular mode (SAM) are explored for the period 1979–2011. In all seasons, high-polarity SAM, which refers to a poleward- shifted eddy-driven westerly jet, results in increased precipitation in high latitudes and decreased pre- cipitation in midlatitudes as a result of the concomitant poleward shift of the midlatitude storm track. In addition, during spring–autumn, high SAM also results in increased rainfall in the subtropics. This subtropical precipitation anomaly is absent during winter. This seasonal variation of the response of subtropical pre- cipitation to the SAM is shown to be consistent with the seasonal variation of the eddy-induced divergent meridional circulation in the subtropics (strong in summer and weak in winter). The lack of an induced divergent meridional circulation in the subtropics during winter is attributed to the presence of the wintertime subtropical jet, which causes a broad latitudinal span of eddy momentum flux divergence due primarily to higher phase speed eddies breaking poleward of the subtropical jet and lower speed eddies not breaking until they reach the equatorward flank of the subtropical jet. During the other seasons, when the subtropical jet is less distinctive, the critical line for both high and low speed eddies is on the equatorward flank of the single jet and so breaking in the subtropics occurs over a narrow range of latitudes. The implications of these findings for the seasonality of future subtropical climate change, in which a shift to high SAM in all seasons is expected to be promoted, are discussed. 1. Introduction Visbeck 2002; Gillett et al. 2006; Sen Gupta and England 2007). Although the SAM is fundamentally an extra- This study addresses the seasonality of the interaction tropical phenomenon, its impacts extend well into of the eddy-driven westerly jet in the Southern Hemi- the subtropics, where the Hadley circulation (e.g., sphere with subtropical precipitation. We concentrate Thompson and Lorenz 2004; Kang and Polvani 2011), on variability of the jet associated with the southern precipitation, and surface temperatures (e.g., Gillett annular mode (SAM), which is the leading mode of et al. 2006; Sen Gupta and England 2007; Hendon et al. circulation variability in the extratropics of the South- 2007; Meneghini et al. 2007; Karpechko et al. 2009; ern Hemisphere at time scales beyond 1 week (e.g., Kang et al. 2011) are affected. Thompson and Wallace 2000). In its high-polarity phase, The magnitude of the regional impacts of SAM on the midlatitude westerlies are shifted poleward and subtropical surface climate is comparable to other well- surface pressure is anomalously low over the polar cap known modes of climate variability such as El Nino–~ and high in the midlatitudes. The associated meridio- Southern Oscillation (ENSO; e.g., Hendon et al. 2013) nal shift in the midlatitude storm tracks (e.g., Karoly and the Madden–Julian oscillation (MJO; e.g., Risbey 1990; Brahmananda Rao et al. 2003) is accompanied by et al. 2009). Furthermore, the SAM is now appreciated changes in precipitation, air temperature, sea surface to be a source of predictability at intraseasonal (Hudson temperature, and ocean surface currents throughout the et al. 2011; Marshall et al. 2012) and seasonal (e.g., extratropics of the Southern Hemisphere (e.g., Hall and Hendon et al. 2013; Lim et al. 2013) time scales. Recent climate change in the Southern Hemisphere has also Corresponding author address: Harry Hendon, CAWCR, GPO been attributed to a forced upward trend in the SAM as Box 1289, Melbourne, VIC 3001, Australia. a result primarily of ozone depletion in austral summer E-mail: [email protected] (e.g., Gillett and Thompson 2003; Arblaster and Meehl DOI: 10.1175/JCLI-D-13-00550.1 Ó 2014 American Meteorological Society Unauthenticated | Downloaded 09/27/21 10:05 AM UTC 1MAY 2014 H E N D O N E T A L . 3447 2006; Polvani et al. 2011; Gillett et al. 2013). Continued divergence and hence shifting the westerly jet pole- increasing greenhouse gases are projected to result in ward. The induced meridional circulation that acts to a robust upward trend of the SAM in all seasons (Yin balance these anomalous momentum fluxes then re- 2005; Lu et al. 2008; Fyfe et al. 2012). Hence, elucidating sults in a poleward shift of the edge of the sinking and understanding how the SAM affects subtropical branch of the Hadley circulation. Ceppi and Hartmann climate and, in particular, subtropical precipitation is (2013) argue that the wintertime-mean westerly jet has fundamental to improved predictions on time scales a sharper latitudinal gradient on its equatorward flank from intraseasonal to climate change. than in summer, so that, for the same increase in speed Although many impacts of variations of the SAM on of the midlatitude eddies, less of a poleward shift of the subtropical climate have been elucidated, some key critical line (where the eddy phase speed matches the questions remain about the mechanism and especially background zonal flow) occurs in winter compared to the seasonality. For instance, high-polarity SAM is as- summer, resulting in less of a poleward shift of the sociated with increased precipitation across subtropical momentum flux divergence and therefore less of a shift Australia during austral spring and summer but with of the westerly jet and Hadley circulation. reduced precipitation across the southernmost portions To explain the response in subtropical precipitation to of the continent during winter (e.g., Hendon et al. 2007). the SAM, it is necessary to account for the meridional An explanation for this seasonality was offered by divergence of the eddy-induced mean meridional flow Hendon et al. (2007): subtropical easterly anomalies (i.e., the anomalous vertical motion) in addition to any attributable to high SAM during winter act to decrease meridional displacement of the edge of the Hadley cell precipitation by deflecting poleward and weakening the (cf. Ceppi and Hartmann 2013). The purpose of this midlatitude storm track that is the source of wintertime paper is thus to establish the seasonality of the response precipitation across the south of the country. In contrast, of subtropical precipitation to the SAM by connecting during summer the anomalous easterlies result in on- this to the seasonality of the induced divergent meridi- shore transport of moist air from the warm Coral Sea, onal circulation so as to offer a dynamical explanation. thereby acting to increase precipitation along and inland The outcomes of this work will provide new insight of much of the eastern (subtropical) seaboard. However, into the seasonality of intraseasonal–interannual pre- Karpechko et al. (2009) point out that this explanation dictability of subtropical climate as well as providing an cannot account for the widespread extension of the improved understanding of the seasonality of impacts anomalous precipitation well inland from the east coast. of future climate change in the subtropics. Furthermore, Kang et al. (2011) show that high SAM The observational datasets and analysis techniques during summer results in increased precipitation through- are described in section 2. Composite behavior of zonal- out the SH subtropics, which suggests the increased pre- mean precipitation and circulation associated with SAM cipitation in the Australian subtropics associated with variations are developed seasonally using regression in high SAM during summer is probably related to the section 3. The zonal-mean zonal momentum budget for hemispheric-wide (i.e., zonally symmetric) changes in the the SAM and its implications for the seasonality of the meridional overturning circulation rather than just local induced divergent meridional circulation in the sub- effects along the Australian coast, but the precise mech- tropics are developed section 4. Conclusions are pro- anism needs to be clarified. vided in section 5. Variations in the latitude of the poleward edge of the sinking branch of the Hadley cell are also found to be more strongly related to the latitude (Kang and Polvani 2. Data and methods 2011) and speed (Ceppi and Hartmann 2013) of the a. Datasets extratropical westerly jet in summer than in winter. An explanation for this seasonality has been proposed by The primary observational data used in this study are Ceppi and Hartmann (2013) that builds on Chen and the Interim European Centre for Medium-Range Weather Held’s (2007) hypothesis to account for the recent pole- Forecasts (ECMWF) Re-Analysis (ERA-Interim; Dee ward shift of the Southern Hemisphere extratropical et al. 2011). We use 6-hourly analyses of u, y,and westerlies. In brief, increases in the speed of the mid- v (pressure vertical velocity) on 28 pressure levels from latitude westerly jet, caused by either external forcing the surface to 10 hPa, which are available on a 1.58 grid. or internal variability, result in faster eddies that then Monthly means are formed and anomalies, when re- break at higher subtropical latitudes as they propagate quired, are created by subtracting the monthly climatol- equatorward from their source region in the extra- ogy for 1979–2011. Transient eddy momentum fluxes are tropics, thereby shifting poleward the momentum flux formed using the 6-hourly analyses by first removing the Unauthenticated | Downloaded 09/27/21 10:05 AM UTC 3448 JOURNAL OF CLIMATE VOLUME 27 TABLE 1. Seasonal correlation coefficients r between SAMI and All analyses are based on the 33-yr period of 1979– ~ the Nino-3.4 index and linear trends expressed as the total change 2011.
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