Seasonal Variations of Subtropical Precipitation Associated with the Southern Annular Mode

3446 JOURNAL OF CLIMATE VOLUME 27

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 precipitation 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 precipitation 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

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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

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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. Because the SAMI exhibits a pronounced upward in monthly standardized units for the period January 1979 through summertime trend for this period (Table 1), we also December 2011. linearly remove the trend from the SAMI in order to DJF MAM JJA SON emphasize anomalies associated with internal variations r (SAMI–Nino-3.4)~ 20.51* 20.07 20.36* 20.32 of the SAM. Hence, for the rest of the analysis, SAMI Trend SAMI 0.39* 0.25 0.06 0.05 refers to the detrended and ENSO-removed index un- Trend Nino-3.4~ 20.11 20.11 0.01 20.06 less otherwise noted. * Value significant at the 95% confidence level. Our key analysis technique is to regress monthly anomalies of precipitation, winds, and momentum fluxes onto the monthly standardized time series of the SAMI. The re- monthly mean from each field. The covariance is then sulting regression coefficients then have units that are re- formed and averaged for each month. Precipitation is flective of a one standard deviation anomaly of the SAMI. taken from the Global Precipitation Climatology Project, To elucidate the seasonality of the anomalies associated version 2 analyses (GPCP; Adler et al. 2003). GPCP is with the SAM, we form the regressions separately using a merged analysis that incorporates surface rain gauge monthly data in the four standard seasons [December– observations and satellite precipitation estimates based February (DJF), March–May (MAM), June–August (JJA), on microwave and infrared data. High-latitude (poleward and September–November (SON)]. Statistical significance of ;508S) precipitation in the GPCP analyses, especially of the regressions is assessed using a t test assuming 33 3 3 in winter, is thought to be less reliable (e.g., Adler et al. degrees of freedom (3 months for each of 33 yr). 2012) and so we include a comparison to the precipitation product provided by ERA-Interim. c. Zonal-mean zonal momentum budget b. SAM indices and regression To provide insight into the dynamical forcing of subtropical precipitation by the SAM, we use the zonal-mean The SAM is defined using monthly index values pro- zonal momentum budget to estimate the zonal-mean vided by the U.S. National Weather Service Climate meridional circulation (and its divergence) that is in Prediction Center (CPC; http://www.cpc.ncep.noaa.gov/ balance with the SAM-induced anomalous momentum products/precip/CWlink/daily_ao_index/aao/monthly.aao. fluxes. Kang et al. (2011) used a similar approach to dy- index.b79.current.ascii). We have also developed our re- namically link externally forced trends in the SAM (i.e., sults using other indices of the SAM (e.g., Marshall 2003) ozone depletion) with trends in subtropical precipitation. and the results are qualitatively similar. The CPC SAM We create a covariant momentum budget for SAM var- index (SAMI) is constructed by projecting monthly 700-hPa iations following the approach of DeWeaver and Nigam height anomalies onto the leading empirical orthogonal (2000) by regressing the monthly zonal momentum function (EOF) of monthly-mean 700-hPa height pole- budget terms onto the monthly time series of SAMI. We 8 ward of 20 S. The height data are from the National use overbars to represent monthly means, asterisks for Centers for Environmental Prediction–National Center departures from the zonal mean, and primes to indicate for Atmospheric Research (NCEP–NCAR) reanalysis departures from the monthly mean (recalling that (Kalnay et al. 1996). The EOF analysis was conducted the transient momentum fluxes are computed using on the base period of 1979–2000. 6-hourly data). Signifying the SAM covariant anomaly SAMI negatively covaries with ENSO (Table 1), with using subscript a and the climatological monthly aver- the strongest relationship in austral summer when La age over all years using subscript c and assuming the ~ Nina conditions promote high SAM by directly forcing anomalous relative vorticity is small compared to the tropical precipitation that acts to shift poleward the mean-state absolute vorticity, the SAM covariant mo- equatorward flank of the subtropical jet, thereby acting mentum budget in spherical coordinates can be ex- to shift poleward the critical latitude of equatorward pressed as propagating eddies (e.g., Seager et al. 2003; Lu et al. 2008; Lim et al. 2013). To focus on internal variations of ! the SAM and to avoid confounding effects of ENSO, we ›u 1 › 2(f 1 z )y 52 v 2 fcos2f[(u0y0) remove the influence of ENSO from the SAMI by sub- c a ›p a cos2f ›f a tracting the linear regression onto the Nino-3.4~ SST in- a e › dex that is formed using extended reconstructed SST, 1 (u*y*) ]g 2 [(u0v0) 1 (u*v*) ], version 3b (ERSST.v3b; Smith et al. 2008), which is avail- a ›p a a able online (http://www.cpc.ncep.noaa.gov/data/indices/). (1)

Unauthenticated | Downloaded 09/27/21 10:05 AM UTC 1MAY 2014 H E N D O N E T A L . 3449 where u, y, and v are the zonal wind, meridional wind, each pressure level and assuming that the vertical mo- and the vertical pressure velocity, respectively; f is lat- tion at 100 hPa is zero, itude; p is pressure; ae is the radius of Earth; f is the ð Coriolis parameter; and z 52›u /a ›f is the climato- 1 600 ›y cosf(p) c c e v (600) 52 eddy dp. (3) logical zonal-mean relative vorticity. We have neglected eddy f ›f ae cos 100 in Eq. (1) any residual attributable to momentum dis- sipation, monthly-mean tendencies, and observational We refer to veddy as the eddy-induced vertical velocity. and computational errors. We compare this to the direct regression of vertical ve- We can develop the anomalous meridional wind y as- locity from the reanalyses onto SAMI in order to con- sociated with the SAM using two different approaches. firm that our approximations are valid. The direct method is to regress y directly on to SAMI to The zonal-mean vertical velocity depends on the lat- obtain ya. The second is to solve Eq. (1) for y in order to itudinal gradient of zonal-mean meridional wind; thus, obtain an estimate of the meridional wind that is induced from Eqs. (2) and (3), veddy depends on the latitudinal by anomalous fluxes of momentum associated with the gradient of the SAM-induced momentum flux divergence. SAM, which we refer to as yeddy. Previous work has emphasized the important implication We will show in section 4, consistent with Hartmann of the zeroes in the latitudinal profile of the momentum and Lo (1998), that the dominant term on the rhs of flux divergence for understanding variations in the extent Eq. (1) for SAM variations in the upper troposphere is of the Hadley cell (i.e., the zeroes in the meridional cir- the meridional flux convergence of zonal momentum by culation coincide with the zeroes in the momentum flux the submonthly transients, divergence; Ceppi and Hartmann 2013). Here we emphasize that it is the meridional gradient of anomalous 1 › 2 cos2f(u0y 0) , momentum flux divergence that is key to understanding a cos2f ›f a e the induced vertical motion that is more directly relevant to the low-latitude precipitation anomalies. which is largely balanced by the anomalous meridional To relate the latitudinal profile of the momentum flux advection of mean-state vorticity [lhs of Eq. (1)]. How- convergence/divergence to eddy propagation and absorp- ever, to better close the momentum budget for the SAM, tion at critical latitudes (where the phase speed of the we will also show in section 4 that there is a nonnegligible eddies equals the zonal wind speed and the eddies break contribution in the extratropics from the meridional flux and act to decelerate the zonal flow), we compute the convergence by anomalous stationary eddies space–time power spectrum of the convergence of eddy 1 › momentum flux and display as a function of latitude and 2 cos2f(u y ) 2f ›f * * a phase speed following Randel and Held (1991).Wecom- ae cos pute the space–time power spectrum by Fourier trans- and that the vertical flux convergence of westerly mo- forming 6-hourly u and y at each latitude for each season mentum by submonthly transients (90 days in summer and 92 days in winter). We then form › the space–time momentum flux cospectra (real part of the 2 u0v0 complex cross power spectrum) at each latitude, compute › ( )a p the meridional convergence, and smooth in frequency with is also nonnegligible in the subtropics during winter. 8 passes of a 1–2–1 smoother. The resulting wavenumber– frequency cospectra (momentum flux convergence) are Thus, to very good approximation we simplify Eq. (1) in 2 rebinned into 1 m s 1 phase speed bins, retaining zonal order to estimate yeddy for the SAM from the observed momentum fluxes as wavenumbers 1–15. The climatological cospectrum is " formed by averaging these cospectra over all 33 yr. The 1 1 › SAM anomaly of the momentum flux convergence co- y 5 fcos2f[(u0y0) eddy 1 z 2f ›f a spectrum is formed by regression of the smoothed co- ( f ) ae cos c # spectrum anomalies each year onto the SAMI. › 1 u y 1 u0v0 ( * *)a]g › ( ) a . (2) p 3. Seasonality of SAM-induced subtropical precipitation and circulation anomalies We then estimate the eddy-induced (pressure) vertical motion at the midtroposphere (600 hPa) by vertically The regression coefficients of zonal-mean precipitation integrating the zonal-mean continuity equation down- from GPCP onto the standardized monthly SAMI time ward from 100 hPa using yeddy derived from Eq. (2) at series are shown in Fig. 1 for each season. In this and all

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FIG. 1. Regression of zonal-mean GPCP precipitation (solid black curve with red for significant anomalies at the 90% confidence level) and ERA-Interim precipitation (solid light blue curves with dark blue for significant anomalies) onto the monthly detrended and ENSO-removed SAM index for (top left) March–May, (top right) June–August, (bottom left) September–November, and (bottom right) December–February. Dashed curves are climatological means (red from GPCP 2 2 and blue from ERA-Interim). Scale for the anomalies is on the left (mm day 1)andforthemeanisontheright(mmday 1). subsequent figures, the display convention is for the high- The middle–high-latitude precipitation dipole associ- polarity phase of the SAM, with the amplitude of the ated with SAM is generally better defined in all seasons anomalies being indicative of a one standard deviation based on ERA-Interim precipitation analyses as com- monthly anomaly for SAMI in each season. pared to that from GPCP, and the wintertime anomaly The high phase of SAM is associated with increased in high latitudes is particularly more prominent. We precipitation centered just south of 608S, decreased conclude then that the distinctive dipole anomaly in rainfall centered at about 458S, and increased precip- middle–high-latitude precipitation, which we interpret itation in the deep subtropics from about 208 to 358S (but to represent a poleward shift of the midlatitude storm not evident in winter). In all seasons, the precipitation track, is a robust feature of SAM variability that occurs anomalies in the middle–high latitudes represent a in all seasons. poleward shift of the climatological storm track (dashed The additional positive precipitation anomaly in the curves in Fig. 1 display the climatological precipitation subtropics during the high phase of the SAM, is highly profile). This poleward shift of precipitation in the storm seasonally dependent, being most pronounced in spring track during high SAM (moistening at high latitudes and and summer and largely absent in winter (Fig. 1). A drying in midlatitudes) is consistent with previous ob- similar seasonality is depicted when using ERA-Interim servational and model studies of the impact of the SAM precipitation. We note that the profile of precipitation on extratropical precipitation (e.g., Gillett et al. 2006; anomaly during summer in Fig. 1 is similar to that de- Sen Gupta and England 2007). scribed by Kang et al. (2011), although they did not The positive precipitation anomaly for high SAM at explicitly remove ENSO from their regression. We have high latitudes as depicted in GPCP (Fig. 1) is much less repeated our analysis without removing the influences of distinctive in winter than in the other three seasons, ENSO from SAMI and obtain a nearly identical result which we think is largely because of the unreliability shown in Fig. 1, except that during summer the pre- of the GPCP precipitation analyses at higher southern cipitation anomalies for high SAM are strongly negative latitudes, especially during winter (e.g., Adler et al. over the equator (because of the association of high 2012). As an independent check, we also display in Fig. 1 SAM with La Nina~ during summer; Table 1) and the the regression using the monthly precipitation product positive anomaly in the subtropics is slightly stronger. from ERA-Interim. The precipitation analyses from Nonetheless, we conclude that a shift to high SAM in- ERA-Interim are a model product but are consistent duces increased precipitation in the subtropics during with the resolved dynamics and moisture fields that are summer (and spring and autumn) independently of any constrained by observations in the assimilation cycle. promotion by ENSO.

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We next explore these precipitation anomalies in re- friction. We also see in summer an additional deep lation to the circulation anomalies associated with the meridional cell in the subtropics with a region of up- SAM. We focus on the differences of behavior between ward motion between about 258 and 358S, which is the winter and summer seasons, which are the seasons largely absent in winter. Comparing to the precip- with the most contrasting behavior of precipitation itation anomalies in the subtropics (Fig. 1), we see good anomaly in the subtropics. We note that the SAM be- agreement between increased precipitation and havior in circulation in the transition spring and autumn anomalous upward motion in the subtropics in summer seasons (not shown) is generally consistent with behav- and note that the absence of a subtropical precipitation ior during summer. In Fig. 2, we display the regression anomaly in winter is consistent with the lack of a sub- coefficients for the monthly anomalies of zonal-mean tropical vertical velocity anomaly. zonal wind, momentum flux convergence, and meridio- In summary, SAM-induced precipitation anomalies in nal circulation for high SAM during winter (left panels) the subtropics exhibit a distinct seasonality that is con- and summer (right panels). For reference, we also in- sistent with the seasonality in the concomitant divergent clude the climatological zonal-mean zonal wind for each meridional circulation in the subtropics. This distinctive season, which emphasizes the distinctive occurrence of seasonality in the vertical velocity is highlighted in the subtropical jet in winter while only a single jet struc- Fig. 3a, which shows the SAM anomalies of vertical ture is apparent in summer. velocity at mid troposphere (600 hPa) for summer and Despite the strong seasonality of the mean-state zonal winter. The subtropical zone of interest is highlighted in wind, high-polarity SAM in both seasons is character- gray. For reference, we also include the climatological ized by an equivalent barotropic meridional shift of the vertical velocity, which also exhibits a strong seasonality middle–high-latitude westerly (eddy driven) jet (Fig. 2, whereby the peak mean subsidence in the subtropics in top). In both seasons, the SAM anomaly in zonal wind winter is equatorward and much stronger than in sum- has a dipole structure centered on the midlatitude eddy- mer. It is clear that the SAM anomalies of vertical ve- driven jet (e.g., Thompson and Wallace 2000). Despite locity in the subtropics in winter are much weaker than the relative lack of seasonality of the SAM signal in the in summer, which together with the much stronger mean middle–high latitudes (e.g., Hartmann and Lo 1998), subtropical subsidence in winter means that there will be a clear seasonality in the subtropics is evident, with the much weaker SAM-induced precipitation in the sub- equatorward extension of the easterly anomalies being tropics in winter compared to summer. greater in winter than in summer. The primary role of the anomalous mean meridional The meridional dipole in zonal wind anomaly in circulation for driving the subtropical precipitation middle–high latitudes coincides with a similar dipole of changes is emphasized in Fig. 3b, which shows the re- transient eddy momentum flux convergence/divergence gression of the zonal-mean monthly root-mean-square in the middle–upper troposphere (Fig. 2, bottom), which (rms) transient vertical velocity onto the SAMI. The elucidates the primary mechanism for promotion of the transient vertical velocity is defined as the 6-hourly de- SAM (i.e., the westerly anomalies coincide with mo- viations from the monthly mean and its rms (averaged mentum flux convergence and the easterly anomalies over 1 month) is taken to be representative of rain- coincide with momentum flux divergence; Hartmann fall variability associated with synoptic-scale weather and Lo 1998). The mean meridional circulation anom- systems. We see in Fig. 3b that the magnitude of the alies indicate that anomalous equatorward flow in the anomalous rms transient vertical velocity is comparable upper troposphere develops to balance the anomalous to the zonal-mean vertical velocity in the extratropics westerly acceleration resulting from momentum flux and its latitudinal profile in the extratropics better convergence, and poleward flow develops to balance the matches the anomalous precipitation profile (Fig. 1), anomalous easterly acceleration resulting from mo- consistent with the notion that the precipitation varia- mentum flux divergence. By mass continuity, anomalous tions attributable to the SAM in the extratropics are upward motion occurs poleward of the region of anom- driven largely by the associated shifts in the storm track. alous momentum flux convergence (poleward of 608S) However, in the subtropics, the anomalous zonal-mean and downward motion occurs between the momentum vertical velocity is much larger than the anomalous rms flux convergence and divergence along about 458S. transient vertical velocity, suggestive that it is the direct Near the surface, return poleward flow occurs in asso- changes in zonal-mean vertical velocity that are more ciation with the westerly anomaly and equatorward relevant to the precipitation changes there. The focus of flowoccursinassociationwiththeeasterlyanomaly, the remainder of this paper is to understand this sea- which indicates the anomalous near-surface zonal flow sonality in the response of the subtropical divergent is sustained by a Coriolis torque acting against surface meridional circulation to the SAM.

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21 FIG. 2. (top) Regression of ERA-Interim zonal-mean zonal wind (m s ; shading, scale on right) onto the monthly detrended and ENSO-removed SAMI for (left) June–August and (right) December–February. The climatological- 2 mean zonal wind is contoured (contour interval of 5 m s 1). (bottom) Regression of ERA-Interim transient eddy 2 2 momentum flux convergence (contour interval of 0.1 m s 1 day 1; convergence is light gray and divergence is black) 2 and mean meridional circulation (vectors). The horizontal component of the meridional circulation vector (m s 1)is 2 meridional wind and vertical component is pressure–vertical velocity [multiplied by 2100 Pa s 1, so that upward (downward) motion is indicated by upward (downward) pointing vectors]. The maximum horizontal (vertical) vector 2 2 2 plotted has a magnitude of 0.3 m s 1 (0.3 3 10 2 Pa s 1). The regressed zonal wind anomaly is repeated in (bottom). Only regressed values significant at the 90% confidence level are shown and vectors are thinned to every other vertical level for clarity.

4. Eddy-induced divergent meridional circulation our approximation to develop Eq. (2). In both summer and winter, the meridional convergence of westerly a. Inferences from zonal momentum budget momentum by the transient eddies, which exhibits a di- Prior to using Eqs. (2) and (3) to infer the eddy- pole structure in the middle–high latitudes that supports induced meridional overturning circulation anomalies, the poleward shift of the westerlies (Fig. 2), is dominant, we ﬁrst look at the individual terms on the rhs of the as expected (cf. Hartmann and Lo 1998). However, as anomalous zonal momentum budget in Eq. (1) for the discussed in section 2, we also see that a modest contri- regression onto the SAMI (Fig. 4) in order to conﬁrm bution is made by the meridional convergence attributable

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with a single peak on the equatorward flank of the single jet. The explanation for this different latitudinal profile of momentum flux divergence and the implication for the induced vertical motion in the subtropics are explored below. In Fig. 5a, we display the vertical-mean (100–600 hPa) eddy-induced meridional flow yeddy for high SAM as derived using Eq. (2) and compare to vertical-mean ya from the direct regression of y from the reanalyses onto SAMI. The correspondence between the eddy-induced anomaly and the actual anomaly is excellent in both summer and winter seasons, confirming the utility of Eq. (2). The key feature of the anomalous negative (poleward) flow in the middle and low latitudes during summer that acts to oppose the eddy-induced easterly acceleration (Fig. 4b) is that it abruptly goes to zero at about 308S. In winter, however, this anomalous poleward flow extends all the way to the equator, reflecting a similar slow drop off of the anomalous momentum flux divergence (easterly acceleration) into the tropics, as seen in Figs. 2 and 4a. The impact of these vastly different latitudinal profiles of meridional flow for the induced vertical motion is demonstrated in Fig. 5b, which shows the eddy-induced vertical velocity at the midtroposphere veddy (600) derived using Eq. (3) and also from the direct regression of vertical velocity from the reanalyses onto SAMI (which FIG. 3. (a) Regression of zonal-mean pressure vertical velocity at is a repeat of that shown in Fig. 3). Again, the agreement the 600-hPa level onto monthly SAMI for JJA (solid blue curve) v v and DJF (solid red curve). Positive values indicate upward motion of eddy with a is seen to be excellent. The rapid me- 2 2 (10 2 Pa s 1; scale on left axis). Dashed curves are climatological ridional decay of the momentum flux divergence into 2 2 means (10 1 Pa s 1; scale on right axis). Shaded region indicates the subtropics during summer results in a strongly di- the key subtropical zone emphasized in the text. (b) As in (a), but vergent meridional flow in the subtropics and hence for the zonal-mean root-mean-square transient vertical velocity at strong upward motion there, whereas the slow meridi- the 600-hPa level (dashed curves, scale on left). Solid curves are repeated from (a). onal decay of the momentum flux divergence into low latitudes during winter results in little induced vertical velocity in the subtropics. to anomalous stationary eddies, which typically opposes Although the slow decay of the meridional divergence the transient contribution in the extratropics. The ver- of transient eddy momentum flux from the midlatitudes tical flux convergence by transient eddies also makes into the subtropics (or equivalently the double-peaked a small contribution in the midlatitudes. The remaining momentum flux divergence on either side of the sub- terms in Eq. (1) are negligible, so Eq. (2) is seen to be tropical jet) appears to be the primary cause of the weak a very good approximation of Eq. (1). SAM rainfall signal in the subtropics during winter, the Importantly, the anomalous easterly forcing in the mean-state vorticity associated with the subtropical jet subtropics during high SAM arising from eddy mo- may also contribute. This possibility is motivated by mentum flux divergence (primarily attributable to examination of the mean-state absolute vorticity profiles transient eddies) is seen to extend deep into low lati- in the upper troposphere for summer and winter (not tudes in winter (Fig. 4a) but more abruptly decays to shown but can be inferred from the climatological-mean zero at about 308S in summer (Fig. 4b). This contrasting zonal-mean wind in Fig. 2). In summer, the absolute behavior is also seen in the momentum flux divergence vorticity has weaker magnitude than f everywhere anomalies in Fig. 2, which emphasizes that in winter the equatorward of about 508S, because the relative vortic- anomalous momentum flux divergence has broad me- ity on the equatorward side of the single midlatitude ridional profile with two peaks on either side of the westerly jet is everywhere anticyclonic. In winter, how- subtropical jet while in summer the profile is narrower ever, the presence of a distinctive subtropical jet contributes

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FIG. 4. Terms in the SAM covariant momentum budget [rhs of Eq. (1)]. Terms have been vertically integrated (100–600 hPa). The sum of all terms on the rhs of Eq. (1) is shown as dark gray dashed curve, and the three-term approximation used in Eq. (2) is the purple dashed curve for (a) winter (JJA) and (b) summer (DJF). Units are meters per second per day. to cyclonic vorticity on its poleward flank, acting to double-peaked momentum flux divergence on either make the absolute vorticity more cyclonic than f in the side of the subtropical jet in winter, which thus results in latitudinal range 308–458S. The very strong drop off of weaker latitudinal gradient of eddy momentum flux di- the subtropical jet on its equatorward flank is associated vergence in the subtropics compared to summer. We now with strong anticyclonic relative vorticity, so the mean will explain this meridionally broader, double-peaked absolute vorticity is markedly more anticyclonic than momentum flux divergence during winter from the per- f equatorward of 308S. spective of the impact of the subtropical jet on eddy From Eq. (2), we can infer that, for a given profile of propagation and wave breaking. meridional momentum flux divergence that extends into b. Impact of subtropical jet on eddy propagation the subtropics, inclusion of the mean-state relative vor- and breaking ticity resulting from the subtropical jet would result in stronger compensating poleward flow developing on the The eddies responsible for transporting momentum equatorward flank of the subtropical jet, where the are well explained to behave like Rossby waves that mean-state absolute vorticity has weaker magnitude are primarily propagating horizontally in the middle– than f, and weaker poleward flow developing on the upper troposphere (e.g., Hartmann 2007). The eddies poleward flank of the subtropical jet, where the mean- flux westerly momentum in the opposite direction of state absolute vorticity has greater magnitude than f. their group velocity and tend to converge westerly mo- Hence, the mean-state relative vorticity resulting from mentum into their source region and diverge momen- the subtropical jet contributes to the convergence of the tum into regions where they break and dissipate. They SAM-induced upper-tropospheric meridional flow, tend to propagate toward large values of the meridional centered on about the latitude of the subtropical jet. gradient of mean-state absolute vorticity b* and break This is confirmed in Figs. 5c and 5d, which show the at their critical latitude where their phase speed ap- eddy-induced meridional wind and vertical velocity proaches u and their group velocity tends to zero (e.g., computed with and without inclusion of the mean-state Hoskins and Karoly 1981). The b* typically increases relative vorticity in the absolute vorticity. In summer, toward the equator where u goes to zero, so there is the effect of the mean-state relative vorticity is negligi- a strong tendency for eddies to propagate equatorward ble, whereas during winter the induced vertical motion from their middle–high-latitude source region, where in the subtropics would be slightly more divergent/ they eventually encounter their critical line at low lati- upward if the subtropical jet were not present. Hence, tudes (e.g., Karoly and Hoskins 1982). the mean-state relative vorticity resulting from the Insight into the meridional profile of anomalous mo- subtropical jet in winter does act to damp the eddy- mentum flux convergence/divergence attributable to the induced upward motion in the subtropics attributable to SAM is provided by decomposing the momentum flux the SAM, but this impact appears to be modest. convergence in the upper troposphere (300 hPa) as The most dominant cause of the weaker SAM-induced a function of latitude and eddy phase speed for winter vertical velocity in the subtropics apparently is the (Fig. 6a) and summer (Fig. 6b). The climatological zonal

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FIG. 5. (a) Vertically integrated (100–600 hPa) meridional wind from direct regression onto SAMI (solid curves: red for summer months and blue for winter months) and derived from the SAMI covariant momentum budget Eq. (2) (dashed curves). (b) Vertical velocity at 600 hPa (positive is upward motion) from direct regression onto SAMI (solid curves) and from vertically integrating the continuity equation using the meridional wind anomalies from the SAMI covariant momentum budget [Eq. (3); dashed curves]. (c) As in (a), but for meridional wind from covariant momentum budget including (dashed curves) and excluding (solid curves) the contribution of mean relative vorticity to the mean absolute vorticity. (d) As in (c), but for the pressure vertical velocity at 600 hPa (positive values indicate upward motion). wind (dashed purple curves) and the zonal wind that higher phase speed eddies while the westerlies on the includes the positive SAM anomaly (solid green curves) poleward side of the subtropical jet appear to be are also shown. The SAM anomalies of eddy momentum a source of lower phase speed eddies (e.g., Trenberth flux convergence/divergence (contours in Fig. 6) repre- 1986; Lee and Kim 2003), the anomalous eddy source sent an intensification and poleward shift of the clima- associated with SAM (i.e., the anomalous momentum tological pattern of convergence/divergence in both flux convergence contoured in Fig. 6) during both winter winter and summer (shading in Fig. 6). The climatological and summer is associated primarily with a poleward shift eddy momentum flux convergence spectrum in both of the middle–high-latitude source region. There is also seasons shows familiar convergence in mid and high lat- an indication of a shift of the momentum flux conver- itudes (indicative of the eddy source region) and di- gence to higher phase speed eddies, which Chen and vergence along the critical line in the subtropics where Held (2007) argue accounts for the poleward shift of the the eddy phase speed approaches the background zonal zonal wind for SAM (higher phase speed eddies break wind and the eddies break (e.g., Chen and Held 2007). at higher latitudes, thereby shifting the momentum flux Because the mean zonal wind decreases toward the equa- and hence the midlatitude jet poleward). However, the tor, the peak eddy momentum flux divergence in low lati- key feature to emphasize here is that the anomalous mo- tudes shifts to slower phase speed with decreasing latitude. mentum flux divergence in the midlatitudes–subtropics Although the climatological eddy source (i.e., mo- during winter (Fig. 6a) clearly shows a preference for mentum flux convergence shaded red in Fig. 6) during breaking to occur on the equatorward flank of the mid- winter is much broader than in summer, suggesting that latitude jet (anomalous momentum flux divergence the middle–high-latitude westerlies act as the source of peaked at about 408S) primarily because of higher phase

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FIG. 6. Latitude–phase speed spectra of meridional momentum flux convergence at 300 hPa for (a) winter and 2 2 (b) summer. Climatological-mean values are shaded (m s 1 day 1 per unit phase speed bin; scale on righ) and re- 2 2 gressed anomalies onto the SAMI are contoured (contour interval of 0.0008 m s 1 day 1 per unit phase speed bin, 2 where negative values are dashed; first contour at 60.0004). The phase speed bin has a width of 1 m s 1. The dashed purple curve is the climatological zonal-mean zonal wind at 300 hPa. The solid green curve is the climatological zonal- mean zonal wind plus one standard SAMI anomaly. The scale for the mean zonal wind is the same as for phase speed 2 on x axis (m s 1). speed eddies but breaking extends onto the equatorward lower phase speed eddies do not break until they en- flank of the subtropical jet (anomalous momentum flux counter their critical line on the equatorward flank of divergence peaked at about 208S) because of lower phase the subtropical jet. speed eddies. In contrast, during summer, the anomalous Close inspection of Fig. 6a suggests peak anomalous eddy momentum flux divergence for eddies of all phase eddy breaking on the equatorward flank of the mid- speed aligns along the equatorward flank of the single jet latitude jet occurs well before the eddies have encoun- but is shifted poleward of its climatological profile. tered their critical line (e.g., peak momentum flux 2 The latitudinal span of anomalous momentum flux di- divergence for eddies with phase speed of ;15 m s 1 at vergence attributable to the SAM thus extends from ;408S occurs some 258 poleward of where the mean 2 about 458 to 158S in winter. In contrast, during summer zonal wind is 15 m s 1 on the equatorward flank of the the region of anomalous momentum flux divergence oc- subtropical jet). A poleward displacement of peak di- curs over a narrower range of latitudes (458–258S). Hence, vergence (eddy breaking) from the critical latitude was the latitudinal gradient of momentum flux divergence in previously noted by Randel and Held (1991), who sug- the subtropics attributable to the SAM is weaker in winter gest that the interaction of the eddies with their critical than in summer, so a weaker divergent meridional circu- line is nonlinear and that the distance the wave breaks lation in the subtropics will be induced. Therefore, the away from the critical line is proportional to the eddy upward motion and precipitation response in the sub- amplitude. However, this apparent breaking well pole- tropics is weaker in winter than in summer. ward of the critical latitudes might possibly be an arte- Breaking of eddies on the flanks of both the eddy- fact of forming the zonal mean across two distinctive driven and subtropical jet during winter has previously regions where one has a strong subtropical jet but with been reported in both idealized model studies and ob- weak midlatitude jet (i.e., the Indo-Pacific sector from servations by Barnes and Hartmann (2012). They show about 908Eto908W) and the other has a strong mid- that breaking between the jets is intensified when the latitude jet but weak subtropical jet (the east Pacific– westerly jet and the subtropical jet are distinct and well Atlantic sector from about 908Wto908E; referred to separated (i.e., like high SAM during winter) but that here as the Atlantic sector). For instance, it is possible some breaking still occurs on the equatorward flank of that most of the breaking (momentum flux divergence) the subtropical jet. We emphasize here that it is pri- of low phase speed eddies on the equatorward side of the marily higher phase speed eddies that break on the subtropical jet might be occurring in the Indo-Pacific equatorward flank of the midlatitude jet where these sector where the subtropical jet is most prominent. eddies apparently first encounter their critical line, while Most of the breaking of higher phase speed eddies on the

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21 21 FIG. 7. Regression of transient eddy momentum flux convergence (contour interval of 0.2 m s day ; convergence is solid and divergence is dotted) onto monthly SAMI in JJA for (a) the Atlantic–east Pacific sector (908W– 908E) and (b) the Indo-Pacific sector (908E–908W). The climatological-mean zonal wind in each sector is shaded (scale on right). equatorward flank of the midlatitude jet might be occurring our notion that higher phase speed eddies break on the in the Atlantic sector, where the midlatitude jet is more critical line set by the midlatitude jet and lower phase prominent. However, as we will show below, this is not the speed eddies break on the critical line set by the sub- case, with a broad profile of momentum flux divergence in tropical jet. In the Indo-Pacific sector, the profile of the subtropics occurring in both sectors. momentum flux divergence (breaking) is more difficult We demonstrate this by recomputing the latitude– to interpret, with maximum divergence occurring on the height regressions of transient eddy momentum flux cyclonic (poleward) side of the more prominent sub- convergence onto SAMI in the two sectors during winter tropical jet but still with a broad extension of divergence (Fig. 7). In the Atlantic sector, where the mean sub- onto the anticyclonic (equatorward) side of the subtropical jet is weaker, the anomalous extratropical mo- tropical jet. This location of maximum momentum flux mentum flux convergence/divergence dipole straddles divergence on the cyclonic (poleward) flank of the the location of the eddy-driven jet, indicative that the subtropical jet might be indicative of refractive effects SAM in this sector is typified by a north–south shift of the resulting from the strong cyclonic vorticity on the eddy-driven jet. In contrast, in the Indo-Pacific sector, poleward flank of the subtropical jet which would act where the subtropical jet is pronounced although an to prevent equatorward propagation of eddies emanat- eddy-driven jet is still apparent but at higher latitude than ing from the midlatitudes (e.g., Thorncroft et al. 1993; in the Atlantic sector, the anomalous momentum flux Eichelberger and Hartmann 2007) and would also help convergence more closely aligns with the eddy-driven jet, account for the pulsing rather than shifting nature of the so the SAM in this sector is better described as a pulse SAM in this sector. However, the key point here is that rather than a shift. These sectoral differences in the breaking (momentum flux divergence) appears to be characteristics of the SAM are consistent with Codron spread into lower latitudes because of the presence of (2007), who used slightly different sectoral averages. the subtropical jet in both sectors so as to result in weak They are also consistent with Eichelberger and Hartmann induced divergent motion in the subtropics, although the (2007), who show that the SAM is characterized as details of eddy breaking and interaction with the mean a pulse when the subtropical jet is strong and as a shift flow in the presence of a distinctive subtropical jet needs when the subtropical jet is weak. further investigation. However, the focus here is on the profile of momentum flux divergence in lower latitudes. We see that in the 5. Conclusions Atlantic sector (Fig. 7a) there is distinctive breaking (divergence) on the anticyclonic (equatorward) side of SAM is primarily characterized as a north–south both the midlatitude jet and the subtropical jet (even fluctuation of the midlatitude, eddy-driven, westerly jet though the subtropical is relatively weak), supportive of with concomitant latitudinal shifts in the midlatitude

Unauthenticated | Downloaded 09/27/21 10:05 AM UTC 3458 JOURNAL OF CLIMATE VOLUME 27 storm track. High–middle-latitude precipitation varies profile of breaking that accounts for the induced di- in concert with these north–south shifts of the storm vergent meridional circulation in the subtropics which track, so that in the high phase of the SAM, when the jet accounts for the subtropical precipitation response to is shifted poleward, precipitation increases on the the SAM. poleward flank of the midlatitude storm track and de- The lack of a subtropical precipitation anomaly during creases on its equatorward flank. However, SAM also winter means that the net impact of high SAM during affects lower-latitude circulation and precipitation. In winter can be characterized as a poleward expansion of particular, during spring–autumn high SAM results in the subtropical dry zone (e.g., Scheff and Frierson 2012). increased precipitation in the subtropics. This sub- In contrast, during summer (and spring and autumn) the tropical precipitation anomaly is absent during winter. net impact of high SAM in the subtropics is better We have shown that the subtropical precipitation vari- characterized as a poleward shift of the subtropical dry ations associated with the SAM are consistent with the zone: the wet-equatorward edge and the dry-poleward eddy-induced divergent meridional circulation in the edge of the subtropical dry zone move poleward in subtropics that is driven by the SAM. The absence of unison (e.g., Kang et al. 2011). the subtropical precipitation anomaly in winter stems This seasonality of impact of the SAM on subtropical from the absence of a concomitant induced divergent precipitation has important implications for predicting meridional circulation. current and future climate. For instance, Hendon et al. We attribute this lack of impact in the subtropics (2013) show that the state of the SAM plays a leading during winter to the presence of the distinctive sub- role in determining the warm season (spring–summer) tropical jet, which acts to cause higher phase speed rainfall anomaly in subtropical eastern Australia during eddies that originate in the higher-latitude westerlies La Nina~ events. The occurrence of high SAM would add during positive excursions of the SAM to break between to the La Nina–induced~ wet anomaly and low SAM the midlatitude and subtropical jets, while lower speed would detract. Thus, limited predictability of the SAM eddies propagate equatorward deep into the subtropics, on seasonal time scale (e.g., Lim et al. 2013) places where they break on their critical line on the equator- a strong constraint on predictability of subtropical ward flank of the subtropical jet. In summer, high and rainfall, even during highly predictable La Nina~ events. low phase speed eddies break in a narrower range of High SAM has also been promoted by ozone depletion latitudes on the equatorward flank of the single mid- (e.g., Arblaster and Meehl 2006; Gillett et al. 2013). latitude westerly jet, resulting in a more rapid meridio- Hence, a primary impact of the recent upward trend in nal decay of momentum flux divergence into the the SAM has been an upward trend in summertime subtropics. Thus, the latitudinal gradient of anomalous subtropical precipitation (Kang et al. 2011), which can momentum flux divergence in the subtropics associated be viewed as a poleward shift of the subtropical dry with the SAM is much weaker in winter than in summer, zone. However, a robust finding of projected future so the eddy-induced anomalous divergent meridional climate in response to increased greenhouse gases is circulation (and therefore upward motion) in the sub- a promotion of high SAM in all seasons (e.g., Yin 2005; tropics is weaker in winter than in summer. Gillett et al. 2013). The emergence of this upward trend This explanation for the seasonality of the eddy- of the SAM in winter would then result in an expanded induced divergent circulation in the subtropics in re- subtropical dry zone in winter (e.g., Scheff and Frierson sponse to the SAM builds on the mechanism proposed 2012), which would have severe consequences for in- by Ceppi and Hartmann (2013) to account for the sea- stance on water resources in subtropical Australia and sonality in the sensitivity of position of the poleward South America, where there is a strong dependence on edge of the Hadley circulation to SAM variations. They wintertime rainfall. argue that, because the wintertime-mean westerly jet Finally, we have emphasized a key role of the has a sharper latitudinal gradient on its equatorward Southern Hemisphere wintertime subtropical jet for side than in summer, the same increase in speed of the determining the impact of SAM variability on sub- midlatitude eddies associated with the SAM leads to less tropical precipitation. Although the impact of variations of a poleward shift of the low-latitude critical line, ulti- of the subtropical jet on the SAM attributable to direct mately resulting in less of a poleward shift of the edge of forcing by ENSO has been previously considered (e.g., the Hadley circulation in winter compared to summer. Seager et al. 2003; L’Heureux and Thompson 2006; Lim Their argument does not take into account the impact of et al. 2013), future changes in the Southern Hemisphere the wintertime subtropical jet on the latitudinal distri- subtropical jet—for instance, as driven by a reduced bution of breaking of the eddies that originate in the outflow from a weakened Asian summer monsoon in midlatitude westerly jet and that it is this latitudinal response to anthropogenic aerosols (e.g., Turner and

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