Structure and Mechanisms of the Southern Hemisphere Summertime Subtropical Anticyclones

Transcription

1 Structure and Mechanisms of the Southern Hemisphere Summertime Subtropical Anticyclones by Takafumi Miyasaka # and Hisashi Nakamura * Department of Earth and Planetary Science University of Tokyo, Tokyo, Japan Submitted to J. Climate, December 2008 Revised, July 2009 Revised further, November 2009 # Corresponding author: Takafumi Miyasaka, Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Tokyo, JAPAN. miyasaka@eps.s.u-tokyo.ac.jp * Additional affiliation: Research Institute for Global Change, JAMSTEC, Yokohama, Japan 1

2 Abstract Three-dimensional structure and dynamics of the climatological-mean summertime subtropical anticyclones in the Southern Hemisphere (SH) are investigated. As in the Northern Hemisphere (NH), each of the surface subtropical anticyclones over the South Pacific, South Atlantic and South Indian Oceans is accompanied by a meridional vorticity dipole aloft, exhibiting barotropic and baroclinic structures in its poleward and equatorward portions, respectively, in a manner dynamically consistent with the observed mid-tropospheric subsidence. Their dynamics is also similar to their NH counterpart. It is demonstrated through our numerical experiments that each of the SH surface anticyclones observed over the relatively cool eastern oceans can be reproduced as a response to a local near-surface cooling-heating couplet. The cooling is due mainly to radiative cooling associated with low-level maritime clouds, and the heating to the east is due to sensible heat flux over the dry, heated continental surface. The low-level clouds act to maintain the coolness of the underlying ocean surface, which is also maintained by the alongshore surface southerlies. As in the NH, the presence of a local atmosphere-ocean-land feedback loop is thus suggested, in which the summertime subtropical anticyclones and continental lows to their east are involved. Both our model experiments and diagnosed upward flux of Rossby wave activity suggest that, in addition to continental deep convective heating, the land-sea heating/cooling contrasts across the west coasts of the three continents can contribute to the formation of the summertime upper-level planetary-wave pattern observed in the entire subtropical SH, characterized by the zonal wavenumber-3 component. Though rather subtle, there are some inter-hemispheric differences in the summertime subtropical anticyclones, including their smaller magnitudes in the SH and stronger equatorward 2

3 propagation of upper-level Rossby wave activity emanating from the SH surface anticyclones. 3

4 1. Introduction Most of the subtropical anticyclones reside over the eastern portions of the subtropical ocean basins throughout the year. In the Northern Hemisphere (NH), they show apparent seasonal variability in their strength and configuration. Each of them strengthens and develops into a basin-wide cell-type configuration in summer. The surface wind stress curl associated with these anticyclones contributes to the maintenance of subtropical oceanic gyres, with warm poleward western boundary currents and cool equatorward currents off the west coasts of the continents. The strong equatorward alongshore winds to the east of the anticyclones also contribute to the maintenance of underlying cool sea-surface temperatures (SSTs) by enhancing surface evaporation, coastal upwelling and entrainment at the bottom of the oceanic mixed layer. Also in the Southern Hemisphere (SH), the summertime surface subtropical anticyclones are in a cell-type configuration (as shown below), suggesting that their dynamics may be understood in essentially the same framework as applicable to their NH counterpart. As argued by Hoskins (1996) and Rodwell and Hoskins (2001), theory of a zonally-symmetric Hadley circulation predicts much weaker subtropical subsidence in the summer hemisphere than in the winter hemisphere (Lindzen and Hou 1988). In relation to the formation of zonally-asymmetric circulation in the subtropics as a manifestation of the climatological summertime planetary waves, the surface NH subtropical anticyclones have been argued as a steady response to monsoonal convective heating to their east (Hoskins 1996; Rodwell and Hoskins 2001) or west (Chen et al. 2001) or to shallow heating/cooling couplets across the west coasts of the subtropical continents (Liu et al. 2004; Nakamura and Miyasaka 2004; Miyasaka and Nakamura 2005, hereafter cited as MN05). 4

5 Specifically, the argument by Hoskins (1996) is based on the monsoon-desert mechanism (Rodwell and Hoskins 1996). Rodwell and Hoskins (2001) concluded that the combined effect of topography and monsoonal heating to the east is of primary importance for the generation of a surface subtropical anticyclone and subsidence aloft (e.g., the Indian, Mexican and South American monsoons for the subtropical anticyclones in the North Atlantic, North Pacific and South Pacific, respectively), with the reinforcement by local cooling over the eastern oceans. However, Wu and Liu (2003) suggested the potential importance of the combined effect of surface sensible heating observed over the dry western portions of subtropical continents and lower-tropospheric radiative cooling off their west coasts in the formation of the subtropical anticyclones. In fact, stationary response to prescribed diabatic heating in the numerical experiments by Liu et al. (2004), Nakamura and Miyasaka (2004) and MN05 have shown the importance of the shallow continental heating and maritime cooling in the formation of surface NH subtropical anticyclones. Through coupled general circulation model (GCM) experiments, Seager et al. (2003) also suggested potential importance of the local air-sea interaction for the subtropical anticyclones in attaining their full strength, although they argued that the monsoonal influence is particularly important. The presence of cool SSTs and a mid-tropospheric subsidence associated with the anticyclone favors the local development of marine stratus in the planetary boundary layer (PBL; Klein and Hartmann 1993). High albedo of the clouds also acts to maintain the underlying cool SSTs (Hartmann et al. 1992), which may imply the involvement of a local air-sea feedback system. The present study is an extension of Nakamura and Miyasaka (2004) and MN05 on the summertime NH subtropical anticyclones into their SH counterparts. Similarly to the NH, 5

6 marine stratus and stratocumulus develop off the west coasts of the SH subtropical continents (Klein and Hartmann 1993), while surface sensible heat flux is strongly upward over the western portions of those continents (Wu and Liu 2003; Liu et al. 2004). In their modeling study, Rodwell and Hoskins (2001) investigated the dynamics of the South Pacific subtropical anticyclone. In the thermal forcing in their model, contributions from monsoonal convective heating and surface sensible heating over the South American continent were not well separated, because they assigned diabatic heating to all the model tropospheric levels. Seager et al. (2003) showed that the monsoon-desert mechanism exerts a weaker impact in the SH than in the NH. They concluded that oceanic processes, which may be driven by the wind stress associated with the subtropical anticyclones, are more important for the maintenance of low SSTs on the eastern portions of the South Pacific and South Atlantic and thus for the formation of the subtropical anticyclones. Through experiments with their regional model in which marine stratiform clouds are reproduced realistically, Wang et al. (2005) showed that radiative cooling associated with those clouds in the eastern South Pacific can reinforce the subtropical anticyclone in austral winter and spring. The aforementioned findings seem consistent with the hypothesis that the subtropical anticyclones in the SH summer can be formed through the essentially same dynamics as in the NH summer. Nevertheless, the three-dimensional structure of each of the SH anticyclones and the relative importance between the low-level, land-sea thermal contrast and monsoonal convective heating in the formation of each of those anticyclones have not been fully understood. In the present study, the three-dimensional structure and mechanisms of the three SH subtropical anticyclones are investigated through data analysis and numerical modeling as 6

7 in MN05. Although the monsoon-desert mechanism argued by Hoskins (1996) and Rodwell and Hoskins (1996, 2001) may be important in maintaining the dryness of the western portions of the continents and thereby setting fundamental zonal asymmetries in the diabatic heating pattern, the present study suggests that the summertime climatological SH subtropical anticyclones can essentially be interpreted in the framework of local atmosphere-ocean-land interactions as their NH counterpart. 2. Data and atmospheric model a. Data and analysis methods In the present study, we use global circulation data based on the National Centers for Environmental Prediction-Department of Energy (NCEP-DOE) reanalyses (Kanamitsu et al. 2002) from 1979 to As in MN05, we examine the surface subtropical anticyclones and their three-dimensional structure in the zonally asymmetric component of the climatological-mean SH circulation for January. In our framework, this component is regarded as being associated with the climatological planetary waves embedded in the zonally uniform westerlies. A wave-activity flux formulated by Plumb (1985) is suited for diagnosing group-velocity propagation of the planetary waves. For the log-pressure coordinate, it may be expressed as 1 *2 * * ( ψ ) 2 2 λ ψ ψ λλ a cos φ pˆ cosφ 1 * * * * W = ( ψ ) 2 λψ φ ψ ψ λφ, (1) 2 a cosφ 2 f 1 * * * * ( ψ ) 2 λψ z ψ ψ λz N a cosφ where ψ denotes geostrophic streamfunction, a the earth s radius, (λ, φ) longitude and 7

8 latitude, respectively, f the Coriolis parameter, N the Brunt-Väisäla frequency, and pˆ pressure (p) normalized by 1000 (hpa). In (1), subscripts and asterisks signify partial derivatives and zonal asymmetries associated with planetary waves, respectively. In theory, the flux W is independent of wave phase and parallel to the local group velocity of planetary waves. The flux tends to diverge out of forcing regions. See Takaya and Nakamura (2001) for the physical meaning of the individual terms of W. Vertical motion associated with vorticity and heat transport by planetary waves can be diagnosed with an omega equation linearized about the zonally-uniform flow: 2 + f S 2 2 p 2 * * [ u ] θ + v [ θ ] * * ([ ug ] ζ gx + vg ( β [ ug ] )) * 2 g x g y f ω = +, (2) yy ( / p) Θ S p where ω denotes pressure velocity, (u g, v g ) geostrophic wind velocity, and θ potential temperature. In (2), brackets signify the zonally-uniform basic-state quantities, and S = ( α / Θ)( Θ / p) represents static stability, where α denotes specific volume and Θ the basic-state θ. As in MN05, the potential-vorticity (PV) inversion technique (Hoskins et al. 1985) is used to assess an impact of a zonally asymmetric distribution of surface temperature upon the tropospheric circulation. It is based on the invertibility principle for PV (q), which indicates that surface θ* can act as PV anomalies (q*). Specifically, warm and cold anomalies at the surface are equivalent to cyclonic and anticyclonic PV anomalies, respectively. In practice, the observed 1000-hPa θ* is assigned as the lower-boundary condition with no q* assigned in the free atmosphere and at the upper boundary. The combination of the PV inversion and the omega-equation diagnosis (2) enables us to evaluate mid-tropospheric vertical motion that would be induced in the presence of a given 8

9 θ* distribution at the surface. b. Atmospheric model As in MN05, a global primitive-equation model with simplified physical processes is used to investigate the atmospheric response to a given zonally-asymmetric distribution of tropospheric diabatic heating Q*. The model, originally developed at the University of Reading (Hoskins and Simmons 1975), has been modified for the study of summertime planetary waves as used by Rodwell and Hoskins (2001) and MN05. Details of the model can be found in Hoskins and Rodwell (1995). The nonlinear spectral model with a triangular spectral truncation at wavenumber 42 (T42) has 30 vertical levels in σ coordinate. In order to resolve shallow Q* confined mainly into the PBL, the model vertical resolution has been set twice as high as in Rodwell and Hoskins (2001). Throughout each of the model integrations, the zonal-mean state was fixed to the climatological-mean state taken from the NCEP-DOE reanalyses. In the model it took 10~15 days for a surface subtropical anticyclone to develop after Q* had been switched on. We therefore examine 10-day mean fields from the 16-th to 25-th day of the integrations. In experiments with orography, the surface elevation had been raised gradually over a 5-day period before Q* was imposed. In our experiments, Q* measured as the rate of a local temperature change was taken from the NCEP-DOE reanalysis data at the 28 σ-levels of the reanalysis system and then interpolated onto the 30 equally-spaced model levels from σ = to σ = In some of our experiments, Q* was assigned with its full intensity only within a given rectangular domain or a latitudinal band. Outside of the domain, the intensity of Q* was reduced linearly in such a manner that Q* vanishes within 10 in latitude or longitude away from 9

10 the domain boundaries. Strictly speaking, the heating outside of the domains does not necessarily be zero locally, since the zonal average of Q* has to remain zero in our experimental design. 3. Observations a. Three-dimensional structure Figure 1 shows the climatological-mean January sea-level pressure (SLP) field for the SH. As in the summertime NH, a cell-type subtropical anticyclone is observed over each of the three SH ocean basins (i.e., the South Pacific, South Atlantic and South Indian Oceans). The anticyclonic center positioned at ~30 S is displaced zonally to the eastern portion of the basin with isobars almost parallel to the west coast of the nearby continent. Strong alongshore equatorward winds are thus observed in the eastern flank of the anticyclone. The vorticity balance near the surface is achieved mainly by the cyclonic planetary vorticity advection due to the alongshore winds and vorticity-tube shrinking associated with mid-tropospheric subsidence (Fig. 1b), in agreement with Rodwell and Hoskins (2001). In the upper troposphere, anticyclonic and cyclonic vorticity centers located above the poleward and equatorward portions of a particular surface anticyclone (Fig. 1d) can both induce anticyclonic vorticity tendencies to their immediate east by the mean zonal advection and by anomalous planetary-vorticity advection, respectively (not shown). These tendencies tend to be in balance with the offsetting effect due to the local subsidence. The upper-level meridional vorticity dipole is a manifestation of barotropic and baroclinic structure in the poleward and equatorward portions, respectively, of a given surface anticyclone. These features are essentially the same as in the summertime NH as observed 10

11 by White (1982), Wallace (1983) and MN05. Reflecting the tendency that upper-level vorticity dipoles in the SH are weaker than in the NH, double-jet structures in upper-level westerlies are less clear above the SH subtropical anticyclones (Fig. 1c), especially over the South Atlantic anticyclone. Nevertheless, in a manner consistent with the presence of the meridional vorticity dipoles, upper-level westerly wind speed (u g ) increases eastward above the equatorward portions of those subtropical anticyclones, including the South Atlantic anticyclone (see the curvature of the isotachs in Fig. 1c). The primary momentum balance u g ( u / x) fv requires g a poleward ageostrophic motion (v a < 0) above the anticyclones where u g / x > 0 (Fig. 1e). The poleward ageostrophic motion appears to be the primary contributor to the upper-level convergence and the associated mid-tropospheric subsidence over the SH surface anticyclones (Fig. 1b). In addition, equatorward ageostrophic motion (v a > 0) from mid-latitudes also contributes to the convergence over the Southeastern Indian Ocean (Fig. 1e). This motion is required in the momentum balance in the downstream regions of the polar-front jet cores (Fig. 1c; Nakamura and Shimpo 2004). Thus, the summertime surface subtropical anticyclones in the SH are similar to their NH counterparts with respect not only to their three-dimensional structure but also to their dynamical coupling with the upper-level vorticity dipoles. Though somewhat weaker than in the reanalysis data, the mid-tropospheric vertical motion, including the subsidence over the subtropical anticyclones, can be reproduced through a diagnosis based on the omega equation (2) (Fig. 2a), which is linearized about the climatological zonal-mean state for the SH summer and forced with the heat and vorticity transport associated with the observed planetary waves (Fig. 2a). This result indicates that those subtropical anticyclones are a fundamental 11

12 element of the climatological planetary wave in the SH summer. b. Wave-activity propagation The structural relationship between the surface subtropical anticyclones and upper-level planetary waves has been investigated in relation to the structure of diabatic heating (Wu and Liu 2003; Liu et al. 2004) and by using Plumb s (1985) wave-activity flux (MN05). We conduct the same wave-activity flux diagnosis as in MN05 to gain some insight into the forcing region of the upper-level planetary waves in focusing particularly on its vertical component in the mid-troposphere. Over the summertime SH, the horizontal component of the wave-activity flux (W) associated with the upper-tropospheric planetary waves shown in Fig. 2b indicates no apparent incoming wave train into any of the core regions of the subtropical anticyclones in the South Pacific and South Indian Oceans, where eastward W diverges out of the vorticity dipoles with its upward component in the mid-troposphere above the surface anticyclones (Fig. 2c). While weak influx of wave activity is observed over the South Atlantic, the mid-tropospheric wave-activity flux is slightly upward to the east of the surface anticyclone. The upward wave-activity flux suggests a source of the SH summertime planetary waves located in the lower troposphere associated with the surface anticyclones, as observed in the NH summer (Nakamura and Miyasaka 2004; MN05). In fact, in the subtropical SH (20 ~40 S), the zonal wavenumber-3 component seems dominant in the upper-level planetary wave field (Fig. 1d), showing some correspondence with the three high-pressure cells at the surface. This feature is in sharp contrast with the dominance of the zonal wavenumber-1 component poleward of 45 S. Interestingly, poleward wave-activity propagation is evident from the 12

13 subtropical anticyclone over the Southeastern Indian Ocean. This propagation appears to add a signature of higher zonal wavenumbers to the mid-latitude planetary wave field to the south of Australia and the western South Pacific. In the SH summer, equatorward wave-activity dispersion over the subtropical anticyclones tends to be stronger than in the NH summer. This is probably because upper-tropospheric zonal-mean westerlies extend more equatorward in the summertime SH than in the summertime NH (see the vertical arrows along the right axis of Fig. 2b). In fact, at the 250-hPa level the critical latitude for stationary Rossby wave over the summertime SH is displaced equatorward by more than 10 in latitude relative to its NH counterpart (13 S versus 25 N). In fact, the wave-activity flux is evident from each of the vorticity dipoles to a cyclonic anomaly located to its northeast (Fig. 2b). The northeastward propagation is particularly strong over the eastern South Pacific. c. Diabatic heating and land-sea thermal contrasts As in the summertime NH, the summertime SH subtropics is characterized by strong land-sea heating/cooling contrasts across the west coasts of the continents (Fig. 3). As pointed out by Wu and Liu (2003), continental heating near the west coast is associated with near-surface sensible heating over the dry land surface heated by strong insolation. In fact, over the South African continent and Australia, diabatic heating Q* is maximized near the surface (Fig. 3b). Over the South American continent, however, both the near-surface sensible heating and deep convective heating above the eastern slope of the Andes contribute to the continental heating Q*. In the reanalysis data, the maritime cooling, which peaks in the lower troposphere at the level of σ = 0.85, is associated mainly with the 13

14 radiative cooling due to marine stratus and/or stratocumulus. In order to confirm the potential impact of the pronounced summertime land-sea thermal contrasts in the SH subtropics upon the maritime anticyclones, continental lows and strong alongshore winds at the surface, the PV inversion technique was applied to the observed field of the zonally asymmetric component of 1000-hPa temperature θ* (Fig. 4a). As in the NH case (MN05), the induced surface winds are the strongest along the west coasts of the subtropical continents across which zonal θ* contrasts are most pronounced. The induced alongshore equatorward winds advect cooler air and negative (i.e., cyclonic) planetary vorticity, which can be balanced if the mid-tropospheric subsidence and associated near-surface divergence are induced, respectively. In fact, subsidence is evident just to the east of each of the surface subtropical anticyclonic centers, in the field of vertical motion (Fig. 4b) as diagnosed by substituting the wind and temperature fields obtained through the PV inversion (Fig. 4a) into the omega equation (2) that has been linearized about the climatological zonal-mean state for January. The diagnosed subsidence is weaker than in the observations, since the PV anomalies were assigned only at the surface. The diagnosed descent is shifted slightly to the east of its observational counterpart (Fig. 1b), because the induced surface wind over the land surface is much stronger than in the real atmosphere due to the lack of surface friction in the PV inversion. Nevertheless, the PV inversion and the subsequent diagnosis of the induced vertical motion have confirmed the potential importance of the surface thermal contrast across the west coast of a subtropical continent in the formation of a SH summer subtropical anticyclone, as has been revealed for the NH summer (MN05). 14

15 4. Model simulations a. Remote versus local influences As in MN05, the primitive-equation planetary-wave model as described in section 2b was used to examine the reproducibility of the zonally asymmetric component of climatological-mean SH summertime tropospheric circulation, including the surface subtropical anticyclones. Compared with their observational counterparts (Fig. 5a), the maritime subtropical anticyclones and continental lows are well reproduced in the model SLP response (Fig. 5b) to the zonally-asymmetric diabatic heating (Q*; cf. Fig. 3a) assigned globally at all the vertical levels in the model with global topography ( control experiment ). This result is in good agreement with Rodwell and Hoskins (2001). Over the South Atlantic and Pacific, the surface anticyclones are reproduced with their full intensities. In another experiment in which the prescribed Q* is limited to a latitudinal band of 40 ~10 S (Fig. 5c), no substantial reduction is found in the magnitude of any of the subtropical anticyclones. Rather, the anticyclone in the South Indian Ocean (i.e., Mascarene high) is enhanced to attain its observed intensity. A comparison between Figs. 5b and 5c reveals that remote influence either from the Tropical convection or from higher latitudes does not act as the primary direct forcing for any of the SH summertime subtropical anticyclones. The same is the case for their NH counterpart (MN05). b. Model response to a low-level heating/cooling couplet in each of the ocean basins In order to substantiate the importance of the local land-sea thermal contrasts in the formation of the SH subtropical anticyclones in January, additional numerical experiments 15

16 were performed with the same model but driven by Q* prescribed only in the lower troposphere (below the σ = level) within the regional domains as indicated with heavy solid lines in Figs. 3b and 3c. In the first model experiment of this kind, the atmospheric response to the low-level Q* prescribed within the Southeastern Pacific/South American domain (Fig. 6a) is investigated. The continental heating is associated mainly with sensible heat flux over the (semi-) desert region in the subtropics and mid-latitudes and convective heating in the Tropics (Fig. 3). Though maximized at the σ = 0.4 level, the convective heating extends into the lower troposphere (Fig. 3b). The offshore cooling confined to the lower troposphere (Fig. 3b), which is associated with marine stratus/stratocumulus, extends more equatorward than in the summertime NH. In the particular experiment, both the maritime cooling and continental heating were assigned together, because the PV thinking predicts that the strength of the alongshore winds should be related to the zonal contrast of the surface temperature, but not solely to either the heating or cooling. This is also because the zonally-integrated Q* is much weaker for the heating/cooling couplet than for either the heating or cooling alone and thus more suited for our planetary-wave model experiment. To isolate the response to Q*, no topography was imposed in the model, although we have confirmed that the model surface response is insensitive to the inclusion of topography (not shown). As a stationary response to the imposed Q* (Fig. 6), a surface temperature contrast across the west coast of the South American continent, though somewhat overestimated, is reproduced reasonably well in the model (Figs. 2d and 6b). The subtropical anticyclone and alongshore equatorward winds are also reproduced reasonably well in the surface response (Fig. 6c), although the simulated anticyclonic center is shifted equatorward by ~10 in 16

17 latitude compared with the observations (Fig. 5a). We have confirmed that the surface anticyclonic response is reproduced well solely with the maritime cooling (not shown). The intensity of the surface anticyclone (Fig. 6c) reaches as much as ~80% of its counterparts in the observations (Fig. 5a) and in the control experiment described in the preceding subsection (Fig. 5b). Correspondingly, subsidence is also induced above the surface anticyclone as observed (Fig. 6d). Unlike in the observations (Fig. 1b), however, the ascent is simulated noticeably poleward of the surface anticyclone (Fig. 6d). In the presence of this dipolar pattern in vertical motion, a meridional vorticity dipole is generated above the simulated surface anticyclone (Fig. 6e). The anticyclonic response that constitutes the dipole is overestimated, probably due to the effect of the unrealistically strong mid-tropospheric ascent and the associated upper-level divergence simulated upstream. This unrealistic equatorward shift of the upper-level dipolar vorticity pattern is consistent with the corresponding equatorward shift of the surface anticyclone in the model response. The node line of this meridional dipole above the surface anticyclone displays a northwest-southeastward tilt as in the observations (Fig. 1d), consistently with the northeastward wave-activity flux (Fig. 6f) as observed (Fig. 2b). The model simulation therefore suggests that the land-sea heating/cooling contrast can contribute to the formation of the surface subtropical anticyclone and the upper-level planetary wave pattern over the southeastern Pacific and South America, although the anticyclonic portion of the upper-level meridional dipole was reproduced in the region of cyclonic vorticity in the observations. As in the NH summer, the contribution from the land-sea heating/cooling contrast to the upper-level planetary-wave pattern is comparable or even weaker than that from the response to the mid- and upper-tropospheric diabatic heating, as suggested by 17

18 another experiment (not shown). Of course, to particular thermal forcing with a given intensity, the upper-level response should be stronger if the forcing is placed at the upper level than in the case where the forcing is placed near the surface. Another factor that can result in the weakness of the upper-level response to the near-surface heating/cooling contrast may be the presence of the zonal-mean easterlies in the region of low-level heating/cooling, especially in the SH summer. Unlike in the case of zonal-mean westerlies, near-resonant stationary Rossby-wave response cannot be realized in the presence of the easterlies. A similar numerical experiment was performed with the lower-tropospheric heating/cooling contrast between the African continent and the South Atlantic (Figs. 3b, 3c and 7a). In the model integration carried out without topography, the near-surface land-sea thermal contrast is reproduced reasonably well in intensity (Fig. 7b). In Fig. 7c, the strength of the surface subtropical anticyclone reaches ~80% of its observational counterpart (Fig. 5a) and ~67% of that in the control experiment (Fig. 5b). The simulated anticyclone is, however, displaced equatorward compared with the observations (Fig. 5a), as in our simulation for the South Pacific (Fig. 6c). Mid-tropospheric subsidence is simulated to the southeast of the surface anticyclone (Fig. 7d), but in reality it is observed over the eastern portions of the anticyclone (Fig. 1b). As in the South Pacific case, unrealistically strong ascent simulated upstream of the subsidence (Fig. 7d) leads to the overestimation of the upper-level anticyclonic anomaly that constitutes a meridional dipole with a cyclonic anomaly to the north (Fig. 7e). Though shifted equatorward in correspondence with the subsidence, the meridional dipole above the surface anticyclone is reproduced upstream of the subsidence (Fig. 7e). A node line of the simulated dipole is tilted 18

19 northwest-southeastward (Fig. 7e) as observed (Fig. 1d), consistent with the northeastward wave-activity flux in the model (Fig. 7f) and in the observations (Fig. 2b). The enhanced upper-level anticyclonic anomalies accompany well-defined poleward wave-activity propagation that is much stronger than in the observation. Our wave-activity flux analysis indicates that the low-level land-sea heating/cooling couplet can act as a local source of planetary waves in the subtropics over the South Atlantic and Africa (Fig. 7f), although the upward flux is somewhat overestimated in the model. The numerical experiment was repeated with the lower-tropospheric heating/cooling contrast between the Southeastern Indian Ocean and the Australian continent (Figs. 3b, 3c and 8a). Again, no surface elevation was imposed. Compared with its observational counterpart (Fig. 2d), the land-sea thermal contrast across the Australian coast is well reproduced in intensity (Fig. 8b). The surface subtropical anticyclone is also reproduced well (Fig. 8c). Although its center is slightly equatorward relative to the observation, the strength of the surface anticyclone in Fig. 8c reaches ~100% of its counterpart in the observations (Fig. 5a) and ~125% of that in the control run (Fig. 5b). Though shifted somewhat eastward relative to the observations (Fig. 2a), the associated mid-tropospheric subsidence is also reproduced (Fig. 8d). However, the model generates unrealistic updraft in the mid- and upper-troposphere above the surface anticyclone. Although the upper-level vorticity dipole (Fig. 8e) is shifted northeastward relative to the observations due to the unrealistic updraft simulated, two Rossby wave trains are reproduced in a realistic manner (Fig. 8e): one propagating northeastward directly into the Tropics and the other propagating southeastward and then refracted northeastward into the western South Pacific. The latter wave train is also evident in the observed SLP field (Fig. 5a) and in the model SLP 19

20 response (Fig. 5f). These wave trains are confirmed through a wave-activity flux diagnosis applied to the model response (Fig. 8f). The aforementioned numerical results suggest that the land-sea heating/cooling couplet can act as one of the local sources of the SH summertime planetary waves with the dominant zonal wavenumber-3 component as observed, while contributing to the formation of the local wave trains as observed in the Australian-South Pacific sector (Figs. 2b and 2c). 5. Summary and discussion In the present study, we have clarified the three-dimensional structure of the climatological-mean summertime subtropical anticyclones observed in the SH. As in the NH summer (MN05), the SH subtropical anticyclones are accompanied locally by pronounced zonal contrasts in surface air temperature and near-surface diabatic heating across the west coasts of the subtropical continents. Using the same nonlinear atmospheric primitive-equation model as used in MN05 for the NH summer driven by a given zonally-asymmetric diabatic heating pattern, we have shown that the model SLP response to a localized near-surface heating/cooling contrast as observed in January can account for more than 80% of the observed strengths of the SH summertime subtropical anticyclones as a component of the climatological planetary waves, although the upper-level vorticity balance in the model response appears to be distorted to a certain degree. Thus, the formation of the summertime subtropical anticyclones and associated upper-level vorticity dipoles in the two hemispheres can be interpreted in the same framework where an anticyclone forms as an atmospheric response to a local thermal forcing. The model response is understandable from a viewpoint of PV thinking, since cool and warm 20

21 anomalies at the ocean and land surfaces act as anticyclonic and cyclonic PV anomalies, respectively, to yield strong equatorward alongshore winds in between. In a steady response, vorticity and heat transport by the alongshore winds requires descent above the eastern portion of the subtropical anticyclone, as observed, to maintain the vorticity and heat balance (Hoskins 1996). This descent acts to maintain the surface anticyclone against the friction and force upper-level planetary waves. In fact, our diagnosis has revealed the upward component of a wave-activity flux of stationary Rossby waves and its upper-level divergence in the vicinities of the surface subtropical anticyclones observed and simulated over the South Pacific, South Atlantic and South Indian Oceans. The upward wave-activity flux is a manifestation of poleward heat transport associated with the cool equatorward alongshore winds. The wave-activity flux diagnosis reveals no substantial influx of Rossby wave activity from upstream into any of the subtropical anticyclones. Our results indicate that the pronounced near-surface land-sea thermal contrasts accompanied by those anticyclones can act as local sources of the planetary waves observed in the summertime subtropical SH, contributing to the formation of a wavenumber-3 pattern prevailing in the subtropics. We have shown that one of the wave trains emanating from the subtropical anticyclone over the South Indian Ocean reaches into mid-latitudes, modifying the mid-latitude planetary wave pattern in the Australian-Southwestern Pacific sector. Each of the observed SH surface subtropical anticyclones accompanies a meridionally-oriented vorticity dipole in the upper troposphere, as found for their counterpart in the NH summer (MN05; The signs of vorticity related to the trough and ridge are opposite between the SH and NH). The summertime subtropical anticyclones in the two hemispheres commonly exhibit barotropic structure in their poleward portions and 21

22 baroclinic structure in their equatorward portions where the low-level zonal-mean flows are easterly. In view of the vorticity and zonal momentum balance, this structure is favorable for inducing descent effectively over a surface anticyclone, particularly over its eastern portion. The land-sea thermal contrast involved in the formation of each of the subtropical anticyclones is mainly a manifestation of the thermal effect by relatively low SSTs over the eastern ocean with radiative cooling by marine stratus and by heating through a sensible heating flux over a dry, heated landmass to the east. The importance of low SSTs has been suggested through coupled GCM experiments by Seager et al. (2003), in which the subtropical anticyclones in the SH summer are not well reproduced because the model SSTs are not as low as observed over the eastern portions of the ocean basins. A regional model experiment by Wang et al. (2005) showed that radiative cooling off the Peruvian coast in austral spring reinforces a surface subtropical anticyclone off the coast. The anticyclonic SLP response in their model reaches ~1 hpa in intensity when the maritime cooling around 20 S is enhanced by ~1 K day -1. This relationship is consistent with our result that ~4-hPa SLP response is induced by the cooling of ~3 K day -1. The temperature inversion forms in association with mid-tropospheric descent, low SSTs and cold advection by the alongshore equatorward winds, all of which can be augmented as the nearby subtropical anticyclone intensifies seasonally into summer. Strong radiative cooling associated with marine stratus and stratocumulus has been shown in the present study to possibly force the subtropical anticyclone. The cooling is thus, in turn, maintained in the presence of the anticyclone itself, suggestive of a local feedback loop in the atmosphere-ocean-land system in the summertime subtropics (Ma et al. 1994; Hoskins 22

23 1996; Seager et al. 2003; Nakamura and Miyasaka 2004; MN05). Reinforcement of continental heating through suppression of both moist convection and cloud formation by descent can also be an additional factor for the feedback loop. In fact, Grotjahn and Osman (2007) showed that intraseasonal anomalies in surface air temperature over the western portion of the US tend to lag the development of the North Pacific subtropical anticyclone. Although the present study and MN05 suggest that the surface summertime subtropical anticyclones in the two hemispheres form mainly through local feedback processes, one may nevertheless wonder why the summertime subtropical anticyclones observed in the SH are weaker than in the NH. Figure 5a indicates that their strength measured by zonally asymmetric SLP is 4~5 hpa, which is only half of their counterpart in the NH (MN05). One of the reasons may be that the SH continents are zonally narrower than their counterpart in the NH. Since the zonally asymmetric surface temperature distribution acts as anomalous PV, the induced circulation should be weaker if the zonal scale of the thermal anomalies is smaller (Hoskins et al. 1985). Another reason may be that climatologically the lower-tropospheric zonal-mean westerlies in summer is more strongly confined meridionally into mid-latitudes over the SH than over the NH. To a given forcing, near-resonant stationary response is possible in the presence of the zonal-mean westerlies but not in the presence of easterlies. Another inter-hemispheric difference is that the upper-tropospheric wave-activity flux that diverges out of the regions around the surface subtropical anticyclones tends to extend equatorward farther in the SH, which is probably because the upper-tropospheric zonal-mean westerlies extend into lower latitudes over the SH (13 S versus 25 N). These inter-hemispheric differences in the climatological westerlies are climatological manifestations of stronger meridional thermal gradient in the 23

24 summertime subtropics over the SH than over the NH, where the Asian monsoon dominates. The importance of the near-surface heating/cooling patterns for the formation of the SH surface subtropical anticyclones has also been confirmed through our other model simulations with deep convective heating imposed as forcing in any of the rectangular domains in Fig. 3a, which cannot induce any noticeable anticyclonic response at the surface in the SH subtropics (not shown). This tendency has been confirmed through a similar numerical experiment but with mid- and upper-tropospheric diabatic heating taken from the Japanese 25-year reanalysis (JRA-25) data set (Onogi et al. 2007), as a product of the cooperative research project of the JRA-25 long-term reanalysis by the Japan Meteorological Agency and Central Research Institute of Electric Power Industry. The wave-activity flux diagnosed from the 500-hPa temperature, wind and geopotential height fields based on the JRA-25 data is downward, as opposed to the corresponding flux diagnosis based on other reanalysis data, including the European Centre for Medium Range Forecasts 40-year global reanalysis (ERA-40; Uppala et al. 2005). The discrepancy is probably due to some uncertainties in the reanalysis data currently available for the SH. It should be pointed out that the corresponding numerical experiments as in Figs. 5-8 but with low-level diabatic heating taken from the JRA-25 data can reproduce the results shown in the upward flux of Rossby wave activity over the surface subtropical anticyclones. Reexamination with a more reliable data set is required in future in order to substantiate the wave-activity propagation in the observed field and the relative importance for the formation of upper-level planetary-wave between the near-surface diabatic heating and the mid- and upper-tropospheric diabatic heating. 24

25 In our model simulation only with local near-surface diabatic heating (Fig. 5d), strength of surface summertime subtropical anticyclone over the South Pacific is somewhat underestimated. One of the reasons for this underestimation is a possible influence of mid-latitude planetary waves characterized by the zonal wavenumber-1 component (Fig. 1d). In association with this structure, there is some enhancement of the upper-level polar-front jet (Fig. 1c) to the south of that surface subtropical anticyclone. In the jet entrance region, transient eddy activity is relatively weak (Fig. 9a). A planetary-wave model simulation with the zonally-asymmetric component of heat and vorticity fluxes associated with transient eddies prescribed as forcing (Fig. 9b) suggests that the eddy forcing acts to suppress the poleward extension of the surface subtropical anticyclone over the South Pacific. In contrast, the eddy forcing contributes positively to the poleward extension of the surface subtropical anticyclones in the South Atlantic and South Indian Oceans (Fig. 9b), where the core region of the SH storm track is located along a prominent oceanic frontal zone (Nakamura and Shimpo 2004). Especially over the South Indian Ocean, upper-level vorticity flux convergence associated with transient eddies directly forces the upper-level meridional vorticity dipole (not shown). The role of the transient eddy feedback for the seasonality of the subtropical anticyclones will be examined in our future study. It is also examined in our future study what mechanisms are involved in the interannual variability of the surface summertime subtropical anticyclones, including the one over the South Pacific (Grotjahn 2004). 25

26 Acknowledgments We are grateful to Prof. B. J. Hoskins for kindly providing the code of the Reading spectral model and giving encouragement and advice. We also thank Dr. T. Enomoto for his technical assistance with the model and his valuable comments. We thank Prof. R. Grotjahn for his useful comments on the earlier version of the paper. We are grateful to Dr. K. Takaya, Prof. S.-P. Xie, Prof. I. Hirota, Prof. Y. Takayabu, Prof. T. Yamagata, Prof. M. Kimoto, Prof. H. Niino, and Prof. M. Takahashi for valuable comments and encouragement. Sound criticism by the two anonymous referees and the editor (Dr. M. Alexander) has led to the substantial improvement of the paper. This work is supported, in part, by the Grant-in-Aid for Scientific Research ( ) of the Japan Society for the Promotion of Science and by the Global Environment Research Fund (S-5) of the Ministry of the Environment, Japan. 26

27 References Grotjahn, R., 2004: Remote weather associated with South Pacific subtropical sea-level high properties. Int. J. Climatol., 24, , and M. Osman, 2007: Remote weather associated with North Pacific subtropical sea-level high properties. Int. J. Climatol., 27, Hartmann, D. L., M. E. Ockert-Bell, and M. L. Michelsen, 1992: The effect of cloud type on Earth s energy balance: Global analysis. J. Climate, 5, Hoskins, B. J., 1996: On the existence and strength of the summer subtropical anticyclones. Bull. Amer. Meteor. Soc., 77, , and A. J. Simmons, 1975: A multi-layer spectral model and the semi-implicit method. Quart. J. Roy. Meteor. Soc., 101, , and M. J. Rodwell, 1995: A model of the Asian summer monsoon. Part I: The global scale. J. Atmos. Sci., 52, , M. E. McIntyre, and A. W. Robertson, 1985: On the use and significance of isentropic potential vorticity maps. Quart. J. Roy. Meteor. Soc., 111, Kanamitsu, M., W. Ebisuzaki, J.Woollen, S.-K. Yang, J. J. Hnilo, M. Fiorino, and G. L. Potter, 2002: NCEP-DOE AMIP-II Reanalysis (R-2). Bull. Amer. Meteor. Soc., 83, Klein, S. A., and D. L. Hartmann, 1993: The seasonal cycle of low stratiform clouds. J. Climate, 6, Lindzen, R. S., and A. Y. Hou, 1988: Hadley circulations for zonally averaged heating centered off the equator. J. Atmos. Sci., 45, Liu, Y. M., G. X. Wu, and R. Ren, 2004: Relationship between the subtropical anticyclone 27

28 and diabatic heating. J. Climate, 17, Ma, C.-C., C. R. Mechoso, A. Arakawa, and J. D. Farrara, 1994: Sensitivity of a coupled ocean-atmosphere model to physical parameterizations. J. Climate, 7, Miyasaka, T. and H. Nakamura, 2005: Structure and formation mechanisms of the Northern Hemisphere summertime subtropical highs. J. Climate, 18, Nakamura, H., and J. M. Wallace, 1990: Observed changes in baroclinic wave activity during the life cycles of low-frequency circulation anomalies. J. Atmos. Sci., 47, , and T. Miyasaka, 2004: Formation of summertime subtropical highs. Bull. Amer. Meteor. Soc., 85, , and A. Shimpo, 2004: Seasonal variations the Southern Hemisphere storm tracks and jet streams as revealed in a reanalysis dataset. J. Climate, 17, Onogi, K., J. Tsutsui, H. Koide, M. Sakamoto, S. Kobayashi, H. Hatsushika, T. Matsumoto, N. Yamazaki, H. Kamahori, K. Takahashi, S. Kadokura, K. Wada, K. Kato, R. Oyama, T. Ose, N. Mannoji and R. Taira 2007: The JRA-25 Reanalysis. J. Meteor. Soc. Japan, 85, Plumb, R. A., 1985: On the three-dimensional propagation of stationary waves. J. Atmos. Sci., 43, Rodwell, M. J., and B. J. Hoskins, 1996: Monsoons and the dynamics of deserts. Quart. J. Roy. Meteor. Soc., 122, , and, 2001: Subtropical anticyclones and summer monsoons. J. Climate, 14, Seager, R., R. Murtugudde, N. Naik, A. Clement, N. Gordon, and J. Miller, 2003: Air-sea 28

29 interaction and the seasonal cycle of the subtropical anticyclones. J. Climate, 16, Takaya, K., and H. Nakamura, 2001: A formulation of a phase-independent wave-activity flux for stationary and migratory quasigeostrophic eddies on a zonally varying basic flow. J. Atmos. Sci., 58, Uppala, S. M., P. Kallberg, A. Simmons, U. Andrae, V. da Costa Bechtold, M. Fiorino, J. Gibson, J. Haseler, A. Hernandez, G. Kelly, X. Li, K. Onogi, S. Saarinen, N. Sokka, R. Allan, E. Andersson, K. Arpe, M. Balmaseda, A. Beljaars, L. van de Berg, J. Bidlot, N. Bormann, S. Caires, F. Chevallier, A. Dethof, M. Dragosavac, M. Fisher, M. Fuentes, S. Hagemann, E. Holm, B. Hoskins, L. Isaksen, P. Janssen, R. Jenne, A. McNally, J.-F. Mahfouf, J.-J. Morcrette, N. Rayner, R. Saunders, P. Simon, A. Sterl, K. Trenberth, A. Untch, D. Vasiljevic, P. Viterbo, and J. Woollen, 2005: The ERA-40 re-analysis. Quart. J. Roy. Meteor. Soc., 131, Wallace, J. M., 1983: The climatological mean stationary waves: Observational evidence. Large-Scale Dynamical Processes in the Atmosphere, B. J. Hoskins and P. R. Pearce Eds., Academic Press, Wang, Y., S.-P. Xie, B. Wang, and H. Xu, 2005: Large-scale atmospheric forcing by southeast Pacific boundary-layer clouds: A regional model study. J. Climate, 18, White, G. H., 1982: An observational study of the Northern Hemisphere extratropics summertime general circulation. J. Atmos. Sci., 39, Wu, G.. X., and Y. M. Liu, 2003: Summertime quadruplet heating pattern in the subtropics and the associated atmospheric circulation. Geophys. Res. Lett., 30 (5), 1201, 29

30 doi: /2002 GL

31 List of Figures Figure 1: Climatological-mean January distribution of (a) sea-level pressure (SLP; contoured for every 4 hpa) and 1000-hPa horizontal wind velocity (arrows), (b) 500-hPa p-velocity (every 0.02 Pa s -1 ; solid lines for subsidence and dashed lines for uprising), (c) 250-hPa zonal wind speed (every 5 m s -1 ), (d) zonally asymmetric component of 250-hPa relative vorticity (unit: 10-5 s -1 ; every ±0.2, 0.6 and 1.0; solid and dashed lines for anticyclonic and cyclonic vorticity, respectively), and (e) horizontal divergence at the 250-hPa surface (every s -1 ) and 250-hPa ageostrophic wind component (arrows) based on the NCEP-DOE reanalyses. In (a) and (e), scaling for arrows is given at the bottom. In (a)-(e), shading is applied where SLP exceeds 1016 (hpa). In (c) and (d), local 250-hPa westerly jet axes are indicated with dots. In (b) and (e), zero lines are omitted. Figure 2: Climatological-mean planetary waves in January, based on zonally asymmetric fields of the NCEP-DOE reanalyses. (a) Wave-associated 500-hPa p-velocity diagnosed with omega equation (2) (contoured for ±0.005, and Pa s -1 ; solid and dashed lines for subsidence and uprising, respectively). (b) Horizontal component of Plumb s (1985) wave-activity flux (arrows) at the 250-hPa level, superimposed on 250-hPa relative vorticity associated with planetary waves (contoured as in Fig. 1d). Scaling (unit: m 2 s -2 ) for arrows is given below this panel. Arrows are plotted where zonal mean westerly is stronger than 2 (m s -1 ) and the horizontal flux is stronger than 1 (m 2 s -2 ). (c) Vertical component of the wave-activity flux (m 2 s -2 ) at the 500-hPa level. Solid lines signify the upward flux. (d) 1000-hPa horizontal wind velocity (arrows; unit: m s -1 ; with scaling given below this panel; 31