Similarities and Differences Among the South Indian Ocean

FebruaryJournal of 2008the Meteorological Society of Japan, Vol. K.86, NINOMIYA No. 1, pp. 141–165, 2008 141

Similarities and Differences among the South Indian Ocean Convergence Zone, North American Convergence Zone, and Other Subtropical Convergence Zones Simulated Using an AGCM

Kozo NINOMIYA

Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan

(Manuscript received 17 May 2007, in final form 30 October 2007)

Abstract

I examined features of the South Indian Ocean convergence zone (SICZ) and the North American convergence zone (NACZ) simulated using an atmospheric general circulation model (AGCM; T106L56: a spectral primitive-equation model with 56 σ levels and triangular spectral truncation at wave-number 106). The 24-year model integration from 1979 to 2002 was constrained by observed sea-surface temperature and sea-ice distribution. I selected a typical case for each zone (SICZ and NACZ) from the 1985–1996 simulation. The AGCM properly simulates African and Indian Ocean monsoon circulation and precipitation. The precipitation zone of the SICZ extends southeastward from the southeastern part of Africa to the southwestern rim of the Mascarene high during Southern Hemisphere summer. North American summer monsoon circulation and precipitation were also correctly reproduced. The precipitation zone of the NACZ extends northeastward along the southeastern coast of North America to the northwestern rim of the Bermuda high during the North American summer monsoon season. I compared the features of the simulated SICZ and NACZ with features of the South Atlantic convergence zone (SACZ) and the Baiu frontal zone (BFZ) simulated using the same AGCM. The SACZ, SICZ, NACZ, and BFZ were characterized as subtropical convergence zones (STCZs) and are commonly sustained along their respective subtropical anticyclones that form over the ocean east of continents. However, their geographical environments differ significantly. Whereas the respective cool oceans at the poleward sides of the SACZ and SICZ provide significant baroclinicity for the SACZ and SICZ, the respective hot continents to the poleward sides of the BFZ and NACZ create weak baroclinicity for the BFZ and NACZ.

ously with the onset of the Indian monsoon (e.g., 1. Introduction Ninomiya and Murakami 1987; Tao and Chen The intertropical convergence zone (ITCZ), the 1987; Ninomiya and Akiyama 1992; Ding 1994). Indian summer monsoon, and the polar frontal As noted by Ninomiya (1984), the BFZ is a quasi- zone are the main precipitation systems over the stationary precipitation zone that forms along the Northern Hemisphere. Although the subtropical northwestern rim of the North Pacific subtropical zone is generally dry, a quasi-stationary precipita- anticyclone. The BFZ is characterized by a strong tion zone called the Meiyu-Baiu frontal zone (BFZ) gradient of specific humidity at its poleward side, appears in subtropical East Asia nearly simultane- an interior thick moist layer with moist neutral stratification, strong moisture flux convergence, Corresponding author: Kozo Ninomiya, Frontier and upward motion. The BFZ is thus regarded Research Center for Global Change, Yokohama 236-0001, Japan as a predominant subtropical moisture front of E-mail: [email protected] Northern Hemisphere summer. ©2008, Meteorological Society of Japan Studies using satellite-observed cloud images 142 Journal of the Meteorological Society of Japan Vol. 86, No. 1 have shown three quasi-stationary cloud zones the BFZ, whereas the cool South Atlantic spreads over subtropical areas of the Southern Hemi- to the poleward side of the SACZ. Thus, the SACZ sphere in summer (e.g., Streten 1973). These exhibits stronger baroclinicity than does the BFZ. zones are recognized as the South Atlantic con- My aim is to compare features of the SICZ and vergence zone (SACZ), the South Indian Ocean NACZ with features of other STCZs simulated us- convergence zone (SICZ; also called the African ing the same AGCM. First, I demonstrate the rea- convergence zone), and the South Pacific conver- sonable reproduction of the basic features of the gence zone (SPCZ). Kodama (1993) reported that SICZ and NACZ by the AGCM. I then compare a significant cloud zone, which he called the “Rain- features of the simulated SICZ and NACZ with fall Area of America,” occurs along the southeast- features of the SACZ and BFZ simulated using the ern coast of North America in northern summer. same AGCM (Ninomiya 2007). AGCM simulation Here, I call that zone the North American conver- data are used for the comparison because simula- gence zone (NACZ). Kodama (1992, 1993) stated tion provides mutually consistent meteorological that the SACZ, SICZ, SPCZ, BFZ, and NACZ quantities that are calculated according to the commonly form along the subtropical jet stream physics incorporated in the model. This compari- on the eastern side of a quasi-anchored trough lo- son will further the understanding of STCZs. cated east-poleward of the respective localized off- 2. Simulation data used for the analysis equatorial heat source. In the lower troposphere, these zones are formed along the poleward rim The simulation data used were obtained using of the respective subtropical anticyclone and are the spectral primitive-equation AGCM T106L56, characterized as poleward boundaries of the moist which has 56 σ levels and triangular spectral tropical airmass. From these characteristics, these truncation at wave-number 106. This AGCM is the zones are recognized as subtropical convergence atmospheric part of the Model for Interdisciplin- zones (STCZs). Based on an aqua-planet simula- ary Research on Climate (MIROC), which is a tion using an atmospheric general circulation coupled general circulation model that has been model (AGCM), Kodama (1999) discussed the collaboratively developed by the Center for Cli- role of the localized off-equatorial heat source in a mate System Research (CCSR) of the University STCZ. of Tokyo, the National Institute of Environmen- Kawatani and Takahashi (2003) reproduced tal Sciences (NIES), and the Frontier Research some of the characteristics of the BFZ using an Center for Global Change (FRCGC) of the Japan AGCM (T106L60: 60 levels and T106 spectral trun- Agency of Marine-Earth Science and Technology cation). Ninomiya et al. (2003) demonstrated that (JAMSTEC). A description of the model has been synoptic- and meso-α-scale features of the BFZ in provided by K-1 Model Developers (2004). T106 June are reasonably reproduced by the T106L52 truncation corresponds to an ~1.1° Gaussian grid. AGCM when large-scale circulations such as the The model layers are ~40 m thick in the lower tro- North Pacific subtropical anticyclone and the In- posphere. The layer thickness increases gradually dian monsoon westerly circulation are reasonably to ~600 m in the lower stratosphere. The model simulated. However, they did not compare the top is at ~1 hPa. This AGCM has been used for simulated BFZ with other STCZs simulated using various studies by CCSR/NIES/FRCGC collabora- the same AGCM. tive research groups. For example, Emori et al. Ninomiya (2007) showed that basic features (2005) reported the validity and parameterization of the BFZ and SACZ were reasonably simulated dependence of daily precipitation simulated using using the T106L56 AGCM as compared with fea- this AGCM. They conducted 24-year integration tures determined in observational studies (e.g., from 1979 to 2002, under constraints of observed Figueroa et al. 1995; Carvalho et al. 2004; Ninomi- sea-surface temperature and sea-ice distribution, ya 2004). Common features of the simulated SACZ using the Earth Simulator of JAMSTEC. This in- and BFZ included their frontal structures, associ- tegration is hereafter referred to as the AMIP234 ated synoptic- and meso-α-scale disturbances, and integration. formation along the poleward rim of the respec- I used two data sets obtained from the AMIP234 tive subtropical anticyclone. However, their geo- integration. The first was daily data for basic- me graphical environments differed significantly. The teorological variables. The second was the month- warm Asian continent lies to the poleward side of ly averaged data for additional physical quantities. February 2008 K. NINOMIYA 143

3. Features of the African and Indian monsoons and the SICZ described in observational studies Numerous studies have documented the seaso nal march of intense rainfall areas across Africa. For example, Kidson (1977) used gauge data col- lected from 1951 to 1975 and Janowiak (1988) used gauge data from 1901 to 1973. The climatological precipitation in January (southern summer) and July (southern winter) obtained by Kidson (1977) are presented in Fig. 1. Rainfall in Africa is strongly seasonal, with a well-defined southern-summer (January) maximum over the southeastern parts of Africa (~15°S and ~30°E) and southern-winter (July) maximum over the latitude zone around ~10°N. This south-north seasonal oscillation of the maximum precipitation zone is associated with that of the ITCZ (Janowiak 1988), which is accompanied by the alternation of the winter and summer monsoons, as seen in Fig. 5.5 of the report by Grotjahn (1993). Trenberth et al. (2006) studied monsoon circulations by averaging National Cen- ters for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data for 1979–2001. Figure 2 is taken from Trenberth et al. (2006). At 850 hPa, there are strong southwesterly winds over the Arabian Sea and strong cross-equatorial southerly winds along the eastern coast of Africa (Somalia low-level jet) in June–July–August (JJA; Fig. 2a), whereas there are strong northeasterly winds over the Arabian Sea and strong cross-equatorial northerly wind along the eastern coast of Africa in December– January–February (DJF; Fig. 2b). The subtropical anticyclone over the South Indian Ocean (Mas- carene high) also indicates significant seasonal change. The Mascarene high extends over the African continent in JJA, whereas the high re- treats eastward in DJF, and a heat low forms over southern Africa. The SICZ is sustained along the southwestern rim of the Mascarene high. Signifi- Fig. 1. Precipitation in January and July av- cant seasonal change is also observed at 200 hPa. eraged for the period 1951–1973. These A large clockwise circulation that appears over the maps are taken from Fig. 1 of Kidson maximum precipitation area of subtropical Asia in (1977). JJA (Fig. 2c) shifts to the tropical western North Pacific in DJF (Fig. 2d). Anticlockwise circulation appears simultaneously over the intense precipita- B, C, and D for the convenience of discussions tion area in the southern part of Africa. in the following sections. System A indicates In Fig. 2b, the four major circulation systems large anticlockwise flows, including northeasterly at 850 hPa in DJF are labeled by the letters A, winds over the Arabian Sea and cross-equatorial 144 Journal of the Meteorological Society of Japan Vol. 86, No. 1

Fig. 2. NCEP/NCAR reanalysis data averaged for 1979–2001. (a) Geopotential height and wind velocity at 850 hPa averaged for JJA. (b) Geopotential height and wind velocity at 850 hPa averaged for DJF. The four major circulation systems are indicated by the letters A, B, C, and D. (c) Geopotential height and wind velocity at 200 hPa averaged for JJA. (d) Geopotential height and wind velocity at 200 hPa averaged for DJF. These maps are taken from Figs. 2.5 and 2.6 of Trenberth et al. (2006). northerly winds along the eastern coast of Africa. indicates the large temporal variability in convec- System B indicates anticyclonic circulation around tion. the Mascarene high. System C indicates westerly Cook (2000) presented mean January precipita- winds in the polar frontal zone. System D indi- tion by averaging satellite-derived precipitation cates circulation around the North Atlantic sub- rates from 1987 to 1998 and mean January pre- tropical anticyclone. cipitation in 1994. These precipitation rates are Lyons (1991) studied features of convective based on infrared (IR) measurements using the clouds over Africa during Southern Hemisphere algorithm described by Arkin and Meisner (1987). summer by analyzing out-going longwave radia- Figure 3 is taken from Cook (2000). In the map tion (OLR) in 1982–1985. The SICZ was recog- of January precipitation from 1987 to 1998 (Fig. nized in the time-averaged OLR map as a zone of 3a), the SICZ is visible as the precipitation area small OLR extending southeastward from the area protruding from the southeastern coast of Africa of minimum OLR over southeastern Africa. The to the South Indian Ocean. The SICZ is observed large standard deviation of OLR within the SICZ more clearly in the map for January 1994 (Fig. 3b) February 2008 K. NINOMIYA 145

4. Features of the simulated SICZ 4.1 A typical case of the SICZ in the monthly averaged field Maps of simulated precipitation averaged for each month during southern summer in the 12 years from 1985 to 1996 (figures not shown) were first compared with the GPI map (Fig. 3b). On the whole, the distribution of simulated precipitation over Africa and the Indian Ocean during southern summer was consistent with the GPI map. In many months, the simulated SICZ appears as an elongated narrow precipitation zone, whereas in other months it appears as a relatively wide precipitation zone. From the 12-year simulation, the simulated SACZ in January 1991 was selected as the case closely resembling Fig. 3b with regard to location and length of the SACZ. First, I examined the seasonal changes in precipitation and circulation in the lower troposphere simulated by the model. Figure 4a shows the simulated precipitation averaged for July 1990 (southern winter). The maximum precipitation zone appears along the ~10°N latitude circle. Rainfall is very small over the African continent between the Fig. 3. (a) Precipitation rate, estimated from Equator and ~35°S. A precipitation zone between IR measurements, averaged for January ~40°S and ~60°S corresponds to the polar frontal from 1987 to 1998. The contour interval is zone. The intense precipitation area shifts and ex- 3 mm d–1. (b) Precipitation from the GPI, pands southward with the seasonal march. Figure averaged for January 1994. These maps are taken from Fig. 1 of Cook (2000). 4b shows the simulated precipitation averaged for January 1991 (southern summer). Although the distribution of simulated precipitation over Africa is generally consistent with the precipita- as the long precipitation zone extending to ~40°S tion shown in Fig. 3, the simulated precipitation and 70°E. The SICZ is clearly separated from the around coastal Nigeria is larger than that shown ITCZ. As Cook (2000) noted, the intermittent ap- in Fig. 3. Intense precipitation appears over south- pearance and large temporal variation in the SICZ eastern Africa between the Equator and ~25°S. causes its relatively obscure appearance on the A precipitation zone of the SICZ extends south- map of long-term averaged precipitation. eastward from the eastern coast of Africa. In this Kodama (1993) conducted a detailed analysis month, ~60% of the precipitation in the ITCZ and for a typical case of the SICZ on 15 January 1981. ~50% of the precipitation in the SICZ north to ~ He noted that the SICZ is characterized as the 35°S is caused by the cumulus parameterization poleward boundary of the moist tropical airmass scheme of the model, whereas a large portion of and has a strong moisture gradient, convergence the precipitation in the SACZ south to ~45°S is with an interior moist layer, and a baroclinic zone. caused by the large-scale condensation scheme In the following section, I compare features of (figure not shown). the SICZ observed in the above studies with the Figure 4c shows wind velocity at 850 hPa simu- features simulated using the AGCM. Although lated in July 1990. There are strong southwesterly numerous researchers have examined the interan- winds over the Arabian Sea and strong cross- nual variability in the African monsoon and SICZ equatorial southerly winds along the eastern coast (e.g., Janowiak 1988; Lyons 1991; Cook 2000), that of Africa. Figure 4d shows wind velocity at 850 is not my focus. hPa in January 1991. There are strong northeast- 146 Journal of the Meteorological Society of Japan Vol. 86, No. 1

Fig. 4. (a) Simulated precipitation (mm d–1) averaged for July 1990. (b) Simulated precipitation averaged for January 1991. (c) Simulated wind velocity at 850 hPa averaged for July 1990. (d) Simulated wind velocity at 850 hPa averaged for January 1991. erly winds over the Arabian Sea and strong cross- heat energy fluxes at the surface averaged for equatorial northerly winds along the eastern coast January 1991 (figures not shown). The total radia- of Africa (circulation system A in Fig. 2b). The tive flux (sum of shortwave and longwave radiative model also properly simulates circulation systems flux) is approximately –200 W –2m (negative value B, C, and D (Fig. 2b). indicates downward flux) over the tropical and Figures 5a and b show the simulated sea-level subtropical areas of the South Indian Ocean and pressure and air temperature at 2 m above the approximately –150 W m–2 over southern Africa. surface averaged for January 1991, respectively. The radiative flux over the continent is partially The center of the heat low of ~1010 hPa with air offset by the sensible heat flux, which warms the temperature of ~24°C appears over the highland low-level atmosphere. The radiative flux over the area (~1000 m above sea level) in the southern tropical and subtropical Indian Ocean is mainly subtropical area of Africa (~20°S, 25°E). The offset by the latent heat flux, which moistens the strong gradient of sea-level pressure is sustained atmosphere. between the heat low and the South Atlantic sub- Figure 6a shows the simulated total heat source –1 tropical anticyclone and the Mascarene high. The Q1 (unit K d ) at 500 hPa averaged for January –1 precipitation zone of the SICZ in Fig. 4b extends 1991. Here, Q1 (unit K d ) is the total heat source along the southeastern rim of the Mascarene in the model obtained by high. Q = dT + dT + dT + dT + dT . (1) I next briefly describe features of the simulated 1 lsc cum rads radl vdf February 2008 K. NINOMIYA 147

Fig. 5. Results of the simulation averaged Fig. 6. Results of the simulation averaged for January 1991. (a) Sea-level pressure for January 1991. (a) Heat energy source (hPa). (b) Air temperature at 2 m above at 500 hPa (K d–1). (b) Wind velocity at the surface (°C). 200 hPa (m s–1).

The terms on the right-hand side of Eq. (1) indi- 4.2 Large-scale characteristics of the SICZ with in- cate the temperature changes due to large-scale tense precipitation condensation, cumulus convection, shortwave The simulated precipitation in the SICZ is most radiation, longwave radiation, and the vertical intense between 11 and 20 January 1991. This pe- eddy transfer of sensible heat, respectively. The riod is examined in more detail. Figures 7a and b maximum heat energy source of ~6 K d–1 appears show the simulated precipitation and the vertical over the area of intense precipitation over Africa. velocity ω at 500 hPa (ω500) averaged for this pe- Figure 6b shows simulated wind velocity at 200 riod. The SICZ is clearly observed as the long pre- hPa averaged for January 1991. At 200 hPa, the cipitation zone extending to ~50°S and 90°E and African anticyclone is formed over ~20°S. A weak is obviously separated from the ITCZ. The SICZ westerly trough is maintained over ~30°S and 45 does not extend directly from the intense precipi- °E. The axes of strong westerly wind are located tation area over the eastern coast of Africa. There along ~50°S. Basic features of the simulated wind is an area of relatively weak precipitation between field at 200 hPa are consistent with the NCEP/ the intense precipitation area over the continent NCAR reanalysis winds shown in Fig. 2d. and the SICZ. Zones of significant subsidence occur at both the northeastern and southwestern 148 Journal of the Meteorological Society of Japan Vol. 86, No. 1

sides of the SICZ. Figure 7c shows the simulated wind velocity at 850 hPa averaged for the period. The major large- scale circulation systems A, B, and C are located over the Indian Ocean, and system D is located over the North Atlantic. The northeasterly winds of system A originated from the Asian winter monsoon easterlies. Strong northeasterly winds (> 10 m s–1), identified as the Somalia winter low-level jet (Findlater 1969), are well simulated along the eastern coast of Somalia. The northeasterly current of circulation system A splits into two branches over the eastern coast of Africa. The western branch intrudes over the continent as an easterly and meets with northeasterly winds (of system D) that originate from the eastern periphery of the North Atlantic subtropical anticyclone. The large precipitation over the eastern coast of Africa around the equator is associated with this confluence. The northeasterly winds, in the eastern branch of system A, turn into the cross-equatorial northerly winds and extend towards Madagascar. The strong northwesterly winds of system B, over ~30°S and 60°E, around the southwestern rim of the Mascarene high, are identified as the SICZ low-level jet (LLJ). The intense precipitation zone of the SICZ is accompanied by the confluence of strong northeasterly streams of system B along the southwestern periphery of the Mas- carene high with the cross-equatorial northerly streams of system A. The northerly winds of system B along the western rim of the Mascarene high meet with the westerlies of system C over ~35°S and 60°E. This confluence sustains the southeastern portion of the SICZ. The basic features of the simulated wind field at 850 hPa are consistent with the winds in NCEP/NCAR reanalysis data shown in Fig. 2b. Figure 8a illustrates the simulated moisture source (evaporation-precipitation; negative value indicates moisture sink). The ITCZ, the intense precipitation area over tropical southern Africa, and the SICZ are major moisture sink regions. The tropical-subtropical South Indian Ocean north to the SICZ, the Arabian Sea, and the Bay of Ben- Fig. 7. Results of the simulation averaged gal are major moisture source regions. for 11–20 January 1991. (a) Precipitation (mm d–1). (b) Vertical velocity ω at 500 Figure 8b shows the simulated “transport of hPa (hPa h–1). (c) Wind velocity at 850 specific humidity” Vq at 850 hPa averaged for hPa (m s–1). the analyzed period. Hereafter, Vq is referred to as horizontal moisture flux. Circulation system A transports the water vapor from moisture source regions over the Arabian Sea and the Bay of Ben- February 2008 K. NINOMIYA 149

eastward moisture transport toward Africa from the South Atlantic occurs only in the latitude zone between the equator and ~15°S.

4.3 Frontal characteristics of the SICZ Figures 9a and b present simulated temperature and specific humidity at 850 hPa averaged for the studied period. At 850 hPa, a very strong moisture gradient appears around the southern part of Africa. In addition, moist air protrudes from southeastern Africa southeastward along the southwestern rim of the Mascarene high. From 35 to 70°E, the poleward side of the SICZ is the boundary of the warm and moist tropical air. In this boundary, the horizontal gradients of temperature and specific humidity are ~3 K (500 km)–1 and ~6 g kg–1 (500 km)–1, respectively. At 70–90°E, the SICZ is located within the polar frontal zone (40–50°S), where the horizontal gradients of temperature and specific humidity are ~4 K (500 km)–1 and ~1 g kg–1 (500 km)–1, respectively. Figure 9c shows the vertical stability in the 850–

500-hPa layer, which is expressed by ([θe500–θe850]/ –1 3.5; K (100 hPa) ). Here, θe500 and θe850 indicate the equivalent potential temperature at 500 and 850 hPa, respectively. The stratification over the southern part of Africa is convectively unstable, whereas that over the ocean is convectively stable. Particularly stable stratification is observed in the Mascarene high. Quasi-moist neutral or weak Fig. 8. Results of the simulation averaged stable stratification is simulated within the ITCZ for 11–20 January 1991. (a) Moisture sink and the SICZ. (evaporation-precipitation; mm d–1). (b) Figures 10a, b, and c show the latitude-height Horizontal moisture flux at 850 hPa (g section of vertical velocity ω, equivalent potential kg–1 m s–1). temperature θe, and zonal wind velocity u along the 60°E meridian, respectively. Strong ascent motion of approximately –8 hPa h–1 appears around gal toward Africa (0 to 15°S) and further south- 600 hPa over the SICZ. Subsiding motion takes ward along the eastern coast of Africa. The largest place in both the northern and southern sides of –1 –1 transport of ~100 g kg m s appears in associa- the SICZ. The large meridional gradient of θe in tion with the Somalia winter LLJ. This west-south- the lower troposphere over ~35°S is caused by ward moisture flux further turns southeastward both the strong moisture and thermal gradients. and supplies a moisture flux of ~40 g kg–1 m s–1 In the SICZ (~35°S), the layer of vertically quasi- into the SICZ. Circulation system B transports uniform θe (~324 K) at 900–600 hPa is formed water vapor of ~80 g kg–1 m s–1 toward the SICZ above the convectively unstable layer from 1000 along the western rim of the Mascarene high. Sig- to 900 hPa. Similar stratification is observed over nificant moisture flux convergence appears over the ITCZ (~10°S). The low θe (~320 K) at 700– the intense precipitation areas (figure not shown). 600 hPa over the subtropical zone (20–30°S) is Circulation system C transports water vapor east- caused by the dry air above the stable layer that ward in the ~50°S zone. In this zone, significant forms in the subsidence of the Mascarene high. moisture flux convergence is observed in associa- In Fig. 10c, the subtropical jet with maximum tion with the confluence of systems B and C. The zonal velocity of ~55 m s–1 at ~150 hPa is located 150 Journal of the Meteorological Society of Japan Vol. 86, No. 1

Fig. 9. Results of the simulation averaged Fig. 10. Results of the simulation averaged for 11–20 January 1991. (a) Temperature for 11–20 January 1991 on the latitude- at 850 hPa (°C). (b) Specific humidity at height section along the 60°E meridian. 850 hPa (g kg–1). (c) Vertical stability in The black triangle ▲ on the abscissa indicates the latitude of the SICZ. (a) Vertical the 850–500-hPa layer ([θ e500–θ e850]/3.5; K –1 (100 hPa)–1). velocity ω (hPa h ). (b) Equivalent potential temperature θ e (K). (c) Zonal wind velocity u (m s–1). February 2008 K. NINOMIYA 151 at ~40°S, whereas the secondary maximum wind zone in the lower troposphere appears at ~33°S, which is identified as the SICZ LLJ. The intense precipitation zone of the SICZ is located between the latitude of the upper jet stream and that of the SICZ LLJ. The wind maximum around 50°S corresponds to the polar jet stream. The frontal characteristics of the simulated SICZ are basi- cally consistent with the observed characteristics shown by Kodama (1993).

4.4 Synoptic- and meso-α-scale variations in the simulated SICZ Figures 11a, b, and c, respectively, show the standard deviations of the simulated precipitation

SD(R), sea-level pressure SD(ps), and vorticity at

850 hPa SD(ζ 850) obtained for the studied period. The standard deviation of variable a, SD(a), is obtained as

2 1/2 SD(a) = [Σ(a－am) /N] , (2) where a is the variable obtained at a 1-day interval, am is the mean a for the analyzed period, N is the total number of data, and Σ indicates the sum- mation of data for the analyzed period. The zone of large SD(R), which coincides with the SICZ, elongates southeastward from southeastern Africa (~20°S, 35°E) to the South Indian

Ocean (~45°S, 90°E). The SD(ω500) (figure not shown) has a distribution similar to that of SD(R). –1 The maximum SD(ω500) of ~8 hPa h appears in the SICZ. However, the zone of large values of SD(ps) and SD(ζ 850) occurs from 50 to 60°S and is caused by the passage of polar frontal disturbances. The SD(ps) is small in the SICZ. The secondary maximum of SD(ζ 850) is observed in association with the large SD(R) in the SICZ. In the equatorward periphery of the Mascarene high,

SD(R), SD(ps), and SD(ζ 850) are very small. Figure 12a presents isohyets of 25 mm d–1 simulated for each day during 11–20 January 1991. The ITCZ over the Indian Ocean and the SICZ corre- spond to the path of intense precipitation systems. –5 –1 Figure 12b shows isolines of ζ 850 of –5×10 s simulated for each day during the period between Fig. 11. Results of the simulation for 11– 11 and 20 January 1991. There are two tracks of 20 January 1991. (a) Standard deviation ζ –1 850 maximums. One is along the SICZ. The other, of precipitation (mm d ). (b) Standard which corresponds to the storm track of the deviation of sea level pressure (hPa). (c) polar frontal disturbances, appears along the ~60° Standard deviation of vorticity at 850 hPa S latitude circle. These two tracks merge into one (10–5 s–1). storm track around 90°E. Many of the precipitation systems in the SICZ form over the eastern 152 Journal of the Meteorological Society of Japan Vol. 86, No. 1

5. Features of the North American summer monsoon and NACZ described in observational studies The North American summer monsoon, which encompasses the Mexican monsoon, has been studied from various viewpoints. Douglas et al. (1993) described the basic features of the North American (Mexican) monsoon, reporting large anticyclonic flow over Arizona and New Mexico in July at 500 hPa. At 900 hPa, southeasterly streams from the Gulf of Mexico do not cross over the Mexican Plateau and the Sierra Madre Oriental. Douglas (1995) studied the summertime low-level flows around the Gulf of California using data observed by pilot balloon and aircraft, obtained for the “Southwest Area Monsoon Project in 1990” (SWAMP-90). The observations indicated that the northwesterly winds from the eastern rim of the North Pacific subtropical anticyclone turned into cyclonic flows around Baja California and then became stronger southerly streams in the lower troposphere over the Gulf of California. Schmitz and Mullen (1996) studied water-vapor transport associated with the North American summer monsoon based on ECMWF analyses. They concluded that most moisture at low levels (below 700 hPa) comes from the northern Gulf of California. Moisture does not enter the Sonora Desert at low levels directly, either from the southern Gulf of Fig. 12. (a) Isohyets of 25 mm d–1 simulated California or the tropical East Pacific. for each day from 11–20 January 1991. (b) Higgins et al. (1998) reported that an exten- Isolines of 850-hPa cyclonic vorticity of – sive region of heavy precipitation first develops –5 –1 5×10 s simulated for each day from 11 over southern Mexico during the spring and then to 20 January 1991. spreads northward along the western slopes of the Sierra Madre Occidental. The “Southwest monsoon” over Arizona and New Mexico typically coast of Africa, whereas the significant vorticity begins in early July. Heating over the elevated ter- cores appear around Madagascar. These features rain of Mexico and the western USA plays a ma- indicate the development of the precipitation sys- jor role in the development and evolution of the tems into the respective meso-scale circulation monsoon. There is a continental-scale mode in the systems during eastward propagation along the warm-season precipitation pattern over the USA, SICZ. consisting of an in-phase relationship between the At 500 hPa, the vorticity cores in the polar southwest and the east coast. Figure 13 repro- frontal zone are more predominant (figure not duces the conceptual model of the North Ameri- shown). During the studied period, no significant can monsoon shown by Higgins et al. (1998). The vorticity core at 850 hPa propagated into southern figure illustrates four major circulation systems in Africa from the eastern South Atlantic in the lati- the lower troposphere. System A indicates large- tude zone between the equator and 35°S. scale anticyclonic flow over the Gulf of Mexico and southern parts of the USA, which originates from the trade easterlies; system B indicates anticyclonic flows around the Bermuda high; February 2008 K. NINOMIYA 153

Fig. 13. Mean (JAS 1975–1995) 925-hPa wind velocity, 200-hPa streamlines, and merged satellite esti- mates and station observations of precipitation (shading, in mm). The position of the North American monsoon anticyclone is indicated by UA. The Bermuda and North Pacific subtropical anticyclone centers are indicated by H. The approximate location of the Great Plains low-level jet stream is shown by the heavy solid arrow. This figure is taken from Higgins et al. (1998). A, B, C, and D indicates the major circulation systems at 925 hPa, respectively. “UA” indicates the upper anticyclone. system C indicates westerlies in the polar frontal over northern Mexico on 23 June 1982, a signifi- zone; and system D indicates the flows around cant NACZ had formed along the southeastern the North Pacific subtropical anticyclone. Barlow coast of the USA. et al. (1998) stated that Mexican monsoon precipi- In the following section, I compare features of tation begins to increase around the beginning of the NACZ described in these previous studies July. At the same time, precipitation over eastern with the features simulated using the AGCM. parts of the USA decreases, whereas precipitation 6. Features of the simulated NACZ along the southeastern coast of the USA increases exceptionally. 6.1 A typical case of the NACZ in the monthly av- The precipitation zone over the southeastern eraged field coast of the USA, as noted by Higgins et al. Maps of simulated precipitation averaged for (1998) and Barlow et al. (1998), corresponds to each month during summer in the 12 years from the NACZ. However, no detailed case studies 1985 to 1996 (figures not shown) were first- com have examined the NACZ, except for the study by pared with maps of precipitation (Fig. 13) and Kodama (1993). His analysis of the NACZ on cloudiness (Fig. 14a). On the whole, the distribu- 23 June 1982 showed that it forms along the tion of simulated precipitation over North America subtropical jet stream on the eastern side of a for June, July, and August is consistent with these quasi-anchored trough lying east-poleward of the maps. From these 12 years, the simulated NACZ respective localized heat source in the tropical in June 1991 was selected as the case that closely zone. In the lower troposphere, it forms along resembles the observed features. the northern rim of the Bermuda high and can Figures 15a and b show the simulated precipita- be characterized as the northern boundary of tion and wind velocity at 10 m in height averaged the moist tropical airmass with a strong moisture for June 1991, respectively. The ITCZ, between the gradient. Figure 14 is reproduced from Kodama’s northeasterly trade winds and the southeasterly (1993) analysis of the NACZ on 23 June 1982. Al- trade winds, accompanies a zone of intense pre- though areas of high cloud had not yet appeared cipitation extending along the latitude circle of ~5 154 Journal of the Meteorological Society of Japan Vol. 86, No. 1

Fig. 15. Results of the simulation averaged for June 1991. (a) Precipitation (mm d–1). (b) Wind velocity at 10 m in height above the surface.

Fig. 14. Features of the NACZ on 23 June 1982. (a) High clouds (clouds higher the whole, the AGCM simulates precipitation and than the 400-hPa level). (b) Height and low-level winds properly, there are some discrep- wind velocity at 1000 hPa. (c) Height and ancies between the simulation and observations. wind velocity at 300 hPa. These maps are The simulated southerly winds over the Bay of taken from Fig. 18 of Kodama (1993). California and the precipitation over the Mexican coast are significantly weak compared to the- ob servations. °N. A zone of intense precipitation is present over In June, ~70% of the precipitation in the ITCZ Central America and southern parts of Mexico. and ~50% of the precipitation in the NACZ is Northwesterly system D winds from the eastern caused by the cumulus parameterization scheme rim of the North Pacific subtropical anticyclone of the model (figure not shown). turn into cyclonic flows around Baja California Figure 16a shows simulated sea-level pressure and then turn into southwesterly streams over the averaged for June 1991. The predominant sub- Pacific coast of Mexico. The precipitation zone of tropical anticyclones are present over the North the NACZ extends along the western and north- Atlantic, eastern North Pacific, South Atlantic, ern peripheries of the Bermuda high. Although on and eastern South Pacific. The low-pressure zone February 2008 K. NINOMIYA 155

Fig. 16. Results of the simulation averaged Fig. 17. Results of the simulation averaged for June 1991. (a) Sea-level pressure for June 1991. (a) Heat energy source at (hPa). (b) Air temperature (°C) at 2 m 500 hPa (K d–1). (b) Wind velocity at 200 above the surface. hPa (m s–1). of the ITCZ forms between these subtropical June 1991 (figures not shown). The total radiative anticyclones over the Northern and Southern flux is approximately –200–2 Wm over tropical hemispheres. The low-pressure area over parts and subtropical oceanic areas and approximately of eastern North America forms between the –175 W m–2 over North America. The radiative Bermuda high and the North Pacific subtropical flux over the continent is partially offset bythe anticyclone. sensible heat flux, which warms the low-level Figure 16b shows simulated temperature at 2 atmosphere. Radiative flux over the tropical and m above the surface averaged for June 1991. The subtropical oceanic areas is mainly offset by the very warm area around the Bay of California re- latent heat flux, which moistens the atmosphere. sults in the heat-low over the Bay of California. Figures 17a and b show the simulated heat –1 The NACZ is not associated with a large thermal energy source (Q1, unit K d ) at 500 hPa and gradient. The zone of strong thermal gradient in wind velocity at 200 hPa averaged for June 1991, the zone between 55 and 45°N corresponds to the respectively. The largest heat energy source of polar frontal zone. ~20 K d–1 appears over Colombia. The heat source I briefly describe the features of the simulated over southern Mexico reaches ~5 K d–1. At 200 heat energy fluxes at the surface averaged for hPa, predominant anticyclonic flows form over the 156 Journal of the Meteorological Society of Japan Vol. 86, No. 1

USA, where a heat source of ~2 K d–1 is simulated at 500 hPa. The 200-hPa westerly wind zone is split into two branches by the upper-level anticyclone. The westerly winds in the northern branch are intensified ~ ( 30 m s–1) along the poleward side of the anticyclonic flows. In the main westerly zone, between 40 and 50°N, a quasi-stationary westerly trough is maintained over ~60°W.

6.2 Large-scale characteristics of the NACZ with intense precipitation During June 1991, the simulated precipitation in the NACZ is most intense between 21 and 30 June. This period is examined in more detail. Figures 18a and b show the simulated precipitation and ω500 averaged for this period, respectively. The NACZ does not extend directly from the intense precipitation zone over Mexico. Subsidence zones occur at both the equatorial and poleward sides of the NACZ. Figure 18c shows the simulated wind velocity at 850 hPa averaged for the period. The four major circulation systems (A, B, C, and D) are reasonably simulated. The clockwise gyre (system A) originates from the trade easterly winds over the Caribbean Sea. These easterly winds turn into southerly winds and then westerly winds over Texas. Circulation system B is anticyclonic flow around the Bermuda high, and system C is the westerly current in the polar frontal zone. The confluence of the westerly winds of systems B and C around 60°W results in strong westerly winds of ~10 m s–1 along the northern periphery of the Bermuda high. Figure 19a shows the simulated moisture source (evaporation-precipitation; negative value indicates moisture sink). The ITCZ, intense precipitation zones over Mexico, and the NACZ are major moisture sink regions. The subtropical At- lantic, the Gulf of Mexico, and the Caribbean Sea are the primary moisture source regions. Figure 19b shows simulated horizontal moisture flux Vq at 850 hPa averaged for the analyzed period. Cir- culation system A transports a large amount of water vapor from the moisture source region over the Caribbean Sea toward the Gulf coast as south- Fig. 18. Results of the simulation averaged –1 –1 for 21–30 June 1991. (a) Precipitation westerly moisture flow of ~60 g kg m s , and (mm d–1). (b) Vertical velocity at 500 hPa further to the NACZ as westerly moisture flow of –1 –1 –1 (hPa h ). (c) Wind velocity at 850 hPa ~20 g kg m s . Circulation system B transports (m s–1). water vapor of ~30 g kg–1 m s–1 from the moisture source region over the subtropical Atlantic toward the NACZ. Significant moisture flux convergence February 2008 K. NINOMIYA 157

Fig. 19. Results of the simulation averaged for 21–30 June 1991. (a) Moisture sink (evaporation-precipitation; mm d–1). (b) Horizontal moisture flux at 850 hPa (g kg–1 m s–1).

(figure not shown) appears around 30°N and 80° W, where the westerly flow of system A meets the southerly flow of system B. Circulation system C transports moisture of ~30 g kg–1 m s–1 westward.

6.3 Frontal characteristics of the NACZ Figures 20a and b show simulated temperature and specific humidity at 850 hPa averaged for the Fig. 20. Results of the simulation averaged studied period, respectively. At 850 hPa, the North for 21–30 June 1991. (a) Temperature at American continent is characterized by warmer 850 hPa (°C). (b) Specific humidity at temperatures, with temperatures reaching ~24°C 850 hPa (g kg–1). (c) Vertical stability in around the State of Wyoming. The vertical exten- the 850–500-hPa layer ([θ e500–θ e850]/3.5; K sion of warm air reaches to ~300 hPa. The pre- (100 hPa)–1). dominant anticyclonic circulation (ridge) observed in Fig. 17b appears over this warm area. The ther- 158 Journal of the Meteorological Society of Japan Vol. 86, No. 1 mal gradient at the northern side of the NACZ over the continent is weak because the North American continent is very warm due to strong insolation in summer. However, over the North At- lantic, a significant thermal gradient is sustained along the southern rim of the cool maritime air. At 850 hPa, moist air protrudes northeastward as the moist tongue along the NACZ. The poleward side of the NACZ corresponds to the northern boundary of the moist tropical air masses. At this boundary, the horizontal gradient of specific humidity is ~3 g kg–1 (500 km)–1. Figure 20c shows the vertical stability in the 850

–500-hPa layer expressed by (θe500–θe850)/3.5 (K [100 hPa]–1). Whereas areas of convectively unstable stratification are present over the continent, areas of convectively stable stratification extend over oceanic areas. Areas of very stable stratification appear within the subtropical anticyclones over the North Atlantic, North Pacific, South Atlantic, and South Pacific. Weak unstable stratification is simulated within the NACZ over the continent, whereas quasi-neutral stratification occurs within the NACZ over the ocean. Figures 21a, b, and c show the latitude-height section of vertical velocity ω, equivalent potential temperature θe, and zonal wind velocity u along 60°W, respectively. A strong ascent motion of approximately –6 hPa h–1 appears in the mid- troposphere over the NACZ. The subsiding motion takes place at both the northern and southern sides of the NACZ. Intense ascent motions also occur in the ITCZ (~3°N) and the polar frontal zone (~50°N). The large meridional gradient of

θe in the lower troposphere over ~40°N is mainly caused by the strong moisture gradient to the north of the NACZ. In the NACZ, the layer of vertically quasi-uniform θe (~320 K) is formed from 800 to 600 hPa above the convectively unstable layer at 1000–800 hPa. In Fig. 21c, the jet stream with maximum zonal velocity of ~30 m s–1 is located at ~42°N, whereas the maximum wind zone in the lower troposphere (the NACZ LLJ) appears at ~38°N.

6.4 Synoptic- and meso-α-scale variations in the Fig. 21. Results of the simulation averaged for 21–30 June 1991 on the latitude-height NACZ section along the 60°W meridian. The Figures 22a, b, and c show the simulated SD(R), black triangle ▲ on the abscissa indicates SD(ps), and SD(ζ 850), which are defined by Eq. the latitude of the NACZ. (a) Vertical (2), obtained for 21–30 June 1991. A zone of large velocity ω (hPa h–1). (b) Equivalent po-

SD(R) appears over the ITCZ. This zone of large tential temperature θ e (K). (c) Zonal wind SD(R), which coincides with the NACZ, elongates velocity u (m s–1). February 2008 K. NINOMIYA 159

Fig. 23. (a) Isohyets of 20 mm d–1 simulated for each day during 21–30 June 1991. (b) Isolines of 850-hPa vorticity of –3×10–5 s–1 simulated for each day during 21–30 June 1991.

northeastward from the State of Florida (~32° N, 80°W) to the North Atlantic (~40°N, 60°W).

The simulated SD(ω500) has a similar distribution

(figure not shown). Maximumω SD( 500) of ~5 hPa h–1 appear over the ITCZ and in the NACZ.

Areas of large SD(ps) and SD(ζ 850) are observed over the polar frontal zone. An area of relatively

Fig. 22. Results of the simulation for 21 large SD(ps) and SD(ζ 850) is also located around –30 June 1991. (a) Standard deviation Florida. Over the Gulf of Mexico and Caribbean –1 of precipitation (mm d ). (b) Standard Sea, SD(R), SD(ps), and SD(ζ 850) are very small. deviation of sea-level pressure (hPa). (c) Tropical disturbances do not influence the activity Standard deviation of vorticity at 850 hPa of the NACZ in this period. –5 –1 (10 s ). Figure 23a presents isohyets of 20 mm d–1 simulated for each day from 21 to 30 June 1991. The NACZ corresponds to the path of the intense 160 Journal of the Meteorological Society of Japan Vol. 86, No. 1

Fig. 24. Schematic illustration of the basic features of the BFZ (a), SACZ (b), NACZ, (c) and SACZ (d) simulated using the AGCM. precipitation systems. Many of the precipitation hPa. Table 1 summarizes the features of large- systems in the NACZ form around Florida. Figure scale conditions around the simulated STCZs. 23b shows isolines of 850-hPa vorticity of 3×10–5 Circulation system A indicates the predominant s–1 simulated for each day during the studied pe- gyre of the respective summer monsoons (Fig. riod. A track of the cyclonic vorticity maximum is 24 and Table 1). This system transports large found along the polar frontal zone; however, there amounts of moisture from the moisture source is no significant track of the cyclonic vorticity regions into the respective STCZs. System A of maximum along the NACZ. the Asian summer monsoon (Fig. 24a) and system A of the Asian winter monsoon (South Indian 7. Discussion Ocean summer monsoon, Fig. 24b) show larger 7.1 Similarities and differences among the BFZ, horizontal extension and larger moisture trans- SICZ, NACZ, and SACZ port, whereas system A in the North American In this section, I compare features of the simu- summer monsoon (Fig. 24c) and system A in the lated SICZ and NACZ with those of the SACZ and South American summer monsoon (Fig. 24d) have BFZ simulated using the same AGCM (Ninomiya smaller horizontal extension and smaller mois- 2007). Figures 24a, b, c, and d show the basic ture transport. The SACZ is, in essence, enclosed features of the BFZ, SICZ, NACZ, and SACZ. within the South America-Atlantic-scale environ- These maps schematically illustrate major precipi- ment, and the NACZ is enclosed within the North tation areas and major circulation systems at 850 America–Atlantic-scale environment. February 2008 K. NINOMIYA 161

Table 1. Large-scale environments of the STCZs. BFZ SICZ NACZ SACZ Max. precipitation Bay of Bengal Around Zambia Over Mexico East side of the Andes ~40 mm d–1 ~12 mm d–1 ~12 mm d–1 ~20 mm d–1

Max dT500 Bay of Bengal Around Zambia Over Mexico East side of the Andes ~20 K d–1 ~7 K d–1 ~5 K d–1 ~12 K d–1 200-hPa anticyclone Tibetan anticyclone Southern Africa Mexico–southwestern Bolivian anticyclone anticyclone U.S. anticyclone 200-hPa westerly jet North to Tibetan South to southern North to Mexico–SW South to Bolivian anticyclone Africa anticyclone U.S. anticyclone anticyclone ~40 m s–1 ~35 m s–1 ~25 m s–1 ~35 m s–1 Quasi-stationary ~125°E (deep) ~50°E (shallow) ~60°W (deep) ~50°W (shallow) trough in westerly jet Circulation system A Asian summer Asian winter Gyre over the South American monsoon gyre monsoon gyre Caribbean Sea and summer monsoon Gulf of Mexico gyre Circulation system B Around N. Pacific Around the Around Bermuda high Around S. Atlantic high Mascarene high high Vq in system A ~160 g kg–1 m s–1 ~100 g kg–1 m s–1 ~60 g kg–1 m s–1 ~40 g kg–1 m s–1 Vq in system B ~80 g kg–1 m s–1 ~80 g kg–1 m s–1 ~30 g kg–1 m s–1 ~80 g kg–1 m s–1

System B indicates the anticyclonic circula- on the eastern side of the quasi-stationary trough. tion along the respective subtropical anticyclone. However, the troughs for the SACZ and SICZ are No significant difference is observed among the very shallow compared to the troughs for the BFZ STCZs or the moisture transport by the systems. and NACZ. However, there is a significant difference in the Table 2 summarizes the frontal characteristics activity of tropical disturbances in the equator of the STCZs. The STCZs have many common fea- sides of the respective anticyclones. Convective tures, including in their frontal structures and as- systems around the Philippines and tropical dis- sociated synoptic- and meso-α-scale disturbances. turbances over the southwestern periphery of the In contrast to the ITCZ in which column moisture North Pacific subtropical anticyclone cause sig- convergence is primarily caused by meridional nificant variation in the North Pacific subtropical wind convergence in the moist environment, pre- anticyclone. Therefore, the BFZ is not always sus- cipitation within the STCZs is also supported by tained in the AGCM and in the real atmosphere in zonal wind and moisture flux convergence (Cook July. In contrast, the development of tropical dis- 2000). turbances is not frequent over the South Atlantic. The BFZ, SICZ, NACZ, and SACZ all form in The area of maximum heating at 500 hPa ap- association with the respective summer monsoon pears in association with intense precipitation of circulation. They elongate east-poleward from the respective summer monsoons. An anticyclone the subtropical eastern part of their respective in the upper troposphere forms over the area of continents and are sustained along the western maximum heating at 500 hPa. Among the summer poleward peripheries of the respective subtropical monsoons, the heating and precipitation of the anticyclones. These features show the strong in- Asian summer monsoon are most predominant. fluence of the continent on the STCZ. Therefore, The 200-hPa westerly jet stream is intensified the BFZ, SICZ, NACZ, and SACZ can be catego- along the poleward side of the upper anticyclone. rized as land-based convergence zones (LBCZs), A quasi-stationary trough in the upper westerly as proposed by Cook (2000). Figueroa et al. (1995) is present in the east-poleward side of the respec- used AGCM experiments to demonstrate how tive maximum heat energy source. The STCZs large-scale topography over South America de- commonly form along the subtropical jet stream termines the formation of the SACZ. Cook (2000) 162 Journal of the Meteorological Society of Japan Vol. 86, No. 1

Table 2. Characteristics of the STCZs BFZ SICZ NACZ SACZ Relation to subtropi- Poleward western Poleward western Poleward western Poleward western cal anticyclone rim of the North rim of the Mascarene rim of the Bermuda rim of the South Pacific anticyclone high high Atlantic anticyclone Precipitation ~24 mm d–1 ~12 mm d–1 ~9 mm d–1 ~12 mm d–1 Convective precept. ~70% ~50% ~50% ~50% ω (hPa h–1) at 500 ~–8 hPa h–1 ~–5 hPa h–1 ~–3 hPa h–1 ~–7 hPa h–1 hPa Low-level jet (LLJ) BFZ LLJ (~15 m s–1) SICZ LLJ (~7 m s–1) NACZ LLJ (~10 m s–1) SACZ LLJ (~7 m s–1) q in moist tongue at ~10 g kg–1 ~10 g kg–1 ~8 g kg–1 ~8 g kg–1 850 hPa T-gradient at 850 hPa ~0 K (500 km)–1 over ~7 K (500 km)–1 ~0 K (500 km)–1 over ~4 K (500 km)–1 China around continent USA around continent ~2 K (500 km)–1 over ~5 K (500 km)–1 over ~3 K (500 km)–1 over ~4 K (500 km)–1 over Japan ocean ocean ocean q-gradient at 850 hPa ~4 g kg (500 km)–1 ~5 g kg (500 km)–1 ~3 g kg (500 km)–1 ~6 g kg (500 km)–1 Stratification Weak convectively ~Moist neutral Weak convectively ~Moist neutral unstable unstable Precipitation system ~30 mm d–1 ~25 mm d–1 ~20 mm d–1 ~20 mm d–1 850-hPa vorticity of ~6×10–5 s–1 ~–5×10–5 s–1 ~3×10–5 s–1 ~–5×10–5 s–1 precipitation system

Table 3. Geographical conditions of the STCZs BFZ SICZ NACZ SACZ Poleward side Warm East Asian Cool South Indian Warm North American Cool South Atlantic continent Ocean continent Ocean Westward side Asian continent African continent North American conti- South American continent nent

also reported that SICZ position and intensity in served around the NACZ. The thermal gradient an AGCM experiment were determined by sur- of the northern boundary of the NACZ over the face conditions over southern Africa. USA is very weak because the North American However, the STCZs show significant differ- continent north of the NACZ is very warm. Large ences in meridional temperature gradients at 850 parts of Africa and South America are located in hPa at their poleward sides. The temperature the tropical zone north to ~35°S, and the cool gradients of the BFZ and NACZ, especially in the south Indian Ocean and cool Atlantic spread to western portions (over land areas), are significant- the pole-ward sides of the SICZ and SACZ, respec- ly weaker compared with those of the SACZ and tively. Thus, the SICZ and SACZ exhibit stronger SICZ. This difference is due to the geographical baroclinicity compared to the BFZ and NACZ. environment shown in Table 3. The warm Asian continent occupies the middle and high latitudes 7.2 Differences between the LBCZ and South Pa- of East Asia. The thermal gradient at the north- cific convergence zones ern boundary of the BFZ, especially over China, The BFZ, SICZ, NACZ, and SACZ all elongate is very weak because the Asian continent north east-poleward from the subtropical eastern parts of the BFZ is very warm. A similar feature is ob- of their respective continents and are sustained February 2008 K. NINOMIYA 163 along the western poleward periphery of the 2007). They commonly elongate east-poleward respective subtropical anticyclones. From these from subtropical eastern parts of their respective features, they are categorized as land-based con- continents and are sustained along the western- vergence zones (LBCZs). poleward peripheries of the respective subtropical The South Pacific convergence zone (SPCZ) anticyclones. From these features, they are cate- does not extend from Australia. It appears over gorized as land-based convergence zones (LBCZs). the South Pacific, between Australia and South They have many common features such as the America, and has long horizontal extension (Vin- frontal structure and associated synoptic- and cent 1994). Cook (2000) suggested that the South meso-α-scale disturbances. These STCZs common- Pacific convergence zone (SPCZ) is not a land- ly form along the poleward rims of the respective based convergence zone. However, in their AGCM subtropical anticyclones. simulation, Kiladis et al. (1989) showed that the However, their large-scale geographical environ- removal of Australia destroys the southern mon- ments differ significantly. The thermal gradients soon and substantially weakens the western part in the northern boundaries of the BFZ and NACZ of the SPCZ. In an aqua-planet AGCM simulation, are weak because of the warm Asian and North Kodama (1999) discussed the role of the localized American continents spreading over the middle off-equatorial heat source in the STCZ. Although and high latitudes. In contrast, the cold South At- the aqua-planet AGCM does not include the conti- lantic and South Indian Ocean spread to the pole- nents, the arbitrarily added localized off-equatorial ward sides of the SACZ and SICZ. Thus, the SACZ heat source suggests a certain implicit influence and SICZ exhibit stronger baroclinicity compared of the continent. At present, it is not clear whether to the BFZ and NACZ. the SPCZ is an LBCZ. To clarify this, the SPCZ There is another important difference between should be simulated with the same AGCM used the Asia monsoon and American monsoons. The here and compared with the STCZs studied here. BFZ is strongly influenced by the predominant This problem will be examined in a following re- Asian summer monsoon circulation, and the SICZ port. is influenced by the predominant Asian winter monsoon circulation. In addition, convective 8. Concluding remarks systems around the Philippines and the western Comparisons with observations indicate that North Pacific also cause significant variation in the African summer monsoon over Africa and the the BFZ through variation in the North Pacific South Indian Ocean and the SICZ are reasonably subtropical anticyclone. In contrast, the SACZ is simulated by the AGCM (CSSR/NIES/FRCGC essentially enclosed within the South America– T106L56). During Southern Hemisphere summer, South Atlantic-scale environment, and the NACZ high amounts of precipitation appear over the is enclosed within the North America–North southern tropical parts of Africa, where strong Atlantic-scale environment. In addition, tropical heating produces the upper tropospheric African disturbances and active convective systems over anticyclone. The SICZ, which extends southeast- the South Atlantic Ocean have very weak influ- ward from southeastern Africa to the South Indian ence on the SACZ. Ocean, forms along the southwestern rim of the Acknowledgements Mascarene high during southern summer. The North American summer monsoon and I express my sincere thanks to the CCSR/ NACZ are also reasonably reproduced compared NIES/FRCGC collaborative research group for to the observations. Strong summer precipitation allowing my use of the AMIP234 simulation data. occurs over Mexico. Strong heating over Mexico I also thank Dr. S. Emori and Dr. A. Hasegawa of and the western USA causes an upper-tropo- the NIES and Dr. K. Suzuki and Dr. T. Nishimura spheric anticyclone to form over these areas. The of the FRCGC for their kind assistance with the NACZ, which extends northeastward from the U.S. AMIP234 data and helpful discussions. Thanks Gulf coast, forms along the northwestern rim of are also due to Prof. Y.-M. Kodama of Hirosaki the Bermuda high during summer. University for informative discussions on STCZs. The features of the simulated SICZ and NACZ I thank two anonymous reviewers for their valu- were compared with those of the BFZ and SACZ able comments and advice. This study was par- simulated using the same AGCM (Ninomiya tially supported by a Grant-in-Aid for Scientific 164 Journal of the Meteorological Society of Japan Vol. 86, No. 1

Research from Japan’s Ministry of Education, Cul- Meteor. Soc. Japan, 81, 113–129. ture, Sports, Science and Technology, Project No. Kidson, J.W., 1977: African rainfall and its relation to C.17540407. the upper air circulation. Quart. J. Roy. Meteor. Soc., 103, 441–456. References Kiladis, G.N., H. von Storch, and H. van Loon, 1989: Origin of the South Pacific convergence zone. J. Arkin, P.A. and B.N. Meisner, 1987: The relationship Climate, 2, 1185–1195. between large-scale convective rainfall and Kodama, Y.-M., 1992: Large-scale common features of cloud cover over the Western Hemisphere dur- subtropical precipitation zones (the Baiu frontal ing 1982–1984. Mon. Wea. Rev., 115, 51–74. zone, the SPCZ, and the SACZ). Part I: charac- Barlow, M., S. Nigam, and E.H. Berbery, 1998: Evolu- teristics of subtropical frontal zones. J. Meteor. tion of the North American monsoon system. J. Soc. Japan, 70, 813-836. Climate, 11, 2238–2257. Kodama, Y.-M., 1993: Large-scale common features Carvalho, L.M.V., C. Jones, and B. Liebmann, 2004: of subtropical convergence zones (the Baiu The South Atlantic Convergence Zone: intensity, frontal zone, the SPCZ, and the SACZ). Part II: form, persistence, and relationships with intra- conditions of the circulations for generating the seasonal to interannual activity and extreme STCZs. J. Meteor. Soc. Japan, 71, 581–610. rainfall. J. Climate, 17, 88–108. Kodama, Y.-M., 1999: Roles of the atmospheric heat Cook, K.H., 2000: The South Indian convergence zone sources in maintaining the subtropical conver- and interannual rainfall variability over southern gence zones—an aqua-planet GCM study. J. At- Africa. J. Climate, 13, 3789–3804. mos. Sci., 56, 4032–4049. Ding, Y.-H., 1994: Monsoon over China, Kluwer Aca- Lyons, S.W., 1991: Origin of convective variability over demic, 419 pp. equatorial southern Africa during austral sum- Douglas, M.W., 1995: The summertime low-level jet mer. J. Climate, 4, 23–39. over the Gulf of California. Mon. Wea. Rev., 123, Ninomiya, K., 1984: Characteristics of the Baiu front 2334–2347. as a predominant subtropical front in the Douglas, M.W., R.A. Maddox, K. Howard, and S. summer Northern Hemisphere. J. Meteor. Soc. Reyes, 1993: The Mexican monsoon, J. Climate, Japan, 62, 880–894. 6, 1665–1677. Ninomiya, K., 2004: Large- and meso-scale features Emori, S., A. Hasegawa, T. Suzuki, and K. Dairaki, of Meiyu-Baiu front associated with intense 2005: Validation, parameterization dependence, rainfalls. East Asia Monsoon, Edit. P.-C. Chang, and further projection of daily precipitation sim- World Scientific, 404–435. ulated with a high resolution atmospheric GCM. Ninomiya, K., 2007: Similarity and difference between Geophy. Res. Lett., 32, L06708, doi:10.1029/2004 the South Atlantic convergence zone and the GL02238. Baiu frontal zone simulated by an AGCM. J. Figueroa, S.N., P. Satyamurty, and P.L. Silva Dias, Meteor. Soc. Japan, 85, 277–299. 1995: Simulations of the summer circulation Ninomiya, K. and T. Akiyama, 1992: Multi-scale fea- over the South American region with an eta co- tures of Baiu, the summer monsoon over Japan ordinate model. J. Atmos. Sci., 52, 1573–1584. and the East Asia. J. Meteor. Soc. Japan, 70, 467– Findlater, J., 1969: Interhemispheric transport of air in 495. the lower troposphere over the western Indian Ninomiya, K. and T. Murakami, 1987: The early sum- Ocean. Quart. J. Roy. Meteor. Soc., 95, 400–403. mer rainy season (Baiu) over Japan. Monsoon Grotjahn, R., 1993: Global atmospheric circulations: ob- Meteorology, Edit. P.-C. Chang and T. N. Krish- servations and theories. Oxford University Press, namurti, Oxford Univ. Press, 93–121. 430 pp. Ninomiya, K., T. Enomoto, T. Nishimura, T. Suzuki, Higgins, R.W., K.C. Mo, and Y. Yao, 1998: Interannual and S. Matsumura, 2003: Synoptic- and meso- variability of the U.S. summer precipitation re- scale variations of the Baiu front simulated in an gime with emphasis on the southwestern mon- AGCM. J. Meteor. Soc. Japan, 81, 1387–1405. soon. J. Climate, 11, 2582–2606. Schmitz, T.S. and S.L. Mullen, 1996: Water vapor trans- Janowiak, J.E., 1988: An investigation of interannual port associated with the summertime North rainfall variability in Africa. J. Climate, 1, 240– American monsoon as depicted by ECMWF 255. analyses. J. Climate, 9, 1621–1634. K-1 Model Developers, 2004: K-1 coupled GCM Streten, N.A., 1973: Some characteristics of satellite- (MIROC) description. K-1 Tech. Rep., 1. CCSR, observed bands of persistent cloudiness over University of Tokyo, Japan, 34 pp. the Southern Hemisphere. Mon. Wea. Rev., 101, Kawatani, Y. and M. Takahashi, 2003: Simulation of 486–495. the Baiu front in a high resolution AGCM. J. Tao, S. and L. Chen, 1987: A review of recent research February 2008 K. NINOMIYA 165

on the East Asia summer monsoon over China. The Asian Monsoon, Springer, 65–87. Monsoon Meteorology, Edit. P.-C. Chang and T.N. Vincent, D.G., 1994: The South Pacific convergence Krishnamurti, Oxford Univ. Press, 50–92. zone (SPCZ): a review. Mon. Wea. Rev., 122, Trenberth, K.E., J.W. Hurrell, and D.P. Stepaniak, 1949–1970. 2006: The Asian monsoon: global perspectives.