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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 sim- ulation. 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 sim- ulated 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 dif- fer 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 strong- ly 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 accom- panied by the alternation of the winter and sum- mer 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) reanal- ysis 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.
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