• Blackmon, M. L., , Y.-H. Lee, , and J. M. Wallace, 1984: Horizontal structure of 500-mb height fluctuations with long, intermediate and short time scales. J. Atmos. Sci., 41, 961980, doi:10.1175/1520-0469(1984)041<0961:HSOMHF>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Cook, K. H., 2000: The south Indian convergence zone and interannual rainfall variability over southern Africa. J. Climate, 13, 37893804, doi:10.1175/1520-0442(2000)013<3789:TSICZA>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Dee, D. P., and et al. , 2011: The ERA-Interim reanalysis: Configuration and performance of the data assimilation system. Quart. J. Roy. Meteor. Soc., 137, 553597, doi:10.1002/qj.828.

    • Search Google Scholar
    • Export Citation
  • Ding, Q., , and B. Wang, 2005: Circumglobal teleconnection in the Northern Hemisphere summer. J. Climate, 18, 34833505, doi:10.1175/JCLI3473.1.

    • Search Google Scholar
    • Export Citation
  • Fauchereau, N., , B. Pohl, , C. J. C. Reason, , M. Rouault, , and Y. Richard, 2009: Recurrent daily OLR patterns in the Southern Africa/Southwest Indian Ocean region, implications for South African rainfall and teleconnections. Climate Dyn., 32, 575591, doi:10.1007/s00382-008-0426-2.

    • Search Google Scholar
    • Export Citation
  • Gill, A. E., 1980: Some simple solutions for heat-induced tropical circulation. Quart. J. Roy. Meteor. Soc., 106, 447462, doi:10.1002/qj.49710644905.

    • Search Google Scholar
    • Export Citation
  • Harrison, M. S. J., 1984: A generalized classification of South African summer rain-bearing synoptic systems. J. Climatol., 4, 547560, doi:10.1002/joc.3370040510.

    • Search Google Scholar
    • Export Citation
  • Hart, N. C. G., 2012: Synoptic-scale rainfall patterns over southern Africa: Scale-interactions with large-scale modes of variability. Ph.D. Thesis, University of Cape Town, 99 pp. [Available from http://www.met.reading.ac.uk/~rb904381/HartNCG_PhD_Dissertation.pdf.]

  • Hart, N. C. G., , C. J. C. Reason, , and N. Fauchereau, 2010: Tropical–extratropical interactions over southern Africa: Three cases of heavy summer season rainfall. Mon. Wea. Rev., 138, 26082623, doi:10.1175/2010MWR3070.1.

    • Search Google Scholar
    • Export Citation
  • Hoskins, B. J., , and T. Ambrizzi, 1993: Rossby wave propagation on a realistic longitudinally varying flow. J. Atmos. Sci., 50, 16611671, doi:10.1175/1520-0469(1993)050<1661:RWPOAR>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Hsu, H.-H., , and S.-H. Lin, 1992: Global teleconnections in the 250-mb streamfunction field during the Northern Hemisphere winter. Mon. Wea. Rev., 120, 11691190, doi:10.1175/1520-0493(1992)120<1169:GTITMS>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Karoly, D. J., 1989: Southern Hemisphere circulation features associated with El Niño–Southern Oscillation events. J. Climate, 2, 12391252, doi:10.1175/1520-0442(1989)002<1239:SHCFAW>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Kiladis, G. N., , and K. M. Weickmann, 1992: Extratropical forcing of tropical Pacific convection during northern winter. Mon. Wea. Rev., 120, 19241939, doi:10.1175/1520-0493(1992)120<1924:EFOTPC>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Liebmann, B., , and C. A. Smith, 1996: Description of a complete (interpolated) outgoing longwave radiation dataset. Bull. Amer. Meteor. Soc., 77, 12751277.

    • Search Google Scholar
    • Export Citation
  • Lindesay, J. A., , and M. R. Jury, 1991: Atmospheric circulation controls and characteristics of a flood event in central South Africa. Int. J. Climatol., 11, 609627, doi:10.1002/joc.3370110604.

    • Search Google Scholar
    • Export Citation
  • Lyon, B., , and S. J. Mason, 2007: The 1997–98 summer rainfall season in southern Africa. Part I: Observations. J. Climate, 20, 51345148, doi:10.1175/JCLI4225.1.

    • Search Google Scholar
    • Export Citation
  • Lyons, S. W., 1991: Origins of convective variability over equatorial southern Africa during austral summer. J. Climate, 4, 2339, doi:10.1175/1520-0442(1991)004<0023:OOCVOE>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Macron, C., , B. Pohl, , Y. Richard, , and M. Bessafi, 2014: How do tropical temperate troughs form and develop over southern Africa? J. Climate, 27, 16331647, doi:10.1175/JCLI-D-13-00175.1.

    • Search Google Scholar
    • Export Citation
  • Matsuno, T., 1966: Quasi-geostrophic motions in the equatorial area. J. Meteor. Soc. Japan, 44, 2543.

  • Pohl, B., , N. Fauchereau, , Y. Richard, , M. Rouault, , and C. J. C. Reason, 2009: Interactions between synoptic intraseasonal and interannual convective variability over southern Africa. Climate Dyn., 33, 10331050, doi:10.1007/s00382-008-0485-4.

    • Search Google Scholar
    • Export Citation
  • Ratna, S. B., , S. K. Behera, , J. V. Ratnam, , K. Takahashi, , and T. Yamagata, 2013: An index for tropical temperate troughs over southern Africa. Climate Dyn., 41, 421441, doi:10.1007/s00382-012-1540-8.

    • Search Google Scholar
    • Export Citation
  • Ratnam, J. V., , S. K. Behera, , Y. Masumoto, , and T. Yamagata, 2014: Remote effects of El Niño and Modoki events on the austral summer precipitation of southern Africa. J. Climate, 27, 3802–3815, doi:10.1175/JCLI-D-13-00431.1.

    • Search Google Scholar
    • Export Citation
  • Reynolds, R. W., , T. M. Smith, , C. Liu, , D. B. Chelton, , K. S. Casey, , and M. G. Schlax, 2007: Daily high-resolution-blended analyses for sea surface temperature. J. Climate, 20, 54735496, doi:10.1175/2007JCLI1824.1.

    • Search Google Scholar
    • Export Citation
  • Richard, Y., , N. Fauchereau, , I. Poccard, , M. Rouault, , and S. Trzaska, 2001: 20th century droughts in southern Africa: Spatial and temporal variability, teleconnections with oceanic and atmospheric conditions. Int. J. Climatol., 21, 873885, doi:10.1002/joc.656.

    • Search Google Scholar
    • Export Citation
  • Sardeshmukh, P. D., , and B. J. Hoskins, 1988: The generation of global rotational flow by steady idealized tropical divergence. J. Atmos. Sci., 45, 12281251, doi:10.1175/1520-0469(1988)045<1228:TGOGRF>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Todd, M. C., , and R. Washington, 1999: Circulation anomalies associated with tropical–temperate troughs in southern Africa and the south west Indian Ocean. Climate Dyn., 15, 937951, doi:10.1007/s003820050323.

    • Search Google Scholar
    • Export Citation
  • Todd, M. C., , R. Washington, , and P. I. Palmer, 2004: Water vapour transport associated with tropical–temperate trough systems over southern Africa and the southwest Indian Ocean. Int. J. Climatol., 24, 555568, doi:10.1002/joc.1023.

    • Search Google Scholar
    • Export Citation
  • Tomas, R. A., , and P. J. Webster, 1994: Horizontal and vertical structure of cross-equatorial wave propagation. J. Atmos. Sci., 51, 14171430, doi:10.1175/1520-0469(1994)051<1417:HAVSOC>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Usman, M. T., , and C. J. C. Reason, 2004: Dry spell frequencies and their variability over southern Africa. Climate Res., 26, 199211, doi:10.3354/cr026199.

    • Search Google Scholar
    • Export Citation
  • Vigaud, N., , B. Pohl, , and J. Crétat, 2012: Tropical–temperate interactions over southern Africa simulated by a regional climate model. Climate Dyn., 39, 28952916, doi:10.1007/s00382-012-1314-3.

    • Search Google Scholar
    • Export Citation
  • Wallace, J. M., , and D. S. Gutzler, 1981: Teleconnections in the geopotential height field during the Northern Hemisphere winter. Mon. Wea. Rev., 109, 784812, doi:10.1175/1520-0493(1981)109<0784:TITGHF>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Washington, R., , and M. Todd, 1999: Tropical–temperate links in southern Africa and southwest Indian Ocean satellite-derived daily rainfall. Int. J. Climatol., 19, 16011616, doi:10.1002/(SICI)1097-0088(19991130)19:14<1601::AID-JOC407>3.0.CO;2-0.

    • Search Google Scholar
    • Export Citation
  • Waugh, D. W., , and L. M. Polvani, 2000: Climatology of intrusions into the tropical upper troposphere. Geophys. Res. Lett., 27, 38573860, doi:10.1029/2000GL012250.

    • Search Google Scholar
    • Export Citation
  • Webster, P. J., , and J. R. Holton, 1982: Cross-equatorial response to middle-latitude forcing in a zonally varying basic state. J. Atmos. Sci., 39, 722733, doi:10.1175/1520-0469(1982)039<0722:CERTML>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 15 15 3
PDF Downloads 10 10 3

Role of Cross-Equatorial Waves in Maintaining Long Periods of Low Convective Activity over Southern Africa

View More View Less
  • 1 Application Laboratory, JAMSTEC, Yokohama, Japan
© Get Permissions
Restricted access

Abstract

Periods of low convective activity over southern Africa during the peak rainy season from December to February are known to be due to the northeastward displacement of the tropical temperate trough (TTT) systems from the landmass. In this study, using Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data, the authors show that the displacement of the TTT systems during long periods of low convective activity has origins in the Northern Hemisphere. Using standardized area-averaged outgoing longwave radiation (OLR) daily anomalies over southern Africa, long periods of low convective activity are defined as periods of positive OLR anomalies lasting consecutively for 5 or more days with a standard deviation of 1 or more. An eddy streamfunction anomaly composite of the periods of low convective activity shows an upper-level anomalous wave originating in the Northern Hemisphere and extending to southern Africa from the eastern Pacific and displacing the tropical–extratropical cloud bands from the southern African landmass into the southwestern Indian Ocean. The wave train is also seen to generate an anticyclonic anomaly over southern Africa, resulting in suppressed convective activity. Understanding the causes of the long periods of low convective activity will help in improving their predictability and also the predictability of seasonal rainfall over southern Africa.

Corresponding author address: J. V. Ratnam, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, Kanagawa 236-0001, Japan. E-mail: jvratnam@jamstec.go.jp

Abstract

Periods of low convective activity over southern Africa during the peak rainy season from December to February are known to be due to the northeastward displacement of the tropical temperate trough (TTT) systems from the landmass. In this study, using Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data, the authors show that the displacement of the TTT systems during long periods of low convective activity has origins in the Northern Hemisphere. Using standardized area-averaged outgoing longwave radiation (OLR) daily anomalies over southern Africa, long periods of low convective activity are defined as periods of positive OLR anomalies lasting consecutively for 5 or more days with a standard deviation of 1 or more. An eddy streamfunction anomaly composite of the periods of low convective activity shows an upper-level anomalous wave originating in the Northern Hemisphere and extending to southern Africa from the eastern Pacific and displacing the tropical–extratropical cloud bands from the southern African landmass into the southwestern Indian Ocean. The wave train is also seen to generate an anticyclonic anomaly over southern Africa, resulting in suppressed convective activity. Understanding the causes of the long periods of low convective activity will help in improving their predictability and also the predictability of seasonal rainfall over southern Africa.

Corresponding author address: J. V. Ratnam, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, Kanagawa 236-0001, Japan. E-mail: jvratnam@jamstec.go.jp
Save