ENSO

• Outline:
  • The Enso Phenomenon in the Tropics
  • Relationship to Mid-Latitude Weather
  • Forecasting ENSO

• The ENSO Phenomenon in the Tropics
  • On-line Sources of Info
    • El Nino update
    • ENSO Information
    • Overview
    • FAQ on ENSO
    • Background Info
    • Technical Background

  • Coupled atmosphere-ocean interactions in the equatorial Pacific

  • Complex interactions between year-to-year fluctuations, the seasonal cycle, and high frequency (intraseasonal-synoptic) fluctuations

  • El Nino- Warm Phase:
    • anomalously warm SST in the central-eastern equatorial Pacific
    • occurs every 2-7 years
    • Big events: 1957-58; 1966; 1973; 1982-83 (biggest to date); 1987; 1991-95 (hyper El Nino); 1997

      

    • weakening of trade winds along equator
    • enhanced convection in central equatorial Pacific; diminished precipitation over maritime continent

      

      

  • La Nina- Cool Phase:
    • anomalously cool SST in the central-eastern equatorial Pacific
    • occurs every 2-7 years
    • Big Events: 1955; 1971; 1974; 1976; 1989;1999

    • strengthening of trade winds along equator
    • diminshed convection in central equatorial Pacific; enhanced precipitation over maritime continent

  • Comparison of 97 and Earlier Events

    

    • Animations

• Relationship to Mid-Latitude Weather
  • ENSO affects mid-latitudes indirectly
  • ENSO causes adjustments to the planetary scale circulation; affects the position of the subtropical and polar jets
  • Dynamically, this adjustment can be explained in terms of:
    • response to anomalous distribution of diabatic heating
      • enhanced convection in central-eastern equatorial Pacific
      • diminished convection over maritime continent
    • direct impacts onto equatorial and subtropical circulation
      • anomalous upper tropospheric easterlies near the equator in the vicinity ofthe excessive diabatic heating
      • anomalously strong westerlies to the north of an accentuated subtropical circulation
      • dispersion of energy away from anomalous energy source regions near equator
        • excessive diabatic heating in the central equatorial Pacific and the diminshed diabatic heating over the maritime continent
        • stationary response to changes in diabatic heating requires energy to be propagated away from source region
        • for energy to propagate from equator requires westerly flow aloft, which is not usually present near equator during NH winter
      • dispersion of energy away from anomalous energy source regions in subtropics
        • adjustment of mass-wind fields in subtropics as a result of diabatic heating anomalies near equator
    • general circulation models capable of reproducing many aspects of response of mid-latitude circulation to anomalous diabatic heating in tropics
    • considerable sensitivity of response in mid-latitudes to:
      • seasonal cycle; timing of onset and demise of event
        • biggest mid-latitude response in late winter (January-March)
        • smallest reponse during mid-latitude summer (July-August)
      • interactions with in-situ mid-latitude climate variations due to other sources (e.g., non-linear dynamical interactions, snow-ice variations)
  • Composite Patterns
  • Climate Anomalies: 1979-1995
  • Seasonal Climate Extreme Risks