IRI Climate Digest
July-August-September Seasonal Forecast
Date and Period of Forecast
In June 2003, the IRI prepared a Climate Outlook for July 2003-December 2003. Here we provide a subset of the June IRI Forecast. The forecasts are updated monthly and can be found in their entirety at http://iri.columbia.edu/climate/forecast/net_asmt/.
This Climate Outlook is dependent on the quality of the sea surface temperature (SST) predictions. For the tropical Pacific, these predictions can be expected to provide useful information, but there is some uncertainty concerning the detailed evolution of SSTs. Spread in global SST predictions is a source of uncertainty in the Outlook provided here. The procedures, models, and data used to derive this Climate Outlook may be somewhat different from those used by National Meteorological Services in particular regions and may differ from the official forecasts issued in those areas.
The current status of seasonal-to-interannual climate forecasting allows prediction of spatial and temporal averages, and does not fully account for all factors that influence regional and national climate variability. This Outlook is relevant only to seasonal time scales and relatively large areas; local variations should be expected. For further information concerning this and other guidance products, users are strongly advised to contact their National Meteorological Services.
The Outlook for other regions of the globe for the seasons July-August-September through October-November-December can be found at Net Assessment forecasts.
Maps show expected precipitation probabilities in tercile classes. The maps indicate probabilities that seasonal precipitation will fall into the wettest third of the years (top number), the middle third of years (middle number) or the driest third of the years (bottom). An outlook of climatology "C" (no color) indicates equal probabilities in each class; i.e., there is no basis for favoring the forecast of any particular category. Boundaries between sub-regions should be considered transition zones, and their location considered to be only qualitatively correct. Color shading indicates which tercile class has the greatest probability of occurrence with darker shading indicating greater likelihood as shown by the legend to the right of the plots.
July-August-September 2003 Global Precipitation Probabilities
This forecast consists of expected probabilities of temperature in tercile classes. The terciles refer to the seasonal temperature falling into the warmest third of the years (top tercile), the middle third of years (middle tercile) or the coldest third of the years (bottom tercile). Boundaries between sub-regions should be considered transition zones, and their location considered to be only qualitatively correct. Color shading indicates which tercile class has the greatest probability of occurrence with darker shading indicating greater likelihood as shown by the legend on the bottom of the plots. Note:
The IRI is in the process of implementing new graphics for its forecast products. Currently, global maps only indicate (by shading) the tercile class with the greatest probability of occurrence.
Please consult the regional maps at Net Assessment forecasts for the probabilities of each tercile class.
July-August-September 2003 Global Temperature Probabilities
Of relevance in the preparation of this outlook is the potential for a developing La Niņa event. In early March 2003 equatorial Pacific sea surface temperatures (SSTs) returned from above-normal to near-normal in the eastern equatorial Pacific, and they continued cooling through early June reaching values on the order of -2°C in the far eastern equatorial Pacific. Weak La Niņa conditions are indicated in the SST predictions on which these climate forecasts are based. See the IRI's ENSO update for a discussion on the La Niņa outlook (see IRI Probabilistic ENSO forecast). Warmer than average SSTs continue to dominate much of the tropical Western Pacific and Indian Ocean (SSTs). These are predicted to decrease through the forecast period. (July-September 2003, August-October 2003, September-November 2003, October-December 2003). Warmer than average SSTs currently exist in the northern and southern sub-tropical Atlantic Ocean. In May the equatorial Atlantic developed a small region of below-normal SSTs. The tropical Atlantic SST forecast slowly damps these anomalies. However, more recent observations show continuing development of cold anomalies on the equator and southward along the west coast of Africa. This discrepancy between the predicted and recently observed SSTs has been accounted for in the final forecasts.
The following procedures and information were used to prepare this Climate
- Coupled ocean-atmosphere model predictions of tropical Pacific SST
-- particularly heavy weighting has been given to the NOAA /NCEP,
Climate Modeling Branch coupled model
- Statistical forecasts of Indian Ocean and Atlantic Ocean sea surface
- The response of Atmospheric global circulation model (GCM) predictions
to the present and predicted SST patterns
- Statistical analyses
- Appropriate Regional Climate Outlook Forum consensus guidance.
Additional sources of information include ACMAD, COLA, CPTEC, CPC/NOAA,
CMC, Department of Natural Resources (Queensland, Australia), NIWA, ECMWF,
Indian Meteorological Department, PAGASA, Bureau of Meteorology, and the
South African Weather Service.