IRI Climate Digest
July-August-September Seasonal Forecast
Date and Period of Forecast
In June 2002, the IRI prepared a Climate Outlook for July-December 2002. 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 June-July-August and September-October-November 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 2002 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 2002 Global Temperature Probabilities
Of relevance in the preparation of this outlook is the prediction of warmer than average conditions
in the equatorial Pacific for the next 6 to 9 months. Currently the sea surface temperatures (SSTs)
across much of the eastern and central equatorial are warmer than their long-term average (SSTs), particularly in
the central and western part of the basin, where warm SST anomalies have been present for nearly a
year, but now also closer to the coast of Peru and Ecuador. Weakly warm equatorial Pacific SST
conditions (approximately 0.5 C) are in effect for the first seasons of the forecast, July-September 2002, August-October 2002. The
central equatorial Pacific is currently
observed to be warmer (SSTs over 1 degree C above normal) than these predictions, and this
discrepancy has been considered in developing the climate forecasts. The predictions attain the level
of a weak El Nino (SSTs between 0.5 and 1 degree C above normal) by the end of the forecast period,
Note that this prediction for a weak El Nino is that given by a particular ENSO prediction model. It
is described here because it served as the boundary forcing for the following climate forecast,
which is primarily based on dynamical global climate models. This should not be confused with the
IRI's ENSO Statement September-November 2002, which while indicating an enhanced potential for an El Nino to develop in 2002 also highlights the uncertainty still present in the prediction at this time. In the other tropical oceans, warmer than average SSTs continue to dominate much of the Indian Ocean, and are not expected to decrease as rapidly as suggested by the SST predictions. However, the actual long-term evolution of the Indian Ocean SSTs is difficult to foresee at this time. The area of above-average temperature in the tropical south Atlantic Ocean is expected to persist through at least the first half of the forecast period. The area of below-average temperature off the western coast of Africa is not expected to persist beyond the first forecast season.
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.