September 2023 Climate Forecast Discussion

Sep 2023 Climate Forecast Discussion for the period Oct-Dec 2023 through Jan-Mar 2024

Note: The IRI seasonal forecasts of precipitation and temperature issued this month are based on an objective calibration procedure that combines the NCEP-CFSv2, CanSIPS-IC3, COLA-RSMAS-CCSM4, and GFDL-SPEAR, and NASA-GEOSS2S models. The climatological base period for normal is 1991-2020. The SST anomalies cited below refer to the OISST.v2.1 SST data set.

In August 2023, sea surface temperatures (SSTs) in the central-eastern equatorial Pacific Ocean were well above the El Niño threshold (the NINO3.4 index value in August 2023 was +1.3 °C). Weekly-averaged SST anomalies in the NINO3.4 region have warmed further and reached +1.6 °C during the week centered on 06 Sep 2023. According to the early-September CPC ENSO forecast, the tropical Pacific is experiencing El Niño conditions and an El Niño Advisory remains in place. Both observations and model forecasts indicate the presence of warm SST anomalies in the Atlantic Ocean, and a positive Indian Ocean Dipole during the forecast period.

Strongly or moderately enhanced probabilities for below-normal precipitation are predicted for most of the northeastern half of South America, the Maritime Continent, and most of Australia during much of the forecast period. Weakly enhanced probabilities of below-normal precipitation are also forecasted for parts of southern and central Africa during Nov-Jan, Dec-Feb, Jan-Mar, 2024. A strong tilt of the odds toward above-normal precipitation is predicted over and around the Greater Horn of Africa, the Middle East, and central Asia during the forecast period. Gradually increasing probabilities for above-normal precipitation are also forecasted over and around Uruguay, southern and southeast US, and northern and eastern China during the forecast period. Many, though not all, of these precipitation patterns are consistent with historical El Niño as well as positive Indian Ocean Dipole events.

Regarding temperature, strongly enhanced probabilities for above-normal temperature are forecasted across most land areas throughout the forecast period. A few local instances of enhanced probabilities for below-normal temperature do appear locally, most notably over southeast Madagascar throughout the forecast period. During boreal winter (Dec-Feb, and Jan-Mar, 2024), the forecast indicates equal chances of the three temperature tercile categories for much of the United States.