The recent multi-year (so-called “triple dip”) La Niña event is over and the tropical Pacific is currently in an ENSO-neutral state. WMO Global Producing Centres of Long-Range Forecasts predict that there is a 40% chance for ENSO-neutral conditions to continue during May-July 2023, and there is a 60% chance for a transition from ENSO-neutral to El Niño during May-July. El Niño conditions are likely to persist thereafter, with current forecasts and expert assessment indicating the chances gradually increasing to about 60-70% in June-August, 70-80% in July-through October 2023, while the chances of ENSO-neutral are around 30-40% and 20-30%, respectively. The current forecasts rule out La Niña development over the next six months. This month’s update therefore indicates a significantly heightened probability of a transition to El Niño conditions this year, while noting that the Northern Hemisphere ‘spring predictability barrier’, a period characterized by somewhat lower predictive skill, continues to lead to enhanced uncertainty associated with predictions made at this time. National Meteorological and Hydrological Services (NMHSs) will closely monitor changes in the state of ENSO over the coming months and provide updated outlooks, as needed.
From February 2023 onwards, there has been a significant increase in sea surface temperatures in the Equatorial Pacific, with notably stronger warming along the coast of South America. During the week centered on 12 April 2023, the sea surface temperatures in the far eastern Pacific region known as the Niño 1+2 region have been 2.7 degrees Celsius above normal. The sustained warming of sea surface temperatures in the eastern Equatorial Pacific is attributed to the warmer subsurface temperatures and a downwelling Kelvin wave. Consequently, Coastal El Niño (Coastal El Niño occurs along the western coast of South America, particularly in the regions of Peru and Ecuador) conditions are considered to have been established.
As of mid-April 2023, the sea surface temperatures and other atmospheric and oceanic indicators in the central-eastern tropical Pacific are consistent with ENSO-neutral conditions. In the atmosphere, convective activity over the equatorial Pacific near the International Date Line is near normal. The Southern Oscillation Index (SOI: defined by the standardized Tahiti minus Darwin sea-level pressure difference), is also within the ENSO-neutral range. Easterly trade winds over the western equatorial Pacific are stronger than normal, while normal or weaker in the eastern Pacific. At upper levels of the atmosphere (200-hPa), strengthened westerly winds are observed across the central and east-central equatorial Pacific. In the ocean, positive sub-surface temperature anomalies have spread across the Pacific Ocean, though remaining at relatively deeper levels (100 to 250 meters) in the western and central Pacific, and between the surface and 100 meters in the eastern Pacific (120°W to 80°W). Overall, the observed oceanic and most atmospheric conditions indicate ENSO-neutral conditions in the Pacific.
The WMO Global Producing Centres of Long-Range Forecasts routinely issue global-scale climate forecasts for the coming months, initializing their dynamical seasonal prediction systems using observations. Their latest forecasts and expert assessment indicate that there is a high probability for the sea surface temperature anomalies in the central and eastern equatorial Pacific to warm further, with 60% chance of reaching El Niño levels in May-June 2023 and persisting throughout the coming months: 60-70% chance in June-August, and 70-80% chances during July-September and August-October 2023. The probability for the ENSO-neutral category is 40% in May-July, decreasing to 30-40% in June-August, and 20-30% during July-September and August-October 2023. The chance of La Niña re-development is near zero.
Weaker trade winds, warmer subsurface temperatures, and significant warming in the eastern equatorial Pacific, are conducive to the onset of El Niño. It is worth noting, however, that the Northern Hemisphere 'spring predictability barrier', a period characterized by somewhat lower predictive skill, has not yet ended. Nevertheless, these recent developments in oceanic and atmospheric conditions in the tropical Pacific, along with consistent multi-model predictions and expert assessments, are indicating a strong likelihood of El Niño onset in early boreal summer and its continuation into the boreal autumn of 2023.
It is important to note that El Niño and La Niña are not the only factors that drive global and regional climate patterns, and further that the magnitudes of ENSO indicators do not directly correspond to the magnitudes of their effects. At the regional level, seasonal outlooks need to assess the relative effects of both the ENSO state and other locally relevant climate drivers. Regionally and locally applicable information is made available via regional and national seasonal climate outlooks, such as those produced by WMO Regional Climate Centres (RCCs), Regional Climate Outlook Forums (RCOFs) and National Meteorological and Hydrological Services (NMHSs).
