Updated: 05-12-2014
Source:
Current Situation and Outlook
Steady warming of the tropical Pacific Ocean over the past two months has resulted in ocean surface temperatures reaching weak El Nino levels. However, the overlying atmosphere is showing a mix of responses, with some indicators exceeding El Nino thresholds, while others remain neutral. Models and expert opinion suggest there is approximately a 70% chance that a weak El Nino event will become established before the end of February 2015. If an event does occur, it is most likely to be weak and persist for the first quarter of 2015. Some El Nino-like impacts have already been observed in several countries, and impacts in other areas may develop regardless of whether an El Nino becomes fully established. National Meteorological and Hydrological Services and other agencies will continue to monitor Pacific Ocean conditions for further El Nino developments, and will assess the most likely local impacts.
In early November, after more than 5 months of warm-neutral to borderline El Nino levels, sea surface temperatures in the east-central tropical Pacific Ocean reached weak El Nino levels. However, despite this oceanic warming, only some of the atmospheric indicators of El Nino have appeared. El Nino is typically associated with and amplified by a difference in the atmospheric pressure between the eastern and western tropical Pacific, and changes in the cloudiness, upper level winds, low level winds, and rainfall across the tropical Pacific. All of these indicators reflect large scale changes in the atmospheric state, which in turn has connections to global weather patterns. At present, the observed changes in these atmospheric indicators are mixed, with some reaching El Nino levels (e.g. surface pressure, upper level winds and, sporadically, the lower level winds) but others remaining closer to normal (most notably, the cloudiness and rainfall patterns). However, heat stored below the surface of the tropical Pacific has clearly increased during November, meaning additional warming of the sea surface is possible in the coming few months. Likewise, average to cooler-than-average ocean temperatures in the far western tropical Pacific combined with increasing eastward positive sea surface temperature anomalies mean west-to-east differences in sea surface temperature anomaly are becoming more El Nino-like than they have been since the possibility of an El Nino first emerged in March/April 2014. Furthermore, some El Nino-like impacts are being observed in parts of South America, Australia, New Zealand, Asia and Africa.
The latest outlooks from climate models and expert opinion favour a weak El Nino in both the ocean and atmosphere during the current season of November 2014 to January 2015, lasting through the northern winter (December to February) and well into the first quarter of 2015. International climate model outlooks suggest a 70% to 75% chance of El Nino during the December to March period. Models also indicate that if an event occurs, it is likely to be weak, though a moderate strength event cannot be completely ruled out. A strong event appears very unlikely. If an El Nino event does occur, the earlier that above-average cloudiness and rainfall appear in the central tropical Pacific Ocean, the greater the likelihood that the event will continue through the first quarter of 2015.
It is important to note that El Nino and La Nina are not the only factors that drive global climate patterns. At the regional level, seasonal outlooks assess the relative impacts of both the El Nino/La Nina state and other locally relevant climate drivers. For example, the state of the Indian Ocean Dipole, or the Tropical Atlantic SST Dipole, may impact the climate in the adjacent land areas. Locally applicable information is available via regional/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).
In summary:
Tropical Pacific Ocean surface temperature anomalies exceeded the threshold for a weak El Nino during November, with values of 0.5 to 1.0 degrees Celsius above normal. Some atmospheric indicators have also become indicative of weak El Nino, while others have remained neutral throughout the recent several months;
Important atmospheric variables that have not shown a pattern indicative of El Nino are cloudiness and rainfall, which have remained near average across the central and eastern tropical Pacific.
As of mid-November 2014, model outlooks suggested an approximately 70-75% chance that oceanic warming will exceed El Nino thresholds during December to March;
Although a range of outcomes is possible, models surveyed and expert opinion favour a weak event, with smaller chances for a near-miss or a moderate strength event. A strong event appears very unlikely.
Regardless of whether or not this event fully develops, some impacts are still likely to be felt.
Editor Shi Long