Island Climate Update 219 - December 2018
Sea surface temperatures (SSTs) across the east-central tropical Pacific have reached weak El Niño conditions. Over the past month, SSTs in the central Pacific (NINO3.4 Index) have continued to warm, increasing to an anomaly of +0.9C. This marks the third consecutive month SST anomalies in the central Pacific have exceeded 0.7 C, which meets NIWA’S technical oceanicdefinition for El Niño.
However, the atmosphere has yet to respond to this additional warmth and become truly coupled with the oceanin a manner typically associated with an El Niño event, and the atmospheric indicators so far fail to signal El Niño conditions. One reflection of this is the Southern Oscillation Index (SOI), which was at –0.1 for November, i.e.in the neutral range.
Trade winds, until recently, have been generally near or weaker than average for the month of November. However, a significant Westerly Wind Burst (WWB) between the Date Line and 120oW is expected through to the first week of December. This WWB will likely result in an additional downwelling Kelvin wave, which will may tip the atmosphere toward El Niño over the coming weeks in what would become an unusually late onset.
The consensus from international models is for the tropical Pacific to transition towards El Niño over the next three-month period (94% chance over December 2018 – February 2019). The probability for El Niño remains high through autumn 2019, with an 85% chance for occurrence in the March – May 2019 period. In fact, the probability of El Nino remains unusually elevated (66% chance) through to the next Southern Hemisphere winter season (signalling the possibility of a protracted El Niño event extending over two years).