Global setting: May 2019

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During May 2019, the atmosphere continued to respond to a warm pool of water in the central and western Pacific, with anomalous rainfall and convection centred along and just west of the International Dateline. The NINO3.4 Index anomaly for May (to the 26th) was +0.72 ̊C, however this was the second consecutive month the value has decreased. The NINO1+2 Index (in the far eastern Pacific) had an anomaly of+0.38 ̊C, down nearly 0.20 ̊C from last month.

Overall, rainfall and SST patterns remained consistent with a weak, central Pacific El Niño. The presence of enhanced convective forcing along the equator in the central and western Pacific remained consistent with El Niño-Modoki conditions.

However, there are signals that a trend toward neutral conditions may occur toward the end of the three-month period.

The Madden-Julian Oscillation (MJO)was active in phases 5-6-7-8 in May. In early June, the MJO is forecast to move through phases 1 and 2. Guidance points to large scale ascent from the Indian Ocean to the International Dateline from mid-June through to early July. This may drive several rounds of wet weather during June and, after a cooler than average start to the month, temperatures could moderate.

The Southern Oscillation Index (SOI) decreased (preliminary value -0.7),compared to April (0.0).The probability for oceanic El Niño conditions, according to the consensus from international models, is 66% for the June –August period. For September –November period, the probability is 56%, and for December 2019 –February 2020, El Niño remains the most likely outcome at 56%.

Sea Surface Temperatures

New Zealand’s coastal water temperatures for May 2019 have cooled for all areas compared to April, but remain above average for the time of year except in the east of the North Island. Sea surface temperatures over the Tasman Sea and far Southwest Pacific remain, in general, above average for the time of year with anomalies of around +1.0 ̊C, with pockets of +2.0 ̊C.

Warmer than average sea surface temperatures are forecast to persist during the coming three-month period. These warm seas may contribute to spells of unseasonable warmth and invigorate low pressure systems as they approach New Zealand from the west.

Monthly values of the Southern Oscillation Index (SOI), a measure of changes in atmospheric pressures across the Pacific, and the 3-month mean (black line). SOI mean values: May SOI -0.77; May-July average -0.5.
Differences from average May surface temperatures in the seas around New Zealand