Global setting: July 2019

During July 2019, SSTs dipped below El Niño thresholds (i.e. +0.70 ̊C) in the central Pacific for the first time in 5 months. The NINO3.4 Index anomaly (in the central Pacific) for the month of July (to the 28th) was +0.54 ̊C. Warmth was steadfast in the NINO4 region with a monthly value of +0.79 ̊C.

Upper-oceanic heat content anomalies remain slightly above average in the west-central equatorial Pacific, but have decreased compared to June. In the eastern part of the basin, heat content was below normal for the time of year. The distribution of ocean heat content across the Pacific reflects ongoing but weak El Niño-Modoki conditions.

Across the global tropics, rainfall patterns remained broadly consistent with a weak, central Pacific El Niño. Rainfall was enhanced just west of the international Dateline as well as between 10-20 ̊N and 10-20 ̊S. A convectively coupled Kelvin wave traversed the far eastern Pacific and South America during the month, which constructively interfered with the base state (i.e. El Niño). This may have contributed to the Southern Oscillation Index holding on the El Niño side of neutral (-0.9).

Trade winds were slightly weaker than normal in the western and central Pacific during July, similar to June. To the contrary, trade winds were stronger than normal in the eastern Pacific and led to the decrease of SSTs in the NINO 1+2 and NINO3 regions.

During August, enhanced trade winds are forecast to progress a bit farther west, extending to near the International Dateline. This could lead to additional cooling of SSTs in the central and eastern equatorial Pacific.

The probability for oceanic El Niño conditions, according to the consensus from international models, is 38% for the August –October period with a 56% chance for the development of ENSO-neutral conditions. This is the first time since July 2018 that ENSO neutral is the most likely outcome. For the November 2019 –January 2020 period, the probability for El Niño is 47% and neutral 42%. For the February –April 2020 period, the probability for El Niño is 51% and neutral 46%.

Sea Surface Temperatures

New Zealand’s coastal water temperatures for July2019 remained above average for five out of the six climate regions. Sea surface temperatures (SSTs) in the Tasman Sea and Southwest Pacific remain above average for the time of year, reflective of a lack of cold southerly outbreaks so far this winter. These warm seas may continue to prevent long-lasting cold spells from late winter into early spring.

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: July SOI -0.53; May-July average -0.8.
Differences from average July surface temperatures in the seas around New Zealand