Global setting: April 2019

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During April 2019, the atmosphere once again responded to a warm pool of water in the central and western Pacific, with anomalous atmospheric lift, rainfall, and convection centred along and just west of the International Dateline. The NINO3.4 Index anomaly for April(to the 28th) was +0.76 ̊C, consistent with what occurred during March. The NINO1+2 Index (in the far eastern Pacific)had an anomaly of +0.57 ̊C.

Rainfall and SST patterns remained consistent with a weak, central Pacific El Niño.

At the end of April, the Madden-Julian Oscillation (MJO) became active and was expected to constructively interfere with the El Niño base-state during the month of May. As this occurs, the atmospheric response may become more aligned with an east-based El Niño(e.g. westerly quarter winds becoming more common for New Zealand).

The Southern Oscillation Index (SOI) was near neutral during April (preliminary value -0.1), an increase compared to March(-0.6). The rising branch of the Walker Circulation focused along and just west of the Dateline and a sinking branch over the central and eastern Pacific. This supports the idea that the ongoing El Niño event is of the central-based type, nudging the synoptic pressure patterns near Darwin and Tahiti away from what might be considered their typical positions.

The probability for oceanic El Niño conditions, according to the consensus from international models, is 83% for the May –July period. For August –October, the probability is 64%. For November 2019 –January 2020, El Niño remains the most likely outcome at 58%. This continues to suggest the potential for a ‘protracted’ event (multi-year duration).

Sea Surface Temperatures

New Zealand’s coastal water temperatures for April 2019 continued above or well above average in all areas. Marine heatwave conditions continued in the Tasman Sea.

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.

Differences from average April surface temperatures in the seas around New Zealand
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: April SOI 0.00; February-April average -0.6.