TECHNICAL
METSERVICE UPDATE LA NIÑA WATCH Words by Georgina Griffiths, MetService Meteorologist
Eyes on the tropics The El Niño Southern Oscillation (ENSO) climate system is an important climate driver for many parts of the world. Because New Zealand lies in the mid-latitudes, on the edge of the tropics, ENSO can influence the types and frequency of weather maps we see here; but New Zealand typically only sees large impacts on our wind flows, rainfall and temperature during intense El Niño or La Niña events. ENSO is a tropical climate system operating along the equator in the Pacific Ocean. For the first half of 2020, the El Niño Southern Oscillation (ENSO) was neutral – neither El Niño nor La Niña.
Sea temperatures at the surface, as well as the sub-surface, running either side of the equator between Australia and South America, are indicators of the ENSO oceanic state. The most commonly used oceanic indicator is the “NINO 3.4” index (Figure 1), which captures sea temperature deviations in the central Pacific Ocean (Figure 2). Sea surface temperatures in the equatorial tropical Pacific Ocean cooled during June (trending towards La Niña thresholds), and were below average in the eastern Pacific region by mid-July. In the central Pacific area, sea surface temperatures remained near average (at the time of writing).
The NINO3.4 Index
Figure 1: The NINO3.4 Index. This index tracks sea surface temperature anomalies (deviations from normal) in the equatorial Pacific Ocean between latitudes 5N and 5S, and between longitudes 120W and 170W. When the sea surface temperatures are below normal (cooler) by an average 0.8C across a 5-month period or more, La Niña conditions are in place. When the sea surface temperatures are above normal (warmer) by an average 0.8C across a 5-month period or more, El Niño conditions are in place.
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NZGROWER : AUGUST 2020
