The NINO3.4 Index anomaly over the last month (to 5 June) was 0.71˚C, near the La Niña threshold and a slight decrease compared to April. The May monthly SOI was +1.9, well within the La Niña range and the equal 2 nd highest May value on record since at least 1876 (May 1917 was higher and May 1956 was equal); this suggests that the atmospheric imprint of La Niña is strong.
In the subsurface equatorial Pacific, warmer than average water was pushing across the western and central part of the basin at 100 150 m depth. Spurred by a downwelling Kelvin Wave, these warmer waters could surface in the eastern Pacific in mid winter. Meanwhile, cooler than average waters continued from the surface to 75 m depth in the central Pacific, reflective of the ongoing La Niña. These signatures were reflected in upper oceanic heat content (OHC). During May, OHC decreased in the eastern equatorial Pacific and increased slightly in the central part of the basin.
Trade winds across the equatorial Pacific were stronger than normal during May, except for a corridor in the north east, west of Central America. A period of reduced trades is expected in early June, followed by a re enhancement of trades from mid month. The latter may induce a cooling effect in the central Pacific during mid winter.
La Niña conditions are favoured to continue during June August (60% chance, an increase of 13% compared to last month). During September November, there is around a 55% chance for La Niña and a 40% chance for ENSO neutral. Should La Niña persist through or redevelop by December February 2022 23, the current event would qualify as a “triple dip”. Since 1900, using an Oceanic Niño threshold of 0.5, the following would qualify as triple dip events: 1998 2000, 1983 1985, 1973 1975, and 1908 1910.
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