Prepare for floods: as El Nino rapidly transitions to La Nina

While still in the grip of a drought, Namibia has to prepare for potential flooding towards the end of the year, when strong La Nina conditions are expected to be present in the equatorial Pacific Ocean.
The latest El Nino update released earlier this week by the National Oceanic and Atmospheric Administration (NOAA) in the US Department of Commerce, shows rapidly declining sea surface temperatures in the equatorial Pacific Ocean close to the South American continent.
Sea surface temperatures between 90°W and 130°W turned neutral earlier in April. By this week however, sea surface temperatures in the equatorial Pacific from 80°W to 110°W have gone below the threshold, registering some 0.6°C below average. This small area from the Ecuadorian coast to the Galapagos Islands is designated Nino zones 1 and 2 by meteorologists. Although this area does not feature in the official definition for an El Nino event, it is an important precursor of what can happen later in the season due to the natural movement of sea currents from east to west, i.e. away from the South American coast westward to Indonesia.
These observations have been released this Monday 25 April by the Climate Prediction Centre of NOAA in its latest report on the so-called ENSO, the El Nino Southern Oscillation, a climate phenomenon describing the oscillation between warmer and colder periods in the Pacific Ocean.
The Pacific Ocean is divided into four zones from Ecuador to Indonesia. These are Nino 1&2, the small area closest to the South American continent, Nino 3 from 90°W to 150°W, Nino 4 from 150°W to 160°E, and an overlapping area, Nino 3.4 from 120°W to 170°W. It is this last overlapping oceanic zone meteorologists employ to define El Nino and La Nina events.
According to this week’s ENSO discussion, El Nino is still present in the Nino 3.4 zone but has been weakening and is expected to return to neutral late in the northern spring or early summer. By implication, it means the Pacific Ocean will be back to normal by more or less the end of June.
These observations refer to surface temperatures. It is however the sub-surface conditions that now merit closer observation.
“A transition to ENSO-neutral is likely during late Northern Hemisphere spring or early summer 2016, with an increasing chance of La Niña during the second half of the year’ said the Climate Prediction Centre in their 14 April update of El Nino.
“Sea surface temperature (SST) anomalies were between 1.0°C and 1.5°C across most of the central and eastern equatorial Pacific Ocean during early April, having weakened appreciably over the last month. The latest weekly values for all of the Niño indices dropped to below 1.5°C” it stated.
Regarding subsurface temperatures, the Centre said “The subsurface temperature anomaly in the central and eastern Pacific decreased to negative values in association with a significant expansion of below-average temperatures at depth.”
In this week’s NOAA update the extent of subsurface cooling is corroborated with various charts showing below average sea temperatures diverging as much as 3°C over a very wide area of the Pacific from 90°W to 120°E at depths of up to 250 metres. This area of below-average subsurface temperatures covers more than 10,000,000 km2 marine space.
Caution has to be applied when interpreting these observations but the forecast models are fairly robust and their is a noticeable consistency between academic models and official forecasts.
“Nearly all models predict further weakening of El Niño with a transition to ENSO-neutral likely during late [northern] spring or early summer 2016. Then, the chance of La Niña increases during the late [northern] summer or early fall. The official forecast is consistent with the model forecasts, also supported by a historical tendency for La Niña to follow strong El Niño events” according to the Prediction Centre’s 14 April release. Going by current conditions, it means the divergence in the subsurface temperatures up to a depth of 250 metres, is more enhanced (-3°C) than the 2015/16 El Nino on the surface (+2.5°C). This colder water has been moving below the surface steadily since November last year, and only reached the South American coast over the past two weeks. The upwelling of this body of colder water is what caused the rapid decline of sea surface temperatures in Nino 1&2. As the colder water moves to the surface, the divergence will become less as the water becomes warmer, but it will still constitute a significant La Nina event. This colder water will be conveyed from east to west by the natural movement of ocean currents at the surface, leading to La Nina conditions during the second half of this year. This event is expected to be a strong La Nina. The last La Nina was in 2010/11, Namibia’s wettest summer on record.

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