Earlier this year, it looked as though El Nino was beginning to form in the tropical Pacific Ocean with warmer-than-normal sea surface temperatures becoming more evident off the west coast of South America. In addition, numerous computer forecast models were predicting moderate-to-strong El Nino conditions for the summer and fall seasons in the equatorial region of the Pacific Ocean. While this can still happen, recent actual observations have raised some questions about those predictions. One of the reasons it is necessary to closely follow the changes in the tropical Pacific Ocean is the fact that it can have an important consequence on the rapidly approaching Atlantic Basin tropical season. In an El Nino year, for example, there is a tendency for less tropical activity in the Atlantic Basin as El Nino-induced vertical wind shear is typically higher-than-normal and this tends to inhibit the growth of tropical storms.
Computer forecast models
The most recent predictions made by a series of dynamical and statistical models for sea surface temperatures in the Nino 3.4 region (central equatorial Pacific Ocean) continued to support the idea of an increasingly strong El Nino this year; however, these forecasts have generally trended lower in recent months as far as probabilities are concerned. In fact, many of the statistical models (represented by circles in plot) now suggest neutral ENSO state (zero line) or even weak La Nina conditions (colder-than-normal) for the summer and fall seasons. Positive values shown on the SST anomaly plot suggest El Nino conditions for the tropical Pacific Ocean and negative values are predictions for La Nina. The yellow line represents an average of all models which hovers near the 1°C temperature anomaly line though much of the second half of the year and this would result in a moderately-strong El Nino – an increasingly questionable prospect based on the factors described below.
Southern Oscillation Index (SOI)
Meteorologists monitor the prospects of an El Nino or La Nina in the tropical Pacific Ocean by using an index known as the Southern Oscillation (SOI). The SOI is calculated using the pressure differences between Tahiti and Darwin. Sustained negative values of the SOI lower than −7 often indicate El Niño episodes. These negative values are usually accompanied by sustained warming of the central and eastern tropical Pacific Ocean, a decrease in the strength of the Pacific Trade Winds, and a reduction in winter and spring rainfall over much of eastern. Sustained positive values of the SOI greater than +7 are typical of a La Niña episode. They are associated with stronger Pacific trade winds and warmer sea temperatures to the north of Australia. Waters in the central and eastern tropical Pacific Ocean become cooler during this time. Together these give an increased probability that eastern and northern Australia will be wetter than normal.
In fact, the SOI has averaged -5.86 for the last 30 days which is a relatively weak signal for an El Nino. During the 3-month period of February, March and April, the SOI was providing mixed signals at best for the prospects of an El Nino. Though the SOI average monthly value was negative for both February and April, they were rather weak (-2.24, -6.31) and the average index value for March was actually in positive (+3.80) territory which is not favorable for El Nino. [Click here for source of SOI index data]
Sea surface temperature anomaly changes
In addition to the Southern Oscillation Index, meteorologists can monitor sea surface temperature changes in the tropical Pacific Ocean to determine if there are important trends favoring the prospects of an El Nino. In this regard and similar to the SOI, there have been mixed signals in recent weeks. Some areas have seen sea surface temperature rises, but others have seen a drop in water temperatures. In the last 7 days, sea surface temperatures have risen (orange) in some areas just off the west coast of South America, but have dropped (blue) in others across the tropical Pacific Ocean.
Meteorologist Paul Dorian