El Nino Signal is Weakening in the Pacific
The probability of El Nino, a warming of ocean surface temperatures in the eastern and central Pacific, developing this year has been downgraded by U.S. government forecasters as sea surface temperatures and wind speeds in the area remain close to their long-term averages.
The Pacific saw a relatively rapid swing in late October from La Nina conditions - characterised as unusually cold temperatures in the equatorial Pacific Ocean - to neutral or even slightly El Nino-leaning conditions by March.
Since then, however, the oceanic and atmospheric signals pointing to a possible El Nino have all weakened.
The U.S. National Oceanic and Atmospheric Administration (NOAA) last week downgraded the probability of El Nino conditions being present in the fourth quarter of 2017, to just 36 percent. That is down from 53 percent at the time of its March forecast.
The agency’s central prediction is now that conditions will be neutral in the fourth quarter, with a probability of 53 percent, with only an outlying chance of La Nina, at 11 percent.
El Nino has a major impact on temperatures and precipitation on countries bordering the Pacific and Indian oceans, and across Latin America, with a smaller impact in some areas of the United States.
The phenomenon is typically strongest during the northern hemisphere autumn and winter and weakest during early spring.
In winter, El Nino typically brings warmer weather to the U.S. Northwest and cooler, wetter weather to the Southwest and Southeast states.
For the purposes of forecasting, meteorologists analyse the surface temperature of the Pacific in a series of boxes stretching from the coast of Peru west along the equator to the international date line and beyond.
The atmospheric and oceanic conditions identified with El Nino and the Southern Oscillation (ENSO) are most closely associated with temperatures in a composite area called Nino region 3.4.
Region 3.4 straddles the equator from 5 degrees north to 5 degrees south and stretches from 120 degrees west to 170 degrees west. (“Why are there so many ENSO indexes, instead of just one?” NOAA, Jan 2015)
El Nino is conventionally defined as at least three or sometimes six consecutive months in which the surface temperature in region 3.4 averages 0.5 degrees or more above normal.
Surface temperatures in region 3.4 have been rising steadily since October, and are now 0.4 degrees Celsius above the long-term seasonal average, but there are indications they may be peaking.
Temperatures further east off the coast of Peru in Nino region 1+2, which stretches from 80 degrees to 90 degrees west, and from the equator to 10 degrees south, have cooled rapidly in the last three months.
Surface temperatures in region 1+2 are now 0.1 degrees Celsius below the long-term average, down from 2.6 degrees above the average in mid-March.
Surface temperatures in region 1+2 are usually a good leading indicator for the warmth of water further out in the ocean in region 3.4 given the westward flow of water on the southern equatorial current.
The rapid cooling of water in region 1+2 indicates temperatures in region 3.4 are likely to cool further in the next few weeks, which makes it much less likely a full El Nino episode will develop in the months ahead.
Other components of ENSO also point to relatively neutral conditions in the near term and mean El Nino will probably not develop before the end of the year.
El Nino conditions are normally propagated by an upwelling of warmer than average water from deep down in the ocean off the coast of Peru.
But there are currently no signs of warm upwelling. Sub-surface waters off the coast of Peru at depths ranging from about 50 metres to 150 metres are close to the long-term average.
The Southern Oscillation, which is the atmospheric counterpart of the El Nino-La Nina cycle, is characterised by variations in atmospheric pressure differentials and wind speeds across the Pacific.
El Nino is usually accompanied by a reduction in pressure differentials between the western and eastern sides of the Pacific and a consequent slackening of the trade winds.
But both sea-level pressure differences and wind speeds remain close to their long-term averages, according to the latest data.
The conditions for a mutually reinforcing warming of the ocean surface and slackening of the trade winds that characterise a warm episode of ENSO are currently absent.