TOKYO -- Indications from the tropical Pacific suggest that the first El Nino event in five years is happening. And this looks to be not any normal El Nino, but one that could rival the abnormal conditions from the spring of 1997 to the spring of 1998. Experts also note that global warming could accelerate if this year's El Nino triggers a "regime shift," the term used for a sudden, massive change in the global climate.
The trade winds in the equatorial Pacific normally blow from east to west. But two strong westerly bursts were recorded in January and February, and two slightly less powerful westerly bursts occurred in March and April.
"The trigger for an El Nino has been pulled. If these westerly bursts continue, it could develop into a powerful event," said Shuhei Maeda, senior coordinator for El Nino information at the Japan Meteorological Agency.
The warm ocean-surface waters carried to the western Pacific by the trade winds can get pushed back eastward by these strong winds. El Nino conditions could develop if this causes ocean temperatures to rise higher than normal across the eastern part of the tropical Pacific and off the waters of Peru.
"The way the westerly winds are blowing and the resulting changes in ocean temperatures resemble what took place in 1997," said Kentaro Ando, a group leader at the Japan Agency for Marine-Earth Science and Technology (Jamstec).
Heat in large amounts accumulates in relatively shallow ocean waters south of Japan and moves eastward along the equator. Ocean water temperatures in the eastern Pacific usually begin rising several months after the amount of heat has reached its maximum in the tropical Pacific.
The occurrence of westerly bursts is influenced by the Madden-Julian oscillation, which is characterized by the eastward movement of massive cumulonimbus clouds, and by the undersea currents known as Kelvin waves heading in the same direction.
It would be hard to predict the strength of the El Nino event that may be developing without closely watching the situation for some more time, but scientists estimate that the episode, if happening, will peak next winter.
When an El Nino event occurs in the summer, the region subject to strong ascending air currents shifts eastward due to rising ocean temperatures, and the high-pressure system in the Pacific weakens.
For Japan, this means a prolonged rainy season and a cooler summer. It also means fewer sunny days, which is bad for crops.
When an El Nino event occurs in the winter, the continental high-pressure system that brings cold air to Japan can fail to gain strength, resulting in a mild winter.
The JMA's forecast for this summer, excluding southern areas such as Okinawa, is for July to have a higher-than-average number of rainy and cloudy days.
There was a powerful El Nino event in the summer of 1997, but June and July were still hot in Japan that year. Although many regions recorded lower-than-normal temperatures in August, the summer as a whole was hot -- which runs counter to many El Nino forecasts.
However, both 1997 and 1998 saw record-mild winters for western Japan.
There has not been such a powerful El Nino since then. There were smaller El Nino episodes between the summer of 2002 and the winter of 2003, and between the summer of 2009 and the spring of 2010.
Japan's weather agency predicted in July 2012 that there was a high probability of an El Nino event occurring that summer, but it did not happen.
Meanwhile, there have been four recent La Nina events, which are the counterparts to El Nino and have cooler-than-normal ocean temperatures in the eastern tropical Pacific. In addition, conditions resembling a La Nina event have remained in place since last year through this year.
This tendency for La Nina episodes helps explain the hot summers and cold winters in Japan in recent years, including the record-setting heatwave in 2010, and the heavy-snowfall winter of 2011-2012.
If conditions flip to a potential El Nino episode this summer, it could amplify the effects of global warming.
It is understood now that global average temperatures are being driven upward by human activity. But in recent years, there has been a hiatus in overall average temperature rise. This can in part be attributed to the series of La Nina episodes that have occurred since the 1997-1998 El Nino.
Time for a shift
The hiatus has stopped global temperatures from rising, and the reason, as explained by University of Tokyo associate professor Masahiro Watanabe, is because conditions now favor the accumulation of heat in the deep waters of the Pacific Ocean. When the ocean can absorb more heat, it takes this heat away from the air, keeping global warming in check. This hiatus is caused by a large-scale change in the distribution of water temperatures in the Pacific.
However, if these favorable conditions break down because of a powerful El Nino episode and make it easier for the atmosphere to warm up, the hiatus in global warming would end.
Predictions are always hard to make, but atmospheric flows could get thrown into confusion, and the world could face an increase in meteorological disasters, bringing about more frequent rainstorms, tornadoes and droughts. This is the potential regime shift.
Every decade to several decades, Earth experiences one of these regime shifts, which involves a large change in air and water temperatures and ocean flows.
The regime shift is heralded by factors such as the Pacific Decadal Oscillation, which is a change in the distribution of water temperatures in the Pacific, and by changes in the occurrence and frequency of El Nino and La Nina.
A regime shift has a major impact on climate, and it also leads to a sudden change in fish catches.
To predict a regime shift requires long-term and precise monitoring of events at sea.
To this end, the Jamstec and the U.S. National Oceanic and Atmospheric Administration are collaborating in the tropical Pacific. But because of budgetary difficulties, the number of monitoring buoys at their disposal has declined in recent years.
As of the spring of last year, 24 of NOAA's 55 buoys were unable to operate and send data, and the situation is no better today.
El Nino conditions may be developing, but a lack of buoys could hinder the ability of scientists to monitor the situation.