Research at the heart of El Nino

prediction

A  thousand miles south of Hawaii, the air at 45,000 feet above the equatorial Pacific was a shimmering gumbo of thick storm clouds and icy cirrus haze, all cooked up by the overheated waters below.

In a Gulfstream jet more accustomed to hunting hurricanes in the Atlantic, researchers with the National Oceanic and Atmospheric Administration (NOAA) were cruising this desolate stretch of tropical ocean where the northern and southern trade winds meet. It’s an area that becalmed sailors have long called the doldrums, but this year it is anything but quiet. This is the heart of the strongest El Nino in a generation, one that is pumping moisture and energy into the atmosphere and, as a result, roiling weather worldwide.

The plane, with 11 people aboard including a journalist, made its way on a long westward tack, steering clear of the worst of the disturbed air to the south. Every 10 minutes, on a countdown from Mike Holmes, one of two flight directors, technicians in the rear released an instrument package out through a narrow tube in the floor. Slowed by a small parachute, the devices, called dropsondes, fell toward the water, transmitting wind speed and direction, humidity and other atmospheric data back to the plane continuously on the way down. The information, parsed by scientists and fed into weather models, may improve forecasting of El Nino’s effect on weather by helping researchers better understand what happens here, at the starting point.

Answering questions

“One of the most important questions is to resolve how well our current weather and climate models do in representing the tropical atmosphere’s response to an El Nino,” said Randall Dole, a senior scientist at NOAA’s Earth System Research Laboratory and one of the lead researchers on the project. An El Nino forms about every two to seven years, when the surface winds that typically blow from east to west slacken. As a result, warm water that normally pools along the Equator in the western Pacific piles up toward the east instead. Because of this shift, the expanse of water — which in this El Nino has made the central and eastern Pacific as much as five degrees Fahrenheit hotter than usual — acts as a heat engine, affecting the jet streams that blow at high altitudes. That, in turn, can bring more winter rain to the lower third of the United States and dry conditions to southern Africa, among El Nino’s many possible effects.

Aided by vast processing power and better data, scientists have improved the ability of their models to predict when an El Nino will occur and how strong it will be. In June, the consensus among forecasters using models developed by NOAA, as well as other US and foreign agencies and academic institutions, was that a strong El Nino would develop later in the year, and it did.

Anthony Barnston, chief forecaster at the International Research Institute for Climate and Society at Columbia University, who has studied the accuracy of El Nino modelling, said that so-called dynamical models, which simulate the physics of the real world, have recently done a better job in predicting whether an El Nino will occur than statistical models, which rely on comparisons of historical data.

With any model, good data is crucial. El Nino models have been helped by the development of satellites and networks of buoys that can measure sea-surface temperatures and other ocean characteristics. When it comes to forecasting El Nino’s weather effects, however, good data can be harder to come by. That’s where the NOAA research project aims to help, by studying a key process in the El
Nino-weather connection: deep tropical convection.

The clouds that the NOAA jet cruised past were a result of this process, in which air over the warm El Nino waters picks up heat and moisture and rises tens of thousands of feet. When the air reaches high altitudes — about the flight level of the Gulfstream — the moisture condenses into droplets, releasing energy in the form of heat and creating winds that flow outward. Scientists know that the energy released can induce a kind of ripple in a jet stream, a wave that as it travels along can affect weather in disparate regions around the world. And they know that the winds that are generated can add a kick to a jet stream, strengthening it.

That’s a major reason California and much of the southern United States tend to be wetter in an El Nino; the winds from convection strengthen the jet stream enough that it reaches California and beyond. But to study convection during an El Nino, data must be collected from the atmosphere as well as the sea surface. That’s a daunting task, because the convection occurs in one of the most remote areas of the planet. As a result, there has been little actual data on convection during El Nino events, Randall said, and most models, including NOAA’s own, have had to make what amount to educated guesses about the details of the process. “Our strong suspicion is that our models have major errors in reproducing some of these responses,” he said.

“The only way we can tell is by going out and doing observations.” When forecasters last year began to predict a strong El Nino, the NOAA scientists saw an opportunity and started making plans for a rapid-response programme of research. Randall estimated that it would normally take two or three years to put together a programme they assembled in about six months. In a way, he said, they were helped by the developing El Nino, which suppressed hurricane activity in the Atlantic last fall.