Last week’s unprecedented rains and devastating floods in Rio de Janeiro are — I think — the final scene in this long season of dramatic weather. The phenomena that produced six months of headlines are fading.
Of course, that leaves us to wonder what comes next.
El Nino, as usual, grabbed most of the attention this year. The cyclical warming of the central and eastern tropical Pacific Ocean contributed to stormy weather that repeatedly swept across the southern United States and up the East Coast, producing unusual snows from Texas to New Jersey.
El Nino also is a big factor in South American weather. Brazil’s Sa√µ Paulo state suffered heavy flooding in February, typically a rainy month there, followed by the record rains that battered Rio in early April, when the wet season should already have wound down. Meanwhile, drought gripped the northern part of the continent from Ecuador to Venezuela, where electricity has been severely rationed as hydroelectric generation remains crippled.
But this year’s El Nino was not the biggest weather story. As El Nino events go, this one was only moderately strong. It was the most powerful thus far in the 21st century, but considerably less spectacular than the historic El Ninos that coincided with the Northern Hemisphere winters of 1972-73, 1982-83 and the granddaddy of all El Ninos, the one that occurred in 1997-98.
On the other hand, the negative phase of the North Atlantic Oscillation, or NAO, that occurred this winter was literally off the charts. As I described here in December, when the NAO was already making itself felt around the globe, the NAO describes the relationship between weather conditions in the far north Atlantic near Greenland and further south, near the Azores.
In its positive phase, the NAO drives powerful westerly winds around the globe at high latitudes. Storm systems follow these westerly winds, keeping frigid air locked over the far north. Many areas in the middle latitudes, including Europe and the eastern United States, stay mild and dry.
A negative NAO relaxes those high-latitude westerlies. Cold air from the Arctic is free to plunge southward and is often accompanied by fierce storms. This year’s record-setting southern snows were spawned when El Nino-induced weather systems reached the Gulf of Mexico and linked up with the cold air that was flooding southward thanks to the negative NAO.
El Nino appears every three to seven years and typically lasts one to two years. The NAO, on the other hand, is continually present (though more significant in winter), and usually swings every few weeks from its negative phase to positive or back. But in some years it can spend much more time negative, in others much more time positive.
This year’s negative NAO was unusually consistent and prolonged. It began in October, with unseasonably early and heavy snows in northern China and the northeastern U.S. It relaxed in November but reappeared with amazing intensity in December and persisted, with only a brief break in late January, into the early part of March.
At its peak, the negative NAO brought prolonged freezing weather to Florida while, at the same time, exceptionally warm midwinter weather produced rain in Greenland. In general it was a warm winter in the high Arctic and a cold one further south. Northern Europe was swept by blizzards while Portugal and other Mediterranean countries grappled with floods.
Despite the relatively warm Arctic season, the Arctic Ocean ice pack steadily expanded, and it kept expanding well into the spring. The National Snow and Ice Data Center reported last week that the polar ice pack did not reach its maximum extent until March 31, the latest date in satellite records that stretch back three decades. The ice pack now is close to the average extent that was measured in the 1980s and 1990s. This is at least a temporary reversal of a strong below-normal trend that has prevailed during the past decade.
Meteorologists measure the strength of El Nino by the Oceanic Nino Index, or ONI, which is calculated every month. This season’s event peaked at a 1.8 ONI in January, which fell to 1.5 in March. (El Nino conditions are considered to begin at 0.5, while the opposite phenomenon, La Nina, commences at —0.5.) Scientists expect El Nino to persist through the spring. Climate models vary in their forecasts beyond spring, with most showing a gradual fade of El Nino while some see it persisting to next autumn. History and recent trends make me believe El Nino will likely be gone by late summer.
Last June, when El Nino was just making itself felt, I wrote that this was a good sign for those of us who are concerned about Atlantic hurricanes. Hurricane seasons tend to be mild in El Nino years thanks to powerful high-level winds that tear apart tropical systems. Sure enough, the Atlantic storm season last year was late and mild.
If El Nino fades away before the peak of the season arrives in late August we may see more favorable conditions for hurricanes this year. This does not mean a major storm is going to strike the United States, since it’s possible for most storms to miss us even in an active year. But the risk is likely to be higher this year than last.
The negative NAO has also been fading in recent weeks. This could mean a warmer, drier spring for storm-weary Easterners, who already enjoyed (if that is the right word) an unseasonable 90-degree heat wave last week. There is some evidence that the NAO is at the beginning of a new long-term cycle in which the negative phase may be more prevalent. This could mean more frequent severe winters than we have seen in the past decade or two.
Despite all the hype about green vehicles and cap-and-trade legislation to protect the climate, in the end what we do about the weather is, mostly, talk about it. This year’s vicious season has given us a lot to talk about. I don’t know about you, but I won’t mind a little bit of peace and quiet.