Is it my imagination, or have the last decade’s winters been uncommonly mild? In the 1970s, single-digit winter mornings were common, and below-zero not unheard-of. I don’t remember so many really cold days in recent years. While I have no doubt these milder recent winters reflect global warming to some degree, in far larger measure they seem to reflect a decades-long weather pattern in the Pacific, a pattern that seems to be changing
This pattern has a cumbersome name, the Pacific Decadal Oscillation. To understand it, you need to first come to grips with something called El Nino. Until about 20 years ago no one talked of El Nino. Now it is used to explain everything from Hurricanes to why my dog runs in circles. Just what is an El Nino?
Every Christmas a tepid current sweeps down the coast of Peru and Ecuador from the tropics, reducing the fish population slightly and giving local fishermen some time off. The local fishermen named this Christmas current El Niño (literally, “the child,” after the Christ Child). Now, though, the term is reserved for a catastrophic version of the same phenomenon, one that occurs every seven years or so and is felt not only locally but on a global scale.
Scientists now have a pretty good idea of what goes on in an El Niño. Think of the Pacific Ocean as a giant saucer of water with a very powerful fan blowing over it. Normally the Pacific Ocean is fanned by constantly blowing east-to-west trade winds that push warm surface water away from the ocean’s eastern side (Peru, Ecuador, and Chile). This surface water piles up in the west, around Australia and the Philippines, making it several degrees warmer and a meter or so higher than the eastern side of the ocean.
El Nino starts as a minor glitch in this picture. One day, if the winds slacken briefly, warm water begins to slosh back across the ocean. Once this happens, ocean and atmosphere conspire to ensure it keeps happening. The warmer the eastern ocean gets, the warmer and lighter the air above it becomes, and hence more similar to the air on the western side. This reduces the difference in pressure across the ocean. Because a pressure difference is what makes winds blow, the easterly trades weaken further, letting the warm water continue its eastward advance.
The end result is to shift the weather systems of the western Pacific Ocean 6000 km eastward. As the warm water moves east, so do the clouds. The tropical rainstorms that usually drench Indonesia and the Philippines now soak the western edge of South America.
St Louis is a long way from South America, but not far enough to escape El Nino’s influence. The enormous air masses of an El Niño event shove other air out of the way, causing effects that propagate like falling dominos across the world’s weather systems. El Niño drives moist Pacific air across Mexico into the Gulf, producing a lot of rain along the Gulf Coast. A lot of this Gulf moisture eventually finds its way up to St Louis, producing heavier-then-normal rains here. In addition, El Nino pushes the polar jet stream up into Canada, so that we in St Louis avoid its icy grip and experience a warmer than usual winter. In essence, then, El Nino is a gardener’s dream, giving St Louisans mild wet winters. 1997 was such an El Nino winter, pleasant with lots of rain.
An El Nino does not last long — too much is out of balance. The correction is called La Nina. La Nina is a Pacific weather pattern that is the opposite of El Nino, driving warm Pacific sea-surface water back westward from South America and so correcting the heat inbalance that the El Nino event produced.
You may have noticed that it’s dry this winter in St Louis, and cold. We are in the grips of La Nina now. The joint U.S./French Topex/Poseidon earth satellite, which measures ocean temperatures, shows us a “cold tongue” in January 2000 that extends out 3,000 miles into the Pacific from Ecuador. With so much warm water flowing toward Asia, the path of the polar jet stream over North America is pulled southward, drawing frigid air down into the U.S. and giving us colder St Louis winters. The tropical jet stream that blows across Mexico and the Gulf weakens, drying out the Gulf states. As a result, there is less moist Gulf air to circulate up to St Louis and produce rain, so we can expect to have less rain than normal this winter.
Now to get back to the Pacific Decadal Oscillation. It is a long-term flip/flop in Pacific Ocean weather patterns. Over a thirty-year interval from the mid-1940s to the mid-1970s, waters in the eastern tropical Pacific tended to be cooler, favoring La Nina patterns (32% of the years were La Nina, 24% El Nino). Then, for a reason no one understands, the pattern flip/flopped. Since the mid-1970s, these waters have been warmer, favoring El Nino events (16% of the years were La Nina, 40% El Nino). Now, based on the last two year’s satellite data, weather scientists tell us the pattern may have flip/flopped again.
If so, our winters are going to flip/flop too, back to the “good old days” of frigid winter mornings. Hope they’re wrong.