El Nino- Southern Oscillation
El Nino is one of those terms that gets slung around the press and I’ve found that referring to it in sound bites and quips doesn’t explain it and just leads to more confusion. When a prominent meteorologist at NASA recently predicted a “Godzilla” El Nino- that didn’t really help. Part of that misunderstanding is inherent in its vague cyclical nature. It doesn’t occur every year like winter- its not as reliable as our change of seasons. I’m going to attempt to explain it in the least vague way possible. Cut through all the bullshit and get to brass tacks. Bear with me as I do a little explaining. And yes, it has a direct influence on our gardens in the PNW.
Hot child- Reversal of winds
The origin of the word Monsoon is Hindi. It doesn’t mean rain- it means a reversal of winds. On the Indian subcontinent that means that following the furnace that is offshore winds in April to June the intertropical convergence zone shifts north- onshore winds that herald rains and an end to the scorching weather.The intertropical convergence zone is a permanent weather feature approximately 20º north of the Equator where a worldwide band of rising air creates precipitation- both north and south of the equator. As the earth tilts on its axis and seasons commence this zone moves north or south. The (ITCZ) shift is as reliable as seasons. A reversal of winds. Remember that because it is not too terribly different than the origin of El Nino.This is predictable and expected almost 100%. What, however, if this reversal of winds was not as reliable as clockwork? Not only does it throw a wrench in forecasts, it affects the whole world and because of its less predictable occurrence we have less of its effects to identify with. Humans like certainty- nature is messy though and we have to acknowledge that to begin to understand El Nino.
What goes east now goes west
The cause of El Nino which is most commonly referred to as EL Nino/Southern Oscillation or ENSO is not fully understood, but its recurring pattern is very well documented. First- El Nino is so named because its most dramatic effects show up in December in the eastern Pacific just off the coast of Peru/Equador primarily around Christmas- hence the name El Nino or the (christ) child. In the equatorial Pacific winds normally blow east to west. This does two things. It forces sun heated surface waters to the west towards SE Asia and N. Australia. In the eastern Pacific this displaces the warm surface water and cold water rises this is called upwelling. Cold water holds more oxygen and this supports much more marine life. Fishing is good. In an El Nino year warm oxygen starved water pools towards that continent – warm water holds less oxygen and it deters marine life- fishing sucks. For an unknown reason every two to seven years (5 years) this wind reverses its flow. Under normal conditions the persistent east to west flow moves the warm surface waters towards the west. In an ENSO event the opposite is true, because of this the entire Pacific basin reacts in various (almost) predictable ways.
Warmer to the North and Cooler to the south- the ultimate reversal.
Several things that are predictable happen in an ENSO event. But no two are completely alike. Just as no two winters are completely alike. In the Pacific basin there is a common effect and it has a profound influence on our winter weather. To begin, warm water has more latent energy and that means the air above it holds more moisture. In northern South America that spells rain for normally parched coastal Peru and Equador. In fact, the southern branch of the jet stream- the subtropical jet becomes energized. This has two effects. This southern branch pulls energy from the normally much more vigorous polar jet to the north (thats what soars over us). This energized jet sets its sights on California and actually all the way across the southern tier of North America. Volatile, soaking storms move in south of us- California can receive a drenching winter. Across northern Mexcio and the southern US precipitation increases and this blocks out sun energy- its cooler there than typical climate averages. At our latitude the paltry polar jet dwindles. Most often a semi-permanent ridge of high pressure sets up off the PNW coast and the weakened storms are split and disintegrate as they are steered inland. The whole Pacific basin is warmer as well so the effects for us are drier, warmer and pretty much bland winter weather. This stretches across the northern tier of the US. The normal mixing and fast motion of the polar jet is perturbed and warm air drifts to the north- all the way in fact to the poles. Check out this winter forecast from the NOAA. It calls for a classic El Nino winter set up. Orange is above normal temperatures and blue is of course colder than normal.
What are the effects?
Warmer drier winters are detrimental to our snow pack. If you are a snow boarder or even if you irrigate in summer this affects you. Less mountain snow pack is less water in our collective water bank for summer. If you are a gardener and you think this means a zone 9 winter and the coast is clear, think again. In approximately 80% of our ENSO winters there is no arctic event. As I said though, like seasons no two El Ninos are alike. We look at the past in climatology to tell us what the future holds. The last ENSO that was as strong as the one we are facing was in the winter of 1997/98. We DID have a short arctic outbreak in that winter so that represents the 20%. Remember that it only takes a few days to produce enough plunging cold effects to mangle tender plants. So while it is not a sure thing that we won’t see arctic conditions 80% are good odds. As for our ‘drought’- which I don’t personally consider it a drought unless rainfall is down for two years in a row, it doesn’t look like the NOAA has high hopes for a typical liquid winter. (See below- brown is drier and green is wetter).
|Click on individual map to view larger format|
A few other facts
As I said ENSO changes the weather all over the globe. For instance one effect is that the energized sub tropical jet that flows over the southern part of our continent and into the Atlantic basin causes upper level wind shear that makes it hard for Hurricanes to form there. The warmer water off the coast of western Mexico actually increases the formation of tropical systems in the eastern Pacific. Far away in Australia and SE Asia cooler waters and stagnant wind flow creates dry conditions. This can be monumental and one knock off effect is drought and higher fire danger. During the last strong ENSO fires scorched huge sections of Indonesia and this loads the atmosphere with carbon- not good for climate change. Its important to note that ENSO events have become not only more frequent in the last 50 years but they have become stronger. Is this a product of climate change? Only the future will answer that question.