There are many natural climate oscillations, but one of the most important is the El Nino/Southern Oscillation (ENSO). El Nino is the ocean oscillation where Pacific waters get warmer than usual off the coast of Peru. NOAA defines El Nino as being when the Peruvian waters are at least half of a degree Celsius warmer than average for at least three months. The Southern Oscillation is the atmospheric oscillation that roughly accompanies the ocean component.
ENSO affects the weather virtually everywhere on the planet. It has caused droughts resulting in massive famines as well as widespread floods. The 1997-1998 El Nino caused the worldwide average temperature to rise more than 2.4 degrees Celsius (4.5 degrees Fahrenheit), making 1998 the hottest year ever recorded at the time. It is still the fourth hottest year on record and was so out of line with the rising temperature trend that global warming deniers continue to cherry pick it as the starting point for trend lines to claim there is no global warming. You can clearly see the isolated peak from 1998 in this graph of global surface temperatures.
|Source: NASA Goddard Institute for Space Studies|
So, it is small wonder that we keep a close eye on the formation of El Nino. The National Weather Service's Climate Prediction Center has a very detailed website with weekly updates on the ENSO conditions, and it is looking more and more likely that 2014 will be an El Nino year. I wrote the other day that El Nino is back. I am a little more lax in my definition of El Nino and jumped the gun in comparison to the official agencies. In fact, forecast centers around the world are saying El Nino, if it occurs, won't occur until later this summer or sometime in the fall. The consensus seems to be about 65% chance of it forming this summer and as much as 80% chance for it this fall. Forecasters are also saying it will be a mild to moderate event and nothing near as strong as some of the large events of the past. I have been reviewing the data and I agree with that assessment. There is a definite warming trend that has been going on this spring. The amount of warming that is being observed in the upper-level of the Pacific Ocean (upper-300 meters) is greater than the weak events of the past, but less than the strongest ones. That is not to say that something couldn't happen that would tip it to being either stronger or weaker. Such an event is certainly possible. But, based on past behavior, this event is proceeding in much the same way that past moderate events have.
What is still unknown is what effect global warming is having on ENSO. That will take some time to figure out.