A slacking of prevailing winds and the resulting slosh of warm water across the Pacific Basin has continued to raise ocean surface temperatures. Now just about every climate model is predicting an El Niño this year, but what exactly does that mean?
We've covered a global view before, and during the Q&A with climate scientist Mike Halpert, we looked at what will happen to California's drought during an El Niño. Now NOAA has released a map of what to expect in North America.
Hadley circulation is the updraft of warm, wet air at the equator and downdraft of cold, dry air at higher latitudes. El Niños amplify this circulation, heating the upper atmosphere over the Pacific at tropical latitudes. That in turn strengthens the whole current system, with more vigorous airflow to the poles and lengthening the west-to-east jet stream over North America. The elongated jet stream carries more storms than is typical for the region, drenching the southern states with more and more intense storms.
How wet California and the southwest will get depends on the intensity of the El Niño. This one is shaping up to be strong, which is historically paired with major storms for the southwestern United States. Here's a chart of the weather patterns within the United States in order from strongest (1997-1998) to weakest (1958-59) El Niño events. Areas that are dark blue-green experienced more than average rainfall, while those that are dark yellow-orange experienced less than average rainfall. The colour intensity reflecting just how anomalous the rainfall was, up to a foot more or less rain than is typical for the region.
Just because something usually happens doesn't mean it will always happen. For example, although a more intense El Niño usually means that California gets soaked, that isn't always the case. 1972-73 and 1965-66 were both strong El Niños, yet California was barely wet or even slightly dryer than normal, while a moderate El Niño in 1968-69 was paired with torrential downpours, and a downright weak El Niño in 1977-78 was still a remarkably wet year.
Weather is complicated. This year really looks like it'll shape up into an El Niño year, but the warming trend could still disintegrate, leaving us all fussing over an event that doesn't ever manifest. And even if the warming trend continues, the El Niño weather patterns may or may not match historical trends. But if you want to guess what's going to happen, it's a pretty good chance that this fall and winter, the southern states will get wetter, the northern states will be warmer, and the northeast will be dryer.
Image credits: NOAA