El Niño Early Season Recap (Oct-Dec 2015)
Looking back at Oct-Dec, did observed weather events correspond with expected (El Niño) climate patterns?
January kicked off with a bang, and the much anticipated mega-Godzilla El Niño is upon us. El Niño conditions have been in place for months (Fig. 1), but has this El Niño event been impacting the weather of the Southwest in the ways that we anticipated? Sort of, but not exactly…
This is probably to be expected, as we and others have been saying that no two El Niño events are alike (which means they are hard to compare). In addition, the October-November-December time period is still very early season for El Niño in the Southwest, so it would be premature to judge it too harshly.
First off, October through December is a period of transition for the Southwest, a time when we move from our summer monsoon thunderstorm season into a more westerly upper-level circulation pattern, and look expectantly for winter storms to bring precipitation to the region. October is also the time when a tropical storm can either directly or indirectly bring heavy precipitation to the region. Even during a ‘normal’ year with no El Niño influence, it is a ‘noisy’ time with lots of moving parts and potential variability, with quite a few dry and wet extremes in the historical record. Long story short, El Niño or not, we expect to see a lot of variability during this transition season, so the larger question is how much does El Niño typically impact this season?
The influence of El Niño during this October–December transition operates through the mechanisms of tropical storm activity and early winter storm tracks, both of which can impact precipitation patterns across the Southwest. Typically, tropical storm activity is enhanced in the east Pacific during strong El Niño events, raising the likelihood of above-average precipitation from these events. In addition, the southward shift in the winter storm track, a hallmark impact of El Niño events, can set up as early as November. Based on this, we largely expected this period to be wetter than average for Arizona and New Mexico and it mostly was (Fig. 2). But when we look a bit more closely, the forces driving this precipitation were a bit unusual and didn’t quite fit the typical El Niño-driven patterns.
For example, a series of cut-off low pressure systems in October wandered across the Southwest (at one point, the same cut-off low visited Arizona twice), picking up abundant tropical moisture and creating widespread precipitation events across Arizona and New Mexico. These cut-off lows formed off of a very busy jet stream pattern across the Pacific Ocean that in part was energized by the tropical storm activity across the basin (see the NOAA discussion). Instead of directly interacting with tropical storms in the east Pacific, El Niño appears to have brought us to above-average precipitation for October in a much more complicated way (Fig. 3).
A highly amplified jet stream pattern continued into November, again partially driven by tropical storm activity that was absorbed into the jet stream in the west Pacific, leaving the Southwest to contend with a busy weather pattern but very little to work with in the way of moisture. A parade of storms marched through the Southwest through the month, but they originated in the Gulf of Alaska and brought very little moisture. Some higher elevation and more northern areas were able to wring out some precipitation with these storm systems, but overall the month was cool and relatively dry for the Southwest (Fig. 4). This was in contrast to the expectation of a much stronger subtropical jet—which was present but south of Arizona and New Mexico—pulling in abundant moisture from lower latitudes.
Similarly, in December, the Southwest saw passing storms and cooler-than-average temperatures but little in the way of precipitation (Fig. 5). New Mexico received higher precipitation totals a bit late in the month when a historic blizzard (compared to a similar snowstorm that hit the region in January of 1983, also a strong El Niño winter,) swept through the area. The final precipitation totals for October–December 2015 across the Southwest are more or less what you would expect for an El Niño year—generally above-average across New Mexico and northern Arizona—and a bit of the unexpected, with drier-than average conditions across parts of western Arizona. But again, maybe that is what we should expect. Two other strong El Niño events (1983, 1997) are very different from each other (Figs. 6-7), illustrating there is considerable variability in the early season El Niño impacts on the Southwest. October–December of 2015 (Fig. 8) establishes itself as yet another flavor of potential precipitation impacts during the fall season across the southwest U.S.