El Nino

The decade ended on a very quiet note weather-wise in the state of California. Despite a strong El Nino event in the Eastern Pacific Ocean, the conditions thus far this winter have been rather cold and dry across the state, and especially in the north. Nearly all of NorCal has seen below-median precipitation for the season thus far, generally in the 65-85% of average range. Much of coastal SoCal has fared relatively better, generally near or somewhat above normal (90-130%), but inland areas have seen less rainfall. The numbers, however, will drop fairly quickly statewide over the next 1-2 weeks as a prolonged dry spell continues and perhaps intensifies. We are entering the climatologically wettest part of the season in California, and the skies above are rather unsettlingly clear.

This season has so far been dominated by fairly strong (but often conflicting) climatological signals. El Nino continues in the E. Pac., and there have been occasional bouts of MJO activity (though some of the MJO index activity has been “artificial” in that it has not been generated by sustained patterns of equatorial convective activity but rather very rapid bursts caused by the El Nino influence). The observed flow pattern in the Northern Hemisphere, however, has not been what one would expect during a strong El Nino event. Instead of a strong zonal jet developing over the Central and Eastern Pacific, the powerful “Pacific Firehose” has been constrained largely to the West Pacific. Further east, an anomalously meridional  jet has brought dramatic Arctic intrusions to much of the United States and Western Europe, with record-breaking cold and blizzards across most of the U.S. (and very cold weather with sea-level snowfall in NorCal in December). The storm track has remained well to the north of CA for the most part, and although some relatively weak systems have made it under the amplified far E. Pacific ridge and brought some rain and snow to the state, the sustained strong subtropical jet characteristic of strong El Ninos has yet to make an appearance anywhere in the U.S. this season. One reason for this appears to be the extremely anomalous pressure patterns over the Arctic this winter. The Arctic Oscillation (AO) has in recent days reached its lowest point in decades, and numerical models foresee that another minimum (perhaps all-time recorded minimum?) could occur in the next week or two. A low AO usually correlates to cold weather over the U.S., and this continues to be the case much as it has been for the entire season so far. The models do foresee a moderation of the extraordinarily low AO values in the coming weeks, which will perhaps allow the El Nino signal to become more prominent. It seems that the anomalously high geopotential heights over nearly the entire Arctic have been suppressing the effects of the warm water in the Pacific, and the AO is going to have to switch signs (or at least become less negative) before this occurs.

4-month three-day running mean of the AO (CPC)

Also worth mentioning is the Pacific/North American teleconnection. Lately, the PNA has been somewhat negative, which tends to correlate to decreased storminess over the E. Pac. and less rain over CA. The models are also forecasting the PNA to go significantly positive over the next two weeks. What does all of this mean? Well, it does appear that a lower-latitude block will strengthen over the next 1-2 weeks, at least initially bringing dry and relatively mild weather to CA. After that, the question becomes whether an energetic southern stream can break through the block over the E. Pac. This is a critical question, as if the subtropical jet becomes stuck under the block it can bring large amounts of moisture to CA for very extended periods of time. I still do expect this to happen at some point, but we do have to keep in mind the possibility that the negative AO is more persistent than expected, and the southern stream never breaks through. If that’s the case, I don’t relish the water restrictions that will come in the later half of the first year of the new decade. There has been talk of the similarities between the current year and the pattern during the late 1970s drought in CA.  This connection is true…to a degree. The current strong El Nino differentiates the present situation to those years, and makes the outlook for the rest of the year very, very uncertain. Stay tuned…it may yet be an interesting season.

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