La Niña and the Weather of WA State
The weather of the Pacific Northwest during the coming months, or otherwise known as the cool season, is impacted by the atmosphere-ocean system of the tropical Pacific. This region is currently experiencing what is termed La Nina, and this condition is expected to persist into 2011. This highlight will briefly describe La Nina, and how it is liable to influence the regional weather during the upcoming fall and winter.
In a nutshell, La Nina is associated with significantly cooler than normal upper-ocean temperatures in the central and eastern equatorial Pacific. The eastern portion of the equatorial Pacific is substantially colder than the western portion on average and this asymmetry is enhanced during La Nina. The atmosphere over the equatorial Pacific is tightly coupled to the state of the underlying ocean, in both a cause and effect manner. During La Nina, the easterly winds are stronger than normal across most of the tropical Pacific, and the deep cumulus convection associated with clusters of thunderstorms is shifted. As a result, these thunderstorms are enhanced over the far western part of the tropical Pacific and Indonesia and are suppressed in the central portion. It is through the changes in the strength and location of this thunderstorm activity that La Nina (and El Nino, of course) impacts the atmospheric circulation and ultimately the weather in other parts of the globe.
The current La Nina began developing earlier this year on the heels of last winter’s El Nino. The sea surface temperature (SST) anomalies in the central-eastern equatorial Pacific switched sign in May, with continued cooling relative to the usual seasonal cycle over the course of the summer of 2010. At the time of this writing, the SST anomalies are approximately -1.5 degrees C, which is highly significant from a statistical point of view. A variety of models have been developed to forecast the state of the equatorial Pacific atmosphere-ocean system; the consensus of these models is that the current La Nina will grow in amplitude and last at least through the end of the year. More details on current conditions and these model forecasts are available at the following web site: http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/enso.shtml. From the perspective of anticipating the weather in Washington State over the course of the approaching cool season, it can be assumed with a high degree of certainty that there will be La Nina, and hence a tendency for the weather that has accompanied these kinds of events in the past.
There have been about 8 La Nina events of moderate to strong intensity over the last 50 years (the exact number depends on the defining criteria). During those periods, the sea level pressure (SLP) in a seasonal mean sense has generally been higher than normal over a broad region of the central North Pacific Ocean basin. The consequence for Washington State has been more atmospheric flow out of the northwest than usual during the fall and winter. In terms of the tangible weather, past La Nina events have been relatively wet and cool, with the precipitation more systematically enhanced during the fall, and the temperature signal more robust in the winter. It bears emphasizing that there are considerable variations in how the different La Nina events have played out, with some cool seasons like 1998-99 being especially wet, and others, like 1988-89 including severe cold snaps. One can think of the tropical Pacific, through La Nina or El Nino, as stacking the deck, but not guaranteeing a particular deal of the cards. The internal variability of the atmospheric circulation at mid-latitudes is substantial, and this variability can serve to reinforce, or counteract, the remote influences of the tropical Pacific in an unpredictable manner. A telling example is provided by the last La Nina, which occurred during 2007-2009. The winter of 2007-08 included much stronger La Nina conditions than the following year, which barely qualified as a La Nina winter, and by some definitions was just on the cool side of the normal range. Yet in many ways the weather was much more extreme in the latter winter, with the unusually heavy lowland snows of December 2008 impacting many residents of the Pacific Northwest. It is also worth noting that La Nina influences the weather for the season as a whole. While it tends to cause a decreased chance of warm, moist southwesterly flow events with heavy rains and high snow levels, and an increased chance of northwesterlies with low snow levels and copious mountain snows, there will certainly be the usual day-to-day and week-to-week fluctuations in the weather.
In summary, La Nina is liable to bring us a relatively wet fall and cool winter. While this may sound like bad news for a region that often experiences rather unpleasant weather this time of year anyway, there are some payoffs. It is obviously good news for winter sports enthusiasts, but that is not all. A greater snowpack means more water for agriculture and hydropower generation and a better overall freshwater habitat for salmon. We will review the fall and winter weather next spring, so stay tuned to see how well 2010-11 compares with past La Nina events.