Despite the presence of a borderline weak / moderate La Nina, the weather across the Red River Valley and Lakes Region has been unusually warm and dry. Typically, by now, the impacts of La Nina would have become more apparent in our region. Typically the weather turns appreciably colder as we transition from fall to winter. Yet so far, we are on track for one of the drier and warmer Decembers on record. What has become of La Nina? It's still there, but there is more to the story.
During the winter of 2010/2011, colder and generally snowier than average conditions prevailed. Last winter featured the switch from an El Nino in early 2010 to La Nina by summer. This fairly rapid switch from El Nino to La Nina caused tremendous amounts of energy to be made available to the atmosphere, generating a more stormy mid-latitude pattern. During the period spring (March - April - May) of 2011 to late fall (October - November) less energy was made available to the atmosphere. This was due to the already cool Pacific Ocean having impacted the atmosphere during the previous years switch from El Nino to La Nina. There are several different measures of this, all of which support the differing amounts of energy available, causing the atmosphere to respond differently this year than last year.
Additionally, this year's La Nina has a slightly different orientation as to the location of the coldest water in the Equatorial Pacific Ocean than in previous La Nina events. As this is the second consecutive year with La Nina,this event is weaker than the 2010/2011 event. Other subtle, but important differences exist which in turn impact the overall atmospheric circulations differently. These subtle differences are difficult to predict, which results in the inherent uncertainty of any long range outlook .
Another significant player in the weather across the northern plains is the Arctic Oscillation (AO). Basically, the Arctic Oscillation is the difference in the pressure between the Polar Region and mid latitudes. When the Arctic Oscillation is in a negative mode it can have significant impacts on the temperature patterns in our region by transporting colder air southward. When the Arctic Oscillation is positive, the colder air is more likely to be bottled up well north of our region. An example is the 2009 / 2010 El Nino, which coincided with record negative values of the Arctic oscillation. As a result the 2009 / 2010 winter was cooler and snowier than in a "normal" El Nino winter. Since the record low values of December 2009 and February 2010, the Arctic Oscillation has been gradually trending towards a positive mode. Late this summer the Arctic Oscillation has become strongly positive, and as of mid December remains in a strong positive mode. Although not always the case, the temperature patterns in our region have switched in concert with the changing mode of the Arctic Oscillation. A detailed, technically oriented paper on the AO is available online from the Climate Prediction Center (CPC) here.
North Atlantic Oscillation
The North Atlantic Oscillation (NAO) consists of a north-south dipole of anomalies, with one center located over Greenland and the other center of opposite sign spanning the central latitudes of the North Atlantic between 35°N and 40°N. The positive phase of the NAO reflects below-normal heights and pressure across the high latitudes of the North Atlantic and above-normal heights and pressure over the central North Atlantic, the eastern United States and western Europe. The negative phase reflects an opposite pattern of height and pressure anomalies over these regions. Both phases of the NAO are associated with basin-wide changes in the intensity and location of the North Atlantic jet stream and storm track, and in large-scale modulations of the normal patterns of zonal and meridional heat and moisture transport (Hurrell 1995), which in turn results in changes in temperature and precipitation patterns often extending from eastern North America to western and central Europe (Walker and Bliss 1932, van Loon and Rogers 1978, Rogers and van Loon 1979). One of the most significant atmospheric variables to affect the winter weather over the northeastern third of the United States is the phase of the NAO. The negative phase of the NAO results in cold and snowy weather, as was the case during December of 2000, when Grand Forks had 19 inches of snow and averaged 11.8F below normal. The following month the NAO went into the positive phase, which typically results in warm and dry weather over our region. January 2001 in Grand Forks was 9.1F above normal with only 3.3 inches of snow! Unfortunately, it is difficult to predict changes in the phase of the NAO, meaning that sharp changes in the winter weather pattern like those of December 2000 to January 2001 can occur without much warning. Recently, the NAO has been strongly positive which favors warmer than normal weather across the northern plains.
So far this early winter season, the combination of the positive AO and NAO have acted together to block the typical La Nina impacts to our weather. Should the AO and NAO switch to negative modes, as some anticipate, the regions weather will likely become much colder and stormier.
Obviously, there are many other atmospheric factors which work to produce "weather" over the region. There are very long term Ocean-Atmospheric signals such as the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO), which as the names imply - operate on decadal time-scales. Research performed by NWS Partner Agencies such as the U.S. Army Corps of Engineers and U.S. Geologic Survey / North Dakota highlight the impacts the AMO and PDO have on our regions weather.
Below are images that show the variation in upper atmospheric conditions, surface pressure and temperature based on various phases of the ENSO, AO and NAO. These are basic examples, and cannot fully represent the literally infinite possible atmospheric variations possible. Some combinations of atmospheric variables are stable, which lead to long term patterns of drought or unusually wet; persistent periods of warm or cold. Some patterns are unstable, which may lead to dramatic changes in the weather that last only a few days.Below are graphics which represent a very small sampling of the atmospheric and oceanic data which we utilize in climate analysis and prediction.
The top image is a comparison of the surface temperatures across the Northern Hemisphere (poleward of 40 degrees latitude) during November 2010 and November 2011. The previous years average has been subtracted to illustrate the change between El Nino (2009 to 2010) and 1st year La Nina to 2nd year La Nina (2010 to 2011)
The second image is the anomaly in the 500 mb heights for November 2010 (left) and November 2011 (right) which illustrates the displacement of the centers of upper level high and low pressure.
The bottom image represent satellite derived Sea Surface Temperature Anomalies from December 19th 2011 (top) and December 20th 2010 (bottom) Both represent a La Nina, the blues and purples along the Equator in the Pacific representing colder than normal water at the surface.
Images Courtesy of the Earth Science Research Laboratory, Boulder CO