Winter 2010 forecast for North America

Summary. During the winter (DJF) of 2010 a pronounced PNA-like temperature pattern is forecast for North America. This pattern is essentially a response to the ongoing El Niño event.  Despite the El Niño dominance in this winter forecast, a significant month-to-month variability should be expected due to other factors, such as the North Atlantic Oscillation (NAO). In December, the NAO is expected to be predominantly in its positive phase, leading to an advection of warm tropical air into the southeastern United States. Therefore, the December temperature anomaly pattern is forecast to be generally opposite to that for the winter. In January, the polar jet stream probably will likely be shifted north of its normal position, and much of the continent will experience above normal temperatures. In February, the NAO is expected to be negative, and its effect on North American temperature will amplify the El Niño effect.

Discussion

Fig. 1. Winter (DJF) 2010 SAT forecast.

In October 2009, the Southern Oscillation Index (SOI) has dropped substantially into the negative territory, and now the oceanic and atmospheric ENSO characteristics are better adjusted to each other and provide a more consistent picture of the current El Niño event. In the North Pacific, the PDO index is on the rise and is expected to be positive during the winter of 2010 (although not strongly positive). As with many other El Niño events, the Aleutian low is likely to be deeper than normal, and the atmospheric circulation over North America will be characterized by an amplified tropospheric ridge-trough system (positive PNA pattern). This will result in above normal surface air temperature (SAT) over the western and central parts of North America and below normal SAT in the southeastern United States (Fig. 1).

This forecast is similar to the official forecast from the CPC, which in turn closely resembles the typical response of North American temperatures to El Niño events, upon which CPC forecasts heavily rely. Forecasts from other parties (IRI, CFS and PSD) are quite unanimous in depicting the El Niño signature in the North American temperature anomaly pattern. One notable exception is the forecast from the U.K. Met Office issued in October, which shows a mosaic distribution of SAT anomalies. Perhaps their November forecast will feature a more pronounced El Niño signal due to recent developments in the tropical Pacific toward a more coherent El Niño picture.

Fig. 2. December 2009 SAT forecast.

Fig. 3. January 2010 SAT forecast.

Fig. 4. February 2010 SAT forecast.

Despite the El Niño dominance this winter and the persistence it introduces into the forecasts, a significant month-to-month variability should be expected due to other factors, such as the North Atlantic Oscillation (NAO). In December, the rules are not unanimous regarding the monthly temperature anomalies in a number of locations, which suggests that one may expect reversals of atmospheric circulation/temperature patters within the month. For example, it is likely that temperature in the southeastern US will decrease compared to December 2008 due to more frequent outbreaks of cold air. It is still expected, however, that it will stay above the 1971-2000 average, partly because the NAO will probably manage to be positive for the month. Similarly, some rules indicate a possibility of above normal temperatures in the Pacific Northwest, as it is normally happens during El Niño winters. However, due to the unusual character of the current El Niño event (in particular, a sharp drop of SLP in Australia in August), temperatures in the Pacific Northwest are more likely to be below normal.  Overall, the December SAT anomaly pattern (Fig. 2) is forecast to be almost opposite to that for the winter as a whole (Fig. 1).

In January, the NAO will be in its neutral phase and its effect on North American temperature will be weaker. The polar jet stream will be shifted north of its normal position and much of the continent will experience above normal temperatures (Fig. 3). It is important to note, however, that the confidence factors for this pattern are relatively low. This is because of a number of conflicting signals, particularly for the Pacific Northwest and southwestern United States. In the Midwest, the temperature is likely to increase compared to the previous winter (which was very cold), but whether it will significantly exceed the 1971-2000 average is less likely. Overall, it will be a transitional pattern between a negative PNA in December and positive PNA in February.

In February, the NAO is expected to become negative. If so, its effect on North American temperature will amplify the El Niño effect, which will probably reach the full strength this month. The SAT anomaly pattern for February (Fig. 4) reflects a strong upper atmospheric ridge on the west and deep trough on the east of the continent. Due to its strength, this February pattern essentially determines the forecast pattern for the entire winter.