Climate Logic

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 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.

During the winter of 2010, mean seasonal temperatures in Europe are forecast to be close to the 1971-2000 average; however, a strong month-to-month variability is expected.  In December, the North Atlantic Oscillation (NAO) will be in the positive phase, and unseasonably warm temperatures will dominate much of Europe, with the strongest positive anomalies in Scandinavia. Colder than normal temperatures are forecast for the Mediterranean region. In January, the westerly winds will start weakening, and the outbreaks of cold arctic air will become more frequent.  As a result temperatures in Europe will be more seasonal. In February, the NAO index will turn negative and the temperature anomaly pattern will be almost opposite to that in December. It will be the coldest month of the season.

As of mid-September 2009, SSTs are above-average across much of the equatorial Pacific indicating weak-to-moderate El Niño conditions. Although every El Niño is different, the current episode seems to be more unusual than one can expect from normal Niño-to-Niño variations. Among the most noticeable abnormalities are the neutral state of SOI, inconsistent El Niño signatures in cloudiness and wind patterns, cooling in subsurface temperature since June, and anomalously warm waters in the Coral Sea, off Australia's northern coasts and in the far western Tropical Pacific.

The 2009 summer temperature anomaly in Europe as a whole was 0.84°C (or 1.98 std) above the 1971-2000 average. The summer was particularly warm in southeast Europe and western Mediterranean, where seasonal temperature anomalies exceeded the 1968-1996 average by more than 2.5°C and 2°C, respectively. Colder or near-normal temperatures were observed in northern Russia and the Baltic countries. A deep atmospheric trough over the eastern North Atlantic contributed to above average precipitation across the British Isles and southern Scandinavia. Drier than normal conditions were observed in Spain, France, Romania, Moldova, Ukraine and southern Russia.

For the summer of 2009, the average temperature over the contiguous United States was 71.7°F, or 0.4°F below the 20th Century average. The national Canadian average temperature was 0.4°C above normal. The strongest negative temperature anomalies were observed in the Great Lakes region, where Michigan experienced its fifth coolest summer since the record began in 1895. Colder than normal temperatures were also observed in the northern Plains States and Northeast. In contrast, the Pacific Northwest and British Columbia saw an exceptionally warm summer. In Vancouver, June 2009 temperature was warmest on record (since 1896).