1. Northern Hemisphere

During December the 500-hPa height pattern featured positive anomalies over the subtropical North Pacific, Canada , and central Europe , and negative anomalies over the Gulf of Alaska and high latitudes of the North Atlantic (Figs. E9, E11). In the subtropics, the 200-hPa streamfunction pattern featured anticyclonic anomalies in both hemispheres flanking the regions of enhanced convection over the central and east-central tropical Pacific (Fig. T22). Anticyclonic anomalies were also evident across the subtropical Atlantic Ocean and Africa in both hemispheres.

Over the North Pacific and North America , these conditions reflected the positive phase of the Pacific/ North American (PNA) teleconnection pattern, and were consistent with El Niño (Table E1, Fig. E7). Over the North Atlantic , the circulation reflected the ongoing positive phase of the East Atlantic (EA) teleconnection pattern, which has been prominent since April.

The main surface temperature departures during December reflected warmer-than-average conditions across most of North America and Eurasia (Fig. E1). For the entire Northern Hemisphere monthly mean land surface temperatures were1.4°C above the 1971-2000 mean, and were the third warmest in the record. The main precipitation anomalies during December included above-average totals in the northwestern, Plains, and Gulf Coast states, and below-average totals in the southwestern U.S. , southern Europe , and Turkey (Figs. E3, E5, E6)

a. Pacific/ North America

Accompanying the positive PNA pattern associated with El Nino, the East Asian jet stream was extended well east of normal, and the associated jet exit region was shifted eastward to the eastern North Pacific (Fig. T21). Also, the Hudson Bay Low was nearly completely absent, and a strong zonal (west-to east) flow was seen throughout the continent.

            This circulation brought an extensive flow of marine air into North America , which prevented arctic air masses from developing in Canada . It also prevented major cold-air outbreaks from occurring in the United States . The result was significantly above-average temperatures across North America , with the largest departures (> 5°C) observed across north-central Canada . In the U.S. , the largest temperature departures (3°-4°C) were observed in the Great Lakes region and the Northeast, where values exceeded the 90^th percentile of occurrences.

The northwestern U.S. and southeastern Canada experienced significantly above-average precipitation during the month, in response to enhanced storminess downstream of the mean upper-level trough and the East Asian jet exit region (Fig. E3). The Great Plains and southeastern U.S. also experienced above-average precipitation within and downstream of a mean upper-level trough situated over the central part of the country.

  b. North Atlantic / Europe

The 500-hPa circulation pattern during December featured below-average heights near Iceland , and above-average heights extending from the eastern U.S. to central Europe . This ongoing positive phase of the EA pattern was associated with a strong southwesterly flow of mild, marine air into Europe . During December, these conditions contributed to well above-average temperatures throughout Eurasia , with the largest departures (4°-7°C) observed in northern Europe , Scandinavia , and central Siberia . They were also associated with an enhanced storm track across northern Europe , which contributed to above-average precipitation in northern Europe and below-average precipitation across southern Europe and Turkey .

Well above-average sea-surface temperatures continued across the high latitudes and tropical latitudes of the North Atlantic during December. This pattern is associated with the ongoing warm phase of the Atlantic multi-decadal mode that began approximately in 1995, and with the warming trend in global temperatures during the past 100 years.

2. Southern Hemisphere

The mean 500-hPa circulation pattern during December featured an anomalous zonal wave-4 structure, with above-average heights across southern Australia and over the central longitudes of the three ocean basins, and below-average heights near New Zealand and south of both South America and South Africa (Fig. E15). In the subtropics, anticyclonic streamfunction anomalies at 200-hPa extended across the South Pacific consistent with ongoing El Niño conditions, and across the South Atlantic and southern Africa .

For extratropical South America , much of the region was situated in an area of enhanced storminess immediately downstream of a mean upper-level trough axis. Precipitation totals exceeded the 70^th percentile of occurrences over much of Chile and Argentina , with some areas of central Argentina recording totals in the highest 90^th percentile of occurrences.

In southern Africa , surface temperatures were again above average during December, and precipitation was near average. The South African rainy season extends from October to April. For the 2006-07 season, totals were below-average in October and near-average in both November and December.

For Australia , the most significant precipitation anomalies again occurred in the east and northeast, where totals remained in the lowest 30^th percentile of occurrences. This below-average precipitation is consistent with El Niño, and has been accentuated at times by large-scale subsidence downstream of an anomalous upper-level ridge.