The 500-hPa circulation pattern during December featured above-average heights across the high latitudes of the North Pacific, the central North Atlantic, and central Siberia, and below-average heights over eastern Canada and the Mediterranean Sea (Figs. E9, E11).  This circulation was associated with a continued strong positive phase of the North Atlantic Oscillation (Table E1, Fig. E7), and the ongoing warm phase of the Atlantic multi-decadal mode.

The main temperature departures during December included warmer than normal conditions over the northwestern United States , and both northern and eastern Europe, and below-average temperatures over eastern Canada and eastern Siberia (Fig. E1). The main precipitation departures included above-average totals in the western and northeastern U.S. , and in south-central Europe (Fig. E3).

a. North America

The mean circulation pattern during December featured weak positive 500-hPa height anomalies in the northwestern U.S. , and negative height anomalies over Hudson Bay (Fig. E9). This pattern prevailed mainly during the second half of the month (Fig. A2.1), when it contributed to above-average temperatures in the northwestern U.S. and below-average temperatures over eastern Canada (Fig. E1).

Precipitation during December was above average from the Great Lakes to New England, and in both the Inter-Mountain and Southern California regions of the United States (Figs. E3, E5). The Inter-Mountain region has recorded above-average precipitation in each of the last four months (Fig. E5), which has helped to moderate the long-term drought conditions plaguing the region. Rainfall in southern California has also been above average during the last three months, with area-mean totals exceeding the 90^th percentile of occurrences during both October and December.

b. North Atlantic/Eurasia

Above-average 500-hPa heights were again observed across the high latitudes of the North Atlantic during December, and below-average heights covered Greenland (Fig. E9). This anomaly pattern reflected a strong positive value (+1.3) of the NAO (Table E1, Fig. E7). These conditions were associated with an enhanced flow of marine air into northern Europe and Scandinavia (Fig. E10), and with a pronounced split-flow pattern across central and southern Europe . A large-amplitude trough embedded within the southern branch of this split flow brought above-average precipitation to south-central Europe, with totals over Italy and the Mediterranean Sea exceeding the 70^th percentile of occurrences.

The North Atlantic also saw a continuation of exceptionally warm SSTs at both high latitudes and in the subtropics. This warmth reflects the ongoing warm phase of the Atlantic multi-decadal mode that began approximately in 1995 (Goldenberg et al. Science, 2001).

In contrast, eastern Siberia experienced well below average surface temperatures during December, with values in many areas ranging from 3°-5°C below average (Fig. E1). These exceptionally cold conditions occurred downstream of a persistent upper-level ridge over central Siberia .

2. Southern Hemisphere

The 500-hPa circulation pattern during December featured a persistent zonal wave-4 pattern in the middle latitudes (Figs. E15, E17). At 200-hPa anticyclonic streamfunction anomalies prevailed across the lower latitudes of the South Pacific (Fig. T22). Over the eastern South Pacific the combination of negative height anomalies in the middle latitudes and positive height anomalies at lower latitudes was associated with an enhanced jet stream extending eastward into southern South America (Fig. T21). This circulation contributed to anomalously warm and wet conditions across the region, with monthly temperature and precipitation departures generally exceeding the 70^th percentile of occurrences.

            In southern Africa the rainy season normally lasts from October to April. During December area-mean precipitation totals were above average (Fig. E4), with the largest departures occurring in central and northeastern parts of the region (Fig. E3). This is the first month that above-average rains were observed during the 2004-2005 rainy season. Exceptionally warm conditions were also observed in South Africa during December, along with near-record sea-surface temperatures (ranging from 1°-2°C above average) surrounding the country. These anomalously warm and wet conditions were associated with positive 500-hPa height anomalies, below-average upper-level westerlies, and below-average surface pressures, across the region (Fig. T19).