The 500-hPa circulation during December featured positive height anomalies over the polar region and in the subtropics, and negative height anomalies in the middle latitudes (Fig. E10). These anomalies project onto several major teleconnection patterns, each of which has persisted since October (Table E1, Figs. E7, E8). Over the Pacific Ocean they reflect the positive phases of the Pacific/ North American (PNA) and West Pacific (WP) patterns. Over the Atlantic Ocean they reflect the negative phase of the North Atlantic Oscillation (NAO) and positive phase of the East Atlantic (EA) pattern. The persistence of these patterns in both ocean basins is consistent with the ongoing El Niño and negative phase of the Arctic Oscillation (Fig. A2.1)

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The prominent temperature anomalies during December included much warmer than normal conditions across the northern half of North America and southern Europe, and cooler than normal conditions across northern Europe, Scandinavia, and western/ central Russia (Fig. E1). Prominent precipitation departures during the month included above-average precipitation across the southern and eastern portions of the United States (Figs. E3, E5, E6) and across southern Europe (Figs. E3, E4), and below-average precipitation over western Canada and northern Europe/ Scandinavia

a. Pacific/North America

The strong positive PNA (+1.1) and West Pacific (+1.4) patterns during December (Table E1) reflected an amplified upper-level trough across the central and high latitudes of the North Pacific, an amplified ridge over western Canada, and a reduced strength of the trough over the subtropical eastern North Pacific (Figs. E10, T22). These anomalies were associated with an extension of the East Asian jet stream across the eastern North Pacific, and with an eastward shift of the mean jet exit region and accompanying split-flow configuration to the western U.S. (Fig. E11). The axis of the East Asian jet stream was also shifted to the extreme southwestern United States, which is well south of its climatological mean position in the Pacific Northwest. This jet stream configuration is consistent with the atmospheric response to mature El Niño conditions.

This circulation was associated with increased storminess in the anomalous jet exit region (Fig. E14). Several of these storms moved into California, strengthened while passing across the southern half of the U.S., and developed into major east-coast cyclones. This anomalous low-latitude storm track resulted in above-average precipitation across California, the Gulf Coast, the Southeast, the mid-Atlantic, and the Northeast (Figs. E3, E5). Each of these regions has recorded above-average precipitation during the last three months (Fig. E5). This combination of above-average precipitation and increased storminess across the south is also consistent with the atmospheric response to El Niño.

A strong flow of marine air into western North America during December contributed to above-average surface temperatures over the northern half of the continent. In Alaska and Canada temperatures ranged from 4-8°C above average while over the northern U.S. they ranged from 2-4°C above average (Fig. E1). This anomalous warmth is also consistent with the ongoing El Niño.

b. North Atlantic and Europe

The circulation over the North Atlantic and Europe featured above-average heights at high latitudes and below-average heights in the middle latitudes (Fig. E10). This circulation was associated with high-latitude blocking and a pronounced split-flow configuration over the eastern North Atlantic and Scandinavia, and with a southward shift of the mean wintertime jet axis to southern Europe (Figs. E11, T21). This overall anomaly pattern has persisted since October, and reflects the ongoing negative phase of the NAO and positive phase of the East Atlantic teleconnection pattern (Table E1, Figs. E7, E8).

During December these conditions were associated with a continuation of warmer and wetter than normal conditions across southern Europe, and cooler and drier than normal conditions in Scandinavia (Figs. E1, E3). The combination of high-latitude blocking over Scandinavia and a very strong upper-level trough over central Russia also contributed to significantly below-averaged surface temperatures across western and central Russia (Fig. E1), with temperatures over western Russia ranging from 4-7°C below average and falling within the lowest 10^th percentile of occurrences.

2. Southern Hemisphere

The upper-level circulation during December featured above-average heights in the middle latitudes and across the subtropical eastern South Pacific, and below-average heights in the polar region (Fig. E16). Over the eastern South Pacific this circulation was associated with enhanced jet stream winds along the poleward flank of the subtropical anticyclonic circulation anomalies, consistent with the ongoing El Niño (Figs. E17, T21).

In South America prominent precipitation anomalies during December reflected the continuation of below-average precipitation in northeastern and northern sections and enhanced precipitation in southern Brazil and northern Argentina (Figs. E3, E4, T25). Below-average precipitation was also observed across northern and northeastern Australia, with monthly totals again below the 10^th percentile. These rainfall patterns are consistent with the ongoing El Niño. Much of eastern Brazil and eastern Australia were again considerably warmer than normal during the month, with temperatures exceeding the 70^th percentile in both areas (Fig. E1).

In South Africa the rainy season typically lasts from October to April. The first three months of the 2002-2003 rainy season have featured below-average rainfall (Fig. E4). During December this dryness was associated with a persistent upper-level ridge/ trough system, with eastern South Africa situated downstream of the mean ridge axis in an area of anomalous sinking motion (Fig. T22).