1. Northern Hemisphere

The 500-hPa circulation pattern during December featured persistent positive height anomalies in the western Canada , and from the high latitudes of the eastern North Atlantic to central Siberia , and persistent negative anomalies over the western and central North Pacific, the southeastern U.S. , and south-central Europe (Figs. E9, E11).

The main surface temperature departures reflected above-average temperatures across western Canada and Alaska , the high latitudes and subtropical latitudes of the North Atlantic , and Scandinavia , and below-average temperatures in the eastern U.S. , south-central Europe , and northern China (Fig. E1). The main precipitation departures included above-average totals in the northwestern U.S. and portions of eastern Europe, and below-average totals across the southern U.S. , the high latitudes of the North Atlantic , and northwestern Europe (Figs. E3, E5, E6).

a. Pacific/ North America

During December, a persistent pattern of below-average heights at 500-hPa covered the western and central Pacific and the southeastern U.S. , while positive height anomalies were observed in western Canada (Figs. E9, E11). This pattern was associated with cyclonic streamfunction anomalies at 200-hPa across the middle latitudes of the North Pacific, and anti-cyclonic anomalies in the subtropics over eastern Asia and near Hawaii (Fig. T22). These overall conditions projected strongly onto the negative phase of the West Pacific teleconnection pattern (-1.8 st. dev) and the positive phase of the Pacific/ North American (PNA) teleconnection pattern (+1.4 st. dev) (Table E1, Fig. E7).

These conditions were accompanied by an amplified East Asian jet stream that extended well east of normal, with the mean jet exit region also shifted eastward to just upstream of the United States (Fig. T21). They also reflected an amplified ridge-trough pattern across the U.S. .

At 200-hPa, the anticyclonic streamfunction anomalies over eastern Asia , along with their SH counterparts across the eastern Indian Ocean and Australia , were consistent with enhanced convection across Indonesia (Fig. T25). However, the circulation anomalies over the eastern half of the North Pacific were opposite to those normally associated with suppressed convection near the date line. Therefore, while the amplified jet stream over eastern Asian has some links to the enhanced tropical convection, the pronounced eastward extension of the jet appears to be un-related to the tropical convection pattern.

This anomalous jet stream led to a focused and persistent storm track over the eastern Pacific, which resulted in series of major storms striking the northwestern U.S. Area-average totals in the Pacific Northwest were near the 100^th percentile of occurrences (Fig. E5), with many locations recording more than double their normal monthly amounts. Conversely, monthly totals across the southwestern quadrant of the U.S. were generally less than 25% of normal, with totals in the south-central region falling in the lowest 10^th percentile of occurrences.

            The mean circulation was also associated with a strong onshore flow of mild air into western Canada and Alaska (Fig. E1), with monthly mean temperatures in these regions generally ranging from 3° to 5°C above average. Conversely, below-average temperatures over the eastern U.S. coincided with the mean position of the amplified upper-level trough.

  b. Europe

      The 500-hPa circulation pattern during December featured above-average heights from the high latitudes of the eastern North Atlantic to central Siberia , and below-average heights across south-central Europe (Fig. E11). These conditions were associated with above-average temperatures across Scandinavia and portions of Siberia , and with an east-to-west dipole of temperature anomalies flanking the upper-level trough. Western Europe was situated between the mean upper-level ridge and trough axes and recorded generally below-average precipitation during the month, while southeastern Europe was situated downstream of the mean trough axis and experienced above-average precipitation.

2. Southern Hemisphere

The mean 500-hPa circulation pattern during December featured above-average heights across Antarctica , and below-average heights from southern Africa to the area south of Australia . At 200-hPa, anticyclonic streamfunction anomalies covered much of the subtropics. The largest anticyclonic anomalies extended from southeastern Africa across Australia , and also across the central South Pacific (Fig. E15).

In Australia , the mean upper-level ridge axis was located over the middle of the continent. Below-average rainfall occurred upstream of the ridge axis and above-average totals occurred downstream over eastern Australia (Fig. E3). Much of eastern Australia also recorded significantly above-average temperatures (2+ °C above average) during the month, with temperatures in many areas exceeding the 90^th percentile of occurrences (Fig. E1). This warmth is consistent with an anomalous westerly flow of low-level air from the interior of the continent, and with below-average precipitation throughout the region (Fig. T20).

In southern Africa the rainy season normally lasts from October to April. Area-average rainfall was above normal during December, thus marking three consecutive months of above-average rains (Fig. E4). However, most of the surplus rainfall was confined to Mozambique during December, with below-average totals observed across southeastern South Africa where isolated areas recorded amounts in the lowest 10^th percentile of occurrences. This rainfall pattern was related to the structure of the subtropical ridge, which was constricted in its poleward extent partially in response to the persistent mid-latitude trough farther south. As a result, southeastern South Africa experienced a mid-latitude circulation regime and below-average precipitation during the month.