Extratropical Highlights – December 2012

 

1. Northern Hemisphere

The 500-hPa circulation during December featured above-average heights near the Aleutian Islands, across eastern Canada, Greenland and Scandinavia. Below-average heights were present over western North America, across the east-central North Atlantic and Europe, and over the middle latitudes of central and eastern Asia (Fig. E9). This overall pattern projected onto a strong positive phase of the Scandinavia teleconnection pattern (Fig. E7, Table E1). It also projected onto continuing negative phases (three months) of the West Pacific and Pacific/ North America teleconnection patterns, and onto the negative phase of the Tropical/ Northern Hemisphere pattern.

The main land-surface temperature signals during December included above-average temperatures across central and eastern North America, and below-average temperatures in Alaska and much of Eurasia (Fig. E1). The main precipitation signals included above-average totals in the northwestern and eastern U.S., northwestern and southeastern Europe, and eastern China, and below-average totals in the south-central U.S. (Fig. E3).

 

a. North America

The mean 500-hPa circulation during December featured above-average heights south of the Aleutian Islands and over eastern Canada, and below-average heights over western Canada (Figs. E9, E11). This pattern projected onto a continuing negative phase (three months) of the Pacific/ North America teleconnection pattern, and also onto the negative phase of the Tropical/ Northern Hemisphere pattern (Fig. E7, Table E1). These conditions reflected a nearly complete disappearance of the mean ridge normally located over western North America. They were also associated with a westward shift of the mean Hudson Bay trough axis to central North America. The resulting upper-level wind pattern reflected anomalous northwesterly flow into Alaska and anomalous southwesterly flow into the western U.S (Fig. E10).

These conditions were associated with above-average surface temperatures across the eastern half of North America, with most regions recording departures in the upper 70^th percentile of occurrences (Fig. E1). In contrast, southwestern and eastern Alaska and northwestern Canada recorded below-average temperatures, with many areas recording departures in the lowest 30^th percentile of occurrences.

Areas downstream of the mean trough axes recorded well above-average precipitation during December, including the U.S. Pacific Northwest and eastern U.S. (Fig. E3). Area-averaged totals in the Pacific Northwest reached the 90^th percentile of occurrences, and were above average for a third straight month (Fig. E5).

In contrast, the south-central U.S. again recorded below-average monthly precipitation (Fig. E3), with large areas of Kansas, Oklahoma, and Texas recording totals less than 25% of normal (Fig. E6). This marks the eighth straight month of below-average precipitation in the Great Plains region (Fig. E5). As a result, much of the region continued to be impacted by extreme or exceptional drought. At the end of December, the “U.S. Drought Monitor” indicated exceptional drought from northern Texas northward to central South Dakota, including portions of eastern Wyoming and southeastern Colorado. Extreme drought persisted in northwestern Arkansas, northwestern Iowa, and southwestern Minnesota.

           

b. North Atlantic/  Europe

The mean 500-hPa circulation during December featured a north-south dipole pattern of height anomalies, with above-average heights across eastern Canada, Greenland, and Scandinavia, and below-average heights over the east-central North Atlantic and Europe (Figs. E9). This pattern was associated with a complete disappearance of the Icelandic Low for a second straight month, and with a strong positive phase (+2.0) of the Scandinavia teleconnection pattern (Fig. E7, Table E1). It was also associated in northern Europe with enhanced jet stream winds, increased storminess (Fig. E13) and above-average precipitation (Fig. E3).

 

c. Asia

The 500-hPa height pattern during December featured an extensive area of negative height anomalies across central and eastern Asia (Fig. E9). Similar conditions were also present in November. In combination with the strong blocking ridge over Scandinavia, this pattern during December contributed to well below-average temperatures across central Asia, with many areas recording departures in the lowest 10^th percentile of occurrences (Fig. E1). Also, eastern China recorded well above-average precipitation for a second straight month (Fig. E3), with area-averaged totals again reaching the 99^th percentile of occurrences (Fig. E5).

 

2. Southern Hemisphere

The mean 500-hPa circulation during December featured above-average heights over the southeastern South Pacific Ocean and in the areas south of both Australia and South Africa, and generally below-average heights in the middle latitudes (Fig. E15)

Much of eastern Australia recorded anomalously warm (Fig. E1) and dry (Fig. E3) conditions in December, with portions of the northeast recording temperature departures in the upper 90^th percentile of occurrences (Fig. E1).

The South African rainy season lasts from October to April. During December 2012, rainfall for the region as a whole was near average, with above-average totals in interior portions of southern Africa and below-average totals in extreme southern South Africa and Madagascar (Fig. E3). To date, the 2012-13 rainy season has been variable, with above-average totals in October, below-average totals in November, and near-average totals in December (Fig. E4).