Extratropical Highlights – December 2014

 

1. Northern Hemisphere

The 500-hPa circulation during December featured above-average heights across Alaska, Canada, the eastern U.S., and the central North Atlantic, and below-average heights over the east-central North Pacific and the high latitudes of the North Atlantic (Fig. E9). Regional aspects of this circulation included an amplified ridge over western North America, a weaker Hudson Bay trough, and a strong positive phase (+1.63) of the North Atlantic Oscillation (NAO, Table E1, Fig. E7).

The main land-surface temperature signals during December included above-average temperatures across Alaska, Canada, the western U.S., and north-central Russia (Fig. E1). The main precipitation signals included above-average totals in the western U.S., and below-average totals in western Europe (Fig. E3).

 

a. North Pacific/ North America

The mean 500-hPa circulation during December featured above-average heights across Alaska, Canada, and the eastern U.S. (Fig. E9). This anomaly pattern reflected an amplified ridge in western Canada, along with a marked weakening of the Hudson Bay trough. This pattern was associated with above-average surface temperatures across most of North America (Fig. E1). The most significant departures were observed in central Alaska and along the west coast of North America, where monthly mean temperatures were in the upper 90^th percentile of occurrences.

Also observed during December was an enhanced trough at 500-hPa over the east-central North Pacific (Fig. E9). Increased storminess downstream of the trough axis resulted in above-average precipitation in the western U.S., with much of the region recording totals in the upper 70^th percentile of occurrences (Fig. E3).

The U.S. Drought Monitor indicated that despite the above-average precipitation, a large area of exceptional drought continued in central/ southern California and western Nevada, with extreme drought extending northward into Oregon. Exceptional or extreme drought also continued in north-central Texas and western Oklahoma.

 

b. North Atlantic/ Europe/ Russia

The 500-hPa circulation featured above-average heights extending from eastern Canada to western Europe, with below-average heights over the high latitudes of the North Atlantic. This north-south dipole pattern projected strongly onto the positive phase (+1.63) of the NAO (Table E1, Fig. E7). It was associated with amplified Icelandic Low, an enhanced ridge over the east-central North Atlantic, and a split flow centered over eastern Europe (Fig. E10). This split flow configuration reflected enhanced jet stream winds extending into northern Europe, and weaker than average winds across southern Europe.

The resulting enhanced flow of mild marine air into the higher latitudes contributed to above-average surface temperatures from Iceland to north-central Russia (Fig. E1). The most significant temperature departures were observed in north-central Russia, exceeding the 90^th percentile of occurrences. Within the southern branch of the jet stream, below-average precipitation was recorded across western Europe in the region downstream of the mean ridge axis (Fig. E3).

 

 

2. Southern Hemisphere

The mean 500-hPa circulation during December featured an anomalous zonal wave-4 pattern, with above-average heights over the central South Atlantic and Indian Oceans, and also over the western and eastern South Pacific Ocean (Fig. E15). Conversely, below-average heights were observed in the area south of Australia, over the central South Pacific, across the high latitudes of the South Atlantic, and throughout the polar region.

In Australia, a mean trough over the eastern part of the continent brought above-average precipitation, with much of the southeast coast recording departures in the upper 70^th percentile of occurrences (Fig. E3).

The South African rainy season lasts from October to April. Rainfall for the region as a whole was above average during December, (Fig. E4), with the main surpluses recorded in the Mozambique and southern Madagascar (Fig. E3). To date, the South African rainy season was below normal during October-November and above normal in December (Fig. E4).