Extratropical Highlights – February 2014

 

1. Northern Hemisphere

The 500-hPa circulation during February featured above-average heights over the central and high latitudes of the North Pacific, the southwestern U.S., the central North Atlantic, Scandinavia, and western Russia, and below-average heights across North America, the high latitudes of the North Atlantic, and central Russia (Fig. E9). Regional aspects of this circulation included enhanced westerly jet stream winds across the United States and North Atlantic Ocean, along with a southward shift of the North Atlantic jet stream (Figs. T21, E10). Other aspects included a marked strengthening of both the Hudson Bay trough and the Icelandic Low.

In the Pacific/ North American sector, the extensive 500-hPa height anomalies projected onto the strong negative phase of the Pacific/ North American (-1.6, PNA) teleconnection pattern. Over the Atlantic and Eurasian sectors, the 500-hPa height anomalies projected onto the strong positive phase of the East Atlantic (EA, +2.2) and Scandinavia (+1.1) patterns, and also onto the negative phase of the East Atlantic/ West Russia (-1.9, EAWR) pattern (Table E1, Fig. E7).

The main land-surface temperature signals during February included well above-average temperatures in the southwestern U.S. and Europe, and below-average temperatures in western Canada, the north-central U.S., and central Russia (Fig. E1). The main precipitation signals included above-average totals in the Pacific Northwest and Northeast regions of the U.S., and in large portions of southern Europe, and below-average totals in the southwestern and south-central U.S., eastern Europe and western Russia (Fig. E3).

 

a. North Pacific/ North America

The mean 500-hPa circulation during February featured an amplified ridge over the central and high latitudes of the North Pacific, and the southwestern U.S., and an amplified trough across central North America (Fig. E9). This circulation projected onto the negative phase of the PNA teleconnection pattern (-1.6) (Table E1, Fig. E7).

This overall pattern was associated with enhanced westerly jet stream winds across the eastern North Pacific and central North America (Fig. T21). It was also associated with anomalous northeasterly winds in western Canada. The northern branch of this confluent flow brought anomalously cold surface temperatures to western and central Canada, and to the north-central U.S. (Fig. E1). The southern branch brought anomalously warm and dry conditions to the southwestern U.S. (Figs. E1, E3).

Above-average precipitation totals in the Pacific Northwest during February were the first since September 2013. In contrast, the Southwest, Southern California, and Great Plains regions of the U.S. have recorded precipitation deficits for at least the last three months (Fig. E5).

These ongoing deficits have contributed to an expansion and worsening of drought conditions in the western U.S. According to the U.S. Drought Monitor (http://droughtmonitor.unl.edu), much of California and northwestern Nevada reported either extreme or exceptional drought throughout the month. Southern Oregon, Idaho, and much of New Mexico reported severe drought.

Drought also worsened and expanded in the Great Plains during February (Fig. E5). By the end of the month moderate or severe drought persisted from Texas to northern Nebraska, with isolated regions of extreme drought present.

In the mid-western U.S., near- or above-average precipitation during the last five months has reduced the severity of ongoing drought conditions (Fig. E5). However, the U.S. Drought Monitor at the end of February indicated the persistence of moderate drought in large portions of the upper Midwest.

 

b. North Atlantic/ Eurasia

The mean 500-hPa circulation during February featured above-average heights across the central North Atlantic, Scandinavia, and western Russia, and below-average heights over the high latitudes of the North Atlantic (Fig. E9). This pattern projected onto the positive phases of the East Atlantic (+2.2) and Scandinavia (+1.1) teleconnection patterns, and also onto the negative phase of the East Atlantic/ West Russia (-1.9) pattern (Table E1, Fig. E7). February marks the third consecutive with a strong positive EA pattern, and the second straight month with a strong negative EAWR pattern

During February, this circulation was again associated with an enhanced North Atlantic jet stream that extended from the eastern U.S. to central Europe. It was also associated with an enhanced flow of marine air into Europe and Scandinavia (Fig. E10). These conditions contributed to above-average temperatures across Europe, with much of the continent recording departures in the upper 70^th percentile of occurrences (Fig. E1). They also contributed to a continuation of below-average temperatures in central Russia, with most areas recording departures in the lowest 30^th percentile of occurrences.

Precipitation within and downstream of the strong ridge axis was again well below-average, with eastern Europe recording totals in the lowest 30^th percentile of occurrences and southwestern Russia recording totals in the lowest 10^th percentile of occurrences (Fig. E3).

 

2. Southern Hemisphere

The mean 500-hPa circulation during February featured above-average heights in the middle latitudes and below-average heights over the high latitudes of the South Atlantic Ocean (Fig. E15). In Australia, precipitation was below average in the northeast (Fig. E3).

The South African rainy season lasts from October to April. During February, much of the region recorded near-average totals (Figs. E3, E4). To date, the 2013-14 rainy season totals have been above-average during October, December, and January, below-average in November, and near-average in February.