Extratropical Highlights – January 2014

 

1. Northern Hemisphere

The 500-hPa circulation during January featured a persistent zonal wave-2 pattern. This pattern included a strong ridge in western North America and another over Greenland and Scandinavia. It also included trough extending from central Asia to the eastern North Pacific and another extending from the eastern U.S. to Europe (Fig. E9). Regional aspects of this circulation included highly amplified wave patterns across North America and Europe, and amplified jet streams across both the central North Pacific and North Atlantic Oceans (Fig. T21).

Over the Pacific/ North American sector, the extensive 500-hPa height anomalies projected onto the strong positive phase (+1.6) of the Tropical/ Northern Hemisphere (TNH) teleconnection pattern. Over the Atlantic and Eurasian sectors, the 500-hPa height anomalies projected onto the strong positive phase of the East Atlantic (EA, +1.4) and Scandinavia (+1.8) patterns, and also onto the negative phase of the East Atlantic/ West Russia (-1.3) pattern (Table E1, Fig. E7). Strong positive TNH and EA patterns were also evident last month.

The main land-surface temperature signals during January included well above-average temperatures across Alaska, western Canada, the western U.S., Europe, and northern China, and below-average temperatures in the eastern half of the U.S. and across central Siberia (Fig. E1). The main precipitation signals included below-average totals along the west coast of North America, the southern and eastern U.S., Scandinavia and northern Russia, and eastern China (Fig. E3).

 

a. North Pacific/ North America

The mean 500-hPa circulation during January featured an extensive trough across the high latitudes of the North Pacific, and a large-amplitude wave pattern across North America (Fig. E9). This circulation projected onto the positive phase of the TNH pattern (+1.6) (Table E1, Fig. E7). A similar large-scale anomaly pattern was also present last month.

Over the North Pacific, the January circulation was associated with an enhanced east Asian jet stream that extended farther east than normal (Fig. T21). Over North America, the wave pattern featured an amplified ridge in the west, which extended into the polar region, and a very strong Hudson Bay trough (Fig. E9). This pattern contributed to exceptionally warm conditions in Alaska, western Canada, and the western U.S. and to below-average temperatures across the eastern half of the U.S. (Fig. E1). The most significant positive departures were observed in Alaska and western Canada, where mean temperatures averaged more than 5^oC above average and were in the upper 90^th percentile of occurrences.

For the central and eastern U.S., exceptionally cold temperatures throughout the month resulted from a strong northwesterly flow of arctic air into the middle of the country. Monthly temperatures throughout these regions were in the lowest 30^th percentile of occurrences.

The strong ridge over western North America also contributed to a continuation of exceptionally dry conditions in the western U.S., in the region immediately downstream of the mean ridge axis (Fig. E3). Anomalous sinking motion and below-average precipitation has prevailed in the western U.S. since October 2013. The Pacific Northwest has recorded totals in the lowest 20^th percentile of occurrences throughout this 4-month period, while southern California has recorded well below-average precipitation during the last two months (Fig. E5). These ongoing precipitation deficits have contributed to a worsening of drought conditions from Washington State to southern California. According to the U.S. Drought Monitor (http://droughtmonitor.unl.edu), much of California and northwestern Nevada reported either extreme or exceptional drought by the end of the month. Most of Oregon and southern Idaho reported severe drought, and Washington State and northern Idaho reported moderate drought.

Anomalously dry conditions also overspread the Plains states during January, with area-average totals in the lowest 20^th percentile of occurrences (Fig. E4). By the end of the month moderate or severe drought persisted from Texas to northern Nebraska, with isolated regions of extreme drought also 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 indicates that moderate or severe drought still persisted in large portions of the upper Midwest at the end of January.

 

b. North Atlantic/ Eurasia

The mean 500-hPa circulation during January featured above-average heights across the central North Atlantic, Greenland and Scandinavia, and below-average heights over eastern Europe (Fig. E9). This pattern projected onto the positive phases of the East Atlantic (+1.4) and Scandinavia (+1.8) teleconnection patterns, and also onto the negative phase of the East Atlantic/ west Russia (-1.3) pattern (Table E1, Fig. E7).

This circulation was 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 with anomalous southerly flow across Scandinavia (Fig. E10). These conditions contributed to above-average temperatures across Europe, with much of the continent recording temperatures in the upper 70^th percentile of occurrences (Fig. E1). In contrast, precipitation within and downstream of the strong ridge axis was well below-average, with the area from central Scandinavia to north-central Russia recording totals in the lowest 10^th percentile of occurrences (Fig. E3).

 

2. Southern Hemisphere

The mean 500-hPa circulation during January featured above-average heights over the high latitudes of the central South Pacific, southern Australia, and the area south of Africa, and below-average heights over the high latitudes of the South Atlantic, the western Indian Ocean and New Zealand (Fig. E15). In Australia, precipitation was above- (below) average in the west (east) (Fig. E3). These areas coincided with the upstream and downstream portions of the mean ridge axis, respectively.

The South African rainy season lasts from October to April. During January, much of the interior region and Madagascar recorded totals in the upper 70^th percentile of occurrences (Fig. E3). For the entire South African monsoon region, area-averaged totals were in the upper 70^th percentile of occurrences (Fig. E4).  To date, the 2013-14 rainy season has produced above-average totals during October, December, and January, and below-average totals in November.