Extratropical Highlights – January 2013

 

1. Northern Hemisphere

The 500-hPa circulation during January featured above-average heights over the polar region, the Gulf of Alaska, the eastern United States, and the subtropical eastern North Atlantic. It also featured below-average heights across the central North Atlantic, southern Europe, and central Russia (Figs. E9, E11).

The main land-surface temperature signals during January included above-average temperatures across Alaska, Canada, and the eastern half of the U.S., and below-average temperatures in the western U.S. and most of Siberia (Fig. E1). The main precipitation signals included above-average totals in the central U.S. and southern Europe, and below-average totals in both the northwestern and extreme eastern U.S., and Scandinavia (Fig. E3). As a result of long-term precipitation deficits, extreme and exceptional drought conditions continued during January in the U.S. Great Plains, despite the above average precipitation recorded during the month (Fig. E5).

 

a. North America

The mean 500-hPa circulation during January featured an amplified wave pattern with above-average heights over the Gulf of Alaska and the southeastern U.S., and a sharp trough extending southwestward from Hudson Bay to the southwestern U.S. (Figs. E9). This pattern strongly influenced the temperature and precipitation patterns across North America. From west-to-east, it was associated with 1) an enhanced flow of mild marine air and above-average temperatures across Alaska and Canada (Fig. E1), 2) a northward shift of the main storm track to Canada (Fig. E13), resulting in well below-average precipitation in the U.S. Pacific Northwest (Fig. E3), 3) enhanced northwesterly flow and below-average temperatures in the western U.S. (Fig. E10), 4) above-average precipitation across the U.S. Plains States and Midwest in the area downstream of the mean upper-level trough axis (Figs. E3, E5, E6), and 5) anomalous southerly flow and above-average temperatures across the eastern half of the U.S., along with below-average precipitation in the extreme east.

January 2013 marks the first month since March 2012 in which the U.S. Great Plains recorded above-average precipitation (Fig. E5). It also marks the first month since March 2012 that the Midwest recorded significantly above-average precipitation. Despite this much-needed precipitation in January, large portions of the central U.S. continued to be impacted by extreme or exceptional drought. At the end of January, the “U.S. Drought Monitor” indicated exceptional drought from portions of Texas northward to central South Dakota, including portions of eastern Wyoming and eastern Colorado. Extreme drought persisted in much of Wyoming, northwestern Iowa, and southern Minnesota.

The U.S. eastern seaboard received well below-average precipitation during January, with both the Mid-Atlantic and Northeast regions recording area-averaged totals in the lowest 5^th percentile of occurrences (Fig. E5). Also, large portions of the extreme southeastern U.S. recorded less than 25% of normal precipitation (Fig. E6). Ongoing deficits have led to extreme or exceptional drought in portions of Georgia, to severe drought from eastern Alabama to northern South Carolina, and to moderate drought across western North Carolina.

 

b. Europe/ Asia

The mean 500-hPa circulation during January featured a ridge near Iceland and below-average heights extending from the central North Atlantic to central Russia (Fig. E9). This pattern contributed to below-average temperatures across northern Russia and Siberia (Fig. E1). It also contributed to above-average precipitation across central and southern Europe, and to below-average precipitation over Scandinavia (Fig. E3).

 

2. Southern Hemisphere

The mean 500-hPa circulation during January featured above-average heights over the high latitudes of the central and eastern South Pacific, and over southern Australia and the central South Atlantic Ocean (Fig. E15).

Over southern Australia, the amplified ridge was associated with a pole-ward shift of the main belt of upper-level westerly winds (Fig. T21, T22), which contributed to well above-average temperatures across much of the continent (Fig. E1). The most significant departures were observed in central and eastern Australia, where they exceeded the upper 90^th percentile of occurrences.

The South African rainy season lasts from October to April. During January 2013, rainfall for the region as a whole was well above average, with area-averaged totals exceeding the 95^th percentile of occurrences (Fig. E4). Much of this surplus was observed in Mozambique and northeastern South Africa (Fig. E3). For the 2012-13 rainy season to date, rainfall for the entire region was above-average precipitation during October and January, below-average in November, and near-average in December.