Extratropical Highlights –January 2017

 

1. Northern Hemisphere

The 500-hPa circulation during January featured above-average heights over the high latitudes of the eastern North Pacific, eastern Canada, the high latitudes of the North Atlantic, and China, and below-average heights over the western U.S., the polar region, and the Mediterranean Sea (Fig. E9). In the subtropics, the 200-hPa circulation featured a pronounced westward retraction of the mean subtropical ridge over Australasia, along with amplified mid-Pacific troughs in both hemispheres (Fig. T22). This pattern is consistent with the La Niña-related pattern of enhanced convection over Indonesia and suppressed convection across the central equatorial Pacific (Fig. T25).

The main land-surface temperature signals during January included above-average temperatures across northern Europe, southeastern Asia, and China and below-average temperatures in the northwestern U.S. and central Russia (Fig. E1). The main precipitation signals included above-average totals in both the western and central U.S., and below-average totals in northern and western Europe (Fig. E3).

 

a. North America

The 500-hPa circulation during January featured above-average heights over the Gulf of Alaska and eastern Canada, and below-average heights in the western U.S. (Fig. E9). This pattern reflected a disappearance of the climatological mean ridge over the western U.S., and a weaker trough over the eastern U.S. These conditions were accompanied by an anomalously zonal flow across the country (Figs. E10, T22).

These conditions contributed to below average surface temperatures in the northwestern U.S. (Fig. E1), and to increased storminess and above-average precipitation in both the western and central U.S. (Fig. E3). Area-averaged precipitation totals were near record (1971-2016) levels in both the Southern California and Inter-Mountain regions during January (Fig. E5), following well above-average precipitation in both regions during December. The Pacific Northwest and Great Plains regions also recorded above-average precipitation during January, with above-average totals in the upper 80^th percentile of occurrences.

According to the U.S. Drought Monitor, the areal extent of long-term exceptional drought in California has ended. However, by the end of January severe drought was still present along the southern half of coastal California. In the southern Plains States, moderate or severe drought covered large portions of Oklahoma and portions of western Kansas. In the southeastern U.S., moderate or severe drought extended from central Alabama to eastern Tennessee and western North Carolina. In the northeastern U.S., severe drought persisted in southern New York, northern New Jersey and portions of New England, with extreme drought evident in Connecticut.

 

b. Eurasia

The 500-hPa circulation during January featured above-average heights across the high latitudes of the North Atlantic and below-average heights over the Mediterranean Sea (Fig. E9). This pattern was associated with a pronounced split-flow pattern over Europe, with anomalous southwesterly flow of milder marine air into northern Europe and an anomalous northwesterly flow into central and southern Europe. These conditions contributed to above-average surface temperatures in Scandinavia (Fig. E1), and to below-average precipitation over large portions of Europe (Fig. E3).

Over eastern Asia, the upper-level circulation featured a broad trough over India and an amplified ridge over eastern China (Fig. T22). This pattern contributed to well above-average precipitation across southeastern Asia in the area between the mean ridge and trough axes (Fig. E3). It also contributed to well above-average surface temperatures in China, with much of the country recording departures in the upper 90^th percentile of occurrences (Fig. E1).

 

2. Southern Hemisphere

The mean 500-hPa circulation during January featured above-average heights over the high latitudes of the central South Pacific, eastern Australia, and the high latitudes of the Indian Ocean, and below-average heights south of Australia and over the eastern Indian Ocean (Fig. E15). In the subtropics, the 200-hPa circulation featured a pronounced westward retraction of the mean subtropical ridge over Australasia, along with amplified mid-Pacific troughs in both hemispheres (Fig. T22). This pattern is consistent with the La Niña-related pattern of enhanced convection over Indonesia and suppressed convection across the central equatorial Pacific (Fig. T25).

In Australia, the overall large-scale circulation pattern contributed to above-average precipitation in the west in the area between the mean trough and ridge axes (Fig. E3). It also contributed to above-average surface temperatures in the east beneath the mean upper-level ridge (Fig. E1).

The South African monsoon season runs from October to April. This area recorded well above-average precipitation during January, with above-average totals near the 90^th percentile of occurrences (Figs. E3, E4). To date, the region received below-average precipitation in October and near-average precipitation during November and December. This region typically records above-average precipitation during La Niña.