Extratropical Highlights –February 2017

 

1. Northern Hemisphere

The 500-hPa circulation during February featured above-average heights over the high latitudes of the central North Pacific, the southern U.S., eastern Europe, and much of central Asia, and below-average heights over western North America and western Russia (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 lingering 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 February included above-average temperatures across most of the U.S., central Canada, portions of eastern Europe, and central/eastern Asia, and below-average temperatures in the northwestern U.S. (Fig. E1). The main precipitation signals included above-average totals in the northwestern and western U.S. U.S., and below-average totals in the southeastern quadrant of the U.S. (Fig. E3).

 

a. North America

The 500-hPa circulation during February featured above-average heights across the central and eastern U.S. and below-average heights in the west (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 above-average temperatures across most of the rest of the country. Much of the eastern half of the U.S. recorded temperature departures in the upper 90^th percentile of occurrences, with actual departures being 4ºC-5ºC in many areas.

The anomalous 500-hPa circulation also contributed to a continuation of above-average precipitation in the northwestern and western U.S., with totals in the upper 90^th percentile of occurrences in the Pacific Northwest, Southern California, and Intermountain regions (Fig. E5). Precipitation surpluses over the last three months have essentially ended the long-term severe-to-exceptional drought that had plagued California for more than two years.

Conversely, below-average precipitation was recorded in the southeastern quadrant of the U.S. during February, with totals in the lowest 10^th percentile of occurrences in the Midwest, Ohio Valley, and Southeast. According to the U.S. Drought Monitor, moderate or severe drought covered large portions of Oklahoma and portions of western Kansas. In the southeastern U.S., severe or extreme drought extended from eastern Alabama to eastern Tennessee and western North Carolina. In the northeastern U.S., moderate drought extended from northeastern Virginia to New Hampshire, with extreme drought evident in western Connecticut and severe drought evident in southern New Hampshire.

 

b. Eurasia

The 500-hPa circulation during February featured an anomalous wave pattern across Eurasia, with above-average heights over eastern Europe and central Asia, and below-average heights over western Russia (Fig. E9). This pattern was associated with well above-average surface temperatures in central and eastern Asia, with departures exceeding the 90^th percentile of occurrences in portions of China (Fig. E1).

 

2. Southern Hemisphere

The mean 500-hPa circulation during February featured above-average heights over the central Indian Ocean and the western South Atlantic, and below-average heights across the high latitudes of the South Pacific (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 lingering La Niña-related pattern of enhanced convection over Indonesia and suppressed convection across the central equatorial Pacific (Fig. T25).

In eastern Australia, the overall large-scale circulation pattern contributed to anomalously warm and dry conditions, with precipitation totals in many regions in the lowest 10^th percentile of occurrences (Fig. E3).

The South African monsoon season runs from October to April. This area recorded well above-average precipitation during February, with totals near the 80^th percentile of occurrences (Figs. E3, E4). To date for the 2016-17 monsoon season, the region received below-average precipitation in October, near-average precipitation in November-December, and above-average totals in January-February. This region typically records above-average precipitation during La Niña.