Extratropical Highlights –January 2018

 

1. Northern Hemisphere

The 500-hPa circulation during January featured an anomalous wave-4 pattern. Above-average heights were present over the high latitudes of the North Pacific, western North America, the central North Atlantic, and northwestern Russia. Below-average heights were present over the Gulf of Alaska, the high latitudes of the North Atlantic, and eastern Asia (Fig. E9).

At 200-hPa, the circulation reflected La Niña. This signal included amplified troughs near and east of the date line across the subtropical Pacific Ocean in both hemispheres, and amplified ridges over Australasia (Fig. T22).  The amplified troughs in both hemispheres reflected the disappearance of deep tropical convection from the central equatorial Pacific (Fig. T25). The amplified subtropical ridges reflected enhanced convection over the western tropical Pacific, Indonesia, and the eastern Indian Ocean.

The main land-surface temperature signals during January included above-average temperatures across western North America, Europe, and western Russia, and below-average temperatures in the southeastern and eastern U.S., and in south-central Russia (Fig. E1). The main precipitation signals included above-average totals in the northwestern U.S. and central Europe, and below-average totals in the southern and southeastern U.S. (Fig. E3).

 

a. North Pacific and North America

The 500-hPa circulation during January featured above-average heights over the high latitudes of the North Pacific and western North America, and below-average heights over the Gulf of Alaska (Fig. E9). This type of highly amplified wave pattern is often seen during La Niña. La Niña produces an enhanced subtropical ridge over southeastern Asia along with an amplified mid-Pacific trough (Fig. T22). These conditions act to retract westward the East Asian jet steam, as seen during January by a confinement of that jet core to the area around Japan and by the location of that jet’s exit region being located well west of the date line (Fig. T21). This anomalous jet structure produces the essence of the downstream anomalous 500-hPa circulation pattern seen during January.

This overall pattern contributed to anomalously warm surface temperatures in western North America, and to anomalously cool conditions in the southern and eastern U.S. (Fig. E1). It also contributed to above-average precipitation in the northwestern U.S., and to below-average precipitation in the southern and southeastern U.S.  (Fig. E3).

 

b. Eurasia

            An amplified wave pattern at 500-hPa extended from the central North Atlantic Ocean to eastern Asia (Fig. E9). Features of this pattern included an anomalous ridges over the central North Atlantic and northwestern Russia, and anomalous troughs over the high latitudes of the North Atlantic and eastern Asia (Fig. E9). This pattern was associated with anomalously warm (Fig. E1) and wet (Fig. E3) conditions in Europe, and with anomalously cool and dry conditions in portions of central Russia (Fig. E3).

 

c. China

            At 200-hPa, the subtropical ridge was much stronger than average across southern Asia and China (Fig. T22). This ridge was associated with an enhanced southerly flow at 850-hPa (Fig. T20) of deep tropical moisture into eastern China, along with anomalous upper-level divergence in that region along the equatorward flank of the East Asian jet entrance region (Fig. T23). As a result, eastern China recorded well above-average precipitation during January, with area-averaged totals in the upper 90^th percentile of occurrences (Fig. E4).

 

2. Southern Hemisphere

The mean 500-hPa circulation during January featured above-average heights in the vicinity of New Zealand, and below-average heights over much of the high latitudes (Fig. E15). At 200-hPa, the subtropical circulation featured an amplified trough over the central and eastern South Pacific Ocean, and an amplified ridge over western Australia and the eastern Indian Ocean (Fig. T22). This anomalous subtropical circulation is typical of La Niña.

On the synoptic scale, the upper-level circulation featured a ridge across western Australia and a trough off the coast of eastern Australia. This pattern was associated with weaker upper-level westerlies across the continent (Fig. T21), and with anomalous upper-level convergence (Fig. T23) and below-average precipitation (Fig. E3) across eastern Australia between the mean ridge and trough axes. It was also associated with well above-average surface temperatures in eastern Australia, with many locations recording temperatures in the upper 90^th percentile of occurrences (Fig. E1).

The South African monsoon season runs from October to April. This area recorded well below-average precipitation during January, with area-averaged totals in the lowest 10^th percentile of occurrences (Fig. E4). The most significant deficits were observed in the northern and western portions of the monsoon region (Fig E3), where they acted to intensify severe drought in areas such as Cape Town. In the north and east, many normally heavy-rainfall areas of Zimbabwe and Mozambique recorded very limited rainfall during January (and December) resulting in 2-month deficits of 400 mm – 500 mm.