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).
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).
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 90th 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 90th
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 10th
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.