Extratropical Highlights –January 2019

 

1. Northern Hemisphere

The 500-hPa circulation during January featured above-average heights over western Canada, the high latitudes of the North Atlantic, and eastern Asia, and below-average heights over the western North Pacific and much of Europe (Fig. E9). At 200-hPa, the circulation featured anticyclonic streamfunction anomalies over the central subtropical North and South Pacific, in association with an eastward extension of the mean subtropical ridge in both hemispheres (Fig. T22). This pattern is consistent with an anomalous eastward extension of deep tropical convection to the International Date Line (Fig. T25).

The main land-surface temperature signals during January included above-average temperatures across most of Alaska, western Canada, the western U.S., and eastern Asia (Fig. E1). The main precipitation signals included above-average totals in the central and northeastern U.S., the mountainous western U.S., and eastern Europe, and below-average totals in Great Britain (Fig. E3).

 

a. North Pacific and America

The 500-hPa circulation during January featured above-average heights over western Canada (Fig. E9), which contributed to above-average surface temperatures across southern Alaska and western Canada (Fig. E1). At 200-hPa, anticyclonic streamfunction anomalies were present over the central subtropical North Pacific, in association with an eastward extension of the mean subtropical ridge (Fig. T22). This pattern was accompanied by an eastward extension of the East Asian jet stream and its jet exit region to well east of the date line (Fig. T21). These conditions contributed to increased storminess and above-average precipitation in portions of the mountainous West (Fig. E3).

According to the U.S. Drought Monitor, severe-to-exceptional drought continued in the four-corner states of Utah, Colorado, Arizona, and New Mexico. Farther north in Oregon, severe or extreme drought persisted across the central part of the state.

 

b. North Atlantic and Eurasia

The 500-hPa height pattern featured above-average heights over the high latitudes of the North Atlantic, and below-average heights over central and eastern Europe (Fig. E9). This pattern resulted in anomalous northerly flow and near-average surface temperatures throughout Europe (Fig. E1), and follows several months of above-average temperatures in that region. It also contributed to exceptionally dry conditions in Great Britain, and to above-average precipitation in eastern Europe (Fig. E3). These regions were situated downstream of the mean ridge and trough axes, respectively.

In eastern Asia, above-average heights at 500-hPa were accompanied by a more zonal wind pattern across central Russia (Fig. T21). These conditions contributed to well above-average surface temperatures in eastern Asia, where departures of +3ºC to +5ºC were generally in the upper 70^th or 90^th percentile of occurrences (Fig. E1).

 

2. Southern Hemisphere

The mean 500-hPa circulation during January featured above-average heights over southeastern Australia, the high latitudes of the eastern South Pacific, and both the central South Atlantic and Indian Oceans (Fig. E15). Below-average heights were observed over the high latitudes of the western South Pacific and also over the high latitudes of the western South Atlantic. At 200-hPa, anticyclonic streamfunction anomalies over the central subtropical South Pacific were associated with an eastward extension of the mean subtropical ridge (Fig. T22). This pattern is consistent with an anomalous eastward extension of deep tropical convection to the International Date Line (Fig. T25).

In Australia, the amplified ridge in the southeast contributed to well above-average surface temperatures in the south-central and southeastern parts of the continent (Fig. E1), with many regions recording departures in the upper 90^th percentile of occurrences. Much of the continent also recorded below-average precipitation during January (Fig. E3).

The South African monsoon season runs from October to April. During January, rainfall was below average in the western portion of the monsoon region, and above average in the eastern portion (Fig. E3). For the monsoon region as a whole, area-average totals in January were slightly above average (Fig. E4). This represents somewhat of an improvement in the monsoon rains from the past three months (October-January), when area-averaged totals were in the lowest 20^th percentile of occurrences.