Extratropical Highlights –April 2020

 

1. Northern Hemisphere

The 500-hPa circulation during April featured above-average heights over the Gulf of Alaska, Europe, and north-central Asia, and below-average heights over the central North Pacific, eastern Canada, western Russia (Fig. E9). At 200-hPa, cyclonic streamfunction anomalies were observed across the Pacific basin in both hemispheres.

The main land-surface temperature signals during April included above-average temperatures in the southwestern U.S., Europe, and north-central Asia, and below-average temperatures in western Canada and portions of the central and northeastern U.S. (Fig. E1). The main precipitation signals included above-average totals in the southeastern and eastern U.S., and southwestern Europe, and below-average totals in the northwestern U.S. and much of Europe (Fig. E3).

 

a. North America

The 500-hPa circulation during April featured above-average heights over the Gulf of Alaska and an enhanced Hudson Bay Low (Fig. E9). This pattern was associated with anomalous northerly jet stream winds across western and central Canada, and with an enhanced subtropical jet stream across the southern U.S. (Fig. T21).

These conditions contributed to exceptionally cool surface temperatures in western and central Canada, with many areas recording temperatures in the lowest 10^th percentile of occurrences (Fig. E1). They also contributed to below-average temperatures in portions of the central and northeastern U.S.

Precipitation was below average in the northwestern U.S. in the area downstream of the mean ridge axis. Precipitation was above average in the southeastern and eastern U.S. (Fig. E3), in response to increased storminess associated with the enhanced subtropical jet stream. In the Pacific Northwest region, precipitation totals have been below average for three past three months (Fig. E5). In contrast, totals in the Midwest region have been above average for the past four months.

 

b. Eurasia

The 500-hPa circulation during April featured an amplified wave pattern across Eurasia, with ridges over Europe and north-central Asia and a deep trough over western Russia (Fig. E9). Well above-average surface temperatures were observed in the vicinity of the ridge axes, with much of Europe and central Siberia recording departures in the upper 90^th percentile of occurrences (Fig. E1). For both areas, this represents a continuation of exceptional warmth that has persisted for several months.

 

2. Southern Hemisphere

The 500-hPa height field during April featured above-average heights across central Australia and southern South America, and below-average heights over southeastern Australia (Fig. E15). In southeastern Australia, this pattern contributed to above-average precipitation, while in western Australia it contributed to below-average precipitation (Fig. E3). In southern South America, it contributed to warmer than average conditions (Fig. E1).

The South African monsoon season runs from October to April. During April 2020, precipitation was above-average across the region. This marks the only month of the 2019-20 monsoon season in which well above-average precipitation was recorded (Fig. E4).