Extratropical Highlights –August 2018

 

1. Northern Hemisphere

The 500-hPa circulation during August featured above-average heights over the Gulf of Alaska, the central North Atlantic and eastern Europe, and below-average heights at high latitudes (Fig. E9). This overall pattern projected strongly onto the positive phases of the North Atlantic Oscillation (NAO, +2.4) and the East Atlantic teleconnection pattern (EA, +1.8) (Fig. E7, Table E1).

The main land-surface temperature signals during August included above-average temperatures in the western and northeastern United States, Europe, and eastern Asia (Fig. E1). The main precipitation signals included above-average totals in the eastern half of the U.S. and eastern China, and below-average totals in the northwestern U.S, southwestern Canada, northern Europe, and western Russia (Fig. E3).

 

a. United States

The 500-hPa circulation during August featured above-average heights over the northwestern and northeastern U.S. (Figs. E9, E11). This pattern was associated with above-average surface temperatures in both regions (Fig. E1), along with below-average precipitation in the Northwest (Fig. E3). The eastern half of the U.S. recorded above-average precipitation during August. The most significant surpluses were observed in the Midwest, where area-averaged totals exceeded the 90^th percentile of occurrences (Fig. E5).

Regarding drought in the western U.S., ongoing anomalously warm and dry conditions have led to a continuation of extreme or exceptional drought in the Southwest, and to an expansion of moderate-to-severe drought in the Northwest. Drought also developed/ intensified in the northern Plains states, while areas of severe and extreme drought expanded in Missouri.

 

b. North Atlantic/ Europe

The 500-hPa height pattern featured above-average heights over the central North Atlantic and Europe, and below-average heights over the high latitudes of the North Atlantic (Fig. E9). This pattern reflected a continued strong positive phase of both the NAO (+2.4 std. dev.) and the East Atlantic (+1.8) teleconnection pattern (Fig. E7, Table E1). A positive NAO pattern has generally prevailed since January 2018.

The overall circulation contributed to a continuation of exceptionally warm surface temperatures across Europe (Fig. E1), with most of the continent recording departures in the upper 90^th percentile of occurrences. It also contributed to a continuation of below-average precipitation in northern Europe, an area which has recorded deficits for the last four months (Fig. E4).

 

c. China

Much of China recorded well above-average surface temperatures during August, with departures generally in the upper 90^th percentile of occurrences (Fig. E1). Area-averaged rainfall totals also exceeded the 90^th percentile of occurrences for the country as a whole (Fig. E4), with the most significant surpluses observed in the east. These conditions were associated with an amplified monsoon ridge (Fig. T22), and with five tropical systems that impacted eastern China.

 

d. West African monsoon

The west African monsoon extends from June through September, with a peak during July-September. During August 2018, the west African monsoon system was enhanced (Fig. E3, Fig. T24) with area-average rainfall totals above the 90^th percentile of occurrences (see Sahel region, Fig. E4). This region has recorded above-average precipitation during June-August.

 

 

2. Southern Hemisphere

The mean 500-hPa circulation during August featured an anomalous zonal wave-3 pattern. Above-average heights were observed over the central portions of the three ocean basins (Fig. E15). Below-average heights were observed south of Australia, over the high latitudes of the South Pacific, and in the area south of Africa. Above-average heights were also observed across Antarctica.

The Antarctic ozone hole typically develops during August and reaches its peak size in September. The ozone hole then gradually decreases during October and November, and dissipates in early December (Fig. S8). During August 2018, the ozone hole was nearly absent for most of the month, but then rapidly intensified and reached 15 million square kilometers by the end of the month. This size is roughly the average for the 2007-2016 period.