Extratropical Highlights –September 2018

 

1. Northern Hemisphere

The 500-hPa circulation during September featured above-average heights over the high latitudes of the North Pacific, eastern North America, and Europe, and below-average heights over western Canada and the high latitudes of the North Atlantic (Fig. E9). For the Atlantic, the pattern projected strongly onto the positive phase of the North Atlantic Oscillation (NAO, +1.8 std. dev.) (Fig. E7, Table E1). A strong positive NAO pattern has been present for almost all of 2018.

The main land-surface temperature signals during September included above-average temperatures in both the western and eastern United States and Europe, and below-average temperatures in western Canada (Fig. E1). The main precipitation signals included above-average totals across the central U.S. and eastern Canada, and below-average totals in the northwestern and southeastern U.S and in much of Europe (Fig. E3).

 

a. United States

The 500-hPa circulation during September featured an amplified ridge in the eastern U.S. and a deep trough in the west (Figs. E9, E11). This anomalous wave pattern was associated with well above-average surface temperatures in the eastern half of the U.S., with many areas recording departures of 3+ºC and in the upper 90^th percentile of occurrences (Fig. E1). Well above-average precipitation occurred across the central U.S., which was situated between the mean trough and ridge axes (Fig. E3). Area-average totals exceeded the 90^th percentile of occurrences in the Great Plains, Midwest, Ohio Valley, and Great Lakes regions (Fig. E5). In the Midwest region, rainfall totals have been above the 90^th percentile of occurrences for the past two months.

According to the U.S. Drought Monitor, severe-to-exceptional drought continued in the southwestern and northwestern U.S. One of the hardest-hit regions is the 4-corner states of Utah, Colorado, Arizona, and New Mexico. Another major drought area includes the states of Oregon and Washington, with extreme drought evident across central Oregon and moderate or severe drought present elsewhere.

 

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 (+1.8 std. dev.) (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 again 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 five months (Fig. E4).

 

c. West African monsoon

The west African monsoon extends from June through September, with a peak during July-September. During September 2018, the monsoon rains were 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 for the last four months.

 

 

2. Southern Hemisphere

The mean 500-hPa circulation during September featured above-average heights over the central portions of the three ocean basins, and below-average heights at high latitudes (Fig. E15). The most significant surface temperature anomalies were present in southern Sough America, where departures were above the 90^th percentile of occurrences (Fig. E1). Much of this area also recorded above-average precipitation (Fig. E3). These conditions were associated with an amplified trough situated over the high latitudes of the South Pacific and located just upstream of continental South America (Fig. E15).

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, top). During the latter half of September 2018, the size of the ozone hole was above average at 23 to 24 million square kilometers. This increased size was associated with larger than average size of the SH polar vortex (Fig. S8, middle), and with an anomalously large area of Polar stratospheric cloud (Fig. S8, bottom).