Extratropical Highlights –October 2018

 

1. Northern Hemisphere

The 500-hPa circulation during October featured above-average heights over Alaska, western Canada, the southeastern U.S., the eastern North Atlantic, Europe, and eastern Siberia. Below-average heights were observed over the central North Pacific, eastern Canada, and the high latitudes of the North Atlantic (Fig. E9). For the Atlantic basin, the anomaly pattern projected strongly onto the positive phase of the North Atlantic Oscillation (NAO, +1.5 std. dev.) (Fig. E7, Table E1). A strong positive NAO pattern has been present for nearly all of 2018.

The main land-surface temperature signals during October included above-average temperatures in Alaska, the southeastern U.S., Europe and northern Asia, and below-average temperatures in much of Canada (Fig. E1). The main precipitation signals included above-average totals across the central U.S. and southern Europe, and below-average totals in northern Europe (Fig. E3).

 

a. United States

The 500-hPa circulation during October featured an amplified wave pattern across the central North Pacific and North America (Fig. E9). This pattern was associated with anomalous southerly flow and exceptionally warm surface temperature in Alaska (Fig. E1). Many areas in Alaska recorded departures of 4+ºC, which were in the upper 90^th percentile of occurrences. Downstream, below-average surface temperatures were observed across much of Canada, with the largest anomalies observed between the mean upper-level ridge and downstream trough axis.

Another aspect of the 500-hPa circulation was anomalous southwesterly flow over the south-central U.S. in the region between the mean ridge and trough axes. These conditions contributed to above-average precipitation the central U.S. (Fig. E3), with totals exceeding the 70^th percentile from Texas to North Dakota. In the Great Plains region, area-averaged rainfall totals were above the 90^th percentile of occurrences for the second consecutive month (Fig. E5). Area-averaged totals also exceeded the 90^th percentile of occurrences in the drought-stricken Inter-Mountain region, and in large portions of the Desert Southwest.

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

 

b. North Atlantic/ Europe

The 500-hPa height pattern featured above-average heights over the eastern North Atlantic and Europe, and below-average heights over the high latitudes of the North Atlantic (Fig. E9). This pattern reflected an ongoing strong positive phase of the NAO (+1.5 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 and western Russia (Fig. E1), with large areas 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 rainfall deficits for the last six months (Fig. E4).

 

2. Southern Hemisphere

The mean 500-hPa circulation during October featured an anomalous zonal wave-3 pattern, with above-average heights in the areas poleward of all three continents and below-average heights over the high latitudes of the central ocean basins. (Fig. E15). The most significant surface temperature anomalies were present in eastern Australia, where many locations recorded departures exceeding 2ºC and in the upper 90^th percentile of occurrences (Fig. E1). The most significant precipitation anomalies during October were observed in central South America, where totals were above the 90^th percentile of occurrences (Fig. E3).

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 October 2018, the size of the ozone hole was well above average at 20 million square kilometers. This increased size was associated with a larger than average size of the SH polar vortex (Fig. S8 middle), and with an anomalously large area of Polar stratospheric cloud (PSC) (Fig. S8 bottom).