1. Northern Hemisphere

During October the 500-hPa circulation pattern featured positive height anomalies over eastern Asia , the eastern North Pacific, Greenland , and the polar region, and negative height anomalies from the north-central U.S. to Spain , and over central Siberia (Figs. E9, E11). For the third straight month, the anomaly pattern from eastern North America to Europe has reflected a strong negative phase of the North Atlantic Oscillation (NAO) (Table E1, Fig. E7). During October the NAO index was -2.2, and for the August-October season the NAO index was -1.9.

In the subtropics, anticyclonic streamfunction anomalies were evident at 200-hPa in both hemispheres over central and east-central Pacific (Fig. T22), which is consistent with strengthening El Niño conditions during the month. Conversely streamfunction anomalies over south-central Asia were consistent with suppressed convection across Indonesia and the eastern half of the equatorial Indian Ocean (Fig. T25).

The main surface temperature departures during October reflected warmer-than-average conditions across northeastern Canada , Greenland , Europe , and China , and cooler-than-average conditions over the central and eastern United States (Fig. E1). The main precipitation anomalies included above-average totals in the Inter-Mountain, Southwest, Ohio Valley , Great Lakes and Northeastern regions of the U.S. (Figs. E3, E5, E6), and below-average totals in the Pacific Northwest U.S. and eastern China (Figs. E3, E4).

a. Pacific/ North America

In the subtropics, the mean 200-hPa circulation pattern during October was consistent with an increasing strength of El Niño, as indicated by anticyclonic streamfunction anomalies flanking the regions of enhanced convection over the central equatorial Pacific (Fig. T22). In the extratropics the 500-hPa circulation pattern featured an anomalous ridge over the eastern North Pacific and an enhanced trough over eastern North America (Fig. T9). This pattern contributed to above-average temperatures over Alaska and to below-average temperatures across central Canada and much of the United States (Fig. E1). The most significant departures occurred in south-central Canada , where monthly temperatures were more than 3°C below average and in the lowest 10^th percentile of occurrences. Farther east, above-average heights covered northeastern Canada and the polar region, which contributed to above average surface temperature throughout the region.

The anomalous precipitation patterns were also generally consistent with the upper-air circulation, with above-average totals in the central and eastern U.S. coinciding with regions of increased storminess in the vicinity of the mean trough axis. The most significant surpluses occurred in the Inter-Mountain, Southwest, Ohio Valley , Great Lakes , and Northeast regions, where totals generally exceeded the 80^th percentile of occurrences (Fig. E6). The Inter-Mountain region has recorded well above-average totals in each of the last four months, and the Southwest region has recorded above-average totals in each of the last three months. In contrast, the Pacific Northwest region of the U.S. recorded well below-average precipitation during October. This area was situated immediately downstream of the mean upper-level ridge axis, and recorded its fourth straight month of below-average precipitation.

b. North Atlantic / Europe

The 500-hPa circulation pattern during October featured an anomalous ridge over Greenland and an anomalous trough across the central North Atlantic (Fig. E9). These anomalies reflected a continuation of the strong negative phase of the NAO (-2.2) that began in August. During October Europe was situated immediately downstream of the mean upper-level trough axis, and again recorded significantly above-average temperatures (Fig. E1). Monthly temperatures across the continent generally exceeded the 90^th percentile of occurrences, with the largest departures of 2°-3°C observed in central and eastern Europe.

c. China

An anomalous upper-level trough was centered over northern India and western China during October. This feature reflected a weakening of the mean subtropical ridge, consistent with suppressed convection across Indonesia and the eastern half of the equatorial Indian Ocean (Fig. T25). The anomalous trough was associated with anomalous southwesterly flow and exceptionally warm surface temperature across China . Temperatures across central and northern China generally averaged 2°-3C during the month, and were in the upper 90^th percentile of occurrences.

2. Southern Hemisphere

The mean 500-hPa circulation pattern during October featured generally above-average heights in the middle latitudes, and below-average heights over the high latitudes of the South Pacific (Fig. E15). In the subtropics, cyclonic streamfunction anomalies were observed across the central and eastern South Pacific, along with an eastward extension of the South Pacific jet stream. These conditions, along with the negative height anomalies farther poleward, are a consistent response to El Niño conditions.

 In Australia , an enhanced mid-and upper-level ridge contributed to a continuation of anomalously warm and dry conditions. The departures were especially significant in the southeast, where monthly temperatures were in the upper 90^th percentile of occurrences and precipitation totals were in the 10^th percentile of occurrences for the third consecutive month.

In central and southern South America anomalous southwesterly flow at upper levels, and a poleward shit of the mean frontal boundary at lower levels, were evident downstream of the mean upper-level trough. These conditions contributed to well above-average temperatures and precipitation during the month.

The Antarctic ozone hole re-developed in late August and remained extensive through the end of October (Fig. S8a). The ozone hole was near record size from late September through October, exceeding 25 million km² in September and early October, and 20 million km² in late October. This extensive ozone hole is consistent with anomalously low stratospheric temperatures (Figs. S3, S4), and with an exceptionally large SH circumpolar vortex (Fig. S8b). Temperatures at both 2 hPa and 10 hPa were among the lowest on record from August through mid-October (Fig. S4), which led to a record large spatial extent of polar stratospheric clouds (PSC, Fig. S8c). PSC’s are the main mechanism by which the ozone hole is created.