1. Northern Hemisphere

The 500-hPa circulation pattern during October featured persistent positive height anomalies across the central North Pacific, eastern Canada , and Scandinavia , and negative height anomalies over the Gulf of Alaska and the central North Atlantic (Figs. E9, E11). The main surface temperature departures reflected above-average temperatures over Canada and the northern tier of the U.S. , the high latitudes and subtropical latitudes of the North Atlantic , and most of Eurasia (Fig. E1). The main precipitation anomalies during October featured well above-average totals over the southeastern and eastern U.S. , and below-average totals in large portions of the Midwestern U.S. and across Scandinavia (Figs. E3, E5, E6).

a. North America

The prominent 500-hPa circulation anomalies during October included above-average heights across the central North Pacific and eastern Canada , and below-average heights over the Gulf of Alaska (Figs. E9, E11). The persistent ridge over eastern North America was associated with exceptionally warm temperatures, with the largest departures of 2°-4°C observed in eastern Canada . This persistent ridge also contributed to below-average precipitation in portions of the Midwestern U.S., where totals were in the lowest 10^th percentile of occurrences.

In contrast, rainfall was well above average in the Gulf Coast and eastern U.S. , partly in response to landfalling tropical storm Tammy in eastern Florida and landfalling hurricane Wilma in western Florida . The northeastern U.S. experienced a series of major storms during mid-October, which resulted in significant flooding in portions of New England . These storms occurred upstream of the mean ridge axis as the persistent ridge temporarily shifted to just east of the region. During 9-17 October, large portions of New England recorded 250+ mm of rain, with some areas recording 300– 400 mm.

b. North Atlantic Hurricanes

The 2005 Atlantic hurricane season was very active during October. Six tropical storms (TS) formed during the month, with four of these becoming hurricanes (H) and two becoming major hurricanes. By the end of the month, 23 tropical storms and 13 hurricanes had occurred during the 2005 hurricane season, breaking the previous records of 21 TS set in 1933 and 12 H set in 1969. The October activity resulted from an inter-related set of conditions that had prevailed throughout the season. These conditions included 1) a persistent upper-level ridge over the southeastern U.S. and western subtropical North Atlantic (Fig. T22), 2) anomalous upper-level easterlies across the tropical Atlantic and Caribbean Sea (Fig. T21), 3) a below-average strength of the tropical easterly trade winds from the eastern North Pacific to Africa (Fig. T20), and 4) ongoing exceptionally warm SSTs across the tropical Atlantic and Caribbean Sea (Fig. T18). These conditions contributed to very low wind shear between 200-850 hPa, which allowed many African easterly waves to develop into tropical storms and hurricanes. Four of the October named storms formed over the Caribbean Sea , which is the main formation region during that month.

c. Europe / Eurasia

The 500-hPa circulation pattern during October featured persistent positive height anomalies across northern Europe and northwestern Russia (Fig. E11). This pattern contributed to warmer and drier conditions in these regions, with temperature departures generally exceeding the 70^th percentile of occurrences and precipitation departures generally falling below the 30^th percentile of occurrences.

2. Southern Hemisphere

The mean 500-hPa circulation pattern during October again featured an anomalous zonal wave-3 pattern in the middle and high latitudes. This pattern featured above-average heights southeast of Australia , over the western South Atlantic , and over the central Indian Ocean , and below-average heights southwest of Australia , over the high latitudes of the central South Pacific, and across the central South Atlantic (Fig. E15).

Positive surface temperature anomalies were observed over southern Africa and the eastern half of Australia , consistent with the persistent pattern of positive height anomalies. The main area of below-average temperatures occurred over central South America , immediately downstream of the mean upper-level trough axis (Fig. E1).

The Antarctic ozone hole and polar stratospheric cloud (PSC) cover during October 2005 were near the 1995-2004 mean (Fig. S8). The average size of the ozone hole during October was 17.5 million  km², with the main area of low ozone shifted toward the high latitudes of the South Atlantic (Fig. S6).