Extratropical Highlights – October 2014

 

1. Northern Hemisphere

The 500-hPa circulation during October featured a zonal wave-4 pattern in the middle latitudes. This pattern included above-average heights over western North America, eastern Canada, Scandinavia, and the high latitudes of the North Pacific, along with below-average heights over the eastern North Pacific, the eastern U.S., the eastern North Atlantic and central Russia (Fig. E9). Also during October, above-average heights were observed throughout the polar region. 

The overall height anomaly pattern projected onto several teleconnection patterns, including the positive phases of the East Atlantic (+1.0), Pacific/ North American (+1.1), and Scandinavia (+1.1) patterns (Table E1, Fig. E7). Additionally, the overall anomaly pattern projected onto the negative phases of the North Atlantic Oscillation (-0.9) and the Arctic Oscillation (-1.1) (Fig. A2.1).

The main land-surface temperature signals during October included above-average temperatures in the western half of the U.S., eastern Canada, Europe, and China, and below-average temperatures across northwestern and north-central Russia (Fig. E1). The main precipitation signals included above-average totals in the midwestern and northeastern U.S., southeastern Europe, and north-central Russia, and below-average totals in southern Alaska and south-central Europe (Fig. E3).

 

a. North Pacific/ North America

The mean 500-hPa circulation during October featured an anomalous wave pattern across the eastern North Pacific and North America, with above-average heights over the western U.S. and eastern Canada and below-average heights over the eastern North Pacific and the eastern U.S. (Fig. E9). This pattern contributed to exceptionally warm temperatures over the western half of the U.S., with many areas recording departures in the upper 90^th percentile of occurrences (Fig. E1). It also contributed to above-average precipitation in the midwestern and northeastern U.S. (Fig. E3), regions located within or downstream of the mean trough axis.

According to the U.S. Drought Monitor, a massive area of exceptional drought continued in central/ southern California and western Nevada, with extreme drought extending northward into Oregon. Exceptional or extreme drought was also observed in northeastern Texas and south-central Oklahoma.

 

b. Europe/ Russia

The 500-hPa circulation featured an anomalous wave pattern extending from the eastern North Atlantic to central Russia, which included a strong ridge over Scandinavia and amplified troughs over the eastern North Atlantic and central Russia (Fig. E9). This pattern impacted surface temperatures and precipitation across the region.

The associated enhanced southwesterly flow of mild marine air across Europe contributed to well above-average surface temperatures, with most areas recording departures in the upper 90^th percentile of occurrences (Fig. E1). In contrast, surface temperatures were well below average across northwestern and north-central Russia, in response to strong northwesterly flow downstream of the mean ridge axis. In addition, monthly precipitation totals were well above average in north-central Russia in the region downstream of the mean trough axis (Fig. E3).

 

2. Southern Hemisphere

The mean 500-hPa circulation during October featured an anomalous zonal wave-3 pattern, which was dominated by above-average heights over southern Australia, the high latitudes of the South Pacific, and south of Africa (Fig. E15).

In southern and eastern Australia, the amplified ridge led to well above average surface temperatures, with much of southwestern and southeastern Australia recording departures in the upper 90^th percentile of occurrences (Fig. E1). Most of southeastern and eastern Australia also recorded exceptionally dry conditions (lowest 10^th percentile of occurrences) in response to anomalous sinking motion downstream of the mean ridge axis (Fig. E3).

The Antarctic ozone hole typically develops during August and reaches its peak aerial extent in September and October. Overall, the 2014 ozone hole was slow to develop during August, and was near the 2004-2013 mean size during both September and October (Fig. S8, top).