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Climate Diagnostics Bulletin
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  Extratropical Highlights

  Table of Indices  (Table 3)

  Global Surface Temperature  E1

  Temperature Anomalies (Land Only)  E2

  Global Precipitation  E3

  Regional Precip Estimates (a)  E4

  Regional Precip Estimates (b)  E5

  U.S. Precipitation  E6

  Northern Hemisphere

  Southern Hemisphere


  Appendix 2: Additional Figures

Extratropical Highlights



Extratropical Highlights October 2013


1. Northern Hemisphere

The 500-hPa circulation during October featured a zonal wave-4 pattern of height anomalies (Figs. E9, E11). This pattern included above-average heights over the Gulf of Alaska, the high latitudes of the North Atlantic, the Mediterranean Sea, and eastern Asia, and below-average heights over the high latitudes of the central North Pacific, the western U.S., the central North Atlantic and central Siberia.

The main land-surface temperature signals during October included above-average temperatures across Alaska, eastern Canada, Europe, and central Asia, and below-average temperatures in the northwestern U.S. (Fig. E1). The main precipitation signals included above-average totals in the southwestern U.S., the north-central U.S., western Alaska, and Mexico, and drier-than average conditions in western Canada, the southeastern and northeastern U.S., large portions of Europe, and eastern Siberia (Fig. E3).


a. North Pacific/ North America

The mean 500-hPa circulation during October featured a strong ridge over the Gulf of Alaska, a broad trough across the continental United States and Canada, and also a trough over the high latitudes of the central North Pacific (Fig. E9). This pattern contributed to well above-average temperatures across Alaska and much of Canada, with much of interior Alaska recording departures that exceeded +5.0oC and were above the 90th percentile of occurrences (Fig. E1). In contrast, below-average temperatures were observed in the northwestern U.S. in the vicinity of the mean upper-level trough axis. The most significant negative departures ranged from -2oC to -3oC and were in the lowest 10th percentile of occurrences.

Several regions with significant precipitation anomalies were also observed during October (Fig. E3). In the west, a pronounced dipole pattern of rainfall anomalies reflected well above-average totals in western Alaska and well below-average totals in western Canada and the northwestern U.S. These regions coincided with the downstream portions of the mean upper-level trough and ridge axes, respectively. Above-average precipitation was also recorded in the north-central U.S., and near-average totals were recorded in drought-stricken regions of the U.S. Midwest (Fig. E5) and southern Plains (Fig. E3). Also in October, below-average precipitation was recorded in both the southeastern and northeastern U.S., where area-averaged totals were in the lowest 10th and 30th percentile of occurrences, respectively (Fig. E5).

In the western half of the U.S., average to above-average precipitation during October again helped to lessen ongoing drought conditions. However, much of the region still recorded severe or extreme drought at the end of the month. Also, moderate or severe drought was present across large portions of the upper Midwest, including Iowa, southern Minnesota, Illinois, Wisconsin, and northern Missouri.


b. North Atlantic/ Europe

Across the North Atlantic Ocean, the mean 500-hPa circulation featured a north-south dipole pattern of height anomalies, with above-average heights at high latitudes and below-average heights in the middle latitudes (Fig. E9). This pattern reflected the negative phase (-0.9) of the North Atlantic Oscillation (NAO) (Table E1). Previously, the positive phase of the NAO had prevailed for the six month period spanning April-September (Fig. E7).


Despite the phase reversal in the NAO, much of Europe remained exceptionally dry during October (Fig. E1), and area-averaged totals in both southern and northern Europe were in the lowest 30th percentile of occurrences (Fig. E4). This marks the fourth consecutive month of below-average precipitation for both regions. Much of Europe also recorded above-average temperatures during October, with portions of western and eastern Europe recording departures in the upper 90th percentile of occurrences (Fig. E1).


2. Southern Hemisphere

The mean 500-hPa circulation during October featured a zonal wave-1 pattern of height anomalies, with above-average heights extending from the central South Pacific to the central South Atlantic Ocean, and below-average heights spanning the southern Indian Ocean to southern New Zealand (Fig. E15). Above-average heights were also observed in the polar region and over southeastern Australia.

Over Australia, the 200-hPa circulation featured anticyclonic streamfunction anomalies and an amplified ridge that spanned the entire continent (Fig. T22). This pattern was associated with a southward shift of the mean jet axis, and resulted above-average surface temperatures across the continent. The eastern half of Australia experienced a continuation of exceptionally warm and dry conditions, with many areas recording temperature departures above the 90th percentile of occurrences (Fig. E1) and precipitation totals in the lowest 10th percentile of occurrences (Fig. E3). In northeastern Australia, area-averaged rainfall was below normal for the past six months, and was in the lowest 20th percentile of occurrences for the past three months (Fig. E4).

The Antarctic ozone hole typically develops during August and reaches its peak aerial extent in September and October. By the end of October 2013, the ozone hole (Fig. S6) spanned 7.5 million square kilometers, which is less than the 2003-2012 mean of 11 million square kilometers (Fig. S8, top). This reduced coverage reflected 1) a below-average size of the SH polar vortex (Fig. S8, middle), as indicated by well above-average heights across Antarctica extending upward to at least 30-hPa (Fig. S1), and 2) a below-average extent of polar stratospheric cloud throughout the month (Fig. S8, bottom).


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Page Last Modified: November 2013
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