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Climate Diagnostics Bulletin
Climate Diagnostics Bulletin - Home Climate Diagnostics Bulletin - Tropics Climate Diagnostics Bulletin - Forecast

 

  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

  Stratosphere

  Appendix 2: Additional Figures

Extratropical Highlights

OCTOBER 2012

1

Extratropical Highlights – October 2012

 

1. Northern Hemisphere

The 500-hPa circulation during October featured above-average heights throughout the polar region and below-average heights in the middle latitudes (Fig. E9). Regional aspects of this pattern included above-average heights across the high latitudes of both the North Pacific and North Atlantic Oceans, which indicates a disappearance of both the Aleutian and Icelandic Lows. The mean 500-hPa circulation also featured below-average heights over the central North Pacific, central North America, and the eastern North Atlantic. The pronounced north-south dipole of height anomalies over the North Atlantic reflected a strong negative phase (-1.7) of the North Atlantic Oscillation (NAO).

The main land-surface temperature signals during October included above-average temperatures in western Russia, and below-average temperatures in western Canada and the north-central United States (Fig. E1). The main precipitation signals included above-average totals in western Canada, the northwestern and Great Lakes regions of the United States, eastern Europe, and western Russia. Below-average totals were recorded in the south-central U.S. and southeastern Alaska (Fig. E3).

 

a. North America

The mean 500-hPa circulation during October featured a persistent north-south dipole pattern of height anomalies over the North Pacific, with above-average heights at high latitudes and below-average heights in the middle latitudes (Figs. E9, E11). This pattern was associated with a confluent height field over the western U.S., and with an amplified trough across central North America. Consistent with these conditions, below-average temperatures were observed in western Canada and the north-central U.S., with areas of western Canada recording departures in the lowest 10th percentile of occurrences (Fig. E1).  Above-average precipitation was recorded in western Canada and the northwestern and Great Lakes regions of the United States, while well below-average precipitation was recorded in southeastern Alaska and the south-central U.S. (Fig. E3).

Large portions of the central U.S. continued to be impacted by extreme or exceptional drought. At the end of October, the “U.S. Drought Monitor” indicated exceptional drought from Oklahoma northward to central South Dakota, including portions of eastern Wyoming, eastern Colorado, and northern Texas. Extreme drought persisted in Arkansas, Iowa, and southern Minnesota.

           

 

b. Europe/ central Asia

The mean 500-hPa circulation during October featured a north-south dipole pattern of height anomalies over the North Atlantic, with above-average heights at high latitudes and below-average heights in the middle latitudes (Figs. E9). This pattern was associated with a complete disappearance of the Icelandic Low, and also projected strongly onto the negative phase (-1.7) of the NAO. This marks the sixth straight month with a negative NAO index.

 

c. African Sahel

The west African monsoon season lasts from May through October, and typically peaks during July-September. The monsoon rains were again enhanced during October 2012 (Fig. E1). The 2012 west African monsoon season featured above-average precipitation throughout the entire season, with area-averaged totals exceeding the 70th percentile of occurrences during all six months (Fig. E4). Overall, the west African monsoon system has been enhanced since 1995, in association with the warm phase of the Atlantic Multi-Decadal Oscillation (AMO). This combination of an enhanced west African monsoon system and a warm AMO has contributed to significantly increased Atlantic hurricane activity seen since 1995.

For the Atlantic hurricane region, 6 Atlantic named storms were present during October, with three reaching hurricane status. The most significant hurricane was Sandy, caused tremendous flooding and damage in the mid-Atlantic and northeastern U.S. in late October. By the end of the month, the 2012 Atlantic hurricane season has produced 19 named storms, with 10 becoming hurricanes and one (Michael) reaching major hurricane status. Also, the seasonal ACE index was 142%  of the 1981-2020 median.

 

 

2. Southern Hemisphere

The mean 500-hPa circulation during October featured an anomalous zonal wave-3 pattern, with above-average heights over the eastern South Pacific Ocean, across the central South Atlantic Ocean, and in the area south of Australia (Fig. E15). Below-average heights were observed over the western South Pacific and in the area south of South America.

In the lower atmosphere, the subtropical high pressure system normally centered east of South America was again enhanced along its westward flank (Fig. T20). The associated anomalous poleward flow over eastern South America led to an anomalous poleward position of the mean cold frontal boundary, and resulted in a continuation of well above-average surface temperatures across that region (Fig. E1). This marks the third straight month in which large portions of eastern South America have recorded surface temperature departures above the 90th percentile of occurrences.

Much of Australia also recorded above-average temperatures in October, while exceptionally dry conditions were present across the eastern half of the continent. The largest precipitation deficits were observed in southeastern Australia, where totals were generally in the lowest 10 percentile of occurrences (Fig. E3).

The South African rainy season lasts from October to April. During October 2012, rainfall for the region as a whole was above average, with much of southeastern Africa recording totals above the 70th percentile of occurrences (Figs. E3, E4).

The Antarctic ozone hole typically develops during August and reaches peak aerial extent in October and early October. During October, the size of the ozone hole was below the 2002-2011 mean, decreasing from 15 million square kilometers at the beginning of the month to 5 million square kilometers at end of the month. (Fig. S8)

Overall, the size of the 2012 ozone hole was comparable to the smallest seen during the 2002-2011 period. This reduced size coincided with a below-average aerial coverage of polar stratospheric clouds throughout July- October, along with a below-average size of the SH polar vortex during October (Fig. S8).

 

 


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