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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 – January 2012


1. Northern Hemisphere

The 500-hPa circulation during January featured a persistent pattern of above-average heights over the eastern North Pacific, the central North Atlantic, and northern Russia, and below-average heights over the western North Pacific, Alaska, western Canada, and the eastern Mediterranean Sea (Figs. E9, E11). Over the subtropical Pacific Ocean, the 200-hPa circulation featured cyclonic streamfunction anomalies in both hemispheres east of the Date Line (Fig. T22). This pattern is linked to La Niña, and reflects enhanced mid-Pacific troughs in both hemispheres flanking the suppressed convection over the central equatorial Pacific (Fig. T25).

The main land-surface temperature signals during January included well above-average temperatures across central North America and northwestern Russia, and below-average temperatures in Alaska and central Asia (Fig. E1). The main precipitation signals (Figs. E3, E6) included above-average totals in the Northwest and Ohio Valley regions of the U.S., and below-average totals along the U.S. eastern sea-board and in the central Plains states, and also in western Europe.


a. North Pacific and North America

The circulation over the North Pacific continued to reflect a strong La Niña influence.  La Niña is associated with deep tropical convection focused over Indonesia and the eastern Indian Ocean, along with a disappearance of tropical convection from the central equatorial Pacific (Fig. T25). This westward retraction in the area of deep convection acts to amplify the mean mid-Pacific troughs at 200-hPa in both hemispheres (Fig. T22), which in the NH results in a westward retraction the east Asian jet stream, along with a westward-shift and amplification of the jet exit region (Fig. T21). During January, the east Asian jet core was focused over Japan, and the jet exit region began well west of the date line (Fig. T21). The jet exit region was also enhanced between 150oE-180, as is indicated anomalous southerly winds immediately south of the jet axis and anomalous northerly winds north of the jet axis.

Over North America, the mean 500-hPa circulation during January featured a deep trough over Alaska and western Canada, and an enhanced ridge over the southwestern U.S. (Fig. E9). This pattern was associated with an enhanced zonal flow of mild marine air into North America (Fig. T21, E10), which resulted in well above-average surface temperatures across the center of the continent (Fig. E1). In many areas temperature departures exceeded +4oC, and were in the upper 90th percentile of occurrences. In Alaska, the deep trough was associated with an enhanced flow of polar air into the western and southern parts of the state, resulting in well below-average surface temperatures with some areas recording totals in the lowest 10th percentile of occurrences.

The circulation also featured a deep trough extending from Hudson Bay to the south-central U.S. Flanking this trough was below-average precipitation over the central Plains states in the area upstream of the mean trough axis, and above-average precipitation extending from eastern Texas to New York in the area downstream of the mean trough axis (Fig. E6).

Also evident during January was below-average precipitation along the U.S. Gulf Coast and extending well up the eastern Seaboard. The combination of above-average precipitation in the Ohio Valley and below-average precipitation along the Gulf coast is consistent with La Niña.

Much of U.S. southern Plains and Southwest continued to experience severe drought in January, with exceptional drought persisting in Texas and western Oklahoma. The area from the western Florida Panhandle to southeastern South Carolina recorded extreme drought in January, with large portions of southern Georgia recording exceptional drought.


b. North Atlantic and Europe

The 500-hPa circulation during January featured strong ridges over the eastern North Atlantic and also over western Russia, and a deep trough extending southward from Scandinavia to the eastern Mediterranean Sea (Figs. E9, E11). This overall circulation was associated with an enhanced northward transport of mild air into Europe, and with below-average precipitation across western Europe (Fig. E3).  It was also associated with a two-celled pattern of precipitation anomalies flanking the amplified ridge in western Russia, with well above-average (below-average) precipitation observed in the area upstream (downstream) of the ridge axis (Fig. E3).


2. Southern Hemisphere

The mean 500-hPa circulation during January featured an anomalous zonal wave-2 pattern in the middle latitudes, along with below-average heights throughout the polar region (Fig. E15). At 200-hPa, the subtropical circulation featured an enhanced mid-Pacific trough in response to suppressed convection over the central equatorial Pacific (Figs. T22, T25).  A similar 200-hPa anomaly pattern was also evident in the Northern Hemisphere, and is consistent with La Niña.

The South African rainy season lasts from October to April. During January, rainfall for the region as a whole was near average (Fig. E4). To date, precipitation for the 2011-12 rainy season was near-average during October through January. Seasonal rainfall is typically above average in this region during La Niña.


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Page Last Modified: February 2012
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