Extratropical Highlights – February 2012

 

1. Northern Hemisphere

The 500-hPa circulation during February featured a persistent pattern of above-average heights over the eastern North Pacific, the eastern North Atlantic, and north-central Russia, and below-average heights over the high latitudes of the North Pacific, northern Africa, southern Russia, and eastern Siberia (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 February included well above-average temperatures in Alaska, Canada, the eastern U.S., and north-central Russia, and below-average temperatures across Europe and western Russia (Fig. E1). The main precipitation signals (Figs. E3, E6) included above-average totals along the U.S. Gulf Coast and southeastern Europe, and below-average totals along the U.S. eastern seaboard, western Europe, and northwestern Russia.

 

a. North Pacific and North America

The circulation over the North Pacific continued to exhibit a 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 February, the east Asian jet core was focused over the western Pacific, and the jet exit region began well west of the date line (Fig. T21). The jet exit region was also enhanced between 150^oE-180, as indicated by anomalous southeasterly winds immediately south of the jet axis.

Over the eastern North Pacific and western North America, the mean 500-hPa circulation during February featured a 4-celled anomaly pattern, with amplified ridges over the eastern North Pacific and northwestern Canada, and troughs over Alaska and the southwestern U.S. (Fig. E9). This pattern was associated with an enhanced flow of mild marine air into North America (Fig. T21, E10), which contributed to well above-average surface temperatures (departures exceeding +5^oC) across eastern Alaska and Canada (Fig. E1).

The circulation also featured an eastward shift of the mean Hudson Bay tough (HBL) to the western North Atlantic, which resulted in a reduced southward transport of cold air from Canada and to well above-average temperatures across eastern North America. This eastward shift of the HBL also affected in two ways the precipitation patterns across the eastern half of the U.S. First, it meant that the eastern seaboard was located upstream (rather than downstream) of the trough axis in an area of below-average precipitation and anomalous descending motion.  Second, it allowed for a southerly flow of marine air and ascending motion into the south-central U.S. and along the Gulf Coast. These conditions contributed to well above-average precipitation along the Gulf Coast (Fig. E5), with southeastern Texas and Louisiana recording totals in excess of 175% of normal (Fig. E6).

Despite this enhanced precipitation much the southern U.S., including Gulf Coast region, continued to experience moderate to exceptional drought during February. Exceptional drought conditions persisted in Texas and western Oklahoma, and extreme drought spanned the area from northern Florida to southeastern South Carolina.

 

b. North Atlantic and Eurasia

At high latitudes, the 500-hPa circulation during February featured an amplified wave pattern that extended from the eastern North Atlantic to eastern Siberia (Figs. E9, E11). Features of this pattern included blocking ridges over the eastern North Atlantic and north-central Russia, and deep troughs over western Russia and eastern Siberia. At lower latitudes, the circulation featured an extensive trough from northwestern Africa to the Caspian Sea.

These overall conditions resulted in a southward and eastward transport of cold air across Europe and western Russia, which resulted in well below-average surface temperatures across the region (Fig. E1). In many areas, monthly temperature departures were in the lowest 10^th percentile of occurrences.

The above circulation anomalies also strongly affected the precipitation patterns, with exceptionally low totals observed over western Europe and north-western Russia downstream of the mean ridge axes, and above-average totals observed over southeastern Europe and southern Russia in association with the mean trough axis (Fig. E3).

 

2. Southern Hemisphere

The mean 500-hPa circulation during February featured generally below-average heights in the middle latitudes and above-average heights over the high latitudes of the central North Pacific (Fig. E15). At 200-hPa, the subtropical circulation featured an enhanced mid-Pacific trough in response to the suppressed convection over the central equatorial Pacific (Figs. T22, T25).  A similar 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 February, rainfall for the region as a whole was below average (Fig. E3), and area-averaged totals were in the lowest 30^th percentile of occurrences (Fig. E4). The most significant deficits were observed in Mozambique, where totals were in the lowest 10^th percentile of occurrences. To date, precipitation for the 2011-12 rainy season has been near average, with near-average totals recorded during October through January and below-average totals observed in February. Seasonal rainfall in this region is often above average during La Niña.