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

FEBRUARY 2011

Beginning with this month, all anomalies reflect departures from the 1981-2010 base period.

1. Northern Hemisphere

The 500-hPa circulation during February featured above average heights across the high latitudes of the North Pacific and portions of the polar region, and below average heights extending from western North America to central Asia (Fig. E9). This pattern projected strongly onto the negative phase (-2.4) of the Pacific/ North American teleconnection pattern (Table E1, Fig. E7). It is notable that for the first month since September 2009, the monthly 500-hPa circulation did not project strongly onto the negative phase of the North Atlantic Oscillation (NAO).

At 200-hPa, the circulation during February featured enhanced troughs in both hemispheres over the tropical and subtropical central/eastern Pacific (Fig. T22).  In the NH, this feature contributed to a westward retraction of the East Asian jet and to an enhanced jet exit region. Such a jet structure favors a westward retraction of the downstream ridge and trough axes, as seen during February by above average 500-hPa heights over the high latitudes of the central North Pacific and a deep trough over the western United States. These features are consistent with the ongoing mature phase of La Ni�a.

Below-average to near-average surface temperatures prevailed over the NH land masses during February (Fig. E1). The most significant negative departures were observed in the western U.S. and western Russia. The main precipitation signals during February included below-average totals along the U.S. Gulf Coast, southeastern Europe, and the southern half of China (Fig. E3).

a. North Pacific and North America

The mean 500-hPa circulation during February featured a strong ridge over the high latitudes of the central North Pacific, and a deep trough over much of the U.S. (Fig. E9). This pattern showed links to La Ni�a, and projected strongly onto the negative phase (-2.4) of the Pacific/ North American teleconnection pattern (Table E1, Fig. E7).

La Ni�a is associated with deep tropical convection focused over Indonesia and the eastern Indian Ocean, along with a disappearance of tropical convection from the western and central equatorial Pacific (Fig. T25). This westward retraction in the area of deep convection acts to amplify the mid-Pacific troughs at 200-hPa in both hemispheres (Fig. T22), which in the NH acts to amplify and retract westward the exit region of the east Asian jet stream (Fig. T21). This jet structure favors corresponding westward shifts in the downstream ridge and trough axes normally located over western and eastern North America, respectively. All of these features were present during February.

In the U.S., the main surface temperature signals during February included below average temperatures in the West, and above average temperatures in the mid-Atlantic region (Fig. E1). The main precipitation anomalies included below average totals in the Pacific Northwest and along the U.S. Gulf Coast (Fig. E5).  This marks the fifth consecutive month with well below average precipitation along the U.S. Gulf Coast (Fig. E5).

b. North Atlantic

Over the North Atlantic, the 500-hPa circulation during February featured below average heights in the middle and high latitudes, and above average heights at lower latitudes.  This pattern projected weakly onto the positive phase of the North Atlantic Oscillation (NAO) (Table E1, Fig. E7). This represents a notable change from the strong negative phase of the NAO that had prevailed from October 2009 � January 2011.

Consistent with this change, the main Atlantic jet stream entered Europe in its more normal position over Great Britain, rather than over southern Europe (or even farther south) as had been seen since October 2009. The associated enhanced northward heat transport brought above average temperatures to much of Europe during February. These conditions differ markedly from those observed in January, when extensive northwesterly flow brought much colder air across Europe.

2. Southern Hemisphere

All anomalies reflect departures from the 1981-2010 base period.

The 500-hPa circulation during February featured above average heights in the middle latitudes and below average heights at high latitudes (Fig. E15). In the subtropics, the upper-level (200-hPa) streamfunction pattern reflected an amplified trough across the central South Pacific (Fig. T22). Over the South Pacific Ocean, the overall height anomaly pattern is consistent with La Ni�a.

The main precipitation signals during February reflected above average totals across Australia, with departures exceeding the 90^th percentile of occurrences across large portions of the south and west (Fig. E3). Much of central South America also recorded above average precipitation.

The South African rainy season lasts from October to April. During February, rainfall for the region as a whole was below average, with the most significant deficits observed in Mozambique and northeastern South Africa (Fig. E4). For the 2010-11 rainy season, precipitation was below average in October and February, near average in November, and above average in both December and January. Seasonal rainfall is typically above average in this region during La Ni�a.