Extratropical Highlights – February 2013

 

1. Northern Hemisphere

The 500-hPa circulation during February featured a general zonal wave-1 pattern at high latitudes and a zonal wave-3 pattern in the middle latitudes (Figs. E9, E11). At high latitudes, above-average heights prevailed over the e North Atlantic and below-average heights prevailed over the North Pacific and Alaska. In the middle latitudes, above-average heights were observed over the central and eastern North Pacific, the eastern North Atlantic, and in the vicinity of the Caspian Sea, and below-average heights covered the eastern U.S., southern Europe, and Mongolia.

This complicated anomaly pattern projected onto several major teleconnection patterns, which explained more than 75% of the pattern. Prominent teleconnection patterns during the month included: positive phases of the Tropical/Northern Hemisphere pattern (+0.9, TNH) and the Scandinavia pattern (+1.0), and negative phases of the West Pacific pattern (-1.5, WP), North Atlantic Oscillation (-1.0, NAO), and East Atlantic/ West Russia pattern (-1.3) (Table E1, Fig. E7).

The main land-surface temperature signals during February included above-average temperatures across eastern Alaska, western Canada, eastern Europe, and western Russia (Fig. E1). Below-average temperatures were observed in the southwestern U.S. and portions of northeastern Asia. The main precipitation signals during February included above-average totals in the central and southeastern U.S., and southeastern Europe, and below-average totals in both the northwestern U.S. and southern China (Fig. E3).

In the U.S., long-term precipitation deficits led to a continuation of extreme and exceptional drought conditions in the Great Plains states. Also, drought expanded and intensified in the southwestern quadrant of the U.S.

 

a. North America

The mean 500-hPa circulation during February featured a 4-celled anomaly pattern, with above-average heights over the eastern North Pacific and eastern Canada, and below-average heights over Alaska and the southeastern U.S. (Fig. E9). This pattern projected onto the positive phase of the TNH teleconnection pattern and the negative phase of the NAO (Table E1, Fig. E7).

These conditions were associated with a strong southwesterly flow (Fig. E10) of mild air and above-average temperatures (Fig. E1) over southeastern Alaska and western Canada, and with below-average temperatures in the southwestern U.S. Also, anomalous sinking motion and below-average precipitation was observed across the U.S. Pacific Northwest and portions of the Southwest in the area within and downstream of the mean ridge axis (Figs. E3, E5).

In the eastern U.S., the Hudson Bay trough was amplified and shifted west of normal. This pattern was associated with an enhanced entrance region of the North Atlantic jet stream and with enhanced jet stream winds over the east-central U.S. (Fig. T21). These overall conditions contributed to above-average precipitation in both the central and southeastern U.S. (Fig. E3).

February marks the second straight month with above-average precipitation in the Great Plains and Midwest regions of the U.S. (Fig. E5), and follows a 9-month period (April-December 2012) with well below-average precipitation in these regions. Despite the much-needed precipitation in January and February, large portions of the central U.S. continued to be impacted by extreme or exceptional drought. At the end of February, the “U.S. Drought Monitor” indicated Exceptional Drought from portions of Texas northward to central South Dakota, including portions of eastern Wyoming and eastern Colorado. Extreme Drought persisted in much of Wyoming, northwestern Iowa, and southwestern Minnesota. In the southwestern quadrant of the U.S., many regions recorded Moderate to Severe Drought, which intensified during February in response to below-average precipitation in parts of the region (Fig. E3).

 

 

b. Europe

The mean 500-hPa circulation during February featured above-average heights extending from eastern Canada to northwestern Russia and then southeastward to the Caspian Sea, and below-average heights covering southern Europe. This pattern projected onto the positive phase of the Scandinavia pattern (+1.0) and the negative phases of the NAO (-1.0, NAO) and East Atlantic/ West Russia pattern (-1.3) (Table E1, Fig. E7). This general north-south pattern of height anomalies was also present last month.

During February, this persistent pattern contributed to well below-average precipitation across the high latitudes of the North Atlantic, with much of the region (including Iceland) recording totals in the lowest 10^th percentile of occurrences (Fig. E3). Conversely, downstream of the mean trough axis, well above-average precipitation was recorded in south-central and southeastern Europe for the second straight month (Fig. E4) and above-average temperatures were recorded across eastern Europe and western Russia (Fig. E1).

 

c. Eastern Asia/ China

            A north-south dipole pattern of 500-hPa height anomalies prevailed across eastern Asia during February, with above-average heights over China and below-average heights over Mongolia (Fig. E9). This pattern was associated at 200-hPa with an enhanced entrance region of the East Asian jet stream, with stronger jet stream winds across northern China, and with a mean ridge axis situated over central China (Fig. T21). These conditions were associated with above-average temperatures across much of China and with below-average precipitation across southern China (Figs. E1, E3). They were also associated with generally colder than average conditions across Mongolia and portions of northeastern Asia.

 

2. Southern Hemisphere

The mean 500-hPa circulation during February featured above-average heights over the high latitudes of the western and eastern South Pacific, and below-average heights over Antarctica (Fig. E15). At 200-hPa, the circulation featured an anomalous trough across southern Australia (Fig. T21). In southeastern Australia, this feature was associated with an enhanced onshore easterly flow at 850-hPa (Fig. T20), along with above-average precipitation (Fig. E3).

The South African rainy season lasts from October to April. During February rainfall for the region as a whole was slightly below average (Fig. E4), but with significant deficits (below the 10^th percentile of occurrences) in the arid west (Fig. E3). For the 2012-13 rainy season to date, rainfall for the entire region was above-average during October and January, below-average in November and February, and near-average in December.