Extratropical Highlights – March 2013

 

1. Northern Hemisphere

The 500-hPa circulation during March featured above-average heights in the polar region and Greenland, along with several extensive zonally-elongated patterns of height anomalies in the middle latitudes (Figs. E9, E11). Features of these extensive patterns included above-average heights across the North Pacific, and extending from northern Africa to northern China, and below-average heights extending from the eastern U.S. to eastern Siberia.

These conditions reflected a north-south oriented dipole pattern over the North Atlantic Ocean, and a north-south oriented tri-pole pattern across Eurasia. They projected strongly onto two teleconnection patterns: the negative phase of the North Atlantic Oscillation (-2.1, NAO) and the positive phase of the East Atlantic/ West Russia pattern (+2.3) (Table E1, Fig. E7).

The main land-surface temperature signals during March included above-average temperatures over eastern Canada and most of southern Eurasia, and below-average temperatures across central North America and much of northern Eurasia (Fig. E1).

The main precipitation signals included above-average totals across southern Europe, and below-average totals in northern Europe and in the Northwest, Gulf Coast and East Coast regions of the U.S (Fig. E3). Also in the U.S., long-term precipitation deficits led to a continuation of extreme and exceptional drought in the Great Plains states, with drought expanding and intensifying in the West.

 

a. North America

The mean 500-hPa circulation during March featured above-average heights over western North America and eastern Canada, and below-average heights over the eastern U.S. (Fig. E9). Over eastern North America, the north-south dipole pattern of anomalies was part of a larger-scale pattern that reflected a strong negative phase of the NAO (Table E1, Fig. E7).

The surface temperature anomalies reflected this negative NAO pattern, with above-average temperatures in eastern Canada and below-average temperatures across central North America and much of the eastern U.S. (Fig. E1). The main precipitation anomalies over North America reflected below-average totals across the U.S. Gulf Coast and east coast, and along much of the west coast (Figs. E3, E5). The precipitation deficits in the eastern U.S. were also consistent with the negative NAO pattern.

Large portions of the U.S. Plains region continued to be impacted by extreme or exceptional drought. At the end of March, the “U.S. Drought Monitor” indicated exceptional drought from portions of Texas northward to central South Dakota, including eastern Wyoming and southeastern 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 or severe Drought, with extreme drought developing across central and eastern New Mexico and northwestern Nevada (Fig. E3).

 

b. North Atlantic and Eurasia

The mean 500-hPa circulation during March featured above-average heights over the polar region and Greenland, and from northern Africa eastward to northeastern China. It also featured below-average heights from the eastern U.S. to eastern Siberia (Figs. E9, E11). Over the North Atlantic, the north-south dipole pattern of height anomalies reflected a strong negative phase of the NAO (-2.1) (Table E1, Fig. E7). Typical negative NAO features included a southward shift and zonal elongation of the North Atlantic jet stream, with the jet core entering Europe over the Mediterranean Sea instead of in its normal location over Great Britain (Figs. T21, E10). They also included an anomalous flow of polar air into northwestern Europe and Scandinavia, and an anomalous flow of cold Canadian air into the north-central U.S. (Fig. T21).

These conditions produced a 4-celled pattern of surface temperature anomalies typical of the negative NAO (http://www.cpc.ncep.noaa.gov/data/teledoc/nao_tmap.shtml), which includes above-average temperatures over eastern Canada and southern Europe/ northern Africa, and below-average temperatures over the eastern half of the U.S. and northern Europe (Fig. E1). They also produced the typical north-south dipole pattern of precipitation anomalies (http://www.cpc.ncep.noaa.gov/data/teledoc/nao_pmap.shtml ) over the eastern North Atlantic and Europe/ western Russia (dry in the north, and wetter/ stormier in the south), along with below-average precipitation in the eastern U.S. (Fig. E3). In all three regions, the observed precipitation anomalies were quite significant. Totals exceeded the 90^th percentile of occurrences across southern Europe and western Russia, while deficits were in the lowest 10^th percentile of occurrences across the U.S. Gulf Coast and Scandinavia.

Over Eurasia, the 500-ha circulation during March also projected onto a strong positive phase of the East Atlantic/ West Russia pattern (-2.3), which includes above-average heights over south-central Asia and below-average heights over western Russia (http://www.cpc.ncep.noaa.gov/data/teledoc/eawruss_map.shtml).  This pattern contributed to the enhanced polar flow of cold air into Scandinavia and northwestern Russia, and also to enhanced jet stream winds across central Russia which transported the anomalously cold air across northern Eurasia. It also contributed to the exceptionally low precipitation over Scandinavia, which was located in an area of anomalous descending motion upstream of the mean trough axis (Fig. E9). Additionally, the persistence of the mean ridge over south-central Asia contributed to well above-average surface temperatures across that region, with departures generally exceeding the 90^th percentile of occurrences (Fig. E1).

 

 

2. Southern Hemisphere

The mean 500-hPa circulation during March featured below-average heights across Antarctica, and an anomalous zonal wave-3 pattern in the middle latitudes (Fig. E15). At 200-hPa, the circulation featured an anomalous trough across southern South America (Fig. T21). This feature was associated with well below average surface temperatures east of the Andes Mountains, with departures along southeastern Argentina falling into the lowest 10^th percentile of occurrences (Fig. E1).

The South African rainy season lasts from October to April. March rainfall for the region as a whole was below average (Fig. E4), with deficits mostly confined to northern Mozambique (Fig. E3). To date for the 2012-13 rainy season, rainfall for the entire region was above-average during October and January, near-average in December, and below-average in November, February and March.