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HOME > Expert Assessments > Climate Diagnostics Bulletin > Extratropical Highlights
 
Extratropical Highlights - March 2005
 

1. Northern Hemisphere

      

     The 500-hPa circulation pattern during March featured above-average heights over the high latitudes of the North Atlantic, the subtropical North Atlantic, and central Russia, and below-average heights from the eastern United States to southern Europe, over western Russia, and over both the western and central North Pacific (Fig. E9). Over the North Atlantic, the circulation pattern reflected a very strong negative phase (-2.0) of the North Atlantic Oscillation (NAO) (Table E1, Fig. E7). The temperature and precipitation anomalies from eastern North America to Europe were consistent with this pattern.

     The main surface temperature departures during March included above-average temperatures over northwestern North America, the central/ high latitudes of the North Atlantic, and central Asia, and below-average temperatures across the eastern United States and eastern Europe/ western Russia (Fig. E1). The main precipitation anomalies reflected above-average totals over the Inter-Mountain region of the western United States, from the southeastern U.S to southern Europe, and in central Russia and the Caspian Sea region, and below-average precipitation over the central U.S. and across the high latitudes of the North Atlantic to northern Europe (Figs. E3, E5, E6).

 

a. North America

     The 500-hPa circulation pattern during March featured above-average heights over western North America and an amplified trough over the eastern United States. These conditions were associated with above-average temperatures in the Pacific Northwest and western Canada, and below-average temperatures in the eastern U.S. (Fig. E1). The combination of the amplified upper-level trough and below-average temperatures in the eastern U.S. is consistent with the strong negative phase of the NAO.

     The Inter-Mountain region of the United States again recorded above-average precipitation during the month (Figs. E3, E5). This area has received well above-average precipitation during October 2004-March 2005, which has led to a dramatic improvement in long-term drought conditions in much of the region.

 

 

b. North Atlantic and Europe

     The 500-hPa circulation pattern featured a pronounced north-south dipole of height anomalies from eastern North America to Europe. The associated area of positive height anomalies reflected a persistent blocking ridge over the high latitudes of the North Atlantic, while the negative anomalies farther south reflected an amplified trough extending from the mid-western U.S. to southern Europe (Fig. E9). This anomaly pattern reflects a strong negative phase of the NAO (Table E1, Fig. E7). In the lower atmosphere, the corresponding anomalous difference in sea-level pressure between the central and high latitudes of the North Atlantic reached 32-hPa (Fig. E8), indicating a pronounced weakening of both the normal sea-level pressure gradient and the low-level westerlies at high latitudes. Associated with these anomalies the mean North Atlantic jet stream and storm track were shifted well south of normal, and extended from the southeastern U.S. to the Mediterranean Sea instead of northeastward to Great Britain as occurs in the climatological mean (Figs. T21, E10).

     The temperature and precipitation anomalies from eastern North America to Europe were consistent with the strong negative phase of the NAO. The key departures included above-average temperatures across the high latitudes of the North Atlantic and below-average temperatures over the southeastern United States (Fig. E1). They also included below-average precipitation across the high latitudes of the North Atlantic and northern Europe, and above-average precipitation across the central North Atlantic and southwestern Europe (Fig. E3).

     The North Atlantic again saw a continuation of exceptionally warm SSTs at both high latitudes and in the subtropics (Fig. T18). This anomaly pattern reflects the ongoing warm phase of the Atlantic multi-decadal mode that began approximately in 1995 (Goldenberg et al. Science, 2001).

 

c. Russia

     In the area immediately downstream of the strong negative NAO, a persistent pattern of below-average 500-hPa heights covered western Russia and above-average heights covered central Russia (Figs. E11, E12). This anomaly pattern contributed to large surface temperature departures in both regions. It was also associated with enhanced high-frequency variability in the 700-hPa height field across northern Russia and Siberia (Fig. E13).

 

2. Southern Hemisphere

     The mean 500-hPa circulation pattern during March featured a north-south oriented tri-pole pattern of height anomalies between Australia and South America (Fig. E15). This pattern included positive height anomalies in the middle latitudes, and negative height anomalies in both the subtropics and high latitudes. The associated South Pacific jet steam was shifted equatorward of normal, with the mean jet axis extending eastward from east-central Australia (Fig. T21). Central and eastern Australia, situated in the area of anomalous descending motion within the right jet entrance region and also upstream of the mean trough axis, recorded well below-average precipitation during the month (Fig. E3).

     In southern Africa the rainy season normally lasts from October to April. During March area-mean precipitation totals were slightly below average (Fig. E4), with the primary rainfall deficits again observed in Mozambique. These deficits reflected an equatorward contraction of the main region of tropical convection (Fig. T25), which occurred in association with an anomalous upper-level cyclonic circulation and enhanced upper-level westerlies across southern Africa (Figs. T22, T21). Exceptionally warm conditions were again observed in coastal South Africa during March, along with a continuation of near-record sea-surface temperatures (ranging from 1-2C above average) surrounding the country. These anomalously warm SSTs began developing during June-August 2004, and have been at near-record levels since November 2004.

 


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