Mean Circulation Highlights and Climate Anomalies
January through April 2008
Anthony Artusa, Meteorologist, Climate Operations Branch,
Climate Prediction Center NCEP/NWS/NOAA
The circulation pattern over the Northern Hemisphere during January was characterized by above normal mid-tropospheric heights across much of the north-central Pacific Ocean, the southern Canadian Maritimes, most of Europe including western Russia, and over the east Asian coast Figure 1. In contrast, below normal midtropospheric heights were observed over eastern Siberia, the Asian side of the Arctic Ocean basin, Alaska, and a large portion of western North America. Sea level pressure (SLP) was lower than average for the month over the Pacific Northwest, and southwest Canada Figure 2.
During February, 500 hPa heights were slightly below normal over the north-central CONUS and central Canada, with somewhat stronger negative anomalies over much of the higher latitudes Figure 3. A quasi-hemispheric belt of above average heights was observed across the mid-latitudes of the Northern Hemisphere from the Date Line to the U.S. West coast, and from the Atlantic and much of Europe southeastward across the Arabian Peninsula and Iran. Sea level pressure during February was well below normal across Alaska and the adjacent northern Gulf region Figure 4.
La Nina conditions intensified as sea surface temperatures (SSTs) continued to cool westward across the central equatorial Pacific in January. La Nina is typically associated with a westward retraction of deep tropical convection from the central Pacific toward Indonesia and an anomalous reduction in deep convection near the Date Line. The SST anomaly in the Nino 3.4 Index region (defined as 5S to 5N, and 170W to 120W) was -1.8C, which represented a decrease of almost half a degree compared to the December anomaly.
In February, SSTs continued to cool across the western equatorial Pacific, as the strong cold event evolved. The SST anomaly in the Nino 3.4 Index region increased slightly from the preceding month to -1.7C. Tropical cyclone activity was confined to the Southern Hemisphere during this period.
The mean circulation for March depicts above normal 500 hPa heights across the mid-latitudes of the Northern Hemisphere from the Middle East eastward through much of northern Asia, and continuing across parts of the northern Pacific to the U.S. West coast Figure 5. Heights were well below-average across Europe, and were associated with near normal temperatures there (not shown). The SLP map shows a nondescript pattern over most of North America, though over Europe and western Russia the SLP values were well below average Figure 6.
In April, well above normal 500 hPa heights were observed from extreme eastern Asia eastward across the mid-high latitudes of the Pacific to the U.S. West coast, as well as from Greenland southwestward to the northeastern CONUS Figure 7. Below average heights were observed over the northeast Atlantic, northern Asia, and west-central portions of North America. In general, sea level pressure in April mirrored the midtropospheric height anomaly pattern Figure 8.
A moderate La Nina event continued in March across the equatorial Pacific. The Nino 3.4 Index region SST anomaly was near -1.0C, which represented a substantial warming in ocean temperatures (at the surface) since the previous month. Tropical cyclone activity occurred south of the Equator, concentrated around Australia during this month, with Ophelia and Pancho.
In April, the oceanic component of the La Nina event showed signs of continued weakening, as SST anomalies in the Nino 3.4 region rose to -0.75C. The atmospheric indicators (such as heavy tropical rainfall, and lower and upper level winds) seemed to lag behind the oceanic indicators, and continued to reflect an energetic La Nina. Typhoon Neoguri, the seasonís earliest and perhaps the strongest early season typhoon to strike China since 1949 developed in the South China Sea at mid-month, reached a peak intensity with winds near 95 kts several days later, and brought torrential rains to the provinces of Hainan and Guangdong near the coast of southern China.
500 hPa Heights and Anomalies: Figures 1,3,5,7Northern Hemisphere mean and anomalous 500-hPa geopotential height (CDAS/Reanalysis). Mean heights are denoted by solid contours drawn at an interval of 6 dam. Anomaly contour interval is indicated by shading. Anomalies are calculated as departures from the 1979-1995 base period monthly means.
Sea-Level Pressure and Anomaly: Figures 2,4,6,8Northern Hemisphere mean and anomalous sea level pressure (CDAS/ Reanalysis). Mean values are denoted by solid contours drawn at an interval of 4 hPa. Anomaly contour interval is indicated by shading. Anomalies are calculated as departures from the 1979-1995 base period monthly means.