Mean Circulation Highlights and Climate Anomalies September through December 2009

Anthony Artusa, Meteorologist, Climate Operations Branch, Climate Prediction Center NCEP/NWS/NOAA

September-October 2009

The 500-hPa circulation pattern over the Northern Hemisphere during September featured above-average heights over Canada, the eastern North Atlantic, and northern Europe, and below-average heights over the Gulf of Alaska, Greenland, and portions of northern Africa and the Middle East Figure 1. Above-average sea-level pressure (SLP) is indicated over northwestern Europe, while anomalously low sea-level pressure is evident over Alaska, the Gulf of Alaska, the Canadian Yukon, Greenland, and far northern portions of Europe Figure 2.

The mid-tropospheric circulation during October 2009 generally featured below-average 500-hPa heights across the mid-latitudes of the Northern Hemisphere, and above-average heights over the polar region Figure 3. This pattern reflected one of the strongest negative phases of the Arctic Oscillation (AO) on record dating back to 1950 (Reference 1). This is also consistent with higher-than-normal SLP in the polar region, and generally lower-than-normal SLP across the temperate latitudes of the Northern Hemisphere Figure 4.

The Tropics

Sea surface temperature (SST) anomalies remained above-average across much of the equatorial Pacific Ocean during September 2009. The oceanic thermocline along the equator, measured by the depth of the 20C isotherm, remained deeper than average across the Pacific Ocean. Atmospheric convection was enhanced over the west-central Pacific, but suppressed over Indonesia. Taken together, these oceanic and atmospheric indicators showed a continuation of weak El Nino conditions. In October, SST anomalies increased across much of the central and eastern equatorial Pacific Ocean, and the oceanic thermocline along the equator remained deeper than average across the Pacific. The pattern of atmospheric convection was similar to that of September. These oceanic and atmospheric anomalies indicated a strengthening El Nino.

September and October 2009 (as well as the 2009 Atlantic hurricane season in general) was relatively inactive. One notable exception was Hurricane Fred, which peaked at category-3 strength in early September. It is the only hurricane on record (in the satellite era) in the basin with an intensity greater than 100 kt when located south of 30N and east of 35W (Reference 2).

November - December 2009

The 500-hPa circulation pattern during November 2009 featured above-average heights across the central North Pacific, much of Canada and the northern contiguous US, and Scandinavia, and below-average heights over the Gulf of Alaska, Alaska, and the eastern North Atlantic. This included a nearly complete disappearance of the mean Hudson Bay trough, and a very strong flow of mild, marine air into central and northern Europe Figure 5. Sea-level pressure for the month was well below-average across Alaska and the adjoining Gulf of Alaska, western Canada, and most of northern Europe including Iceland Figure 6. Sea-level pressure was above-average over the western and central mid-latitude Pacific, the central North Atlantic, and a large portion of northern Siberia.

The month of December was dominated by a record-breaking, negative phase of the Arctic Oscillation (AO). In fact, the AO index value of -2.4 reflects the strongest negative phase of the AO for any December since 1950 (Reference 3). The well defined mid-tropospheric pattern featured above-average heights in the polar region, and below-average heights in the middle latitudes Figure 7. The SLP pattern largely mirrored the 500-hPa height anomaly pattern over the Northern Hemisphere Figure 8.

The Tropics

Atmospheric and oceanic indicators reflected a moderate strength El Nino in November 2009. SST departures during the month continued to increase across much of the central and eastern equatorial Pacific Ocean, and the oceanic thermocline remained deeper than average across the equatorial Pacific. In addition, the atmospheric circulation over the tropics featured intra-seasonal activity associated with the Madden-Julian Oscillation (MJO), which masked the El Nino-related pattern of enhanced convection over the central equatorial Pacific. By December, a strong El Nino was in progress, which featured suppressed convection across Indonesia, and enhanced convection over the western and central equatorial Pacific.

The tropical disturbance that would later become Hurricane Ida formed off eastern Costa Rica in early November 2009. Ida moved inland over eastern Nicaragua and Honduras and weakened as it traversed the high terrain of both countries. The storm later re-emerged over the western Caribbean Sea and re-strengthened to a category-2 hurricane with 90 kt winds as it moved through the Yucatan Channel. Upon entering the Gulf of Mexico, Ida encountered a strong shearing environment, and was downgraded to a tropical storm. After more short-term fluctuations in intensity, Ida finally weakened over the cooler waters of the northern Gulf, and became extra-tropical shortly before the system moved onshore near Mobile Bay, Alabama, on November 10th. The cyclone dissipated over the Florida Panhandle the following day. Ida's remnants contributed to the formation of a separate, strong extra-tropical low that affected the U.S. East Coast during the following few days (Reference 4).

References

  1. Climate Diagnostics Bulletin October 2009
  2. National Hurricane Center (2009 Archives on Hurricane Fred)
  3. Climate Diagnostics Bulletin December 2009
  4. National Hurricane Center (2009 Archives on Hurricane Ida)
Mean Circulation Figure 1
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Mean Circulation Figure 2
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Mean Circulation Figure 3
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Mean Circulation Figure 4
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Mean Circulation Figure 5
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Mean Circulation Figure 6
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Mean Circulation Figure 7
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Mean Circulation Figure 8
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500 hPa Heights and Anomalies: Figures 1,3,5,7 Northern 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 2,4,6,8 Northern 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.

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