Friday map discussion, 13 February 2009
Ross A. Lazear

After a brief discussion of the rare tornado touchdown at Kapolei golf course on the southwest side of Oahu, Hawaii on Wednesday 2/11, the primary focus of the discussion was on the current state of ENSO and its effects on the mid-latitude longwave pattern, specifically over the North Pacific.

In accordance with the weak La Niña conditions, sea surface temperature anomalies in the eastern tropical Pacific have been below normal throughout the past few months.  Figure 2 from CPC's latest ENSO discussion show that each region in the eastern tropical Pacific has had below normal SSTs since the middle of boreal autumn, with the exception of the Niño 1+2 region (off the Peru/Ecuador coast), which experienced a later decline in SSTs, as well as a sudden increase towards the end of January.  In addition, the western Niño regions (3-4) experienced a decrease in SSTs slightly before the eastern regions (1-2), indicative of an eastward-propagating upwelling Kelvin wave.

Figure 3 from the same report shows equatorial upper-ocean heat content anomalies in the eastern equatorial Pacific.  Heat content anomalies in this region have been negative since August 2008, but a slight increase (likely from the extreme eastern Pacific) is noted after a minimum of -1.5° C in late December.

Moving on to the mid-latitudes...  Maps of monthly temperature and precipitation anomalies for January 2009 (courtesy of NCDC) show warmer and drier than normal conditions for most of the western U.S., as well as the Southern Plains.  This has worsened drought conditions in parts of Texas and the southwest U.S.

A "typical" mid-latitude response to a La Niña event features a weakened westerly jet over the southwest U.S., a broad ridge over the northwest Pacific (Gulf of Alaska), and a strong westerly jet over the Pacific Northwest, bringing abundant rains/snows to the region.  For January 2009, a maximum in 500-mb height anomalies (from CDC's NCEP/NCAR Reanalysis) was located in the northwest Pacific, just west of Washington.  300-mb zonal wind anomalies for the same time period show a marked northward shift in the jet over the northwest Pacific, coincident with the region of large 500-mb height anomalies. 

If the prominent northeast Pacific ridge that existed throughout much of January was at least partly forced by the weak La Niña, its position was  atypical from that of ridging normally associated with a La Niña.  This is evidenced by the fact that the Pacific northwest was relatively dry and warm during January 2009.  If anything, the unusual cold (and snows) in the Pacific Northwest during December 2008 was more in line with what is expected during a La Niña winter.  One can imagine that if January's large anomalous ridge over the northeast Pacific were shifted to the west, it would have resulted in a major change in sensible weather over the Pacific Northwest.

By the end of January, the large ridge finally began to break down, which indirectly led to the first U.S. severe weather outbreak of 2009.  A 2.0 PVU Dynamic tropopause map (thanks to Ron McTaggart-Cowan) of the northern hemisphere at 1200 UTC on 18 January shows a high-amplitude ridge with its axis along the western Canada/U.S. coast.  Likely aided by diabatic heating upstream, 2.0 PVU theta of 370 K extended up to southern Alaska and northwest Canada.  The ridging temporarily weakened as the flow became more zonal, but by 0000 UTC 26 January, another anticyclonic wave-break event occurred farther west than the previous, which resulted in a Rex block pattern over the northeast Pacific. 

By 1200 UTC 2 February, with continued blocking off the west coast of the United States, a major cyclogenesis event occurred east of the Kamchatka Peninsula (eastern Russia).  It is possible that this event helped to force the trough at 150W eastward, thus moving the downstream ridge over the central U.S. by 6 February.