Monday, 12 January 2004
The planetary and synoptic scale interactions in southeast Pacific blocking using Potential Vorticity diagnostics
The synoptic and planetary-scale forcing in two blocking anticyclones occurring over the Southeast Pacific Ocean are examined using Potential Vorticity diagnostics. While many studies have examined the dynamic and thermodynamic forcing associated with blocking events in the Northern Hemisphere (NH), very few studies have examined blocking in the Southern Hemisphere (SH). Climatological analysis suggests SH blocking events in the Pacific Region have similar characteristics to their NH counterparts. However, the occurrence of blocking is rare elsewhere in the SH, and these events are relatively short lived. Some studies of NH blocking dynamics have also shown that the extent to which planetary and synoptic-scale, and planetary-synoptic-scale interaction forcing contribute to the genesis and maintenance of Pacific and Atlantic region events can be quite different. Thus, a study of the relevant atmospheric dynamics associated with blocking events in the SH is carried out in order to determine whether or not these events are associated with similar dynamic forcing mechanisms to those in the NH. Using the NCEP re-analyses data set and applying a low-pass filter to the relevant variables, a study of the scale interactions associated with two blocking events which occurred during July and August 1986 and applying Potential Vorticity diagnostics. Results demonstrate that blocking in the Southeast Pacific is associated with similar forcing mechanisms on the planetary and synoptic scales to those of North Pacific blocking events rather than those occurring over the NH Atlantic region. However, these results also demonstrate that blocking events in the NH are associated with synergistically interacting synoptic and planetary-scale waves, while in the SH, blocking events are merely associated with the superposition of synoptic and planetary waves. This explains the paucity of blocking occurrences over much of the SH.