Easterly Waves

After much effort and little success with classifying easterly waves with the methodologies of Knippertz (2003) I have decided to classify these features with the criteria of Stensrud et al. (1997). I think the issue with using Knippertz methods is that the easterly waves typically have shorter wavelengths and are often tilted horizontally at angles that could prove to be difficult for identification with his strictly zonal calculations. Probably a more important factor is that these waves are identified at a lower level, usually 600 or 700 hPa, which makes topography an issue. Either way, I plan on coming back to this issue at a later time. For now, let's see what results I have found with Stensrud et al. (1997).

This identification methodology uses time-dependent changes in 600 or 700 hPa meridional winds to classifty easterly waves. They classified passage of an easterly wave when a northerly wind on Day 1 was followed by a southerly wind on Day 2, as reference to trough axis passage, at 110 W. They used Hovmoller diagrams to show how this methodology 'looked.' So, below is an example of 600 hPa for 1 June through 24 Aug 1987. Winds are at 22.5 N and 145 to 60 W. Warm (cool) colors correspond to southerly (northerly) flow.
Their study used winds that were averaged over 10 to 22.5 N. Douglas and Leal (2003) showed that high amplitude waves over western Mexico typically migrate across the region between 10 and 30 N. Therefore, I chose to limit my domain to 20 (~Mexico City, MX) to 25 N (~Brownsville, TX) so that I would capture features inline with their findings as well as ones more associated spatially with the central and northern portions of the NAM. Many recent papers have started to use winds at 600 hPa in lue of 700 hPa so that an influences from the topography of Mexico does not affect the signatures of these wave features. I performed analysis on both levels for comparison and my own interest in their correlation.

Results:
Fuller and Stensrud (2000) using Stensrud et al. (1997)methods -- 85 easterly waves over 14 years (only July and August)
=> 6.07/year and 3.03/month
My work -- 260 (243) easterly waves at 600 (700) hPa over 29 years (only July and August)
=> 8.9/year and 4.4/month(600 hPa)
=> 8.4/year and 4.2/month(700 hPa)

How did the two levels compare with one another? Well, 161 waves (62% and 66% of waves at 600 and 700 hPa, respectively) were identified at both levels on the same day. I am happy with these results in the context of how they compare with previous work, but I believe we can definitely improve the validaty of these results by using a more robust identification scheme like those in Knippertz. In the case where unforeseen events come into play, at least we have results to use in our reserach that are in line with previous research.

-jamie