Guest essay by Michael Wallace, Hydrologist
I’m sharing something which likely has been observed and perhaps even written widely about, yet I cannot find examples in the popular or peer review literature. These images are just a sample from the growing collection I’ve been compiling as time permits. These examples are limited for now to the monthly averaged values of EP for the full atmosphere. EP stands for “Evaporation minus Precipitation” in units of mm/day, as developed at the ERA Interim (ERAI) resources. This is nominally equivalent to atmospheric moisture, without accounting for the clouds.
I find this full atmosphere coverage helpful, but I always try to be mindful of limitations. For the most part a full atmosphere approximation shows great fidelity to common meteorological phenomena. I’ve provided several examples of this quasi geostrophic approximation in a previous post at WUWT:
I’ve been working on a paper which in part compares this EP time series coverage since 1979 to solar cycles and continental streamflows. My paper is back in review and I thought I would take advantage of the break to continue to explore some of the many patterns of additional interest. Earlier this year I had begun to notice some wave-like patterns in 2D contour plots of the EP. I thought they might be an artifact of some form of aliasing. Now that I’ve been plotting them via a Matlab surface plot option in the highest resolution available from the ERAI source (.703125 degrees per grid cell) I no longer believe that is the case.
Figures 1 through 4 show these interesting wave-like patterns as averaged for the months of January and February from 1979 across most of the Northern Hemisphere (NH). Some ‘disclaimers’; I cannot yet program my Matlab application to consistently view and light these surfaces from the exact same camera location and so my manual application is why the views and hues vary somewhat for each screen capture. Also, I left some annotations in some of these images which are legacy of other work, such as the circles in the Western US. Since they are not germane to this discussion, I’m only noting that.
Of the initial four images, the first two only cover the value of EP. In addition to applying a high resolution surface coverage, I also employed a Matlab “Winter” color scheme. Given the definition of EP, the more negative the value, the greater the relative amount of atmospheric moisture. In my implementation, the greater the moisture, the deeper and bluer the surface. One can confirm these surface plots are reasonable by examining regions where high moisture zones are well known to persist, such as the Pacific Northwest and the Equatorial Trough.
In addition to those notable moisture concentrations, it would seem hard for anyone to miss the north – south trending ridges (I will call them megaripples) and troughs, thousands of kilometers long, which dominate the ocean footprints on either side of North America and which overlie that continent as well. With some notable exceptions such as the northeast coast of the US over January, the waves are not always expressed well within the lower moisture, higher pressure green zones which tend to dominate over summer months. Accordingly, to profile these waves, I’ve chosen to focus on a boreal winter season to start. A later image addresses the austral winter in the Southern Hemisphere, for the same reason.
Figure 1. EP for Full Atmosphere, January 1979
Figure 2. EP for Full Atmosphere, February 1979
Figure 3. EP for Full Atmosphere with streamlines, January 1979
Figure 4. EP for Full Atmosphere with streamlines, February 1979
These appear to be real patterns of moisture and they happen to resemble more widely known gravity waves. But they are based on monthly averages. In contrast, the typical residence times of normal gravity waves are much less than a single day. Even so called “infra gravity” waves have periods shorter than a day. Also, gravity waves are often defined in association with the effects of orographic uplifts. Yet the megaripples can be found crossing the very flat Great Plains and Midwest of the U.S.
So, perhaps they are not classic gravity waves. I explored some material at hand to see if they might be Rossby waves. At least Rossby waves are attributed to express much longer residence times than mere days. Rossby waves are defined in some sources as a Coriolis dominated type of inertial gravity wave in any case. Yet one would be challenged to find a graphic of a Rossby wave that resembled these megaripples in any way, shape or form. I am familiar with popular representations (including the Wikipedia page) of Rossby waves that appear to define them as the fluctuating curving boundaries of the Polar Vortex which is largely aligned with the North American Jet Stream.
In other words, Rossby waves are popularly defined to primarily oscillate zonally and meridionally, not vertically. Accordingly, unless those popular definitions are inaccurate, then these EP features cannot be Rossby waves either. Rossby waves are also asserted to never migrate east. Given that, I hoped to track a single wave or two from month to month and determine if it moved east or west at all. Unfortunately, although many patterns and trends can be seen, there is not clear continuity of any individual wave form one month to the next, at least so far as I could tell.
I decided to explore this in somewhat more detail by adding a set of streamlines in Figures 3 and 4 for the same EP surfaces. The blue streamlines originate near the left end of the global grid and the magenta streamlines originate near the right end. The default elevation for the streamlines is zero. Given the coincidental range for EP which extends slightly above and below zero, this leads to the appearance and disappearance of the streamlines according to the sign of the surface value. The streamlines for January 1979 near the Arctic do show the typical vortex pattern and accordingly, a Rossby wave near Alaska. Moreover, by the next month of February as expected, that wave has indeed migrated to the west and not east. But there doesn’t appear to be any clear correlation between that wave and the atmospheric ripples of my focus.
One could note that the ripples appear to terminate where the vortex begins. There is at least that. But there are also the megaripples in the Southern Hemisphere as captured by Figure 5. The blue color scheme no doubt helps, but I think it is safe to say that these megaripples resemble nothing less than the typical waves of an ocean at closer view, or perhaps a transverse wind driven dune field.
Figure 5. Figure 1. EP for Full Atmosphere, July 1979
My best guess at this point is that these are composites of gravity waves. In this view, the many gravity waves over time over any area, when averaged over a month, simply yield these interesting patterns. . In any case, even if they are merely a composite artifact, they might be arguably “real” at a climatological, or at least a monthly scale.
As I continue to read and research the lines of hydroclimatology most germane to my interests, I know that deeper and more analytical expressions of all planetary waves are surely in my academic future. But I’m not there yet and felt that sharing these observations is a way to also advance my learning. In other words, I don’t know what these waves are. Can anyone enlighten?
As usual, many thanks to the Las Cruces Atmospheric Forum for their sounding board services.
 http://www.cgd.ucar.edu/cas/catalog/newbudgets/index.html#ERBEFs downloaded file named ‘ERAI.EP.1979-2014.nc ‘
Michael’s blog is at www.abeqas.com.