876 
found in or near the trough so long as the waves are of 
small amplitude. There is, of course, horizontal velocity 
convergence in the lower layers, associated with the 
wave motion, but it may be said to be “unorganized.” 
As the waves move toward the west, some of them at 
least become unstable, growing in amplitude. The wave 
in the pressure field can now be detected with good 
observations, and an asymptote of convergence usually 
appears behind the “trough” of a wave and a corre- 
sponding asymptote of divergence ahead of it. The 
asymptote of convergence may be detected in flight as 
a line of cumulonimbus or tall cumulus. With further 
increase in amplitude of the wave a small vortex appears 
in the wave pattern, usually, but not always, in the 
equatorial trough itself. With the formation of this 
vortex, westerly winds near the equator appear for the 
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Fic. 3—Streamline and isovel analysis of the mean resultant surface winds, tropical Pacific Ocean, Northern Hemisphere ~ 
6 
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SZ 
a 
‘TROPICAL METEOROLOGY 
west; there the passage of so many of them north and 
south of the equator and in both the wet and the dry 
season results in a mean west wind on the equator in 
the western part of the ocean. Since this west wind is the 
statistical result of the vortices it is clearly fallacious 
to import them into a hypothetical general circulation 
which is then perturbed to produce the vortices. Our 
task is to explain the mean maps as the integration of 
numerous synoptic maps, not to explain the synoptic 
map in terms of the mean and its perturbations. In 
other words synoptic models must always be checked by 
what is known of the mean circulation, but we cannot, 
except under very special circumstances, infer the syn- 
optic models from the mean map. Figures 3 and 4 
show schematically the correct explanation for the 
tropical Pacific. Neglect of this principle led to the 
yz 
=—N 
= 
Sea 
autumn. Speed in meters per second. Data derived from Deppermann and Werenskiold, and analyzed by G. A. Dean. 
first time. The vortex proceeds on its westward track; 
it may deepen very rapidly, forming a typhoon or 
storm, or it may remain weak, being accompanied in 
this case by one of the slight depressions that are 
characteristic of the doldrums in the western Pacific. 
When the vortex is well developed a complicated system 
of asymptotes accompanies it and the movement of 
these ‘‘equatorial fronts” is dependent not on the winds 
on either side of them but on the motion of the vortex 
itself. This brief summary of the equatorial waves and 
vortices is based upon recent study of the data collected 
during the Bikini and Eniwetok bomb tests and the 
whole subject has been more fully treated in recent 
publications [47, 48]. 
We are now in a position to reconcile the conflicting 
views that have been held concerning oceanic equatorial 
meteorology. We see first that the equatorial westerlies, 
as long ago realized by Piddington and Reid, are merely 
the mean expression of the fact that the vortices form 
in the central parts of the Pacific and move toward the 
conception of the continuous equatorial front; because 
it can easily be detected on the mean maps, it was 
assumed that it would be found on the synoptic maps. 
Recent research has shown that the synoptic conver- 
gence lines are quite different from the mean con- 
vergence line; the latter, however, can be derived as 
the statistical result of the properties and positions of 
the former. 
The special conditions under which we can pass from 
the mean map to the synoptic map obviously hold in 
the eastern parts of the oceans. Here we know that 
departures from the mean are very small compared with 
the mean. And it seems that in these regions the thermal 
effects long ago described by the climatological school 
must play at least the dominant part in driving the 
southeast and northeast trades. Here the direct-cell 
model is probably a good approximation to what 
actually occurs in the atmosphere. The result of the 
action of the direct cell, however, is not a mass ascent of 
the trades at the equator, and their return aloft as anti- 
