Global Atmospheric Circulations

As mentioned previously, solar heating drives atmospheric circulation. Thus, if earth were a uniform substance constantly tilted perpendicular with its orbital plane (and perhaps another requirement being that Earth rotated sufficiently slow such that the 'Coriolis force' were much smaller than presently), a thermal circulation would be expected (also called a direct circulation) as a simple diagram (referenced site) (main site) indicates. This is similar with other types of thermal circulations, e.g., sea and land breezes. We don't live on such a planet though, as previously mentioned regarding Earth orbit. The Northern Hemisphere contains the majority of land, and the Southern Hemisphere is primarily water. It also rotates quite quickly - a rotation per day meaning a speed of 1038 mph at the equator, decreasing to about 732 mph at 45 latitude, 518 mph at 60 latitude, to 0 at the poles. Each of these factors alter global air circulation from the idealized situation previously mentioned.

Observational studies suggested that several basic wind regimes exist, which correspond with atmospheric pressure observations, the thermal circulation being one. Average global air circulations indicate that such a thermal circulation may exist in tropical regions, called Hadley cells. The supposed Hadley circulation extends to the mid-latitudes. Low pressure tends to be near the equator (the monsoon trof or Intertropical Convergence Zone), and high pressure where the topics and mid-latitudes meet, so the observation of surface flow from high to low pressure conveniently fits this theory. Air convergence at equatorial regions requires a poleward return flow aloft as illustrated. The Coriolis force is supposed responsible for its termination, where subsidence occurs. Poleward of the Hadley cell is supposed a Ferrel cell - an 'indirect' circulation (reverse sense of a thermal circulation) - a consequence of subsidence at its equatorward end and ascent at its poleward end because of similar coriolis turning and supposed convergence. North of the Ferrel cell, a Polar cell is supposed, which completes the 3-cell idealized global circulation shown. Why is the coriolis force assumed responsible for the transitions occurring at approximately 30 and 60 latitude when it is sin(60) = .866 the maximum polar value at 60 latitude and sin(30) = .5 that value at 30 latitude, instead of 42 latitude and 19 latitude which would equally spilt the coriolis difference ? That's a good question which I can't answer, not being a global circulation specialist; which an analysis of the atmospheric equations of motion may answer. I have perhaps a better explanation for the observed circulation below.

Before I mention that though, what is the observed average global circulation ? I.e., how well does the 3-cell theory mentioned above describe observations ? Let's look at some. First, I display average January and July temperatures for each hemisphere. These plots are 5-day averages for 1979-1995 centered about the day shown each year (85 total days) obtained from the Climate Diagnostics Center's Atmospheric Variables Plotting Page. I include these first for 2 main reasons : 1) So users unfamiliar with polar stereographic data plots can gain perspective using an intuitive concept such as temperature (latitude varies on the plot from the equator at the edge to the pole at the center, and continents are shown), 2) So you can imagine how a consequential thermal circulation might appear during warmest and coldest times of year. Perhaps the first things you notice viewing these plots (you can save them for later viewing using your right mouse button) are that temperature gradients are much greater during the cold than the warm season in each hemisphere, and the maximum temperature gradient during the cold season occurs at poleward locations of mid-latitudes. Thus, a purely thermal circulation would perhaps be over mid-latitudes rather than tropical regions - temperature gradients are not so large there.

So if a thermal circulation is perhaps imagined, then what type of circulation does exist ? To help answer that, we can examine plots of average surface pressure and wind. A necessary concept regarding this is zonal wind and meridional wind. Zonal wind refers to a west-to-east direction as positive, meridional wind refers to a south-to-north wind as positive (unfortunately, no matter which hemisphere). The zonal wind is called its u-component and the meridional wind its v-component. Let's consider surface pressure (corrected to sea level) plots. Among most notable features are regions of average high pressure at subtropical oceanic regions, particularly eastern portions of large oceans. You can also see semi-permanent Lows near arctic oceanic regions, particularly during winter - an Aleutian Low south of Alaska and an Icelandic Low near that continent. These semi-permanent features follow the subsolar point to some extent. Comparing Northern & Southern Hemisphere plots, you can also see a much more uniform pressure distribution over the Southern than Northern Hemisphere, because it is much more similar to the idealized situation I previously mentioned. The monsoon trof near the equator is evident, though admittedly difficult to see on these plots which end there. The corresponding near-surface wind distributions are interesting. Because the plotted wind vectors are almost impossible to see, I plotted zonal and meridional components previously mentioned. First examining zonal winds, you can see evidence of the 3 regimes previously mentioned. Easterly winds (negative values) dominate equatorial regions, and westerlies (positive values) at mid-latitudes. The polar easterlies are not so evident. They are in the Antarctic, where high pressure resides over a cold mass of ice encircled by warmer ocean and air - most likely a consequence of those factors more than rotational dynamics (though I am prepared to be proven wrong regarding that). Let's peek at the meridional circulation. Peek is about all we're doing. You can probably surmise that these average characteristics are a sum of the many greatly variable conditions which occur. These might be surprising after staring at them for awhile. The Intertropical Convergence Zone, though evident, does not seem like that which may be imagined. In quite a few locations, equatorial easterlies exist, but not necessarily trade winds. You can see a tendency for poleward flow at mid-latitudes, especially during summer, an 'indirect' circulation, considering the temperature gradient previously seen. Again, equatorward (northerly) flow from the frigid South Pole (positive values) is seen, but not so well-organized from the North Pole, if existent. An interesting plot is that for April in the Northern Hemisphere. The meridional flow is primarily poleward over the continents. A possible reason is a temporary thermal circulation caused as rapid surface heating occurs as the subsolar point quickly moves northward (24 hour days occur at many Arctic locations then).

What about the flow aloft ? First I show zonal wind plots for 500 mb (approximately halfway into the atmosphere according to weight - Southern Hemisphere is similar). These indicate westerly winds at all locations, ranging from near 0 at equatorial regions and the poles to as much as 35 m/sec (average !) at mid-latitudes to arctic locations. The poleward movement of maximum winds is evident during summer, and equatorward movement during winter, with stronger winds associated with the larger temperature gradient. Meridional winds are comparatively small and not particularly organized (though many reasons exist for each maximum and minimum). I'll leave it as an exercise for the reader to obtain those plots

One more series of plots is quite helpful - meridional plots of winds further aloft - near ther tropopause. Any well-organized Hadley circulation would require a return poleward flow aloft corresponding with the supposed trade wind convergence (regarding which I showed some contrary evidence also). I chose 2 levels for these plots - 250 mb & 150 mb - corresponding approximately with the subtropical and equatorial tropopause. The lack of such a return flow during July might be disappointing to a believer of the 3-cell theory, but you should keep in mind that this is only a few plots at a few levels. If trade wind convergence was absent for some locations, perhaps return flow should also be (though the trade winds are more absent for the Northern Hemisphere and the return flow for the Southern Hemisphere). The maps for October are similar, thought the maps for April similar with January's, indicating return flow.

A much greater examination of detailed mass flux calculations are required to rigorously critique Hadley circulations, determine global wind characteristics, etc., but we have examined some of the basic flows. What is a good explanation for the observed atmospheric circulation ? Perhaps the most noticeable aspect of daily weather are the westerly-traveling cyclones and anticyclones people from the subtropics to arctic regions often experience. Theory provides a reason why such are not often experienced near the equator nor the poles (Coriolis force approaches 0 at the equator, thus no large scale circulations), though some quite intense tropical storms can be very near the equator (Typhoon Paka is at about 7 N as I type this, with estimated sustained 85 knot winds !) and easterly waves are common easterly-traveling tropical disturbances. Tropical cyclones do not often form because of baroclinic instability though, and require other processes to become hurricanes. Baroclinic cyclones cause air at mid-latitudes to preferentially rise while moving poleward and sink while moving equatorward. The same is true regarding anticyclones, around which cold air tends to subside from poleward regions and rise from equatorward regions. Winds with cyclones are the primary contributors to wind, anticyclones being much more tranquil. Thus, a combined Hadley cell and mid-latitude cyclone regime is imagined, with a less organized polar regime.

Text is copyright of Joseph Bartlo, though may be used with proper crediting.

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