Global climate models, or GCM's, can be used for many
applications in the atmospheric sciences to help meteorologists to
better understand the atmosphere. One of these GCM's is the Goddard
Earth Observing System, or GEOS-1. This is the model that was used by
Arthur Y. Hou and Andrea Molod in an article in the August 1, 1995 issue
of the Journal of the Atmospheric Sciences. In this article, titled
"Modulation of Dynamic Heating in the Winter Extratropics Associated
with the Cross-Equatorial Hadley Circulation," Hou and Molod use the
GEOS-1 GCM to determine the effect that changes in the Hadley
circulation have on the heating in the extratropics, particularly
focusing on the poleward transfer of heat in the winter hemisphere.
This study was actually a follow-up study to one done by Hou in 1993. In the first study, Hou used an idealized GCM, which essentially left moisture out of the physics of the model (Hou 1993). Moisture is, of course, abundant in the atmosphere, so this second study was conducted to make the data more realistic. Each study was going towards the same hypothesis, though, which is to show that there is a correlation between the Hadley circulation and the heating in the extratropics.
The model for this study was the same as in the first study, but this one did not alter the model physics. Therefore, moisture was included. The run was only for the northern hemisphere during the winter. To make the experiment accurate, they needed to change the Hadley circulation in a way that did not directly impact the extratropics. This was done by rearranging the stratospheric ozone in the tropics, which increased the summer side solar radiation and decreased the winter side radiation, thus changing the Hadley circulation, since solar radiation is the main driving force of Hadley cells. The study used 3-month forecasts for 10 northern winters for both the standard ozone and the modified ozone. The standard ozone was the control group. They used January 1 as the starting date, instead of the more logical December 1 because apparently the GEOS-1 GCM lags in establishing the northern winter Hadley circulation. When starting on January 1, the Hadley cell is firmly established. The ten runs are then started and completed. The results consisted of a large amount of data, but the main conclusions are summarized in the following paragraph.
Using these model runs, Hou and Molod discovered a positive correlation between intensification in the winter Hadley cell and increased heat transport toward the winter pole. This heat transport was inferred from the fact that the models showed increased cooling in the midlatitudes and warming in the high latitudes throughout the winter hemisphere troposphere. The most important conclusion from this study is that "a persistent shift or concentration in tropical convection in such a way as to produce a more intense cross-equatorial Hadley circulation can lead to enhanced upper-level easterlies in the Tropics, a slightly stronger subtropical winter jet, and increased poleward heat transport in the winter extratropics." (Hou and Molod 1995). This is an important finding for global climate change, because some of the changes that are occurring in the atmosphere for anthropogenic reasons could possibly alter the tropical convection, thereby changing the heat transport and altering the things mentioned above.
The findings in this study serve to show how fragile the balance of the atmosphere is. Simply changing the tropical convection can affect processes of the atmosphere over an entire hemisphere, changing the strength of the jets and altering the transport of heat. We tend to think that the actions of certain populations can only affect their immediate surroundings, but this is unfortunately not the case. We can't judge the consequences of our actions simply by what is happening around us. More studies such as this one are needed so we can truly know what problems we are causing throughout the planet.
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