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Overturning Circulation
Figure 1: . Zonally averaged temperature in January and July from 1948 to 2016. The warmest air is always at the tropics, but the temperature gradient is strongest over the mid-latitude in the northern hemisphere in January and in the southern hemisphere is July.
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Because the incoming solar radiation in January and July is not equal in southern and northern hemisphere, the Hadley cell will not be two symmetrical cells over the equator. One cell will be weaker and one stronger, where it is easier to observe the stronger cell.
Figure 2. Zonally averaged vertical wind in January and July over 1948 to 2016. Positive direction is downward. The strongest upward flow is in the tropics, shifted to the south in January and to the north in July. The downward flow is strongest in the subtropics in the northern hemisphere in January and in the southern hemisphere in July.
Figure 3. Zonally averaged meridional wind in January and July from 1948 to 2016. Positive direction is northward. In January, the strongest meridional winds are the wind in the northern tropics flowing toward North Pole in the upper layers and the wind flowing south toward the equator above the surface. In July, the strongest meridional winds are the wind in the southern tropics flowing toward South Pole in the upper layers and the wind flowing north toward the equator above the surface.
In January the temperature peak is in the southern tropics, so the strongest rising air is in the southern tropics (Fig 2). The air then moves northward following the largest temperature gradient into the northern hemisphere on the upper layers (Fig 3), then sinks down in the sub-tropics (Fig 2) and flow southward back to the equator near surface (Fig 3) to complete the circulation.
Similarly in July, the stronger Hadley cell has hot air rising from the northern tropics (Fig 2), moving southward following the stronger temperature gradient in the upper layers (Fig 3), sinking in the southern sub-tropics (Fig 2), and flowing back toward the equator near the surface (Fig 3).
Figure 4. Zonally averaged potential temperature in January and July from 1948 to 2016. A dome of cold air is located over the pole. The temperature gradient is strongest over the mid-latitudes in the northern hemisphere in January and in the southern hemisphere is July.
Even thought the circulation consists of both the air flow poleward and equatorward, the amount of heat transport poleward is larger than the amount of heat transported toward the equator. From figure 4, the potential temperature in the tropics, where the Hadley cell .
Thermal Wind
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circulation happens, has a rather flat gradient, showing that the Hadley cell is efficient in transporting heat.
Thermal Wind
From project 2, we know that thermal wind is strongest where the temperature gradient is largest. From Figure 1, the strongest temperature gradient is in the mid-latitudes of both hemispheres, and from figure 5 the strongest wind or jet stream is over the mid-latitudes.
Figure 5. Zonally averaged zonal wind in January and July over 1948 to 2016. Positive direction is eastward. The strongest winds are eastward winds over the mid-latitudes in upper layers in both hemispheres, with the peak in the northern hemisphere in January and in southern hemisphere in July.
In the Hadley cell when air flows from the equator toward the pole, to conserve the angular momentum the wind speed increases. Moreover, the large temperature gradient in the mid-latitudes also marks the location where the heat flux from the Hadley cell stops, hence the temperature difference. The location and speed of the jet stream in the upper layers in the mid-latitudes also matches with the location and wind speed of a corner of Hadley cell where the air flow changes direction from poleward to downward.
Eddies
Transient eddies is the characteristic heat flux in the mid-latitudes. Even though mean circulation also occurs in this area (called the Ferrel cell), the transient eddy flux, which can be calculated by subtract the mean heat flux from the total heat flux, is more effective in transporting heat.
Figure 6. Transient heat flux at the level of 850 and 250 mbar respectively. Positive direction is going northward. Peaks exist in both levels of transient heat flux located in the mid-latitudes with flux going northward in the north hemisphere and going southward in the south hemisphere, but the magnitude of the peak flux is higher at the 250 mbar level.
From Figure 6, the transient heat flux has its peaks in the mid-latitude at both the 250 and 850 mbar levels. This contrasts with the heat transport in the Hadley cell circulation where the heat is transported poleward on the upper level and partially transported back toward the equator on the lower level. This characteristic also shows in figure 7 where the transient heat flux in the mid-latitude has the same direction in each hemisphere with the peak at 850 and 200 mbar levels.
Figure 8. Zonally averaged transient heat flux is highest in the mid-latitudes with flux going northward in the north hemisphere and going southward in the south hemisphere at the 850 mbar and 200 mbar levels.
When the meridional transient heat flux is vertically integrated, the effectiveness of transient heat flux by eddies is shown by the clear peaks in the mid-latitudes, as shown in Figure 9. The eddies transport the heat to the north in the northern hemisphere, and to the south in the southern hemisphere.
Figure 9. Anomaly of zonally and vertically integrated transient energy transport shows a peak in the mid-latitudes with flux going northward in the northern hemisphere and southward in the southern hemisphere.
When the heat fluxes from the Hadley cell circulation and transient eddies are combined, the heat is transferred effectively from the equator toward the pole in both southern and northern hemisphere. These two systems govern the climate pattern in their respective regions. In the tropics, the climate is largely due to the convective rising air into rainclouds. In the sub-tropics, the dry, warm sinking air creates dry deserts. In the mid-latitudes, the weather is influenced by fronts and hurricanes.
Tank Experiment
Two rotating tank experiments were set up. Tank rotation speed was the important parameter varied: one tank rotated at a speed of 1
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