Explain how the five labeled ocean gyres in Fig. 6.38 arise from the atmospheric general circulation, being explicit about vertical atmospheric circulation cells and surface pressure centers.
To understand how the five labeled ocean gyres arise from the atmospheric general circulation, it is important to consider the concept of atmospheric circulation cells and surface pressure centers. Here's an explanation of how these factors contribute to the formation of ocean gyres:
1. Atmospheric Circulation Cells: The Earth's atmosphere is divided into three major circulation cells: Hadley cells, Ferrel cells, and Polar cells. These cells result from the uneven distribution of solar radiation and the Earth's rotation.
- Hadley Cells: These cells exist between the equator and approximately 30 degrees latitude in both hemispheres. Warm air rises at the equator due to intense solar heating, creating a region of low pressure. As the air rises, it cools, releases moisture, and forms clouds, resulting in heavy rainfall in tropical regions. The upper-level winds move poleward and descend around 30 degrees latitude, creating a region of high pressure.
- Ferrel Cells: Found between 30 and 60 degrees latitude in both hemispheres, Ferrel cells are driven by the interaction between polar and tropical air masses. Air from the Polar cells (mostly cold and dry) converges with air from the Hadley cells (warmer and moister) around 60 degrees latitude. This convergence causes uplift, cloud formation, and precipitation. The upper-level winds of the Ferrel cells move equatorward and descend around 30 degrees latitude.
- Polar Cells: These cells exist at the poles and extend to approximately 60 degrees latitude. Cold air descends from the Polar regions, creating a high-pressure area. As it moves toward lower latitudes, it warms slightly, leading to the formation of low-level cold and dry winds.
2. Surface Pressure Centers: Surface pressure centers are regions where air either converges (low-pressure areas) or diverges (high-pressure areas). These pressure systems result from the interaction of vertical atmospheric circulation cells.
- Low-Pressure Centers: In the vicinity of the intertropical convergence zone (ITCZ), a region where air from the Northern and Southern Hemisphere Hadley cells converge, a low-pressure center is formed. Additionally, low-pressure centers can develop in the mid-latitudes due to the interaction of Ferrel cells and Polar cells.
- High-Pressure Centers: High-pressure centers occur around 30 degrees latitude in both hemispheres, resulting from the descending air in the Hadley cells. These regions are known as subtropical highs and are associated with dry and stable weather conditions.
Now, considering how atmospheric general circulation influences the oceans:
1. Ekman Transport: The interaction between wind and ocean surface causes a phenomenon called Ekman transport. Surface winds drag on the ocean surface, causing a net movement of water at an angle to the wind direction. In the Northern Hemisphere, this movement is to the right of the wind, while in the Southern Hemisphere, it is to the left. Ekman transport results in a net transport of surface water away from the shore in coastal regions.
2. Geostrophic Currents: As the ocean surface water is displaced by Ekman transport, a pressure gradient force arises due to the imbalance in the ocean's mass. This pressure gradient force acts perpendicular to the isobars (lines of equal pressure) and drives ocean surface currents. These currents are called geostrophic currents and flow parallel to the isobars. In the Northern Hemisphere, the geostrophic currents flow clockwise around high-pressure centers and counterclockwise around low-pressure centers. In the Southern Hemisphere, the flow is opposite.
3. Ocean Gyres: The combination of Ekman transport and geostrophic currents results in the formation of large circular patterns of ocean circulation, known as gyres. In the Northern Hemisphere, the gyres rotate clockwise around subtropical high-pressure centers, and in the Southern Hemisphere, they rotate counterclockwise around low-pressure centers. There are five major ocean gyres: North Atlantic gyre, South Atlantic gyre, North Pacific gyre, South Pacific gyre, and the Indian Ocean gyre.
In conclusion, the atmospheric general circulation, driven by the uneven distribution of solar radiation and the Earth's rotation, creates vertical atmospheric circulation cells and surface pressure centers. These factors, in turn, influence the wind patterns and pressure gradients, leading to Ekman transport and the development of ocean gyres.