Global Weather & Ocean Circulation

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How are global weather patterns and ocean circulation patterns formed?

What causes hurricanes, ocean currents and other "big weather" events? These are big picture questions that require you to zoom out and look at the Earth as a planet. The equator receives an above-average amount of incoming solar energy, while the poles receive below average amounts. This respective high and low pressure imbalance is the basis for a general gradient along which air masses circulate -- warm air rises at the equator, where it is heated and moves towards the poles, where it sinks and moves back towards the equator. While this general trend is true, since the Earth is rotating, the Coriolis Effect causes the larger trend of convection to be broken into smaller convecting cells that follow the same concept of high-pressure divergence and low-pressure convergence.

The Coriolis Effect is observed when a frictionless object is moving in a straight line across a rotating disk. There is an apparent “turning” of the object in the direction counter to the direction of disk rotation, as measured from the rotating surface. The Earth’s sphere can be conceptually broken up into a series of stacked disks, their radii decreasing as you move away from the equator. As you approach 30 degrees S from the equator, the Coriolis effect has turned your momentum 90 degrees to the East. Simultaneously to the turning by Coriolis Effect, the air mass is moving onto disks with a smaller radius as it moves away from the equator. Since angular momentum must be conserved, the velocity of the air mass relative to the Earth’s surface will increase. This fast, horizontal-moving air is geostrophic -- the Coriolis Effect is balanced out by the force from the pressure gradient. It is boxed in by neighboring air masses, and has nowhere to go but down.

Combining the concepts of high pressure divergence and the Coriolis Effect allows you to predict the fast winds observed traveling at 30 degrees South (Trade Winds) and at 60 degrees South (Westerlies). Southern Trade Winds travel to the East: At 30 degrees, the Coriolis effect has turned the high altitude air masses moving to the South away from the equator, 90 degrees to the left. They are now moving directly to the East, and their speed has increased significantly as their angular momentum is conserved. The wind has cooled, increasing in density; it descends and heads back towards the Equator. These are the Trade Winds. Southern Westerlies travel to the West: High pressure areas in the Horse latitude cause air to sink and diverge, sweeping air masses towards the poles, a low pressure zone. During this journey poleward they increase in speed, as described above. Since these air masses are moving towards the poles, the Coriolis effect turns them, and by 60 degrees S, the masses are moving quickly 90 degrees to the West.

Water in motion along the Earth’s surface is also subject to the Coriolis effect. Here we examine how this works in the Southern Hemisphere in the South Pacific Subtropical Gyre. The force of wind on the water (drag) results in a surface current speed equal to 3% of the original wind speed. As the water mass travels over large distances, the Coriolis effect applies, forcing the water to the left of the original wind vector. This direct effect only penetrates to the top shallow layer of the water, but an indirect effect causes movement as deep as 100m. Like the drag on the surface caused by the winds, the surface layer exerts a frictional force on the layer of water below it. But again, the Coriolis Force comes into play, and the net motion of this second layer of water is to the left of the upper layer exerting the drag. Although some speed is lost with every transfer of motion, the transfer continues to spiral with depth, with the lowest effected levels traveling slowly in the opposite direction from the original wind direction. When these vectors, from surface to ~100m, are added up, the net motion of the water mass is 90 degrees to the left of the wind direction. This is called Ekman Transport.

Now consider the winds above the South-Pacific Subtropical Gyre, drawn into a counter-clockwise formation by the Trade Winds and Westerlies. If the water is moving in a net direction to the left of the wind direction, these counterclockwise winds cause Ekman transport to push water towards the center of the Gyre, raising the central water level. In clockwise Southern clockwise gyres (such as the one circumscribing the Antarctic) Ekman transport pushes water away from the center of the circular structure, creating a lower topography and low pressure zone.

In wind-driven systems such as the South-Pacific Subtropical Gyre, neither thermal vertical convection nor density-driven vertical convection applies; Instead, Ekman Transport creates a centralized high pressure system that drives vertical mixing. The counterclockwise motion of the wind and surface currents create a net inward water movement and subsequent bulge, as described earlier. “Down” is the only direction available for the water being piled up centrally to move.

This has implications for the biological productivity of the Gyre. Whereas an upwelling of water caused by a Southern clockwise Gyre draws water up from lower levels, bringing nutrients up from the lower levels of the ocean, downwelling does the opposite. Chlorophyll levels, lower in the center of downwelling systems, confirm this and create a dearth early in the food chain. This leads to smaller populations of larger organisms that depend on algae and other primary producers for food.

The prevailing currents in the South Pacific Subtropical Gyre are geostrophic currents. At a point of equilibrium, the Ekman transport (using the Coriolis Effect) masses enough water in the high-topography center to cause an outward-pushing high pressure gradient that results in a net zero central force. This leaves the net current movement as a counterclockwise circle.

Momentum of these Gyre currents is generally proportional to wind speed, which is in turn caused by the intensity of the gradient formed by the high and low air-pressure areas described earlier.

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