Ocean Circulation and Weather

This is an article from the American Meteorological Association explaining the role the oceans play in our climate.  It is rather long, however it is a good explanation of how the ocean currents affect our weather.  The ocean is just another piece of the puzzle albeit a large one when figuring our forecasts.  I hope you take the time to read it and see how it figures into the big picture on how our atmosphere works.

This is all explained in one of the books I have also by the AMS which covers all aspects of the weather and how everything works – luckily I found this article on the AMS website which was freely available for publication.

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The lower atmospheric circulation and the upper oceanic circulation are closely linked, with the sun being the ultimate source of energy for both circulations. Unequal heating of the atmosphere produces atmospheric circulation and wind. The wind blowing over the surface of the water drives the ocean’s major surface currents. These currents, along with the wind, transfer heat from tropical regions, where there is a surplus, to polar regions, where there is a deficit. The ocean, in turn, releases energy to the atmosphere, which helps maintain the general atmospheric circulation.

The main features of the wind-driven surface circulation are the large, roughly circular current systems, called gyres, which are found in most ocean basins. Driven by the prevailing wind systems and deflected by continental boundaries and Earth’s rotation, the gyres help to redistribute heat from low to high latitudes. Along the western margins of ocean basins, warm ocean currents, such as the Gulf Stream, transport heat poleward. The meanders of these fast moving currents give rise to large rotating warm and cold water rings which are similar to high and low pressure cells observed in the atmosphere. Along the eastern margins of the major ocean basins, currents, such as the California Current, transport cold water to the lower latitudes.

Related to the gyre circulation patterns are other surface circulation features. In the land-locked higher latitudes of the Northern Hemisphere, the prevailing winds drive smaller gyres which redistribute heat to the polar regions. In the higher latitudes of the Southern Hemisphere where there are no blocking continents, strong westerly winds drive the largest volume surface current in the world around Antarctica. Limited by land boundaries, the surface circulation in the northern Indian Ocean actually reverses itself, driven by the seasonally reversing monsoon winds.

Along with widespread horizontal currents at the surface of the world ocean, there are limited regions where vertical circulation is set in motion by persistent winds. In some coastal and equatorial regions, the wind transports warm surface water away from the coast or equator, to be replaced by water from below. This process, called upwelling, brings to the sunlit surface cool, nutrient-rich water which can lead to an increase in biological production.  Recent research is investigating the global connections between the wind-driven surface circulation and the density-driven deep circulation. This is a possible mechanism by which small changes in the surface circulation can influence not only the weather, but climate and climate change.

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The Role of the Sun

1.The Sun is the ultimate source of energy that brings about the surface circulation of the ocean.

2.Because of astronomical and atmospheric factors, ocean surfaces in tropical regions receive more of the Sun’s energy over the course of a year than do those at higher latitudes.

3.In tropical regions, the radiant energy received from the Sun exceeds the radiant energy lost from Earth to space. At higher latitudes, radiation loss from Earth is greater than the solar input.

4.The imbalance between radiant energy gains and losses at different latitudes results in a poleward flow of heat that is almost entirely accomplished by atmospheric and oceanic motions.

The Role of the Atmosphere

1.Unequal heating of the atmosphere from underlying ocean and land surfaces produces atmospheric circulations and winds.

2.The frictional effects between the resulting winds and the ocean surface produce the broad-scale horizontal water movements of the ocean’s surface, called surface currents, that tend to resemble the patterns of surface winds.

3.If the Earth were not rotating, the friction of wind blowing on the ocean surface would push a thin layer of the water in the same direction, but at a small percent-age of the wind’s speed. This layer, in turn, would push on the layer beneath and push it into motion. This would continue downward through successive layers, like pages in a book, each with a lower speed than that of the layer above.

Ekman Circulation

1.Since the Earth does rotate, the shallow layer of surface water set in motion by the wind will be deflected to the right of the wind’s direction in the Northern Hemisphere and to the left of the wind’s path in the Southern Hemisphere. This deflection is called the Coriolis effect.

2.Except at the Equator, where there is no Coriolis effect, each layer of water put into motion by the layer above will be further turned in direction because of the Earth’s rotation.

3.When viewed from above, the changes in current direction and decreased speed with increased depth form a spiral pattern called the Ekman spiral.

4.Although the motion of the surface water layer can be up to 45 degrees to the right or left of the wind, the Ekman spiral phenomenon can cause the direction of net transport of water in around the top 100 meters of the ocean to be at right angles to the wind direction. This is called the Ekman transport.

Gyre Circulation

1.The surface circulation of most of the major ocean basins is dominated by large, roughly circular patterns, called gyres, centered at about 30 degrees latitude. As seen from above, they flow clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. The locations are largely determined by the locations of the Earth’s major wind belts, the blocking effects of the continents, and the effects of the Earth’s rotation.

2.Due to Ekman transport, water at and near the surface is moved towards the middle of the gyres from all sides, producing a broad mound of water as high as one meter at the gyres’ centers.

3.In the Northern Hemisphere, water flowing down the sloping mound of the gyres is deflected to the right, due to the Coriolis effect. This reinforces the clockwise circulation of the gyres. In the Southern Hemisphere, the left turning helps to maintain the counterclockwise pattern of the gyres.

4.An added effect of the Earth’s eastward rotation is the westward displacement of the ocean gyre mounds within their ocean basin. The resulting steeper ocean-surface slopes on the western sides of the gyres help produce higher water speeds in those regions. These western boundary currents that flow poleward include the fastest currents of the ocean. They play a major role in transporting heat from the tropics towards the poles.

5.The fast moving western boundary currents are also distinguished by the formation of large rotating warm and cold water rings that form as meanders in the current become pinched off to form eddies. These eddies, which may reach a size of 200 miles (325 km), a rotation speed of over 1 knot (0.5 m/sec), and ex-tend to the seafloor, have similarities with high and low pressure cells observed in the atmosphere.

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Surface Circulation Features

1.In the northern Indian Ocean which extends to only 20 degrees North Latitude, the surface circulation is dominated by the monsoon wind systems and under-goes dramatic seasonal reversals in direction.

2.In tropical latitudes, warm surface water is piled up in the western margin of the ocean basins by the steady trade winds and westward flowing equatorial currents. The resulting downhill flows of water in an easterly direction are called equatorial counter currents.

3.Around the Antarctic, where there are no blocking continents, flows the largest volume surface current, the Antarctic Circumpolar Current. It is also known as the West Wind Drift, after the strong winds driving it.

4.In the high latitudes of the Northern Hemisphere, where current flow is driven by cyclonic wind systems and modified by land masses, smaller counterclockwise subpolar gyres form, bringing cold water currents south to the western ba-sins. One of these, the Labrador Current, carries icebergs, like the one instrumental in the sinking of the Titanic, into the North Atlantic shipping channels.

5.Farthest north, the Arctic Sea, which is almost surrounded by land and covered by ice, has a clockwise gyre.

Upwelling

1.There are regions where the winds and surface currents cause surface water to move away from the area and to be replaced by the vertical movement of colder water from below. Such occurrences are called upwelling.

2.In coastal regions, upwelling can result from water being moved by Ekman transport away from the coast by winds along the shore.

3.The Coriolis effect, acting on the westward flowing equatorial currents, moves water away from the equator, to be replaced by equatorial upwelling.

4.By replacing warm surface water with cooler nutrient-rich water from below, upwelling increases the concentration of nutrients in sunlit water that can lead to an increase in biological productivity.

Whole Ocean Circulation

1.On a global scale, the connections between wind-driven surface currents, density-driven deep circulation, and the atmosphere are being investigated.

2.This research into whole-ocean circulation may help to identify the mechanisms by which small changes in surface circulation can influence not only the weather, but climate and climate change as well.

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Above it the moon beginning to set over lake Michigan this morning….

We have a large area of rain mainly from Grand Rapids to Lansing this morning.  There is a small chance of rain south of that line today with a better chance of rain and storms developing along the I69 corridor this afternoon.

Today we will see highs in the mid 70s.  Rain chances will be around right on through the weekend with increasing temperatures each day with the mid 80s being more the norm from Sunday on into next week.  We will also see humidity increase…

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