proc.ess.es (pros'es'iz), n. 1. a system of operations in producing something. 2. A series of actions, changes or functions that achieve an end or result. 3. Ongoing movement: PROGRESSION
(Webster's New College Dictionary, 1995)

 

About Coastal Ocean Processes


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Wave ForcastOregon's ocean area is a small segment of a much larger region affected by oceanographic and atmospheric forces that operate across the NE Pacific Ocean. Comprehension of this context is vital to understanding the mechanics of how the various subsystems of the coastal ocean interact, and ultimately affect coastal communities.

Ocean circulation in the north Pacific Ocean reflects large-scale air movements in the atmosphere, specifically those relating to 2 major atmospheric pressure cells: the North Pacific High and the Aleutian Low. Storm systems spiral in rotary fashion around these pressure gradients (clockwise around the high, counterclockwise around the low) and deliver energy to the surface of the ocean, resulting in the waves which impact the coast.

In Winter, the large Aleutian low is dominant, causing storms which blow onshore from the southwest. In the summer the North Pacific High moves northward off Oregon, causing winds to blow north to south along the Oregon Coast. El Nino and La Nina years impose still different conditions. All these factors combine to affect the physics and the biology of the coast itself. An better understanding of these interactions can help coastal communities anticipate and prepare for the surprises that the ocean can deliver.


Circulation and Physical Properties of the Coastal Ocean

Although the churning surf driven before a winter storm is an unforgettable sight, the ocean's most important feature, its internal composition, cannot be seen by the naked eye. The composition of this fluid mass is far from homogeneous in source or structure, and the dynamics which result from the physical interaction of air, water, temperature, salinity, density and tides drive the engine of the entire ocean system.

Horizontal Circulation

Analysis of current meter data indicates that ocean circulation off the Oregon coast responds principally to wind fluctuations and sea level. Currents over the shelf tend to flow in the same direction as the wind. On average currents over the continental shelf and slope flow south at the surface and north along the bottom. This general behavior of ocean currents can be further refined for seasonal conditions. The summertime southward-flowing surface current called the California current, responds to the northwest winds, and is broad and weak (between 4 and 8 kilometers a day). It's counterpart, the California undercurrent is narrower and faster than the overlying California current. Winter flow is northward at all depths over the continental shelf in response to southwest winds, and is called the Davidson current.

Vertical Circulation

In Oregon, strong northwest winds begin in the spring and usually continue into the fall. As these winds blow south along the coastline, they produce currents which divert water to the right of the wind directions a result of the Coriolis force (an effect of the earth's rotation). Studies off the central Oregon coast show that the surface layer in which the wind-driven offshore transport occurs is relatively thin - less than 20 meters deep. This off shore flow of water is replaced by an onshore flow of colder, nutrient rich water that rises from depths of 100 to 200 meters. Vertical velocities of this onshore flow vary but have been estimated to be as much as 17.3 meters per day. Vertical flow is strongest near the surface and decreases rapidly with distance from shore. As the local wond stress varies from day to day and from season to season, significant variations in the strength of coastal upwelling occur. Most upwelling occurs as a series of pulses or "events" with a time scale or days to weeks, as long as winds are favorable. When these northwest winds cease, warmer sea surface temperatures return to the area in a matter of days.

Sea Surface Temperature

Mean annual temperature variations at the surface of the Pacific Ocean are relatively small. Because of the sun's seasonal effect, the mean summer temperature is 14 dgrees C while the mean winter temperature is 9 degress C. Summer upwelling brings colder, deep waters to the surface along the central and south coast shoreline and supresses the normally high summer surface temperatures. During the summer, the warmer Columbia River waters are pushed seaward.

Sea Surface Salinity

The salinity of the surface layers off the Pacific Northwest reflects the freshwater inflow from the Columbia River. In summer a lower salinity plume spreads southwest. Its shoreward boundary may be pushed to sea by colder, highly saline, upwelled water. In winter, the Columbia River discharge creates a band of lower-salinity water along the coastline from Tillamoook to the Strait of Juan de Fuca.

Sea Surface Density

Density is a result of temperature and salinity. Summer surface densty is clearly influenced by the large freshwater flow from the Columbia River over a broad area and by the upwelling of cold, saline waters along the coastline. Likewise in winter the freshwater discharges from the Columbia and other coastal rivers result in a band of less dense water along the northern Oregon and Washington coastlines.

Tides

Tides are the regular rise and fall of sea level, and are caused by the gravitational pull on the earth by the moon and the sun. The gravitational attraction of the moon pulls ocean water nearest it away from the earth and at the same time pulls the earth away from the water farthest away. Two equal tidal bulges result on opposite sides of the earth. Lead by the moon's orbit about the earth, the tidal bulges move across the ocean as a long-period wave. A particular location in the earth will come under the influence of the moon each 24 hours and 50 minutes, slightly longer than one day. Since the moon's orbit is offset from the equator by approximately 23.5 degrees, the Oregon Coast experiences a procession of unequal tides spaced approximately 6 hous apart. These tides move northwards up the coast; Coos Bay has high and low tides approximately 20 to 30 minutes earlier than those on the Columbia River.


This text abridged from The Oregon Ocean Book and other materials provided by Bob Bailey, OCMP
 

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