INTRODUCTORY
OCEANOGRAPHYINTRODUCTORY
OCEANOGRAPHY
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The last part of this lesson will discuss how coastal waters differ from ocean waters, how a coastal geostrophic current may be formed at the mouth of a major river, how estuaries are classified and how marginal seas are created adjacent to the major oceans.
Coastal waters are adjacent to land and at the extremes of oceanic margins. They are influenced by continental process more than their parent ocean because of this proximity, and because they are shallow. As a result, changes in the salinity and temperature structures of the coastal ocean is generally more rapid than in the ocean. Also, river runoff and the blowing of dry offshore winds from the continents may offset each other to produce isohaline conditions throughout its depth.
As we learned in the lesson on ocean circulation, geostrophic currents may be set up on a sloping sea surface when the CE balances the slope gradient force (or down-slope component of gravity). When a similar density slope is created in coastal waters, a coastal geostrophic current may be created. This density slope may be created by the wind when waters are blown against the coast and/or by the flow of fresh water into the coastal ocean from large rivers. The Davidson Current off of the Washington coast, is one such current. Here water flowing into the coastal ocean from two rivers is deflected to the right by the CE, which creates both a seaward slope of low salinity water and, when it balances the slope gradient, a coastal geostrophic current.
How would this coastal current compare (in depth, speed and volume transport) with its cousin in the ocean (such as the Gulf Stream)?
An estuary is a semi-enclosed basin of water in which fresh water mixes with, and significantly dilutes, coastal ocean water. Normally the flow of ocean water is restricted as it enters the estuary, either through a narrow inlet or over a shallow sill. Estuaries can be classified in at least two distinct ways (by Origin and By Degree of Mixing):
Four
major classes of estuaries may be identified on the basis of their
origin (Fig. 15.29):
There
also are at least four major classes of estuaries identified by the
degree of mixing of fresh water with salt water (Fig. 15.30):
I am more interested in you remembering the estuaries classified by origin than by degree of mixing - see the study guide.
Marine wetlands, which border estuaries and other shore areas protected from the ocean, are biologically productive areas delicately in tune with natural shore processes. There are two basic types of marine wetlands -- salt marshes and mangrove swamps. Salt marshes are filled with a variety of grasses and are found from the equator to as high as 65 deg. latitude. Mangrove swamps are restricted to latitudes below 30 deg. Once mangrove swamps colonize an area, they normally outgrow and replace marsh grasses.
Left alone, marine wetlands serve as nursery grounds for more than half the species of commercially important fishes in the SE United States. Other fishes use them for feeding and overwintering, and oysters, scallops, clams and other types of fish are located directly in the marshes. Wetlands also remove inorganic nitrogen compounds and metals from groundwater polluted by land sources. Key to the effectiveness of marine wetlands is the natural circulation and flushing provided by the tides.
Humans destroy wetlands at their peril!
At the margins of the ocean are bodies of water set-off from their parent ocean by sills or island arcs that provide varying degrees of restriction to the circulation that exists between the marginal sea and the ocean. We will discuss just two of the many marginal seas.
The
American Mediterranean includes the four
basins of the Caribbean Sea and the Gulf of Mexico as
shown in the figure to the right. The Caribbean Sea is set off from
the Atlantic Ocean by an island arc called the Antillean Chain (which
includes the islands of Cuba, Hispaniola, Puerto Rico, and Jamaica,
and the Greater and Lesser Antilles). The deepest channel (of about
2300 m) between the Caribbean and the Atlantic is just east of the
Virgin Islands. Much of the water that circulates through the
American Mediterranean comes from both the North and South Equatorial
Currents. It leaves the Caribbean and enters the Gulf of Mexico
through the Yucatan Straits. Some of this water circulates in a
clockwise gyre in the Gulf, and then joins other water flowing
directly through the Yucatan Straits, to flow through the Florida
Straits as the Florida Current. The Florida
Current joins with the remainder of the water from the N. Equatorial
Current that flows east of the Caribbean in the Antilles Current to
form the Gulf Stream north of Miami.
Sometimes, the flow through the Yucatan Straits forms a CW looping current in the eastern end of the Gulf of Mexico. Occasionally, this Loop Current will intensify and turn with such a sharp curve as the current approaches the Florida Straits, that a clockwise rotating ring and warm core eddy is broken off. This eddy usually moves toward the western Gulf as it spins down.
As
is shown in the figures to the right and below, the basin that forms
the Red Sea is being created by the separation of the Arabian
Peninsula from the African mainland. It is separated from the Gulf of
Aden and the Indian Ocean by a narrow strait and a shallow sill of
only 125 m in depth, but has a depth of over 2300 m at its deepest
point. The Red Sea is surrounded by highly arid desert so evaporation
is very high. In fact, the evaporation is so high (more than a 200 cm
loss per year) that all water flowing into the Red Sea through the
narrow straits is used to replace the evaporation loss. The surface
salinity is 42.5‰ and a temperature of 30 C during the summer,
but at depths below 2000 m, the salinity exceeds 250‰ and the
temperature exceeds 36 C (note that the very high salinity dominates
the high temperature of the water, so a very dense water mass is
created that sinks to the bottom of the sea). These
brine pools are so dense that
they do not move from their location. However, water enters these
pools through the porous crust, is heated and dilutes the brine.
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