Physical oceanography
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Physical oceanography is the study of physical conditions and physical processes within the ocean, especially the motions and physical properties of ocean waters.
Physical oceanography is one of several sub-domains into which oceanography is divided; others include biological, chemical and geological oceanographies.
[edit] The physical setting
The pioneering oceanographer Matthew Maury said in 1855 "Our planet is invested with two great oceans; one visible, the other invisible; one underfoot, the other overhead; one entirely envelopes it, the other covers about two thirds of its surface." The fundamental role of the oceans in shaping Earth is acknowledged by ecologists, geologists, geographers and others interested in the physical world. An Earth without oceans would truly be unrecognizable.
Roughly 97% of the planet's water is in its oceans, and the oceans are the source of the vast majority of water vapor that condenses in the atmosphere and falls as rain or snow on the continents (Pinet 1996), (Hamblin 1998). The tremendous heat capacity of the oceans moderates the planet's climate, and its absorption of various gases affects the composition of the atmosphere (Hamblin 1998). The ocean's influence extends even to the composition of volcanic rocks through seafloor metamorphism, as well as to that of volcanic gases and magmas created at subduction zones (Hamblin 1998).
[edit] Vertical and horizontal dimensions
The oceans are far deeper than the continents are tall; the average elevation of Earth's landmasses is only 840 meters, while the ocean's average depth is 3800 meters (Way, Hypsographic curve). Though this apparent discrepancy is great, for both land and sea, the respective extremes such as mountains and trenches are rare (Pinet 1996).
| Body | Area (106km²) | Volume (106km³) | Mean depth (m) | Maximum (m) |
| Pacific Ocean | 165.2 | 707.6 | 4282 | -10911 |
| Atlantic Ocean | 82.4 | 323.6 | 3926 | -8605 |
| Indian Ocean | 73.4 | 291.0 | 3963 | -8047 |
| Southern Ocean | 20.3 | -7235 | ||
| Arctic Ocean | 14.1 | 1038 | ||
| Caribbean Sea | 2.8 | -7686 |
[edit] Temperature, salinity and density
Because the vast majority of the world ocean's volume is deep water, the mean temperature of seawater is low; roughly 75% of the ocean's volume has a temperature from 0° - 5° C (Pinet 1996). The same percentage falls in a salinity range between 34-35 ppt (3.4-3.5%) (Pinet 1996). There is still quite a bit of variation, however. Surface temperatures can range from below freezing near the poles to 35°C in restricted tropical seas, while salinity can vary from 10 to 41 ppt (1.0-4.1%) (Marshak 2001).
The vertical structure of the temperaturecan be divided into three basic layers, a surface mixed layer, where gradients are high, a thermocline where gradients are high, and a poorly stratified abyss.
In terms of temperature, the ocean's layers are highly latitude-dependent; the thermocline is pronounced in the tropics, but nonexistent in polar waters (Marshak 2001). The halocline usually lies near the surface, where evaporation raises salinity in the tropics, or meltwater dilutes it in polar regions (Marshak 2001). These variations of salinity and temperature with depth change the density of the seawater, creating the pycnocline (Pinet 1996).
[edit] Density
- Thermohaline circulation (Density-driven)
Also see:
[edit] The general circulation of the ocean
The ultimate energy source for the ocean circulation (and for the atmospheric circulation) is the sun. The amount of sunlight absorbed at the surface varies strongly with latitude, being greater at the equator than at the poles, and this engenders fluid motion in both the atmosphere and ocean that acts to redistribute heat from the equator towards the poles, thereby reducing the temperature gradients that would exist in the absence of fluid motion. Perhaps three quarters of this heat is carried in the atmosphere; the rest is carried in the ocean.
The atmosphere is heated from below, which leads to convection, the largest expression of which is the Hadley circulation. By contrast the ocean is heated from above, which tends to suppress convection. Instead ocean deep water is formed in polar regions where cold salty waters sink in fairly restricted areas. This is the beginning of the thermohaline circulation.
Oceanic currents are largely driven by the surface wind stress; hence the large-scale atmospheric circulation is important to understanding the ocean circulation. The Hadley circulation leads to Easterly winds in the tropics and Westerlies in mid-latitudes, which creates an anticyclonic wind stress curl over the subtropical ocean. This leads to slow equatorward flow throughout most of a subtropical ocean basin (the Sverdrup balance). The return flow occurs in an intense, narrow, poleward western boundary current. Like the atmosphere, the ocean is far wider than it is deep, and hence horizontal motion is in general much faster than vertical motion. In the southern hemisphere there is a continuous belt of ocean, and hence the mid-latitude westerlies force the strong Antarctic Circumpolar Current. In the northern hemisphere the land masses prevent this and the ocean circulation is broken into smaller gyres in the Atlantic and Pacific basins.
[edit] The Coriolis Effect
The Coriolis effect results in a deflection of fluid flows (to the right in the northern hemisphere and left in the Southern Hemisphere). This has profound effects on the flow of the oceans. In particular it means the flow goes around high and low pressure systems, permitting them to persist for long periods of time. As a result, tiny variations in pressure can produce measurable currents. A slope of one part in one million in sea surface height, for example, will result in a current of 1 cm/s at mid-latitudes. The fact that the Coriolis effect is largest at the poles and weak at the equator results in sharp, relatively steady western boundary currents which are absent on eastern boundaries. Also see secondary circulation effects.
The Coriolis effect is also responsible for coastal upwelling as wind-driven currents tend to forced to the right of the winds in the Northern Hemisphere and to the left of the winds in the Southern Hemisphere. When winds blow either equatorward along an eastern ocean boundary or poleward along a western ocean boundary, water is driven away from the coasts (the so called Ekman transport), and denser water rises from below to replace it.
[edit] Pressure-driven flows
[edit] Angular momentum and the ocean circulation
[edit] Ocean - atmosphere interface
At the ocean-atmosphere interface, the ocean and atmosphere exchange fluxes of heat, moisture and momentum.
[edit] Heat
The important heat terms at the surface are the sensible heat flux, the latent heat flux, the incoming solar radiation and the balance of long-wave (infra red) radiation. In general, the tropical oceans will tend to show a net gain of heat, and the polar oceans a net loss, the result of a net transfer of energy polewards in the oceans.
The oceans' large heat capacity moderates the climate of areas adjacent to the oceans, leading to a maritime climate at such locations. This can be a result of heat storage in summer and release in winter; or of transport of heat from warmer locations: a particularly notable example of this is Western Europe, which is heated at least in part by the north atlantic drift.
[edit] Momentum
Surface winds tend to be of order meters per second; ocean currents of order centimeters per second. Hence from the point of view of the atmosphere, the ocean can be considered effectively stationary; from the point of view of the ocean, the atmosphere imposes a significant wind stress on its surface, and this forces large-scale currents in the ocean.
[edit] Moisture
The ocean can gain moisture from rainfall, or lose it through evaporation. Evaporative loss leaves the ocean saltier; the Mediterranean for example has strong evaporative losses; the resulting plume of dense salty water may be traced through the Straits of Gibraltar into the Atlantic. At one time, it was believed that evaporation/precipitation was a major driver of ocean currents; it is now known to be only a very minor factor.
[edit] Equatorial effects
[edit] Planetary waves in the ocean
[edit] Kelvin Waves
[edit] Rossby Waves
[edit] Climate variability
![December 1997 chart of ocean surface temperature anomaly [°C] during the last strong El Niño](../../../upload/thumb/5/5a/El-nino.gif/350px-El-nino.gif)
The interaction of ocean circulation, which serves as a type of heat pump, and biological effects such as the concentration of carbon dioxide can result in global climate changes on a time scale of decades. Known climate oscillations resulting from these interactions, include the Pacific decadal oscillation, North Atlantic oscillation, and Arctic oscillation. The oceanic process of thermohaline circulation is a significant component of heat redistribution across the globe, and changes in this circulation can have major impacts upon the climate.
[edit] La Niña - El Niño
La Niña
The Walker circulation is seen at the surface as easterly trade winds which move water and air warmed by the sun towards the west. This also creates ocean upwelling off the coasts of Peru and Ecuador and brings nutrient-rich cold water to the surface, increasing fishing stocks.
The western side of the equatorial Pacific is characterized by warm, wet low pressure weather as the collected moisture is dumped in the form of typhoons and thunderstorms. The ocean is some 60 cm higher in the eastern Pacific as the result of this motion.
The water and air are returned to the east. Both are now much cooler, and the air is much drier. An El Niño episode is characterised by a breakdown of this water and air cycle, resulting in relatively warm water and moist air in the eastern Pacific.
In the Pacific, La Niña is characterized by unusually cold ocean temperatures in the eastern equatorial Pacific, compared to El Niño, which is characterized by unusually warm ocean temperatures in the same area.
[edit] Antarctic Circumpolar Wave
This is a coupled ocean/atmosphere wave that circles the Southern Ocean about every eight years. Since it is a wave-2 phenomenon (there are two peaks and two troughs in a latitude circle) at each fixed point in space a signal with a period of four years is seen. The wave moves eastward in the direction of the Antarctic Circumpolar Current.
[edit] Ocean currents
These global thermodynamic forces drive ocean currents:
- Antarctic Circumpolar Current
- Deep ocean (density-driven)
- Western boundary currents
[edit] Antarctic Circumpolar Current
The ocean body surrounding the Antarctic is currently the only continuous body of water to circumnavigate the globe about the polar axis. It interconnects the Atlantic, Pacific and Indian oceans, and provide an uninterrupted stretch for the prevailing westerly winds to significantly increase wave amplitudes. It is generally accepted that these prevailing winds are primarily responsible for the circumpolar current transport. This current is now thought to vary with time, possibly in an oscillatory manner.
[edit] Deep ocean currents (abyssal circulation)
In the Norwegian Sea evaporative cooling is predominant, and the sinking water mass, the North Atlantic Deep Water (NADW), fills the basin and spills southwards through crevasses in the submarine sills that connect Greenland, Iceland and Britain. It then flows along the western boundary of the Atlantic with some part of the flow moving eastward along the equator and then poleward into the ocean basins. The NADW is entrained into the Circumpolar Current, and can be traced into the Indian and Pacific basins. Flow from the Arctic Ocean Basin into the Pacific, however, is blocked by the narrow shallows of the Bering Strait.
Also see marine geology about that explores the geology of the ocean floor including plate tectonics that create deep ocean trenches.
[edit] Western boundary currents
An idealised subtropical ocean basin, forced by an anticyclonic wind stress, acquires a gyre circulation with slow steady flows everywhere except in the region of the western boundary, where a thin fast polewards flow called a western boundary current develops. Flow in the real ocean is more complex, but the Gulf stream, Agulhas and Kuroshio are examples of such currents. They are narrow (approximately 100 km across) and fast (approximately 1.5 m/s).
Equatorwards western boundary currents occur in tropical and polar locations, e.g. the East Greenland current.
[edit] Gulf stream
The Gulf Stream, together with its northern extension, North Atlantic Drift, is a powerful, warm, and swift Atlantic ocean current that originates in the Gulf of Mexico, exits through the Strait of Florida, and follows the eastern coastlines of the United States and Newfoundland to the northeast before crossing the Atlantic Ocean.
[edit] Kuroshio
The Kuroshio Current is an ocean current found in the western Pacific Ocean off the east coast of Taiwan and flowing northeastward past Japan, where it merges with the easterly drift of the North Pacific Current. It is analogous to the Gulf Stream in the Atlantic Ocean, transporting warm, tropical water northward towards the polar region.
[edit] Overflows
[edit] Ocean eddies
[edit] Coastal and nearshore processes
[edit] Introduction
Nearshore circulation and wave dynamics are involved in rip current formation. (Wave characteristics, types, and life cycles are discussed in greater detail in other COMET modules.)
[edit] Nearshore Terminology and Circulation
[edit] Zones
Waves absorb energy from the wind. That energy is transmitted across the water surface by waves. At the shoreline, waves break and unleash that energy on the beach.
There are four defined zones in the nearshore environment. When a wave passes from deep to shallow water, it enters the shoaling zone. In this zone the depth of the water is less than half the wavelength. The vertical orientation of the wave changes as it begins to feel the effect of the lake or ocean bottom. As the wave nears the shore, its steepness increases and the wave eventually breaks.
Where the wave begins to break is known as the breaker zone. Not all waves begin to break in the same location due to wave height variation. The breaker zone is where the majority of waves reach their steepness limit for a given wave spectrum. After the wave begins to break, it enters the surf zone where water is transported toward the beach in the form of smaller, broken waves known as bores. These bores can be thought of as continually breaking waves. As bores reach the beach, water particles are pushed onshore and then retreat seaward. This area of run-up and backwash of water is known as the swash zone.
[edit] Circulation - Onshore Flow
In the deeper water beyond the shoaling zone, water particles of non-breaking waves have a decreasing orbital motion to a depth equal to half the wavelength, but little to no net flow in the wave direction. As waves move into shallower water, the circular orbits become progressively more distorted and upon breaking are highly disrupted. Following the wave breaking, water particles still oscillate moving landward with the wave crest and seaward with the trough. The average of these currents onshore causes a rise in the mean water level above the still water level known as wave set-up. The other parts of the nearshore circulation system stem from this onshore flow of water.
[edit] Circulation - Longshore Current
When waves break on the shoreline, they create currents parallel to the shoreline called longshore currents. Longshore currents occur most often when waves approach the shoreline at an angle. The angle of the incoming wave causes a progressively breaking wave that moves along the shoreline and a longshore current that moves in the same direction as the breaking wave. The longshore current spans the entire width of the surf zone. It reaches maximum strength in the middle of the surf zone and diminishes in strength as it moves farther offshore.
Larger waves create faster longshore currents. The angle of wave approach at breaking also affects the speed of the current. Peak currents occur when the wave approaches from 45 degrees. Higher or lower angles produce slower currents. Waves breaking parallel to the shoreline will have no longshore current generated by the wave angle.
Like rip currents, longshore currents are subtle but can be seen or felt while standing in the surf zone. Longshore currents will always be present with rip currents as part of the nearshore circulation system.
Large-scale currents moving at slower speeds in the nearshore can also be generated from persistent synoptic-scale winds as opposed to locally-breaking waves.
[edit] Circulation - Rip Currents
Rip currents are jet-like currents of water that typically extend from near the shoreline out past the line of breaking waves.
Rip currents can be caused by several wave phenomena that will be presented later in this module. These include offshore flow through channels in sandbars, natural variability of breaking wave heights, and longshore current interaction with man-made structures. Rip currents are a natural part of the dynamic nearshore circulation system. A portion of the longshore current enters into "feeder currents," which are the segments on the shore-side of a rip current. A rip current also has a neck and a head.
[edit] Surf Zone - Characteristics
Of the four zones defined in the nearshore environment, the surf zone is the most important for the marine forecaster’s analysis of rip current potential. There are several aspects of the surf zone that must be assessed, including:
- Variable and fixed bathymetry
- Multispectrum waves
- Wave height
- Slope of the beach
[edit] Surf Zone - Variable and Fixed Bathymetry
Beaches exist because of the erosion and recovery of sand as shorelines adjust to the forces that shape them. Varying weather and storm patterns that recur every year cause seasonal fluctuations in the beach width and the bathymetry.
Rip current potential is affected by the interaction between waves and the bathymetry of the surf zone. Beaches can contain man-made bathymetric structures such as groins, jetties, and piers and natural bathymetric features such as canyons, ridges, and sandbars.
[edit] Surf Zone - Multispectrum Waves
Multispectrum waves are present in the surf zone, and the forecaster needs to understand the numerous and complex motions occurring within the wave spectra.
It should be emphasized that most offshore wave observation data systems report only the significant wave height, thus making it necessary for forecasters to look at the full spectral wave analysis to obtain a comprehensive depiction of the sea state environment affecting the surf zone.
[edit] Surf Zone - Wave Height
Surf zone wave height and period correlate reasonably well with offshore buoy measurements of directional wave spectral data. Given the lack of any real-time shoreline data in many locations, distant buoy data can be used to forecast waves and, hence, the rip current potential in the surf zone. The process of determining this correlation involves the use of large amounts of data to find a regression equation.
[edit] Surf Zone - Slope of the Beach
Another important aspect of the surf zone is its slope.
In general, fine sands result in flatter beaches. Coarse sand results in a steeper beach face. Variations in sand size occur due to the source of the sand, most often resulting from the underlying or adjacent geology that is being reworked by the waves.
The slope of the beach has a direct effect on the size of the surf zone. For a given wave height, steeper beaches will have a narrower surf zone and, hence, reduce the flux of water in the longshore current. This decreased flow tends to cause weaker rip currents on steep beaches (Murray et al. 2003).
[edit] Nearshore Waves
[edit] Wave Energy Transformation - Shoaling
Wave shoaling occurs as waves travel toward shore in shallow water. Shoaling is the changes in wave characteristics that occur when a wave reaches shallow water. The decreasing depth causes:
- An increase in wave height. The conservation of energy results in more energy forced into a smaller area. Since wave energy is proportional to wave height squared, this increases wave height as it propagates toward shore even though some of the energy is dissipated by bottom friction.
- A decrease in wave speed. Remember that waves in shallow water have speeds that are dependent on the square root of water depth. As the depth decreases, so too will the wave speed.
As the wave moves into shallower water, shoaling affects the wave form by slowing its base while having less effect on the crest. At some point, the crest of the wave is moving too fast for the bottom of the wave form to keep up. The wave then becomes unstable and breaks.
[edit] Wave Energy Transformation - Refraction Due to Depth
While change in water depth affects wave height, it can also affect the direction of wave propagation. If a wave is moving in a direction that is not perpendicular to bathymetry contours, then the wave speed will vary along the length of the wave crest. The section of the wave over deeper water will move faster than other parts of the wave. This causes a turning or “refraction” of the wave direction. This refraction occurs before the surf zone and within it. Refraction of waves in the surf zone implies a lower wave incidence angle from shore, which is not conducive to rip current formation.
Submarine canyons will cause a convergence of longshore currents. This area is favorable for rip current formation due to the localized decrease in wave heights near shore. The opposite is true for a submarine ridges, which tend to focus the waves and cause a divergence of the longshore current.
[edit] Wave Energy Loss and Dissipation - Diffraction
Wave diffraction occurs when wave crests bend around barriers. Wave energy can be diffracted into the shadow of an object similar to light bending around corners. Waves will diffract on the lee side of small islands and breakwaters.
[edit] Wave Energy Loss and Dissipation - Reflection
Reflection is dependent on the breaking state of the wave as well as the slope and hardness of the surface. It can vary from 0% for breaking waves on low-sloped, porous beaches to nearly 100% for non-breaking waves in deep water on a hard, vertical surface such as a sea wall.
[edit] Wave Energy Loss and Dissipation - Breaking
Wave breaking will occur at or prior to a wave steepness of 1/7 (wave height/wavelength). Remember that wave breaking is also dependent on water depth. Waves are forced to break nearshore when the wave height is approximately 78% of the water depth.
Breaking waves are the key to rip current production because they help produce the wave set-up which piles water onshore. The non-uniform height of wave set-up within the surf zone helps generate longshore currents which, in turn, promote the formation of rip currents.
[edit] Rip Currents
[edit] Characteristics of Rip Currents - Rip Current Circulation
Rip currents are jet-like flows of water moving away from the beach with a root at the shore, a neck across the breaker zone, and a mushrooming head. A rip current neck can be very narrow or more than 50 yards in width. A rip current head is typically seen just beyond the breaker zone.
The seaward extent of rip currents can vary from just beyond the line of breaking waves to hundreds of yards offshore, extending up to a maximum of 2.5 times the surf zone width.
[edit] Characteristics of Rip Currents - Visual Clues
Clues for identifying a rip current include the following:
- Channel of churning, choppy water
- Difference in water color (Suspended sediments may be transported back to sea in the rip current.)
- Line of foam, seaweed, or debris moving out to sea
- Break in the incoming wave pattern
The rip current head is often brown and foamy from sediment that has been disturbed. When viewed from above, this head provides a clear indication that a rip current is present. However, rip currents may not be as evident from the perspective of a swimmer standing on a beach. This risk can be minimized by swimming in beach areas monitored by lifeguards.
[edit] Characteristics of Rip Currents - Wave Angle
Rip currents are frequently generated when the incoming wave direction is nearly perpendicular to the shoreline. The orientation of the shoreline is therefore a key feature to note when assessing the potential for rip current formation at a particular beach. Some studies suggest the ideal wave angle for rip current formation is within 20 degrees of normal. However, in the vicinity of shoreline structures waves approaching at angles of greater than 20 degrees create faster longshore currents and are therefore more likely to cause rip currents.
[edit] Characteristics of Rip Currents - Location
Rip currents are commonly observed on beaches with a relatively gentle slope. This occurs because there is generally a wider surf zone on gently sloping beaches. A gently sloping beach has a typical slope of around 1/40 or 0.025. The wider surf zone allows waves to break for a longer period of time and, hence, transport more water toward the beach. Steep beaches are less likely to have rip currents. Rip currents that do occur on steep beaches will be weaker due to waves breaking close to shore and the lack of a wide surf zone. Note that this relationship with beach slope puts the public at greater risk since more people tend to swim at gently sloping beaches.
[edit] Characteristics of Rip Currents - Spacing
A single-cell rip current can occur solely in the vicinity of jetties or other man-made structures as we will discuss in the next section.
Multicell rip currents can form in many different conditions. Here are a few common examples of when multiple rip currents occur:
One current is being created as another dissipates near the same location. Multiple gaps in the sandbar exist. Wave angles are close to normal on a cuspate beach. Due to their nature, the spacing between multicell rip currents is generally observed to be less than 500 m and will vary based upon beach slope, shoreline orientation, wave height, and wave period.
[edit] Characteristics of Rip Currents - Duration
Rip currents are transitory and temporal. The pulsation or intermittency of an individual rip current is approximately 10 to 20 minutes. It is rare to see a rip current sustained for an hour or more. This is not to say that an area of beach will not be affected by a series of rip currents for more than 10-20 minutes as there could be multicell development that occurs over time along a stretch of beach.
[edit] Characteristics of Rip Currents - Velocity
Rip current velocity is intermittent and may rapidly increase within minutes due to larger incoming wave groups or nearshore circulation instabilities. It is important to understand that changes in rip current velocity occur in response to changes in incoming wave height and period as well as changes in water level. The rapid increase in velocity can catch unwary beach goers and swimmers off guard. While rip current velocities average 1-2 ft/s, moderate to strong rip currents can have speeds at or over 8 ft/s.
Without the use of velocity measurements, one can attempt to estimate the potential strength of rip currents based on their spacing along a single beach. A single rip current in a given area of wave height usually indicates high offshore velocity. Multiple rip currents in the same area of given wave height tend to reduce the velocity. In general, the larger the spacing between rip currents on a single beach, the larger the potential velocity in the current. However, under the right wave and water level conditions, high velocities should be a concern on any beach, no matter what the spacing of the rip currents.
[edit] Rip Current Forcing Mechanisms
[edit] Longshore Variations in Incoming Waves - Wave Set-up
When waves break on a beach, they produce a set-up, or rise in the mean water level above the still water level. Wave height and period variations among incoming waves cause the water level in the surf zone to have areas of non-uniformity. This varying of high and low set-up causes a horizontal pressure gradient that induces the current flow from the high to low set-up areas. Convergence of the longshore current at a low set-up point causes a rip current to form and transport the displaced mass of water back offshore.
[edit] Longshore Variations in Incoming Waves - Wave-Wave Interactions
The waves coming into the surf zone are generated by multiple sources and can arrive from different directions. For example, the southeastern U.S. coast can experience long-period swells from a distant tropical cyclone traveling toward shore from the southeast while wind waves are impacting the same shore from the northeast. These bidirectional incoming wave components can produce variations in the water transported through the surf zone, leading to rip current formation. Rip currents will develop in the areas between wave intersections due to the low point in water level at these locations. The rip currents will migrate along the beach relative to these wave intersections unless the wave groups have periods and wavelengths that develop a constant intersection point.
[edit] Longshore Variations in Incoming Waves - Other Temporal Modulations
In addition to large-scale wave interactions in the surf zone, the following factors may also influence the type, duration, and strength of rip currents:
- Sea breezes
- Infra-gravity waves
- Current-refracted waves
- Energy from wave groups
These motions are beyond the scope of this module, but are mentioned to illustrate the complexity of the surf zone environment.
[edit] Wave-Boundary Interaction - Surf Zone Bathymetry
Changes in surf zone bathymetry can cause incoming wave variations along the shore. We have already talked about the fact that submarine canyons and ridges can refract wave energy, changing the direction of wave propagation. Therefore, coastal zones with complex or significant bathymetry will have areas where wave energy converges and diverges for a given wave path. These areas, if predetermined, can be used as a guide for forecasting areas of potential rip current formation.
[edit] Wave-Boundary Interaction - Coastal Structures
Natural and man-made obstructions frequently occur in the shoaling and surf zones. Waves that intersect shore-connected piers, offshore groins and jetties, and points of land are bent (partially diffracted) around these objects. Some of the energy from the wave is bent into the “shadow region” of the object. This results in wave height variations as the diffracted wave energy interacts with other significant waves in the surf zone.
[edit] Wave-Boundary Interaction - Longshore Sandbar
Often, rip currents form where a cut in a longshore sandbar is already present. Typically, the incoming waves will break on the sandbar. The sandbar acts as a dam that holds water deposited by the breaking waves. As wave set-up occurs and the longshore current develops, the low spots in the sandbar become the path of least resistance for the return flow of water. The speed of the rip current is strong enough to move bottom sand and carve out a rip channel across the sandbar's low point. For these reasons, natural changes (sediment transport and major storms) as well as manmade changes (major sand replenishment projects) can significantly impact the surf zone environment, leading to changes in rip current frequency and intensity.
[edit] Tidal Forcing - Modulation by Tides
Rip currents are modulated by tides due to changes in the position of the breaker zone and the size of the surf zone. Higher water levels during high tide cause the breaker zone to move closer to shore, while the opposite effect occurs at low tide. Lower water levels during low tide produce a wider surf zone and, hence, larger water mass transport. If a sandbar is present in the surf zone, high tide allows easier transport of water away from the beach than at low tide due to a greater depth between the surface of the water and the top of the sandbar.
There is some evidence suggesting rip currents are more prevalent in the hours immediately before and after the time of low tide. However, rip currents can occur at high tide depending on various factors, and the forecaster should not assume that the danger of rip currents will not be present.
[edit] Modeling the ocean general circulation
[edit] Oceanic heat flux and the climate connection
[edit] Heat storage
[edit] Sea level change
Tide gauges and satellite altimetry suggest an increase in sea level of 1.5-3 mm/yr over the past 100 years.
The IPCC predicts that by 2100, global warming will lead to a sea level rise of 110 to 880 mm.
[edit] Rapid variations in the ocean
[edit] Ocean tides
The rise and fall of the oceans due to tidal effects is a key influence upon the coastal areas. Ocean tides on the planet Earth are created by the gravitational effects of the Sun and Moon. The tides produced by these two bodies are roughly comparable in magnitude, but the orbital motion of the Moon results in tidal patterns that vary over the course of a month.
The ebb and flow of the tides produce a cyclical current along the coast, and the strength of this current can be quite dramatic along narrow estuaries. Incoming tides can also produce a tidal bore along a river or narrow bay as the water flow against the current results in a wave on the surface.
Tide and Current (Wyban 1992) clearly illustrates the impact of these natural cycles on the lifestyle and livlihood of Native Hawaiians tending coastal fishponds. Aia ke ola ka hana meaning . . . Life is in labor.
Tidal resonance occurs in the Bay of Fundy since the time it takes for a large wave to travel from the mouth of the bay to the opposite end, then reflect and travel back to the mouth of the bay coincides with the timing between this repeating wave that is also reinforced by the tidal rhythm producing the world's highest tides.
[edit] Tsunamis
A series of surface waves can be generated due to large-scale displacement of the ocean water. These can be caused sub-marine land slips, seafloor deformations due to earthquakes, or the impact of a large meteorite.
The waves can travel with a velocity of up to several hundred km/hour across the ocean surface, but in mid-ocean they are barely detectable with wavelengths spanning hundreds of kilometers.
The primary impact of these waves is along the coastal shoreline, as large amounts of ocean water are cyclically propelled inland and then drawn out to sea. This can result in significant modifications to the coastline regions where the waves strike with sufficient energy.
[edit] References
- Gill, Adrian E. (1982) Atmosphere-Ocean Dynamics, Academic Press, ISBN 978-0-12-283522-3.
- Hamblin, W. Kenneth and Eric H. Christiansen (1998)
Earth's Dynamic Systems, 8th ed., Upper Saddle River: Prentice-Hall ISBN 0-13-018371-7 (8th ed.)
- Marshak, Stephen. (2001) Earth: Portrait of a Planet, New York: W.W. Norton & Company, ISBN 0-393-97423-5
- Maury, Matthew F. (1855) The Physical Geography of the Seas and Its Meteorology.
- Pinet, Paul R. (1996) Invitation to Oceanography, St. Paul, MN: West Publishing Co., ISBN 0-7637-2136-0 (3rd ed.)
- Way, John H., Hypsographic curve, http://www.lhup.edu/jway/101/101.sg/hypsographic_curve.htm Accessed 10 January 2006
- Wyban, Carol Araki (1992) Tide and Current: Fishponds of Hawai'I University of Hawaii Press :: ISBN 0-8248-1396-0
[edit] See also
[edit] External links
- Oceanography Image of the Day, from the Woods Hole Oceanographic Institution
- NASA Oceanography
- Ocean World (digital book)
- Pacific Disaster Center
- Pacific Tsunami Museum Hilo, Hawaii
- Science of Tsunami Hazards (journal)
- NEMO academic software for oceanography
