Density Differences. Fluctuations in both temperature and salt content lead different regions of ocean water to have different densities. Higher temperatures, such as near the equators, cause a given mass of water to expand and therefore drop in density.
Also, lower salt content causes a given mass of water to be lower in density. Gravity causes the more dense water to fall, pushing away the less dense water, which shoots sideways and rises. Giant convection loops of ocean currents form as the lighter hotter, less salty regions of water rise and flow to replace the heavier colder, more salty regions of water. The effect of density-driven currents is fundamentally a result of the interplay heating from the sun, earth's gravity, and salinity differences.
Differences in the gravitational field of the moon from one location to the next causes tidal forces. Differences in the gravitational field of the sun also causes tidal forces.
Tidal forces push water towards the axis connecting the earth and moon, and the axis connecting the earth and sun. The net change in the angle between the shallowest bin and the deepest bin, or through the wind driven layer, was 0. The estimates of standard error for the bins do not overlap, indicating a statistically meaningful description of the velocity spiral has been achieved. Using an ensemble-mean m velocity field derived from the motion of a large number of drifting buoys in the tropical Pacific between and and removing a climatological mean geostrophic velocity, a field of ageostrophic velocity was derived.
A comparison of the ageostrophic velocity with the local mean wind stress shows deflection to the right left in the Northern Hemisphere Southern Hemisphere. The ageostrophic current was clearly recognizable as a current derived from almost any dynamical model of Ekman balance in which the turbulent stress decayed with depth. A series of dynamical models were fit to this data in the least square sense with the intention of identifying the physical process that leads to the development of the turbulent mixing in the subtropical Pacific.
The results of the analysis showed that the wind-driven layer on the long-term mean does not move as a uniform slab within the layer of uniform temperature. The data supports, in general, that the strongest ageostrophic currents occur in regions of strongest winds, closest to the equator.
A model of the turbulent, nonstratified Ekman layer Caldwell et al. A model in which the wind creates as strong a current as possible so that the Richardson number of a mixing layer in a weakly stratified upper ocean remains marginally below unity, as proposed by Pollard et al. In Ekman layer theories, the vertical scale of the decay of the current strength with depth is the same as the vertical scale of the rotation of the vector with depth. A statistically significant Ekman spiral shows there is a rotation of a slablike velocity field throughout the upper ocean.
This examination of the locally wind-driven flow relied on the assumption that the near-surface velocity field can be decomposed into a geostrophic component and a wind-driven component. Further work that relies of the use of the satellite sea-level variations for estimating time-dependent geostrophic currents would be valuable in understanding time-dependent models of the surface currents.
Citation: Journal of Physical Oceanography 29, 9; The constants found for the linear regression models [Eq. The set a,b,c represents the exponents for the friction velocity, the Coriolis parameter, and the depth of the thermocline. See the text for a description of the seven different models.
Sign in Sign up. Advanced Search Help. Journal of Physical Oceanography. Sections Abstract 1. Introduction 2. Data 3. Ageostrophic velocity 4. The Ekman spiral 5. Summary and discussion. Export References. View in gallery Ageostrophic velocity magnitude ordinate vs modeled velocity absicca. View raw image Ageostrophic velocity magnitude ordinate vs modeled velocity absicca. Chart I. Tracks of Centers of Anticyclones, December, Author: P.
Next Article. Editorial Type: Article. Wind-Driven Currents in the Tropical Pacific. Ralph 1 and Pearn P. Niiler 1. CO;2 Page s : — Article History. Received: 02 Jul Published Online: Sep Download PDF. Full access. Email: llo d. Introduction The momentum balance of large spatial scale, time-mean near-surface circulation of the ocean is between the Coriolis force, pressure gradient, and the vertical convergence of the turbulent stress due to the winds.
Under the fair-weather conditions that typically dominate the tropical Pacific, upper-ocean stratification is expected to play a role in the determination of the thickness and structure of the wind-driven Ekman layer.
Ageostrophic velocity The horizontal distribution of the ageostrophic velocity relative to the wind stress shows a pattern of deflection to the right left of the wind stress in the Northern Hemisphere Southern Hemisphere Fig.
First, consider a model in which there is a layer H in which the turbulent stress is a linear function of depth, vanishing at its base. This is a model of Ekman balance suggested by Pollard and Millard in a study of wind-driven inertial motions in the mixed layer. The data in Fig. To examine this parameter dependence further, the amplitude of the ageostrophic current is considered first. The Ekman spiral The most remarkable result of the ageostrophic velocity computation was that, at every location, it was directed to the right of the wind stress in the Northern Hemisphere and to the left in the Southern Hemisphere Fig.
Summary and discussion Using an ensemble-mean m velocity field derived from the motion of a large number of drifting buoys in the tropical Pacific between and and removing a climatological mean geostrophic velocity, a field of ageostrophic velocity was derived. Download Figure Download figure as PowerPoint slide. Table 1.
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Plastic is ubiquitous in our everyday lives. Some plastics we can reuse or recycle—and many play important roles in areas like medicine and public safety—but other items, such as straws, are designed for only one use. In fact, more than 40 percent of plastic is used only once before it is thrown away, where it lingers in the environment for a long, long time.
It often breaks down into smaller and smaller particles, called microplastics, which can be ingested by both animals and people.
Fortunately, there are things we can do to help—like stop using plastic bags, straws, and bottles, recycling when we can, and disposing of waste properly. Use these classroom resources to teach about ocean plastics and check back for more coming later this year!
Scientists across the globe are trying to figure out why the ocean is becoming more violent and what, if anything, can be done about it. Ocean currents, including the ocean conveyor belt, play a key role in determining how the ocean distributes heat energy throughout the planet, thereby regulating and stabilizing climate patterns.
A current is the steady, predictable movement of a fluid within a larger body of that fluid. Fluids are materials capable of flowing and easily changing shape. A gyre is a circular ocean current formed by the Earth's wind patterns and the forces created by the rotation of the planet. Join our community of educators and receive the latest information on National Geographic's resources for you and your students.
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