The tidal effect, as its known, doesn't just affect water. There's an Earth tide as well, where the solid Earth changes its shape directly due to the pressures of the Sun and the Moon. But that's not as noticeable as what happens in the ocean. Anything in the universe that has mass also has its own gravitational field. Sometimes, in the case of humans, that gravitational field is so tiny that they're irrelevant to our everyday lives.
But when the mass starts increasing, changes start to take place. The Earth, for example, has enough of a gravitational field to keep things on the ground, and to keep the Moon rotating around the planet. The Moon, in turn has its own gravitational field. This field is strong enough to create a tug on the Earth's oceans, and because the Moon is in rotation around the Earth, the strength of this tug varies by location and time of day.
The Moon is mostly responsible for high tide, when there's more water in areas, and low tide, when there's less. The Moon is the biggest player in creating tides, but it's not the only planetary body involved.
There's also the body with the biggest gravitational pull in the solar system, the Sun. Even though its closeness to Earth means the Moon has the bigger impact, the Sun's affect on tides is noticeable.
During new, or full, moons, the Earth, Moon, and Sun are all in alignment. That alignment allows all of those gravitational forces to join together, creating stronger tides known as spring tides. They're not associated with the Spring season at all as they occur every month. But no alliance can last forever. At the LS, strong diurnal and semidiurnal ITs are generated when barotropic tides flow over the double ridges Wang et al. Coherent ITs are phase-locked with barotropic tides at the generation site.
Variability of coherent ITs is primarily explained by spring-neap cycles in barotropic tides. During their propagation, incoherence grows and ITs lose coherence to surface tides 3 , 4 , 17 , Eich et al. Many scales of motions, such as large-scale circulations, near-inertial waves and mesoscale eddies, are active in the SCS, which induce the complex background currents and stratification in the SCS and further modulate the incoherent feature of ITs 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , Xu et al.
They also suggested that semidiurnal ITs are more incoherent than diurnal ITs 3. Liu et al. Cao et al. ITs produce disturbances to sea surface height SSH. Therefore, ITs can be detected by the altimeter But altimeters can only detect the coherent ITs While incoherent ITs play important roles in cascading tidal energy to turbulent mixing. High-mode ITs have large current shear, which eventually break and drive turbulent mixing Coherent ITs propagated thousands of kilometers without significant loss of energy.
Coherent ITs appear to have weak dissipation, therefore they can hardly participant in ocean mixing Incoherent ITs make contributions to tide-induced mixing Investigating coherent and incoherent features of ITs in the northern SCS can not only provide better understanding of ITs, but also make contributions to the improvement of parameterization of ocean turbulent mixing in the northern SCS.
In this study, month from May to March moored current observations are used to investigate variability of coherent and incoherent ITs in the north SCS. Then simulations based on the Massachusetts Institute of Technology General Circulation Model MITgcm are performed to better understand the coherent and incoherent variations. The paper is organized as follows. Water depth of mooring place is 2, m. The velocity data were used for analysis.
The velocity data were interpolated onto uniform levels with 5 m intervals. The hourly averaged data from May to March were examined in this study.
Mooring position and topography of the northern SCS are shown in Fig. The barotropic currents can hardly be represented by depth-averaged currents because of the limitation of observation which do not cover the whole water depth. As a result, the barotropic tidal currents were extracted from the regional solution for the China Sea of the Oregon State University inverse barotropic tidal model OTIS The baroclinic currents were calculated by removing the barotropic currents from the raw currents.
Based on the least square method, harmonic analysis was applied to baroclinic currents to separate tidal components of diurnal K 1 , O 1 , P 1 , Q 1 and semidiurnal M 2 , S 2 , N 2 , K 2 ITs, which were coherent with surface tides at the LS. A fourth-order Butterworth filter was introduced to time series of baroclinic currents at diurnal 0. Incoherent ITs were obtained by subtracting the coherent ITs from the band pass filtered baroclinic tidal currents.
MITgcm ocean model is used in this study to investigate the feature of ITs and the influence of mesoscale eddies on the incoherent signals. In the vertical direction from 0 m at the top to 5, m at the bottom , there are 60 uneven vertical layers Fig. The initial temperature and salinity profiles are derived from the monthly mean climatology of Generalized Digital Environmental Model, version 3 GDEMv3.
The initial fields are set to be horizontally homogeneous using temperature and salinity at site The model is forced by barotropic tidal currents at the open boundaries.
Diurnal and semidiurnal ITs are simulated separately. For the diurnal ITs, only the dominant K 1 and O 1 are considered. For the semidiurnal ITs, the M 2 and S 2 are taken into consideration. The amplitudes and phases of these constituents are extracted from OTIS.
For eddy-tide simulations which simulate both a mesoscale eddy and ITs, the initial fields are described according to Zhang et al. The internal tidal energy is calculated according to Wang et al. When the simulation is stable, the simulated energy in the model is no longer varying, and TEN and ADV can be negligible. The equation can be approximated as. Distributions of baroclinic current velocities at diurnal and semidiurnal frequency bands are displayed in Fig.
Diurnal ITs follow obvious spring-neap cycles, which has strong day periodic variations. While semidiurnal ITs nearly lose that feature in most of the observation period. In vertical, within the ADCP observing depths, diurnal baroclinic tidal currents tend to be obviously surface-intensified with larger velocities appearing above m depth. But surface-intensification of semidiurnal ITs is not as obvious as that of diurnal ITs at the mooring site.
Distributions of baroclinic current velocities at a diurnal and b semidiurnal frequency bands at the mooring site. Vertically averaged KE of coherent and incoherent signals for diurnal and semidiurnal ITs are shown in Fig. The KE of corresponding barotropic tidal currents at the LS The results indicate that variability of both coherent diurnal and semidiurnal ITs can be largely explained by the barotropic tidal forcing at the LS. Incoherent signals of both diurnal and semidiurnal ITs are not phase-locked to the surface tides and exhibit intermittent behaviors.
Time series of vertically averaged coherent and incoherent baroclinicKE blue lines, orange lines present period-smoothed KE at diurnal and semidiurnal frequency bands at the mooring stie. Time series of vertically averaged diurnal and semidiurnal barotropic KE blue lines, orange lines present period-smoothed KE at the LS KE of coherent semidiurnal ITs raises to 2.
Ratios of coherent baroclinic KE to barotropic KE are 2. It is interesting to note that the strength of coherent semidiurnal ITs at the mooring site is obviously amplified. In other words, variations of baroclinic tides will obviously cause the corresponding changes of coherent signals.
Therefore, the increase of coherent intensity can be explained by the amplification of baroclinic tides. The internal wave regime is classified by the parameter of criticality CR. CR is a nondimensional parameter, which represents the possibility of generating of ITs. ITs are more likely to generate in regions with CR larger than 1. The topography is more favor of generating ITs in the diurnal frequency bands than those in the semidiurnal frequency bands, which is incapable of explaining the amplification of semidiurnal ITs.
Therefore, to better understand those variations, numerical modellings based on MITgcm are used to simulate the diurnal and semidiurnal ITs respectively. Simulations are conducted with stratification of January and July, which are regard as winter and summer runs respectively. Figure 7 shows comparison of simulated amplitudes, which are averaged of winter and summer runs, and observed results of eastward velocities of M 2 , S 2 , K 1 and O 1 at the mooring site.
Table 1 displays averaged differences vertical mean absolute errors of amplitudes between observations and simulations. For semidiurnal ITs, errors of S 2 are smaller than those of M 2. For diurnal ITs, the simulated amplitudes of K 1 is weaker than observation and making the difference larger than other constituents. Whereas the simulated O 1 is much closer to observation. The enhancement of K 1 has been observed by previous investigations, which can be attributed to the intrusion of Kuroshio In our simulation, the Kuroshio are not involved in the model, which probably leads to the larger error of K 1.
Winter and summer runs averaged eastward baroclinic tidal amplitudes of simulations black asterisk as well as those of observations blue line at frequency of a M 2 and b S 2 c K 1 and d O 1 at the mooring site. According to our simulated results, during the period of spring-tide in winter,there are In summer, generation rates for diurnal and semidiurnal ITs are Energy generated are obviously larger than those dissipated at the LS, therefore intensity of baroclinic tides are dominated by the conversion.
Our simulated baroclinic energy budget is similar to Alford et al. Idealized twin experiments are carried out to further investigate the difference between diurnal and semidiurnal ITs.
Idealized experiment 1 IE1 is designed with a single east ridge. In IE1, depth in region of the west ridge is set to be 3, m. Idealized experiment 2 IE2 is carried out with a single west ridge. In IE 2, the east ridge is removed and the depth is set to be 3, m. In IE1 and IE2, model setups are the same as those in the double-ridge simulation except for the topography. Area-integrated conversion of baroclinic tidal energy are shown in Table 2. For both diurnal and semidiurnal ITs, conversions at the east ridge of the LS IE1 are much larger than those at the west ridge IE2 , which indicates that ITs are largely generated on the east ridge.
The distribution of CR also indicates that both diurnal and semidiurnal ITs are largely generated at the east ridge owing to the larger topography gradient Fig. It is notable that the conversion rates in the double-ridge simulation are much larger than the sums of single east and single west simulations for semidiurnal ITs in both winter and summer, which can be attributed to the resonance of semidiurnal ITs at the LS 43 , At the LS, tidal waves from the opposing ridges interfere with each other, influencing the conversion of ITs.
The phase difference between the remote generated ITs and local baroclinic flow can either enhance or weaken the barotropic to baroclinic energy conversion compared to when remote generated ITs are absent Not only the semidiurnal ITs, but the diurnal ITs can also be affected by interference.
However, interference of diurnal ITs at the LS is rarely discussed. Results show that in both winter and summer, conversions of diurnal internal tidal energy in the double-ridge cases are much smaller than the sums of single ridge cases, suggesting interference of diurnal ITs weakens the generation of diurnal ITs.
Figure 8 shows density perturbation and vertical barotropic velocity at the depth of m for both diurnal and semidiurnal ITs in the central of the LS The conversion of ITs is largely governed by the amplitude of density perturbation and the phase difference between density perturbation and vertical barotropic velocity. For diurnal ITs, the interference decreases the conversion by weakening the amplitude of density perturbation and enlarging the phase difference.
Whereas, the resonance of semidiurnal ITs enhances the conversion mainly by engendering the density perturbation more in phase with the local barotropic tide. Interference weakens the conversion of diurnal ITs but strengthen the generation of semidiurnal ITs, which leads to the amplification of coherent semidiurnal baroclinic tides. Density perturbation and vertical barotropic velocity at the depth of m in IE1 of winter run for a diurnal and b semidiurnal ITs, as well as those c , d in the double-ridge simulation of winter run.
Semidiurnal ITs are more incoherent than diurnal counterparts within the observation period. However, those differences of incoherence are rarely investigated and the underlying mechanism remains unclear.
Incoherence of ITs can be influenced by background currents and stratification during propagation. Incoherent ITs are extracted from simulations of both winter and summer runs: Harmonic analysis was applied to simulated currents to obtain coherent signals for ITs.
Band-pass filter was introduced to time series at diurnal and semidiurnal bands. Simulated results of last 10 days are used to extract the incoherent ITs. To avoid errors from band pass filter, results at the two ends 36 h for each end are not considered for investigation, only results in the 7 middle days are used.
Table 3 shows simulated incoherence of diurnal and semidiurnal ITs at the mooring site. Incoherent ITs remain weak and vary slightly from winter and summer runs, indicating that variation of vertical stratification or surface tide forcing has little effect on the incoherent signals of ITs.
Following the pull of gravity, ocean water moves from the built-up areas of high pressure down to the valleys of low pressure. But as the water moves from hills to valleys, it does so in a curved trajectory, not a straight line.
On Earth, movement in a straight line over long distances is harder than it may seem. From our perspective, stationary objects are just that, unmoving. It also influences the movement of ocean currents. Scientists refer to this bending as the Coriolis Effect. It is easiest to understand this phenomenon when thinking about travel in a northern or southern direction. As you get closer and closer to the poles, the distance traveled in one rotation gradually shrinks until it reaches zero at either pole.
Therefore, an object on the surface will gradually spin slower the closer it gets to a pole. But leave the surface of the planet, and the anchor keeping you in sync with the land beneath you disappears. Any moving object plane, boat, hot air balloon, water will begin its travels at the rotating speed of the location where it took off from. If it should travel north or south, the ground beneath it will be traveling at a different speed. Travel North from the Equator, and the ground will gradually spin slower beneath you.
This causes an object attempting to travel in a straight line to veer to the right in the Northern Hemisphere and veer to the left in the Southern Hemisphere relative to the direction traveling. Understanding how the rotating Earth affects movement to the west or east is a bit trickier.
Envision an elastic string attached to a ball on one end and an anchored point at the other. The faster the ball is spun around the anchor, the more the elastic stretches and the farther the ball travels from the center point. An object traveling on Earth behaves the same way.
If the object moves east, in the direction that Earth is spinning, it is now traveling around the axis of Earth faster than it was when it was anchored—and so, the object wants to move out and away from the axis. Still tethered by gravity, the object does so by moving toward the equator, the place on Earth that is the greatest distance from the axis.
It does so by moving toward the pole. This again appears as a bend to the right in the Northern hemisphere and to the left in the Southern hemisphere. In the Northern Hemisphere, surface water curves to the right and in the Southern Hemisphere it curves to the left of the direction it is forced to move.
There are 5 major gyres—expansive currents that span entire oceans—on Earth. Similar to surface waters, Northern gyres spin clockwise to the right while gyres in the south spin counterclockwise to the left. The center of the gyres are relatively calm areas of the ocean. The Sargasso Sea, known for its vast expanses of floating Sargassum seaweed, exists in the North Atlantic gyre and is the only sea without land boundaries. Today, gyres are also areas where marine plastic and debris congregate.
The most famous one is known as the Great Pacific Garbage Patch , but all five gyres are centers of plastic accumulation. Wind moving across the ocean moves the water beneath it, but not in the way you might expect. The Coriolis Effect, the apparent force created by the spinning of Earth on its axis, affects water movement, including movement instigated by wind. Recall that Coriolis causes the trajectory of a moving object to veer to the right or the left depending upon the hemisphere it is located in.
Wind blowing over water will move the ocean water underneath it in an average direction perpendicular to the direction the wind is traveling.
As wind blows over the surface layer of water, friction between the two pulls the water forward. The top most layer of water will bend away from the direction of the wind at about 45 degrees.
For simplicity, we will assume that this scenario is in the Northern Hemisphere and all movement bends to the right. As the top layer of water begins to travel, it in turn pulls on the water layer beneath it, just as the wind had. Now this second water layer begins to move, and it travels in a direction slightly to the right of the layer above it. This effect continues layer by layer as you move down from the surface, creating a spiral effect in the moving water. In addition to a change in direction, each sequential layer down loses energy and moves at a slower speed.
Friction causes the water to move, but drag resists that movement, so as we travel from the top layer to the next, some of the energy is lost. When all the layers down the spiral are accounted for, the net direction of the water is perpendicular to the direction of the wind.
The ocean is connected by a massive circulatory current deep underwater. This planetary current pattern, called the global conveyor belt , slowly moves water around the world—taking 1, years to make a complete circuit. It is driven by changes in water temperature and salinity, a characteristic that has scientists refer to the current as an example of thermohaline circulation. Saltier and colder water is heavier and denser than less salty or fresher , warmer water. Around the globe there are areas where the heat and saltiness of ocean water and therefore, its density change.
The most important of these areas is in the North Atlantic. As warm Atlantic water from the Equator reaches the cold polar region in the North via the Gulf Stream, it rapidly cools. This region is also cold enough that the ocean water freezes, but only the water turns to ice. As the water freezes it leaves the salt behind, causing the surrounding water to become saltier and saltier. The cold, salty water then sinks in a mass movement to the deep ocean. It is this sinking that is a main driver for the entire deep-water circulation system that moves massive quantities of water around the globe.
Cooling also occurs near Antarctica, but not to the extremes that happen in the Northern Hemisphere. In this area, evaporation is the main driver that changes the salinity of the ocean water. As water in the Mediterranean evaporates, it leaves the salt behind. This super salty ocean water then bleeds into the Atlantic via the thin mouth of the Mediterranean, also known as the Strait of Gibraltar. When cold, salty water circulates the globe and gradually becomes warmer, it begins to rise.
Scientists worry that the melting ice caused by global warming may weaken the global conveyer belt by adding extra fresh water in the Arctic. A study found that the massive ocean current that courses around the Atlantic Ocean, called the Atlantic Meridional Overturning Circulation, has decreased in strength by about 15 percent since AD and is now the weakest it has been in 1, years. Ironically, despite an overall increase in global temperatures, many places in North America and Europe may get colder as a result.
Not all currents occur at such a large scale. Individual beaches may have rip currents that are dangerous to swimmers. Rip currents are strong, narrow, seaward flows of water that extend from close to the shoreline to outside of the surf zone. Rip currents are formed when there are alongshore variations in wave breaking. In particular, rip currents tend to form in regions with less wave breaking sandwiched between regions of greater wave breaking. This can occur when there are gaps in sand bars nearshore, from structures like piers or jetties, or from natural variations in how waves are breaking.
Rip currents can move faster than an Olympic swimmer can swim, at speeds as fast as eight feet 2. At these speeds, a rip current can easily overpower a swimmer trying to return to shore.
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