Employees in the News. Emergency Management. Survey Manual. The oceans cover about 70 percent of the Earth's surface, and that about 97 percent of all water on and in the Earth is saline—there's a lot of salty water on our planet. Find out here how the water in the seas became salty.
Why is the ocean salty? Rivers discharge mineral-rich water to the oceans. One way minerals and salts are deposited into the oceans is from outflow from rivers, which drain the landscape, thus causing the oceans to be salty.
You may know that the oceans cover about 70 percent of the of Earth's surface, and that about 97 percent of all water on and in the Earth is saline—there's a lot of salty water on our planet. By some estimates, if the salt in the ocean could be removed and spread evenly over the Earth's land surface it would form a layer more than feet meters thick, about the height of a story office building Source: NOAA. But, where did all this salt come from? If you get into folk stories and mythology you will see that almost every culture has a story explaining how the oceans became salty.
The answer is really very simple. Salt in the ocean comes from rocks on land. Here's how it works The rain that falls on the land contains some dissolved carbon dioxide from the surrounding air.
This causes the rainwater to be slightly acidic due to carbonic acid. The rain physically erodes the rock and the acids chemically break down the rocks and carries salts and minerals along in a dissolved state as ions. The ions in the runoff are carried to the streams and rivers and then to the ocean. Many of the dissolved ions are used by organisms in the ocean and are removed from the water.
Others are not used up and are left for long periods of time where their concentrations increase over time. The two ions that are present most often in seawater are chloride and sodium. By the way, the concentration of salt in seawater salinity is about 35 parts per thousand. In other words, about 35 of 1, 3. Land masses have a higher emissivity than the ocean, so any measurement close to land tends to be skewed by its brightness.
Over time, the Aquarius research team should be able to calibrate the measurements and develop mathematical tools to better distinguish the salt signal. But for now, the measurements are so new that the team is still working on the big picture of ocean salinity. Aquarius is the first NASA instrument specifically designed to study surface ocean salinity from space, and it does so at a rate of , measurements per month.
It uses three passive microwave sensors, called radiometers, to record the thermal signal from the oceans' top 10 millimeters about 0. Most global precipitation and evaporation events take place over the ocean and are very difficult to measure. Animation by Robert Simmon. One year after its launch, the Aquarius instrument is giving ocean sciences its first global view of sea surface salinity.
Image of the Day Water. Examining temperatures from the depths of the ocean, JPL scientists have found that lower layers of the Western Pacific and Indian Oceans grew much warmer during a decade when surface temperatures cooled. Image of the Day Heat Water. Warren , B. Evolution of Physical Oceanography , B. Warren and C. Wunsch, Eds. Weaver , A. Bitz , A. Fanning , and M. Holland , : Thermohaline circulation: High-latitude phenomena and the difference between the Pacific and Atlantic.
Earth Planet. Wills , R. Schneider , : Stationary eddies and the zonal asymmetry of net precipitation and ocean freshwater forcing. Young , W. Zhang , R. Vallis , : The role of bottom vortex stretching on the path of the North Atlantic western boundary current and on the northern recirculation gyre. Surface salinity anomaly, referenced to 35 psu, for zonally symmetric surface forcing in the top W2N geometry and bottom W3N geometry.
Thick gray lines represent boundaries. Contours are every 0. The dashed lines show the freshwater flux used to induce sinking in the wide basin in the W2N geometry, and the dotted lines show the freshwater flux used to induce sinking in the wide basin in the W3N geometry. Where these lines split, the upper dashed—dotted line is applied to the narrow basin and the lower one is applied to the wide basin. For wide sinking, the wide-basin freshwater flux is reduced by 0.
In all plots, the reentrant channel region left of the thick black vertical line shows the total streamfunction integrated over all longitudes. Meridional transport, zonally and vertically integrated within each sector above the isopycnal b m for top W2N and bottom W3N.
The solid lines are for narrow sinking, and the dashed lines are for wide sinking. The gray box marks the location of sinking. For tracer buoyancy, the depth of b m is still calculated using the salinity.
The isopycnal b m outcrops in the white area. The isopycnal b m outcrops in the yellow area. The gray box in a and d marks the location of sinking.
Streamfunction Sv associated with U gyre and V gyre. Contours are every 5 Sv. Arrows are not plotted if the magnitude of the depth-integrated velocity is less than 0. In a , the blue solid line and the blue dashed line are on top of each other. The surface salinity in the North Atlantic controls the position of the sinking branch of the meridional overturning circulation MOC ; the North Atlantic has higher salinity, so deep-water formation occurs there rather than in the North Pacific.
Here, it is shown that in a 3D primitive equation model of two basins of different widths connected by a reentrant channel, there is a preference for sinking in the narrow basin even under zonally uniform surface forcing.
The southward western boundary current associated with the wind-driven subpolar gyre has higher velocity in the wide basin than in the narrow basin.
It overwhelms the northward western boundary current associated with the MOC for wide-basin sinking, so freshwater is brought from the far north of the domain southward and forms a pool on the western boundary in the wide basin.
The fresh pool suppresses local convection and spreads eastward, leading to low salinities in the north of the wide basin for wide-basin sinking. This pool of freshwater is much less prominent in the narrow basin for narrow-basin sinking, where the northward MOC western boundary current overcomes the southward western boundary current associated with the wind-driven subpolar gyre, bringing salty water from lower latitudes northward and enabling deep-water mass formation. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy www.
In the current climate system, deep water is formed in the North Atlantic but not in the North Pacific, resulting in a meridional overturning circulation MOC that transports heat northward everywhere in the Atlantic. Deep-water formation in the North Pacific is prevented by fresh, buoyant surface waters, much fresher than surface waters at comparable latitudes in the Atlantic Warren ; Broecker , with the salinity component dominating over the temperature component the colder and fresher North Pacific surface water is lighter than the warmer and saltier North Atlantic surface water.
Several reasons have been advanced for the salinity difference between the Atlantic and the Pacific [see Weaver et al. Craig et al. The North Pacific experiences slightly higher precipitation because of the more effective orographic blockage of moist air in the Pacific sector Broecker et al.
However, Kamphuis et al. This implies that the orographic blockage of moisture transport by the American continent alone does not set the location of deep-water formation. The northward transport of moisture by the Asian monsoon Geay et al. The precipitation footprint of water evaporating from the narrow Atlantic basin extends into the wider Pacific basin, while most of the water evaporating from the Pacific precipitates in the same sector Ferreira et al.
In addition, a larger fraction of the area of the Atlantic is exposed to dry air coming off the continents, which may also increase evaporation over the Atlantic Schmitt et al.
Some of the geographical differences between the two ocean basins are thought to favor deep-water formation in the North Atlantic. Most obviously, the Atlantic extends farther north than the Pacific, and it is more connected to the Arctic, where cold dense water is formed during winter Warren Mixing with the outflow from the Mediterranean Sea a semienclosed basin with net evaporation may increase the salinity of the northward branch of the MOC Reid ; Warren , although the importance of this process has been questioned McCartney and Mauritzen ; Talley The larger width of the Pacific, and associated stronger wind-driven circulation and east—west temperature contrasts, may produce a larger poleward heat transport by the gyres in the Pacific relative to the Atlantic, removing the need for a MOC-mediated heat transport Wang et al.
The lower-latitude position of the tip of South Africa relative to the tip of South America is a favorable configuration for transporting high-salinity water from the Indo-Pacific sector into the Atlantic Reid ; Gordon et al.
Finally, the surface branch of the MOC advects higher-salinity waters from the subtropics to the high latitudes, further enhancing the salinity in the North Atlantic relative to the North Pacific. At least some of the explanations for a saltier Atlantic rely on the Atlantic meridional overturning circulation AMOC , which is itself enabled by increased salinity in the North Atlantic.
Salt and heat advection feedbacks act to maintain the MOC and higher salinity and temperature in the region where the deep-water formation takes place. Higher temperatures decrease the surface density and thus tend to reduce the overturning rate, that is, a negative feedback.
Higher salinities increase the surface density and thus directly increase the overturning rate, while higher temperature tends to decrease the overturning rate. In principle, the salt advection feedback could act to produce deep overturning in the Pacific [Pacific meridional overturning circulation PMOC ] rather than in the Atlantic. However, both coupled Ferreira et al. Huisman et al. It is important to understand the reasons why the Atlantic is saltier than the Pacific in order to predict the fate of the MOC under global climate change.
Even if atmospheric transport were the most important factor today, this may change in the future because of greenhouse gas forcing Seager et al. In this study, we focus on one of the more obvious asymmetries between the Atlantic and the Pacific: the difference in basin widths. Specifically, we examine the consequences of this geometrical asymmetry on the oceanic flow and its repercussions on the salinity distribution.
The impacts of atmosphere-only processes or atmosphere—ocean feedbacks on the asymmetry in salinity distribution between the basins have been examined using ocean—atmosphere models e. These processes are excluded here by applying fixed surface forcings.
Few studies on the location of deep-water formation have considered the role of the wind-driven gyres. Warren attributes low salinities in the North Pacific to the small rate of northward flow between the subtropical and subpolar gyres, but he does not consider a scenario with deep-water formation in the North Pacific.
An important result relevant for this work is that the transport of the MOC is essentially independent of the sinking location Jones and Cessi This is because the MOC transport is determined by three sources: the northward Ekman transport entering the basins from the Southern Ocean, minus the southward eddy thickness transport exiting the same region, plus the global diapycnal upwelling into the upper branch of the MOC.
All these sources add up to the total sinking, regardless of its location. Upon entering the sinking basin, the northward flow of the upper branch of the MOC forms a western boundary current, with a velocity that is independent of the sinking location.
This western boundary current is superimposed on the western boundary current associated with the wind-driven gyres. In the simplified geometry presented here, the MOC northward velocity is larger than the southward SPG western boundary current in the narrow basin with width similar to the Atlantic but not in the wide basin with width similar to the Pacific. The net result is that salty water from the subtropical gyre is carried into the western portion of the SPG when sinking occurs in the narrow basin but not when sinking occurs in the wide basin.
Instead, in the wide basin, the southward western boundary current in the SPG brings freshwater from the far north, where the freshwater flux is maximum. The resulting fresh pool suppresses local deep-water formation, and the faster zonal velocity efficiently spreads the freshwater eastward, causing the water in the SPG of the wide basin to become fresher than the water in the SPG of the narrow basin. This process, diagnosed in the 3D model experiments, is documented in section 2. In section 3 , a 2D advection—diffusion model of the upper branch of the MOC is used to explore the salinity distribution for various flow fields and zonal arrangements of deep-water formation.
Further idealized experiments with the same 2D model show that neither the MOC western boundary current alone nor the wind-driven gyres alone can produce different salinity fields based on the basin width. However, salt advection by the combined velocity fields, and the associated feedback on deep-water formation, selects the narrow basin as the preferred deep-water mass formation site.
Section 4 provides a summary and draws conclusions. Citation: Journal of Physical Oceanography 47, 11; The Redi tensor is tapered to horizontal diffusion in regions of weak stratification, as described by Danabasoglu and McWilliams Each simulation was run for at least years, until equilibrium was reached.
Additional details of the model configuration are given in Jones and Cessi Two configurations are considered in the 3D model: one in which the wide basin is twice as wide as the narrow basin W2N and one in which the wide basin is 3 times as wide as the narrow basin W3N. These geometries are shown in Fig. Under zonally uniform forcing, deep-water formation occurs in the narrow basin only, regardless of the initial condition, and the surface salinity in the SPG of the narrow basin is higher for W3N than for W2N.
Sinking in the wide basin can be coerced by reducing the freshwater flux at the northern end of the wide-basin sector, while compensating this reduction by a uniform freshwater flux increase everywhere else.
A larger asymmetry in freshwater flux is needed to force wide sinking in the W3N geometry than in the W2N geometry see the bottom panel of Fig. For both W2N and W3N, the wide sinking state reverts to narrow sinking when the forcing is slowly over 20 years returned to zonal symmetry.
In summary, wide sinking is unstable under zonally uniform freshwater forcing. In practice, this isopycnal contour is chosen to pass as close as possible through the maxima of both the deep overturning cell in the sinking basin and the shallow overturning cell in the nonsinking basin. The upwelling across this isopycnal contour is approximately fixed by wind stress in the Southern Ocean plus eddy transport of buoyancy and global diapycnal diffusion Gnanadesikan ; Allison ; Jones and Cessi , setting the cross-equatorial northward transport of the upper branch of the MOC in the sinking basin to approximately 11 Sv regardless of the location of sinking Fig.
This cross-equatorial transport, augmented by the diapycnal upwelling across b m in the Northern Hemisphere of the sinking basin, determines the maximum transport of the MOC.
In the nonsinking basin, diffusive upwelling feeds a shallow cell in the Northern Hemisphere and an abyssal cell mostly in the Southern Hemisphere. The upwelled water flows northward, sinking to about m at high northern latitudes, and then returns southward.
The meridional transport in the nonsinking basin integrated zonally and above b m is shown in Fig. The numerical simulations indicate why wide sinking is unstable when the surface freshwater flux is symmetric: Fig. The values in the sinking region of the wide basin red dashed lines in the gray box of Figs.
In other words, it appears that it is the zonal asymmetry in freshwater forcing that keeps the wide basin slightly saltier than the narrow basin.
Without this asymmetry, the salinities of the basins reverse and narrow sinking occurs. To quantify how the salinity might be distributed under zonally uniform forcing for wide sinking, we advect and diffuse a passive tracer with the velocity, diffusivity, and convective adjustment time series from the wide sinking state. Unlike salt, the tracer is forced with the zonally uniform surface flux given by the solid line in Fig.
The resulting tracer field vertically averaged above b m is shown in the bottom panel of Fig. Compared to the salinity anomaly middle panel of Fig.
To make a more quantitative comparison between the three cases, it is useful to examine the salinity and tracer concentrations averaged above b m and then zonally averaged. Figure 6 shows that, for wide sinking, the tracer concentration anomaly at the latitudes of sinking is larger in the narrow basin than in the wide basin cf.
At high latitudes, the temperature is approximately independent of the location of sinking and thus does not contribute directly to the preference for narrow over wide sinking. However, at the sinking latitudes, the temperature of the sinking basin is slightly higher than the temperature of the nonsinking basin, so it partially counteracts the effects of salinity on the buoyancy.
Consequently, when the salinity in the sinking region is only marginally larger than the salinity in the nonsinking basin, the negative temperature advection feedback destabilizes the wide sinking state.
The buoyancy is displayed in the top panels of Fig. For wide sinking, the buoyancy in the sinking region is lower than at the same latitudes of the narrow basin cf.
In the bottom panels of Fig. For wide sinking, the tracer buoyancy at high latitudes is indeed lower in the narrow basin than in the wide basin cf. This confirms that the wide sinking solution is unstable under zonally uniform surface salt flux forcing.
Salinity in turn affects the distribution of convective adjustment, 2 which is rather different for narrow and wide sinking. Convective adjustment is the main process determining the diapycnal velocity across the buoyancy b m. Thus, we use the diapycnal velocity, denoted with following the notation of Young , henceforth referred to as WRY12 , as a measure of deep-water formation.
As illustrated in Fig. For wide sinking, is confined to the eastern two-thirds of the domain, whereas for narrow sinking, it is spread throughout the whole width of the basin. This pattern reflects the zonal distribution of salinity cf. In the western third of the wide sinking basin, the surface is especially fresh, while it is relatively salty almost everywhere in the narrow sinking basin.
We now demonstrate that the contrast in the zonal distribution of salinity results from differences in the velocities near the western boundary of the sinking basin. As shown in Fig. Therefore, salinity is transported farther north into the western portion of the SPG for narrow sinking.
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