What is the average salinity of water of arabian sea

A Simulated monthly basin-average salinity. D Simulated climatological monthly lateral volume flux into the Gulf from the Indian Ocean positive and out of the Gulf to the Indian Ocean negative in the vertical overturning circulation. Consequently, the overturning circulation between the Gulf and the Indian Ocean that is associated with Ic1 and Ic2 equilibrium states have different temporal and vertical structures at the Strait.

In Ic1 equilibrium, lateral inflow to the Gulf and outflow from the Gulf have their respective maximum and minimum in February and October Figure 5D : lateral inflow is in top layers while the outflow is in bottom layers Figures 6A,B , as evidenced in situ observations Johns et al. The annual structure and direction of Ic1 overturning circulation, year-round counterclockwise inflow in top layers and outflow in bottom layers, thus represents an anti-estuarine circulation.

Figure 6. Simulated climatological February and October salinity and net lateral velocity velocity in minus velocity out into positive and out negative of the Gulf at the Strait of Hormuz in the overturning circulations associated with: Ic1 equilibrium A,B and Ic2 equilibrium C,D ; colored contour is salinity and line contour is net lateral velocity.

Unlike Ic1 simulation, lateral fluxes in Ic2 simulation have a much weak seasonality Figure 5D. More importantly, in February, flow into the Gulf is in the northern shallow Strait region and flow out of the Gulf is in the deep southern channel Figures 6C,D ; whereas in October the opposite vertical flow structure occurs.

Therefore, the annual structure and direction of Ic2 overturning circulation at the Strait, which switches between counterclockwise and clockwise patterns, represents a mixture of estuarine and anti-estuarine circulations. Furthermore, whereas the overturning circulation associated with Ic3 equilibrium is very similar to that of Ic1 equilibrium anti-estuarine circulation , the overturning circulations associated with Ic4—Ic6 equilibria have vertical structures that are similar to the Ic2 vertical structure: mixture of anti-estuarine and estuarine circulation.

Figure 7. Simulated Gulf dynamical properties of three equilibria Ic4—Ic6 intermediate between Ic1 and Ic2 equilibria. B Climatological monthly basin-average salinity. D climatological monthly lateral volume flux into the Gulf from the Indian Ocean positive and out of the Gulf to the Indian Ocean negative in the associated overturning circulation. An important follow-up question is the stability of the different dynamical structures associated with Ic1—Ic6 simulations.

A system in an equilibrium state is said to be stable if the system returns to its equilibrium state after a small, externally induced disturbance; whereas in an unstable equilibrium, natural feedbacks enhance small perturbations, and move the system away from the original equilibrium state. In the following section, we test the stability of Ic1—Ic6 equilibria to salinity perturbation in order to understand the resilience of the Gulf basin salinity to brine discharge from seawater desalination plants.

Model results show that the simulated Gulf system, when initialized with different salinities, exhibits both unstable and stable equilibrium salinity states in response to continuous seawater desalination perturbation.

Ic1 equilibrium salinity state is stable because the Gulf system returns to this state after a seawater desalination perturbation was introduced starting from either a lower— Although multiple possible equilibrium salinity states were simulated when the GARM was initialized with different salinities in Ic1—Ic6 experiments, these states are not all stable. For example, Ic2 simulation was initialized with salinity of This is because when seawater desalination perturbation was introduced D1a experiment , Gulf basin salinity continued to increase, crossing over several equilibria Ic4—Ic6 , until it reached Ic1 equilibrium salinity state—the current Gulf equilibrium salinity Figure 8A.

Whereas in D1a experiment the lateral fluxes became stronger during the adjustment process to Ic1 equilibrium salinity state, in D1b experiment the lateral fluxes became weaker during the adjustment process Figure 8B.

Figure 8. Simulated A Gulf monthly basin-average salinity in Ic1, D1a, and D1b experiments; and B the corresponding lateral volume inflow and outflow. See Table 1 for experiment design details. At the annual time scale Ic1 equilibrium salinity and net lateral flow are also stable Table 2.

However, although Gulf equilibrium salinity in D1b experiment converges to Ic1 equilibrium salinity, the standard deviation of depth-average annual mean salinity in these two experiments are different Figure 9 , indicating that brine discharge causes substantial changes in regional salinity maxima and minima.

These regional salinity changes, which has been quantified and discussed in Ibrahim and Eltahir , are very likely to have significant impact on local marine ecosystems. Accordingly, D1b experiment identifies Gulf subdomains that should be monitored for ecosystem stress due to brine discharge.

Table 2. Gulf annual mean basin-average salinity and lateral inflow in , outflow out , and net exchange fluxes a in Ic1 experiment. Figure 9. Simulated climatological, depth-average annual mean salinity and corresponding temporal standard deviation: A,B Ic1 experiment — C,D D1b experiment — Note that the large standard deviation in the northwestern Gulf is a consequence of the Tigris and Euphrates Rivers freshwater discharge into this region.

We performed non-linear regression analysis, using the exponential model fitting, and linear regression analysis to further examine whether the Gulf has reached or approaches a steady equilibrium salinity state in D1a and D1b simulations. As the first step in both regression analyses, the climatological seasonal cycle is removed from the monthly mean basin-average salinity. For D1b experiment, the robustness of the non-linear regression model is visually evident Figure 10B.

Figure Exponential model to fit basin-average salinity in D1a A and D1b B experiments. GARM simulation results from — red dots are used to build the regression model, while results from — black dots are used to test the predictive capability of the regression model.

Consequently, no statistically significant trend can be confirmed, reinforcing the conclusion that the Gulf has reached an equilibrium salinity state. The reason why Gulf equilibrium salinity in D1a simulation has not reached the steady state is because of the small magnitude of seawater desalination perturbation Table 1.

Note that D1a and D1b simulations are designed to examine the stability of Gulf equilibrium salinity state: D1a simulation shows the transition from an unstable Ic2 to the current stable Ic1 equilibrium salinity state with small desalination, and D1b simulation shows that even when the Gulf system is initialized with a higher-than-present salinity and with a large desalination perturbation, Ic1 equilibrium salinity state is restored Figure 8.

The third salinity perturbation experiment, D1c simulation, is the same as D1a simulation but a large perturbation flux is applied as in D1b. In summary, the statistical analyses of D1a and D1b simulation experiments show the existence of a single stable Gulf equilibrium salinity of about Discharge of brine into the Gulf may cause costly environmental problems to marine ecosystems.

To ensure the long-term sustainability of seawater desalination, which has grown rapidly in the last five decades, it is necessary and urgent to determine current Gulf equilibrium salinity and its response to seawater desalination perturbation. Model results from Ic1 simulation show a close agreement with multiple observational datasets including reanalysis temperature and salinity, and satellite-derived Gulf WSE, which is also directly influenced by the Gulf basin salinity and temperature.

Thus, Ic1 simulation is a high-fidelity representation of the present Gulf system. The current overturning circulation between the Gulf and the Indian Ocean is an anti-estuarine circulation lateral inflow and outflow in the low- and high-density layers, respectively with the year-round counterclockwise overturning circulation. Strong evaporation removes freshwater excessively from the Gulf surface, resulting in high salinity dense water that drive more Gulf bottom waters to the Indian Ocean.

Consequently, the anti-estuarine overturning circulation in the Gulf depends upon an adjustment of two opposite tendencies: thermal expansion, a function of Gulf heat content, and haline contraction, a function of Gulf salt content.

If a positive salinity perturbation is applied to Gulf equilibrium salinity, such as in Ic3 and D1b experiment Figures 7A , 8 , then the resulting haline contraction causes flow out of the Gulf to increase until the equilibrium salinity is restored.

Therefore, the anti-estuarine overturning circulation between the Gulf and the Indian Ocean is a dynamical upper limit, associated with a stable equilibrium salinity under the current climate.

The hydrodynamic characteristics of this dynamical upper limit i. The salt in the Gulf comes from the Indian Ocean, thus a higher-salinity state will occur only if the anti-estuarine overturning circulation between the Gulf and Indian Ocean brings more salt into the Gulf.

This circulation controls Gulf salt content; hence the circulation also controls Gulf salinity stability. Our results show that the present equilibrium salinity is stable and unlikely to change under current climate ocean and atmosphere boundary conditions.

Thus, D1b simulation shows that seawater desalination comparable to current capacity does not increase Gulf annual mean basin-average salinity from its current level of However, under a changing climate, if seawater desalination capacity in the Gulf continues to expand to meet the projected freshwater demand in the region World Bank, , then it is possible that a new equilibrium salinity may be established in the future.

However, the impact of changing future climate on Gulf equilibrium salinity is beyond the scope of this work and should be examined in future studies. HI and EE conceived and designed the experiments. HI performed the modeling experiments.

All authors contributed to the article and approved the submitted version. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

This is Contribution No. We thank Chenfu Huang for his assistance in the model data processing. Antonov, I. World Ocean Atlas , Salinity. Government Printing Office. Google Scholar. Ashkenazy, Y. Box modeling of the eastern mediterranean Sea. A Stat. Bathiany, S. Implications of climate variability for the detection of multiple equilibria and for rapid transitions in the atmosphere-vegetation system.

Brewer, P. Chemical oceanography of the Persian Gulf. Broecker, W. Does the ocean-atmosphere system have more than one stable mode of operation. Nature , 21— Budyko, M. The effect of solar radiation variations on the climate of earth. Tellus 21, — Carton, J. A simple ocean data assimilation analysis of the global upper ocean Part I: methodology. Oceanogr 30, — A reanalysis of ocean climate using simple ocean data assimilation SODA. Quadfasel , and J. Sun , C. Swift , S. Varma , K. Das , and A.

Indian J. Wang , Z. DiMarco , A. Jochens , and S. Webster , P. Yang , : Monsoon and Enso - selectively interactive systems. Wentz , F. Liu , B. Bao , A. Duan , and F. Jin , : Thermal controls on the Asian summer monsoon. Wyrtki , K. National Science Foundation , pp.

Yao , F. Formation and export of Persian Gulf water. Weller , : Objectively analyzed air—sea heat fluxes for the global ice-free oceans — The bold rectangle in a shows the region used for the average in b.

Values of DMI in c larger than 0. Vertical salinity maxima and minima psu obtained from Argo profiles during —17 and the distribution and numbers of the profiles in the northern Arabian Sea and the Gulf of Oman. Black dots in a — c showed the geographical distribution of ARGO hydrographic profiles. Temporal variability of the vertical salinity maxima and minima psu in the left Argo profiles gray dots and right seasonal cycles curves in the northern Arabian Sea and the Gulf of Oman.

Bold blue and dark curves in d — f are seasonal cycles of monthly mean during —17 and during —13, respectively. The blue lines in a and b are the mixed layer depths. As in Fig. St represents the tendency of the salinity, Fs represents the freshwater flux term, and Adv represents the total advection terms.

Salinity psu observed by Argo floats in the elliptical area near the mouth of the Gulf of Oman. The red ellipses in a and b show the elliptical area. The salinity data observed outside the elliptical area are removed in c and d. The light blue and light red wind vectors indicate the negative and positive wind curls, respectively.

A prolonged high-salinity event in the northern Arabian Sea, to the east of the Gulf of Oman, during —17 was identified based on Argo datasets. The prolonged event was manifested as enhanced spreading of the surface Arabian Sea high-salinity water and the intermediate Persian Gulf water.

We used satellite altimetric data and geostrophic current data to understand the oceanic processes and the salt budget associated with the high-salinity event. The results indicated that the strengthened high-salinity advection from the Gulf of Oman was one of the main causes of the salinity increase in the northern Arabian Sea. The changes of the seasonally dependent eddies near the mouth of the Gulf of Oman dominated the strengthened high-salinity advection during the event as compared with the previous 4-yr period: the westward shifted cyclonic eddy during early winter stretched to the remote western Gulf of Oman, which carried the higher-salinity water to the northern Arabian Sea along the south coast of the Gulf.

An anomalous eddy dipole during early summer intensified the eastward Ras Al Hadd Jet and its high-salinity advection into the northern Arabian Sea. In addition, the weakened low-salinity advection by coastal currents along the Omani coast caused by the weakened southwest monsoon contributed to the maintenance of the high-salinity event.

This prolonged high-salinity event reflects the upper-ocean responses to the monsoon change and may affect the regional hydrography and biogeochemistry extensively. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy www. The region is predominantly influenced by the monsoon and characterized by an excess of evaporation over precipitation and complex ocean circulation Fig. Then, it is transported southward and the low-salinity water masses are advected northward by the West Indian Coastal Current during winter monsoon and the western currents along the Omani coast during summer monsoon to maintain the long-term salinity balance Jensen ; Schott and McCreary ; Stramma et al.

Citation: Journal of Physical Oceanography 50, 4; During the winter monsoon October—March , the prevailing wind is the northeast monsoon, which triggers a southward coastal current near the western boundary of the Arabian Sea and cyclonic eddies off the coastal region. In the summer season April—September , the southwest monsoon prevails in the Arabian Sea, triggering strong upwelling and a negative sea surface height anomaly along the east Omani coast and an anticyclonic eddy in the off coastal regions Manghnani et al.

The upwelling occurs in the northern edge of the anticyclone, which advects low-salinity water originated in the southern Indian Ocean to the Omani coast region. Then, the upwelled low-salinity water is carried into the northern Arabian Sea and the Gulf of Oman by the coastal currents and eddies.

These currents favor the spreading of the high-salinity water from the Gulf of Oman to the northern Arabian Sea.

Observations and modeling studies have suggested that eddies play an important role in the spreading of the PGW water in the Gulf of Oman and the northern Arabian Sea Carton et al. Previous works studied the water mass and the related dynamical processes based on sparse observations obtained from the International Indian Ocean Expedition IIOE in the s Levitus ; Wyrtki and cruises after that Banse and Postel ; Varma et al.

After , the Array for Real-Time Geostrophic Oceanography Argo project provides more observations in the northern Arabian Sea to conduct detailed studies on the water mass and related ocean dynamics. They suggested that both the ocean dynamics and the atmospheric coupling can affect the salinity variation. Wang et al. In this study, we depict a prolonged high-salinity event in the northern Arabian Sea: the surface and intermediate layer high-salinity water increased in volume and the low-salinity water between them decreased in volume during —17, which caused significant positive salinity anomaly in the upper m relative to the seasonal climatology derived from the Argo data during —18 and persisted for more than three years.

We used all the Argo float profiles from December to January and the gridded Argo datasets to analyze the ocean dynamics related to the prolonged high-salinity event. Our results suggested that the changes of the seasonally dependent eddies near the mouth of the Gulf of Oman were one of the most important factors for the high-salinity event during — The remainder of this paper is as follows.

Section 2 describes the datasets and methods. The mean state and salinity anomalies in the northern Arabian Sea are presented in section 3. The dynamic processes associated with the high-salinity event and the salt budget analysis are described in section 4. Section 5 shows an example of the high-salinity event captured by two Argo floats and discusses the possible processes affecting the event. Conclusions are given in section 6. All the Argo profile data collected from December to January were used in this study to show the spatial and temporal variations of the water mass in the northern Arabian Sea.

About 69 floats have been deployed in the northern Arabian Sea, and 12 Argo profiles were collected in the region for the research period, of which 12 profiles were used in this study after quality controls.

The profile number dramatically increased after when the dataset satisfied the needs of our research on seasonal and interannual variations. The Argo floats measure temperature and salinity from a designed depth of m up to a typical surface value at 5 m and record the data at vertical resolutions of 1—5 m in the upper ocean and coarser resolutions at depths.

This product spans from to present and has 58 levels in vertical m, with a vertical resolution of 10 m in the upper and 20 m between and m. This product provides a baseline of temperature and salinity data in the upper m ocean for the estimates of the interannual to decadal variability. The dataset spans from to present with a 0.

The precipitation data span from to present with a 2. The level We use data from the same period as that of Argo data in this study. The P-vector inverse method is used to derive the four-dimensional velocities on the isopycnic surfaces based on Argo product. This method was first proposed in Chu and then was tested to be reliable in many studies Chu and Li ; Chu et al. The P-vector method calculates the absolute geostrophic circulations on the isopycnic surfaces by two steps.

The sea surface salinity SSS pattern in the tropical Indian Ocean shows a significant contrast from west to east and from south to north. The excess of evaporation over precipitation in the Arabian Sea and the subtropical southern Indian Ocean leads to the formation of the high-salinity water mass in the west and south, while the excessive precipitation and river runoffs in the Bay of Bengal and the eastern Indian Ocean lead to the low-SSS water in the east Fig.

Previous studies suggested that the ocean advection might modulate the seasonality of the SSS in the region Joseph and Freeland ; Morrison The mean circulation in the northern Indian Ocean changes from clockwise flowing during summer to anticlockwise during winter caused by the monsoon forcing Schott and McCreary , which carries the ASHSW southward and brings the low-salinity water into the northern Arabian Sea twice a year Jensen Both the salinity of the intermediate PGW and the subsurface low-salinity water show strong seasonal cycles in the northern Arabian Sea Fig.

Intense interannual variability of the water salinity occurred in the northern Arabian Sea compared to the regional air—sea freshwater flux variations Fig. Considering the changes of the typical water masses in the northern Arabian Sea, a prolonged high-salinity event was identified during —17, lasting for more than 3 years, when both the volumes of the surface ASHSW and the intermediate PGW increased, with the volume of the low-salinity water between them reduced Fig.

Using all the profiles shown in Fig. Here, based on yr Argo profiles collected since , the results showed that the salinity maxima in the surface and the minima in the subsurface can be used to illustrate the advection of the ASHSW and the low salinity. Note that Argo profiles covered most of the northern Arabian Sea, especially the northeastern region Fig. The density of Argo profiles in the Gulf of Oman was higher than that in most regions of the northern Arabian Sea, which was sufficient to detect the high-salinity advection associated with the eddies.

These two regions are characterized by an excess of evaporation over precipitation and the shallow water mass which favored the formation of the high-salinity water Fig.

The surface high-salinity water in the Gulf of Oman is saltier than that in the northeastern region. This implies that the spreading of the surface high-salinity water from the Gulf of Oman can also cause an increase of salinity in the northern Arabian Sea, although the regional freshwater flux has no significant change Fig.

In the subsurface layer, the relatively low-salinity water spreads northward to the northern coastal region through the central northern Arabian Sea Fig. Besides, there are many fragments of the high- and low-salinity signals distributed in the layers of three water masses, which implies the influence of eddies on the spreading of the three water masses Carton et al.

The seasonal cycles of the salinities in all three layers were more credible after since there were sufficient Argo observations in the northern Arabian Sea and the Gulf of Oman Fig.

Thus, in order to explain the drivers for the prolonged high-salinity event during —17, we take the conditions in —13 as the background climatology. The salinity in the three layers in —13 showed similar seasonal variability to the regional mean SSS, which indicated that the processes that impacted the surface may also impact the subsurface and the intermediate layer Figs.

Both the values of monthly mean salinity and the numbers of the values higher than twice the standard deviation dramatically increased during —17 Figs.

The mean values of the salinity seasonal cycle in the three layers during —17 was higher than that during —13 by 0. The analysis based on Argo profiles showed that a prolonged high-salinity event may indeed have occurred during —17, and it had a strong seasonal dependence.

The mouth of the Gulf of Oman is a key chokepoint for the high-salinity advection to the northern Arabian Sea. It was restricted to a near-surface layer from July to November, deepened during winter, when convective mixing occurred due to heat and freshwater loss, and then shoaled after spring Fig. During winter for —13, the mixed layer depth deepened to m in February Fig. The difference of the salinity on the transect between the periods of —17 and —13 showed that the strengthened advection of the surface ASHSW occurred in April—July and November—February, which was consistent with a seasonal increase of the regional high salinity Fig.

Further, the surface salinity in April—July and November—February and the 2-month leading eddy processes during —13 and —17 were shown in Fig. The southern flank of the eddy advected the high-salinity water to the northern Arabian Sea along the Omani coast.

The salinity near the mouth of the Gulf of Oman during November—January for —17 was higher than that during November—January for —13 Fig.

The increase of the salinity was highly related to the westward shifted and weakened cyclonic eddy near the mouth of the Gulf of Oman, as compared to that during — The weakened cyclonic eddy caused an anomalous southward flow on the eastern flank of the eddy, which led to less northward advection of the relatively low-salinity water, further contributing to the salinity increase Fig.

The northern flank of anticyclone formed the eastward Ras Al Hadd Jet, favoring the spreading of the high-salinity water and the southern flank advected the relatively low salinity into the south of the Gulf.

The low-salinity water upwelled to the surface near the south Omani coast Fischer et al. In May—July, an eddy dipole off the south Omani coast advected this upwelled low-salinity water northward, then fed into the anticyclonic eddy near the mouth of the Gulf of Oman Fig. In the eastern part, the high-salinity water spread southward by the southward flowing currents during the season Fig.

In the same season for the high-salinity event in —17, the salinity dramatically increased in most regions of the northern Arabian Sea caused by an anomalous eddy dipole with a cyclone in the north and an anticyclone in the south near the mouth of Gulf of Oman, as compared to that during —13 Figs.

Along the south Omani coast, the anticyclone and its northward low-salinity advection weakened during —17 than that during —13, which also led to the increase of salinity in the northern Arabian Sea Fig. The last one was much stronger than the previous two, indicating that more PGW spread to the northern Arabian Sea during boreal fall.

The seasonal cycle of the PGW on the transect showed some phase shift during the high-salinity event in —17 Fig. However, the conditions in November—January during the high-salinity event in —17 showed some differences compared to that during — the water was saltier and the cyclonic eddy was weaker and westward shifted Fig.

The salter PGW in the western part of the Gulf of Oman was advected by the southern flank of the cyclonic eddy to the northern Arabian Sea, causing significant positive salinity anomaly Fig. The eddy structures and their high-salinity advection were much clearer in the intermediate layer than that in the surface layer Figs.

In the same season during the high-salinity event in —17, an eddy dipole near the mouth of the Gulf of Oman Fig. Besides, the influence of the weakened coastal summer monsoonal current and its reduced low-salinity advection also appeared in the intermediate layer Fig. Both in the surface and intermediate layers, the high-salinity water spreading from the Gulf of Oman to the northern Arabian Sea were strengthened by the change of the eddies near the mouth of the Gulf of Oman, including the weakened and west shifted cyclonic eddy during fall and early winter and an eddy dipole during spring and early summer, which might be an important reason for the developing of the prolonged high-salinity event in — The northward advection of the low-salinity water is another important factor influencing the salt balance in the northern Arabian Sea.

This low-salinity water may be transported by eddies from the upwelling region near the Omani coast or carried by the West Indian Coastal Current from the equator Fig. In the same season during the high-salinity event in —16, the salinity near the northern coastal region was about 0.

The West Indian Coastal Current was weaker than that during —12, causing weakened northward low-salinity advection and a dramatic salinity increase all over the northern Arabian Sea.

The weakened northward coastal currents along the Omani coast and its low-salinity water advection caused a significant increase of salinity all over the northern Arabian Sea during April—June in —17, as compared to that during —13 Fig. The mean tendency of the average salinity within the control volume was very small during —13, although the seasonal and interannual variability was intense Fig.

The residual is rather large, which might be caused by undersampled boundary currents, and uncertainties with the datasets. However, the observed Argo data were very scarce within the km along the shore Fig.

In hot and dry regions, where evaporation is high, the salinity sometimes reaches to Comparatively Low salinity regions In the estuaries enclosed mouth of a river where fresh and saline water get mixed and the Arctic, the salinity fluctuates from 0 — 35, seasonally fresh water coming from ice caps. Pacific The salinity variation in the Pacific Ocean is mainly due to its shape and larger areal extent.

Atlantic The average salinity of the Atlantic Ocean is around The equatorial region of the Atlantic Ocean has a salinity of about Near the equator, there is heavy rainfall , high relative humidity, cloudiness and calm air of the doldrums.

The polar areas experience very little evaporation and receive large amounts of fresh water from the melting of ice. This leads to low levels of salinity, ranging between 20 and It gradually decreases towards the north. Indian Ocean The average salinity of the Indian Ocean is The low salinity trend is observed in the Bay of Bengal due to influx of river water by the river Ganga. On the contrary, the Arabian Sea shows higher salinity due to high evaporation and low influx of fresh water.

Marginal seas The North Sea , in spite of its location in higher latitudes, records higher salinity due to more saline water brought by the North Atlantic Drift. Baltic Sea records low salinity due to influx of river waters in large quantity.

The Mediterranean Sea records higher salinity due to high evaporation. Salinity is, however, very low in Black Sea due to enormous fresh water influx by rivers. Their water becomes progressively more saline due to evaporation. The oceans and salt lakes are becoming more salty as time goes on because the rivers dump more salt into them, while fresh water is lost due to evaporation.

Cold and warm water mixing zones Salinity decreases from 35 — 31 on the western parts of the northern hemisphere because of the influx of melted water from the Arctic region. Sub-Surface Salinity With depth, the salinity also varies, but this variation again is subject to latitudinal difference. The decrease is also influenced by cold and warm currents.