Moreover, brine exclusion intensifies the cooling by wind. Today there are three major deep ocean masses. This cooled water seeps through the ridge and downwells. This water mass is With this source, AABW is amongst the coldest water in the ocean with a temperature of This water is relatively fresh average Some of the water mass spills over into the eastern part of the South Atlantic, while the remainder travels into the equatorial channel between South America and Africa.
AIW is produced near the Antarctic Convergence or Polar Front where downwelling occurs as a result of the convergence of surface currents. AIW has a temperature of oC and a salinity of These two steady states were used to study the western Mediterranean deep water formation.
From the model velocity fields, using PaTATo, we computed 3D Lagrangian trajectories of water particles released in the Sicily Strait at a depth of m, which is the bottom layer of the strait.
We tracked the water particles, released in two successive model years, in monthly changing velocity fields for 3 years for the passive and active Adriatic DWF steady states. Below, we describe and analyze the western Mediterranean intermediate and deep water properties under the active and the passive Adriatic DWF state found in Amitai et al. The difference between both states at the intermediate depth of m Figure 2 shows that there is a substantial influence of the Adriatic DWF on the Adriatic itself as expected and on the western Mediterranean's Tyrrhenian Basin.
When the Adriatic is active, its intermediate water has similar characteristics as the surface signal; hence, it is warmer and saltier than when the Adriatic is passive. In the Tyrrhenian Basin, however, the intermediate water is colder and fresher when the Adriatic is active.
Another difference between the two states is the appearance of a warm, saline strip in the northwestern Ionian Basin when the Adriatic is passive. This water strip may be due to LIW entrainment Astraldi et al. Figure 2. We released virtual particles in the bottom layer of the Sicily Strait m in two successive model years; this release was repeated for the two flow patterns created by the two steady states Figure 3.
The virtual water particles were released in the beginning of each year and were tracked for 3 years. This analysis was repeated several times, starting from various months and years, resulting in pattern similar to the one shown in Figure 3.
When the Adriatic is passive, the water coming from the Sicily Strait widely disperses in the Tyrrhenian, following along Sardinia's eastern coast and recirculate mainly in intermediate levels before reaching the GoL region.
However, when the Adriatic is active, the water flowing along the eastern Tyrrhenian, dilute with deep Tyrrhenian's water, hence less trajectories seen reaching the GoL region. It seems that most water particles leaving the Sicily Strait under active Adriatic state tend to follow the deeper barotropic streamlines in the eastern Tyrrhenian but this is not the case under passive Adriatic state.
The difference in the observed circulation pattern is related to the density of the water leaving the eastern Mediterranean and may also be related to the differences in the Sicily Strait's flow regime.
Figure 3. The water particles were tracked for 3 years in an active Adriatic state A and a passive Adriatic state B. The colorbar indicates the depth of the particles at the given time. Next, we examine the properties of the flow in the Sicily Strait along the transect marked in Figure 1. Transect of Sicily Strait water temperature left column in Figure 4 and salinity right column in Figure 4 are examined with contours of density left column in Figure 4 and velocity right column in Figure 4 superimposed above.
Since the transect is diagonal, the velocity fields were rotated so that the velocity examined is the component that is perpendicular to the transect. The dashed lines in Figures 4B,D indicate southeastward flow, and the solid lines indicate northwestward flow. For both states, the known depiction of low salinity density water entering the eastern Mediterranean through the upper layer and high salinity density water leaving mostly through the bottom layer is seen.
However, in the passive Adriatic state Figure 4D , there is water outflowing from the eastern to the western Mediterranean northwestward velocity in very shallow depths around longitude This signal might correspond to the uplift of deep and intermediate water masses reported during the EMT Roether et al. When looking at the differences between the active and passive states Figures 4E,F , the most prominent feature is the positive salinity and density anomalies around m in the middle of the strait that emerges together with velocity anomalies along the transect.
It appears that when the Adriatic is passive, the flow from the eastern to the western Mediterranean in m depth at the strait southwestern part is stronger but with lighter water positive anomaly contours in Figure 4E. In the bottom layer of the strait, the flow strength is unchanged between states, but the density of the water leaving the eastern Mediterranean is a bit higher negative anomaly contour beneath m at Figure 4E , as was evidenced during the EMT.
We also note that the isopycnal contours follow the salinity slopes rather than the temperature slopes along the transect, emphasizing the role of LIW salinity in dictating the Sicily Strait's flow regime. Figure 4. Five-year means of salinity left and temperature right of water passing through the Sicily Strait along the transect plotted in Figure 1.
Contours of density left and velocity right along the transect are superimposed, in an active Adriatic state A,B , passive Adriatic state C,D , and their difference E,F. The dashed negative lines indicate southward flow, while the solid positive lines indicate northward flow. To summarize, the shallow water in the transect is denser when the Adriatic is active, but the flow regime in the shallow layers when the Adriatic is passive is such that the water exchanges more rapidly velocity anomalies from both sides of the transect between eastern and western Mediterranean.
The deep water flux denser than This verifies that most water that leaves the eastern Mediterranean in a passive Adriatic state 50 vs. We next study the temperature and salinity differences between the active and passive Adriatic states at depths of and 1, m Figure 5 in comparison to the differences between the states at a depth of m Figures 2E,F.
At the m depth, similar to the m depth, the Tyrrhenian is still warmer and saltier when the Adriatic is passive negative anomaly in both properties. However, the Balearic and Alboran Basins of the western Mediterranean exhibit positive anomalies of temperature and salinity that are absent from the m layer. These anomalies are enhanced and spread eastward to the Tyrrhenian in the 1, m layer. This result does not accord with the observations reported by Gasparini et al. However, this result is implied by the density transect through the Sicily Strait Figure 4 , which indicate a significant density difference that is confined to shallow above m depths.
Figure 5. Active and passive Adriatic states' differences in salinity and temperature, at depths of m A,B and 1, m C,D. Deep water formation often affects the mixed layer depth MLD , and we now concentrate on the MLD's characteristics in the GoL region under active and passive Adriatic states.
This MLD criteria was chosen since the coupling between temperature and salinity is significant in the presence of the warm and saline LIW in the GoL deep convection process Houpert et al. It shows that the difference in maximal MLDs between the passive and active Adriatic states can vary between and m, suggesting deeper convection for the passive Adriatic state. All the above indicates that the passive Adriatic MOC results in circulation that enables a deeper convection. This result might be explained by the LIW's pathway in a passive Adriatic state, where the flow is through the Sicily Strait at shallow—intermediate levels Figure 4 and, after recirculating in the Tyrrhenian Figure 3B , reaching rapidly the region of convection in the GoL.
There it enhances the production of deep water Lacombe et al. Figure 6. Monthly mean of maximal MLD in the region depicted in the upper panels with standard deviation as error bars in both states C. Stratification Index difference between states D.
We note that in all the simulations, the atmospheric forcing over the western Mediterranean is monthly climatology, such that the forcing has no interannual variability.
To further understand the difference between the two states of the Adriatic we calculated the difference in the stratification index SI between the active and passive Adriatic states Figure 6D. The larger the SI is, the stronger the vertical stratification is. Positive anomaly in the map of active Adriatic SI minus passive Adriatic SI Figure 6D emphasizes our former result that under active Adriatic there are less deep convection events in the GoL since it is more stratified than in the passive Adriatic state.
Our main result, based on our GCM simulations, is that the western Mediterranean is largely influenced by the DWF's location in the eastern Mediterranean. We focus on two cases under the same forcing : active and passive sources of deep water in the Adriatic Sea. When the Adriatic DWF is active, it produces deep water in the eastern Mediterranean, and the deep layer outflow to the western Mediterranean through the Sicily Strait is larger.
However, the exchange of water at depths shallower than m is smaller. This flow regime creates a state where warm, saline eastern Mediterranean water significantly affects the intermediate water of the western Mediterranean under passive Adriatic state. Eventually, these eastern intermediate water reaches the GoL area and modifies the GoL's stratification, hence significantly affects the properties shape and strength of the western Mediterranean deep convection.
Another important result of our analysis is that although intense local convection events are simulated in a passive Adriatic DWF, as was observed after the EMT Gasparini et al.
This result highlights the role of LIW in enabling or disabling the deep convection that appears in the GoL region Margirier et al. Furthermore, we found that the Tyrrhenian Basin in the western Mediterranean acts as a buffer zone between the western and the eastern Mediterranean, and dilution of LIW may take place there under an unusual flow regime, as was observed during the EMT Sparnocchia et al. To summarize, although our results were derived from simulations with idealized boundary conditions that only partially resemble the observed climatic transient, they contribute to the understanding of the interaction between the western and eastern Mediterranean and to characterize the propagating signals between them.
YAm conducted the model simulations and data analysis. All authors contributed to the experimental design, manuscript writing and revision, and read 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. Amitai, Y. Multiple equilibria and overturning variability of the Aegean-Adriatic Seas.
Change , 49— T-S diagrams can be used to identify water masses. Since each major water mass has its own characteristic range of temperatures and salinities, a deep water sample that falls into that range can presumably have come from that water mass. Figure 9. To investigate water masses, oceanographers can take a series of temperature and salinity measurements over a range of depths at a particular location.
If the water column was highly stratified and there was no mixing between or within the layers, as the probe was lowered your would get a series of constant temperature and salinity readings as you moved through the first water mass, followed by a sudden jump to another set of different but constant readings as you moved through the next water mass.
Plotting temperature vs. However, in reality, the water masses will show some mixing within and between layers. So as the probes are lowered, they will encounter water that shows traits intermediate between the two points.
Therefore, with increasing depth, the points on the T-S diagram will gradually move from one point to the other, creating a line connecting the two points, illustrating mixing between those two water masses.
AABW is the deepest water mass, at depths of about m. Notice that as the recordings get deeper in Figure 9. This is because the densest water should be located at the bottom, with the other layers stratified according to their density, otherwise the water column would be unstable. The bottom water from the Weddell Sea and Greenland Sea does not just circulate through the Atlantic. Together these water masses move eastwards into the Indian and Pacific Oceans.
The deep Common Water moves northwards into the Pacific and Indian Oceans and gradually mixes with the warmer water, causing it to eventually rise to the surface. As surface water, it makes its way back to the North Atlantic through the surface currents of the Pacific and Indian Oceans. For one, it is vital to the transport of heat around the globe, bringing warm water towards the poles, and cold water to the tropics, stabilizing temperature in both environments. The conveyor belt also helps deliver oxygen to deep water habitats.
The deep water began as cold surface water that was saturated with oxygen, and when it sank it brought that oxygen to depth. Thermohaline circulation carries this oxygen-rich deep water throughout the oceans, where the oxygen will be used by deep water organisms. Bottom water in the Atlantic is relatively high in oxygen, as it still retains much of its original oxygen content, but as it travels over the seafloor the oxygen is used up, so that deep water in the Pacific Ocean has much less oxygen than deep Atlantic water, with Indian Ocean water somewhere in between.
At the same time, deep water will accumulate nutrients as organic matter sinks and decomposes. Atlantic bottom water is low in nutrients because it has not had much time to accumulate them, and the original surface water was nutrient-poor.
0コメント