However, these characteristics are only valid for parts of the year. Especially at BATS, the export pathway exists only in spring and the system reverts to a regeneration loop in summer and fall. This is a consistent result given the strong summer-time stratification and the resulting low levels of nutrient input. Heat and freshwater precipitation-evaporation fluxes, combined with the gyre circulation, are the dynamic drivers for SST and sea surface salinity SSS spatial and temporal variability that highly influence vertical mixing and thus the depth of the mixed layer.
The Aquarius SSS global map shows that the surface salinity within the Atlantic gyres is greater than the salinity in the Pacific gyres. The minimum SSS occurs in summer-fall and the maximum in winter-spring for all gyres, except for the SATL gyre where the SSS seasonal cycle is in phase opposition with the other 4 gyres.
This may be due to a different seasonality pattern of precipitation-evaporation in the sub-tropical SATL. The variability in surface water density due to changes in SST and SSS, combined with wind stirring, are effective drivers of vertical mixing, which in turn control the renewal of nutrients within the euphotic zone.
Figure 2. There is a large spatial variability of MLD globally and within the gyres, a result of the interplay of the driving factors described above.
The anti-cyclonic circulation patterns within the gyres are clearly shown in the climatological global map of Aviso dynamic height Figure 3B , with the strongest western boundary currents being the Gulf Stream in the North Atlantic and the Kuroshio Current in the North Pacific, both originating from the western limbs of the gyres.
Figure 3. Note that the time axes have been adjusted to provide synchronization of seasons between the northern and southern hemispheres, so winter, summer, spring and autumn appear in phase in all gyres. Summer and autumn, a period of relatively shallower MLDs, are the seasons with lowest biomass Chl-a , while in the peak of winter strong vertical mixing drives the elevated biomass shown by the higher values of Chl-a.
Figure 4. Values were averaged within the polygons shown in Figure 1. The trends and corresponding statistical results are summarized in Table 1. Table 1. There is a general agreement of magnitude and sign of trends among the different sensors and algorithms, with some exceptions.
The statistical significance for the trends is improved for some gyres when the longer MODIS-SeaWiFS year combined record is used, except for the SPAC gyre which still shows a weaker positive trend with low statistical significance.
The largest differences in trends between the IOCE and NATL gyres using the two different algorithms may be a result of stronger variability in the Chl-a anomalies in these 2 gyres see Figure 5. There are several factors that contribute to the uncertainty associated with the estimation of the trends in the subtropical gyres, including sensor and algorithm accuracies and the length of available ocean color records. Figure 5. Gregg and Rousseaux analyzed decadal trends in global pelagic chlorophyll by integrating multiple satellites, in situ data, and models.
As shown in Table 1 , the Chl trends derived in this study are very close to those reported by Gregg and Rousseaux The analysis was done using monthly data for the period of —, except for the MLD which was limited by data availability — The units for the trends are chosen to enable uniform magnitude range and number of decimal places for all variables. The time series of monthly anomalies with superposed linear trends are shown in Figure 5 for all variables analyzed.
Table 2. As previously mentioned, the SPAC is the only gyre with a positive trend. All the other gyres have negative trends indicating an expansion of the oligotrophic areas. Thus, as the MLD shallows negative trend , the average Chl-a concentration in the gyres is expected to decrease negative trend following the dynamics of forcing vs.
Therefore, a relatively small mixed layer deepening in a region of the gyres where the nutricline is much deeper see Figure 1B has a significant effect in phytoplankton production. Our year analyses of Chl trends in the oligotrophic regions of the subtropical gyres are consistent with the biogeochemical response to changes in the forcing factors affecting the gyre dynamics. The new export production in the gyres is controlled by inorganic nutrient inputs into the euphotic zone, which in turn result from seasonal vertical mixing driven by winter-spring convective overturning.
During summer, the upper ocean waters re-stratify leading to shallow mixed layers and phytoplankton production is significantly reduced and primarily driven by ecosystem nutrient regeneration.
In this study, we showed that these changes are indeed occurring and that the subtropical gyres are becoming more oligotrophic as a result of the forcing changes.
Our analyses revealed warming trends in all 5 gyres, as well as an increase in sea level height. Warming was more intense in the IOCE gyre with a year trend of 0. Dynamic effects other than surface warming and increase in sea level are probably influencing the somewhat weak upward trend in Chl-a, but the upward trend in Chl-a associated with increasing MLD appears to be coherent with our original forcing vs. The upward trends in SLA for all the gyres can be an indication that the thermocline, and thus the nutricline are getting deeper.
Turk et al. Our year upward trends in SLA Table 2 are potential indicators that new production is being reduced in all gyres. There is a debate in the literature Letelier et al. This effect has the potential to introduce uncertainties in the determination of biomass concentration from Chl-a.
Mignot et al. In each of the regions investigated in their study, the Chl-a at the DCM increases from spring to summer and then decreases from summer to fall. Encyclopedic Entry Vocabulary. Also known as thermohaline circulation , the ocean conveyor belt is essential for regulating temperature , salinity and nutrient flow throughout the ocean.
Wind drags on the ocean surface, causing water to move in the direction the wind is blowing. This deflection is a part of the Coriolis effect. The Coriolis effect shifts surface currents by angles of about 45 degrees. In the Northern Hemisphere , ocean currents are deflected to the right, in a clockwise motion.
In the Southern Hemisphere , ocean currents are pushed to the left, in a counterclockwise motion. Beneath surface currents of the gyre, the Coriolis effect results in what is called an Ekman spiral. While surface currents are deflected by about 45 degrees, each deeper layer in the water column is deflected slightly less.
This results in a spiral pattern descending about meters feet. The massive South Pacific Gyre, for instance, includes hundreds of kilometers of open ocean. In contrast, the northern Indian Ocean Gyre is a much smaller ocean gyre. Unlike the South Pacific Gyre, its extent is determine d largely by landmasses. The Equator forms its southern boundary , but it is bounded elsewhere by the Horn of Africa, Sri Lanka and the Indian subcontinent , and the Indonesian archipelago.
Subpolar gyre s form in the polar regions of the planet. They sit beneath an area of low atmospheric pressure. Wind drives the currents in subpolar gyres away from coastal areas.
These surface currents are replaced by cold, nutrient-rich water in a process called upwelling. The Northern Hemisphere has several subpolar gyres, bounded by islands such as Iceland, Greenland, and the Aleutians; and the northern reaches of Scandinavia , Asia, and North America. Tropical gyre s form near the Equator. The Coriolis effect is not present at the Equator, and winds are the primary creators of currents.
For this reason, tropical gyres tend to flow in a more east-west instead of circular pattern. These form between the polar and equatorial regions of Earth. Subtropical gyres circle areas beneath regions of high atmospheric pressure. These are placid ocean areas thousands of kilometers in diameter. Unlike coastal zones, these central regions are relatively stable. The ocean water generally stays in one place while the currents of the gyre circulate around it.
Each gyre has a powerful western boundary current and a weaker eastern boundary current. The Gulf Stream is the western boundary current of the gyre. This entire circle and the water within it is the North Atlantic Gyre. Most ocean gyres are very stable and predictable. Some gyres experience seasonal variation, however. At the same time, between o latitude the westerlies move surface water towards the east.
The Coriolis Effect and the presence of the continents deflect the currents towards the equator, creating eastern boundary currents on the eastern side of the ocean basins.
These currents come from high latitude areas, so they deliver cold water to the lower latitudes. Together, these currents combine to create large-scale circular patterns of surface circulation called gyres. In the Northern Hemisphere the gyres rotate to the right clockwise , while in the Southern Hemisphere the gyres rotate to the left counterclockwise.
The Kuroshio flows into the North Pacific Current which moves east towards North America, where it becomes the California Current to complete the gyre. Near Antarctica the circulation is somewhat different. Because there is little in the way of continental land masses between o south, the surface current created by the westerly winds can make its way completely around the Earth, creating the Antarctic Circumpolar Current ACC or West Wind Drift WWD that flows from west to east Figure 9.
Source: Library of Congress. Today, scientists can study the Gulf Stream from above, using satellites. Satellite images of sea surface temperature can show the path of the warm Gulf Stream current with great precision.
Knowing the sea surface temperature can give scientists information about what is happening in and around the ocean. Changes in this temperature can influence the behavior of fish, cause the bleaching of corals, and affect weather along the coast.
0コメント