[1] Taking advantage of the rapid advance in ocean modeling, this study investigates the sea surface salinity maximum in the North Atlantic, using results from a model of the Consortium for Estimating the Circulation and Climate of the Ocean (ECCO). Salinity budget terms were computed at the model's integration time step and archived as monthly averages. The simulated mixed layer salinity budget provides the first quantitative evidence for the ocean's role in governing the sea surface salinity maximum in the North Atlantic. Our analysis reveals that ocean dynamics explains about half of the sea surface salinity variance, being of equal importance as surface forcing. The sea surface salinity maximum varies both seasonally and interannually, as a consequence of interplay among surface flux, advection, and vertical entrainment. Contribution from eddies and small-scale processes is relatively weak but not negligible. These results may provide useful hints for the design and interpretation of future observations in the region.
The origin and pathway of the thermostad water in the eastern equatorial Pacific Ocean, often referred to as the equatorial 138C Water, are investigated using a simulated passive tracer and its adjoint, based on circulation estimates of a global general circulation model. Results demonstrate that the source region of the 138C Water lies well outside the tropics. In the South Pacific, some 138C Water is formed northeast of New Zealand, confirming an earlier hypothesis on the water's origin. The South Pacific origin of the 138C Water is also related to the formation of the Eastern Subtropical Mode Water (ESTMW) and the Sub-Antarctic Mode Water (SAMW). The portion of the ESTMW and SAMW that eventually enters the density range of the 138C Water (25.8 , s u , 26.6 kg m 23 ) does so largely by mixing. Water formed in the subtropics enters the equatorial region predominantly through the western boundary, while its interior transport is relatively small. The fresher North Pacific ESTMW and Central Mode Water (CMW) are also important sources of the 138C Water. The ratio of the southern versus the northern origins of the water mass is about 2 to 1 and tends to increase with time elapsed from its origin. Of the total volume of initially tracer-tagged water in the eastern equatorial Pacific, approximately 47.5% originates from depths above s u 5 25.8 kg m 23 and 34.6% from depths below s u 5 26.6 kg m 23 , indicative of a dramatic impact of mixing on the route of subtropical water to becoming the 138C Water. Still only a small portion of the water formed in the subtropics reaches the equatorial region, because most of the water is trapped and recirculates in the subtropical gyre.
[1] The formation of salinity maximum water in the North Atlantic is investigated using a simulated passive tracer and its adjoint. The results reveal that most salinity maximum water in the North Atlantic comes from the northwestern part of the subtropical gyre, and direct contribution from the evaporation-precipitation maximum region via the surface Ekman current is minor. Water originating from the evaporation-precipitation maximum region has to recirculate in the subtropical gyre before entering the sea surface salinity maximum region from the northwest. Once subducted, some portion (~10%) of the salinity maximum water enters the equatorial region in the shallow subtropical cell, but most (~70%) of it appears to turn northward to join the North Atlantic Deep Water. The latter pathway involves a three-dimensional circulation. When the warm, fresh surface water flows northward along the western boundary, it turns eastward in the northern subtropical gyre. As a result of the large excess of evaporation over precipitation, this water gradually gains its salinity on the route, until it reaches the sea surface salinity maximum region in the central subtropical gyre. From there, the salinity maximum water is subducted and flows back to the western boundary in the depth range of the thermocline. With its high-salinity nature, a major portion of this water penetrates into the subpolar region and directly contributes to the deep thermohaline circulation.Citation: Qu, T., S. Gao, and I. Fukumori (2013), Formation of salinity maximum water and its contribution to the overturning circulation in the North Atlantic as revealed by a global general circulation model,
Both a quasi‐biennial variability and an overall linearly increasing trend are identified in the Sub‐Antarctic Mode Water (SAMW) subduction rate across the Southern Hemisphere ocean, using the Argo data during 2005–2019. The quasi‐biennial variability is mainly due to variability of the mixed layer depth. Variability of wind stress curl in the SAMW formation regions associated with the Southern Annular Mode plays a critical role in generating the quasi‐biennial variability of the mixed layer depth and consequently the SAMW subduction rates. The SAMW subduction rate across the Southern Hemisphere ocean, long‐term mean totaling 56 Sv, has increased at 0.73 ± 0.65 Sv year−1 over the past 15 years. The increase has directly contributed to the observed increase in the total SAMW volume. Much of this increasing trend can be explained by the deepening mixed layers, which in turn are primarily forced by the strengthening westerly winds under an increasing Southern Annular Mode.
[1] Using existing high-resolution CTD observations, complemented by a large amount of recently available Argo floating data, this study provides a detailed description of the subduction of South Pacific waters. With a significantly improved climatological dataset on the mixed layer properties, we obtain an annual subduction rate of 48.8 Sv (1 Sv = 10 6 m 3 s À1) from 10°S to 60°S in the South Pacific. Two peaks stand out in this subduction rate sorted by winter mixed layer density: one corresponds to the formation of eastern Subtropical Mode Water (STMW) and part of the Subtropical Underwater (SUW) between 25.0 and 25.5 s q , and the other has a density range between 26.6 and 27.1 s q , representing the formation of Sub-Antarctic Mode Water and Antarctic Intermediate Water (SAMW/ AAIW). The subduction in eastern STMW/SUW range is 12.7 Sv, only slightly smaller than that (14.6 Sv) associated with the SAMW/AAIW. Sandwiched between these two peaks, roughly in the density range between 26.0 and 26.5 s q , is the southwestern STMW, with a relatively small annual subduction of 5.6 Sv. Uncertainties of these estimates are discussed.
The subduction and equatorward pathways of North Pacific Tropical Water (NPTW) are investigated using a simulated passive tracer of the consortium Estimating the Circulation and Climate of the Ocean (ECCO). The results demonstrate that the subduction of NPTW occurs in a large area that extends from about 150°E to 130°W between 20°N and 30°N, but the main subduction region lies in its eastern part. After subduction, the main body of NPTW first spreads westward in the North Equatorial Current. Then it splits into two branches. One flows northward in the Kuroshio upon reaching the western boundary, and the other enters the tropical Pacific either via its western boundary pathway (WBP) or interior pathway (IP). Less than half of the transport through the WBP can eventually reach the central and eastern Pacific by the Equatorial Undercurrent, while the rest is seen to flow into the Indian Ocean by the Indonesian Throughflow. The IP is found to play a significant role in equatorward transport of the NPTW. About 30% of the NPTW that reached the equatorial Pacific is transported through the IP.
The asymmetry of mixed layer salinity (MLS) anomalies in response to El Niño and La Niña events in the tropical Pacific is examined for the first time based on Argo observations and Estimating the Circulation and Climate of the Ocean simulation. The difference of MLS anomalies between El Niño and La Niña shows a dipole structure, with El Niño featuring strong negative salinity anomalies east of 160°E while La Niña shows remarkable positive salinity anomalies west of 160°E. A salinity budget analysis suggests that nonlinear zonal advection plays a dominant role in generating the asymmetric MLS structure. This dipole MLS structure acts to generate a dipole structure of barrier layer thickness and thus likely plays an important role in the development of El Niño–Southern Oscillation asymmetries in sea surface temperature.
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