Pacific Salinity Research Articles

Ocean dynamics contribute to the amplifying Atlantic–Pacific salinity contrast

Ocean dynamics contribute to the amplifying Atlantic–Pacific salinity contrast

North Atlantic–Pacific salinity contrast enhanced by wind and ocean warming

North Atlantic–Pacific salinity contrast enhanced by wind and ocean warming

Observation impact statement on satellite sea surface salinity data from two operational global ocean forecasting systems

ABSTRACT Observing system experiments have been carried out with two operational ocean data assimilation systems to assess the impact of assimilating satellite sea surface salinity (SSS) data. The SSS data reduces the errors in tropical Pacific salinity by 4% and 8% in the Met Office and Mercator Ocean systems respectively with an impact down to about 50 m depth on average. Salinity errors were reduced by up to 30% in the tropical Atlantic in both systems. There were significant reductions in near-surface temperature and sea level anomaly errors in both the central tropical Pacific and Amazon outflow regions in the Met Office system. The experiment results and characteristics of the operational ocean forecasting systems lead to a new set of requirements for satellite salinity data. The product accuracy required to significantly improve real-time ocean analysis and forecasts should be less than 0.2 pss at a resolution of at least 50 km for daily products. Improvements to spatial resolution down to the order of 10 km would represent an even more significant breakthrough. To be assimilated in near-real time, products should be available within 24 h and estimates of the systematic and random components of the uncertainty should be provided with each measurement.

Spreading of the South Pacific Tropical Water and Antarctic Intermediate Water Over the Maritime Continent

AbstractThe three‐dimensional climatological spreading of South Pacific waters over the Maritime Continent is examined using eddy‐resolving simulations and a passive tracer method. Four pathways are summarized for the South Pacific Tropical Water and the Antarctic Intermediate Water through the Maritime Continent, including two direct pathways, namely, the Eastern Path and the Torres Strait Path, and two indirect pathways, namely, the Western Path and the South China Sea Path. Among them, according to tracer concentration distribution, the former two contribute most of the volume transport of the Indonesian Throughflow (ITF) originating from the South Pacific. The latter two experience two retroflections (i.e., the South Equatorial Current‐Equatorial Undercurrent and the North Equatorial Countercurrent‐North Equatorial Current) before entering the Indonesian Seas. The vertical distributions of the South Pacific waters along each path throughout the Indonesian Seas are also analyzed, revealing that the South Pacific salinity extrema greatly affect the intermediate and deep layers of the Indonesian Seas and ITF as well as the upper thermocline. Vertical fluxes, especially vertical advection, are vital to these Indo‐Pacific water exchange processes. Sensitivity tests are conducted for the vertical diffusion coefficient and initial conditions, the results of which are robust. According to current study, the South Pacific has substantial impacts on the water properties throughout the Maritime Continent and the mass‐energy exchanges between the Pacific and Indian Oceans.

Salinity changes in the World Ocean since 1950 in relation to changing surface freshwater fluxes

Global hydrographic and air–sea freshwater flux datasets are used to investigate ocean salinity changes over 1950–2010 in relation to surface freshwater flux. On multi-decadal timescales, surface salinity increases (decreases) in evaporation (precipitation) dominated regions, the Atlantic–Pacific salinity contrast increases, and the upper thermocline salinity maximum increases while the salinity minimum of intermediate waters decreases. Potential trends in E–P are examined for 1950–2010 (using two reanalyses) and 1979–2010 (using four reanalyses and two blended products). Large differences in the 1950–2010 E–P trend patterns are evident in several regions, particularly the North Atlantic. For 1979–2010 some coherency in the spatial change patterns is evident but there is still a large spread in trend magnitude and sign between the six E–P products. However, a robust pattern of increased E–P in the southern hemisphere subtropical gyres is seen in all products. There is also some evidence in the tropical Pacific for a link between the spatial change patterns of salinity and E–P associated with ENSO. The water cycle amplification rate over specific regions is subsequently inferred from the observed 3-D salinity change field using a salt conservation equation in variable isopycnal volumes, implicitly accounting for the migration of isopycnal surfaces. Inferred global changes of E–P over 1950–2010 amount to an increase of 1 ± 0.6 % in net evaporation across the subtropics and an increase of 4.2 ± 2 % in net precipitation across subpolar latitudes. Amplification rates are approximately doubled over 1979–2010, consistent with accelerated broad-scale warming but also coincident with much improved salinity sampling over the latter period.

Removing the North Pacific halocline: Effects on global climate, ocean circulation and the carbon cycle

A well-pronounced halocline is a key feature of today's subarctic North Pacific. There is indirect paleo-evidences from the last glacial termination as well as from the early and middle Pliocene that this halocline has not always been there. To study the effects of North Pacific salinity on global climate, ocean circulation and the marine carbon cycle, we perform idealized experiments using an Earth system model of intermediate complexity (LOVECLIM). Imposing a negative freshwater flux in the northern North Pacific, the halocline vanishes and a deep Pacific meridional overturning circulation (PMOC) establishes. The associated increase of meridional heat transport in the Pacific leads to a bipolar seesaw response in temperature, with warming in the North Pacific and over North America and cooling in the Southern Ocean. As a result of the formation of North Pacific deep water (NPDW), the surface branch of the global conveyor belt circulation weakens. Transport through the Indonesian Seas decreases by 50% as the warm and saline waters of the equatorial Pacific are diverted into the North Pacific.In our idealized experiments, the enhanced global deep water formation is balanced by an increase in diapycnal mixing. As a result nutrient concentrations in the euphotic zone increase by about 25% globally, leading to a 20% increase in global export production. The effect of greater export production on atmospheric pCO2 is, however, compensated by the enhanced transport of dissolved inorganic carbon (DIC) to the surface. As a result, the atmospheric CO2 concentration increases by only 5ppmv. Our results further suggest that the absence of the subarctic halocline for instance during Heinrich event 1 and the Pliocene may have exerted a strong influence on global climate and the carbon cycle.

Water mass transformation along the Indonesian throughflow in an OGCM

The oceanic pathways connecting the Pacific Ocean to the Indian Ocean are described using a quantitative Lagrangian method applied to Eulerian fields from an ocean general circulation model simulation of the Indonesian seas. The main routes diagnosed are in good agreement with those inferred from observations. The secondary routes and the Pacific recirculation are also quantified. The model reproduces the observed salt penetration of subtropical waters from the South Pacific, the homohaline stratification in the southern Indonesian basins, and the cold fresh tongue which exits into the Indian Ocean. These particular water mass characteristics, close to those observed, are obtained when a tidal mixing parameterization is introduced into the model. Trajectories are obtained which link the water masses at the entrance and at the exit of the Indonesian throughflow (ITF), and the mixing along each trajectory is quantified. Both the ITF and the Pacific recirculation are transformed, suggesting that the Indonesian transformation affects both the Indian and Pacific stratification. A recipe to form Indonesian water masses is proposed. We present three major features of the circulation that revisit the classical picture of the ITF and its associated water mass transformation, while still being in agreement with observations. Firstly, the homohaline layer is not a result of pure isopycnal mixing of the North Pacific Intermediate Water and South Pacific Subtropical Water (SPSW) within the Banda Sea, as previously thought. Instead, the observed homohaline layer is reproduced by the model, but it is caused by both isopycnal mixing with the SPSW and a dominant vertical mixing before the Banda Sea with the NPSW. This new mechanism could be real since the model reproduces the SPSW penetration as observed. Secondly, the model explains why the Banda Sea thermocline water is so fresh compared to the SPSW. Until now, the only explanation was a recirculation of the freshwater from the western route. The model does not reproduce this recirculation but instead shows strong mixing of the SPSW within the Halmahera and Seram Seas, which erodes the salinity maximum so that its signature is not longer perceptible. Finally, this work highlights the key role of the Java Sea freshwater. Even though its annual net mass contribution is small, its fresh salinity contribution is highly significant and represents the main reason why the Pacific salinity maxima are eroded.

Freshwater transport estimates and the global overturning circulation: Shallow, deep and throughflow components

Meridional ocean freshwater transports and convergences are calculated from absolute geostrophic velocities and Ekman transports. The freshwater transports are analyzed in terms of mass-balanced contributions from the shallow, ventilated circulation of the subtropical gyres, intermediate and deep water overturns, and Indonesian Throughflow and Bering Strait components. The following are the major conclusions: 1. Excess freshwater in high latitudes must be transported to the evaporative lower latitudes, as is well known. The calculations here show that the northern hemisphere transports most of its high latitude freshwater equatorward through North Atlantic Deep Water (NADW) formation (as in [Rahmstorf, S., 1996. On the freshwater forcing and transport of the Atlantic thermohaline circulation. Climate Dynamics 12, 799–811]), in which saline subtropical surface waters absorb the freshened Arctic and subpolar North Atlantic surface waters (0.45 ± 0.15 Sv for a 15 Sv overturn), plus a small contribution from the high latitude North Pacific through Bering Strait (0.06 ± 0.02 Sv). In the North Pacific, formation of 2.4 Sv of North Pacific Intermediate Water (NPIW) transports 0.07 ± 0.02 Sv of freshwater equatorward. In complete contrast, almost all of the 0.61 ± 0.13 Sv of freshwater gained in the Southern Ocean is transported equatorward in the upper ocean, in roughly equal magnitudes of about 0.2 Sv each in the three subtropical gyres, with a smaller contribution of <0.1 Sv from the Indonesian Throughflow loop through the Southern Ocean. The large Southern Ocean deep water formation (27 Sv) exports almost no freshwater (0.01 ± 0.03 Sv) or actually imports freshwater if deep overturns in each ocean are considered separately (−0.06 ± 0.04 Sv). This northern–southern hemisphere asymmetry is likely a consequence of the “Drake Passage” effect, which limits the southward transport of warm, saline surface waters into the Antarctic [Toggweiler, J.R., Samuels, B., 1995a. Effect of Drake Passage on the global thermohaline circulation. Deep-Sea Research I 42(4), 477–500]. The salinity contrast between the deep Atlantic, Pacific and Indian source waters and the denser new Antarctic waters is limited by their small temperature contrast, resulting in small freshwater transports. No such constraint applies to NADW formation, which draws on warm, saline subtropical surface waters . 2. The Atlantic/Arctic and Indian Oceans are net evaporative basins, hence import freshwater via ocean circulation. For the Atlantic/Arctic north of 32°S, freshwater import (0.28 ± 0.04 Sv) comes from the Pacific through Bering Strait (0.06 ± 0.02 Sv), from the Southern Ocean via the shallow gyre circulation (0.20 ± 0.02 Sv), and from three nearly canceling conversions to the NADW layer (0.02 ± 0.02 Sv): from saline Benguela Current surface water (−0.05 ± 0.01 Sv), fresh AAIW (0.06 ± 0.01 Sv) and fresh AABW/LCDW (0.01 ± 0.01 Sv). Thus, the NADW freshwater balance is nearly closed within the Atlantic/Arctic Ocean and the freshwater transport associated with export of NADW to the Southern Ocean is only a small component of the Atlantic freshwater budget. For the Indian Ocean north of 32°S, import of the required 0.37 ± 0.10 Sv of freshwater comes from the Pacific through the Indonesian Throughflow (0.23 ± 0.05 Sv) and the Southern Ocean via the shallow gyre circulation (0.18 ± 0.02 Sv), with a small export southward due to freshening of bottom waters as they upwell into deep and intermediate waters (−0.04 ± 0.03 Sv). The Pacific north of 28°S is essentially neutral with respect to freshwater, −0.04 ± 0.09 Sv. This is the nearly balancing sum of export to the Atlantic through Bering Strait (−0.07 ± 0.02 Sv), export to the Indian through the Indonesian Throughflow (−0.17 ± 0.05 Sv), a negligible export due to freshening of upwelled bottom waters (−0.03 ± 0.03 Sv), and import of 0.23 ± 0.04 Sv from the Southern Ocean via the shallow gyre circulation. 3. Bering Strait’ssmall freshwater transport of <0.1 Sv helps maintains the Atlantic–Pacific salinity difference. However, proportionally large variations in the small Bering Strait transport would only marginally impact NADW salinity, whose freshening relative to saline surface water is mainly due to air-sea/runoff fluxes in the subpolar North Atlantic and Arctic. In contrast, in the Pacific, because the total overturning rate is much smaller than in the Atlantic, Bering Strait freshwater export has proportionally much greater impact on North Pacific salinity balances, including NPIW salinity.

A role for North Pacific salinity in stabilizing North Atlantic climate

A simple ocean/atmosphere feedback may reduce the amplitude of climate variability in around the North Atlantic during interglacial compared to glacial states. When climate is warm in the North Atlantic region, the Intertropical Convergence Zone has a relatively northward position, and moisture is exported from the tropical Atlantic to the tropical Pacific. At the same time the east Asian summer monsoon is strong, which helps maintain a positive balance of precipitation over evaporation in the subpolar North Pacific. This is thought to account for lower salinity in the North Pacific relative to the North Atlantic, which, in turn, drives northward flow through the Bering Strait to the northern North Atlantic. Freshening in the North Atlantic by water of Pacific origin suppresses the meridional overturning circulation and reduces the heat flux. The opposite situation exists during cold climate. Thus the combination of atmospheric vapor transport and flow through Bering Strait tends to cool the North Atlantic region when warm and warm the region when cool.

Seawater isotope constraints on tropical hydrology during the Holocene

Paleoceanographic data from the low latitude Pacific Ocean provides evidence of changes in the freshwater budget and redistribution of freshwater within the basin during the Holocene. Reconstructed Holocene seawater δ18O changes compare favorably to differences predicted between climate simulations for the middle Holocene (MH) and for the pre‐Industrial late Holocene (LH). The model simulations demonstrate that changes in the tropical hydrologic cycle affect the relationship between δ18Osw and surface salinity, and allow, for the first time, quantitative estimates of western Pacific salinity change during the Holocene. The simulations suggest that during the MH, the mean salinity of the Pacific was higher because less water vapor was transported from the Atlantic Ocean and more was transported to the Indian Ocean. The salinity of the western Pacific was enhanced further due both to the greater advection of salt to the region by ocean currents and to an increase in continental precipitation at the expense of maritime precipitation, the latter a consequence of the stronger Asian summer monsoon.

Trends in Caribbean Paleoproductivity related to the Neogene closure of the Central American Seaway

This study tests the hypothesis that the late Miocene to early Pliocene constriction and closure of the Central American Seaway (CAS), connecting the tropical Atlantic and East quatorial Pacific (EEP), caused a decrease in productivity in the Caribbean, due to decreased coastal upwelling and an end to the connection with high-productivity tropical Pacific waters. The present study compared paleoceanographic proxies for the interval between 8.3 and 2.5 Ma in 47 samples from south Caribbean ODP Site 999 with published data on EEP DSDP Site 503. Proxies for Site 999 include the relative abundance of benthic foraminiferal species representing bottom current velocity and the flux of organic matter to the sea floor, the ratio of infaunal/epifaunal benthic foraminiferal species and benthic foraminifer accumulation rates (BFARs). In addition, we calculated % resistant planktic foraminifers species and used the previously published % sand fraction and benthic carbon isotope values from Site 999. During early shoaling of the Isthmus (8.3–7.9 Ma) the Caribbean was under mesotrophic conditions, with little ventilation of bottom waters and low current velocity. The pre-closure interval (7.6–4.2 Ma) saw enhanced seasonal input of phytodetritus with even more reduced ventilation, and enhanced dissolution between 6.8 and 4.8 Ma. During the post-closure interval (4.2–2.5 Ma) in the Caribbean, paleoproductivity decreased, current velocity was reduced, and ventilation improved, while the seasonality of phytodetrital input was reduced dramatically, coinciding with the establishment of the Atlantic–Pacific salinity contrast at 4.2 Ma. Our data support the hypothesis that late Miocene constriction of the CAS at 7.9 Ma and its closure at 4.2 Ma caused a gradual decrease in paleoproductivity in the Caribbean, consistent with decreased current velocity and seasonality of the phytodetrital input.

Seasonal and ENSO variations of sea surface salinity and temperature in the South Pacific Convergence Zone during 1976–2000

Sea surface salinity (SSS) and temperature (SST) data collected from voluntary observing ships over 25 years (1976–2000) are analyzed in the Southwestern Tropical Pacific (10°S–24°S/160°E–140°W). This region lies under the South Pacific Convergence Zone (SPCZ), at the southern edge of the western Pacific warm pool between Tahiti and Darwin, the two places whose atmospheric sea level pressure difference is used to define the Southern Oscillation Index (SOI). Complementary data such as precipitation are used to assist in the analysis. The mean and seasonal variations of these parameters are described. An empirical orthogonal function (EOF) analysis of low‐pass filtered time series is then performed to extract the interannual variability. All parameters show an interannual signal which correlates well with the SOI. The Southwestern Tropical Pacific Ocean is saltier and colder during El Niño than during La Niña events. In the southwestern part, there is a shortage (excess) in precipitation during El Niño (La Niña) events. The greatest anomalies appeared during the last La Niña, in 1996 and 1999 as regards SST and in 1999 and 2000 as regards SSS. SST and precipitation El Niño–Southern Oscillation (ENSO)‐related anomalies are an order of magnitude smaller than seasonal anomalies, while the SSS ENSO‐related signal is twice as strong as the seasonal signal. These facts reflect the northeastward (southwestward) shift of the SPCZ during El Niño (La Niña) events. While consistent with precipitation changes, the ENSO‐related variability in SSS can also be partly explained by the displacement of the salinity front that separates fresh, warm pool waters from salty subtropical waters. Computation of surface geostrophic current anomalies from Geosat (1987–1988) and TOPEX/Poseidon (1993–2000) indicates that westward current anomalies developed during the 1987/88 and 1997/98 El Niño and are linked to the displacement of the salinity front. The Southwestern Tropical Pacific salinity front moves westward (eastward) in contrast to the equatorial salinity front which moves eastward (westward) during an El Niño (La Niña) event.

Seasonal and internannual variability of estuarine circulation in a box model of the Strait of Georgia and Juan de Fuca strait

A box model of the Strait of Georgia and Juan de Fuca Strait, British Columbia, Canada is developed to investigate the estuarine circulation driven by Fraser River runoff but modulated by tidal mixing in Ham Strait which connects the two straits. The water mass exchange between the Juan de Fuca Strait and the Pacific Ocean is parametrized by a restoring salinity boundary condition. In addition to horizontal advection of water masses, the salinity in the lower layer of Juan de Fuca Strait is relaxed to the Pacific salinity. For realistic values of the fresh water flux and vertical eddy diffusivity, the model predicts a seasonal cycle of salinities and water mass transports in reasonable agreement with the observations. The salinity distribution in the estuary is determined by the combined effect of river runoff and vertical mixing. Tidal mixing in Haro Strait not only reduces stratification but also causes a spring‐neap tidal modulation of salinities. Multi‐year calculations show a rapid response of the estuarine circulation to interannual variability in the fresh water forcing and in the properties of the continental shelf water which enters the deep layer of the Juan de Fuca Strait. The salinities and volume fluxes adjust to the new river runoff within a year, consistent with observational estimates of a short flushing time in the estuary.

Aluminium as a depth-sensitive tracer of entrainment in submarine hydrothermal plumes

HYDROTHERMAL plumes formed by entrainment of ambient sea water into buoyant high-temperature vent fluids, are important features of ocean-ridge hydrothermal systems1,2. An understanding of the mixing processes in plumes is essential for evaluating their role in modifying the chemical impact of hydrothermal activity on sea water and for calculating the heat and chemical budgets of ocean ridges. Central to all plume models is the rate-of-entrain-ment function which is used to determine the volume of water entrained by the buoyant plume at different depths3–5. In the Pacific salinity can be used as a tracer of entrainment6 to a limited extent, but in the Atlantic Median Valley salinity does not vary appreciably near the sea bed. Here we show that aluminium can be used as a sensitive tracer as it varies markedly with depth in the immediate vicinity of hydrothermal vents and the aluminium signal from vent fluids is small, so that the aluminium entrainment signal is not masked.