Saline Water Mass Research Articles

Atoll Groundwater Movement and Its Response to Climatic and Sea-Level Fluctuations

Groundwater resources of low-lying atoll islands are threatened due to short-term and long-term changes in rainfall, wave climate, and sea level. A better understanding of how these forcings affect the limited groundwater resources was explored on Roi-Namur in the Republic of the Marshall Islands. As part of a 16-month study, a rarely recorded island-overwash event occurred and the island’s aquifer’s response was measured. The findings suggest that small-scale overwash events cause an increase in salinity of the freshwater lens that returns to pre-overwash conditions within one month. The overwash event is addressed in the context of climate-related local sea-level change, which suggests that overwash events and associated degradations in freshwater resources are likely to increase in severity in the future due to projected rises in sea level. Other forcings, such as severe rainfall events, were shown to have caused a sudden freshening of the aquifer, with salinity levels retuning to pre-rainfall levels within three months. Tidal forcing of the freshwater lens was observed in electrical resistivity profiles, high-resolution conductivity, groundwater-level well measurements and through submarine groundwater discharge calculations. Depth-specific geochemical pore water measurements further assessed and confirmed the distinct boundaries between fresh and saline water masses in the aquifer. The identification of the freshwater lens’ saline boundaries is essential for a quantitative evaluation of the aquifers freshwater resources and help understand how these resources may be impacted by climate change and anthropogenic activities.

Sensitivity of Atlantic meridional overturning circulation to the dynamical framework in an ocean general circulation model

The horizontal coordinate systems commonly used in most global ocean models are the spherical latitude–longitude grid and displaced poles, such as a tripolar grid. The effect of the horizontal coordinate system on Atlantic meridional overturning circulation (AMOC) is evaluated by using an OGCM (ocean general circulation model). Two experiments are conducted with the model—one using a latitude–longitude grid (referred to as Lat_1) and the other using a tripolar grid (referred to as Tri). The results show that Tri simulates a stronger North Atlantic deep water (NADW) than Lat_1, as more saline water masses enter the Greenland–Iceland–Norwegian (GIN) seas in Tri. The stronger NADW can be attributed to two factors. One is the removal of the zonal filter in Tri, which leads to an increasing of the zonal gradient of temperature and salinity, thus strengthening the north geostrophic flow. In turn, it decreases the positive subsurface temperature and salinity biases in the subtropical regions. The other may be associated with topography at the North Pole, because realistic topography is applied in the tripolar grid while the latitude–longitude grid employs an artificial island around the North Pole. In order to evaluate the effect of the filter on AMOC, three enhanced filter experiments are carried out. Compared to Lat_1, an enhanced filter can also augment NADW formation, since more saline water is suppressed in the GIN seas, but accumulated in the Labrador Sea, especially in experiment Lat_2_S, which is the experiment with an enhanced filter on salinity.

Hurricane Ingrid and Tropical Storm Hanna’s effects on the salinity of the coastal aquifer, Quintana Roo, Mexico

There is a lack of information on aquifer dynamics in anchialine systems, especially in the Yucatán Peninsula of Mexico. Most of our knowledge is based on “spot” measurements of the aquifer with no long-term temporal monitoring. In this study spanning four years (2012–2016), sensors (water depth and conductivity (salinity)) were deployed and positioned (−9 and −10m) in the meteoric Water Mass (WM) close to the transition with the marine WM (halocline) in 2 monitoring sites within the Yax Chen cave system to investigate precipitation effects on the salinity of the coastal aquifer. The results show variation in salinity (<1ppt) of the freshwater over seasonal cycles of wet and dry (approx. 6.5–7.25ppt), depending on the position of the halocline. The aquifer response to larger precipitation events (>95mm) such as Hurricane Ingrid (2013) and Tropical Storm Hanna (2014) shows meteoric water mass salinity rapidly increasing (approx. 6.39 to >8.6ppt), but these perturbations have a shorter duration (weeks and days). Wavelet analysis of the salinity record indicates seasonal mixing effects in agreement with the wet and dry periods, but also seasonal effects of tidal mixing (meteoric and marine water masses) occurring on shorter time scales (diurnal and semi-diurnal). These results demonstrate that the salinity of the freshwater lens is influenced by precipitation and turbulent mixing with the marine WM. The salinity response is scaled with precipitation; larger more intense rainfall events (>95mm) create a larger response in terms of the magnitude and duration of the salinity perturbation (>1ppt). The balance of precipitation and its intensity controls the temporal and spatial patterning of meteoric WM salinity.

Trichodesmium latitudinal distribution on the shelf break in the southwestern Atlantic Ocean during spring and autumn

AbstractWe report Trichodesmium slicks in the South Atlantic associated with fronts produced by the southward displacement of the Brazil Current. For the first time, a relatively high spatial sampling grid of trichomes density is shown over an undersampled region with regard to Trichodesmium. In this study, the distributions of Trichodesmium along the southeastern Brazilian continental shelf break were characterized during four oceanographic cruises and related to seawater temperature, upper layer nutrient concentrations and wind dynamics. Meridional changes in the densities of trichomes followed an expected trend with higher (lower) values found in warmer (colder) and more saline (less saline) water masses. Between 23° and 28°S, large accumulations of trichomes were associated with high temperature, increased phosphate concentrations (&gt;0.08 μM), nitrogen below 2.4 μM, and low wind speeds (&lt;11 m s−1). Maximum densities of 212.6 × 105 trichomes L−1, which are considerably larger than those reported in other studies in subtropical oceans, occurred at stations showing weak water column stability and higher phosphate levels over the upper 25 m. We suggest that shelf break dynamics, which drive and modulate the intensity of the shelf break upwelling, as well as wind divergence at spatial scales of approximately 600 km2, associated with mesoscale activities, may favor Trichodesmium growth and accumulation. On the other hand, Trichodesmium densities were low at latitudes between 28° and 33°S, likely due to either or both the lowest sea surface temperatures and/or out‐competition by the faster‐growing nondiazotrophs.

Salinity Structure Within The Estuary Of Bintuni Bay, At The Southern Part Of Bird Head Of West Papua, Indonesia

Bintuni Bay is a semi-closed bay which a lot of big rivers flow into the bay and bring a huge amount of freshwater. This bay is not directly connected to the open sea but separated by the Berau Bay. The bay of Berau has wider bay mouth as well as deeper than Bintuni Bay, the deepest part more than 70 m located at the center part of bay. Tidal force generated offshore is able to push higher salinity water (HSW) mass further into the inner bay, otherwise the low salinity water (LSW) mass pushed out toward the head of the bay. In the area where LSW and HSW masses meet each other is found steep salinity gradient (salinity front). Two times field observations were conducted with lowered CTD (Conductivity Temperature Depth) sensor from the surface layer until one meter above seabed at 30 stations, either at the end of north-east monsoon (March 2013) or at south-east monsoon (July-August 2013). The HSW mass was dominant found at deeper layer and southern side of the bay, while the LSW mass occupied surface layer at the northern side of the bay.

Multidecadal freshening and lightening in the deep waters of the Bransfield Strait, Antarctica

AbstractThe deep waters of the Bransfield Strait receive considerable amounts of water from the Weddell Sea continental shelf. The restricted connections to the surrounding ocean and relatively easier access makes the Bransfield Strait an important proxy region for monitoring changes in the dense Weddell Sea shelf water masses, which are an important precursor of Antarctic Bottom Water (AABW). Long‐term hydrographic data from the period 1960s–2010s showed freshening and lightening of the deep water masses of the Bransfield Strait, which was likely caused by large freshwater inputs originating from the western shelf of the Weddell Sea. The rates of freshening and lightening were −0.0010 ± 0.0005 yr−1 and −0.0016 ± 0.0014 kg m−3 yr−1 for the central basin, respectively, and −0.0010 ± 0.0006 yr−1 and −0.0029 ± 0.0013 kg m−3 yr−1 for the eastern basin, respectively. The deep waters showed a high degree of interannual thermohaline variability, which appeared to be caused by changes in the proportions of source water mass mixing between the years. Statistically significant negative correlations between salinity/neutral density fields and the Southern Annular Mode (SAM) were observed (−0.56 and −0.62 for the central basin, respectively, and −0.58 and −0.68 for the eastern basin, respectively) between 1980 and 2014. During SAM positive phases, communication between the Weddell Sea and the Bransfield Strait is reduced, which leads to less saline and lighter water masses in the Bransfield Strait; however, the opposite trends are observed during SAM negative phases.

Observations of seasonal subduction at the Iceland‐Faroe Front

AbstractThe polar front in the North Atlantic is bound to the ridge between Iceland and the Faroe Islands, where about one‐half of the northward transport of warm Atlantic Water into the Nordic Seas occurs, as well as about one sixth of the equatorward dense overflow. We find a low salinity water mass at the surface of the Iceland‐Faroe Front (IFF), which in wintertime subducts along outcropping isopycnals and is found in much modified form on the Atlantic side of the Iceland‐Faroe Ridge (IFR) crest. The features found on the Atlantic side of the crest at depth have temperature and salinity characteristics which are clearly traceable to the surface outcrop of the IFF. The presence of coherent low salinity layers on the Atlantic side of the IFR crest has not been previously reported. Warm waters above the IFR primarily feed the Faroe Current, and injection of a low salinity water mass may play an early role in the water mass transformation taking place in the Nordic Seas. The seasonality of the intrusive features suggests a link between winter convection, mixed layer instability and deep frontal subduction. These low salinity anomalies (as well as a low oxygen water mass from the Iceland Basin) can be used as tracers of the intermediate circulation over the IFR.

Origins of suspended particulate matter based on sterol distribution in low salinity water mass observed in the offshore East China Sea

The molecular composition and distribution of sterols were investigated in the East China Sea to identify the origins of suspended particulate matter (SPM) in offshore waters influenced by Changjiang River Diluted Water (CRDW). Total sterol concentrations ranged from 3200 to 31,900pgL−1 and 663 to 5690pgL−1 in the particulate and dissolved phases, respectively. Marine sterols dominated representing 71% and 66% in the particulate and dissolved phases, respectively. Typical sewage markers, such as coprostanol, were usually absent at ~250km offshore. However, sterols from allochthonous terrestrial plants were still detected at these sites. A negative relationship was observed between salinity and concentrations of terrestrial sterols in SPM, suggesting that significant amounts of terrestrial particulate matter traveled long distance offshore in the East China Sea, and the Changjiang River Diluted Water (CRDW) was an effective carrier of land-derived particulate organic matter to the offshore East China Sea.

Ocean-current controlled sedimentation: the Lofoten Contourite Drift, Norwegian Sea

Sediment deposition from ocean-bottom or contour currents leads to the development of contourite drifts. On the continental slope offshore of Norway, contourite drifts have developed from sediment deposition from the alongslope-flowing Norwegian Current which brings warm and saline water masses into the Norwegian Sea. The present pattern of circulation of the Norwegian–Greenland Sea water masses developed as a result of the opening of the Fram Strait separating Greenland and Svalbard to the north from the general subsidence of the Greenland–Scotland Ridge to the south (e.g. Laberg et al. 2005). The Lofoten Contourite Drift is located on the continental slope offshore of northern Norway (Fig. 1). It is a mounded, elongate, separated giant drift ( sensu Faugeres et al. 1999) up to 360 m thick, about 150 km long (along drift axis) and is characterized by a layered, continuous, parallel to slightly divergent seismic signature of medium-amplitude reflections (Fig. 1d). The uppermost part of the drift includes mainly homogeneous or bioturbated mud and sandy mud that is …

Migration Process of Megalopae of the Japanese Mitten Crab Eriocheir japonica (De Haan) from Open Sea to Tidal River

The migration process of megalopae from the sea to a tidal river is an important subject for catadromous Brachyura. The process of invasion and settlement of the megalopae of the Japanese mitten crab Eriocheir japonica was investigated in a small river in temperate Japan. First, to elucidate the relationship between settlement and lunar rhythm, megalopae were collected using artificial grass mats (900 cm2) in the upper tidal river area every 3–4 days from November to December. An evident semi-lunar rhythm was detected, and the settlement was concentrated in the spring tide around the days of full moon and new moon. All the megalopae completed metamorphosis within 5 days before the next spring tide. Second, the relationship between settlement and tidal cycles within a day was investigated in the upper tidal river by collecting megalopae using artificial grass mats every 2 h during 1 day in November. Megalopae exhibited in their settlement an evident nocturnal behavior and synchronization with the tidal cycles. A mass of megalopae suddenly began settling on the mat during the night when the tide came in and high salinity water (ca. >10‰) reached the sampling site. In contrast, after the sunrise, very few megalopae were collected even during the flood tide. Third, the upstream migration process of megalopae in the tidal river was surveyed by hand-net sweeping at five stations during the night every hour for 7 h in May. No larvae were collected during the ebb tide before sunset, but swimming megalopae suddenly gathered at the rivermouth just after sunset. Aggregated megalopae shifted upstream with the flow of the highly saline water mass until the latter reached a weir in the uppermost tidal area. In the subsequent ebb tide, the megalopae remained in the uppermost area. Therefore, they utilized a selective tidal transport mechanism; megalopae migrated upstream from the sea preferably during the night around the spring tide and reached the uppermost tidal river during a single flood tide. Combined with previous data showing the distribution pattern of parent crabs and the settled megalopae, it was estimated that all megalopae were supplied from the sea’s offshore area, and river population dynamics depend on the supply of megalopae from the sea. This also suggests the larval exchange and gene flow between local river populations of E. japonica within a metapopulation structure.

Role of Southern Ocean stratification in glacial atmospheric CO2 reduction evaluated by a three-dimensional ocean general circulation model

Atmospheric carbon dioxide (CO2) concentration during glacial periods is known to be considerably lower than during interglacial periods. However, previous studies using an ocean general circulation model (OGCM) fail to reproduce this. Paleoclimate proxy data of the Last Glacial Maximum indicate high salinity (>37.0 psu) and long water mass residence time (>3,000 years) in the Southern Ocean, suggesting that salinity stratification was enhanced and more carbon was stored there. Reproducibility of salinity and water mass age is considered insufficient in previous OGCMs, which might affect the reproducibility of atmospheric CO2 concentration. This study investigated the role of increased stratification of the Southern Ocean in the glacial CO2 variation using an OGCM. We found that deep water formation in East Antarctica is required to explain high salinity in the South Atlantic. Saltier deep Southern Ocean resulted in increased atmospheric CO2 concentration against previous estimates. This is partly due to increased volume transport of Antarctic Bottom Water and associated decrease in the water mass age of the deep Pacific Ocean. On the other hand, weakening of vertical mixing contributed to increase of the vertical gradient of dissolved inorganic carbon and decrease of atmospheric CO2 concentration. However, we show that it is unable to explain all of the glacial CO2 variations by the contribution of the Southern Ocean. Our findings indicate that detailed understanding of the impact of enhanced stratification in the Southern Ocean on the Pacific Ocean might be crucial to understanding the mechanisms behind the glacial-interglacial ocean carbon cycle variations.

INTER-annual/decadal variability of the north Aegean Sea hydrodynamics over 1960-2000

Results from a high-resolution hindcast model experiment, supported by available observations, reveal an increasing salinity trend in the north Aegean during the Eastern Mediterranean Transient (EMT), largely controlled by increases in the flow rate and salinity of water masses of Levantine origin entering the domain through the Myconos-Ikaria strait as a response to an acceleration of the Aegean thermohaline cell. Changes in the Dardanelles inflow (increasing salinity) and in the surface freshwater flux (increasing Evaporation-Precipitation), although both contribute to a higher salt content of the basin during the EMT, play a minor role in the inter-annual/decadal variability of the freshwater budget. A long-term decreasing temperature trend is observed from the 1960s to the early 1990s. It is superimposed on the salinity-preconditioning phase over the 1980s and early 1990s. Both signals are, concomitantly, favouring conditions for intense Dense Water Formation (DWF) in the north Aegean Sea. In addition, the northward displacement of the Black Sea Water front over the EMT, leads to the expansion of convective cells towards the north and to higher formation rates associated with both colder and saltier surface waters.

Metatranscriptomics of N2-fixing cyanobacteria in the Amazon River plume.

Biological N2 fixation is an important nitrogen source for surface ocean microbial communities. However, nearly all information on the diversity and gene expression of organisms responsible for oceanic N2 fixation in the environment has come from targeted approaches that assay only a small number of genes and organisms. Using genomes of diazotrophic cyanobacteria to extract reads from extensive meta-genomic and -transcriptomic libraries, we examined diazotroph diversity and gene expression from the Amazon River plume, an area characterized by salinity and nutrient gradients. Diazotroph genome and transcript sequences were most abundant in the transitional waters compared with lower salinity or oceanic water masses. We were able to distinguish two genetically divergent phylotypes within the Hemiaulus-associated Richelia sequences, which were the most abundant diazotroph sequences in the data set. Photosystem (PS)-II transcripts in Richelia populations were much less abundant than those in Trichodesmium, and transcripts from several Richelia PS-II genes were absent, indicating a prominent role for cyclic electron transport in Richelia. In addition, there were several abundant regulatory transcripts, including one that targets a gene involved in PS-I cyclic electron transport in Richelia. High sequence coverage of the Richelia transcripts, as well as those from Trichodesmium populations, allowed us to identify expressed regions of the genomes that had been overlooked by genome annotations. High-coverage genomic and transcription analysis enabled the characterization of distinct phylotypes within diazotrophic populations, revealed a distinction in a core process between dominant populations and provided evidence for a prominent role for noncoding RNAs in microbial communities.

DOC and POC in the southern Baltic Sea. Part II – Evaluation of factors affecting organic matter concentrations using multivariate statistical methods

Summary This study is a continuation of the research on organic matter sources, distribution and dynamics in the southern Baltic Sea described in detail by Maciejewska and Pempkowiak (2014) . In this paper, cluster analysis, principal component analysis and segment analysis were used to assess relations among factors influencing organic matter concentration in the Baltic sea-water. The following sea-water properties, salinity (Sal), temperature (Temp), pH, concentrations of chlorophyll a (Chla) and phaeopigment a (Feo), were assessed, while dissolved (DOC) and particulate (POC) organic carbon were used as organic matter measures. Water samples were collected in the course of a three-year study (2009–2011) from the Gdansk Deep, the Gotland Deep and the Bornholm Deep (Southern Baltic). As a result, relations among both DOC and POC and the measured water properties were revealed. The cluster analysis leads to the discovery of the following structure of the analyzed water properties: DOC-pH, POC-Chla, without providing interpretation why the structure exists. Using the principal component analysis, factors influencing DOC and POC concentrations were classified as plankton activity and the inflows of saline and freshwater water masses as the study area. Segment regression analysis revealed that organic matter consists of labile and stable fractions and led to the quantification of relations between DOC and the measured sea-water properties. The following contributions to the DOC fluctuations were calculated: salinity – 11%, chlorophyll a – 26%, phaeopigment a – 26%, POC – 38% in the growing season and 31%, 33%, 21% and 22% respectively in the non-growing season.

An algorithm based on sea-level pressure fluctuations to identify major Baltic inflow events

Major Baltic inflows are an important process to sustain the sensitive steady state of the Baltic Sea. We introduce an algorithm to identify atmospheric variability favourable for major Baltic inflows. The algorithm is based on sea-level pressure (SLP) fields as the only parameter. Characteristic SLP pattern fluctuations include a precursory phase of 30 days and 10 days of inflow period. The algorithm identifies successfully the majority of observed major Baltic inflows between 1961 and 2010. In addition, the algorithm finds some occurrences which cannot be related to observed inflows. In these cases with favourable atmospheric conditions, inflows were precluded by contemporaneously existing saline water masses or strong freshwater supply. Moreover, the algorithm clearly identifies the stagnation periods as a lack of SLP variability favourable for MBIs. This indicates that the lack of inflows is mainly a consequence of missing atmospheric forcing during this period. The only striking inflow which is not identified by the algorithm is the event in January 2003. We demonstrate that this is due to the special evolution of SLP fields which are not comparable with any other event. Finally, the algorithm is applied to an ensemble of scenario simulations. The result indicates that the number of atmospheric events favourable for major Baltic inflows increases slightly in all scenarios.

An Arctic seal in temperate waters: History of the ringed seal (Pusa hispida) in the Baltic Sea and its adaptation to the changing environment

The ringed seal ( Pusa hispida) is an early immigrant in the Baltic Basin and has since its arrival experienced substantial changes in the climate, salinity and productivity of the Basin. In this paper, we discuss the dispersal and distribution of the ringed seal during different stages of the Baltic Sea in relation to past and ongoing environmental changes. Subfossil ringed seal remains around the Baltic Sea and the Danish Straits were radiocarbon dated in order to map the distribution of the species in different time periods. The δ13C data were used in evaluating the changes in the marine character of the Baltic Basin. The sequence of the dates indicates a continuous presence of the species in the Baltic Basin. The earliest ringed seal finds come from the Skagerrak/Kattegat area (Denmark, Swedish west coast) and date to the full glacial period and Baltic Ice Lake. In the Baltic Basin, the species appears in the subfossil record during the Ancylus period, but the main part of the remains date to the Littorina stage. During the Littorina stage, the distribution of the species was at least periodically wider than today, covering also southern parts of the Baltic. The presence of breeding populations in southern parts of the Baltic during the Holocene Thermal Maximum (HTM) indicates that the winters were at least periodically cold enough for winter ice. The changes in the marine influence in the Baltic Basin can be seen in the seal collagen δ13C values, which serve as a proxy for qualitative changes in water mass salinity.

Dense intermediate water outflow from the Cretan Sea: A salinity driven, recurrent phenomenon, connected to thermohaline circulation changes

AbstractData collected from different platforms in the Cretan Sea during the 2000s decade present evidence of gradually increasing salinity in the intermediate and deep intermediate layers after the middle of the decade. The observed gradual salt transport toward the deeper layers indicates contributions of dense water masses formed in various Aegean Sea subbasins. The accumulation of these saline and dense water masses in the Cretan Sea finally led to outflow from both Cretan Straits, with density greater than typical Levantine/Cretan Intermediate water but not dense enough to penetrate into the deep layers of the Eastern Mediterranean. We name this outflowing water mass as dense Cretan Intermediate Water (dCIW). A retrospective analysis of in situ data and literature references during the last four decades shows that similar events have occurred in the past in two occasions: (a) in the 1970s and (b) during the Eastern Mediterranean Transient (EMT) onset (1987–1991). We argue that these salinity‐driven Aegean outflows are mostly attributed to recurrent changes of the Eastern Mediterranean upper thermohaline circulation that create favorable dense water formation conditions in the Aegean Sea through salinity preconditioning. We identify these phenomena as “EMT‐like” events and argue that in these cases internal thermohaline mechanisms dominate over atmospheric forcing in dense water production. However, intense atmospheric forcing over an already salinity preconditioned basin is indispensable for creating massive deep water outflow from the Cretan Sea, such as the EMT event.

Abundance and distribution of scyllarid phyllosoma larvae (Decapoda: Scyllaridae) in the Balearic Sea (Western Mediterranean)

The distribution, abundance, and interannual variation of scyllarid phyllosoma larvae were studied in waters of the Balearic Sea (Western Mediterranean) from plankton samples collected between 2001 and 2005. Over the study period, 342 phyllosomas were collected, all belonging to Scyllaridae. Scyllarus arctus (Linnaeus, 1758) was the most abundant (89%), with representation of all developmental stages (I-X), except stage IX, plus two nistos. Scyllarus pygmaeus (Bate, 1888) phyllosoma included middle stages (IV-VII) and two final stages X. Finally, a stage II phyllosoma of Scyllarides latus (Latreille, 1802) was recorded for the first time in the Western Mediterranean. The majority of the phyllosomas were early stage S. arctus found in near shore stations, while advanced stages were common farther away from the coast, especially to the south of the Balearic Sea. That was also the case of S. pygmaeus, most of which stage V, observed to the southwest of the archipelago. Phyllosomas were scarce in low salinity (S<37.2) water masses of recent Atlantic origin, and interannual differences in distribution reflected the intrusions of low salinity Atlantic water from the South.

Variations of alkenone temperature in the Sea of Japan during the last 170 ka and its paleoceanographic implications

Two sediment cores, KCES1 and ODP797, which were recovered from the Sea of Japan (JS), were measured for alkenone-derived sea surface temperatures (\(U^{{\text{K}}^{\prime}}_{37} {\text{-SSTs}}\)). Our results revealed that the SSTs closely follow the glacial-interglacial cycles during the last 170 ka, except in the last glacial maximum (LGM), during which the SST was higher than in the Holocene. The anomalous high temperature in the LGM is considered as an effect of the intrusion of a low salinity water mass into the JS when the sea level was almost below 130 m. On the glacial-interglacial to orbital timescale, the \( U^{{\text{K}}^{\prime }}_{37} {\text{-SSTs}} \) record in the JS correlated well with the benthic foraminiferal δ18O record and solar insolation, which suggests the dominant control of solar insolation and its related sea ice development on the SST in the JS. On the sub-orbital/millennial timescale, reduced SST corresponds to an enhanced east asian winter monsoon (EAWM) during the last glacial period (MIS3 and MIS4), indicating the dominant control of sea ice expansion due to the enhanced EAWM on the SST in the JS. In contrast, during the last interglacial period (MIS5), the SST in the JS was controlled by variations in the east Asian summer monsoon. These results highlight the key role of solar insolation and associated glacial-interglacial conditions in the variations of the SST in the JS since the last 170 ka.

Seasonal variability of the equatorial undercurrent termination and associated salinity maximum in the Gulf of Guinea

The termination of the Equatorial Undercurrent (EUC) in the eastern equatorial Atlantic during boreal summer and fall, and the fate of the associated saline water masses, are analyzed from in situ hydrological and currents data collected during 19 hydrographic cruises between 2000 and 2007, complemented by observations from Argo profiling floats and PIRATA moorings, and from a numerical simulation of the Tropical Atlantic Ocean for the period 1993–2007. An intense variability of the circulation and hydrological properties is evidenced from observations in the upper thermocline (24.5–26.2 isopycnal layer) between June and November. During early boreal summer, saline water masses are transported eastward in the upper thermocline to the African coast within the EUC, and recirculate westward on both sides of the EUC. In mid-boreal summer, the EUC weakens in the upper thermocline and the equatorial salinity maximum disappears due to intense mixing with the surface waters during the upwelling season. The extra-equatorial salinity maxima are also partially eroded during the boreal summer, with a slight poleward migration of the southern hemisphere maximum until late boreal summer. The upper EUC reappears in September, feeding again the eastern equatorial Atlantic with saline waters until boreal spring. During December–January, numerical results suggest a second seasonal weakening of the EUC in the Gulf of Guinea, with a partial erosion of the associated equatorial salinity maximum.