Mixed Layer Salinity Research Articles

Dinoflagellate cyst based modern analogue technique at test — A 300 year record from the Gulf of Taranto (Eastern Mediterranean)

To test the performance of dinoflagellate cyst based palaeoenvironmental reconstructions derived via the modern analogue technique (MAT) and a non-quantitative method, we have compared, reconstructions of mixed layer temperature, sea surface temperature and mixed layer salinity of a well dated site from the Gulf of Taranto, to instrumental data of air temperature, precipitation and river discharge. Two hypotheses are tested: the reconstructions of dinoflagellate cyst based MAT improve when a) a local reference dataset rather than an extra-regional reference dataset is used, b) the reference and downcore datasets are corrected for species specific preservation prior to analysis. This is achieved by executing four experiments based on different reference datasets 1) a North Atlantic dataset including all species, 2) a North Atlantic dataset including degradation resistant species, 3) a Mediterranean dataset including all species, 4) a Mediterranean dataset including degradation resistant species only.We show that MAT based reconstructions improve when a local rather than an extra-regional reference dataset is used. Exclusively including species resistant to degradation improves the reconstruction if an extra-regional reference dataset is used, but leads to considerable loss of variability if the local reference dataset is used.Both MAT-based and qualitative reconstructions correlate to instrumental data for industrial times. For pre-industrial times, MAT reconstructions of variability in temperature and salinity do not co-vary with variability in the instrumental data, whereas qualitative reconstructions provide a relatively good fit. We suggest this to be a result of the anthropogenic influence on coastal marine ecosystems existing today at the majority of the coastal sample sites in the reference dataset. As a result, conditions at these sites do not correspond to pre-industrial conditions. We advise users of dinoflagellate cyst transfer functions to pay more attention to this aspect.

Decadal variations in mixed layer salinity in the Kuroshio Extension recirculation gyre region: influence of precipitation during the warm season

The salinity of the mixed layer in the Kuroshio Extension recirculation gyre (KERG) region (28°–35°N, 141°–160°E) was examined over a period of 20 years (1993–2012), and was found to show low-frequency variations on a decadal (~10 years) timescale: salinity decreased during the periods 1995–2001 and 2006–2009, but increased over the periods 2002–2005 and 2010–2012. Salinity anomalies at the sea surface during the warm season (June–October) influenced salinity in the following winter. These warm-season anomalies were caused by precipitation changes related to the number of low-pressure systems passing over the KERG associated with a westward elongation/eastward shrinkage of the summertime North Pacific subtropical high, which is driven by the Pacific–Japan (PJ) teleconnection pattern.

Pacific Ocean surface freshwater variability underneath the double ITCZ as seen by satellite sea surface salinity retrievals

AbstractThe salinity budget of the upper tropical Pacific Ocean underneath the double Intertropical Convergence Zone (ITCZ) is studied using the Soil Moisture and Ocean Salinity (SMOS) and Aquarius surface salinity observations as well as in situ salinity measurements. In this shallow mixed layer region of the ocean, precipitation effects on the near‐surface salinity budget are large, typically leading to a band of fresh sea surface salinity (SSS) between March and June. The role of precipitation during the freshening period is documented here through a direct correlation between the SMOS SSS fields and the monthly accumulated precipitation. During the same period, the mixed layer salinity budget is impacted by advection, which, based on in situ observations, is found to be another important mechanism for the evolution of the near‐surface salinity as documented through a connection between the north equatorial eastern Pacific Fresh water pool and this south equatorial freshwater pattern in boreal spring. However, given the information at hand, the near‐surface salinity budget cannot be closed, suggesting that other processes are important too, such as nonlinear effects, mixing, and entrainment.

Millennial-scale tropical atmospheric and Atlantic Ocean circulation change from the Last Glacial Maximum and Marine Isotope Stage 3

Abrupt, millennial-scale climate oscillations, known as Dansgaard–Oeschger (D–O) cycles, characterized the climate system of the last glacial period. Although proxy evidence shows that D–O cycles resulted in large-scale changes in atmospheric circulation patterns around the planet, an understanding of how Atlantic Meridional Overturning Circulation (AMOC) varied across these events remains unclear. Here, we take advantage of the fact that both tropical atmospheric circulation changes corresponding to north–south shifts in the Intertropical Convergence Zone (ITCZ) and large-scale changes in ocean circulation associated with AMOC variability can be reconstructed in the same sediment core from the Florida Straits to examine the relationship between atmospheric and ocean circulation changes across D–O events. To reconstruct surface water conditions, Mg/Ca-paleothermometry and stable isotope measurements were combined on the planktonic foraminifera Globigerinoides ruber (white variety) from sediment core KNR166-2 JPC26 (24°19.61′N, 83°15.14′W; 546 m depth) to reconstruct a high-resolution record of sea surface temperature and δOseawater18 (a proxy for upper mixed layer salinity) during Marine Isotope Stages (MIS) 2 and 3 from 20–35 ka BP. As an additional proxy for upper water column salinity change, we also generate a faunal abundance record of the salinity-sensitive planktonic foraminifera Neogloboquadrina dutertrei. Our results suggest that rapid reductions in sea surface salinity occurred at the onset of D–O interstadials, while stadials are characterized by increased surface salinities. The most likely cause of these salinity changes was variation in the strength and position of the ITCZ across D–O events. Finally, we examine the relationship between millennial-scale atmospheric circulation changes recorded in the planktonic records and ocean circulation changes inferred from the benthic δO18 record from our core. Our results provide some of the first evidence that AMOC strength did vary across at least one of the millennial-scale D–O cycles of MIS 3.

On the seasonal variations of salinity of the tropical Atlantic mixed layer

AbstractThe physical processes controlling the mixed layer salinity (MLS) seasonal budget in the tropical Atlantic Ocean are investigated using a regional configuration of an ocean general circulation model. The analysis reveals that the MLS cycle is generally weak in comparison of individual physical processes entering in the budget because of strong compensation. In evaporative regions, around the surface salinity maxima, the ocean acts to freshen the mixed layer against the action of evaporation. Poleward of the southern SSS maxima, the freshening is ensured by geostrophic advection, the vertical salinity diffusion and, during winter, a dominant contribution of the convective entrainment. On the equatorward flanks of the SSS maxima, Ekman transport mainly contributes to supply freshwater from ITCZ regions while vertical salinity diffusion adds on the effect of evaporation. All these terms are phase locked through the effect of the wind. Under the seasonal march of the ITCZ and in coastal areas affected by river (7°S:15°N), the upper ocean freshening by precipitations and/or runoff is attenuated by vertical salinity diffusion. In the eastern equatorial regions, seasonal cycle of wind forced surface currents advect freshwaters, which are mixed with subsurface saline water because of the strong vertical turbulent diffusion. In all these regions, the vertical diffusion presents an important contribution to the MLS budget by providing, in general, an upwelling flux of salinity. It is generally due to vertical salinity gradient and mixing due to winds. Furthermore, in the equator where the vertical shear, associated to surface horizontal currents, is developed, the diffusion depends also on the sheared flow stability.

Mixed-layer salinity budget in the tropical Indian Ocean: seasonal cycle based only on observations

The mixed-layer salinity (MLS) budget in the tropical Indian Ocean is estimated from a combination of satellite products and in situ observations over the 2004–2012 period, to investigate the mechanisms controlling the seasonal MLS variability. In contrast with previous studies in the tropical Indian Ocean, our results reveal that the coverage, resolution, and quality of available observations are now sufficient to approach a closed monthly climatology seasonal salt budget. In the South-central Arabian Sea and South-western Tropical Indian Ocean (SCAS and STIO, respectively), where seasonal variability of the MLS is pronounced, the monthly MLS tendency terms are well captured by the diagnostic. In the SCAS region, in agreement with previous results, the seasonal cycle of the MLS is mainly due to meridional advection driven by the monsoon winds. In the STIO, contrasting previous results indicating the control of the meridional advection over the seasonal MLS budget, our results reveal the leading role of the freshwater flux due to precipitation.

Ocean salinity changes in the northwest Pacific subtropical gyre: The quasi‐decadal oscillation and the freshening trend

AbstractOcean salinity changes play an important role in modulating ocean and climate variability. Analyses of the repeating observations along PN, TK, and 137°E sections reveal that both surface and subsurface salinity in the Northwest Pacific Subtropical Gyre (NPSG) had clear quasi‐decadal oscillation (QDO) of ∼10 year and a sustained freshening trend during 1987–2012. Surface salinity in the NPSG troughed in 1989, 1999, and 2008, and peaked in 1993, and 2003. The peak‐to‐tough range of surface salinity oscillation can reach 0.3 psu. Meanwhile, surface salinity decreased about 0.10 psu from 1987 to 2012 with a freshening trend of −0.0042 psu yr−1. These surface salinity anomalies were subducted into the subsurface layer mainly in the ventilated zone along the Kuroshio Extension, and advected over the NPSG. The QDO of the subsurface salinity maximum (Smax) lagged that of the surface salinity by about 1∼2 years. Both the peak‐to‐tough range of the Smax oscillation (0.15 psu) and its freshening trend (−0.0036 psu yr−1) are smaller than those of the surface salinity. Salinity changes in the NPSG likely began in the mid‐1970s associated with the North Pacific regime shift during 1976/1977. Analyses of mixed layer salinity budget indicated that air‐sea freshwater flux change in the NPSG is the leading factor controlling the surface salinity anomalies, while change of large‐scale ocean circulation (Geostrophic advection) also plays a minor role. Salinity and air‐sea freshwater flux changes in the NPSG are all closely related to the Pacific Decadal Oscillation, indicating the large‐scale ocean‐atmosphere interaction.

English

English

Mixed layer salinity budget in the tropical Pacific Ocean estimated by a global GCM

AbstractThe mixed layer salinity (MLS) budget of the tropical Pacific is investigated using results from a model of the Consortium for Estimating the Circulation and Climate of the Ocean (ECCO). The results focusing on the western Pacific freshwater pool indicate that the long‐term averaged surface freshwater flux is well balanced by ocean dynamics, in which the subsurface processes account for the major part. The MLS budget shows significant seasonal and interannual variability, as a consequence of interplay among surface freshwater flux, advection, mixing, and vertical entrainment. On seasonal time scale, both the MLS and mixed layer depth are largely controlled by surface freshwater flux. The opposite phase between the subsurface processes and the barrier layer thickness confirms the important influence of the barrier layer on vertical mixing and entrainment from below. On interannual time scale, all the MLS budget terms show significant ENSO signal, which in turn is highly correlated with the salinity front and barrier layer thickness in the equatorial Pacific.

An upper ocean response to Typhoon Bolaven analyzed with Argo profiling floats

In situ observations from Argo profiling floats combined with satellite retrieved SST and rain rate are used to investigate an upper ocean response to Typhoon Bolaven from 20 through 29 August 2012. After the passage of Typhoon Bolaven, the deepening of mixed layer depth (MLD), and the cooling of mixed layer temperature (MLT) were observed. The changes in mixed layer salinity (MLS) showed an equivalent number of increasing and decreasing because the typhoon-induced salinity changes in the mixed layer were influenced by precipitation, evaporation, turbulent mixing and upwelling of thermocline water. The deepening of the MLD and the cooling of the MLT indicated a significant rightward bias, whereas the MLS was freshened to the left side of the typhoon track and increased on the other side. Intensive temperature and salinity profiles observed by Iridium floats make it possible to view response processes in the upper ocean after the passage of a typhoon. The cooling in the near-surface and the warming in the subsurface were observed by two Iridium floats located to the left side of the cyclonic track during the development stage of the storm, beyond the radius of maximum winds relative to the typhoon center. Water salinity increases at the base of the mixed layer and the top of the thermocline were the most obvious change observed by those two floats. On the right side of the track and near the typhoon center when the typhoon was intensified, the significant cooling from sea surface to a depth of 200×104 Pa, with the exception of the water at the top of the thermocline, was observed by the other Iridium float. Owing to the enhanced upwelling near the typhoon center, the water salinity in the near-surface increased noticeably. The heat pumping from the mixed layer into the thermocline induced by downwelling and the upwelling induced by the positive wind stress curl are the main causes for the different temperature and salinity variations on the different sides of the track. It seems that more time is required for the anomalies in the subsurface to be restored to pretyphoon conditions than for the anomalies in the mixed layer.

Modeled mixed-layer salinity balance in the Gulf of Guinea: seasonal and interannual variability

A regional numerical simulation and observations were used to investigate the various processes controlling mixed-layer salinity balance on seasonal and interannual time scales in the Gulf of Guinea. Processes were quantified using a mixed-layer salt budget. Model results correctly reproduced the mean, phase, and amplitude of observed seasonal near-surface salinity. The results indicated that on seasonal time scales, the mixed-layer salinity balance differed from one region to another. The surface salinity seasonal cycle was characterized by strong salinization during May for coastal areas north and south of the equator. Model results suggested that vertical mixing controls the mixed-layer salinity increase at the equator during May, while both vertical mixing and vertical advection contribute to the salinity increase in coastal regions. We also determined that freshening from horizontal advection and freshwater flux tended to balance the salinization effects of vertical diffusion and vertical advection during the seasonal cycle. On interannual time scales, based on the mixed-layer salinity balance and sensitivity experiments, we determined that for the northern and equatorial Gulf of Guinea, changes in near-surface salinity were largely due to changes in precipitation and winds. For the southern Gulf of Guinea, only wind changes were determined to be important for explaining near-surface salinity changes.

Causes for the recent increase in sea surface salinity in the north-eastern Gulf of Guinea

In situ sea surface salinity (SSS) observations showed an increase >0.5 over the period 2002–2009 in the Gulf of Guinea, off the Niger Delta. Observed changes in the Niger River runoff were not consistent with this increase in SSS, but the increase was reproduced in a regional numerical simulation with climatological river runoff. The simulated mixed-layer salinity budget was used to identify the mechanisms responsible for the increase. When comparing the period 2002–2009 with the period 1993–2001, significant changes in the salt budget were identified. The increase in SSS in the more recent period appeared to be driven by changes in the atmospheric freshwater flux, mainly attributed to a regional decrease in precipitation. Horizontal advection partly compensated for the effect of freshwater flux through changes in zonal currents and zonal SSS gradients.

Formation and Subduction of North Pacific Tropical Water and Their Interannual Variability

Abstract Formation and subduction of the North Pacific Tropical Water (NPTW), its interannual variability, and its associated mechanisms were investigated by using gridded Argo-profiling float data and various surface flux data in 2003–11. The NPTW has two formation sites in the center of the North Pacific subtropical gyre, corresponding to two regional sea surface salinity maxima. Mixed layer salinity variations in these two NPTW formation sites were found to be significantly different. While seasonal variation was prominent in the eastern formation site, interannual variation was dominant in the western site. The mixed layer salinity variation in the eastern site was controlled mainly by evaporation, precipitation, and entrainment of fresher water below the mixed layer and was closely related to the seasonal variation of the mixed layer depth. In the western site, the effect of entrainment is small due to a small vertical difference in salinity across the mixed layer base, and excess evaporation over precipitation that tended to be balanced by eddy diffusion, whose strength varied interannually in association with the Pacific decadal oscillation. After subduction, denser NPTW that formed in the eastern site dissipated quickly, while the lighter one that formed in the western site was advected westward as far as the Philippine Sea, transmitting the interannual variation of salinity away from its formation region.

Entrainment‐driven modulation of Southern Ocean mixed layer properties and sea ice variability in CMIP5 models

The evolution of the upper Southern Ocean hydrographic structure in response to the representative concentration pathways 4.5 (RCP 4.5) forcing scenario is analyzed using model data drawn from the coupled model intercomparison project phase 5 (CMIP5) archive. A robust freshening trend is evident, associated with an increase in stratification and decoupling of the upper ocean as the mixed layer gains buoyancy at a faster rate than the underlying ocean. The magnitudes of the individual terms of the salinity and heat budgets are evaluated. Convection‐driven entrainment from the thermocline into the mixed layer is found to play a significant role in modulating the mixed layer salinity, whilst the heat budget of the mixed layer is dominated by a primary balance between atmospheric warming and the entrainment‐modulated supply of oceanic heat from below the mixed layer. The relationship between oceanic heat storage below the mixed layer, ice thickness and atmospheric temperature is investigated, and a very disparate response noted amongst the models considered here. Based on this analysis, we hypothesize that the balance between the entrainment‐modulated supply of oceanic heat from below the mixed layer and the heat supplied by the atmosphere may play an important role in determining the simulated sea ice variability.

Interannual sea surface salinity variations observed in the tropical North Pacific Ocean

AbstractAnalysis of observational data sets reveals pronounced interannual variations of sea surface salinity (SSS) in the tropical North Pacific (7°N–15°N) during 2000–2012. SSS anomalies with maximum magnitudes >0.2 occur in the central Pacific and translate westward at a speed of 15–20 cm s–1. The signals are weakened during their westward movement but reinforced in the Philippine Sea. Budget analysis for the mixed layer salinity suggests that in the central Pacific, El Niño–Southern Oscillation‐related atmospheric freshwater forcing and ocean advection changes are both important in generating and maintaining these large SSS anomalies. In the advection term, the most contributing component is the meridional Ekman advection induced by trade winds. These SSS anomalies are subsequently carried westward by the North Equatorial Current, which is the primary cause of SSS variations in the Philippine Sea. Freshwater forcing is also at work in the Philippine Sea, but its role is generally secondary.

An assessment of the mixed layer salinity budget in the tropical Pacific Ocean. Observations and modelling (1990–2009)

This paper investigates mechanisms controlling the mixed-layer salinity (MLS) in the tropical Pacific during 1990–2009. We use monthly 1° × 1° gridded observations of salinity, horizontal current and fresh water flux, and a validated ocean general circulation model with no direct MLS relaxation in both its full resolution (0.25° and 5 days) and re-sampled as the observation time/space grid resolution. The present study shows that the mean spatial distribution of MLS results from a subtle balance between surface forcing (E − P, evaporation minus precipitation), horizontal advection (at low and high frequencies) and subsurface forcing (entrainment and mixing), all terms being of analogous importance. Large-scale seasonal MLS variability is found mainly in the Intertropical and South Pacific Convergence Zones due to changes in their meridional location (and related heavy P), in the North Equatorial Counter Currents, and partly in the subsurface forcing. Maximum interannual variability is found in the western Pacific warm pool and in both convergence zones, in relation to El Nino Southern Oscillation (ENSO) events. In the equatorial band, this later variability is due chiefly to the horizontal advection of low salinity waters from the western to the central-eastern basin during El Nino (and vice versa during La Nina), with contrasted evolution for the Eastern and Central Pacific ENSO types. Our findings reveal that all terms of the MLS equation, including high-frequency (<1 month) salinity advection, have to be considered to close the salinity budget, ruling out the use of MLS (or sea surface salinity) only to directly infer the mean, seasonal and/or interannual fresh water fluxes.

Seasonal mixed‐layer salinity balance in the tropical Atlantic Ocean: Mean state and seasonal cycle

AbstractWe investigate the causes of the seasonal cycle of the near‐surface salinity using a mixed‐layer salinity model and a combination of satellite products, atmospheric reanalyses, and in situ observations for the period 2000–2008, in the tropical Atlantic Ocean. We find that the balance differs from one region to another. In the western tropical Atlantic, it is controlled by horizontal advection from March to November and by freshwater flux and entrainment for the rest of the year. In the central tropical Atlantic, it is mainly due to the strong contribution of precipitation in agreement with previous results. In the northeastern tropical Atlantic, all terms contribute to the mixed layer salinity between December and March; during the rest of the year, precipitation and zonal advection mainly control the balance. In the Gulf of Guinea, it is driven by freshwater flux from October to February; from March to July, it is controlled by horizontal advection and entrainment; from August to September, mixed‐layer salinity variability is weak. Finally, in the Congo region, it is driven by freshwater flux (precipitation and runoff from Congo River) from September to December, by horizontal advection during January to March, and by vertical entrainment during the rest of the year (April to August). There are some discrepancies between observed and modeled salinity tendencies. Some of them are due to our model formulation, which does not explicitly account for the effect of vertical diffusion. Uncertainties of observation products, which force the model, are also sources of errors.

Temperature and salinity variability in the south-eastern corner of the Bay of Biscay (NE Atlantic)

Temperature- and salinity-balance equations together with a one-dimensional model are used in this work to investigate the governing processes of the surface temperature and salinity variability at low and high frequencies in the southern Bay of Biscay. We found that the temporal evolution of mixed layer temperature is dominated by its seasonal cycle, mostly explained through a vertical balance (80–90%) that is mostly driven by air–sea heat exchange and contributed by entrainment, while horizontal advection plays a secondary role (≤12%). A substantial imbalance can be noticed in spring, which surprisingly strongly diminishes from 2003 onward, coinciding with apparent shifts in surface heat flow. The uncertainties of the air–sea heat fluxes and the advective and entrainment terms seem to be responsible for the imbalance of the temperature budget. On the contrary, the interannual variability dominates over the seasonal cycle of mixed layer salinity, and advection accounts for more than 70% of the budget terms, with the sum of vertical processes playing a secondary role. The different contributions of air–sea exchanges on temperature and salinity (heat vs. freshwater) are the ultimate cause of the different patterns observed for both of the parameters.

Freshwater flux product reconstruction based on Argo data and mixed layer model

Sea surface freshwater flux(FWF, evaporation minus precipitation) plays an important role in understanding and modeling atmosphere and ocean processes, but there exists strong disagreement different freshwater flux estimates. In this study, a weekly sea surface FWF product is reconstructed for the period from 2002 through 2008 based on the mixed layer salinity balance model for the first time, by combining the Argo data with OSCAR current data. Compared with the other precipitation and evaporation data from satellite product and TAO observations, the reconstructed FWF can capture not only the seasonal cycle of freshwater flux, but also some high-frequency features of freshwater flux. The FWF product provides a useful tool for studying the climate variation over the ocean.

A global relationship between the ocean water cycle and near-surface salinity

[1] Ocean evaporation (E) and precipitation (P) are the fundamental components of the global water cycle. They are also the freshwater flux forcing (i.e., E-P) for the open ocean salinity. The apparent connection between ocean salinity and the global water cycle leads to the proposition of using the oceans as a rain gauge. However, the exact relationship between E-P and salinity is governed by complex upper ocean dynamics, which may complicate the inference of the water cycle from salinity observations. To gain a better understanding of the ocean rain gauge concept, here we address a fundamental issue as to how E-P and salinity are related on the seasonal timescales. A global map that outlines the dominant process for the mixed-layer salinity (MLS) in different regions is thus derived, using a lower-order MLS dynamics that allows key balance terms (i.e., E-P, the Ekman and geostrophic advection, vertical entrainment, and horizontal diffusion) to be computed from satellite-derived data sets and a salinity climatology. Major E-P control on seasonal MLS variability is found in two regions: the tropical convergence zones featuring heavy rainfall and the western North Pacific and Atlantic under the influence of high evaporation. Within this regime, E-P accounts for 40–70% MLS variance with peak correlations occurring at 2–4 month lead time. Outside of the tropics, the MLS variations are governed predominantly by the Ekman advection, and then vertical entrainment. The study suggests that the E-P regime could serve as a window of opportunity for testing the ocean rain gauge concept once satellite salinity observations are available.