Eddy-resolving Ocean General Circulation Model Research Articles

An ensemble-based data assimilation system for forecasting variability of the Northwestern Pacific ocean

AbstractAn adjoint-free four-dimensional variational (a4dVar) data assimilation (DA) is implemented in an operational ocean forecast system based on an eddy-resolving ocean general circulation model for the Northwestern Pacific. Validation of the system against independent observations demonstrates that fitting the model to time-dependent satellite altimetry during a 10-day DA window leads to substantial skill improvements in the succeeding 60-day hindcast. The a4dVar corrects representation of the Kuroshio path variation south of Japan by adjusting the dynamical balance between amplitude/wavelength of the meander and flow advection. A larger ensemble spread tends to reduce the skill in representing the observed sea surface height anomaly, suggesting that it is possible to use the ensemble information for quantifying the forecast error. The ensemble information is also utilized for modification of the background error covariance (BEC), which improves the accuracy of temperature and salinity distributions. The modified BEC yields the skill decline of the Kuroshio path variation during the 60-day hindcast period, and the ensemble sensitivity analysis shows that changes in the dynamical balance caused by the ensemble BEC result in such skill deterioration.

Subsurface eddies east of the Philippines: Geographic characteristics, vertical structures, volume and thermohaline transport

Energetic subsurface eddies (SSEs) play a significant role in regulating the subthermocline circulation east of the Philippines. However, due to the paucity of targeted observations, they remain largely unexplored. By analyzing the outputs from an eddy-resolving ocean general circulation model (OGCM), this study investigated the statistics of SSEs east of the Philippines, including their geographic characteristics, vertical structures, and eddy-induced transport. During the period of 2009–2019, approximately 1927 and 1176 SSEs were detected to be anticyclonic and cyclonic, respectively, indicating the predominance of subsurface anticyclonic eddies (SSAE). The hotspot area of SSEs is in the latitude band of 6° N–15° N off the Philippine coast, especially around 10° N and 14° N–15° N prevailing abundant SSEs. Most SSEs originate at ∼138° E furthest with a mean lifespan of 55 days and a westward translation speed of 6 cm/s. The composite SSAE exhibits a typical subsurface-intensified feature with a velocity core at ∼520 m, while the subsurface cyclonic eddy (SSCE) has a relatively shallow core at ∼420 m and exhibits considerable strength near the surface with a rotating speed is around 5 cm/s. Temperature anomalies induced by SSEs show a dual-core structure associated with lens-like isopycnal undulations, and salinity anomalies are characterized by an alternating positive and negative signal owing to the complexity of the water masses. SSEs-induced meridional volume transport mainly occurs off the Philippine coast, where the northward transport is approximately 0.3 Sv and the southward transport is approximately 0.5 Sv. The zonal volume transport of SSEs is 1–2 Sv per latitude and mainly occurs in the North Equatorial Current (NEC) region. SSEs-induced stirring heat/salt transport is 1–2 orders larger than the trapping component and is mainly concentrated near the Mindanao coast. The meridional stirring heat transport is equatorward, which is up to 2.7 × 107 W/m, and the salt transport is northward with a magnitude of ∼100 kg·m−1·s−1 between 200 m and 2000 m. At the 8°N section, the mean salt transport associated with SSEs is about 24% of that induced by the Mindanao Undercurrent (MUC).

Energetics of eddy–mean flow interactions in the deep western boundary current off the northeastern coast of Brazil

Thirty-six years output of a 1/10° eddy-resolving Ocean General Circulation Model are used to analyze the energetics of eddy–mean flow interactions in the Deep Western Boundary Current (DWBC) region of the tropical South Atlantic between 5°S and 16°S. The DWBC flow has a coherent structure between 5°S and 8°S but breaks up into a train of eddies downstream of a region of strong bathymetric curvature at 8°S. In the train of eddies area, the seasonal cycle of eddy kinetic energy (EKE) exhibits poleward phase propagation from May to September. The connection between the seasonal cycle of mean kinetic energy and EKE indicates an intensification of the downstream eddy field associated with enhanced upstream mean flow. The magnitudes of the baroclinic conversion and vertical eddy density flux terms are small in the DWBC core layer depth but somewhat elevated 500 m above and below the core. Eddy processes, including eddy generation and propagation, are accompanied by high EKE and large barotropic conversion. While in the global ocean baroclinic conversion is thought to dominate the energy transfer to EKE, our results suggest that barotropic energy conversion is the primary source of EKE and modulates its variability in the DWBC region of the deeper ocean.

Cold- versus warm-season-forced variability of the Kuroshio and North Pacific subtropical mode water

The ocean responds to atmospheric variations. Changes in sea surface winds, surface air temperature, and surface air humidity cause upper ocean variability by modulating air-sea momentum and heat exchanges. Upper ocean variability in the mid-latitudes on inter-annual and longer timescales has previously been considered to be attributable to atmospheric variations in the cold season, because atmospheric forcing is stronger in the cold season than in the warm season. However, this idea has not been sufficiently confirmed yet. Although the ocean model is a useful tool to evaluate the impact of the atmospheric forcing in each season, there are no past studies having examined ocean model responses respectively to the cold- and warm-season atmospheric forcing. In this study, we performed numerical experiments with an eddy-resolving ocean general circulation model and investigated oceanic responses to cold- and warm-season atmospheric forcing, focusing on the Kuroshio and North Pacific subtropical mode water (STMW) in the western mid-latitude North Pacific. We found that temporal variations of net Kuroshio transport and STMW distribution/temperature are dominantly controlled by atmospheric forcing in the cold season. These results suggest that cold-season atmospheric variations are key to obtaining insights into large-scale upper ocean variability in the North Pacific subtropical gyre.

Simulated decadal variations of surface and subsurface phytoplankton in the upstream Kuroshio Extension region

Using outputs from an ecosystem model embedded in an eddy-resolving ocean general circulation model that can realistically simulate decadal modulations of the Kuroshio Extension (KE) between stable and unstable states, decadal variations of phytoplankton concentration in the upstream KE region are investigated. During stable states of the KE, surface phytoplankton concentrations are anomalously suppressed to the south of the KE front, while those to the north are anomalously enhanced. Although the surface phytoplankton concentration anomalies are prominent only during winter to spring, significant subsurface anomalies centered around 60 m depth persist even in summer and autumn. Anomalies persist throughout the year in phytoplankton biomass integrated over the upper 100 m despite the strong surface anomalies during the spring bloom season. An analysis of nitrogen concentration anomalies suggests that the vertical movement of the isopycnal surfaces, vertical mixing of nutrients, and meridional shifts in the KE jet contribute to the anomalous phytoplankton biomass.

Seasonal variability of eddy kinetic energy in the East Australian current region

The East Australian Current (EAC) is an important western boundary current of the South Pacific subtropical Circulation with high mesoscale eddy kinetic energy (EKE). Based on satellite altimeter observations and outputs from the eddy-resolving ocean general circulation model (OGCM) for the Earth Simulator (OFES), the seasonal variability of EKE and its associated dynamic mechanism in the EAC region are studied. High EKE is mainly concentrated in the shear-region between the poleward EAC southern extension and the equatorward EAC recirculation along Australia's east coast, which is confined within the upper ocean (0-300 m). EKE in this area exhibits obvious seasonal variation, strong in austral summer with maximum (465±89 cm² s-²) in February and weak in winter with minimum (334±48 cm² s-²) in August. Energetics analysis from OFES suggests that the seasonal variability of EKE is modulated by the mixed instabilities composed of barotropic and baroclinic instabilities confined within the upper ocean, and barotropic instability (baroclinic instability) is the main energy source of EKE in austral summer (winter). The barotropic process is mainly controlled by the zonal shear of meridional velocities of the EAC southern extension and the EAC recirculation. The poleward EAC southern extension and the equatorward EAC recirculation are synchronously strengthened (weakened) due to the local positive (negative) sea level anomalies (SLA) under geostrophic equilibrium, and the barotropic instability dominated by zonal shear is enhanced (slackened), which results in a high (low) level of EKE in the EAC region.

Energetics Analysis of the Eddy–Kuroshio Interaction East of Taiwan

Abstract The three-dimensional energetics evolution during eddy–Kuroshio interactions east of Taiwan is systematically investigated in a time-dependent theoretical framework using outputs from an eddy-resolving ocean general circulation model. Composite analyses are conducted based on 17 anticyclonic eddies (AEs) and 19 cyclonic eddies (CEs). These westward propagating mesoscale eddies impinge on the Kuroshio at ∼22°N, ∼124.5°E and interact with the Kuroshio with a mean duration of ∼70 days. During the interaction, all the eddy energy reservoirs and eddy–mean flow energy conversions exhibit complex spatial–temporal variations. In particular, during the strong interaction period (days 18–54), both AEs and CEs are deformed into an elliptic shape, with the major axis in the northeast–southwest direction due to the squeeze of surrounding eddies, and obtain kinetic energy from the mean flow. Overall, the eddies are weakened gradually after encountering the Kuroshio, with the energy of CEs decreased more rapidly than that of AEs. The eddies decay through two pathways: transferring ∼8% of eddy available potential energy (EPE) to the mean flow, and converting ∼64% of EPE to eddy kinetic energy (EKE) via the baroclinic instability with the majority of the EKE finally dissipated. The results suggest that although the time-dependent energy conversion terms vanish upon time averaging, they play important but opposite roles in the evolution of AEs and CEs. The analysis in this work is on the synoptic and intraseasonal time scales; hence, it provides a basis for understanding the long-term variations of the eddy–Kuroshio interaction and associated climate change.

Two Flavors of Intraseasonal Variability and Their Dynamics in the North Equatorial Current/Undercurrent Region

Significant surface-intensified and subsurface-intensified intraseasonal variability (ISV) of the North Equatorial Current/Undercurrent with different periods are detected to coexist with mooring ADCP measurements at 13°N, 130°E. The ISV of the currents in the upper 200 m has a relatively shorter period of 45 days, while the period of the subsurface-intensified ISV between 400 and 800 m is around 85 days. By combining with sea surface height measurements from satellite altimeters and outputs from an eddy-resolving ocean general circulation model, the origin and dynamic mechanism of the two flavors of ISV are investigated. Eddy trajectory tracking and energy analysis indicate that both the surface-intensified and subsurface-intensified ISV are related to locally generated meso-scale eddies near the mooring sites. Stability analysis suggests the surface-intensified ISV is related to the baroclinic instability induced by the vertical velocity shear of the North Equatorial Current. While the generation of the subsurface-intensified ISV is more complex and is partly related to the enhanced barotropic instability induced by the intensified horizontal shear of the subsurface zonal background flow.

Seasonal Variability of Subthermocline Eddy Kinetic Energy East of the Philippines

AbstractSeasonal modulation of subthermocline eddy kinetic energy (EKE) east of the Philippines and its associated dynamics are studied, using mooring measurements and outputs from an eddy-resolving ocean general circulation model for the period from 2000 to 2017. Significantly high EKE appears below the thermocline in the latitude band between 5° and 14°N east of the Philippines. Separated by 10°N, the EKE in the northern and southern parts of the region shows nearly opposite seasonal cycles, with its magnitude reaching a maximum in early spring and minimum in summer in the northern part and reaching a maximum in summer and minimum in winter in the southern part of the region. Further investigation indicates that both baroclinic and barotropic instabilities are essential in generating the subthermocline eddies, but the seasonal variation of subthermocline EKE is mainly caused by the seasonal modulation of barotropic instability. The seasonal modulation of barotropic instability in the northern and southern part of the region is associated with the seasonal evolution of North Equatorial Undercurrent and Halmahera Eddy, respectively.

On the statistics of the zonal jets in the eastern equatorial Pacific and eastern North Pacific in an ensemble of eddy-resolving ocean general circulation model runs

It has been known for some time that the ocean basins are populated by what is known as “zonal jets”, “deep zonal jets”, or “striations”. Since the oceanic flow is, at least weakly, chaotic, it is not known whether the positions of the jets are “deterministic”, that is, entirely determined by external parameters. A number of theories have been proposed to explain them, some of them predicting zonal jets at fixed latitudes and others implying that the positions of the jets are random. To investigate how deterministic the zonal jets are in the eastern North Pacific, a ten-member ensemble of long-term integrations of a semi-global, eddy-resolving ocean general circulation model is analyzed. The positions of the equatorial jets, even their variability, seem to obey deterministic dynamics and some of the jets in the tropics (5°–15°N) migrate poleward coherently (similarly between ensemble members). The jets in the subtropics (15°–45°N) systematically migrate equatorward but their positions are less coherent; the jets in the subpolar region (45°N–) are random and without systematic migration. Jets near the coast of North and South America tend to have shorter meridional wavelengths than interior ones and those in the northern hemisphere are fairly coherent whereas those in the southern hemisphere seem more random. There are a few quasi-barotropic jets which are anchored to steep bottom topographic features and which also appear to trap shallower counter-flows on their poleward and equatorward flanks.

Atmospheric-Driven and Intrinsic Interannual-to-Decadal Variability in the Kuroshio Extension Jet and Eddy Activities

To investigate influences of oceanic intrinsic/internal variability and its interannual-to-decadal modulations on the Kuroshio Extension (KE) jet speed and associated eddy activity, a ten-member ensemble integration of an eddy-resolving ocean general circulation model forced by 1965-2016 atmospheric reanalysis is conducted. We found a distinct time-scale dependence in the ratio of forced and intrinsic variability of the KE jet speed. On decadal time scale, the ratio of the magnitude of intrinsic variability to that of atmospheric-driven variability is 0.69, suggesting it is largely atmospheric-driven. In contrast, on interannual time scales, the KE jet speed has large ensemble spread, indicating that it is strongly affected by intrinsic variability and has substantial uncertainty. For eddy activity, ratios of variability in ensemble average and spread also depend on region. In the downstream KE [32°-38°N, 153°-165°E], variability in the ensemble average eddy activity dominates (1.5 times) over rms of its ensemble spread on decadal time scale, and is positively correlated with current speed. Ensemble mean eddy activity in the downstream KE region is correlated (r=0.59) with ensemble mean current speed variability in the central North Pacific 4 years earlier, consistent with westward propagation of wind-driven jet speed anomalies. This linkage is robust even for each ensemble member with the significant lagged correlation also found in seven out of ten ensemble members, suggesting possibility of prediction of the eddy activity. In contrast, eddy activity in the upstream KE [32°-38°N, 141°-153°E] shows very large intrinsic and limited atmospheric-driven variability with a ratio of the former to the latter of 1.96. These results suggest intrinsic variability needs to be considered in interannual variability of strong ocean jet. The dependence of these findings to the model specificities need to be further explored.

Effects of Kuroshio intrusion optimization on the simulation of mesoscale eddies in the northern South China Sea

The impacts of Kuroshio intrusion (KI) optimization on the simulation of meso-scale eddies (MEs) in the northern South China Sea (SCS) were investigated based on an eddy-resolving ocean general circulation model by comparing two numerical experiments with differences in their form and intensity of KI due to the optimizing topography at Luzon Strait (LS). We found that a reduced KI reduces ME activities in the northern SCS, which is similar to the observations. In this case, the biases of the model related to simulating the eddy kinetic energy (EKE) west of the LS and along the northern slope are remarkably attenuated. The reduced EKE modeling bias is associated with both the reduced number of anti-cyclonic eddies (AEs) and the reduced amplitude of cyclonic eddies (CEs). The EKE budget analysis further suggests that the optimization of the KI will change the EKE by changing the horizontal velocity shear and the slope of the thermocline, which are related to barotropic and baroclinic instabilities, respectively. The former plays the key role in regulating the EKE in the northern SCS due to the changing of the KI. The EKE advection caused by the KI is also important for the EKE budget to the west of the LS.

Generation Mechanism of Tropical Instability Waves in the Equatorial Pacific Ocean

AbstractTropical instability waves (TIWs) are prominent features in the equatorial Pacific, propagating westward at a speed of ~0.5 m s−1 with a wavelength of ~1000 km. In this study, we show that a linear stability analysis using a 1.5-layer shallow water model can predict successfully an unstable mode whose wavelength, phase speed, growth rate, and meridional structure are all consistent with those of the TIWs simulated by an eddy-resolving ocean general circulation model (OGCM). This unstable mode can be interpreted as resulting from the coupling of two Rossby waves, namely, one trapped just north of the equator (~1°–3.5°N) and the other trapped farther north (~3.5°–8°N). Although these two Rossby waves have opposite intrinsic phase propagation directions reflecting the negative and positive local meridional potential vorticity (PV) gradients, respectively, their actual propagation direction can be adjusted through the advection by the South Equatorial Current and the North Equatorial Countercurrent such that they might propagate westward at the same speed and with the same zonal wavenumber yielding the largest growth rate of TIWs. The unstable mode does not appear during the period in which the negative PV gradient is absent, which demonstrates its essential role in generating TIWs. Indeed, the seasonal and interannual variability of the TIWs simulated by the OGCM is shown to be significantly controlled by the strength of the negative PV gradient just north of the equator, suggesting that it could be a key parameter toward a dynamically based parameterization of the heat and momentum transfer associated with TIWs in coarse-resolution OGCMs.

A Geometric Interpretation of Southern Ocean Eddy Form Stress

AbstractAn interpretation of eddy form stress via the geometry described by the Eliassen–Palm flux tensor is explored. Complimentary to previous works on eddy Reynolds stress geometry, this study shows that eddy form stress is fully described by a vertical ellipse, whose size, shape, and orientation with respect to the mean flow shear determine the strength and direction of vertical momentum transfers. Following a recent proposal, this geometric framework is here used to form a Gent–McWilliams eddy transfer coefficient that depends on eddy energy and a nondimensional geometric parameter α, bounded in magnitude by unity. The parameter α expresses the efficiency by which eddies exchange energy with baroclinic mean flow via along-gradient eddy buoyancy flux—a flux equivalent to eddy form stress along mean buoyancy contours. An eddy-resolving ocean general circulation model is used to estimate the spatial structure of α in the Southern Ocean and assess its potential to form a basis for parameterization. The eddy efficiency α averages to a low but positive value of 0.043 within the Antarctic Circumpolar Current, consistent with an inefficient eddy field extracting energy from the mean flow. It is found that the low eddy efficiency is mainly the result of that eddy buoyancy fluxes are weakly anisotropic on average. The eddy efficiency is subject to pronounced vertical structure and is maximum at ~3-km depth, where eddy buoyancy fluxes tend to be directed most downgradient. Since α partly sets the eddy form stress in the Southern Ocean, a parameterization for α must reproduce its vertical structure to provide a faithful representation of vertical stress divergence and eddy forcing.

Impacts of the mid-latitude westerlies anomaly on the decadal sea level variability east of China

The present analysis has revealed that both the altimetry and tide gauge records show a prominent sea level rise from 1993 to 2016 in the coastal seas east of China, including Bohai, Yellow Sea, and East China Sea (BYECS). The regional mean sea level rise rate for the BYECS continental shelf area is ~ 3.2 mm/year, which is close to the global mean (2.9 mm/year). Besides the rising trend, the BYECS sea level anomaly (SLA) also shows obvious interannual and decadal oscillation. It is found that the mid-latitude westerlies, rather than the steric height variability, are responsible for these oscillations. By analyzing the 67-year (1950–2016) hindcast outputs from an eddy-resolving ocean general circulation model, a 14.2 years decadal period is found to dominate the BYECS SLA variability, and the decadal oscillation is positively correlated with the North Pacific Gyre Oscillation (NPGO). The composite analysis and causality analysis demonstrate that during the positive phase of the NPGO, the mid-latitude westerlies decrease and a positive wind stress curl anomaly at the same latitude of the East China Sea’s Kuroshio occurs to the south of the westerlies. Consequently, the Kuroshio Current volume transport and sea surface height difference between the two sides of the Kuroshio decrease, that is, the sea level in the BYECS rises.

Long-term evolution of Caspian Sea thermohaline properties reconstructed in an eddy-resolving ocean general circulation model

Abstract. Decadal variability in Caspian Sea thermohaline properties is investigated using a high-resolution ocean general circulation model including sea ice thermodynamics and air–sea interaction forced by prescribed realistic atmospheric conditions and riverine runoff. The model describes synoptic, seasonal and climatic variations of sea thermohaline structure, water balance, and sea level. A reconstruction experiment was conducted for the period of 1961–2001, covering a major regime shift in the global climate during 1976–1978, which allowed for an investigation of the Caspian Sea response to such significant episodes of climate variability. The model reproduced sea level evolution reasonably well despite the fact that many factors (such as possible seabed changes and insufficiently explored underground water infiltration) were not taken into account in the numerical reconstruction. This supports the hypothesis relating rapid Caspian Sea level rise in 1978–1995 with global climate change, which caused variation in local atmospheric conditions and riverine discharge reflected in the external forcing data used, as is shown in the paper. Other effects of the climatic shift are investigated, including a decrease in salinity in the active layer, strengthening of its stratification and corresponding diminishing of convection. It is also demonstrated that water exchange between the three Caspian basins (northern, middle and southern) plays a crucial role in the formation of their thermohaline regime. The reconstructed long-term trends in seawater salinity (general downtrend after 1978), temperature (overall increase) and density (general downtrend) are studied, including an assessment of the influence of main surface circulation patterns and model error accumulation.

The three-dimensional structure of the Leeuwin Current System in density coordinates in an eddy-resolving OGCM

The Leeuwin Current System (LCS) consists of the Leeuwin Current (LC), a surface, poleward current along the west coast of Australia; the Leeuwin Undercurrent (LUC), a subsurface, equatorward current on the continental slope beneath the LC; and zonal flows in the interior ocean that interact with the LC and LUC. To understand the three-dimensional structure of the LCS circulation, we construct a mean flow over twenty years in z coordinates and density coordinates from three-day snapshots of an eddy-resolving ocean general circulation model (OGCM) and carry out a “leak-free” volume budget analysis on the mean flow field in both coordinate systems. The mean flow in z coordinates forms a zonal and a meridional overturning, where a near-surface eastward flow enters the LC, flows poleward and at the same time sinks to the LUC depths, flows equatorward, and finally moves westward out of the LCS, as found in a previous study. On the other hand, the mean flow in density coordinates is mostly isopycnic, except that the upper part of the LC becomes denser as it flows poleward partly owing to sea-surface cooling. The lower part of the LC becomes denser, not because of cooling but because denser water joins the LC from the interior ocean. The LUC becomes lighter as it flows equatorward because its lower part flows westward out of the LCS and its top part is joined by eastward flows from the interior. The mean downwelling in z coordinates is therefore isopycnic and is likely a result of temporal variability.

Study of subsurface eddy properties in northwestern Pacific Ocean based on an eddy-resolving OGCM

A method based on the Okubo–Weiss parameter was used to detect subsurface eddies (SSEs) with an eddy-resolving ocean general circulation model. Statistical analyses showed that SSEs are ubiquitous in the northwestern Pacific Ocean. Three regions were found to have high probability of SSE, which are as follows: the latitudinal band between 9°N and 17°N, the Kuroshio extension region, and the area east of the Ryukyu Islands. Although surface eddies (SEs) were found distributed widely within the zonal band of the Subtropical Counter Current, few SSEs were found there. In contrast, few SEs were found to the east of The Philippines, whereas SSEs were abundant. The kinetic energy contained within SSE was found comparable in magnitude with that of SE. During 1993–2013, about 2569 and 2099 SSEs (at a depth of about 400 m) were observed to be anticyclonic and cyclonic, respectively; thus, SSEs tended to be anticyclonic. The mean radius, lifespan, and propagation speed of SSE in this study were about 60 km, 50 days, and 6.6 cm/s, respectively. The propagation speed showed a wave-like decrease with increasing latitude. Some long-lived SSEs were found to persist for longer than 4 months and to move thousands of kilometers. About 89% of SSEs were nonlinear for at least half their lifespan, which implies that SSE can trap interior fluid during translation. Trajectories revealed that SSEs propagate nearly due west with only small meridional deflection. The findings of this study will contribute to the enrichment of our knowledge regarding SSE in the northwestern Pacific Ocean.

Analysis of mean solution characteristics of an eddy-resolving numerical model simulating tropical instability waves in the Pacific Ocean

Using an eddy-resolving ocean general circulation model called INMIO, we study mean thermohydrodynamic conditions in which the equatorial convergence of eddy meridional heat transport (MHT) is formed by tropical instability waves (TIWs). In nature the TIWs warm the well-known cold tongue of the East Pacific waters. The model has sufficient resolution to reproduce the TIWs, but in the vicinity of 110ºW it shows significantly underestimated convergence values compared to estimates of direct observations. Analysis of mean temperature distributions in comparison with the WOA09 climatology suggests that the underestimation of the eddy MHT convergence is a model bias, rather than an error of the observational data.

Impacts of sea-surface salinity in an eddy-resolving semi-global OGCM

To explore the impacts of sea-surface salinity (SSS) on the interannual variability of upper-ocean state, we compare two 10-year runs of an eddy-resolving ocean general circulation model (OGCM): in one, SSS is strongly restored toward a monthly climatology (World Ocean Atlas ’98) and in the other, toward the SSS of a monthly gridded Argo product. The inclusion of the Argo SSS generally improves the interannual variability of the mixed layer depth; particularly so in the western tropical Pacific, where so-called “barrier layers” are reproduced when the Argo SSS is included. The upper-ocean subsurface salinity variability is also improved in the tropics and subtropics even below the mixed layer.To understand the reason for the latter improvement, we separate the salinity difference between the two runs into its “dynamical” and “spiciness” components. The dynamical component is dominated by small-scale noise due to the chaotic nature of mesoscale eddies. The spiciness difference indicates that as expected from the upper-ocean general circulation, SSS variability in the mixed layer is subducted into the thermocline in subtropics; this signal is generally advected downward, equatorward, and westward in the equator-side of the subtropical gyre. The SSS signal subducted in the subtropical North Pacific appears to enter the Indian Ocean through the Indonesian Throughflow, although this signal is weak and probably insignificant in our model.