Eastern Subtropical Mode Water Research Articles

Spiciness anomalies in the upper North Pacific based on Argo observations

The density-compensated salinity anomalies (spiciness anomalies) in the upper North Pacific were investigated using Argo float profiles during 2004–2018. The freshening of the subtropical thermocline was found within the Central Mode Water (CMW) and the North Pacific Intermediate Water (salinity minimum). Meanwhile, the increase of salinity was found in the lighter layer within the North Pacific Tropical Water (salinity maximum). The interannual and longer spiciness anomalies were interpreted with three-dimensional evolutions and were linked to the fate of three mode waters (i.e., Subtropical Mode Water (STMW), Eastern Subtropical Mode Water (ESTMW), and CMW) for the first time. In the STMW, the salinity was dominated by a quasi-decadal variability, which was consistent with the KE variability with a 1-year lag, and did not show rapid freshening. In the CMW, the salinity decreased with a quasi-decadal variability, which was weaker and out of the phase compared with the STMW. In the ESTMW, the salinity was dominated by year-to-year variability. The spiciness anomalies originated mainly in the outcrop region of the isopycnals, where they were accompanied by the formation and the subduction of the mode waters. They also propagated and decayed downstream the geostrophic currents. However, a few of the interannual anomalies found in the northern part of the CMW were almost dampened before their spread further south. In addition, some anomalies in the ESTMW appeared and intensified without a connection to the mixed layer. These anomalies occurred far from the outcrop line, suggesting these were caused likely by salt fingering associated with the modification of the mode waters. Furthermore, the propagation of the spiciness anomalies in the western to the central subtropics was significantly faster than the geostrophic current and inclined to the inner side of streamlines. These indicated the transport by eddies in addition to the mean geostrophic currents.

An Anti‐Phase Relationship Between the Subtropical Underwater and Eastern Subtropical Mode Water in the South Pacific During 2004–2020

AbstractUsing the Roemmich‐Gilson Argo data set, this study investigates variability of the Subtropical Underwater (STUW) and eastern Subtropical Mode Water (ESTMW) in the South Pacific during 2004–2020. The STUW volume decreased during 2004–2013 and increased during 2013–2020, while the volume of the ESTMW shows the opposite phase. On interannual time scales, there is also a significant negative correlation in volume between the STUW and ESTMW. This anti‐phase relationship is attributed to changes in their volumetric subduction rates, which are in turn closely related to variability in the mixed layer depth (MLD). ENSO directly contributes to variability of the subduction rates by modifying the MLD. Equatorward propagation of spiciness anomalies is identified along isopycnal surfaces of the STUW and ESTMW cores. These spiciness anomalies in the downstream region are correlated with changes in volume of both water masses, and significant spiciness anomalies can reach the tropical Pacific.

The Characteristics of Near‐Equatorial North Pacific Low PV Water and Its Possible Influences on the Equatorial Subsurface Ocean

AbstractUsing SODA oceanic reanalysis data, the subduction rate and low potential vorticity (PV) water masses in the North Pacific are investigated from January 1870 to December 2008. A new low PV water mass with a large subduction rate is found near‐equatorial North Pacific, which is named as the near‐equatorial North Pacific low PV water mass (NELPVW), and it is the only one low PV water mass in North Pacific that appears an increasing trend under global warming. Among all the low PV water masses in North Pacific, only Eastern Subtropical Mode Water (EMW) and NELPVW can migrate to the equatorial Pacific and effect subsurface oceanic temperature (SOT) there. Further research shows that the subducted low PV water mass in EMW generation region can migrate to equatorial Pacific through two transport paths, which are called the Western‐EMW path and the Central‐EMW path. The analysis of Lagrange tracking shows that EMW can migrate and bring SOT anomaly to equatorial Pacific along these paths in 7 years and lead to the almost precisely opposite distributions of equatorial Pacific SOT anomaly between the strong and weak EMW subduction years, while it takes the newly discovered NELPVW less than 3 years to reach the equatorial Pacific along the Western‐NELPVW path or the Eastern‐NELPVW path after subduction. The distributions of equatorial Pacific SOT anomaly in the strong and weak NELPVW subduction years are approximately reverse too. The actual distribution of the equatorial Pacific SOT is affected by these two low PV water masses at the same time.

Mesoscale Eddy Effects on the Subduction of North Pacific Eastern Subtropical Mode Water

AbstractThe effects of mesoscale eddies on the subduction of the North Pacific eastern subtropical mode water (ESTMW) are studied by comparing observations with results from the Ocean Model for the Earth Simulator (OFES). Both observations and the OFES show that mesoscale eddies exist in the ESTMW subduction region. Compared to the observations, the OFES reproduces the characteristics of the ESTMW and subduction quite well. Subduction of the ESTMW mainly occurs within a low‐density advection region, which leads to the formation of low potential vorticity water. The eddy subduction rate accounts for nearly 56% of the total subduction rate, creating high peaks in the ESTMW subduction volume histogram. Case study results show that the propagation of eddies is responsible for the shallowing of the mixed‐layer depth, which allows low potential vorticity water to subduct under the mixed‐layer bottom and prevents it from being entrained back into the mixed layer. The eddy subduction process investigated here has important implications for the further study of ESTMW subduction and transport, which is helpful for better understanding the interior transportation between the tropics and subtropics.

Contribution of Mesoscale Eddies to the Subduction and Transport of North Pacific Eastern Subtropical Mode Water

This study investigates the contribution of mesoscale eddies to the subduction and transport of North Pacific Eastern Subtropical Mode Water (ESTMW) using the high-frequency output of an eddy-resolved ocean model spanning the period 1994–2010. Results show that the subduction induced by mesoscale eddies accounts for about 31% of the total subduction of ESTMW formation. The volume of ESTMW trapped by anticyclonic eddies is slightly larger than that trapped by cyclonic eddies. The ESTMW trapped by all eddies in May reaches up to about 2.8 × 1013 m3, which is approximately 16% of the total ESTMW volume. The eddy-trapped ESTMW moves primarily westward, with its meridional integration at 18°–30°N reaching about 0.17 Sv, which is approximately 18% of the total zonal ESTMW transport in this direction, at 140°W. This study highlights the important role of eddies in carrying ESTMW westward over the northeastern Pacific Ocean.

Interannual Variability of Upper Ocean Water Masses as Inferred From Argo Array

AbstractInterannual variability of Ocean Heat Content (OHC) is intimately linked to ocean water mass changes. Water mass characteristics are imprinted at the ocean surface and are modulated by climate variability on interannual to decadal time scales. In this study, we investigate the water mass change and their variability using an isopycnal decomposition of the OHC. For that purpose, we address the thickness and temperature changes of these water masses using both individual temperature‐salinity profiles and optimal interpolated products from Argo data. Isopycnal decomposition allows us to characterize the water mass interannual variability and decadal trends of volume and OHC. During the last decade (2006–2015), much of interannual and decadal warming is associated with Southern Hemisphere Subtropical Mode Water and Subantarctic Mode Water, particularly in the South Pacific Eastern Subtropical Mode Water, the Southeastern Indian Subantarctic Mode Water, and the Southern Pacific Subantarctic Mode Water. In contrast, Antarctic Intermediate Water in the Southern Hemisphere and North Atlantic Subtropical Mode Water in the Northern Hemisphere have cooled. This OHC interannual variability is mainly explained by volume (or mass) changes of water masses related to the isopycnal heaving. The forcing mechanisms and interior dynamics of water masses are discussed in the context of the wind stress change and ocean adjustment occurring at interannual time scale.

Decadal Variability of North Pacific Eastern Subtropical Mode Water

AbstractA significant decadal variability of the North Pacific Eastern Subtropical Mode Water (NPESTMW), related to the Pacific Decadal Oscillation, is identified from the gridded Argo and EN4 observational data sets. To investigate the decadal variability of the NPESTMW volume, the high‐resolution Estimating the Circulation and Climate of the Ocean, Phase II simulation is used to diagnose the volume budget of the NPESTMW density layer in the NPESTMW formation region (20°–36°N, 160°–120°W) between 1992 and 2016. Results show that NPESTMW is formed through subduction of low potential vorticity (PV) water. This low‐PV water subduction is mainly determined by mixed layer depth change and lateral induction, which account for approximately 58% and 32%, respectively. The decadal NPESTMW volume anomaly is originated from the Pacific Decadal Oscillation‐induced decadal anomaly of air‐sea buoyancy flux. This decadal buoyancy flux anomaly drives the decadal anomaly of mixed layer depth change, then causes anomalous subduction, and leads to anomalous low‐PV water subduction. The decadal NPESTMW volume anomaly is finally generated by this anomalous low‐PV water subduction.

Properties, formation, and dissipation of the North Pacific Eastern Subtropical Mode Water and its impact on interannual spiciness anomalies

The properties, formation, and dissipation of the North Pacific Eastern Subtropical Mode Water (ESTMW), their interannual variability, and impact on spiciness anomalies in the upper permanent pycnocline were investigated using Argo profiling float data in 2005–2015. The core temperature and salinity of ESTMWs were horizontally compensated to a constant density, and core potential density concentrates in a range of 24.5–25.2 kg m−3 with two distinct peaks. ESTMWs showed different spatial distribution and persistence for its core potential density. Denser ESTMWs with a potential density of 24.9–25.2 kg m−3 were formed in winter mixed layer depth maximum centered at 30°N, 140°W and lighter ESTMWs of 24.5–24.9 kg m−3 were formed south and east of it. After formation through shoaling of the winter mixed layer, the former persisted until the following autumn and a small part of it subducted in winter, while the latter dissipated in summer. The formation region of ESTMW corresponded to the summer sea surface density maximum resulting from its poleward sea surface salinity front. Sea surface density maximum maintains weak stratification during summer, preconditioning the deepening of the winter mixed layer and hence the formation of ESTMWs. A relationship between the ESTMW formation region and the summer sea surface density maximum was also found in the North Atlantic and the South Pacific, implying the importance of sea surface salinity fronts and the associated summer sea surface density maximum to ESTMW formation. Interannual variations of ESTMW reflected that of the winter mixed layer in its formation region, and the thickness of ESTMW was related to the Pacific decadal oscillation. ESTMW contributed to the occurrence of spice injection and affected spiciness anomalies in the upper permanent pycnocline through its formation and dissipation.

North and equatorial Pacific Ocean circulation in the CORE-II hindcast simulations

We evaluate the mean circulation patterns, water mass distributions, and tropical dynamics of the North and Equatorial Pacific Ocean based on a suite of global ocean-sea ice simulations driven by the CORE-II atmospheric forcing from 1963-2007. The first three moments (mean, standard deviation and skewness) of sea surface height and surface temperature variability are assessed against observations. Large discrepancies are found in the variance and skewness of sea surface height and in the skewness of sea surface temperature. Comparing with the observation, most models underestimate the Kuroshio transport in the Asian Marginal seas due to the missing influence of the unresolved western boundary current and meso-scale eddies. In terms of the Mixed Layer Depths (MLDs) in the North Pacific, the two observed maxima associated with Subtropical Mode Water and Central Mode Water formation coalesce into a large pool of deep MLDs in all participating models, but another local maximum associated with the formation of Eastern Subtropical Mode Water can be found in all models with different magnitudes. The main model bias of deep MLDs results from excessive Subtropical Mode Water formation due to inaccurate representation of the Kuroshio separation and of the associated excessively warm and salty Kuroshio water. Further water mass analysis shows that the North Pacific Intermediate Water can penetrate southward in most models, but its distribution greatly varies among models depending not only on grid resolution and vertical coordinate but also on the model dynamics. All simulations show overall similar large scale tropical current system, but with differences in the structures of the Equatorial Undercurrent. We also confirm the key role of the meridional gradient of the wind stress curl in driving the equatorial transport, leading to a generally weak North Equatorial Counter Current in all models due to inaccurate CORE-II equatorial wind fields. Most models show a larger interior transport of Pacific subtropical cells than the observation due to the overestimated transport in the Northern Hemisphere likely resulting from the deep pycnocline.

Response of North Pacific eastern subtropical mode water to greenhouse gas versus aerosol forcing

Abstract Mode water is a distinct water mass characterized by a near vertical homogeneous layer or low potential vorticity, and is considered essential for understanding ocean climate variability. Based on the output of GFDL CM3, this study investigates the response of eastern subtropical mode water (ESTMW) in the North Pacific to two different single forcings: greenhouse gases (GHGs) and aerosol. Under GHG forcing, ESTMW is produced on lighter isopycnal surfaces and is decreased in volume. Under aerosol forcing, in sharp contrast, it is produced on denser isopycnal surfaces and is increased in volume. The main reason for the opposite response is because surface ocean-to-atmosphere latent heat flux change over the ESTMW formation region shoals the mixed layer and thus weakens the lateral induction under GHG forcing, but deepens the mixed layer and thus strengthens the lateral induction under aerosol forcing. In addition, local wind changes are also favorable to the opposite response of ESTMW production to GHG versus aerosol.

Interannual Variation of Annual Subduction Rate in the North Pacific Estimated from a Gridded Argo Product

AbstractSpatiotemporal variability of the subduction rate in the North Pacific from 2005 to 2012 is examined based on the Argo observational data. The subduction rate in the subtropical North Pacific varies significantly from year to year between 25 and 50 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1), and it is well correlated with the Pacific decadal oscillation. The temporal change of the subduction rate is largely determined by that of the late winter mixed layer depth through the lateral induction term. The increase (decrease) in the subduction rate in the subtropical mode water areas accompanies densification (lightening) of the mode density class of the subducted water. The subduction rate variability in the central mode water and eastern subtropical mode water regions is anticorrelated as found in the previous study using the output from an ocean GCM. The subduction rate in the central mode water density range changes dramatically, which is very large in 2005 and 2010 but almost disappears in 2009. The subduction rate variability in the western subtropical mode water regions seems to be correlated with the Pacific decadal oscillation with a lag of a few years.

North Pacific Eastern Subtropical Mode Water simulation and future projection

The present climate simulation and future projection of the Eastern Subtropical Mode Water (ESTMW) in the North Pacific are investigated based on the Geophysical Fluid Dynamics Laboratory Earth System Model (GFDL-ESM2M). Spatial patterns of the mixed layer depth (MLD) in the eastern subtropical North Pacific and the ESTMW are well simulated using this model. Compared with historical simulation, the ESTMW is produced at lighter isopycnal surfaces and its total volume is decreased in the RCP8.5 runs, because the subduction rate of the ESTMW decreases by 0.82×10−6 m/s during February-March. In addition, it is found that the lateral induction decreasing is approximately four times more than the Ekman pumping, and thus it plays a dominant role in the decreased subduction rate associated with global warming. Moreover, the MLD during February-March is banded shoaling in response to global warming, extending northeastward from the east of the Hawaii Islands (20°N, 155°W) to the west coast of North America (30°N, 125°W), with a maximum shoaling of 50 m, and then leads to the lateral induction reduction. Meanwhile, the increased northeastward surface warm current to the east of Hawaii helps strengthen of the local upper ocean stratification and induces the banded shoaling MLD under warmer climate. This new finding indicates that the ocean surface currents play an important role in the response of the MLD and the ESTMW to global warming.

Control of Decadal and Bidecadal Climate Variability in the Tropical Pacific by the Off-Equatorial South Pacific Ocean

Abstract Delayed negative feedback processes determining intrinsic decadal and bidecadal time scales for the tropical variability in the Pacific are investigated based on climate model experiments. By comparing a control run driven by preindustrial forcing and partial blocking runs driven by the same forcing but with ocean temperature and salinity restored to climatology in selected regions, subsurface oceanic signals of South Pacific origin are shown to precede SST variability in the Niño-3.4 region. Using a linear reduced-gravity ocean model driven only by wind stress changes and an offline tracer model, oceanic wave adjustment triggered by changes of wind stress curl in the South Pacific extratropics is suggested to be essential for the decadal component of the equatorial SST, while slower isopycnal advection of subsurface temperature anomalies from the formation region of South Pacific Eastern Subtropical Mode Water controls the bidecadal component. The intrinsic time scales of the tropical variability are regulated by simple linear ocean dynamics.

Interannual and decadal variability of the subtropical mode water formation in the South Pacific Ocean

The variability of the subtropical mode water (STMW) formation in the South Pacific Ocean from 1980 to 2004 is investigated in this study, using a high-resolution numerical model and a 3D Lagrangian trajectory model. Subduction rate, subduction-rate-weighted geometric centers and mean densities of mode waters all exhibit a significant interannual and decadal variability. The interannual change of the subduction rate is predominately due to the lateral induction, resulting from the strong spatial and temporal variations of the wintertime mixed layer depth. The upward trend in the subduction-rate-weighted mean density of western STMW is largely caused by the wintertime (September) cooling of the net heat flux in this area. Variations of subduction rate in the mode waters are closely linked to the El Niño-Southern Oscillation (ENSO) cycle. Two different methods of defining mode waters (fixed region versus thermodynamic approach) yield similar patterns of subduction rate and almost identical correlations with the annual Southern Oscillation Index. The eastern STMW could potentially affect the ENSO cycle through the interior communication window that was identified from the virtual streamfunction. Its location and width are closely related to the ENSO cycle. The deep westward penetration of the western edge of the window at the equatorial Pacific is evident during the 1998 La Niña event.

Interannual variations of the Hawaiian Lee Countercurrent induced by potential vorticity variability in the subsurface

Interannual variations of the Hawaiian Lee Countercurrent (HLCC) in the 2000s were investigated using satellite and Argo profiling float observations. The satellite-observed sea surface height shows that the geostrophic eastward current was anomalously strong to the west away from Hawaii in 2003 and 2005. However, the trade winds and the orographic wind curl dipole in the lee of Hawaii that drives the climatological mean HLCC were not particularly strong in these years, suggesting that the accelerations of the HLCC were not caused by the wind stress curl forcing around Hawaii and subsequent Rossby wave propagation. Using Argo observations, we found negative potential vorticity (PV) anomalies in the subsurface north of the HLCC in these 2 years. The pycnocline is lifted northward as low PV waters of different densities stack up in the vertical, and the HLCC is then accelerated via the thermal wind. The intensification and/or southward intrusion of the eastern subtropical mode water and subtropical mode water seem to have induced negative PV anomalies in 2003 and 2005, respectively. Using high-resolution ocean simulations, we confirmed the migrations of PV anomalies and their contributions to the HLCC accelerations. Although the HLCC is located away from the cores of major mode waters, our results suggest that interannual variations of the HLCC are affected by those of mode waters.

Variability of subduction rates of the subtropical North Pacific mode waters

The climatology subduction rate for the entire Pacific is known, but the mechanism of interannual to decadal variation remains unclear. In this study, we calculated the annual subduction rates of three types of North Pacific subtropical mode waters using a general circulation model (LICOM1.0) for the period of 1958–2001. The model experiments focused on interannual variations of ocean dynamical processes under daily wind forcings and seasonal heat fluxes. The mode water formation region was defined by a potential vorticity minimum at outcrop locations. The model results show that two subduction rate maxima (>100 m/a) were located in the Subtropical Mode Water (STMW) and the Central Mode Water (CMW) formation regions. These regions are consistent with a climatologically calculated value. The subduction rate in the Eastern Subtropical Mode Water (ESTMW) formation region was smaller at about 75 m/a. The subduction rate shows clear interannual and decadal variations associated with oceanic dynamic variabilities. The average subduction rate of the STMW was much smaller during the period of 1981–1990 compared with other periods, while that of the CMW had a negative anomaly before 1975 and a positive anomaly after 1978. The variability agreed with Ekman and geostrophic advections and mixed layer depths. The interannual variability of the subduction rate for the ESTMW was smallest during 1970–1990, as a result of a weak wind stress curl. This paper explores how interannual signals from the atmosphere are stored in different parts of the ocean, and thus may contribute to a better understanding of feedback mechanisms for the Pacific Decadal Oscillation (PDO) event.

Roles of mode waters in the formation and maintenance of central water in the North Pacific

This study describes the three-dimensional distributions of the Turner angle (Tu) and the potential vorticity (PV) of the main pycnocline water in the subtropical North Pacific (10–50°N, 120°E–120°W) using a large in situ CTD data set taken by the Argo profiling floats during June to October of 2001–2009 to clarify the detailed distribution of the central water and the mode waters as well as the relationship between these water masses. The ventilated part of the main pycnocline water (σθ < 26.7 kg m-3) in the subtropical gyre generally displays a sharp peak in Tu value of 59° in the histogram. The Tu histograms for 10° × 10° geographical boxes mostly show that the mode for the Tu value is 59° too, but they also show some regional differences, suggesting some types of relations with the North Pacific mode waters. To further investigate this relationship, the appearance probability density function of the central water (defined as the main pycnocline water with Tu = 56°–63°) and those of the mode waters with PVs lower than the critical value on each isopycnal surface were analyzed. The distribution area of the central mode water (CMW) corresponds so well with that of the central water that a direct contribution of the CMW to the formation and maintenance of the central water is suggested. On the other hand, the distribution areas of subtropical mode water (STMW), Eastern STMW, and transition region mode water do not correspond to that of the central water. Nevertheless, indirect contributions of these mode waters to the formation and maintenance of the central water through salt finger type convection or diapycnal mixing are suggested.

Increase of South Pacific eastern subtropical mode water under global warming

[1] The response of South Pacific Eastern Subtropical Mode Water (SPESTMW) to global warming is investigated by comparing solutions from a set of Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) coupled models between a present-day climate and a future, warmer climate. Under the warmer climate scenario, the SPESTMW extends southwestward and is significantly increased in volume. This is because all the local surface forcing mechanisms (i.e., wind stress, heat and freshwater fluxes) in the eastern subtropical South Pacific tends to de-stratify the upper ocean and thus deepen the mixed layer. Further, a suite of process-oriented experiments with an ocean general circulation model suggest that it is the intensified southeast trade winds under the warmer climate that promotes more heat flux from the ocean into the atmosphere that then results in a deepening of the mixed layer in the eastern subtropics of the South Pacific.

Anti‐correlated variability in subduction rate of the western and eastern North Pacific Oceans identified by an eddy‐resolving ocean GCM

Analysis of results from an eddy‐resolving ocean general circulation model has revealed the existence of an anti‐correlated variability in subduction rate of the western and eastern North Pacific Oceans, consistent with the leading mode of Pacific Decadal Oscillation (PDO). A composite analysis shows that during the positive phase of PDO, the winter mixed layer depth (MLD) generally gets deeper in the west and shallower in the east, being closely related to the variability in surface heat flux stemming from an enhanced Aleutian Low, and the situation during the negative phase of PDO is reversed. Changes in MLD are primarily responsible for the anti‐correlated variability in subduction rate, though changes in subtropical gyre circulation also play a role. The result also provides useful evidence for the interior and western boundary pathways, through which variability in eastern subtropical mode water can be conveyed into the equatorial region.

Interannual variability of North Pacific eastern subtropical mode water formation in the 1990s derived from a 4-dimensional variational ocean data assimilation experiment

Abstract The interannual variability of eastern subtropical mode water (ESTMW) formation in the North Pacific is examined using a new ocean dataset constructed by a 4-dimensional variational data assimilation experiment covering the decade of the 1990s. The volume of newly formed ESTMW varies due to interannual variability in the following three physical processes taking place in the surface layer: (1) convergence in the transport of surface saline water induced by Ekman flow in the vicinity of the formation region, (2) thermal stratification in the preconditioning phase in association with the insolation anomaly induced largely by low-level cloud coverage, and (3) wintertime surface cooling in the eastern subtropics. We find that, in addition to the surface forcing, the properties of both the ESTMW and the upper mixed-layer water are broadly controlled by the volume of the new ESTMW component, and that the variations in the upper mixed-layer water affect the properties of ESTMW formed in the following winter. Due to the combined effect of these processes, the ESTMW subducts down to subsurface layers with a wide range of σ θ values lying between 24.8 and 25.4 and with significant interannual variation in water mass formation.