Pacific Eastern Subtropical Mode Water Research Articles

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.

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.

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.

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.

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.

Structure and Modification of the South Pacific Eastern Subtropical Mode Water

Abstract Using all available temperature and salinity profiles obtained by Argo floats from July 2004 to June 2007, this study investigated the structure and modification of the South Pacific Eastern Subtropical Mode Water (SPESTMW). Based on the observed characteristics of the vertical minima of potential vorticity over the subtropical South Pacific, SPESTMW is defined as water with potential vorticity magnitude less than 2.5 × 10−10 m−1 s−1 and thickness exceeding 40 m. It is found between 35°–5°S and 160°–70°W and has a temperature of 13°–26°C, salinity greater than 34.0, and density of 24.5–25.8 kg m−3 at its core. This study confirmed that vertical changes in temperature and salinity tend to compensate for each other in terms of density changes, resulting in favorable salt fingering conditions, as previously reported. By analyzing many profiles of Argo data in spring immediately after the SPESTMW formation period, its temperature and salinity are vertically uniform in the formation region, but large vertical gradients of temperature and salinity are found downstream from that region, even in the SPESTMW core. Consequently, the low potential vorticity signature of SPESTMW spread much wider than its signature as a thermostad. The Argo data also captured the seasonal changes of the vertical gradients of temperature and salinity at the SPESTMW core; these gradients increased as the seasons progressed, even in the formation region. Therefore, SPESTMW is truly vertically uniform water (i.e., thermostad, halostad, and pycnostad simultaneously) only immediately after the formation period. Afterward, it is only pycnostad. This seasonal evolution is related to temperature and salinity diffusion due to salt fingering in a manner similar to the rapid modification of interannual anomalies as shown by previous research. The temperature and salinity near the SPESTMW core and lower region decreased soon after its formation.

Structure and Modification of the South Pacific Eastern Subtropical Mode Water

Using all available temperature and salinity profiles obtained by Argo floats from July 2004 to June 2007, this study investigated the structure and modification of the South Pacific Eastern Subtropical Mode Water (SPESTMW). Based on the observed characteristics of the vertical minima of potential vorticity over the subtropical South Pacific, SPESTMW is defined as water with potential vorticity magnitude less than 2.5 × 10−10 m−1 s−1 and thickness exceeding 40 m. It is found between 35°–5°S and 160°–70°W and has a temperature of 13°–26°C, salinity greater than 34.0, and density of 24.5–25.8 kg m−3 at its core. This study confirmed that vertical changes in temperature and salinity tend to compensate for each other in terms of density changes, resulting in favorable salt fingering conditions, as previously reported. By analyzing many profiles of Argo data in spring immediately after the SPESTMW formation period, its temperature and salinity are vertically uniform in the formation region, but large vertical gradients of temperature and salinity are found downstream from that region, even in the SPESTMW core. Consequently, the low potential vorticity signature of SPESTMW spread much wider than its signature as a thermostad. The Argo data also captured the seasonal changes of the vertical gradients of temperature and salinity at the SPESTMW core; these gradients increased as the seasons progressed, even in the formation region. Therefore, SPESTMW is truly vertically uniform water (i.e., thermostad, halostad, and pycnostad simultaneously) only immediately after the formation period. Afterward, it is only pycnostad. This seasonal evolution is related to temperature and salinity diffusion due to salt fingering in a manner similar to the rapid modification of interannual anomalies as shown by previous research. The temperature and salinity near the SPESTMW core and lower region decreased soon after its formation.

Further evidence for non-reemergence of winter SST anomalies in the North Pacific eastern subtropical mode water area

Following our previous study (Sugimoto and Hanawa, 2005b), we further investigate the reason why reemergence of winter sea surface temperature anomalies does not occur in the North Pacific eastern subtropical mode water (NPESTMW) area, despite its occurrence in the North Pacific subtropical mode water and North Pacific central mode water areas. We use vertical temperature and salinity profiles of the World Ocean Circulation Experiment Hydrographic Program and Argo floats with high vertical and temporal resolution, together with heat flux data through the sea surface. We point out first that one of the causes for non-occurrence of reemergence is that the thickness of NPESTMW is very thin. In addition to this basic cause, two major reasons are found: a vigorous mixing in the lower portion of NPESTMW and less heat input from the atmosphere in the warming season. Since, in the lower portion of NPESTMW and deeper, the stratification is favorable for salt-finger type convection to occur compared with the other mode water areas, vigorous mixing takes place. This is confirmed by both a large Turner Angle there and the existence of staircase structures in vertical temperature and salinity profiles. From the viewpoint of heat input, the NPESTMW area gradually gains heat in the warming season compared with other mode water areas. As a result, NPESTMW cannot be capped so quickly by the shallow summer mixed layer, and water properties of NPESTMW are to be gradually modified, even in the upper portion.

Sensitivity of the Ventilation Process in the North Pacific to Eddy-Induced Tracer Transport*

Abstract A coarse-resolution isopycnal model coupled with a bulk mixed layer model is used to examine the effect of isopycnal thickness diffusion, which parameterizes the subgrid-scale eddy-induced tracer transport, on ventilation of the North Pacific Ocean. Three numerical experiments with thickness diffusivities of 0 m2 s−1 and around 500 and 2000 m2 s−1 are carried out. The model successfully reproduces a deep winter mixed layer in the subarctic North Pacific, leading to well-formed mode waters and the subtropical countercurrent in the experiment with thickness diffusivity around 500 m2 s−1. The annual-mean subduction rate has peaks at densities of 25.0–25.4 and 26.4 σθ. The former peak spans the densities of North Pacific Subtropical Mode Water and North Pacific Eastern Subtropical Mode Water, whereas the latter peak is centered near the density of North Pacific Central Mode Water. The annual mean obduction rate also has the former peak and a slight enhancement corresponding to the latter peak. The Kuroshio plays a crucial role in obduction of North Pacific Subtropical Mode Water by transferring it northward from the permanent pycnocline to the seasonal pycnocline around the Kuroshio Extension, the importance of which has been overlooked in previous studies. In contrast to the simple expectation that the eddy-induced tracer transport enhances the ventilation process, stronger circulation with lower thickness diffusion increases the annual-mean subduction rate by carrying the subducted water quickly away from the seasonal pycnocline into the permanent pycnocline, as well as the annual-mean obduction rate by transferring much water from the permanent pycnocline to the seasonal pycnocline. As thickness diffusivity increases, the former peaks in the subduction and obduction rates occur at lighter densities, whereas the latter peak in the subduction rate is shifted toward higher densities.

Generation and Initial Evolution of a Mode Water θ–S Anomaly*

Abstract Generation and evolution of an isopycnal potential temperature–salinity (θ–S), or spiciness, anomaly is studied around 20°–23°S, 110°W in the austral winter of 2004. Two profiling CTD floats deployed in the region in January 2004 provide the observations. The anomaly (defined as relative to water properties of the preceding summer) is very large (initially about 0.35 in S and about 0.9°C in θ). It is associated with the winter ventilation of a thick, low-potential-vorticity layer known as South Pacific Eastern Subtropical Mode Water. Regional lateral θ and S distributions at the surface predispose the ocean to formation of this water mass and allow significant anomalies to be generated there with relative ease. The water mass is potentially important for climate in that, after northwestward advection in the South Equatorial Current, it contributes to the Equatorial Undercurrent and eventually resurfaces in the cold tongue of the eastern equatorial Pacific Ocean. The anomaly studied is strong enough to predispose a portion of the water column to salt fingering, increasing vertical mixing. Although lateral processes are no doubt important in evolution of the anomaly, the vertical mixing appears to be sufficiently vigorous to reduce it significantly within 6 months after its formation by spreading it to denser horizons through diapycnal fluxes. By that time the anomaly is most likely sufficiently diffuse so that subsequent evolution from diapycnal fluxes is significantly reduced as it makes its way toward the equator.

Why does reemergence of winter sea surface temperature anomalies not occur in eastern mode water areas?

In terms of buoyancy flux, winter mixed layer depth, and mode water thickness in summer, we explore the causes why reemergence does not occur in the formation areas of the North Pacific eastern subtropical mode water (NPESTMW) and the North Atlantic Madeira mode water (NAMMW). It is found that as season goes by, the thicknesses of these mode waters become quickly thinner due to vigorous salt‐finger type convection throughout the water column in contrast to those of the other mode waters. Correspondingly, mode waters lying under the shallow summer mixed layer quickly lose their water properties. In addition, it is found that in these areas, net buoyancy flux integrated through a year is almost zero, while in the other mode water areas where reemergence occurs, net buoyancy flux is negative (the ocean's loss) to some degree. Based on these facts, we can conclude that the formation areas of NPESTMW and NAMMW are the regions having unfavorable conditions for an occurrence of reemergence.

Preconditioning of winter mixed layer in the formation of North Pacific eastern subtropical mode water

The preconditioning of winter mixed layer in the formation of the eastern subtropical mode water (ESTMW) in the North Pacific Ocean is investigated using an eddy‐permitting ocean general circulation model. The result shows that convergence in the northward Ekman transport of saltier water and weak summertime heat fluxes due to the presence of stratus cloud in the ESTMW formation region are both central to the initiation of significant mixed layer deepening that subsequently evolves into the ESTMW pycnostad in the main thermocline. These two factors originate from air‐sea interactions inherent in the region of Northeast Pacific Basin. The approach described here has significant potential for determining the water‐mass distribution and may lead to an explanation of the Sverdrup's Eastern Gyral in the subtropical North Pacific.

South Pacific Eastern Subtropical Mode Water*

Abstract The structure, formation, and destruction of South Pacific Eastern Subtropical Mode Water (SPESTMW) are analyzed. Geographic extent and water properties are discussed by using high-quality CTD sections collected between 1991 and 1996. Defined as having a planetary potential vorticity magnitude of less than 3 × 10−10 m−1 s−1, SPESMTW has a volume of about 1.1 × 1015 m3, estimated from CTD data. The ventilation of this mode water is described by using data from a high-resolution XBT section in concert with 30-month time series from profiling CTD floats, some of the first Argo deployments. Published subduction rates allow a mode-water formation rate estimate of 8.7 × 106 m3 s−1. Combining this estimate with the volume yields a residence time of about 4 years. The density-compensating covarying patterns of late winter surface temperature and salinity in the ventilation region of SPESTMW are shown to contribute to the strength of the mode water. However, while the destabilizing salinity gradient in SP...