Subtropical Countercurrent Research Articles

Eddy properties in the Subtropical Countercurrent, Western Philippine Sea

An array of six oceanographic moorings with acoustic and environmental sensors was deployed in the central Philippine Sea from April 2010 to April 2011. The location spanned 18−23°N, 124 – 130°E in the Subtropical Countercurrent (STCC). The most prominent feature of the data set was a densely-packed eddy field with about equal numbers of cyclones and anticyclones moving westward at 6–12kmd−1. Eddies of either sign displaced the thermocline about ±50m and had surface velocities exceeding 110cms−1. While warm eddies were slightly larger than cold eddies, the distance to maximum radial velocity was similar for both at about 65km, close to the local Rossby radius of deformation. The steepness parameter U/c in the eddies ranged from 3 to 10, accompanied by relative vorticity of order 0.1–0.3f, suggesting nonlinear, quasigeostrophic features with trapped cores rather than linear waves. This was borne out by the water mass analysis which showed high salinity, high spice North Pacific Tropical Water (NPTW) being transported westward in the warm eddy cores. The total KE and APE in eddies of both signs was about 1×1015J with 85% of the APE and 74% of the KE located above 250m depth. This equipartitioning of energy suggests mature eddies near equilibrium, that had been evolving for some time as they propagated into the area from the east.

Net-phytoplankton communities in the Western Boundary Currents and their environmental correlations

This study investigated net-phytoplankton biomass, species composition, the phytoplankton abundance horizontal distribution, and the correlations between net-phytoplankton communities and mesoscale structure that were derived from the net samples taken from the Western Boundary Currents during summer, 2014. A total of 199 phytoplankton species belonging to 61 genera in four phyla were identified. The dominant species included Climacodium frauenfeldianum, Thalassiothrix longissima, Rhizosolenia styliformis var. styliformis, Pyrocystis noctiluca, Ceratium trichoceros, and Trichodesmium thiebautii. Four phytoplankton communities were divided by cluster analysis and the clusters were mainly associated with the North Equatorial Counter Current (NECC), the North Equatorial Current (NEC), the Subtropical Counter Current (STCC), and the Luzon Current (LC), respectively. The lowest phytoplankton cell abundance and the highest Trichodesmium filament abundance were recorded in the STCC region. The principal component analysis showed that T. thiebautii preferred warm and nutrient poor water. There was also an increase in phytoplankton abundance and biomass near 5°N in the NECC region, where they benefit from upwellings and eddies.

The dynamical impact of mesoscale eddies on migration of Japanese eel larvae.

In this study, we explore the dynamical role of mesoscale eddies on fish larvae migration using the example of Subtropical Counter Current eddies and the migration of Japanese eel larvae in the western North Pacific Ocean. An idealized experiment is conducted to isolate the effects of eddies, and use a three-dimensional particle-tracking method to simulate virtual eel larvae (v-larvae) migration, including both horizontal and vertical swimming behaviors. The impact of eddies strongly depends on the swimming speed of v-larvae relative to the eddy speed. Eddies accelerate the movement of v-larvae that swim slower than the propagation speed of the eddy, whereas faster-swimming v-larvae are dragged by eddies. A modified stream function that incorporates biological swimming ability explains the non-uniform trapping of v-larvae in mesoscale eddies. A high swimming speed and/or a small eddy rotation speed results in a weak trapping capacity. Simulations of v-larvae migration in realistic cases of eddy fields indicate that the abundance of eddies significantly affects the duration of larval migration, with the effects being largely dependent on the larvae swimming speed. We noted a negative relationship between the observed annual eel recruitment index in Taiwan and the eddy index subtropical countercurrent (STCC) region, which suggests a potentially important role of mesoscale eddies in eel larvae migration.

Decadal Variability in the South Pacific Subtropical Countercurrent and Regional Mesoscale Eddy Activity

AbstractDecadal variability of eddy activity in the western, subtropical South Pacific is examined using the past two decades of satellite altimetry data. Between 21° and 29°S, there is a band of heightened eddy activity. In this region, the eastward South Pacific Subtropical Countercurrent (STCC) overlays the westward South Equatorial Current (SEC). This vertically sheared STCC–SEC system is subject to baroclinic instabilities. By using the European Centre for Medium-Range Weather Forecasts (ECMWF) Ocean Reanalysis System, version 4 (ORAS4), data and verifying with the gridded Argo float data, low-frequency variations in the state of the ocean in this region are investigated. It is found that the low-frequency changes in the shearing and stratification of the STCC–SEC region simultaneously work to modulate the strength of baroclinic instabilities, as measured through the baroclinic growth rate. These changes in the strength of the instabilities consequently affect the observed eddy activity. Using a linearization of the baroclinic growth rate, the contribution to the variability from the changes in shearing is found to be roughly twice as large as those from changes in stratification. Additionally, changes in the temperature and salinity fields are both found to have significant impacts on the low-frequency variability of shearing and stratification, for which salinity changes are responsible for 50%–75% of the variability as caused by temperature changes. However, the changes in all these parameters do not occur concurrently and can alternately work to negate or augment each other.

Spatial variation of the zooplankton community in the western tropical Pacific Ocean during the summer of 2014

Knowledge of the zooplankton community in the western tropical Pacific Ocean is poor compared to that of the communities in the central and eastern Pacific Ocean. The zooplankton composition, abundance, biomass and community structure in the western Pacific Ocean were studied based on data collected during a synoptic cruise (August-September 2014). Four zooplankton communities were determined via cluster analysis, and these four clusters were mainly spatially related to four different currents: the Luzon Current (LC), Subtropical Countercurrent (STCC), North Equatorial Current (NEC) and North Equatorial Countercurrent (NECC). The estimated mean abundance and biomass of the zooplankton for the whole surveyed area were 146.7±178.1 ind/m3 and 36.9±40.3mg/m3, respectively. The zooplankton abundance was dominated by small copepods, such as Clausocalanus furcatus, C. pergens, Oncaea mediterranea and Oithona plumifera. The zooplankton abundance and biomass values were lowest in the STCC region and highest in the NECC region. BEST analysis based on surface environmental factors showed that chlorophyll a (chl a), pH, temperature and salinity were the environmental variables that best explained the distribution pattern of the zooplankton community (pw=0.372). The zooplankton abundance was higher south of the salinity front at 16°N, in accordance with the relatively higher nutrient and chl a levels. Maximum zooplankton biomass was found in regions on the periphery of the cyclonic Mindanao Eddy (ME) and anticyclonic Halmahera Eddy (HE).

Regional patterns of δ13C and δ15N stable isotopes of size-fractionated zooplankton in the western tropical North Pacific Ocean

Zooplankton play a prominent role in the biogeochemical cycles of marine ecosystems. Little is known about the trophodynamics of zooplankton in response to geographic patterns in isotopic baselines and physical processes in the western tropical North Pacific. In this study, stable isotope ratios of five size fractions of zooplankton (100 to >2000µm) from different current regions in the western tropical North Pacific Ocean were analyzed. Both δ13C and δ15N isotopic values increased with zooplankton size class. The largest zooplankton group (>2000µm), with a diverse composition, showed relatively higher stable isotope signatures, covering a wider range. Regional variations in the zooplankton stable isotope signatures were similar across all size classes, with generally higher values in the North Equatorial Counter Current (NECC) and the North Equatorial Current (NEC) and lower values in the Subtropical Counter Current (STCC). These regional patterns of zooplankton isotope signatures were consistent with the variation of oceanographic features (temperature, salinity, nutrients, chlorophyll a) and were also related to the isotopic baselines of particulate organic matter (POM) in the different current regions. Moreover, the nitrogen-fixing cyanobacteria Trichodesmium spp. may be the main contributor to low δ15N values in the STCC. The results of this study demonstrate the influence of physical processes on the stable isotopic signatures of zooplankton. This baseline information is crucial for future food web studies in the western tropical North Pacific Ocean.

Interannual variability of the subtropical countercurrent eddies in the North Pacific associated with the Western-Pacific teleconnection pattern

The connection and the relevant dynamical processes between oceanic eddies in the North Pacific Subtropical Countercurrent (STCC) region and the atmospheric Western-Pacific (WP) teleconnection is investigated on interannual timescales. North of the STCC region, the local northerly surface wind anomalies cool the ocean surface during negative phases of the WP teleconnection. The local surface cooling modifies the meridional gradient of sea surface temperature (SST), strengthening the SST front at its south. In the STCC region, we show the meridional gradient of surface-heat-flux forcing caused by the local surface cooling is the same order as the Ekman-convergence forcing. The strengthened SST front then leads to the pycnocline shoaling in the STCC region, which can also enhance the growth of baroclinic instability to produce more oceanic eddies, in addition to the enhanced STCC proposed previously. These dynamics are reversed during the positive phases of WP teleconnection.

Seasonal variations of sea level anomaly in the subtropical front zone based on satellite-derived data from 2003–2009

We investigated seasonal variations in the sea level anomaly in the northwestern Pacific subtropical front zone using weekly satellite-derived products and in situ observations from 2003 to 2009. The sea level anomalies had different seasonal cycles in the cold, front and warm zones. In the former two zones, peak values appeared (∼15 cm) in July–August and hollow values (∼−10 cm) in February–March; in the warm zone, sea level anomalies had no apparent seasonal variations. We found that the correlation between sea level anomaly and sea surface temperature reached values of 0.76 in the cold and front zones, but only 0.38 in the warm zone. The steric height anomaly explained 70% variances of sea level anomaly in the former two zones but only 25% in the warm zone. We examined the mechanisms of the different patterns of sea level anomaly variation among the three different zones and concluded that the steric height anomaly induced by air–sea heat exchanges controlled the seasonal variations of sea level anomaly in the former two zones, while the barotropic term accompanied with subtropical countercurrent lead to the obscure seasonal variations in the warm zone. These high frequency variations merit future study.

북서태평양 열대해역에서 관측된 수괴의 경년변동성

The interannual variability of the water masses was analyzed from the CTD data measured in the tropical northwestern Pacific from 2006 to 2014. There are two typical water masses NPTW and NPIW that reveal the interannual variability in the survey area, in addition to two other water masses; the surface water mass TSW with a large seasonal variability and the deep water mass AACDW with a constant temperature-salinity characteristic at the depths deeper than 2,000 meters. In 2012 and 2014 NPTW was the most widely extended horizontally and thicker than 100 meters vertically, which was found over the entire survey area. However, NPTW was reduced and became much narrower in 2009 than in the other years. NPIW seemed to expand southwards from the north of 21˚N to 15oN in 2008 and in 2012, which showed the salinity minimum in 2013 (＜ 34.15 psu). The sea surface height estimated by Absolute Dynamic Topography (ADT) approximately along 135˚E section showed the high peaks (＞ 1.45 dyn•m) between 16˚N and 18˚N during the periods between 2007 and 2009 and between 2012 and 2013; the former peak lasted wider and longer in latitude and time (about three times) than the latter. The vertical section of the geostrophic currents in the upper 1,000 meters shows that there was a mesoscale pattern of repeated eastward and westward flows a few times in some years (2010 and 2014), which seemed to disappear in some other years (2008 and 2012); the former was closely related to the mesoscale eddies and the latter implied the pattern with the permanent currents. The persistent eastward flow between 17oN and 19oN seems to be related to the Subtropical Countercurrent (STCC).

Role of horizontal density advection in seasonal deepening of the mixed layer in the subtropical Southeast Pacific

The mechanisms behind the seasonal deepening of the mixed layer (ML) in the subtropical Southeast Pacific were investigated using the monthly Argo data from 2004 to 2012. The region with a deep ML (more than 175 m) was found in the region of (22°–30°S, 105°–90°W), reaching its maximum depth (~200 m) near (27°–28°S, 100°W) in September. The relative importance of horizontal density advection in determining the maximum ML location is discussed qualitatively. Downward Ekman pumping is key to determining the eastern boundary of the deep ML region. In addition, zonal density advection by the subtropical countercurrent (STCC) in the subtropical Southwest Pacific determines its western boundary, by carrying lighter water to strengthen the stratification and form a “shallow tongue” of ML depth to block the westward extension of the deep ML in the STCC region. The temperature advection by the STCC is the main source for large heat loss from the subtropical Southwest Pacific. Finally, the combined effect of net surface heat flux and meridional density advection by the subtropical gyre determines the northern and southern boundaries of the deep ML region: the ocean heat loss at the surface gradually increases from 22?S to 35?S, while the meridional density advection by the subtropical gyre strengthens the stratification south of the maximum ML depth and weakens the stratification to the north. The freshwater flux contribution to deepening the ML during austral winter is limited. The results are useful for understanding the role of ocean dynamics in the ML formation in the subtropical Southeast Pacific.

Impacts of Interannual Ocean Circulation Variability on Japanese Eel Larval Migration in the Western North Pacific Ocean.

The Japanese eel larvae hatch near the West Mariana Ridge seamount chain and travel through the North Equatorial Current (NEC), the Kuroshio, and the Subtropical Countercurrent (STCC) region during their shoreward migration toward East Asia. The interannual variability of circulation over the subtropical and tropical regions of the western North Pacific Ocean is affected by the Philippines–Taiwan Oscillation (PTO). This study examines the effect of the PTO on the Japanese eel larval migration routes using a three-dimensional (3D) particle tracking method, including vertical and horizontal swimming behavior. The 3D circulation and hydrography used for particle tracking are from the ocean circulation reanalysis produced by the Japan Coastal Ocean Predictability Experiment 2 (JCOPE2). Our results demonstrate that bifurcation of the NEC and the strength and spatial variation of the Kuroshio affect the distribution and migration of eel larvae. During the positive phase of PTO, more virtual eels (“v-eels”) can enter the Kuroshio to reach the south coast of Japan and more v-eels reach the South China Sea through the Luzon Strait; the stronger and more offshore swing of the Kuroshio in the East China Sea leads to fewer eels entering the East China Sea and the onshore movement of the Kuroshio to the south of Japan brings the eels closer to the Japanese coast. Significant differences in eel migration routes and distributions regulated by ocean circulation in different PTO phases can also affect the otolith increment. The estimated otolith increment suggests that eel age tends to be underestimated after six months of simulation due to the cooler lower layer temperature. Underestimation is more significant in the positive PTO years due to the wide distribution in higher latitudes than in the negative PTO years.

Effects of the STCC eddies on the Kuroshio based on the 20-year JCOPE2 reanalysis results

In this study, 20years of model reanalysis data are analyzed to study the effects of the subtropical countercurrent (STCC) eddies on the upstream Kuroshio, from east of Luzon to east of Taiwan. The effects are assessed from individual events to interannual time scales. The wind-driven Kuroshio is modified by the STCC eddies, with high spatiotemporal variations. The mass balance in the composite eddy events indicates that the strengthening and weakening of the Kuroshio transport are locally caused by the mass convergence and divergence produced by the eddies. The same analogy applies to the interannual time scale. In the eddy-rich years, the upstream Kuroshio is generally stronger because of wind forcing, yet the strengthening is nonuniform because of modification by the eddies. The larger number of warm eddies to the east of Taiwan and Luzon Island further strengthen the jet, whereas the larger number of cold eddies to the east of the Luzon Strait weaken the Kuroshio in the Luzon Strait. Drifter trajectories show larger Luzon Strait intrusion during the occurrence of cold eddies. The local weakening of the Kuroshio by cold eddies leads to a weaker potential vorticity jump, producing favorable conditions for the intrusion of a water mass into the South China Sea.

Wind- versus Eddy-Forced Regional Sea Level Trends and Variability in the North Pacific Ocean

Abstract Regional sea level trend and variability in the Pacific Ocean have often been considered to be induced by low-frequency surface wind changes. This study demonstrates that significant sea level trend and variability can also be generated by eddy momentum flux forcing due to time-varying instability of the background oceanic circulation. Compared to the broad gyre-scale wind-forced variability, the eddy-forced sea level changes tend to have subgyre scales and, in the North Pacific Ocean, they are largely confined to the Kuroshio Extension region (30°–40°N, 140°–175°E) and the Subtropical Countercurrent (STCC) region (18°–28°N, 130°–175°E). Using a two-layer primitive equation model driven by the ECMWF wind stress data and the eddy momentum fluxes specified by the AVISO sea surface height anomaly data, the relative importance of the wind- and eddy-forced regional sea level trends in the past two decades is quantified. It is found that the increasing (decreasing) trend south (north) of the Kuroshio Extension is due to strengthening of the regional eddy forcing over the past two decades. On the other hand, the decreasing (increasing) sea level trend south (north) of the STCC is caused by the decadal weakening of the regional eddy momentum flux forcing. These decadal eddy momentum flux changes are caused by the background Kuroshio Extension and STCC changes in connection with the Pacific decadal oscillation (PDO) wind pattern shifting from a positive to a negative phase over the past two decades.

Seasonal Mesoscale and Submesoscale Eddy Variability along the North Pacific Subtropical Countercurrent

Abstract Located at the center of the western North Pacific Subtropical Gyre, the Subtropical Countercurrent (STCC) is not only abundant in mesoscale eddies, but also exhibits prominent submesoscale eddy features. Output from a ° high-resolution OGCM simulation and a gridded satellite altimetry product are analyzed to contrast the seasonal STCC variability in the mesoscale versus submesoscale ranges. Resolving the eddy scales of >150 km, the altimetry product reveals that the STCC eddy kinetic energy and rms vorticity have a seasonal maximum in May and April, respectively, a weak positive vorticity skewness without seasonal dependence, and an inverse (forward) kinetic energy cascade for wavelengths larger (shorter) than 250 km. In contrast, the submesoscale-resolving OGCM simulation detects that the STCC eddy kinetic energy and rms vorticity both appear in March, a large positive vorticity skewness with strong seasonality, and an intense inverse kinetic energy cascade whose short-wave cutoff migrates seasonally between the 35- and 100-km wavelengths. Using a 2.5-layer, reduced-gravity model with an embedded surface density gradient, the authors show that these differences are due to the seasonal evolution of two concurring baroclinic instabilities. Extracting its energy from the surface density gradient, the frontal instability has a growth time scale of O(7) days, a dominant wavelength of O(50) km, and is responsible for the surface-intensified submesoscale eddy signals. The interior baroclinic instability, on the other hand, extracts energy from the vertically sheared STCC system. It has a slow growth time scale of O(40) days, a dominant wavelength of O(250) km, and, together with the kinetic energy cascaded upscale from the submesoscales, determines the mesoscale eddy modulations.

Variation of the southern Subtropical Countercurrent related to sea surface height and eddies in the northwest tropical pacific

The ADCP records obtained at about 18°N, 135°E show the southern branch of the Subtropical Countercurrent (STCC). The sea surface heights (SSH) show that there is a tendency to increase and decrease in the south/north of STCC, respectively. So the variability of SSH ultimately contributes to the strengthening of STCC through geostrophic balance. The southern STCC branch distinctly persists from winter to spring. Since 2005, the southern STCC exists almost throughout the year, and the STCC is clearly stronger to the east of 145°E. Anticyclonic and cyclonic eddies exist seemingly as bands around 17.5°N and 20.5°N, respectively. The STCC flowing eastward, which is formed by the geostrophic balance, is maintained with the interaction between geostrophic currents and anticyclonic-cyclonic eddies. The rotating eddies exert an additional driving force to maintain the eastward flow of STCC, and then the STCC reveals a meandering movement due to the interaction with the eddies. The trajectories of surface drifters together with the altimeter data analysis in June 2009 dictate the variability of the STCC induced by the interaction between eddies and the eastward flow. These results suggest that the southern STCC slowly changes from an intra-seasonal event an annual one with time duration of over 21 months.

Instability of the North Pacific Subtropical Countercurrent

Abstract The North Pacific Subtropical Countercurrent (STCC) has a weak eastward velocity near the surface, but the region is populated with eddies. Studies have shown that the STCC is baroclinically unstable with a peak growth rate of 0.015 day−1 in March, and the ~60-day growth time has been used to explain the peak eddy kinetic energy (EKE) in May observed from satellites. It is argued here that this growth time from previously published normal-mode instability analyses is too slow. Growth rates calculated from an initial-value problem without the normal-mode assumption are found to be 1.5 to 2 times faster and at shorter wavelengths, due to the existence of (i) nonmodal solutions and (ii) sea surface temperature front in the mixed layer in winter. At interannual time scales it is shown that because of rapid surface adjustments, the STCC geostrophic shear, hence also the instability growth, is approximately in phase with surface forcing, leading to EKE modulation that peaks approximately 10 months later. However, the EKE can only be partially explained by this mechanism of modulation by baroclinic instability. It is suggested that the unexplained variance may be caused additionally by modulation of the EKE by dissipation.

Analysis of STCC eddies using the Okubo–Weiss parameter on model and satellite data

The North Pacific Subtropical Counter Current (STCC) is a weak zonal current comprising of a weak eastward flow near the surface (with speeds of less than 0.1 m/s and a thickness of approximately 50–100 m) and westward flow (the North Equatorial Current) beneath. Previous studies (e.g., Qiu J Phys Oceanogr 29: 2471–2486, 1999) have shown that the STCC is baroclinically unstable. Therefore, despite its weak mean speeds, nonlinear STCC eddies with diameters ~300 km or larger and rotational speeds exceeding the eddy propagation speeds develop (Samelson J Phys Oceanogr 27: 2645–2662, 1997; Chelton et al. Prog Oceanogr 91: 167–216, 2011). In this study, the authors present numerical experiments to describe and explain the instability and eddy-generation processes of the STCC and the seasonal variation. Emphasis is on finite-amplitude eddies which are analyzed based on the parameter of Okubo (Deep-Sea Res 17: 445–454, 1970) and Weiss (Physica D 48: 273–294, 1991). The temperature and salinity distribution in March and April offer the favorable condition for eddies to grow, while September and October are unfavorable seasons for the generation of eddies. STCC is maintained not only by subsurface front but also by the sea surface temperature (SST) front. The seasonal variation of the vertical shear is dominated by the seasonal surface STCC velocity. The SST front enhances the instability and lead to the faster growth of STCC eddies in winter and spring. The near-surface processes are therefore crucial for the STCC system.

Distribution of ultraphytoplankton in the western part of the North Pacific subtropical gyre during a strong La Niña condition: relationship with the hydrological conditions

Abstract. The distribution of ultraphytoplankton was investigated in the western North Pacific Subtropical Gyre (NPSG) during La Niña, a cold phase of El Niño Southern Oscillation (ENSO). Observations were conducted in a north-south transect (33.6–13.25° N) along the 141.5° E meridian in order to study the ultraplankton assemblages in various oligotrophic conditions. Analyses were performed at the single cell level by analytical flow cytometry. Five ultraphytoplankton groups (Prochlorococcus, Synechococcus, picoeukaryotes, nanoeukaryotes and nanocyanobacteria-like) defined by their optical properties were enumerated in three different areas visited during the cruise: the Kuroshio region, the subtropical Pacific gyre and a transition zone between the subtropical Pacific gyre and the Warm pool. Prochlorococcus outnumbered the other photoautotrophs in all the investigated areas. However, in terms of carbon biomass, an increase in the relative contribution of Synechococcus, picoeukaryotes and nanoeukaryotes was observed from the centre of the subtropical gyre to the Kuroshio area. In the Kuroshio region, a peak of abundance of nanoeukaryotes observed at the surface suggested an increase in nutrients likely due to the vicinity of a cold cyclonic eddy. In contrast, in the salinity front along the isohaline 35 and anticyclonic eddy located around 22.83° N, the mainly constant distribution of Prochlorococcus from the surface down to 150 m characterised the dominance by these microorganisms in high salinity and temperature zone. Results suggested that the distribution of nanocyanobacteria-like is also closely linked to the salinity front rather than low phosphate concentration. The maximum abundance of ultraphytoplankton was located above the SubTropical Counter Current (STCC) at depths > 100 m where higher nutrient concentrations were measured. Finally, comparison of the ultraphytoplankton concentrations during El Niño (from the literature) and La Niña (this study) conditions seems to demonstrate that La Niña conditions lead to higher concentrations of Synechococcus in the Subtropical gyre and a lower abundance of Synechococcus in the Kuroshio region. Our results suggest that the west part of NPSG is a complex area, where different water masses, salinity fronts and eddies lead to a heterogeneous distribution of ultraphytoplankton assemblages in the upper layer of the water column.

A modified method to estimate eddy diffusivity in the North Pacific using altimeter eddy statistics

The method proposed by Stammer (1998) is modified using eddy statistics from altimeter observation to obtain more realistic eddy diffusivity (K) for the North Pacific. Compared with original estimates, the modified K has remarkably reduced values in the Kuroshio Extension (KE) and North Equatorial Counter Current (NECC) regions, but slightly enhanced values in the Subtropical Counter Current (STCC) region. In strong eastward flow areas like the KE and NECC, owing to a large difference between mean flow velocity and propagation velocity of mesoscale eddies, tracers inside the mesoscale eddies are transported outside rapidly by advection, and mixing length L is hence strongly suppressed. The low eddy probability (P) is also responsible for the reduced K in the NECC area. In the STCC region, however, L is mildly suppressed and P is very high, so K there is enhanced. The zonally-averaged K has two peaks with comparable magnitudes, in the latitude bands of the STCC and KE. In the core of KE, because of the reduced values of P and L, the zonally-averaged K is a minimum. Zonally-integrated eddy heat transport in the KE band, calculated based on the modified K, is much closer to the results of previous independent research, indicating the robustness of our modified K. The map of modified K provides useful informationfor modeling studies in the North Pacific.

Response of mode water and Subtropical Countercurrent to greenhouse gas and aerosol forcing in the North Pacific

The response of the North Pacific Subtropical Mode Water and Subtropical Countercurrent (STCC) to changes in greenhouse gas (GHG) and aerosol is investigated based on the 20th-century historical and single-forcing simulations with the Geophysical Fluid Dynamics Laboratory Climate Model version 3 (GFDL CM3). The aerosol effect causes sea surface temperature (SST) to decrease in the mid-latitude North Pacific, especially in the Kuroshio Extension region, during the past five decades (1950–2005), and this cooling effect exceeds the warming effect by the GHG increase. The STCC response to the GHG and aerosol forcing are opposite. In the GHG (aerosol) forcing run, the STCC decelerates (accelerates) due to the decreased (increased) mode waters in the North Pacific, resulting from a weaker (stronger) front in the mixed layer depth and decreased (increased) subduction in the mode water formation region. The aerosol effect on the SST, mode waters and STCC more than offsets the GHG effect. The response of SST in a zonal band around 40°N and the STCC to the combined forcing in the historical simulation is similar to the response to the aerosol forcing.