Kuroshio Extension Front Research Articles

Evidence of enhanced double‐diffusive convection below the main stream of the Kuroshio Extension

AbstractIn this study, a Navis‐MicroRider microstructure float and an EM‐APEX float were deployed along the Kuroshio Extension Front. The observations deeper than 150 m reveal widespread interleaving thermohaline structures for at least 900 km along the front, presumably generated through mesoscale stirring and near‐inertial oscillations. In these interleaving structures, microscale thermal dissipation rates χ are very high ( K2s−1), while turbulent kinetic energy dissipation rates ϵ are relatively low ( Wkg−1), with effective thermal diffusivity of ( m2s−1) consistent with the previous parameterizations for double‐diffusion, and, is two orders of magnitude larger than the turbulent eddy diffusivity for density . The average observed dissipation ratio Γ in salt finger and diffusive convection favorable conditions are 1.2 and 4.0, respectively, and are larger than that for turbulence. Our results suggest that mesoscale subduction/obduction and near‐inertial motions could catalyze double‐diffusive favorable conditions, and thereby enhancing the diapycnal tracer fluxes below the Kuroshio Extension Front.

Argo data assimilation and its effect on climate state estimation and forecasting in the western North Pacific using a coupled model

AbstractIn this study, we investigated the effects of ocean subsurface data (Argo data) on climate state estimation and forecasting, focusing on the reproduction of North Pacific subtropical mode water (STMW) using a four‐dimensional variational data assimilation system with a coupled model. We produced two reanalysis plus forecast data sets for the ocean and atmosphere in 2010 using a 3 month assimilation period: the first including Argo data (Argo case) and the second did not include Argo data (control case). In the control case, the Kuroshio, Kuroshio Extension front, and recirculation gyres along the front were not adequately reproduced. Consequently, there were large biases in temperature and salinity in the western North Pacific. The assimilation of Argo data effectively corrected these biases and significantly improved reproduction of the Kuroshio fronts and recirculation gyres, resulting in a more realistic reproduction of the winter mixed layer and STMW. The correction of these biases is critical to the 1–3 year predictions of the STMW core properties, and the assimilation of Argo data enables prediction of these properties for more than a year. We showed that assimilation of Argo data affects the surface atmospheric temperature above the STMW formation region.

Marine atmospheric boundary layer and low-level cloud responses to the Kuroshio Extension front in the early summer of 2012: three-vessel simultaneous observations and numerical simulations

Intensive atmospheric observations were carried out with five research vessels in total across the sea surface temperature (SST) front along the Kuroshio Extension in the early summer of 2012, to identify the effects of the front on the thermal structure and cloud formation in the marine atmospheric boundary layer (MABL). Three of the vessels were aligned together along the 143°E meridian with latitudinal separation of as small as 30′ or 45′, going back and forth across the SST front for in situ observations during 2–6 July. The SST front was quite sharp and moved northward by about 50 km in 3 days, which was not well represented in objectively analyzed SST data sets. The observations captured rapid changes of the mesoscale MABL structure across the SST front, which were particularly evident in cloud base height and downward longwave radiation (DLR) at the surface. The higher base of low-level clouds observed over the warmer water resulted from stronger turbulent mixing in the MABL, which became prominent under the northerlies. The most frequently measured DLR value was greater by 20 W m−2 to the south of the SST front than to the north. High-resolution atmospheric model experiments conducted with and without the frontal SST gradient have confirmed its critical importance for the MABL structure and low-level clouds. These imprints of the SST front simulated in the models are sensitive to SST data assigned at the lower boundary of the model.

Synoptic observation of Central Mode Water in its formation region in spring 2003

Hydrographic data east of Japan from five research cruises and Argo profiling floats in spring 2003 have been analyzed to examine the relationship of the formation of Central Mode Water (CMW) and Transition Region Mode Water (TRMW) in late winter 2003 to thermohaline fronts and mesoscale eddies. TRMW and the denser variety of CMW (D-CMW) were formed continuously just south of the subarctic frontal zone between 155°E and 165°W with little relation to eddies, suggesting that the absence of the permanent thermocline and halocline in this area is essential for the formation. The lighter variety of CMW (L-CMW) was formed south of the Kuroshio bifurcation front and east of 165°E, partly in an anticyclonic eddy associated with the Kuroshio Extension. Some portion of D-CMW and L-CMW likely had been subducted to the permanent pycnocline by crossing southward the Kuroshio bifurcation front and the Kuroshio Extension front, respectively. In contrast, the formation of these waters in the western regions was inactive and was significantly different from that described previously using multiyear Argo float data. West of 155°E, TRMW and D-CMW were formed only in two anticyclonic eddies that had been detached from the Kuroshio Extension 1–2 years ago. L-CMW was hardly formed west of 165°E, which might be related to the upstream Kuroshio Extension being in its stable state characterized by low regional eddy activity.

Atmospheric pressure response to mesoscale sea surface temperature variations in the Kuroshio Extension region: In situ evidence

Several research cruises were conducted across the Kuroshio Extension front to examine the low-level atmospheric responses to mesoscale variations in sea surface temperature (SST). Surface meteorological observations, including sea level pressure (SLP) and SST, were collected at two moored buoys that were located on either side of the Kuroshio Extension, and from a research vessel, as it moved between the two buoys during the various cruises. Spatial perturbations in SLP along the ship transects, calculated by subtracting moored-buoy SLP from that of the moving ship, tend to be positive (negative) where SST is lower (higher) on spatial scales of about 100 km, and the magnitude of these SLP perturbations near the SST front can exceed 1.0 hPa. Radiosonde data also show that the atmospheric boundary layer thins (thickens) over lower (higher) SST. Although the contribution of across-track component in the wind cannot be calculated, the along-track component of divergence suggests low-level convergence over higher SST. The thermally induced pressure gradient is important in the momentum budget, suggesting that sea breeze-like local circulations formed over the SST fronts. Indeed, the pressure adjustment mechanism can sometimes dominate even on a scale of 100 km, although it is not always observed. The time scale in which the boundary layer thickness adjusted to SST is estimated to be 1 day or less.

Distribution of sei whales (Balaenoptera borealis) in the subarctic–subtropical transition area of the western North Pacific in relation to oceanic fronts

The subarctic–subtropical transition area of the western North Pacific is an important summer feeding grounds of sei whales. The oceanographic structure and circulation of this area are largely determined by strong oceanic fronts and associated geostrophic currents, namely the Polar Front (PF), Subarctic Front (SAF) and Kuroshio Extension Front (KEF). The relationship between the distribution of sei whales and oceanographic fronts was investigated using a generalized additive model (GAM), and the cetacean sighting survey data and oceanographic observations in July from 2000 to 2007 were used in the analysis. The number of individual sei whales was used as the response variable while the distances from the PF, SAF, and KEF to the whales were used as explanatory variables along with the longitude values. Sei whales were concentrated north and south of the SAF and the areas from 250 to 300km north and from 100 to 200km south of the SAF were estimated as high-density areas of sei whales. The entire inter-frontal zone between the PF and SAF featured an elevated concentration of sei whales, and the area south of the PF and along the SAF was identified as an important feeding ground of sei whales in July from 2000 to 2007.

The response of the Pacific storm track and atmospheric circulation to Kuroshio Extension variability

An index of the Kuroshio Extension front strength is produced using a maximum covariance analysis between sea‐surface temperature (SST) and sea‐surface height (SSH) gradient observations, and composites of the atmospheric state are presented during its positive and negative phases using reanalysis data (1992–2011).It is found that when the Kuroshio Extension is less (more) meandering, with a stronger (weaker) SST front, the atmospheric heat transport by transient eddies is increased in the western (eastern) Pacific region, consistent with an increase (decrease) in low‐level baroclinicity. Analysis of the eddy–mean flow interaction shows that this zonal shift in heat transport forces anomalous barotropic flow in the Eastern Pacific, where blocking frequency is strongly influenced.The above relationships cannot be reconciled with the known response of the North Pacific storm track to remote forcing from the Tropical Pacific, nor can they be explained by the response of the ocean to atmospheric forcing via surface heat fluxes or winds. Rather, the zonal shift in the storm track highlighted here, and the associated changes in the large‐scale circulation, are interpreted as a response to the interannual variability of the Kuroshio Extension front.

Cloud Response to the Meandering Kuroshio Extension Front

A unique set of observations on board research vessel (R/V) Mirai in April 2010 captured a striking cloud hole over a cold meander of the Kuroshio Extension (KE) east of Japan as corroborated by atmospheric soundings, ceilometer, shipboard radiation data, and satellite cloud images. Distinct differences were also observed between the warm meander farther to the north and warm water south of the KE. The atmosphere is highly unstable over the warm meander, promoting a well-mixed marine atmospheric boundary layer (MABL) and a layer of solid stratocumulus clouds capped by a strong inversion. Over the warm water south of the KE, MABL deepens and is decoupled from the ocean surface. Scattered cumulus clouds develop as captured by rapid variations in ceilometer-derived cloud base. The results show that the meandering KE front affects the entire MABL and the clouds. Such atmospheric response can potentially intensify the baroclinicity in the lower atmosphere.

Transport simulation of the radionuclide from the shelf to open ocean around Fukushima

We simulate radionuclide distribution in the ocean due to direct emission from the Fukushima Daiichi nuclear power plant (FNPP) for the period from 21 March to 6 May 2011. Dispersion of Cesium-137 is modeled by a transport equation with advection and diffusion by three dimensional ocean current. The ocean current data are provided from a data-assimilative, tide-resolving ocean general circulation model with horizontal resolution of 1/36°. Results show that the radionuclide expands from the shelf region into open ocean in April by the ocean currents and farther transported eastward along the Kuroshio Extension front in May. Sensitivity experiments demonstrate that the mesoscale geostrophic currents basically governed the transport processes in the open ocean after April. In the shelf region, on the other hand, the wind-driven currents facilitate the north–south extension of the radionuclide distribution through the repeated generations of the shelf waves. Influences of tide and river discharges cannot be neglected.

Atmospheric impact on the northwestern Pacific under a global warming scenario

Eleven climate models, one high‐resolution and ten low‐resolution, were analyzed to investigate the response of the northwestern Pacific under a global warming scenario. Application of scenario A1B of the Special Report on Emission Scenarios (SRES) weakens (intensifies) the southern (northern) part of the interior subtropical gyre both in high‐resolution and low‐resolution model. Such a dipole type change is mainly due to a basin‐scale dynamic atmosphere‐to‐ocean process. Namely, under global warming the Hadley circulation is weakened and expanded poleward. The Ferrel circulation is also displaced poleward, leading to weakening of ascending (descending) air motion and a high (low) sea level pressure anomaly in the northwestern (southeastern extratropical) North Pacific. Finally, a negative wind stress curl anomaly developed along the zero wind stress curl line of the present‐day climate to enhance the northern part of the gyre. The high‐resolution model results show greater changes in the structure of the Kuroshio and Kuroshio Extension, with strong intensification of the Kuroshio Extension front and jet, while in the low‐resolution models the changes are small. The Kuroshio between Taiwan and the southern coast of Japan is significantly intensified in the high‐resolution model results, but is slightly weakened in the ensemble of the low‐resolution models.

Surface Heat Flux Variations across the Kuroshio Extension as Observed by Surface Flux Buoys

Abstract Wintertime sea surface heat flux variability across the Kuroshio Extension (KE) front is analyzed using data from the Kuroshio Extension Observatory (KEO) buoy in the Kuroshio recirculation gyre south of the KE front and from the Japan Agency for Marine–Earth Science and Technology KEO (JKEO) buoy in the north of the front. The coincident data used are from periods during two winters (2007 and 2008), when both buoys had a complete suite of meteorological data. In these two winter periods, the focus of this research is on three types of typical weather patterns referred to here as the northerly wind condition, the monsoon wind condition, and the normal condition. During the northerly wind condition, latent and sensible heat fluxes were large and often varied simultaneously at both sites, whereas during the monsoon wind condition the latent heat flux at the KEO site was significantly larger than that at the JKEO site. The difference between these heat flux patterns is attributed to the different airmass transformations that occur when prevailing winds blow across the KE front versus along the front. Reanalysis products appear to reproduce these heat flux spatial patterns at synoptic scales. It is suggested that the relative frequencies of these different types of weather conditions result in anomalous spatial patterns in the heat fluxes on monthly time scales.

Ocean Frontal Effects on the Vertical Development of Clouds over the Western North Pacific: In Situ and Satellite Observations*

Abstract A suite of shipboard and satellite observations are analyzed and synthesized to investigate the three-dimensional structure of clouds and influences from sea surface temperature fronts over the western North Pacific. Sharp transitions are observed across the Kuroshio Extension (KE) front in the marine atmospheric boundary layer (MABL) and its clouds. The ocean’s influence appears to extend beyond the MABL, with higher cloud tops in altitude along the KE front than the surroundings. In winter, intense turbulent heat release from the ocean takes place on the southern flank of the KE front, where the cloud top penetrates above the MABL and reaches the midtroposphere. In this band of high cloud tops, frequent lightning activity is observed. The results of this study suggest a sea level pressure mechanism for which the temperature gradient in the MABL induces strong surface wind convergence on the southern flank of the KE front, deepening the clouds there. In early summer, sea fog frequently occurs on the northern flank of the subtropical KE and subarctic fronts under southerly warm advection that suppresses surface heat flux and stabilizes the surface atmosphere. Sea fog is infrequently observed over the KE front even under southerly conditions, as the warm ocean current weakens atmospheric stratification and promotes vertical mixing. The KE front produces a narrow band of surface wind convergence, helping support a broad band of upward motion at 700 hPa that is associated with the eastward extension of the baiu rainband from Japan in June–July.

Observations of Marine Atmospheric Boundary Layer Transitions across the Summer Kuroshio Extension*

Abstract The baiu and Kuroshio Extension (KE) fronts, both zonally oriented and nearly collocated east of Japan, are the dominant summertime features of the atmosphere and ocean, respectively, over the midlatitude northwest Pacific. An atmospheric sounding campaign was conducted on board the R/V Roger Revelle during the 2005 summer. Transects of soundings across the KE front are analyzed to study its effects on the atmosphere, along with continuous surface meteorological and ceilometer cloud-base observations. While the KE front remained nearly stationary during the cruise, the baiu front displayed large meridional displacements that changed wind direction across the KE front. The presence of sharp sea surface temperature (SST) gradients anchored by the KE enhanced the thermal and moisture advection, causing substantial changes in the marine atmospheric boundary layer (MABL) structure. When the baiu front was displaced north of the KE front, southwesterly winds advected warm, humid air from the subtropics over the cold water, producing a surface inversion favorable to fog formation. When the baiu front was to the south, on the other hand, northerly winds across the KE front destabilized the MABL, leading to the formation of a solid low-cloud deck beneath a strong capping inversion. The wind changes with the meridional displacement of the baiu front thus caused large variations in near-surface atmospheric stability and surface turbulent heat fluxes, with potential feedback on deep convection and fog/low-cloud formation around the front.

Recruitment Variability of Small Pelagic Fish Populations in the Kuroshio-Oyashio Transition Region of the Western North Pacific

Climatic and ecosystem changes in the transition region between the subarctic Oyashio and the subtropical Kuroshio Extension fronts may be the key to understanding recruitment variability of small pelagic fish populations in the Pacific waters off Japan. The transition region is characterized by complex oceanographic structure with the meandering fronts, eddies and streamers creating large spatial and temporal variability in the environment. Juveniles of small pelagic fishes such as Japanese sardine (Sardinops melanostictus), Pacific saury (Cololabis saira), and Japanese anchovy (Engraulis japonicus) migrate from the subtropical Kuroshio Current area to the subarctic Oyashio Current area across the transition region. A precipitous decline in number of age 0 recruits of Japanese sardine after the end of 1980s appears to be related to the extremely high mortality rate of young-of-the-year fish in the transition region. In contrast, populations of Pacific saury and Japanese anchovy increased after 1988. These synchronous changes in populations seem to be associated with the weakening of the Aleutian Low Pressure in the North Pacific and weakening of the Oyashio Current in winter, and a sea surface temperature (SST) rise in the Kuroshio-Oyashio transition region in spring. The contrasting responses of the fish populations to the SST rise can be explained by different temperature preference in terms of growth rate in larval and early juvenile stages; cool temperatures are preferred by sardine and warm temperatures are preferred by saury and anchovy.

The Kuroshio Extension Front from satellite sea surface temperature measurements

The position and strength of the surface Kuroshio Extension Front (KEF), defined as the sea surface temperature (SST) gradient maximum adjacent to the Kuroshio Extension (KE) axis (approximated by a specific SSH contour consistently located at, or near, the maximum of the SSH gradient magnitude), have been studied using weekly, microwave SST measurements from the later 1997 to early 2008. The mean KEF meanders twice around ∼36°N between the east coast of Japan and 153°E. It then migrates southeast to ∼34°N, just before reaching the Shatsky Rise (∼158°E), then progresses mostly eastward. Spatially, the KEF is strongest near the Japan coast, while it is seasonally strongest in winter and weakest in summer. Low-frequency variations of its strength, most notably in its upstream region, can be related to the known bimodal states of the KE. During 2003–2005, when the KE was in its stable state, the winter KEF SST gradient exceeded 10°C/100 km.

Numerical modeling of the cross-section structure of the subarctic/subtropical boundary in the North Pacific

A simple variational model of the zonal region between waters has been developed with a specified density gradient and a convergence zone. An approximate analytical solution has been constructed. The boundary between waters is shown to divide into two fronts, a convergence front and a density front. The density front should be shifted equatorward relative to the convergence axis. Such a structure corresponds to the subarctic/subtropical boundary in the Pacific Ocean: the subarctic front and the Kuroshio Extension front that goes into the North Pacific Current.

Decadal Variability of the Kuroshio Extension: Observations and an Eddy-Resolving Model Hindcast*

Abstract Low-frequency variability of the Kuroshio Extension (KE) is studied using observations and a multidecadal (1950–2003) hindcast by a high-resolution (0.1°), eddy-resolving, global ocean general circulation model for the Earth Simulator (OFES). In both the OFES hindcast and satellite altimeter observations, low-frequency sea surface height (SSH) variability in the North Pacific is high near the KE front. An empirical orthogonal function (EOF) analysis indicates that much of the SSH variability in the western North Pacific east of Japan is explained by two modes with meridional structures tightly trapped along the KE front. The first mode represents a southward shift and to a lesser degree, an acceleration of the KE jet associated with the 1976/77 shift in basin-scale winds. The second mode reflects quasi-decadal variations in the intensity of the KE jet. Both the spatial structure and time series of these modes derived from the hindcast are in close agreement with observations. A linear Rossby wave model forced by observed wind successfully reproduces the time series of the leading OFES modes but fails to explain why their meridional structure is concentrated on the KE front and inconsistent with the broadscale wind forcing. Further analysis suggests that KE variability may be decomposed into broad- and frontal-scale components in the meridional direction—the former following the linear Rossby wave solution and the latter closely resembling ocean intrinsic modes derived from an OFES run forced by climatological winds. The following scenario is suggested for low-frequency KE variability: basin-scale wind variability excites broadscale Rossby waves, which propagate westward, triggering intrinsic modes of the KE jet and reorganizing SSH variability in space.

Simultaneous seismic reflection and physical oceanographic observations of oceanic fine structure in the Kuroshio extension front

New simultaneous seismic reflection and physical oceanographic observations east of Japan demonstrate the utility of the seismic reflection method in mapping oceanic fine structure. Synthetic seismograms calculated from temperature and salinity data confirm that seismic reflections correlate with physical oceanographic structures. Seismic reflections at the boundary between the warm Kuroshio and the cold Oyashio water masses correspond to well developed, ∼10 m scale, temperature fine structure. Vertical current profiles suggest that this fine structure is caused by interleaving of these two water masses. We compare our seismic images with acoustic Doppler current profiler (ADCP) intensity maps and discuss similarities and the differences between seismic images and the ADCP maps. Our study demonstrates that even relatively low‐energy seismic sources, in this case a 3.4 l (210 in3) generator‐injector (GI) airgun, can be used to image upper oceanic fine structure.

Larval mesopelagic fish assemblages in the Kuroshio–Oyashio transition region of the western North Pacific

We examined larval mesopelagic fish assemblages, their distribution, and seasonal occurrence patterns in the Kuroshio–Oyashio transition region of the western North Pacific where complex hydrographic structures are observed due to the confluence of the Kuroshio Extension and Oyashio current. Larvae of the dominant families Myctophidae, Gonostomatidae, Bathylagidae, Sternoptychidae, and Phosichthyidae were represented by 31 species or types belonging to 24 genera. Based on species composition analysis using the Morishita–Horn similarity index, five assemblages were recognized: Oyashio, Spring Transition, Summer Transition, Kuroshio, and Slope Water assemblages. The distribution patterns of these assemblages corresponded closely with hydrographic structures such as position of the Oyashio and Kuroshio Extension fronts, warm core rings and streamers. Spring Transition (April) and Summer Transition (July–October) assemblages were the most important larval assemblages in the transition region. Larval abundances were low during late autumn and winter. The Spring Transition and Summer Transition assemblages were composed of subtropical, transitional, subarctic, and slope-water species, suggesting the importance of the transition region as nursery grounds of mesopelagic fishes of various origins from subarctic to subtropical waters. Larval fish transport by the Kuroshio, Oyashio, and Tsugaru Warm currents into the transition region is discussed.

Decadal Variability in the Kuroshio–Oyashio Extension Simulated in an Eddy-Resolving OGCM

Abstract Through analysis of a hindcast integration of an eddy-resolving quasi-global ocean general circulation model, decadal variability in the Kuroshio–Oyashio Extension region is investigated, with particular emphasis on that of the subarctic (Oyashio) and the Kuroshio Extension (KE) fronts. The KE front is deep and is accompanied by a sharp sea surface height (SSH) gradient with modest sea surface temperature (SST) gradient. In contrast, the subarctic front is shallow and is recognized as a zone of tight gradient in SST but not SSH. As a decadal-scale change from a warm period around 1970 to a cool period in the mid-1980s, those fronts in the model migrate southward as observed, and the associated pronounced cooling is confined mainly to those frontal zones. Reflecting the distinctive vertical structure of the fronts, the mixed layer cooling is the strongest along the subarctic front, whereas the subsurface cooling and the associated salinity changes are most pronounced along the KE front. Concomitantly with their southward migration, the two fronts have undergone decadal-scale intensification. Associated with reduced heat release into the atmosphere, the cooling in the frontal zones can be attributed neither to the direct atmospheric thermal forcing nor to the advective effect of the intensified KE, while the advective effect by the intense Oyashio can contribute to the cooling in the subarctic frontal zone. In fact, their time evolution is not found to be completely coherent, suggesting that their variability may be governed by different mechanisms. Decadal SSH variability in the KE frontal zone seems to be largely explained by propagation of baroclinic Rossby waves forced by anomalous Ekman pumping in the central North Pacific. This process alone cannot fully explain the corresponding variability in the subarctic frontal zone, where eastward propagating SSH anomalies off the Japanese coast seem to be superimposed on the Rossby wave signals.