Shinya Kouketsu

Simulated Rapid Warming of Abyssal North Pacific Waters

Warming the Deep The coldest ocean waters are located at the bottoms of the major ocean basins, and, because it takes a long time for water to sink from the surface to these regions, they are relatively isolated from the warming trends that are now occurring at shallower depths. However, warming in these deep waters has recently been observed, sooner than anticipated. Masuda et al. (p. 319, published online 24 June) performed computer simulations of ocean circulation and found that internal waves are able to transport heat rapidly from the surface waters around Antarctica to the bottom of the North Pacific, which can occur within four decades, rather than the centuries that conventional mechanisms have suggested. Computer simulations suggest a possible reason for the warming of North Pacific bottom water during the past four decades. Recent observational surveys have shown significant oceanic bottom-water warming. However, the mechanisms causing such warming remain poorly understood, and their time scales are uncertain. Here, we report computer simulations that reveal a fast teleconnection between changes in the surface air-sea heat flux off the Adélie Coast of Antarctica and the bottom-water warming in the North Pacific. In contrast to conventional estimates of a multicentennial time scale, this link is established over only four decades through the action of internal waves. Changes in the heat content of the deep ocean are thus far more sensitive to the air-sea thermal interchanges than previously considered. Our findings require a reassessment of the role of the Southern Ocean in determining the impact of atmospheric warming on deep oceanic waters.

The role of meso-scale eddies in mixed layer deepening and mode water formation in the western North Pacific

Distributions of mixed layer depths around the centers of anti-cyclonic and cyclonic eddies in the North Pacific Ocean were composited by using satellite-derived sea surface height anomaly data and Argo profiling float data. The composite distributions showed that in late winter, deeper mixed layers were more (less) frequently observed inside the cores of the anti-cyclonic (cyclonic) eddies than outside. This relationship was the clearest in the region of 140°E–160°W and 35°N–40°N, where the temperature and salinity of the deep mixed layers were similar to those of the lighter variety of central mode water (L-CMW). A simple one-dimensional bulk mixed layer model showed that both strong sea-surface heat and momentum fluxes and weak preexisting stratification contributed to formation of the deep mixed layer. These conditions were associated with the anti-cyclonic eddies, suggesting that these eddies are important in the formation of mode waters, particularly L-CMW.

Heat balance and regime shifts of the mixed layer in the Kuroshio Extension

Abstract We investigated the seasonal variations of the mixed layer depth and temperature in the Kuroshio Extension region (145–180°E, 30–36°N), and studied the causes of the mixed layer depth and temperature ‘regime shifts’ which occurred in the late 1980s, using upper ocean thermal and heat flux datasets incorporated with a bulk mixed layer model. The mixed layer from fall to winter is cooled by the net surface heat flux, the Ekman transport and the entrainment, and warmed by the horizontal heat convergence resulting from the Kuroshio heat advection. The mixed layer depth is controlled by the entrainment and the horizontal transport divergence/convergence which act to slow the mixed layer deepening from fall to early winter and then in winter deepen the mixed layer. The entrainment velocity is significantly influenced by the temperature difference between the mixed layer and the layer below. The mixed layer shift from its deep to shallow phase occurred in 1985. This shift is preceded the SST-shift in 1988 by three years, and was caused by the horizontal transport divergence anomaly. The horizontal heat convergence of the Kuroshio Extension caused the SST-shift in 1988, whereas its anomaly had been already positive since 1983. The delay from 1983 to 1987 can be attributed to the effects of the negative fall–SST anomaly, stronger surface heat flux and Ekman cooling and the shoaling of the mixed layer depth. It is suggested that the Kuroshio current system plays a major role in forcing the SST-shift and thus the subsequent climate regime shift.

Southward spreading of the Fukushima-derived radiocesium across the Kuroshio Extension in the North Pacific

The accident of the Fukushima Dai-ichi nuclear power plant in March 2011 released a large amount of radiocesium into the North Pacific Ocean. Vertical distributions of Fukushima-derived radiocesium were measured at stations along the 149°E meridian in the western North Pacific during the winter of 2012. In the subtropical region, to the south of the Kuroshio Extension, we found a subsurface radiocesium maximum at a depth of about 300 m. It is concluded that atmospheric-deposited radiocesium south of the Kuroshio Extension just after the accident had been transported not only eastward along with surface currents but also southward due to formation/subduction of subtropical mode waters within about 10 months after the accident. The total amount of decay-corrected 134Cs in the mode water was an estimated about 6 PBq corresponding to 10–60% of the total inventory of Fukushima-derived 134Cs in the North Pacific Ocean.

Formation and Subduction of Central Mode Water Based on Profiling Float Data, 2003–08

Abstract Temperature and salinity data from Argo profiling floats in the North Pacific during 2003–08 have been analyzed to study the structure of winter mixed layer north of the Kuroshio Extension and the subsurface potential vorticity distribution in the subtropical gyre in relation to the formation and subduction of the central mode water (CMW). In late winter, two zonally elongated bands of deep mixed layer extend at 33°–39° and 39°–43°N, from the east coast of Japan to 160°W. These correspond to the formation region of the lighter variety of CMW (L-CMW) and that of the denser variety of CMW (D-CMW) and the recently identified transition region mode water (TRMW), respectively. In the western part of the L-CMW and D-CMW–TRMW formation regions west of 170°E, the winter mixed layer becomes deeper and lighter to the east (i.e., to the downstream). As a result, the formed mode water is reentrained into the mixed layer in the farther east in the following winter and modified to the lighter water and is thus...

Decadal seesaw of the Central and Subtropical Mode Water formation associated with the Kuroshio Extension variability

Available Argo profiling float data from 2002 to 2011 were analyzed to examine the effect of the Kuroshio Extension (KE) current system variability on the formation of the Central Mode Water. Just north of the upstream portion of the KE at 140–152°E, formation of a lighter variety of the Central Mode Water in winter was active during the unstable period of the upstream KE in 2006–2009 and was reduced when the upstream KE was in the stable period of 2002–2005 and 2010–2011. This decadal formation variability is out of phase with that of the Subtropical Mode Water just south of the KE.

Mesoscale eddy effects on temporal variability of surface chlorophyll a in the Kuroshio Extension

We investigated the relationship between chlorophyll a (Chl-a) concentrations estimated from satellite observations and the activity of eddies in the Kuroshio Extension region. High (low) area-averaged Chl-a concentrations were frequently observed in the core of cyclonic (anticyclonic) eddies. Such relationships between Chl-a concentrations and eddy cores were not frequently observed in the southern part of the recirculation gyre, and advection of background meridional gradient of Chl-a by eddy-edge currents accounted for Chl-a spatial variability. Decadal-scale changes of Chl-a concentrations around the Kuroshio Extension were strongly affected by eddy activity and transport but not by large-scale near-surface isopycnal heaving. We also found that decadal changes of nutrient concentrations near the main stream could affect Chl-a concentrations in the southern part of the recirculation gyre via southward transport of eddies and mean flow.

Three-Dimensional Structure of Frontal Waves and Associated Salinity Minimum Formation along the Kuroshio Extension

Abstract High-resolution hydrographic observations were conducted in September 2002 to describe the detailed structure of frontal waves along the Kuroshio Extension. Frontal waves were observed both in the upper and intermediate layers, which corresponded to the depths of North Pacific Subtropical Mode Water and North Pacific Intermediate Water, respectively. The frontal wave in the upper layer preceded the intermediate-depth frontal wave by about 1/4 wavelength. The vertical phase lag created situations in which the low-salinity Oyashio water in the intermediate layer was superimposed below the upper-layer high-salinity Kuroshio water, thus forming a vertical salinity minimum. The frontal waves with a wavelength of about 200 km and a phase speed of 0.2–0.3 m s−1 propagating in the downstream direction were consistent with those from satellite images. Vertical velocity fields were estimated with the quasigeostrophic omega equation. Downwelling around the trough (from the crest to trough) of the intermedia...

Physical oceanographic conditions around the S1 mooring site

We describe physical oceanographic conditions around the S1 biogeochemical mooring site (30°N, 145°E) between February 2010 and July 2013. At the S1 mooring site, there is a clear seasonal variability of the mixed layer depth, wind forcing as well as horizontal kinetic energy in a near-inertial band. Interannual variability of the winter mixed layer was observed. The winter mixed layer depth was shallower in early 2010 and became deeper afterwards. Several mesoscale eddies and typhoons passed by the S1 mooring sites every year. Based on observed events, we suggest that those physical processes possibly affected biogeochemical properties around the S1 mooring site.

Unstable Frontal Waves along the Kuroshio Extension with Low-Potential Vorticity Intermediate Oyashio Water

Abstract A linear stability analysis was conducted for a three-layer primitive equation model including viscosity with a basic state, which modeled the stratification and velocity fields with the vertical and horizontal variations across the Kuroshio Extension. An unstable wave with a wavelength of 220 km and a phase speed of 0.24 m s−1 propagating in the downstream direction was found to grow the fastest. Characteristics of this unstable baroclinic wave were similar to those of waves observed along the Kuroshio Extension. The growth rate of the fastest-growing waves became greater with an increase of the cross-stream difference of the potential vorticity (PV) in the intermediate layer. For a cross-frontal stratification structure without the PV gradient in the intermediate layer, which is similar to that in the Gulf Stream, the wavelength of the fastest-growing unstable wave changed to 390 km and the unstable wave had a much different structure. Thus, the unstable frontal waves observed along the Kuroshi...