Shusaku Sugimoto

Oceanic fronts and jets around Japan: a review

This article reviews progress in our understanding of oceanic fronts around Japan and their roles in air–sea interaction. Fronts associated with the Kuroshio and its extension, fronts within the area of the Kuroshio-Oyashio confluence, and the subtropical fronts are described with particular emphasis on their structure, variability, and role in air–sea interaction. The discussion also extends to the fronts in the coastal and marginal seas, the Seto Inland Sea and Japan Sea. Studies on oceanic fronts have progressed significantly during the past decade, but many of these studies focus on processes at individual fronts and do not provide a comprehensive view. Hence, one of the goals of this article is to review the oceanic fronts around Japan by describing the processes based on common metrics. These metrics focus primarily on surface properties to obtain insights into air–sea interactions that occur along oceanic fronts. The basic characteristics derived for each front (i.e., metrics) are then presented as a table. We envision that many of the coupled ocean-atmosphere global circulation models in the coming decade will represent oceanic fronts reasonably well, and it is hoped that this review along with the table of metrics will provide a useful benchmark for evaluating these models.

Roles of SST Anomalies on the Wintertime Turbulent Heat Fluxes in the Kuroshio–Oyashio Confluence Region: Influences of Warm Eddies Detached from the Kuroshio Extension

AbstractVariations of turbulent heat fluxes (sum of sensible and latent heat fluxes) in the North Pacific during 16 winters from December 1992/February 1993 to December 2007/February 2008 are investigated because the months from December to February correspond to the period having peak winter conditions in the atmosphere field. Turbulent heat fluxes are calculated from the bulk formula using daily variables [surface wind speed, surface air specific humidity, surface air temperature, and sea surface temperature (SST)] of the objectively analyzed air–sea flux (OAFlux) dataset and bulk coefficients based on the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) bulk flux algorithm 3.0. The winter turbulent heat fluxes over the Kuroshio–Oyashio Confluence Region (KOCR; 142°–150°E, 35°–40°N) have the largest temporal variances in the North Pacific. The relative contributions among observed variables in SST, surface air temperature, and surface wind speed causing turb...

Long-Term Variations of the Kuroshio Extension Path in Winter: Meridional Movement and Path State Change

AbstractAs an indicator of the Kuroshio Extension (KE) path, the KE northern boundary (KENB) was detected based on the position of the strong winter sea surface temperature (SST) gradient between 142° and 155°E, using high spatial resolution satellite-derived SST for the 30 winters (January to March) from 1982 to 2011. The KE path showed meridional movement with a period of 10–15 yr and an amplitude of about 2° latitude. The changes in latitudinal position of the KE path were initiated by a north–south shift of the Aleutian low (AL). Negative wind stress curl anomalies around 35°N in the eastern North Pacific associated with a northward shift of the AL induced a deepening of the main thermocline depth, and then this deepening signal propagated westward, reaching the KE region after about 3 yr, where it caused the KE path to move northward. The path state of the KE (straight path/convoluted path) modulated on a time scale of 8–12 yr, but this was not significantly correlated with the meridional movement of...

Impact of Remote Reemergence of the Subtropical Mode Water on Winter SST Variation in the Central North Pacific

Abstract Using long-term datasets of sea surface temperature (SST), core-layer temperature (CLT) of the North Pacific subtropical mode water (NPSTMW), and the North Pacific index, an impact of remote reemergence of NPSTMW on winter SST variation in the central North Pacific is quantitatively investigated. A running correlation analysis between CLT and winter SST in the remote reemergence area clearly shows that an occurrence of remote reemergence of NPSTMW strongly depends on the specific time period: occurrence period and nonoccurrence period. It is found that background conditions, such as formation rate of NPSTMW, winter mixed layer depth, ocean heat content, and buoyancy flux, play a crucial role in the period-dependent remote reemergence. In the occurrence (nonoccurrence) periods, since a large positive (negative) upper-ocean heat content anomaly is located around the central North Pacific, a deeper (shallower) winter mixed layer is formed in both the formation area and the reemergence area of NPSTMW...

Influence of SST Anomalies on Winter Turbulent Heat Fluxes in the Eastern Kuroshio–Oyashio Confluence Region

AbstractVariations in the turbulent heat flux (THF; the sum of the sensible and latent heat fluxes) in the eastern Kuroshio–Oyashio confluence region (EKOCR; 36°–40°N, 155°–160°E) were investigated over a period of 27 consecutive winters (December–February) from 1985/86 to 2011/12. The THF was calculated from a bulk formula using daily variables [surface wind speed, surface air specific humidity, surface air temperature, and sea surface temperature (SST)] of the objectively analyzed air–sea fluxes (OAFlux) dataset and bulk coefficients based on the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) bulk flux algorithm 3.0. The winter THF over the EKOCR showed low-frequency variations, with larger THF values in the early 2000s and smaller values in the late 1990s and late 2000s. The heat release in the early 2000s was up to ~40% greater than that in the late 1990s and late 2000s. By performing experiments using combinations of daily raw data values and daily clim...

Influence of Kuroshio Path Variation South of Japan on Formation of Subtropical Mode Water

AbstractDistributions of subtropical mode water (STMW) in the northwestern part of the North Pacific Subtropical Gyre were investigated, using temperature–salinity profiles from 2005 to 2011, with particular reference to the Kuroshio meander and non-meander path states, south of Japan. In spring of meander years, warm STMW with a potential temperature of 19°–20°C (potential density anomaly of 24.6–24.9 kg m−3) was found in the Shikoku Basin, whereas cold STMW below 19°C was distributed throughout the southern region of Japan in non-meander years. The warm STMW was formed in a spatially isolated and warm winter mixed layer (ML) in the Shikoku Basin, where a local recirculation developed in association with the Kuroshio meander path; both the absence of horizontal mixing with a cold ML south of the Kuroshio Extension because of the spatially isolated ML and an increase in horizontal heat advection due to the westward flow associated with this local recirculation caused the ML warming in the Shikoku Basin. A...

Decadal Variation in Winter Mixed Layer Depth South of the Kuroshio Extension and Its Influence on Winter Mixed Layer Temperature

AbstractThe long-term behavior of the wintertime mixed layer depth (MLD) and mixed layer temperature (MLT) are investigated in a region south of the Kuroshio Extension (KE) (30°–37°N, 141°–155°E), an area of the North Pacific subtropical gyre where the deepest MLD occurs, using historical temperature profiles of 1968–2014. Both the MLD and MLT in March have low-frequency variations, which show significant decadal (~10 yr) variations after the late 1980s. Observational data and simulation outputs from a one-dimensional turbulent closure model reveal that surface cooling is the main control on winter MLD in the late 1970s and 1980s, whereas there is a change in the strength of subsurface stratification is the main control after ~1990. In the latter period, a weak (strong) subsurface stratification is caused by a straight path (convoluted path) of the KE and by a deepening (shallowing) of the main thermocline depth due to oceanic Rossby waves formed as a result of positive (negative) anomalies of wind stress...

Quasi-Decadal Variation in Intensity of the Western Part of the Winter Subarctic SST Front in the Western North Pacific: The Influence of Kuroshio Extension Path State

AbstractVariations of the subarctic front (SAF), which is extracted as a strong sea surface temperature (SST) gradient in the longitudinal band of 146°–152°E, were investigated over 30 winters (January–March) from 1982 to 2011, using high-spatial-resolution satellite-derived SST dataset and hydrographic observations. The SAF had little meridional movement in this longitudinal band, lying between 40° and 41°N throughout the analysis period. The SAF intensity had a dominant quasi-decadal time scale (about 8 yr); its intensity in the strong phase was up to about 50% higher than in the weak phase. The SAF intensity strongly reflected upper-ocean conditions in the Kuroshio–Oyashio Confluence (KOC) region just south of the SAF. In the strong SAF phase, large positive anomalies in both temperature and salinity were found in the KOC region, not only at the sea surface but also below 300 m, and the water properties in the KOC region were those of the Kuroshio water. The warm, salty water in the KOC region was supp...

Local atmospheric response to warm mesoscale ocean eddies in the Kuroshio–Oyashio Confluence region

In the extratropical regions, surface winds enhance upward heat release from the ocean to atmosphere, resulting in cold surface ocean: surface ocean temperature is negatively correlated with upward heat flux. However, in the western boundary currents and eddy-rich regions, the warmer surface waters compared to surrounding waters enhance upward heat release–a positive correlation between upward heat release and surface ocean temperature, implying that the ocean drives the atmosphere. The atmospheric response to warm mesoscale ocean eddies with a horizontal extent of a few hundred kilometers remains unclear because of a lack of observations. By conducting regional atmospheric model experiments, we show that, in the Kuroshio–Oyashio Confluence region, wintertime warm eddies heat the marine atmospheric boundary layer (MABL), and accelerate westerly winds in the near-surface atmosphere via the vertical mixing effect, leading to wind convergence around the eastern edge of eddies. The warm-eddy-induced convergence forms local ascending motion where convective precipitation is enhanced, providing diabatic heating to the atmosphere above MABL. Our results indicate that warm eddies affect not only near-surface atmosphere but also free atmosphere, and possibly synoptic atmospheric variability. A detailed understanding of warm eddy–atmosphere interaction is necessary to improve in weather and climate projections.

The Wintertime Wind Stress Curl Field in the North Atlantic and Its Relation to Atmospheric Teleconnection Patterns

Abstract Adopting a rotated empirical orthogonal function (REOF) analysis and a maximum covariance analysis (MCA), characteristics of the wintertime wind stress curl (WSC) anomaly field in the North Atlantic are investigated. In terms of both temporal variation and spatial distribution, the first four leading modes of WSC show a one-to-one relation with four atmospheric teleconnection patterns over the North Atlantic sector: the North Atlantic Oscillation (NAO) and the east Atlantic (EA), tropical–Northern Hemisphere (TNH), and Pacific–North American (PNA) patterns. These four patterns characterize the WSC variations over the different regions in the North Atlantic: NAO and EA over the eastern side of the basin, TNH over the central part of the basin, and PNA over the western side of the basin.