Masami Nonaka

Covariations of Sea Surface Temperature and Wind over the Kuroshio and Its Extension: Evidence for Ocean-to-Atmosphere Feedback*

Abstract Satellite microwave measurements are analyzed, revealing robust covariability in sea surface temperature (SST) and wind speed over the Kuroshio Extension (KE) east of Japan. Ocean hydrodynamic instabilities cause the KE to meander and result in large SST variations. Increased (reduced) wind speeds are found to be associated with warm (cold) SST anomalies. This positive SST–wind correlation in KE is confirmed by in situ buoy measurements and is consistent with a vertical shear adjustment mechanism. Namely, an increase in SST reduces the static stability of the near-surface atmosphere, intensifying the vertical turbulence mixing and bringing fast-moving air from aloft to the sea surface. South of Japan, the Kuroshio is known to vary between nearshore and offshore paths. These paths are very persistent and can last for months to years. As the Kuroshio shifts its path, coherent wind changes are detected from satellite data. In particular, winds are high south of Tokyo when the Kuroshio takes the near...

An Eddy-Resolving Hindcast Simulation of the Quasiglobal Ocean from 1950 to 2003 on the Earth Simulator

An eddy-resolving hindcast experiment forced by daily mean atmospheric reanalysis data covering the second half of the twentieth century was completed successfully on the Earth Simulator. The domain covers quasiglobal from 75°S to 75°N excluding arctic regions, with horizontal resolution of 0.1° and 54° vertical levels. Encouraged by high performance of the preceding spin-up integration in capturing the time-mean and transient eddy fields of the world oceans, the hindcast run is executed to see how well the observed variations in the low- and midlatitude regions spanning from intraseasonal to decadal timescales are reproduced in the simulation. Our report presented here covers, among others, the El Nino and the Indian Ocean Dipole events, the Pacific and the Pan-Atlantic decadal oscillations, and the intraseasonal variations in the equatorial Pacific and Indian Oceans, which are represented well in the hindcast simulation, comparing with the observations. The simulated variations in not only the surface but also subsurface layers are compared with observations, for example, the decadal subsurface temperature change with narrow structures in the Kuroshio Extension region. Furthermore, we focus on the improved aspects of the hindcast simulation over the spin-up run, possibly brought about by realistic high-frequency daily mean forcing.

Interdecadal Thermocline Variability in the North Pacific for 1958–97: A GCM Simulation*

An ocean general circulation model is forced with the NCEP reanalysis wind stress for 1958‐97 to understand mechanisms of ocean subsurface variability. With relatively high horizontal (1 83 18) and vertical (41 levels) resolutions, the model produces mode waters on a range of density surfaces in the western, central, and eastern North Pacific, in qualitative agreement with observations. These mode waters appear as a thermostad or a region of weak stratification in the upper thermocline as they flow southward from their formation regions in the Kuroshio and its extension. In the model, subsurface temperature variability in the central subtropical gyre reaches a maximum within the thermostad, in contrast to what might be expected from the linear baroclinic Rossby wave theory. This variance maximum is associated with the longitudinal shift in the path of mode waters. In particular, deepened mixed layer and accelerated eastward currents in the Kuroshio Extension by wind changes in the mid-1970s act cooperatively to move the central mode waters toward the east, causing large subsurface temperature anomalies. Besides the local maximum in the central North Pacific subtropical gyre, two additional maxima of the subsurface anomaly are identified in the northwestern and southern parts of the gyre, respectively. Among these subsurface anomaly centers, the one in the northwestern North Pacific has a strong effect on the model sea surface temperature, suggesting that the Kuroshio and its extension are a key region to decadal/interdecadal ocean‐atmosphere interaction. Finally, possible effects of atmospheric thermal forcing are discussed.

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...

Air-Sea Interaction over the Eastern Pacific Warm Pool: Gap Winds, Thermocline Dome, and Atmospheric Convection*

High-resolution satellite observations are used to investigate air‐sea interaction over the eastern Pacific warm pool. In winter, strong wind jets develop over the Gulfs of Tehuantepec, Papagayo, and Panama, accelerated by the pressure gradients between the Atlantic and Pacific across narrow passes of Central American cordillera. Patches of cold sea surface temperatures (SSTs) and high chlorophyll develop under these wind jets as a result of increased turbulent heat flux from the ocean and enhanced mixing across the base of the ocean mixed layer. Despite a large decrease in SST (exceeding 38C in seasonal means), the cold patches associated with the Tehuantepec and Papagayo jets do not have an obvious effect on local atmospheric convection in winter since the intertropical convergence zone (ITCZ) is located farther south. The cold patch of the Panama jet to the south, on the other hand, cuts through the winter ITCZ and breaks it into two parts. A pronounced thermocline dome develops west of the Gulf of Papagayo, with the 208C isotherm only 30 m deep throughout the year. In summer when the Panama jet disappears and the other two wind jets weaken, SST is 0.58C lower over this Costa Rica Dome than the background. This cold spot reduces local precipitation by half, punching a hole of 500 km in diameter in the summer ITCZ. The dome underlies a patch of open-ocean high chlorophyll. This thermocline dome is an ocean dynamic response to the positive wind curls south of the Papagayo jet, which is optimally oriented to excite ocean Rossby waves that remotely affect the ocean to the west. The meridionally oriented Tehuantepec and Panama jets, by contrast, only influence the local thermocline depth with few remote effects on SST and the atmosphere. The orographical-triggered air‐sea interaction described here is a good benchmark for testing high-resolution climate models now under development.

Influences of the Kuroshio/Oyashio Extensions on Air–Sea Heat Exchanges and Storm-Track Activity as Revealed in Regional Atmospheric Model Simulations for the 2003/04 Cold Season*

Abstract Influences of oceanic fronts in the Kuroshio and Oyashio Extension (KOE) region on the overlying atmosphere are investigated by comparing a pair of atmospheric regional model hindcast experiments for the 2003/04 cold season, one with the observed finescale frontal structures in sea surface temperature (SST) prescribed at the model lower boundary and the other with an artificially smoothed SST distribution. The comparison reveals the locally enhanced meridional gradient of turbulent fluxes of heat and moisture and surface air temperature (SAT) across the oceanic frontal zone, which favors the storm-track development both in winter and spring. Distinct seasonal dependency is found, however, in how dominantly the storm-track activity influences the time-mean distribution of the heat and moisture supply from the ocean. In spring the mean surface sensible heat flux (SHF) is upward (downward) on the warmer (cooler) side of the subarctic SST front. This sharp cross-frontal contrast is a manifestation of...

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. Concomitan...

Seasonal Evolutions of Atmospheric Response to Decadal SST Anomalies in the North Pacific Subarctic Frontal Zone: Observations and a Coupled Model Simulation

AbstractPotential impacts of pronounced decadal-scale variations in the North Pacific sea surface temperature (SST) that tend to be confined to the subarctic frontal zone (SAFZ) upon seasonally varying atmospheric states are investigated, by using 48-yr observational data and a 120-yr simulation with an ocean–atmosphere coupled general circulation model (CGCM). SST fields based on in situ observations and the ocean component of the CGCM have horizontal resolutions of 2.0° and 0.5°, respectively, which can reasonably resolve frontal SST gradient across the SAFZ. Both the observations and CGCM simulation provide a consistent picture between SST anomalies in the SAFZ yielded by its decadal-scale meridional displacement and their association with atmospheric anomalies. Correlated with SST anomalies persistent in the SAFZ from fall to winter, a coherent decadal-scale signal in the wintertime atmospheric circulation over the North Pacific starts emerging in November and develops into an equivalent barotropic an...

Air-Sea Heat Exchanges Characteristic of a Prominent Midlatitude Oceanic Front in the South Indian Ocean as Simulated in a High-Resolution Coupled GCM

Abstract An integration of a high-resolution coupled general circulation model whose ocean component is eddy permitting and thus able to reproduce a sharp gradient in sea surface temperature (SST) is analyzed to investigate air–sea heat exchanges characteristic of the midlatitude oceanic frontal zone. The focus of this paper is placed on a prominent SST front in the south Indian Ocean, which is collocated with the core of the Southern Hemisphere storm track. Time-mean distribution of sensible heat flux is characterized by a distinct cross-frontal contrast. It is upward and downward on the warmer and cooler flanks, respectively, of the SST front, acting to maintain the sharp gradient of surface air temperature (SAT) that is important for preconditioning the environment for the recurrent development of storms and thereby anchoring the storm track. Induced by cross-frontal advection of cold (warm) air associated with migratory atmospheric disturbances, the surface flux is highly variable with intermittent en...

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...