Published papers

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.

The relative importance of remote and local influences in the development of Benguela Ninos is examined using observations and a coupled general circulation model capable of simulating interannual variability in the tropical Atlantic. While previous studies have emphasized the role of equatorially excited Kelvin waves, the present study finds that meridional wind anomalies along the southwest African coast contribute substantially. Both observations and model output indicate that sea-surface temperatures along the southwest African coast respond rapidly to changes in meridional wind stress. These wind anomalies form part of a basin-scale weakening of the subtropical anticyclone that extends to the equator. As the weakening begins three months before the peak of the event it might have some predictive potential. Results also indicate that the close correlation between Benguela and Atlantic Ninos in observations might result from the large spatially coherent wind stress anomalies associated with the weakened anticyclone. Citation: Richter, I., S. K. Behera, Y. Masumoto, B. Taguchi, N. Komori, and T. Yamagata (2010), On the triggering of Benguela Ni os: Remote equatorial versus local influences, Geophys. Res. Lett., 37, L20604, doi:10.1029/2010GL044461.

Decadal variability of Antarctic sea ice and a role of ocean dynamics are examined with sea ice concentration (SIC) and sea surface temperature (SST) derived from satellite microwave observation and obtained by a high resolution coupled ocean-atmosphereice general circulation model (CGCM). Sea ice observations revealed a circumpolar variability of sea ice edge (SIE) on quasi-decadal time scale. SST also showed variation on similar time scale with warm (cool) anomaly roughly corresponding to retreat (extension) of SIE at negative (positive) southern annular mode (SAM). CGCM run without anthropogenic forcing and volcanic eruptions revealed that the leading mode of SIC is quasi-circumpolar pattern with a dominant time scale of 12-17 years and the leading mode of SST also has a similar pattern with the SIC showing a high degree of inverse correlation. The modeled SAM significantly correlates with these leading modes of both SIC and SST, representing the same structure with the observations. This indicates that the oceanic natural variability is the key to understand the quasi-decadal variability in sea ice.

Potential 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 degrees and 0.5 degrees, 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 anomaly pattern similar to the Pacific North American (PNA) pattern. The PNA-like signal with the weakened (enhanced) surface Aleutian low correlated with positive (negative) SST anomalies in the SAFZ becomes strongest and most robust in January, under the feedback forcing from synoptic-scale disturbances migrating along the Pacific storm track that shifts northward (southward) in accord with the oceanic SAFZ. This PNA-like signal, however, breaks down in February, which is suggestive of a particular sensitivity of that anomaly pattern to subtle differences in the background climatological-mean state. Despite its collapse in February, the PNA-like signal recurs the next January. This subseasonal evolution of the signal suggests that the PNA-like anomaly pattern may develop as a response to the persistent SST anomalies that are maintained mainly through ocean dynamics.

In this study, we introduce a new seasonal prediction system using an atmosphere–ocean-coupled general circulation model called CFES (hereafter referred to as CFES ESPreSSO). We compare its prediction skill of the interannual variability of the surface air temperature (SAT) and precipitation anomalies with that of the SINTEX-F2 seasonal prediction system. We find that CFES ESPreSSO has a higher skill in predicting the SAT variability in January-February-March over East Asia and northeastern North America than SINTEX-F2, while the following season (April-May-June), SINTEX-F2 provides better predictions of the SAT variability over the Maritime Continent and subtropical North Pacific. Meanwhile, CFES better predicts the SAT variability in July-August-September over Eurasia and Arctic, and it continues to be so over the following season (October-November-December) over Eurasia. However, the prediction skill of SINTEX-F2 is generally better in the tropics (e.g., SAT in the subtropical North Pacific, SAT and precipitation in the Maritime Continent). Regarding climate indices, CFES shows a better prediction skill for the Atlantic Niño and Ningaloo Niño indices, whereas SINTEX-F2 is generally better for El Niño and the Indian Ocean dipole mode. These results suggest that for improved seasonal forecasting, it is beneficial to consider a multi-model approach, leveraging the respective strengths of each model.

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 enhancement of the upward (downward) flux predominantly on the warmer (cooler) flank of the front. Indeed, several intermittent events of cold (warm) air advection, whose total duration accounts for only 21% (19%) of the entire analysis period, contribute to as much as 60% (44%) of the total amount of sensible heat flux during the analysis period on the warmer (cooler) flank. This antisymmetric behavior yields the sharp cross-frontal gradient in the time-mean flux. Since the flux intensity is strongly influenced by local magnitude of the SST-SAT difference that tends to increase with the SST gradient, the concentration of the flux variance to the frontal zone and cross-frontal contrasts in the mean and skewness of the flux all become stronger during the spinup of the SST front. Synoptically, the enhanced sensible heat flux near the SST front can restore SAT toward the underlying SST effectively with a time scale of a day, to maintain a frontal SAT gradient against the relaxing effect of atmospheric disturbances. The restoration effect of the differential surface heating at the SST front, augmented by the surface latent heating concentrated on the warm side of the front, represents a key process through which the atmospheric baroclinicity and ultimately the storm track are linked to the underlying ocean.

A sensitivity experiment has been performed by assimilating altimetric data into a 1.5-layer primitive equation model as a first attempt to examine the impact of initialization on forecasts of the Kuroshio path variability south of Japan. By exploiting the advantage of an adjoint model, our approach clearly shows that a small meander off Shikoku Island has a large impact on the prediction of meander growth in the Kuroshio region. Further, the strengthening of the Kuroshio current and its recirculation clearly becomes an important factor in the development of the meander. These results demonstrate the effectiveness of our assimilation approach in identifying efficient initialization schemes on numerical forecasting of the Kuroshio south of Japan and should help in the construction of an effective observing system for improving the forecasting.

In order to investigate the deep ocean inertia-gravity waves, a high-resolution global coupled atmosphere-ocean simulation is carried out with a coupling interval of 20 minutes. Large (similar to 10(-3) m s(-1)) root-mean-square variability of vertical velocity is found in middepths (2000-4000 m), which is not reported in previous studies using realistic ocean simulations. Horizontal distribution of the large variability roughly corresponds to the wintertime atmospheric storm tracks and is stretched equatorward due to beta-dispersion in open ocean with some "shadow regions'' behind the obstacles. Frequency spectrum of vertical velocity has strong peaks at around f and 2f (f is the local inertial period) in midlatitudes, and has additional peak at around (3/2) f or 3f at some points. These results suggest necessity of re-evaluation of wind-induced near-inertial energy with high-frequency atmospheric forcing.

Oceanic alternating zonal jets at depth have been detected ubiquitously in observations and ocean general circulation models (GCMs). Such oceanic jets are generally considered as being generated by purely oceanic processes. Here we explore a possible air-sea interaction induced by surface signatures of the deep zonal jets using an eddy-permitting coupled atmosphere-ocean GCM (CGCM). The 23-year CGCM integration reproduces bands of latitudinally-narrow alternating jets in the Southeast Pacific. They extend from the sea surface to well below the main thermocline and are embedded in the large-scale westward-flowing South Equatorial Current, the latter mostly confined above the thermocline. These jets generate fine-scale sea surface temperature (SST) anomalies through the advection of zonal temperature gradients. The atmospheric boundary layer appears to respond thermally to this fine-scale SST field, which induces fine-scale wind stress anomaly through atmospheric pressure adjustment, as indicated by a good spatial correlation between the SST Laplacian field and the fine-scale wind stress curl. A Sverdrup calculation on the wind stress field of the CGCM predicts fine-scale zonal currents driven by the meridional gradient of the fine-scale wind stress curl. The positions of these Sverdrup currents are generally coincident with those of the original zonal jets and the Sverdrup prediction explains roughly half of the amplitudes of the jets. While the original cause of the deep zonal jets simulated in our CGCM is unidentified, this analysis suggests that there is likely a positive air-sea feedback: the jets generate fine-scale wind stress curl that reinforces themselves through the Sverdrup dynamics. Citation: Taguchi, B., R. Furue, N. Komori, A. Kuwano-Yoshida, M. Nonaka, H. Sasaki, and W. Ohfuchi (2012), Deep oceanic zonal jets constrained by fine-scale wind stress curls in the South Pacific Ocean: A high-resolution coupled GCM study, Geophys. Res. Lett., 39, L08602, doi:10.1029/2012GL051248.

Abstract. A quasi-global eddying ocean hindcast simulation using a new version of our model, called OFES2 (Ocean General Circulation Model for the Earth Simulator version 2), was conducted to overcome several issues with unrealistic properties in its previous version, OFES. This paper describes the model and the simulated oceanic fields in OFES2 compared with OFES and also observed data. OFES2 includes a sea-ice model and a tidal mixing scheme, is forced by a newly created surface atmospheric dataset called JRA55-do, and simulated the oceanic fields from 1958 to 2016. We found several improvements in OFES2 over OFES: smaller biases in the global sea surface temperature and sea surface salinity as well as the water mass properties in the Indonesian and Arabian seas. The time series of the Niño3.4 and Indian Ocean Dipole (IOD) indexes are somewhat better in OFES2 than in OFES. Unlike the previous version, OFES2 reproduces more realistic anomalously low sea surface temperatures during a positive IOD event. One possible cause of these improvements in El Niño and IOD events is the replacement of the atmospheric dataset. On the other hand, several issues remained unrealistic, such as the pathways of the Kuroshio and Gulf Stream and the unrealistic spreading of salty Mediterranean overflow. Given the worldwide use of the previous version and the improvements presented here, the output from OFES2 will be useful in studying various oceanic phenomena with broad spatiotemporal scales.

Abstract Impacts of temperature assimilation on Venusian thermal tides are investigated by the observing system simulation experiments assuming Akatsuki Longwave Infrared Camera (LIR) observations. Synthetic temperature data are prepared by a Venusian general circulation model (VGCM) to test if LIR temperature data resolves a discrepancy in the structure of thermal tides between observations and the VGCM. They are assimilated at 70 km altitude with several combinations of frequency and horizontal region. The result shows that the three-dimensional structure of thermal tides is significantly improved not only in temperature but also in horizontal wind, even if observations are available only at a limited frequency of 6-hourly or on the dayside. The zonal–mean zonal wind and temperature fields are also modified at 60–80 km altitudes globally through the vertical momentum transport of thermal tides. It would be promising to assimilate Akatsuki LIR observations to produce realistic objective analysis of the Venus atmosphere.

Abstract Horizontal diffusion and resolution are important factors to generate and maintain the super-rotation in the general circulation model (GCM) because waves which transport angular momentum are sensitive to them. Here, we investigated how the super-rotation depends on the magnitude of horizontal hyper diffusion using a Venus atmospheric GCM with medium and high resolutions. In both the runs, we found a parameter range where the structure of fully developed super-rotation is almost independent of the magnitude of horizontal diffusion. Spectral analysis shows that unrealistically strong super-rotation is developed when medium-scale disturbances are dissipated by stronger horizontal diffusion. On the other hand, artificially weak super-rotation is also realized because spurious small-scale disturbances are accumulated when the horizontal diffusion is too weak. Graphical Abstract

AbstractAn ensemble-based forecast sensitivity to observations (EFSO) diagnosis has been implemented in an atmospheric general circulation model–ensemble Kalman filter data assimilation system to estimate the impacts of specific observations from the quasi-operational global observing system on weekly short-range forecasts. It was examined whether EFSO reasonably approximates the impacts of a subset of observations from specific geographical locations for 6-hour forecasts, and how long the 6-hour observation impacts can be retained during the 7-day forecast period. The reference for these forecasts was obtained from 12 data denial experiments in each of which a subset of three radiosonde observations launched from a geographical location was excluded. The 12 locations were selected from three latitudinal bands comprising (i) four Arctic regions, (ii) four midlatitude regions in the Northern Hemisphere, and (iii) four tropical regions during the Northern Hemisphere winter of 2015/16. The estimated winter-averaged EFSO-derived observation impacts well corresponded to the 6-hour observation impacts obtained by the data denials and EFSO could reasonably estimate the observation impacts by the data denials on short-range (6-hour to 2-day) forecasts. Furthermore, during the medium-range (4-day to 7-day) forecasts, it was found that the Arctic observations tend to seed the broadest impacts and their short-range observation impacts could be projected to beneficial impacts in Arctic and midlatitude North American areas. The midlatitude area located just downstream of dynamical propagation from the Arctic toward the midlatitudes. Results obtained by repeated Arctic data-denial experiments were found to be generally common to those from the non-repeated experiments.