Mikitoshi Hirabara

Formation mechanism of the Weddell Sea Polynya and the impact on the global abyssal ocean

An experiment using a global ocean–ice model with an interannual forcing data set was conducted to understand the variability in the Southern Ocean. A winter-persisting polynya in the Weddell Sea (the Weddell Polynya, WP) was simulated. The process of WP breaking out after no-WP years was explored using the successive WPs found in the late 1950s. The results suggested that the anomalously warm deep water, saline surface layer, and a cyclonic wind stress over the Maud polynya region in early winter are essential for the surface layer to be dense enough to trigger deep convections which maintain a winter-persisting polynya; also, the reanalyzed surface air temperature (SAT) over the observed polynya region is too high for an ocean–ice model’s bulk formula to yield sufficient upward heat fluxes to induce WP formation. Therefore the Weddell Polynya, a series of WPs observed from satellite in the mid-1970s, is reproduced by replacing the SAT with a climatological one. Subsequent to the successive WP events, density anomalies excited in the Weddell Sea propagate northward in the Atlantic deep basins. The Antarctic Circumpolar Current (ACC) is enhanced through the increased meridional density gradient. The enhanced ACC and its meandering over the abyssal ridges excite buoyancy anomalies near the bottom at the southwestern end of the South Pacific basin. The buoyancy signals propagate northward and eventually arrive in the northern North Pacific.

Decadal variability of the Pacific Subtropical Cells and its relevance to the sea surface height in the western tropical Pacific during recent decades

Decadal variability of the Pacific Subtropical Cells (STCs) and associated sea surface height (SSH) in the western tropical Pacific during recent decades are examined by using an historical OGCM simulation. The model represents decadal variations of the STCs concurrent with tropical Pacific thermal anomalies: the eastern tropical Pacific is warmer when the STCs are weaker and cooler when they are stronger. The spatial patterns of the SSH in the western tropical Pacific show different features, depending on events associated with decadal variability. During the warm phase (1977–1987), the SSH anomalies exhibit deviations from a meridionally symmetric distribution, with weakly positive (strongly negative) anomalies in the western tropical North (South) Pacific. Analysis of the heat budget in the upper tropical Pacific indicates that the termination of the warm phase around 1985 results from a poleward heat transport anomaly that is induced by a horizontal gyre associated with the SSH anomalies. During the cold phase (1996–2006), in contrast, the SSH anomalies are nearly meridionally symmetric, with positive anomalies in both hemispheres. Enhanced easterly wind anomalies contribute to the development of the cold phase after the late 1990s.

Development of an operational coastal model of the Seto Inland Sea, Japan

We have developed a coastal model of the Seto Inland Sea, Japan, for a monitoring and forecasting system operated by the Japan Meteorological Agency (JMA). We executed a hindcast experiment using reanalysis datasets for the atmospheric and lateral boundaries without ocean initialization by data assimilation. The seasonal variability is verified to be realistic by comparing sea surface temperature and salinity of the hindcast experiment with observations. With a horizontal resolution of approximately 2 km, the model represents explicitly various coastal phenomena with a scale of 10–100 km, such as the Kuroshio water intrusion into Japanese coasts. This leads to good representation of intramonthly variations. For example, intensity of the sea level undulations with a period shorter than 23 days shows 1.6-fold improvement, as compared to the present model of JMA with the horizontal resolution of approximately 10 km. In addition to the increased resolution, the model is optimized for coastal modeling as follows. Incorporation of a tidal mixing parameterization reduces a high temperature bias in the Bungo Channel (a western channel of the Seto Inland Sea) and contributes to formation of a frontal structure. An accurate dataset of the river discharges is used for runoff, which has a strong impact on salinity. Enhancement of coastal friction improves surface currents. Owing to the increased resolution and these optimizations, the model shows realistic variability in a wide temporal range from several days to seasons. Root-mean-square errors of sea surface temperature and heights are evaluated as 1–2 K and 7–10 cm, respectively, without data assimilation. In the eastern part, however, the predictability is relatively low, which might be related to representation of an eastward mean flow in the Seto Inland Sea.

Impact of Solar Radiation Data and Its Absorption Schemes on Ocean Model Simulations

Since absorption of solar radiation plays a major role in heating the upper ocean layers, it is essential for modeling physical, chemical and biological processes (e.g., ocean general circulation or marine carbon cycle). In order to simulate the upper ocean thermal structures as realistically as possible, an ocean general circulation model (OGCM) requires accurate solar radiation data, used as the surface boundary condition. In this sense, it is important to recognize the quality of the solar radiation data being expected or suitable for OGCMs beforehand. The appropriate choice of absorption schemes of solar radiation is also important for ocean modeling in the upper ocean. The absorption of solar radiation is greatly affected by many factors, such as the wavelength of sunlight, the zenith angle and ocean optical properties in the ocean interior. Many absorption schemes have attempted to mimic these processes, but the impact of those schemes on the upper ocean thermal structures is not yet fully understood.