Hidetaka Kobayashi

Role of Southern Ocean stratification in glacial atmospheric CO2 reduction evaluated by a three‐dimensional ocean general circulation model

Atmospheric carbon dioxide (CO2) concentration during glacial periods is known to be considerably lower than during interglacial periods. However, previous studies using an ocean general circulation model (OGCM) fail to reproduce this. Paleoclimate proxy data of the Last Glacial Maximum indicate high salinity (>37.0 psu) and long water mass residence time (>3,000 years) in the Southern Ocean, suggesting that salinity stratification was enhanced and more carbon was stored there. Reproducibility of salinity and water mass age is considered insufficient in previous OGCMs, which might affect the reproducibility of atmospheric CO2 concentration. This study investigated the role of increased stratification of the Southern Ocean in the glacial CO2 variation using an OGCM. We found that deep water formation in East Antarctica is required to explain high salinity in the South Atlantic. Saltier deep Southern Ocean resulted in increased atmospheric CO2 concentration against previous estimates. This is partly due to increased volume transport of Antarctic Bottom Water and associated decrease in the water mass age of the deep Pacific Ocean. On the other hand, weakening of vertical mixing contributed to increase of the vertical gradient of dissolved inorganic carbon and decrease of atmospheric CO2 concentration. However, we show that it is unable to explain all of the glacial CO2 variations by the contribution of the Southern Ocean. Our findings indicate that detailed understanding of the impact of enhanced stratification in the Southern Ocean on the Pacific Ocean might be crucial to understanding the mechanisms behind the glacial-interglacial ocean carbon cycle variations.

Doppler Spectra of Microwave Backscatter From Water Surface

This paper describes the experimental results of microwave backscattering at water surfaces. Active microwave remote sensing is one of the useful techniques for sea surface measurement. For example, it enables us to know the wind vector on global scale. A principle of measurement is that the microwave backscattering depends on the wind speed. Therefore understanding of the phenomena of microwave scattering at sea surface in detail is indispensable for improvement of measuring accuracy. The purpose of the research is to investigate the characteristics of microwave scattering at various water surface conditions. Water surface was generated by wind and currents, microwave backscattering at that surface was measured by X and C-band microwave scatterometer. The experimental results were summarized in scattering coefficients and Doppler spectra. X-band microwave was more sensitive at wind wave surface than C-band. The mean frequency of Doppler spectrum of backscattering microwave was corresponded to the phase velocity of the mean water surface wave and the bandwidth of Doppler spectrum had close relation to the orbital velocity of the mean wave. A current had no effect on the scattering coefficient, but the Doppler spectrum was shifted to the side corresponding to current direction.Copyright