16:30 〜 16:45
[AOS12-05] Seasonal and Interannual Variations of Indian Ocean Subtropical Mode Water Based on the Argo Data
キーワード：Indian Ocean, Subtropical Mode Water, Argo Float, Air-Sea Interaction
Subtropical Mode Water (STMW) is a water mass that is formed during winter season and located in the western part of subtropical gyre. STMW covers a large area both in horizontal and vertical, and distinguished by the extreme uniformity of water properties. Extensive studies have been done for STMWs in the Pacific and Atlantic Oceans, meanwhile, there are only few studies that specifically discuss about Indian Ocean Subtropical Mode Water (IOSTMW). Sparseness of observational data was the obstacle for studying the Indian Ocean. However, this situation has been changing due to Argo program started in 1999, where numerous Argo floats were deployed to collect temperature and salinity data globally. Until now, the increasing Argo data in Subtropical Indian Ocean region has not been fully utilized. The seasonal and interannual variability of IOSTMW is still not clear yet. Therefore, by using the Argo data, this study investigates the interannual variability of IOSMTW in a longer time period and the mechanisms behind the variability. In Situ Analysis System (ISAS) gridded fields of temperature and salinity, which entirely based on in situ measurements, are used to examine the temporal variability of IOSTMW. Additionally, the turbulence heat flux data from OAFlux (Yu et al., 2018) and the radiative heat flux data from CERES (Kato et al., 2018) are used to see the relationship between winter atmospheric cooling and the formation of IOSTMW.
The definition of IOSTMW layer itself is not clearly defined yet. IOSTMW has been defined differently by each of the previous study. In this study, IOSTMW layer is defined by using the temperature gradient (dT/dz) and temperature range. Based on the previous studies, there are four IOSTMW definitions that are analyzed in this study: (1) dT/dz < 1.5°C/100 m with temperature of 15-18°C, (2) dT/dz < 1.5°C/100 m with temperature of 16-18°C, (3) dT/dz < 1°C/100 m with temperature of 15-18°C, (4) dT/dz < 1°C/100 m with temperature of 16-18°C.
The correlation coefficients between late-winter IOSTMW thickness and both winter (Jun-Aug) heat flux and ocean summer (Jan-Mar) stratification are examined. Significant correlation coefficients are found between winter heat flux and IOSTMW thickness for the definitions with dT/dz < 1°C/100 m (p < 0.01), higher than the definitions with dT/dz < 1.5°C/100 m. This implies IOSTMW defined as layer with dT/dz < 1.5°C/100 m, or the older and more stratified mode water, is less sensitive to the atmospheric forcing in the same year. Moreover, the comparison between cumulative winter heat flux and the difference of ocean heat content at the beginning of the winter and the end of winter also indicates the strong connection between atmospheric forcing and the upper ocean stratification. On the other hand, low correlation coefficients are found between IOSTMW thickness of each definition and summer stratification (0-150 m).
Therefore, the atmospheric cooling is found more dominant in determining the formation of IOSTMW, while the summer stratification is less influential. Layer with dT/dz < 1°C/100 m and temperature of 16-18°C is chosen as the definition of IOSTMW in the further analysis because this layer is sensitive to air-sea interaction and correlates relatively well with the summer stratification among all definitions.
The definition of IOSTMW layer itself is not clearly defined yet. IOSTMW has been defined differently by each of the previous study. In this study, IOSTMW layer is defined by using the temperature gradient (dT/dz) and temperature range. Based on the previous studies, there are four IOSTMW definitions that are analyzed in this study: (1) dT/dz < 1.5°C/100 m with temperature of 15-18°C, (2) dT/dz < 1.5°C/100 m with temperature of 16-18°C, (3) dT/dz < 1°C/100 m with temperature of 15-18°C, (4) dT/dz < 1°C/100 m with temperature of 16-18°C.
The correlation coefficients between late-winter IOSTMW thickness and both winter (Jun-Aug) heat flux and ocean summer (Jan-Mar) stratification are examined. Significant correlation coefficients are found between winter heat flux and IOSTMW thickness for the definitions with dT/dz < 1°C/100 m (p < 0.01), higher than the definitions with dT/dz < 1.5°C/100 m. This implies IOSTMW defined as layer with dT/dz < 1.5°C/100 m, or the older and more stratified mode water, is less sensitive to the atmospheric forcing in the same year. Moreover, the comparison between cumulative winter heat flux and the difference of ocean heat content at the beginning of the winter and the end of winter also indicates the strong connection between atmospheric forcing and the upper ocean stratification. On the other hand, low correlation coefficients are found between IOSTMW thickness of each definition and summer stratification (0-150 m).
Therefore, the atmospheric cooling is found more dominant in determining the formation of IOSTMW, while the summer stratification is less influential. Layer with dT/dz < 1°C/100 m and temperature of 16-18°C is chosen as the definition of IOSTMW in the further analysis because this layer is sensitive to air-sea interaction and correlates relatively well with the summer stratification among all definitions.