Research For The Characteristics And Mechanism Of The Variations Of The North Pacific Subtropical Mode Waters Subduction Rate

The North Pacific subtropical mode waters are formed in late winter due to ocean-atmosphere interaction. Climate change signal can be sent in to the permanent pycnocline through subduction of these mode waters, and then transferred to the tropical area. It is significant to study the features and mechanism of the variations of subtropical subduction, since it would help further study the exchange between subtropical and tropical oceans. And subduction is one of the issues of great interests among the researches of ocean-atmosphere interaction.Most of past researches of subtropical subduction variations were completed with numerical model data, but few are studied with observational data. However, Argo system has continued until now since 2001, and valid data since 2003 is available for large scale ocean researches as the Argo floats deployed more and more concentrated. In this work, variability and variation mechanism of subduction of the North Pacific subtropical mode waters, including the western subtropical mode water (STMW), the eastern subtropical mode water (ESTMW), and the central mode water (CMW), are analyzed with monthly mean Argo data from 2003 to 2013 and SODA data from 1960 to 2010. The major results are as follows.Firstly, according to the climate mean distribution of North Pacific subtropical subduction, choose 28°-35°,140°-180° as the STMW subduction region,20°-28°N, 150°-120° as the ESTMW subduction region, and 36°-45°N,145°-160°W as the CMW subduction region. The spatial and temporal distribution both showed that lateral induction term anomalies, corresponding to the horizontal gradient of mixed layer depth, have played the most important roles in subduction rate anomalies of STMW, ESTMW and CMW. It presented that, in North Pacific, late winter mixed layer variation is important in controlling the subduction rate variation.Diagnostic analysis of mixed layer variation was made by controlling either the annual variation or climate mean of temperature and salinity. The results showed that late winter mixed layer depth variation was mainly due to the mixed layer temperature variation in STMW and ESTMW subduction region. In the CMW subduction region, late winter mixed layer temperature and salinity changing both contribute to the mixed layer variation.Heat storage analysis was made of the later winter North Pacific mixed layer. In the STMW and CMW subduction region, heat storage advection term variations corresponds well with the heat storage tendency, and trend of the advection term also contributed most to the positive trend of the heat storage tendency during 2003-2013 according to Argo data. In the ESTMW subduction region, it is the entrainment term correlating with heat storage tendency significantly. According to the Argo data analysis, it is also the trend of entrainment term contributed to the negative trend of heat storage in the ESTMW subduction region during 2003-2013. Salinity budget analysis was also made of the late winter mixed layer salinity in the CMW subduction region. The result presented that horizontal advection effect also play an important role in the late winter mixed layer salinity variation in the CMW region.Since the results of variability and variation mechanism analysis of SODA and Argo correspond well with each other, we combined the SODA result during 1960-2009 with the Argo result during 2003-2013 to discuss decadal variation of North Pacific subduction rate. Around the abrupt climate change in late 1970s, all of the subduction rates of STMW, ESTMW and CMW showed a strong trend. Later the North Pacific subduction rates have showed weak trends for a long time, as the PDO index varied gently since 1980s. According to the Argo data analysis, subduction rates of STMW and CMW presented strong weakening trends during 2003-2013, and subduction rate of ESTMW presented enhanced strengthening trend during 2003-2013. Abrupt climate change might have happened after 2000, as a PDO phase transition was found, and the North Pacific subduction rates have responded well with this change.