Title: The seasonally varying ventilation of the ocean: a model-data synthesis
Abstract: This dissertation is a study of the climatological mean seasonal cycle of the ventilation of the ocean. Firstly, I have developed a computationally efficient 4-D variational assimilation system, called CYCLOCIM. It assimilates monthly mean temperature and salinity data from the World Ocean Atlas, transient CFC-11 and CFC12 tracers, and natural radiocarbon measurements for the deep ocean from the GLODAPv2 database. CYCLOCIM is a new variational assimilation system that is specifically designed for the problem of estimating the climatological seasonal cycle of the residual mean circulation. I independently obtain the modeled overturning circulation, and the meridional heat and fresh water transport that largely agree with existing studies. Moreover, CYCLOCIM improves the model fit to the observation in the upper ocean compared to previous studies that ignored the seasonal cycle. The main product of the assimilation system is a set of 12 monthly data-constrained of tracer transport operators. I then calculate transit-time distribution (TTD) using the monthly transport operators to quantify "Stommel's demon", that is, the seasonality of the ventilation in the main thermocline. The study provides an accurate estimate of effective subduction and obduction months and maps of the water fraction in the main thermocline was last subducted from the surface or will be first obducted back to the surface. For example, effective subduction mainly happens in the Southern Ocean (64%), which lasts from June to November and peaks in September (21%) and October (20%). The subduction also happens in the North Pacfic (12%) and the North Atlantic (16%), which lasts from January to April and peaks in March (15%). Effective obduction mainly happens in ACC regions (48%) which starts from April to November and peaks in August, and the upwelling regions of the tropical ocean (35% ) which lasts the whole year. Furthermore, I have applied the TTD method to estimate the ventilation of the deep water masses. It is shown that the main sinking of NADW, AABW, PDW, IDW happens in the Southern Ocean (respectively, 15%, 69%, 56%, 53%) during summer and fall, and the Atlantic Ocean (respectively, 84%, 39%, 37%, 41%) during winter and spring.