Subtropical Mode Water And Its Formation Mechanism In The North Pacific

Mode water is a special type of water mass characterized by its vertical homogeneity, such as temperature, density and especially the potential vorticity, which is acquired during the wintertime (fall and winter) deep convection processes. As one of the products of air-sea interaction, it has great impacts on the upper ocean circulation pattern and the thermal structure. As more attentions being paid to the interaction between the tropical and subtropical ocean and the recently decadal variations of the climate, mode water, for it may contain the long-term memory of the external atmospheric forcing and thus, has come into the spot in the future. Because the mode water is formed due to the ventilation process of the thermocline and the characteristics of the mode water embody the water properties of the mixed layer during later winter, thus, to found the formation mechanism of the depth and temperature of wintertime deep mixed layer in the mode water formation region are main objects in this paper. Some main new research results are listed as follows:1. Using one-dimensional turbulent kinetic energy (TKE) model, quantitative estimations of the external atmospheric forcing (sea surface heat fluxes, wind stress and evaporation minus precipitation) on the formation of the later winter mixed layer are presented in the mode water formation region. Enhancement (weakening) about 50% of the seasonal cycle of the sea surface heat fluxes tends to result in thedeepening (shallow) of the later winter mixed layer of about 50% and 45%, respectively, in the western subtropical mode water (NPSTMW) and central mode water (CMW) formation region, while its impact on the later winter mixed layer depth (MLD) in the eastern subtropical mode water (ESMW) formation is no more than 5%. Besides, it is found that if the heat losses in winter in three mode water formation regions are all increased by 25 Wm-2, most distinguished deepening of the MLD (70 m) is located in the CMW formation, while the deepening of the MLD in the ESMW formation region is the smallest (10 m). Meanwhile, the "precondition" mechanism due to the residue of the NPSTMW, which is favorable for the wintertime deepening of the mixed layer, is also proposed.2. Diagnostics of the upper ocean mixed layer heat budget equation in the three mode waters formation regions are executed using NCEP data. It is found that the surface heat flux and vertical entrainment are both the dominant terms in determining the coldness tendency of the wintertime mixed layer in all three mode water formation regions, with relative contributions as 67% and 19%, respectively in the NPSTMW formation region; 53% and 20%, respectively in the CMW formation region; 65% and 30%, respectively in the ESMW formation region. Besides, both the Ekman advection and geostrophic advection are warm advection in the ESMW formation region, while only geostrophic advection is warm advection in the NPSTMW and CMW formation. Meanwhile, it is estimated that the contributions of surface heat flux and geostrophic advection are 62% and 32%, respectively, on the interannual and decadal variations of the wintertime mixed layer temperature tendency in the NPSTMW formation region. In the CMW formation region, the interannual and decadal variations of the wintertime mixed layer temperature tendency are the result of the cooperation of the Ekman advection (32%), geostrophic advection (30%) and the surface heat flux (25%). While the surface heat flux is themost dominant term (67%) in affecting the interannual and decadal variations of the wintertime mixed layer temperature tendency in the ESMW formation region.3. Based on former researches, a "stability gap" (weak ocean stratification) confined between 170E-160W at about 40N in the upper central North Pacific (< 125 m) in September and October is detected and quantified by analyzing the historical datasets such as Levitus (1994), NCEP and SODA. It is found that this region [170E-160W, 38-42N] corresponds directly to the formation area of the CMW and there exists an e...