Ventilation of the North Pacific subtropical pycnocline and mode water formation
Ventilation of the North Pacific subtropical pycnocline and mode water formation
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DOI：
10.1016/j.pocean.2006.12.005
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发表时间：
2008-06
影响因子：
4.1
通讯作者：
T. Suga;Y. Aoki;H. Saito;K. Hanawa
中科院分区：
文献类型：
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作者：
T. Suga;Y. Aoki;H. Saito;K. Hanawa
北太平洋的年俯冲率是根据等周期平均水文气候学(Water Base)、高分辨率冬季混合层气候学(NWMLC)以及来自船舶报告、数值天气预报产品和卫星产品的各种风应力气候学计算的。计算使用拉格朗日坐标，与以前的工作相同，只是使用了不太平滑的海洋气候学(Helbase和NWMLC)，而不是世界海洋地图集。风应力气候的差异对俯冲速度的估计几乎没有影响。密度等级的俯冲率普查显示，对应于副热带模式水(STMW)、中央模式水(CMW)和东部副热带模式水(ESTMW)的峰值。与先前的估计相比，较深的混合层和相关的较尖锐的混合层锋面导致较大的侧向感应，从而提高了俯冲速率，特别是较轻的短波(25.0<；σθ<；25.2 kgm−3)和较轻的σθ(26.0<；σθ<；26.2 kgm−3)的位势密度异常(σθ)范围。永久跃层水的更新时间被估计为水的体积除以每一σθ级的下降率：2-4年σθ(24.5<；σθ<；25.2 kgm−3)，2年(25.0<；σθ<；25.3 kgm−3)，5-9年(25.3<；σθ<；25.6 kgm−3)，10-20年(26.0<；σθ<；26.2kgm−3)，20-30年为中等CMW(26.2kgm；σθ<；26.3kgm−3)，60年或更长时间(26.3<；σθ<；26.6kgm−3)。潜在温盐(θ-S)空间的水量和俯冲速率的比较表明，上层的跃层水(25.0<；σθ<；26.2 kgm−3)直接由冬季地表水的非扩散性俯冲所维持，包括STMW和较轻的CMW。较低的永久跃层水(26.2kgm；σθ<；26.6kgm−3)可以通过来自亚北极-亚热带过渡区的较冷和较新鲜的水俯冲并随后与较咸和较暖的水混合来保持。对俯冲水的位涡诊断表明，STMW的低位涡主要是由于俯冲率较大所致，而ESTMW和CMW的位涡则主要是由于密度平流率较小(横流)所致。此外，相对较大的俯冲速率可能是在38°N和170°W(28°N和145°W)附近形成的部分较轻的CMW(ESTMW)的PV较低的原因，该地区冬季混合层相对较厚，并伴随着混合层锋面，导致较大的横向诱发率。
The annual subduction rate of the North Pacific was calculated based on isopycnally averaged hydrographic climatology (HydroBase), high-resolution winter mixed-layer climatology (NWMLC), and various wind stress climatologies from ship reports, numerical weather prediction products, and satellite products. The calculation was performed using Lagrangian coordinates in the same manner as in previous works, except a less smoothed oceanic climatology (HydroBase and NWMLC) was used instead of a World Ocean Atlas. Differences in the wind stress climatologies have very little effect on subduction rate estimates. The subduction rate census for density classes showed peaks corresponding to subtropical mode water (STMW), central mode water (CMW), and eastern subtropical mode water (ESTMW). The deeper mixed layer and the associated sharper mixed-layer fronts in the present climatology resulted in a larger lateral induction, which boosted the subduction rate, especially for the potential density anomaly (σθ) range of the lighter STMW (25.0<σθ<25.2kgm−3) and lighter CMW (26.0<σθ<26.2kgm−3), compared to previous estimates. The renewal time of permanent pycnocline water was estimated as the volume of water divided by the subduction rate for each σθclass: 2–4 years for ESTMW (24.5<σθ<25.2kgm−3), 2 years for the lighter STMW (25.0<σθ<25.3kgm−3), 5–9 years for the denser STMW (25.3<σθ<25.6kgm−3), 10–20 years for the lighter CMW (26.0<σθ<26.2kgm−3), 20–30 years for the middle CMW (26.2<σθ<26.3kgm−3), and 60 years or longer for the denser CMW (26.3<σθ<26.6kgm−3). A comparison of the water volume and subduction rate in potential temperature–salinity (θ–S) space indicated that the upper permanent pycnocline water (25.0<σθ<26.2kgm−3) was directly maintained by nondiffusive subduction of winter surface water, including STMW and lighter CMW. The lower permanent pycnocline water (26.2<σθ<26.6kgm−3) may be maintained through the subduction of fresher and colder water from the subarctic–subtropical transition region and subsequent mixing with saltier and warmer water. Diagnosis of the potential vorticity (PV) of the subducted water demonstrated that the low PV of STMW was mainly due to the large subduction rate, whereas that of both ESTMW and CMW was due mainly to the small density advection rate (cross-isopycnal flow). Additionally, a relatively large subduction rate probably contributes to the low PV of part of the lighter CMW (ESTMW) formed in the region around 38°N and 170°W (28°N and 145°W), which is characterized by a relatively thick winter mixed layer and an associated mixed-layer front, causing a large lateral induction rate.