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首页> 外文期刊>Journal of Physical Oceanography >Effects of the Mixed Layer Time Variability on Kinematic Subduction Rate Diagnostics
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Effects of the Mixed Layer Time Variability on Kinematic Subduction Rate Diagnostics

机译：混合层时间变异性对运动俯冲速率诊断的影响

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摘要

An eddy-resolving primitive equation general circulation model is used to estimate water-mass subduc-tion rates in the North Atlantic Ocean subtropical gyre. The diagnostics are based on the instantaneous kinematic approach, which allows the calculation of the annual rate of water-mass subduction at a given density range, following isopycnal outcrop positions over the annual cycle. It is shown that water-mass subduction is effected rapidly (～1-2 months) as the mixed layer depth decreases in spring, consistent with Stommel's hypothesis, and occurs mostly over the area of deep late-winter mixed layers ( ≥ 150 m) across the central North Atlantic in the density range 26 ≤ σ ≤ 27.2. Annual subduction rates O(100-200 m yr~(-1) are found south and east of the Gulf Stream extension in the density range of subtropical mode waters from roughly 26.2 to 26.6. In the northeastern part of the subtropical gyre, annual subduction rates are somewhat larger, O(250 m yr~(-1)), from a density of about 26.9 east of the North Atlantic Current to 27.4 (upper cutoff in this study). The overall basin-integrated subduction rate for subtropical mode waters (26.2 ≤ σ ≤ 26.6) is about 12.2 S_v (S_v ≡ 10~6 m~3 s~(-1)), comparable to the total formation rate inferred from the surface density forcing applied in the model of roughly 11 Sv in this density range. In contrast, basin-integrated rates for denser central water (26.8 ≤ σ ≤ 27.2) provide a vanishingly small net subduction. In this range, eddy correlations ( < 30 days) between the surface outcrop area and the local subduction rate counteract the net subduction by the mean flow (deduced from monthly averaged model fields). Comparison with estimates of the annual subduction rate based on the annual mean velocity and late-winter mixed layer properties alone, as is usual in climatological and coarse-resolution model analyses, indicates a mismatch of at least 8 Sv in the density range where the model forms subtropical mode water. This mismatch is primarily due to time-varying mixed layer processes rather than small-scale mixing not resolved explicitly by the model. Our diagnostics based on the instantaneous kinematic approach provide a more complete picture of the water-mass formation process than diagnostics based only on air-sea flux or late-winter mixed layer model data. They reveal the crucial importance of both the seasonal mixed layer cycle and mesoscale eddies to the overall formation rate and provide thus a valuable tool for the analysis of water-mass formation rates in eddy-resolving numerical simulations at basin scale.
机译：求解涡旋的原始方程组总环流模型用于估算北大西洋亚热带回旋区的水质俯冲率。诊断是基于瞬时运动学方法的，该方法可以在给定的密度范围内，根据一年周期中的等渗露头位置，计算出水质俯冲的年速率。结果表明，随着春季混合层深度的减小，水体俯冲作用迅速发生（〜1-2个月），这与Stommel的假说相符，并且主要发生在深冬混合层（≥150 m）的区域。在北大西洋中部的密度范围为26≤σ≤27.2。在亚热带模式水域的密度范围从大约26.2到26.6的墨西哥湾流扩展的南部和东部发现年俯冲率O（100-200 m yr〜（-1）。在亚热带回旋区的东北部，年俯冲从北大西洋海流以东约26.9的密度到27.4（在本研究中为上截流），密度为O（250 m yr〜（-1））较大。 （26.2≤σ≤26.6）约为12.2 S_v（S_v≡10〜6 m〜3 s〜（-1）），相当于从该模型中应用的大约11 Sv的表面密度强迫推断的总形成速率。相比之下，较稠密的中央水的盆地综合速率（26.8≤σ≤27.2）提供的净俯冲几乎消失，在此范围内，地表露头面积与局部俯冲率之间的涡旋相关性（<30天）抵消了平均流量的净俯冲（从月平均模型字段推导）。仅根据年平均速度和冬末混合层特性估算年俯冲率，这在气候和粗分辨率模型分析中很常见，表明模型形成的密度范围内至少有8 Sv不匹配亚热带模式的水。这种不匹配主要是由于时变的混合层过程而不是模型没有明确解决的小规模混合。与仅基于海气通量或冬末混合层模型数据的诊断相比，基于瞬时运动学方法的诊断可提供更完整的水团形成过程图。它们揭示了季节性混合层周期和中尺度涡旋对总体形成速率的至关重要性，因此为在盆地规模的涡旋解析数值模拟中分析水体形成速率提供了有价值的工具。

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