The effect of uncertain river forcing on the thermohaline properties of the North West European Shelf Seas
The effect of uncertain river forcing on the thermohaline properties of the North West European Shelf Seas
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DOI：
10.1016/j.ocemod.2023.102196
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发表时间：
2023-04
期刊：
影响因子：
3.2
通讯作者：
S. Zedler;J. Polton;Robert R. King;S. Wakelin
中科院分区：
文献类型：
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作者：
S. Zedler;J. Polton;Robert R. King;S. Wakelin
模拟研究和观测表明，西北欧陆架海（NWESS）的热盐特性对地表风和热通量强迫以及将淡水从陆地输送到海洋的河流流出非常敏感。在以前的研究中，人们假设可以通过高分辨率、亚中尺度、长期（即30年）的确定性后验来充分采样热盐特性对河流流出的响应。在本研究中，我们假设河流强迫的统计分布，而不是强迫本身的时间序列，充分受到28年历史（1991年至2018年）的约束。通过这种方式，我们创建了一个由10个低分辨率（≈7公里）的短期（即2.5年）后推模型组成的集合，这些模型是由随机扰动的河流流出量和10个ECMWF ERA5再分析的地表通量组成的集合（“测试”集合），以及一个由ERA5地表强迫通量组成的伙伴集合，但没有扰动的河流流出量（“基础”集合），时间跨度为2016年6月至2018年12月。在这两个集合中，模拟的25小时平均（部分滤除潮汐）温度和盐度的演变是真实的，海面温度的峰值在夏季，盐度的峰值在冬季，年振幅与在NWESS的其他研究中发现的相似。试验集合中海面和海底盐度的平均值和标准偏差的增加在一定程度上是一种假设的产物，即河流强迫的误差具有对数正态分布，这种分布模拟了河流流出的偶然性，平均值为正，形状不对称，长尾朝向大值。对于地表密度，测试集合的标准差在0.5 kg/m3以下，覆盖面积全年大于基地集合。该区域密度面积范围的年周期夏季最大，冬季最小，与河流流出强迫的年周期相一致。总体而言，不确定的河流强迫对本研究中温盐性质的影响很小。为了了解河流强迫的真正影响，需要对河流流出进行更好的时空观测。
Modeling studies and observations show that the thermohaline properties of the North West European Shelf Seas (NWESS) are sensitive to surface wind and heat flux forcing, as well as river outflows that transport fresh water from land to the ocean. In previous studies, it was assumed that the variability of the thermohaline properties in response to river outflow could be adequately sampled with a high-resolution, submesoscale permitting, long-term (ie, 30-year) deterministic hindcast. In this study, we assume that the statistical distribution of the river forcing, rather than the time series of forcing itself, is adequately constrained by a 28-year history (1991 to 2018) of river forcing created specifically for our domain. In this way, we created an ensemble of 10 lower-resolution (≈ 7-km), short-term (ie, 2.5 years) hindcast models that are forced with randomly perturbed river outflows and an ensemble of surface fluxes from the 10-member ECMWF ERA5 reanalysis (the ‘Test’ensemble) as well with a companion ensemble that is forced with the ERA5 surface forcing fluxes but unperturbed river outflows (the ‘Base’ensemble) for the June 2016 through December 2018 time period. In both ensembles, the modeled evolution of 25-hour averaged (to partially filter out tides) temperature and salinity is realistic with peaks in summer for sea surface temperature and in winter for salinity, and annual amplitudes that are comparable to those found in other studies of the NWESS. The increased mean and standard deviation of the sea surface and bottom salinity in the Test ensemble are partly an artifact of the assumption that the errors in river forcing have a log-normal distribution that mimics the episodic nature of river outflow with a positive mean and an asymmetrical shape with a long tail toward large values. For surface density, the standard deviation in the Test ensemble was below 0.5 kg/m3, covering an areal extent larger than that for the Base ensemble throughout the year. The annual cycle of the areal extent of density in that range had a peak in summer and minima in winter, in phase with that of the river outflow forcing. Overall, the effect of uncertain river forcing on the thermohaline properties in this study is small. In order to understand the true impact of river forcings, better temporal and spatial observations of river outflow are needed.