Time scales of circulation and mixing processes of San Francisco Bay waters
Time scales of circulation and mixing processes of San Francisco Bay waters
复制标题
旧金山湾水域环流和混合过程的时间尺度
DOI：
10.1007/bf00048685
复制
发表时间：
1985
期刊：
影响因子：
2.6
通讯作者：
T. Conomos
中科院分区：
文献类型：
--
作者：
R. Walters;R. T. Cheng;T. Conomos
在旧金山弗朗西斯科的北方和南部河段，潮汐周期和海平面、海流和混合过程的低频变化的概念模型描述了这些海湾的对比特征和不同的过程和速率：北方河段为部分混合型河口，南湾是一个与北方河段有密度驱动交换的潮汐振荡泻湖，混合半日潮是前进波和驻波的混合。南湾相对简单的振荡几乎是驻波，能量沿着水道传播并分散到广阔的浅滩区域。北方河段的潮汐具有前进波的一般特性，但在海湾收缩处发生变化，并逐渐向上游方向变化为前进波和驻波的混合。春季和小潮变化的潮汐是显着的，并导致每两周变化的潮流，影响混合和盐度分层的水柱。风应力的水面上，淡水流入，和潮流与复杂的海湾配置的相互作用是主要的本地强迫机制，创造低频变化的海平面和电流。这些本地强迫机制驱动的剩余流量，潮汐扩散，控制在河口的水置换率。在北方河段，纵向密度梯度驱动的河口环流的渠道，和潮汐振幅的空间变化产生潮汐驱动的剩余环流。相比之下，南湾在一年中的大部分时间里表现出风驱动环流和潮汐驱动剩余环流之间的平衡。然而，在冬季，可以有足够的密度变化，以驱动多层（2至3）流在通道的南湾混合模型（包括扩散和分散过程）是基于时间尺度与盐的变化在边界和那些与当地强迫机制，而变化的空间尺度取决于配置的海湾。在河口环流较强的北方河段，盐通量主要由平均盐场的平均平流输送。在盐度梯度较大的地方，盐通量的潮汐相关部分与平流部分具有相同的量级。我们对南湾的混合和汇率的了解很差。由于该海湾接近等盐线，盐通量完全由平均盐场的平均平流控制。在河流流量高峰期间和之后，水混合变得更加动态，强烈的密度驱动电流产生了水质量和盐的净交换。这些交换在小潮期间更强。水团的停留时间随季节而变化，并且在河段之间也不同。在北方河段，冬季河流高流量的停留时间为几天，夏季为几个月。南湾的停留时间在夏季相当长（大约几个月），而在冬季通常较短（不到一个月），此时发生密度驱动的交换。
Conceptual models for tidal period and low-frequency variations in sea level, currents, and mixing processes in the northern and southern reaches of San Francisco Bay describe the contrasting characteristics and dissimilar processes and rates in these embayments: The northern reach is a partially mixed estuary whereas the southern reach (South Bay) is a tidally oscillating lagoon with density-driven exchanges with the northern reach.The mixed semidiurnal tides are mixtures of progressive and standing waves. The relatively simple oscillations in South Bay are nearly standing waves, with energy propagating down the channels and dispersing into the broad shoal areas. The tides of the northern reach have the general properties of a progressive wave but are altered at the constriction of the embayments and gradually change in an upstream direction to a mixture of progressive and standing waves. The spring and neap variations of the tides are pronounced and cause fortnightly varying tidal currents that affect mixing and salinity stratification in the water column.Wind stress on the water surface, freshwater inflow, and tidal currents interacting with the complex bay configuration are the major local forcing mechanisms creating low-frequency variations in sea level and currents. These local forcing mechanisms drive the residual flows which, with tidal diffusion, control the water-replacement rates in the estuary. In the northern reach, the longitudinal density gradient drives an estuarine circulation in the channels, and the spatial variation in tidal amplitude creates a tidally-driven residual circulation. In contrast, South Bay exhibits a balance between wind-driven circulation and tidally-driven residual circulation for most of the year. During winter, however, there can be sufficient density variations to drive multilayer (2 to 3) flows in the channel of South Bay.Mixing models (that include both diffusive and dispersive processes) are based on time scales associated with salt variations at the boundaries and those associated with the local forcing mechanisms, while the spatial scales of variations are dependent upon the configuration of the embayments. In the northern reach, where the estuarine circulation is strong, the salt flux is carried by the mean advection of the mean salt field. Where large salinity gradients are present, the tidal correlation part of the salt flux is of the same order as the advective part. Our knowledge of mixing and exchange rates in South Bay is poor. As this embayment is nearly isohaline, the salt flux is dominated entirely by the mean advection of the mean salt field. During and after peaks in river discharge, water mixing becomes more dynamic, with a strong density-driven current creating a net exchange of both water mass and salt. These exchanges are stronger during neap tides.Residence times of the water masses vary seasonally and differ between reaches. In the northern reach, residence times are on the order of days for high winter river discharge and of months for summer periods. The residence times for South Bay are fairly long (on the order of several months) during summer, and typically shorter (less than a month) during winter when density-driven exchanges occur.