Statistical properties of near‐surface horizontal velocity gradients are obtained from four drifter experiments conducted in the Gulf of Mexico during Summer 2012 and Winter 2016. The data density provided by the near‐simultaneous deployments of 90‐326 surface drifters in each allows direct, drifter‐based estimates of the scale dependence of velocity gradients at separation scales ranging from 200 m to 5 km. The robustness of these estimates, derived from uniquley sampled, nearly equilateral triplets, is confirmed by comparisons with estimates produced from larger drifter clusters, and with estimates based on concurrent Eulerian X‐band radar observations. The winter launches were deployed above a ∼80 m deep mixed layer, one in a region with nearly homogeneous horizontal density structure, the other in a region of strong surface density gradients associated with filaments of fresh Mississippi River water. The summer launches occurred in a shallow (10m) mixed layer, one launched across a mesoscale frontal jet separating regions of horizontally homogeneous density and the other, similar to the corresponding winter launch, also in a region filamented by shallow lenses of cold, fresh water. Seasonal differences are observed, with larger velocity fluctuations and greater variance in divergence and vorticity, especially at the smallest scales, in winter. Differences between same‐season launches are, however, as large as seasonal differences. In both seasons, observations sampling regions directly impacted by fresh water fluxes show strongly skewed vorticity distributions, with cyclonic vorticity dominating strain. For the other launches, one in each season, strain dominated minimally skewed vorticity.

中文翻译：

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墨西哥湾北部夏季和冬季地表流动的亚中尺度运动学特征

从2012年夏季和2016年冬季在墨西哥湾进行的四个漂移实验获得了近地表水平速度梯度的统计特性。通过同时部署90-326个地面漂移器而提供的数据密度允许直接使用漂移器基于分离范围为200 m至5 km的速度梯度的尺度相关性估计。这些估计值的稳健性来自于单股抽样的几乎等边的三元组，通过与较大的漂移星团产生的估计值以及基于并发欧拉X波段雷达观测值的估计值进行比较，得到了证实。冬季发射升空到〜80 m深的混合层，一个在一个具有几乎均匀的水平密度结构的区域中，另一个在与新鲜的密西西比河水的细丝相关的强表面密度梯度的区域中。夏季发射发生在浅层（10m）混合层中，一个发射穿过中尺度的前喷流，将水平均匀密度的区域分开，另一个发射类似于冬季发射，也发生在由浅淡淡水冷的淡水透镜状的区域。观察到季节差异，冬季的速度波动更大，发散和涡度变化更大，尤其是在最小尺度上。但是，同一季节发射之间的差异与季节差异一样大。在两个季节里 观测结果表明，受淡水通量直接影响的采样区域显示出强烈偏斜的涡度分布，其中旋风涡度占主导。对于其他发射，每个季节中的一个，应变占最小程度的偏斜涡度。