Lagrangian characterization of the southwestern Atlantic from a dense surface drifter deployment

IF 2.3 3区 地球科学 Q2 OCEANOGRAPHY Deep-Sea Research Part I-Oceanographic Research Papers Pub Date : 2024-05-08 DOI:10.1016/j.dsr.2024.104319
M. Saraceno , N. Bodnariuk , L.A. Ruiz-Etcheverry , M. Berta , C.G. Simionato , F.J. Beron-Vera , M.J. Olascoaga
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Abstract

The Southwestern Atlantic (SWA) is characterized by its large Eddy Kinetic Energy as the result of the confluence of two major western boundary currents, the northward flowing Malvinas Current (MC) and the southward flowing Brazil Current. The SWA study was addressed in the literature based on altimetry data, in situ measurements, regional models and ocean reanalysis. The present study constitutes the first effort to sample a portion of the SWA, with a dense drifter array (N = 62) deployment. The drifters, drogued at 15 m depths, were deployed across the MC and the Argentine Continental Shelf along two zonal transects located at 47°S and 47.25°S, between the 8th and the September 9, 2021. Drifters were set to deliver their position every 10 and 60 min, providing accurate Lagrangian trajectories that provide information on a large range of space and time scales of the surface currents. Three regions are clearly identified based on the analysis of the speed of the drifters, of their trajectories and of the spectral density of their velocities: the continental shelf, the slope and the open ocean. The comparison of the trajectories of the drifters with satellite altimetry images shows that, in general, drifters follow mesoscale features that are detectable in satellite altimetry maps. The analysis of the drifter trajectories also allowed us the study of submesoscale features of the flow (1–10 km) that are not observable in satellite altimetry data. Comparison with cloud-free, high-resolution color images, shows that drifter trajectories organized by the mesoscale flow might also locally follow sub-mesoscale features. In frontal regions it was found that drifter velocities double satellite altimetry geostrophic velocities, which suggests that the dynamics at those regions is largely dominated by ageostrophic components. The ageostrophic Ekman component might explain the direction of the drifters when strong winds from a given direction prevail for several days and the drifters are not in a region with large sea surface height (SSH) gradients. The joint analysis of drifters’ trajectory and SSH clearly depicts that mesoscale features on the open ocean region control the cross-shelf exchanges between the MC and open ocean regions as well as the strength and width of the MC. Finally, the spatial density distribution of the drifters during the first hours after deployment and within a small eddy also allowed us to characterize the flow in terms of its divergence, vorticity and strain, indicating that the MC is geostrophic and has a jet-like behavior while the eddy is largely ageostrophic and has a dominant vorticity component over strain. We conclude observing that the analysis of a dense array of drifters provides valuable information of the flow that cannot be attained solely based on satellite data.

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从密集的海面漂流器部署中了解西南大西洋的拉格朗日特征
西南大西洋(SWA)的特点是涡动能大,这是两个主要的西部边界洋流(北流向马尔维纳斯群岛洋流(MC)和南流向巴西洋流)汇合的结果。文献根据测高数据、现场测量、区域模型和海洋再分析结果对西南大西洋进行了研究。本研究是首次对西南大西洋的部分海域进行采样,部署了密集的漂流器阵列(N = 62)。2021 年 8 月 8 日至 9 月 9 日期间,沿位于南纬 47 度和 47.25 度的两条带状横断面,在 MC 和阿根廷大陆架上部署了水深 15 米的漂流器。漂流器设定每 10 分钟和 60 分钟提供一次位置,提供精确的拉格朗日轨迹,从而提供大范围的表层海流空间和时间尺度信息。根据对漂流器的速度、轨迹及其速度频谱密度的分析,可以清楚地确定三个区域:大陆架、斜坡和公海。将漂流物的轨迹与卫星测高图像进行比较后发现,一般来说,漂流物是沿着卫星测高图上可探测到的中尺度特征漂流的。通过对漂流器轨迹的分析,我们还研究了卫星测高数据无法观测到的次中尺度(1-10 公里)气流特征。与无云高分辨率彩色图像的比较表明,由中尺度气流组织的漂移器轨迹也可能局部跟随次中尺度特征。在锋面区域,漂移器速度是卫星测高地转速度的两倍,这表明这些区域的动力学主要由老化逆温成分主导。当某一方向的强风持续数天,而漂流物又不在海面高度梯度较大的区域时,老龄逆温 Ekman 成分可以解释漂流物的方向。对漂流物轨迹和 SSH 的联合分析清楚地表明,开阔洋区域的中尺度特征控制着 MC 和开阔洋区域之间的跨大陆架交换,也控制着 MC 的强度和宽度。最后,我们还利用漂流器在布放后最初几小时内以及在一个小涡内的空间密度分布,从发散、涡度和应变等方面分析了海流的特征,表明 MC 是地营流,具有类似喷流的行为,而涡在很大程度上是老营流,涡度分量大于应变。最后,我们观察到,对密集漂流器阵列的分析提供了有价值的流动信息,而这些信息仅靠卫星数据是无法获得的。
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来源期刊
CiteScore
4.60
自引率
4.20%
发文量
144
审稿时长
18.3 weeks
期刊介绍: Deep-Sea Research Part I: Oceanographic Research Papers is devoted to the publication of the results of original scientific research, including theoretical work of evident oceanographic applicability; and the solution of instrumental or methodological problems with evidence of successful use. The journal is distinguished by its interdisciplinary nature and its breadth, covering the geological, physical, chemical and biological aspects of the ocean and its boundaries with the sea floor and the atmosphere. In addition to regular "Research Papers" and "Instruments and Methods" papers, briefer communications may be published as "Notes". Supplemental matter, such as extensive data tables or graphs and multimedia content, may be published as electronic appendices.
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