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Bay of Bengal Sea surface salinity variability using a decade of improved SMOS re-processing
Remote Sensing of Environment ( IF 13.5 ) Pub Date : 2020-10-01 , DOI: 10.1016/j.rse.2020.111964
V.P. Akhil , J. Vialard , M. Lengaigne , M.G. Keerthi , J. Boutin , J.L. Vergely , F. Papa

Monsoon rain and rivers bring large freshwater input to the Northern Bay of Bengal (BoB), yielding low Sea Surface Salinity (SSS) after the monsoon. The resulting sharp upper-ocean salinity stratification is thought to influence tropical cyclones intensity and biological productivity by inhibiting vertical mixing. Despite recent progresses, the density of in situ data is far from sufficient to monitor the BoB SSS variability, even at the seasonal timescale. The advent of satellite remotely-sensed SSS (SMOS, Aquarius, SMAP) offers a unique opportunity to provide synoptic maps of the BoB SSS every ~8 days. Previous SMOS SSS retrievals did not perform well in the BoB. Here, we show that improved systematic error corrections and quality control procedures yield a much better performance of the new “debiased v4” CATDS level-3 SSS from SMOS (~0.8 correlation, 0.04 bias and 0.64 root-mean-square difference to more than 28,000 collocated in situ data points over 2010–2019). The SMOS product now performs equivalently to Aquarius, and is slightly inferior to SMAP over the BoB. In particular, SMAP and SMOS are able to capture salinity variations close to the east coast of India (r > 0.8 within 75–150 km of the coast). They thus capture the seasonal freshening there, associated with equatorward advection of the Northern BoB low-salinity water by the East Indian Coastal Current (EICC) after the summer monsoon. The 10-year long SMOS record further allows to describe the BoB interannual SSS variability, which is strongest in boreal fall in relation with the Indian Ocean Dipole (IOD). Positive IOD events induce a weakening of the southward export of freshwater by the EICC, and hence negative SSS anomalies in the Northern BoB and positive ones along the East Indian coast. This confirms results from earlier studies based on modelling, sparse in situ data, or shorter satellite records, but this time from a 10-year long SSS record. Overall, our study indicates that the new SMOS retrieval can be confidently used to monitor the BoB SSS and to study its mechanisms. We end by a brief description of the BoB SSS anomalies associated with the extreme 2019 IOD event and highlight the very good performance over the BoB of a new multi-satellite product developed by the European Space Agency merging SMOS, Aquarius and SMAP data.

中文翻译:

孟加拉湾海面盐度变化使用十年改进的 SMOS 再处理

季风降雨和河流为孟加拉北部湾 (BoB) 带来大量淡水输入,季风过后产生低海面盐度 (SSS)。由此产生的急剧上层海洋盐度分层被认为通过抑制垂直混合来影响热带气旋强度和生物生产力。尽管最近取得了进展,但原位数据的密度远不足以监测 BoB SSS 的变化,即使在季节性时间尺度上也是如此。卫星遥感 SSS(SMOS、Aquarius、SMAP)的出现提供了一个独特的机会,可以每约 8 天提供一次 BoB SSS 的天气图。以前的 SMOS SSS 检索在 BoB 中表现不佳。在这里,我们展示了改进的系统误差校正和质量控制程序产生了来自 SMOS 的新“去偏差 v4”CATDS 3 级 SSS(~0. 8 相关性、0.04 偏差和 0.64 均方根差,与 2010-2019 年间的 28,000 多个并置的原位数据点相关)。SMOS 产品现在的性能与 Aquarius 相当,但在 BoB 上略逊于 SMAP。特别是,SMAP 和 SMOS 能够捕捉印度东海岸附近的盐度变化(海岸 75-150 公里内的 r > 0.8)。因此,他们捕捉到了那里的季节性清新,与夏季季风后东印度海岸流 (EICC) 将北 BoB 低盐度水向赤道平流有关。长达 10 年的 SMOS 记录进一步允许描述 BoB 的年际 SSS 变化,这在与印度洋偶极子 (IOD) 相关的北方下降中最强。正 IOD 事件导致 EICC 向南出口淡水的减弱,因此,BoB 北部出现负 SSS 异常,东印度海岸出现正异常。这证实了早期基于建模、稀疏原位数据或较短卫星记录的研究结果,但这一次来自 10 年长的 SSS 记录。总的来说,我们的研究表明,新的 SMOS 检索可以自信地用于监测 BoB SSS 并研究其机制。最后,我们简要描述了与 2019 年 IOD 极端事件相关的 BoB SSS 异常,并强调了由欧洲航天局合并 SMOS、Aquarius 和 SMAP 数据开发的新型多卫星产品在 BoB 上的出色表现。但这一次来自长达 10 年的 SSS 记录。总的来说,我们的研究表明,新的 SMOS 检索可以自信地用于监测 BoB SSS 并研究其机制。最后,我们简要描述了与 2019 年 IOD 极端事件相关的 BoB SSS 异常,并强调了由欧洲航天局合并 SMOS、Aquarius 和 SMAP 数据开发的新型多卫星产品在 BoB 上的出色表现。但这一次来自长达 10 年的 SSS 记录。总的来说,我们的研究表明,新的 SMOS 检索可以自信地用于监测 BoB SSS 并研究其机制。最后,我们简要描述了与 2019 年 IOD 极端事件相关的 BoB SSS 异常,并强调了由欧洲航天局合并 SMOS、Aquarius 和 SMAP 数据开发的新型多卫星产品在 BoB 上的出色表现。
更新日期:2020-10-01
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