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Abnormal features of oceanographic characteristics in upwelling Vietnam waters under impact of El Niño events

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The analysis of spatial variations of oceanography fields derived from HYCOM/NCODA/FEM system along zonal and meridional sections and vertical profiles as well as the results obtained from water mass analysis allow us to identify in details the abnormal oceanic characteristics of deeper water layers during the declining El Niño phase. | Vietnam Journal of Earth Sciences, 39(3), 225-239, DOI: 10.15625/0866-7187/39/3/10268 Vietnam Academy of Science and Technology Vietnam Journal of Earth Sciences (VAST) http://www.vjs.ac.vn/index.php/jse Abnormal features of oceanographic characteristics in upwelling Vietnam waters under impact of El Niño events Tong Phuoc Hoang Son*1, Tran Van Chung1, Nguyen Huu Huan1, Ngo Manh Tien1, Vu Van Tac1, Nguyen Hoang Thai Khang1, Nguyen Truong Thanh Hoi1, Marine Herrmann2, Eko Siswanto3 1 Institute of Oceanography, Vietnam Academy of Science and Technology LEGOS, IRD-CNES-CNRS-Université de Toulouse, University of Sciences and Technology of Hanoi, Vietnam Academy of Science and Technology 3 Japan Agency for Marine-Earth Science and Technology (JAMSTEC) 2 Received 28 March 2017. Accepted 5 June 2017 ABSTRACT The summer upwelling that occurs in coastal waters of South Central Vietnam is one of the major hydrographic features in the East Sea. A weakening of the upwelling after major El Niño events was observed in the literature for previous El Niño events and was verified here from the analysis of new satellite image data sets of sea surface temperature (SST) and surface wind. The analysis of empirical orthogonal function (EOF) from of monthly SST as well as of temporal and spatial variations of SST and wind force allow us to identify abnormal characteristics in ocean surface water that happened after El Niño episode, in agreement with previous studies. Those abnormal characteristics in Vietnam upwelling waters appeared mainly during the summers of 1998, 2003, 2010 and 2016 years for the El Niño decline phase. The upwelling weakening during El Niño decline episodes is associated with the following signals: (1) Wind force and Ekman pump are very weak; (2) the cold and high chlorophyll-a tongue is shifted northward but not extended eastward; (3) for years when El Niño occurs, SST strongly increases and reaches a peak in May or early June of next year, during the .