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Monsoon-induced upwelling off the Vietnamese coast

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Abstract

During the southwest monsoon from July 8 to 28, 2003, an interdisciplinary cruise took place in the central area of Vietnamese upwelling with “MV Nghien Cuu Bien” in the South China Sea. Physical observations in the upwelling area are analyzed with respect to local/regional wind forcing and far field forcing. Nutrients and phytoplankton measurements are discussed with respect to exchange processes between different water masses. The wind-induced coastal upwelling by local wind forcing is much weaker than in the previous years due to weaker-than-normal winds. This can be attributed to the far field forcing of the 2002/2003 El Niño event which modulates the upwelling intensity. The atmospheric conditions reflect the typical situation after an El Niño event which weakens the wind-induced coastal upwelling, reduces the latent heat flux, and results in higher-than-normal sea-surface temperatures. The general circulation pattern during SW monsoon is driven by the spatial asymmetry in the monsoon forcing. The flow pattern is characterized by an upwelling-induced northward undercurrent and a recently detected southward countercurrent. The resulting stretching deformation of this flow pattern forms an offshore jet between ∼12°N and 12.5°N and causes a local enhancement of the upwelling intensity. The upwelling due to stretching deformation is a peculiarity, which makes the Vietnamese upwelling area different to other upwelling areas. A budget of the upwelling components is presented: the strongest contribution in 2003 to the Vietnamese upwelling is the dynamical upwelling due to the clockwise rotation of the northward undercurrent. The internal radius of deformation separates the upwelling area from the offshore area as well as different water masses. Mekong River and the Gulf of Thailand waters which are offshore show nutrient depletion. Therefore, high chlorophyll maxima cannot be explained by nutrient supply from river runoff. The dynamical upwelling brings in nutrient-rich Maximum Salinity Water into the euphotic zone. This causes a subsurface chlorophyll maximum between 20 and 40 m water depth along the northward undercurrent. Deflection from the Redfield ratio in the C:N ratio and negative excess nitrogen identifies the region as nitrogen-limited which may favor cyanobacteria blooms. The consequence is a unique feature in new production: in the upwelling area, new production is based on upwelled nitrate, whereas offshore in the nutrient-depleted Mekong and Gulf of Thailand water, new production is based in addition on nitrogen fixation.

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Acknowledgments

The German–Vietnamese cooperation has been financed by the German Research Foundation (DFG), the Federal Ministry for Economic Cooperation and Development (BMZ), and the Vietnamese Ministry of Science and Technology (MOST), which is greatly acknowledged. The observations were carried out on board of “MV Nghien Cuu Bien”. We thank Nguyen Ngoc Lam and Rolf Peinert for their organization before and during the cruise. TMI data are produced by Remote Sensing Systems and sponsored by the NASA Earth Science REASON DISCOVER Project. Data are available at http://www.remss.com. Bui Hong Long provided the meteorological observations. The topography of the SCS and adjacent areas was provided by Delft Hydraulics which is greatly acknowledged. The authors are indebted to Eberhard Hagen, Joe Montoya, and Maren Voss for their helpful comments and discussions and Ines Staeglich and Sabine Schnell for their support with data processing. We also want to thank an anonymous reviewer whose comments clearly improved the paper.

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Correspondence to Joachim W. Dippner.

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Dippner, J.W., Nguyen, K.V., Hein, H. et al. Monsoon-induced upwelling off the Vietnamese coast. Ocean Dynamics 57, 46–62 (2007). https://doi.org/10.1007/s10236-006-0091-0

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