Noble gases sampled from three cruises in the eastern tropical Atlantic

Oceanic upwelling velocities are too small to be measured directly. Deviations of the He-3/He-4 ratio in the mixed layer from solubility equilibrium provide an indirect means to infer vertical velocities at the base of the mixed layer. This method is applied to the Mauritanian upwelling region for data from three cruises in summer 2006 and winter 2007 and 2008. Diapycnal mixing coefficients are estimated from microstructure measurements, reaching from 10**-3 m**2/s over the shelf break to 10**-5 m**2/s in the open ocean. The resulting upwelling velocities in the onshore region (upto 50 km from the 50 m isobath) are of the order of 2 x 10**-5 m/s}, in agreement with Ekman theory. Further offshore, in some cases the vertical velocities inferred from the helium isotope disequilibrium exceed the values derived from the wind stress curl by one order of magnitude. The Mauritanian coastal area as part of the Canary Current upwelling system belongs to the most productive ocean regions in the world. Nutrient fluxes into the mixed layer (both advective and diffusive) are equivalent to a net community production of about 1 g C/d, and associated heat fluxes vary between 183 +/- 62 W/m**2 in summer and 97 +/- 25 W/m**2 in winter. Regarding the flux into the mixed layer, the contribution of diffusion and advection are of similar magnitude for both heat and nutrients. The upwelling, however, provides the supply of cold and nutrient rich water from below. The large offshore vertical velocities inferred from the helium method are associated with nutrient fluxes of the same order as for the onshore region, and may be responsible for observed patches of high productivity in that area. The offshore heat fluxes due to upwelling and diapycnal mixing are smaller than 70 W/m**2 for all cruises.

海洋上升流流速过小，无法直接测量。混合层（mixed layer）内氦3/氦4比值（He-3/He-4 ratio）与溶解度平衡的偏差，为推断混合层底部的垂向流速提供了间接手段。本方法应用于毛里塔尼亚上升流区，所用数据取自2006年夏季、2007年冬季与2008年冬季的三次航次观测。跨等密度面混合（diapycnal mixing）系数通过微结构观测（microstructure measurements）估算得到，取值范围为陆架坡折（shelf break）处的10^-3 m²/s至开阔大洋中的10^-5 m²/s。近岸区域（距50米等深线不超过50公里范围）的推算上升流流速约为2×10^-5 m/s，与埃克曼理论（Ekman theory）相符。在离岸更远的区域，部分案例中由氦同位素不平衡推算得到的垂向流速，较风应力旋度（wind stress curl）推算结果高出一个数量级。作为加那利流上升流系统（Canary Current upwelling system）的一部分，毛里塔尼亚近岸海域属于全球生产力最高的海洋区域之一。进入混合层的营养盐通量（包括平流输送与扩散输送）约相当于1 g C/d的净群落生产力；与之相关的热通量在夏季为183±62 W/m²，冬季为97±25 W/m²。就进入混合层的通量而言，扩散与平流对热通量和营养盐通量的贡献量级相近。然而上升流从下层海域输送了低温且富含营养盐的水体。通过氦同位素法推算得到的离岸大垂向流速，其对应的营养盐通量与近岸区域处于同一量级，这或可解释该区域观测到的高生产力斑块分布现象。所有航次观测中，由上升流与跨等密度面混合导致的离岸热通量均小于70 W/m²。