The deepwater oxygen deficit in stratified shallow seas is mediated by diapycnal mixing

Seasonally stratified shelf seas are amongst the most biologically productive on the planet. A consequence is that the deeper waters can become oxygen deficient in late summer. Predictions suggest global warming will accelerate this deficiency. Here we integrate turbulence timeseries with vertical profiles of water column properties from a seasonal stratified shelf sea to estimate oxygen and biogeochemical fluxes. The profiles reveal a significant subsurface chlorophyll maximum and associated mid-water oxygen maximum. We show that the oxygen maximum supports both upward and downwards O2 fluxes. The upward flux is into the surface mixed layer, whilst the downward flux into the deep water will partially off-set the seasonal O2 deficit. The results indicate the fluxes are sensitive to both the water column structure and mixing rates implying the development of the seasonal O2 deficit is mediated by diapcynal mixing. Analysis of current shear indicate that the downward flux is supported by tidal mixing, whilst the upwards flux is dominated by wind driven near-inertial shear. Summer storminess therefore plays an important role in the development of the seasonal deep water O2 deficit.

季节性分层的陆架海是地球上生物生产力最高的区域之一。其结果是，在夏末时较深的水域可能会出现缺氧情况。预测表明全球变暖将加速这种缺氧状况。在此，我们将湍流时间序列与来自一个季节性分层陆架海的水柱特性垂直剖面相结合，以估算氧气和生物地球化学通量。这些剖面显示出一个显著的次表层叶绿素最大值以及相关的中层水氧气最大值。我们表明，氧气最大值既支持向上的也支持向下的O₂通量。向上的通量进入表层混合层，而向下进入深水的通量将部分抵消季节性的O₂亏损。结果表明，通量对水柱结构和混合速率都很敏感，这意味着季节性O₂亏损的发展是由斜压混合介导的。对水流切变的分析表明，向下的通量由潮汐混合支持，而向上的通量主要由风驱动的近惯性切变主导。因此，夏季的风暴活动在季节性深水O₂亏损的发展中起着重要作用。