Seasonality and trend of the global upper-ocean vertical velocity over 1998–2017

The ocean vertical circulation has been historically underappreciated compared to the lateral circulation, largely due to the poor availability of the ocean vertical-velocity information. With the advent of high-performance ocean models, especially those constrained by the most available observations, it is now possible and incentive to dig into the vertical branch of ocean circulation. In this study, we used a state-of-the-art and dynamically-consistent ocean state estimate to investigate the seasonal variations and trend of the global upper-ocean (in the top 200 m) vertical velocity, with emphasis on the widely recognized upwelling and downwelling systems. Significant seasonal variations were noted. All around the global ocean, the North Indian Ocean and the Equator exhibited the strongest seasonality. There existed an equatorial Rossby wave propagating the equatorial Pacific upwelling at a phase speed of approximately −0.60 m/s (westward). Over 1998–2017, there were not basin-scale patterns of statistically-significant trend in the upper-ocean vertical velocity. In addition, our results did not support the classical Bakun's 1990 hypothesis on the upwelling intensification along the major eastern boundary upwelling systems in the context of global warming. This, however, may be due to the short period considered in this study. Four extended datasets were also examined. Patterns of seasonal variations were largely robust among these datasets. Results from these extended datasets further confirmed that there were not basin-scale patterns of statistically significant intensification or weakening of vertical circulations in the top 200 m of the global ocean during 1998–2017.