Browsing by Author "Li, Yun"
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Item The changing CO2 sink in the western Arctic Ocean from 1994 to 2019(Global Biogeochemical Cycles, 2021-12-28) Ouyang, Zhangxian; Li, Yun; Zhong, Wenli; Murata, Akihiko; Nishino, Shigeto; Wu, Yingxu; Jin, Meibing; Kirchman, David; Chen, Liqi; Cai, Wei-JunThe Arctic Ocean has turned from a perennial ice-covered ocean into a seasonally ice-free ocean in recent decades. Such a shift in the air-ice-sea interface has resulted in substantial changes in the Arctic carbon cycle and related biogeochemical processes. To quantitatively evaluate how the oceanic CO2 sink responds to rapid sea ice loss and to provide a mechanistic explanation, here we examined the air-sea CO2 flux and the regional CO2 sink in the western Arctic Ocean from 1994 to 2019 by two complementary approaches: observation-based estimation and a data-driven box model evaluation. The pCO2 observations and model results showed that summer CO2 uptake significantly increased by about 1.4 ± 0.6 Tg C decade−1 in the Chukchi Sea, primarily due to a longer ice-free period, a larger open area, and an increased primary production. However, no statistically significant increase in CO2 sink was found in the Canada Basin and the Beaufort Sea based on both observations and modeled results. The reduced sea ice coverage in summer in the Canada Basin and the enhanced wind speed in the Beaufort Sea potentially promoted CO2 uptake, which was, however, counteracted by a rapidly decreased air-sea pCO2 gradient therein. Therefore, the current and future Arctic Ocean CO2 uptake trends cannot be sufficiently reflected by the air-sea pCO2 gradient alone because of the sea ice variations and other environmental factors.Item Coastal phytoplankton blooms expand and intensify in the 21st century(Nature, 2023-03-01) Dai, Yanhui; Yang, Shangbo; Zhao, Dan; Hu, Chuanmin; Xu, Wang; Anderson, Donald M.; Li, Yun; Song, Xiao-Peng; Boyce, Daniel G.; Gibson, Luke; Zheng, Chunmiao; Feng, LianPhytoplankton blooms in coastal oceans can be beneficial to coastal fisheries production and ecosystem function, but can also cause major environmental problems1,2—yet detailed characterizations of bloom incidence and distribution are not available worldwide. Here we map daily marine coastal algal blooms between 2003 and 2020 using global satellite observations at 1-km spatial resolution. We found that algal blooms occurred in 126 out of the 153 coastal countries examined. Globally, the spatial extent (+13.2%) and frequency (+59.2%) of blooms increased significantly (P < 0.05) over the study period, whereas blooms weakened in tropical and subtropical areas of the Northern Hemisphere. We documented the relationship between the bloom trends and ocean circulation, and identified the stimulatory effects of recent increases in sea surface temperature. Our compilation of daily mapped coastal phytoplankton blooms provides the basis for global assessments of bloom risks and benefits, and for the formulation or evaluation of management or policy actions.Item Hidden Upwelling Systems Associated With Major Western Boundary Currents(Journal of Geophysical Research: Oceans, 2022-03-03) Liao, Fanglou; Liang, Xinfeng; Li, Yun; Spall, MichaelWestern boundary currents (WBCs) play an essential role in regulating global climate. In contrast to their widely examined horizontal motions, less attention has been paid to vertical motions associated with WBCs. Here, we examine vertical motions associated with the major WBCs by analyzing vertical velocity from five ocean synthesis products and one eddy-resolving ocean simulation. These data reveal robust and intense subsurface upwelling systems, which are primarily along isopycnal surfaces, in five major subtropical WBC systems. These upwelling systems are part of basin-scale overturning circulations and are likely driven by meridional pressure gradients along the western boundary. Globally, the WBC upwelling contributes significantly to the vertical transport of water mass and ocean properties and is an essential yet overlooked branch of the global ocean circulation. In addition, the WBC upwelling intersects the oceanic euphotic and mixed layers, and thus likely plays an important role in ocean biological and chemical processes by transporting nutrients, carbon and other tracers vertically inside the ocean. This study calls for more research into the dynamics of the WBC upwelling and their role in the ocean and climate systems. Plain Language Summary: This study shows that intense upwelling systems exist along the major western boundary currents (WBCs) around the global ocean. In contrast to other well-known oceanic upwelling systems (e.g., equatorial, coastal upwelling), these WBC upwelling systems, which are essential branches of the global ocean circulation, have been largely unrecognized in the literature. This intense upwelling and the associated overturning circulation in the subtropical ocean basins can transport nutrients, carbon, and heat inside the ocean, and consequently act as an important yet unexplored route through which the oceanic biological, chemical, and physical processes, and consequently the climate system, will be affected. This study calls for more research into the dynamics of the WBC upwelling and its role in the ocean and climate systems.