Li Yun

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Variations in rates of biological production in the Beaufort Gyre as the arctic changes: Rates from 2011 to 2016

(American Geophysical Union, 2019-04-30) Ji, Brenda Y. ; Sandwith, Zoe O. ; Williams, William J. ; Diaconescu, Oana ; Ji, Rubao ; Li, Yun ; Van Scoy, Emma ; Yamamoto-Kawai, Michiyo ; Zimmermann, Sarah ; Stanley, Rachel H. R.

The Arctic Ocean is experiencing profound environmental changes as the climate warms. Understanding how these changes will affect Arctic biological productivity is key for predicting future Arctic ecosystems and the global CO2 balance. Here we use in situ gas measurements to quantify rates of gross oxygen production (GOP, total photosynthesis) and net community production (NCP, net CO2 drawdown by the biological pump) in the mixed layer in summer or fall from 2011 to 2016 in the Beaufort Gyre. NCP and GOP show spatial and temporal variations with higher values linked with lower concentrations of sea ice and increased upper ocean stratification. Mean rates of GOP range from 8 ± 1 to 54 ± 9 mmol O2·m−2·d−1 with the highest mean rates occurring in summer of 2012. Mean rates of NCP ranged from 1.3 ± 0.2 to 2.9 ± 0.5 mmol O2·m−2·d−1. The mean ratio of NCP/GOP, a measure of how efficiently the ecosystem is recycling its nutrients, ranged from 0.04 to 0.17, similar to ratios observed at lower latitudes. Additionally, a large increase in total photosynthesis that occurred in 2012, a year of historically low sea ice coverage, persisted for many years. Taken together, these data provide one of the most complete characterizations of interannual variations of biological productivity in this climatically important region, can serve as a baseline for future changes in rates of production, and give an intriguing glimpse of how this region of the Arctic may respond to future lack of sea ice.
Wind-induced interannual variability of sea level slope, along-shelf flow, and surface salinity on the Northwest Atlantic shelf

(John Wiley & Sons, 2014-04-16) Li, Yun ; Ji, Rubao ; Fratantoni, Paula S. ; Chen, Changsheng ; Hare, Jonathan A. ; Davis, Cabell S. ; Beardsley, Robert C.

In this study, we examine the importance of regional wind forcing in modulating advective processes and hydrographic properties along the Northwest Atlantic shelf, with a focus on the Nova Scotian Shelf (NSS)-Gulf of Maine (GoM) region. Long-term observational data of alongshore wind stress, sea level slope, and along-shelf flow are analyzed to quantify the relationship between wind forcing and hydrodynamic responses on interannual time scales. Additionally, a simplified momentum balance model is used to examine the underlying mechanisms. Our results show significant correlation among the observed interannual variability of sea level slope, along-shelf flow, and alongshore wind stress in the NSS-GoM region. A mechanism is suggested to elucidate the role of wind in modulating the sea level slope and along-shelf flow: stronger southwesterly (northeastward) winds tend to weaken the prevailing southwestward flow over the shelf, building sea level in the upstream Newfoundland Shelf region, whereas weaker southwesterly winds allow stronger southwestward flow to develop, raising sea level in the GoM region. The wind-induced flow variability can influence the transport of low-salinity water from the Gulf of St. Lawrence to the GoM, explaining interannual variations in surface salinity distributions within the region. Hence, our results offer a viable mechanism, besides the freshening of remote upstream sources, to explain interannual patterns of freshening in the GoM.
Hidden upwelling systems associated with major western boundary currents

(American Geophysical Union, 2022-03-03) Liao, Fanglou ; Liang, Xinfeng ; Li, Yun ; Spall, Michael A.

Western 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.
Remote silicate supply regulates spring phytoplankton bloom magnitude in the Gulf of Maine

(Association for the Sciences of Limnology and Oceanography (ASLO), 2022-02-24) Zang, Zhengchen ; Ji, Rubao ; Liu, Zongguang ; Chen, Changsheng ; Li, Yun ; Li, Siqi ; Davis, Cabell S.

Spring phytoplankton blooms in the Gulf of Maine (GoM) are sensitive to climate-related local and remote forcing. Nutrient supply through the slope water intrusion has been viewed as critical in regulating the GoM spring blooms, with an assumption that nitrogen is the primary limiting nutrient. In recent years, this paradigm has been challenged, with silicate being recognized as another potential limiting nutrient, but the source of silicate and its associated water mass remain difficult to be determined. In this study, a time series of spring bloom magnitude was constructed using a self-organizing map algorithm, and then correlated with the fluctuation of water composition in the deep Northeast Channel. The results reveal the importance of silicate supply from previously less-recognized deep Scotian Shelf Water inflow. This study offers a new hypothesis for spring bloom regulation, providing a better understanding of mechanisms controlling the spring bloom magnitude in the GoM.
Circumpolar analysis of the Adélie Penguin reveals the importance of environmental variability in phenological mismatch

(John Wiley & Sons, 2017-03-20) Youngflesh, Casey ; Jenouvrier, Stephanie ; Li, Yun ; Ji, Rubio ; Ainley, David G. ; Ballard, Grant ; Barbraud, Christophe ; Delord, Karine ; Dugger, Katie M. ; Emmerson, Louise M. ; Fraser, William R. ; Hinke, Jefferson T. ; Lyver, Philip O'B. ; Olmastroni, Silvia ; Southwell, Colin J. ; Trivelpiece, Susan G. ; Trivelpiece, Wayne Z. ; Lynch, Heather J.

Evidence of climate-change-driven shifts in plant and animal phenology have raised concerns that certain trophic interactions may be increasingly mismatched in time, resulting in declines in reproductive success. Given the constraints imposed by extreme seasonality at high latitudes and the rapid shifts in phenology seen in the Arctic, we would also expect Antarctic species to be highly vulnerable to climate-change-driven phenological mismatches with their environment. However, few studies have assessed the impacts of phenological change in Antarctica. Using the largest database of phytoplankton phenology, sea-ice phenology, and Adélie Penguin breeding phenology and breeding success assembled to date, we find that, while a temporal match between Penguin breeding phenology and optimal environmental conditions sets an upper limit on breeding success, only a weak relationship to the mean exists. Despite previous work suggesting that divergent trends in Adélie Penguin breeding phenology are apparent across the Antarctic continent, we find no such trends. Furthermore, we find no trend in the magnitude of phenological mismatch, suggesting that mismatch is driven by interannual variability in environmental conditions rather than climate-change-driven trends, as observed in other systems. We propose several criteria necessary for a species to experience a strong climate-change-driven phenological mismatch, of which several may be violated by this system.
Synchronicity between ice retreat and phytoplankton bloom in circum-Antarctic polynyas

(John Wiley & Sons, 2016-03-05) Li, Yun ; Ji, Rubao ; Jenouvrier, Stephanie ; Jin, Meibing ; Stroeve, Julienne

Phytoplankton in Antarctic coastal polynyas has a temporally short yet spatially variant growth window constrained by ice cover and day length. Using 18-year satellite measurements (1997–2015) of sea ice and chlorophyll concentrations, we assessed the synchronicity between the spring phytoplankton bloom and light availability, taking into account the ice cover and the incident solar irradiance, for 50 circum-Antarctic coastal polynyas. The synchronicity was strong (i.e., earlier ice-adjusted light onset leads to earlier bloom and vice versa) in most of the western Antarctic polynyas but weak in a majority of the eastern Antarctic polynyas. The west-east asymmetry is related to sea ice production rate: the formation of many eastern Antarctic polynyas is associated with strong katabatic wind and high sea ice production rate, leading to stronger water column mixing that could damp phytoplankton blooms and weaken the synchronicity.
Pan-Antarctic analysis aggregating spatial estimates of Adélie penguin abundance reveals robust dynamics despite stochastic noise

(Nature Publishing Group, 2017-10-10) Che-Castaldo, Christian ; Jenouvrier, Stephanie ; Youngflesh, Casey ; Shoemaker, Kevin T. ; Humphries, Grant ; McDowall, Philip ; Landrum, Laura ; Holland, Marika M. ; Li, Yun ; Ji, Rubao ; Lynch, Heather J.

Colonially-breeding seabirds have long served as indicator species for the health of the oceans on which they depend. Abundance and breeding data are repeatedly collected at fixed study sites in the hopes that changes in abundance and productivity may be useful for adaptive management of marine resources, but their suitability for this purpose is often unknown. To address this, we fit a Bayesian population dynamics model that includes process and observation error to all known Adélie penguin abundance data (1982–2015) in the Antarctic, covering >95% of their population globally. We find that process error exceeds observation error in this system, and that continent-wide “year effects” strongly influence population growth rates. Our findings have important implications for the use of Adélie penguins in Southern Ocean feedback management, and suggest that aggregating abundance across space provides the fastest reliable signal of true population change for species whose dynamics are driven by stochastic processes.
Coastal phytoplankton blooms expand and intensify in the 21st century

(Nature Research, 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, Lian

Phytoplankton blooms in coastal oceans can be beneficial to coastal fisheries production and ecosystem function, but can also cause major environmental problems -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.
Stratification breakdown in Antarctic coastal polynyas, part I: influence of physical factors on the destratification timescale

(American Meteorological Society, 2023-08-28) Xu, Yilang ; Zhang, Weifeng Gordon ; Maksym, Ted ; Ji, Rubao ; Li, Yun

This study examines the process of water-column stratification breakdown in Antarctic coastal polynyas adjacent to an ice shelf with a cavity underneath. This first part of a two-part sequence seeks to quantify the influence of offshore katabatic winds, alongshore winds, air temperature, and initial ambient stratification on the time scales of polynya destratification through combining process-oriented numerical simulations and analytical scaling. In particular, the often-neglected influence of wind-driven circulation on the lateral transport of the water formed at the polynya surface—which we call Polynya Source Water (PSW)—is systematically examined here. First, an ice shelf–sea ice–ocean coupled numerical model is adapted to simulate the process of PSW formation in polynyas of various configurations. The simulations highlight that (i) before reaching the bottom, majority of the PSW is actually carried away from the polynya by katabatic wind–induced offshore outflow, diminishing water-column mixing in the polynya and intrusion of the PSW into the neighboring ice shelf cavity, and (ii) alongshore coastal easterly winds, through inducing onshore Ekman transport, reduce offshore loss of the PSW and enhance polynya mixing and PSW intrusion into the cavity. Second, an analytical scaling of the destratification time scale is derived based on fundamental physical principles to quantitatively synthesize the influence of the physical factors, which is then verified by independent numerical sensitivity simulations. This work provides insights into the mechanisms that drive temporal and cross-polynya variations in stratification and PSW formation in Antarctic coastal polynyas, and establishes a framework for studying differences among the polynyas in the ocean.
Stratification breakdown in Antarctic coastal polynyas, part II: influence of an ice tongue and coastline geometry

(American Meteorological Society, 2023-08-28) Xu, Yilang ; Zhang, Weifeng Gordon ; Maksym, Ted ; Ji, Rubao ; Li, Yun

This is Part II of a study examining wintertime destratification in Antarctic coastal polynyas, focusing on providing a qualitative description of the influence of ice tongues and headlands, both common geometric features neighboring the polynyas. The model of a coastal polynya used in Part I is modified to include an ice tongue and a headland to investigate their impacts on the dispersal of water formed at the polynya surface, which is referred to as Polynya Source Water (PSW) here. The model configuration qualitatively represents the settings of some coastal polynyas, such as the Terra Nova Bay Polynya. The simulations highlight that an ice tongue next to a polynya tends to break the alongshore symmetry in the lateral return flows toward the polynya, creating a stagnant region in the corner between the ice tongue and polynya where outflow of the PSW in the water column is suppressed. This enhances sinking of the PSW and accelerates destratification of the polynya water column. Adding a headland to the other side of the polynya tends to restore the alongshore symmetry in the lateral return flows, which increases the offshore PSW transport and slows down destratification in the polynya. This work stresses the importance of resolving small-scale geometric features in simulating vertical mixing in the polynya. It provides a framework to explain spatial and temporal variability in rates of destratification and Dense Shelf Water formation across Antarctic coastal polynyas, and helps understand why some polynyas are sources of Antarctic Bottom Water while others are not.
It's about time: a synthesis of changing phenology in the Gulf of Maine ecosystem

(Wiley, 2019-04-22) Staudinger, Michelle D. ; Mills, Katherine E. ; Stamieszkin, Karen ; Record, Nicholas R. ; Hudak, Christine A. ; Allyn, Andrew ; Diamond, Antony ; Friedland, Kevin D. ; Golet, Walter J. ; Henderson, Meghan Elisabeth ; Hernandez, Christina M. ; Huntington, Thomas G. ; Ji, Rubao ; Johnson, Catherine L. ; Johnson, David Samuel ; Jordaan, Adrian ; Kocik, John ; Li, Yun ; Liebman, Matthew ; Nichols, Owen C. ; Pendleton, Daniel ; Richards, R. Anne ; Robben, Thomas ; Thomas, Andrew C. ; Walsh, Harvey J. ; Yakola, Keenan

The timing of recurring biological and seasonal environmental events is changing on a global scale relative to temperature and other climate drivers. This study considers the Gulf of Maine ecosystem, a region of high social and ecological importance in the Northwest Atlantic Ocean and synthesizes current knowledge of (a) key seasonal processes, patterns, and events; (b) direct evidence for shifts in timing; (c) implications of phenological responses for linked ecological‐human systems; and (d) potential phenology‐focused adaptation strategies and actions. Twenty studies demonstrated shifts in timing of regional marine organisms and seasonal environmental events. The most common response was earlier timing, observed in spring onset, spring and winter hydrology, zooplankton abundance, occurrence of several larval fishes, and diadromous fish migrations. Later timing was documented for fall onset, reproduction and fledging in Atlantic puffins, spring and fall phytoplankton blooms, and occurrence of additional larval fishes. Changes in event duration generally increased and were detected in zooplankton peak abundance, early life history periods of macro‐invertebrates, and lobster fishery landings. Reduced duration was observed in winter–spring ice‐affected stream flows. Two studies projected phenological changes, both finding diapause duration would decrease in zooplankton under future climate scenarios. Phenological responses were species‐specific and varied depending on the environmental driver, spatial, and temporal scales evaluated. Overall, a wide range of baseline phenology and relevant modeling studies exist, yet surprisingly few document long‐term shifts. Results reveal a need for increased emphasis on phenological shifts in the Gulf of Maine and identify opportunities for future research and consideration of phenological changes in adaptation efforts.
Influence of physical factors on restratification of the upper water column in Antarctic coastal polynyas

(American Geophysical Union, 2024-03-03) Xu, Yilang ; Zhang, Weifeng Gordon ; Maksym, Ted ; Ji, Rubao ; Li, Yun ; Walker, Catherine C.

Antarctic coastal polynyas are hotspots of biological production with intensive springtime phytoplankton blooms that strongly depend on meltwater-induced restratification in the upper part of the water column. However, the fundamental physics that determine spatial inhomogeneity of the spring restratification remain unclear. Here, we investigate how different meltwaters affect springtime restratification and thus phytoplankton bloom in Antarctic coastal polynyas. A high-resolution coupled ice-shelf/sea-ice/ocean model is used to simulate an idealized coastal polynya similar to the Terra Nova Bay Polynya, Ross Sea, Antarctica. To evaluate the contribution of various meltwater sources, we conduct sensitivity simulations altering physical factors such as alongshore winds, ice shelf basal melt, and surface freshwater runoff. Our findings indicate that sea ice meltwater from offshore is the primary buoyancy source of polynya near-surface restratification, particularly in the outer-polynya region where chlorophyll concentration tends to be high. Downwelling-favorable alongshore winds can direct offshore sea ice away and prevent sea ice meltwater from entering the polynya region. Although the ice shelf basal meltwater can ascend to the polynya surface, much of it is mixed vertically over the water column and confined horizontally to a narrow coastal region, and thus does not contribute significantly to the polynya near-surface restratification. Surface runoff from ice shelf surface melt could contribute greatly to the polynya near-surface restratification. Nearby ice tongues and headlands strongly influence the restratification through modifying polynya circulation and meltwater transport pathways. Results of this study can help explain observed spatiotemporal variability in restratification and associated biological productivity in Antarctic coastal polynyas.

Li Yun

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