Li
Yizhen
Li
Yizhen
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ArticleVariability of deep water in Jordan Basin of the Gulf of Maine: influence of Gulf Stream warm core rings and the Nova Scotia Current(American Geophysical Union, 2021-05-06) Du, Jiabi ; Zhang, Weifeng G. ; Li, YizhenAs the nutrient-rich subsurface slope water intruding into the deep basin of the Gulf of Maine (GoM) supports the high biological productivity in the semi-enclosed gulf, it is important to understand the process and time scale of such slope water intrusion. This study focuses on variations of the GoM deep water on seasonal to interannual time scales and the influences of open ocean processes on the temporal variation of the deep water properties. Based on long-term monitoring data, it is found that the deep water at Jordan Basin (one of three major basins in the GoM) is persistently warmer in winter than in summer, which is distinctly different from the seasonality of surface water in the basin and the deep water on neighboring shelf seas. The unique seasonality in the deep GoM reflects a time-lagged response to shoreward intrusion of the subsurface slope water off the GoM. Both observation-based lag-correlation analyses and numerical simulations confirm a timescale of approximately 3 months for the intruding subsurface slope water to flow from Northeast Channel to Jordan Basin. Properties of the intruding slope water at the Northeast Channel were significantly correlated with the Gulf Stream position and dramatically impacted by episodic warm-core rings shed from Gulf Stream. Inside the deep GoM, the intruding slope water was also indirectly affected by the fresher water input from Nova Scotia Current. Spreading of the fresher water inside the gulf strengthens near-surface stratification, suppresses deep convection, and preserves heat and salt in the deep GoM during the wintertime.
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ArticleDynamics of an intense Alexandrium catenella red tide in the Gulf of Maine: satellite observations and numerical modeling(Elsevier, 2020-10-26) Li, Yizhen ; Stumpf, Richard P. ; McGillicuddy, Dennis J. ; He, RuoyingIn July 2009, an unusually intense bloom of the toxic dinoflagellate Alexandrium catenella occurred in the Gulf of Maine. The bloom reached high concentrations (from hundreds of thousands to one million cells L−1) that discolored the water and exceeded normal bloom concentrations by a factor of 1000. Using Medium Resolution Imaging Spectrometer (MERIS) imagery processed to target chlorophyll concentrations (>2 µg L−1), patches of intense A. catenella concentration were identified that were consistent with the highly localized cell concentrations observed from ship surveys. The bloom patches were generally aligned with the edge of coastal waters with high-absorption. Dense bloom patches moved onshore in response to a downwelling event, persisted for approximately one week, then dispersed rapidly over a few days and did not reappear. Coupled physical-biological model simulations showed that wind forcing was an important factor in transporting cells onshore. Upward swimming behavior facilitated the horizontal cell aggregation, increasing the simulated maximum depth-integrated cell concentration by up to a factor of 40. Vertical convergence of cells, due to active swimming of A. catenella from the subsurface to the top layer, could explain the additional 25-fold intensification (25 × 40=1000-fold) needed to reach the bloom concentrations that discolored the water. A model simulation that considered upward swimming overestimated cell concentrations downstream of the intense aggregation. This discrepancy between model and observed concentrations suggested a loss of cells from the water column at a time that corresponded to the start of encystment. These results indicated that the joint effect of upward swimming, horizontal convergence, and wind-driven flow contributed to the red water event, which might have promoted the sexual reproduction event that preceded the encystment process.
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ArticleVariational data assimilative modeling of the Gulf of Maine in spring and summer 2010(John Wiley & Sons, 2015-05-19) Li, Yizhen ; He, Ruoying ; Chen, Ke ; McGillicuddy, Dennis J.A data assimilative ocean circulation model is used to hindcast the Gulf of Maine [GOM) circulation in spring and summer 2010. Using the recently developed incremental strong constraint 4D Variational data assimilation algorithm, the model assimilates satellite sea surface temperature and in situ temperature and salinity profiles measured by expendable bathythermograph, Argo floats, and shipboard CTD casts. Validation against independent observations shows that the model skill is significantly improved after data assimilation. The data-assimilative model hindcast reproduces the temporal and spatial evolution of the ocean state, showing that a sea level depression southwest of the Scotian Shelf played a critical role in shaping the gulf-wide circulation. Heat budget analysis further demonstrates that both advection and surface heat flux contribute to temperature variability. The estimated time scale for coastal water to travel from the Scotian Shelf to the Jordan Basin is around 60 days, which is consistent with previous estimates based on in situ observations. Our study highlights the importance of resolving upstream and offshore forcing conditions in predicting the coastal circulation in the GOM.
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PreprintOceanographic and biological influences on recruitment of benthic invertebrates to hard substrata on the Oregon shelf( 2018-04) Meyer, Kirstin S. ; Li, Yizhen ; Young, Craig M.The number of anthropogenic substrata in the ocean – structures like oil rigs and offshore renewable energy generators – is increasing. These structures provide hard-bottom habitat in areas previously dominated by sand or mud, so they have the potential to alter species distributions or serve as “stepping-stones” between other hard-bottom habitats. It is thus important to understand what factors influence the composition and abundance of benthic fauna recruiting at these sites. We examined recruitment to hard substrata (fouling panels) deployed on sand at various distances from a large rocky reef (~60 m isobath) on the southern Oregon coast in 2014 – 2015. Recruitment was dominated by the acorn barnacle Hesperibalanus hesperius. For the majority of the study period in 2014, an anti-cyclonic eddy was present near the deployment sites. However, anomalously high recruitment of H. hesperius during August – early October 2014 coincided with dissipation of the eddy, slower bottom currents, and a positive convergence index, suggesting that H. hesperius larvae from the adjacent area may have been accumulated and retained near our study sites. Other sessile species, including hydroids and bryozoans, recruited to the fouling panels in low abundances, and most of these species have long-range dispersal and fast growth. Mobile invertebrates observed on the fouling panels included gastropods and nudibranchs, most of which also have long-range dispersal and fast growth, and are predators as adults. Thus, a community with two trophic levels assembled on the fouling panels in a relatively short time period (<12 weeks). None of the common hard-bottom species from the adjacent rocky reef recruited to the panels, suggesting that there is a specialized assemblage of species that can exploit hard-bottom habitats surrounded by sandy plains. Our results raise many questions about the influences of dispersal and oceanographic conditions on recruitment to hard substrata.
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ArticleTechnical note : Common characteristics of directional spreading–steepness joint distribution in freak wave events(Copernicus Publications on behalf of the European Geosciences Union, 2016-06-09) Liu, Shouhua ; Li, Yizhen ; Yue, XinyangSeven freak wave incidents previously documented in the real ocean in combination with model hindcast simulations are used to study the variations associated with freak-wave-related parameters, such as wave steepness, directional spreading, and frequency bandwidth. Unlike the strong correlations between the freak wave parameters and freak waves' occurrence which were obtained in experimental and physical research, the correlations are not clear in the freak waves occurring in the real ocean. Wave directional spreading–steepness joint distribution is introduced and common visual features were found in the joint distribution when freak waves occur among seven “freakish” sea states. The visual features show that freak wave incidents occur when the steepness is large and directional spreading is small. Besides large steepness and small directional spreading, a long-duration, relatively rough sea state is also necessary for the freak wave generation. The joint distribution is more informative than any single statistical wave parameter. The continuous sea states of local large steepness and small directional spreading are supposed to generate freak waves, and two-dimensional distribution visualization is found to be a useful tool for freak waves' forecast. The common visual features of joint distributions supply an important cue for the theoretical and experimental research.
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ArticleImpact of Gulf Stream warm-core rings on slope water intrusion into the Gulf of Maine(American Meteorological Society, 2022-08-01) Du, Jiabi ; Zhang, Weifeng G. ; Li, YizhenIntruding slope water is a major source of nutrients to sustain the high biological productivity in the Gulf of Maine (GoM). Slope water intrusion into the GoM is affected by Gulf Stream warm-core rings (WCRs) impinging onto the nearby shelf edge. This study combines long-term mooring measurements, satellite remote sensing data, an idealized numerical ocean model, and a linear coastal-trapped wave (CTW) model to examine the impact of WCRs on slope water intrusion into the GoM through the Northeast Channel. Analysis of satellite sea surface height and temperature data shows that the slope sea region off the GoM is a hotspot of ring activities. A significant linear relationship is found between interannual variations of ring activities in the slope sea region off the GoM and bottom salinity at the Northeast Channel, suggesting the importance of WCRs in modulating variability of intruding slope water. Analysis of the mooring data reveals enhanced slope water intrusion through bottom-intensified along-channel flow following impingements of WCRs on the nearby shelf edge. Numerical simulations qualitatively reproduce the observed WCR impingement processes and associated episodic enhancement of slope water intrusion in the Northeast Channel. Diagnosis of the model result indicates that baroclinic CTWs excited by the ring–topography interaction are responsible for the episodically intensified subsurface along-channel inflow, which carries more slope water into the GoM. A WCR that impinges onto the shelf edge to the northeast of the Northeast Channel tends to generate stronger CTWs and cause stronger enhancement of the slope water intrusion into the GoM.
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PreprintProcesses influencing formation of low-salinity high-biomass lenses near the edge of the Ross Ice Shelf( 2016-07) Li, Yizhen ; McGillicuddy, Dennis J. ; Dinniman, Michael S. ; Klinck, John M.Both remotely sensed and in situ observations in austral summer of early 2012 in the Ross Sea suggest the presence of cold, low-salinity, and high-biomass eddies along the edge of the Ross Ice Shelf (RIS). Satellite measurements include sea surface temperature and ocean color, and shipboard data sets include hydrographic profiles, towed instrumentation, and underway acoustic Doppler current profilers. Idealized model simulations are utilized to examine the processes responsible for ice shelf eddy formation. 3-D model simulations produce similar cold and fresh eddies, although the simulated vertical lenses are quantitatively thinner than observed. Model sensitivity tests show that both basal melting underneath the ice shelf and irregularity of the ice shelf edge facilitate generation of cold and fresh eddies. 2-D model simulations further suggest that both basal melting and downwelling-favorable winds play crucial roles in forming a thick layer of low-salinity water observed along the edge of the RIS. These properties may have been entrained into the observed eddies, whereas that entrainment process was not captured in the specific eddy formation events studied in our 3-D model—which may explain the discrepancy between the simulated and observed eddies, at least in part. Additional sensitivity experiments imply that uncertainties associated with background stratification and wind stress may also explain why the model underestimates the thickness of the low-salinity lens in the eddy interiors. Our study highlights the importance of incorporating accurate wind forcing, basal melting, and ice shelf irregularity for simulating eddy formation near the RIS edge. The processes responsible for generating the high phytoplankton biomass inside these eddies remain to be elucidated.
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ArticleDissolved organic matter dynamics in the epipelagic Northwest Pacific low-latitude western boundary current system: insights from optical analyses(American Geophysical Union, 2021-08-17) Wang, Chao ; Li, Yizhen ; Li, Yan ; Zhou, Hui ; Stubbins, Aron ; Dahlgren, Randy A. ; Wang, Zhiheng ; Guo, WeidongHigh-resolution horizontal and vertical distribution of dissolved organic carbon (DOC), chromophoric, and fluorescent dissolved organic matter (CDOM and FDOM) were investigated in the western boundary current system of the tropical Northwest Pacific (<200 m) in autumn 2017. A strong correlation between DOC and stratification index indicated that the vertical DOC profile was primarily regulated by physical processes. The association of high aCDOM(254) with the maximum chlorophyll (Chl a) layer infers phytoplankton-sourced dissolved organic matter (DOM). The aCDOM(325) and humic-like FDOM (FDOMH) showed an accumulation in the deeper layer and positive correlations with apparent oxygen utilization and Chl a concentration at the maximum chlorophyll layer, suggesting that these components are related to microbial degradation of biogenic materials. Elevated Chl a at the frontal area between the North Equatorial Current (NEC) and cold Mindanao Eddy enhanced DOM production. Input waters from the NEC showed higher DOC, but lower FDOMH, than inflow waters from the New Guinea Coastal Current/Undercurrent (NGC(U)C). A mass balance model estimated a 6-times higher lateral DOC flux from the NEC tropical-gyre branch (12°N–7.5°N) than that from the subtropical-gyre branch (12°N–17°N). Based on comparison with long-term (1994–2015) average DOC fluxes for the same season, eddy and upstream processes contributed 38%, 46% and 40% of lateral DOC fluxes for the NEC tropical-gyre branch, NGC(U)C and export North Equatorial Counter Current, respectively. These results demonstrated that the quasi-permanent Mindanao and Halmahera eddies greatly enhance lateral export of DOM with altered properties throughout this large conjunction area.
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ArticleStorm impact on sea surface temperature and chlorophyll α in the Gulf of Mexico and Sargasso Sea based on daily cloud-free satellite data reconstructions(John Wiley & Sons, 2016-12-14) Shropshire, Taylor ; Li, Yizhen ; He, RuoyingUpper ocean responses to tropical storms/hurricanes have been extensively studied using satellite observations. However, resolving concurrent sea surface temperature (SST) and chlorophyll α (chl α) responses along storm tracks remains a major challenge due to extensive cloud coverage in satellite images. Here we produce daily cloud-free SST and chl α reconstructions based on the Data INterpolating Empirical Orthogonal Function method over a 10 year period (2003–2012) for the Gulf of Mexico and Sargasso Sea regions. Daily reconstructions allow us to characterize and contrast previously obscured subweekly SST and chl α responses to storms in the two main storm-impacted regions of the Atlantic Ocean. Statistical analyses of daily SST and chl α responses revealed regional differences in the response time as well as the response sensitivity to maximum sustained wind speed and translation speed. This study demonstrates that SST and chl α responses clearly depend on regional ocean conditions and are not as universal as might have been previously suggested.
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ArticleHydrological and biogeochemical controls on absorption and fluorescence of dissolved organic matter in the northern South China Sea(John Wiley & Sons, 2017-12-29) Wang, Chao ; Guo, Weidong ; Li, Yan ; Stubbins, Aron ; Li, Yizhen ; Song, Guodong ; Wang, Lei ; Cheng, YuanyueThe Kuroshio intrusion from the West Philippine Sea (WPS) and mesoscale eddies are important hydrological features in the northern South China Sea (SCS). In this study, absorption and fluorescence of dissolved organic matter (CDOM and FDOM) were determined to assess the impact of these hydrological features on DOM dynamics in the SCS. DOM in the upper 100 m of the northern SCS had higher absorption, fluorescence, and degree of humification than in the Kuroshio Current of the WPS. The results of an isopycnal mixing model showed that CDOM and humic-like FDOM inventories in the upper 100 m of the SCS were modulated by the Kuroshio intrusion. However, protein-like FDOM was influenced by in situ processes. This basic trend was modified by mesoscale eddies, three of which were encountered during the fieldwork (one warm eddy and two cold eddies). DOM optical properties inside the warm eddy resembled those of DOM in the WPS, indicating that warm eddies could derive from the Kuroshio Current through Luzon Strait. DOM at the center of cold eddies was enriched in humic-like fluorescence and had lower spectral slopes than in eddy-free waters, suggesting inputs of humic-rich DOM from upwelling and enhanced productivity inside the eddy. Excess CDOM and FDOM in northern SCS intermediate water led to export to the Pacific Ocean interior, potentially delivering refractory carbon to the deep ocean. This study demonstrated that DOM optical properties are promising tools to study active marginal sea-open ocean interactions.