Li Xingwen

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Li
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Xingwen
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  • Article
    An adjoint sensitivity study of chlorofluorocarbons in the North Atlantic
    (American Geophysical Union, 2004-01-03) Li, Xingwen ; Wunsch, Carl
    Adjoint sensitivities of CFC-11 concentrations and CFC-11/CFC-12 ratio ages in a North Atlantic general circulation model are analyzed. These sensitivities are compared with those of spiciness, T − (β/α) S, where α, β are the thermal and haline expansion coefficients, respectively. High-sensitivity fields are candidates for providing the most powerful constraints in the corresponding inverse problems. In the dual (adjoint) solutions all three variables exhibit the major ventilation pathways and define the associated timescales in the model. Overall, however, spiciness shows the highest sensitivity to the flow field. In the North Atlantic Deep Water, sensitivities of CFC properties and spiciness to the isopycnal mixing and thickness diffusion are of the same order of magnitude. In the lower subtropical thermocline, sensitivities of CFC properties to the isopycnal mixing and thickness diffusion are higher. The utility of this sensitivity is undermined by the need to reconstruct their boundary conditions. Given the influence of T, S measurements on the density field, they produce the most powerful constraints on the model on the large scale. It still remains possible, however, that transient tracers can provide a larger relative information content concerning the mixing process between the near-surface boundary layer and the thermocline but dependent upon the ability to reconstruct accurate initial and boundary conditions.
  • Preprint
    Biological control of the vernal population increase of Calanus finmarchicus on Georges Bank
    ( 2006-03-14) Li, Xingwen ; McGillicuddy, Dennis J. ; Durbin, Edward G. ; Wiebe, Peter H.
    An adjoint data assimilation approach was used to quantify the physical and biological controls on Calanus finmarchicus N3 to C stages on Georges Bank and its nearby environs. The mean seasonal cycle of vertically-averaged distributions, from 5 years of the GLOBEC Georges Bank Broad-Scale Surveys between January and June, was assimilated into a physical-biological model based on the climatological circulation. Large seasonal and spatial variability is present in the inferred supply sources, mortality rates, computed molting fluxes, and physical transports. Estimated mortalities fall within the range of observed rates, and exhibit stage structure that is consistent with earlier findings. Inferred off-bank initial conditions indicate that the deep basins in the Gulf of Maine are source regions of early-stage nauplii and late-stage copepodids in January. However, the population increase on Georges Bank from January to April is controlled mostly by local biological processes. Magnitudes of the physical transport terms are nearly as large as the mortality and molting fluxes, but their bank-wide averages are small in comparison to the biological terms. The hypothesis of local biological control is tested in a sensitivity experiment in which upstream sources are set to zero. In that solution, the lack of upstream sources is compensated by a decrease in mortality that is much smaller than the uncertainty in observational estimates.
  • Thesis
    Constraining the North Atlantic circulation with transient tracer observations
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2003-02) Li, Xingwen
    The capability of transient tracers to constrain the ocean circulation in the North Atlantic is explored. Study of an idealized tracer shows that inferences of circulation properties from transient state distributions are impacted by uncertainties in the time-varying boundary conditions and sparse data coverage. Comparison of CFC, tritium, temperature and salinity (T-S) observations with model results in the North Atlantic shows that regions of important model-data disagreements in the transient tracer fields can also be readily identified in the T-S distributions. In the model, excessive vertical penetration of convective adjustment, leads to problematic production and outflow of the NADW, again appearing in both transient tracer and T-S fields. Sensitivities of the model fields are determined using the adjoint model. In the dual solutions, CFC-ll, CFC-ll/CFC-12 ratio age, and T - (ß/α)S (α and ß are thermal and haline expansion coefficients, respectively) exhibit the major ventilation pathways and the associated timescales, in the model. High sensitivity fields are candidates for providing the most powerful constraints in the corresponding inverse problems. Assimilation of both CFC and tritium data, with different input histories, sampling distributions, and radioactive decay constants, shows that by adjusting only initial-boundary conditions of CFCs and tritium, a 1° x 1° offline model and the transient tracer data can be brought into near-consistency, in the domain between 4.5°S and 39.5°N of the North Atlantic. Constraining a GCM with transient tracers is thus fully practical. However, the large uncertainties in the time-varying boundary conditions of transient tracer concentrations, and in their interior distributions, renders the transient tracers less-effective in determining the circulation than are more conventional steady tracers, and known oceanic dynamics.