Over the past few decades, various algorithms have been developed for the retrieval of water constituents from the
measurement of ocean color radiometry, and one of those approaches is spectral optimization. This approach defines an
error function (or cost function) between the observed spectral remote sensing reflectance and an estimated spectral
remote sensing reflectance over the range of observed wavelengths, with the latter modeled using a few variables that
represent the optically active properties (such as the absorption coefficient of phytoplankton and the backscattering
coefficient of particles). The values of the variables when the error function reaches a minimum are the optimized
properties. The applications of this approach implicitly assume that there is only one global minimum condition, and that
any local minimum (if exist) can be avoided through the numerical optimization scheme. Here, with data from numerical
simulations, we show the shape of the error surface as a mechanism to visualize the solution space for the model
variables. Further, using two established models as examples, we demonstrate how the solution space changes under
different model assumptions as well as the impacts on the quality of the retrieved water properties.
Three models were used to estimate primary productivity (PP) in the Southern Ocean for the summer of 2003-2007.
They are the widely accepted model VGPM, a carbon-based model CbPM and a new type of model which uses
phytoplankton absorption coefficient as input variable in stead of chlorophyll concentration. It was found that the degree
of agreement among the results from three models was low, but the difference appeared relatively small with regard to
previous reports. Nevertheless, the results were comparable to that from a PP model parameterized specifically for use in
Southern Ocean waters. Among the three models, the output from CbPM differed the most from that estimated by the
other two models. The different PP estimates were primarily attributed to the different ways these models treat
phytoplankton physiology, along with the difference in input variables.
This study deals with the interannual variation of summer upwelling in the Taiwan Strait (TS), based on the empirical
orthogonal function analysis. NOAA AVHRR sea surface temperature dataset from 1985-2005 and hydrographic records
at two coastal stations from 1970-2001 are used. The results indicate that the first mode (85.3%) of the spatial variance
shows a persistent front, which is generally aligned northeast-southwestward in the western TS. The eigenvector time
series show that the variability of this front with time is closely correlated with the change in the wind stress anomaly of
alongshore wind component derived from 17 years of ERS and QuickSCAT wind dataset from 1992-2005. The records
of the water temperature and salinity anomaly at Pingtan Is. located in the northwestern TS, and Dongshan Is. located in
the southwestern TS show that a negative temperature anomaly appears along with a positive salinity anomaly in some
years. This suggests a dominant influence of cold and saline upwelling water at the surface. The years for notable
cooling events derived from the station measurements are generally consistent with the time series of EOF Mode 1.
These results indicate that for the entire west TS, the summer coastal upwelling was strong in 1987, 1993, and 1998
during the period from 1985 to 2005. A delayed ENSO effect is suggested as a major mechanism for the interannual
variability of TS coastal upwelling.
This study employs SeaWiFS data over the waters off the southeastern China to evaluate a semi-analytical algorithm for
euphotic zone depth (Ze). This algorithm is based on water's inherent optical properties (IOPs), which can be
near-analytically calculated from spectral remote-sensing reflectance, where remote-sensing reflectance can be derived
from the normalized water-leaving radiance provided by SeaWiFS. In the Taiwan Strait, compared with in situ Ze (±3
hour within SeaWiFS collection), average error (ε) is 15.0 % and root mean square error (RMSE) is 0.074, with Ze in a
range of 14-34 m from field measurements. In the South China Sea, compared with in situ Ze (±48 hour within SeaWiFS
collection),ε is 5.1 % in summer and 22.6 in winter, while RMSE is 0.032 in summer and 0.129 in winter, with Ze in a
range of 10-82 m from field measurements. For comparison, we also evaluate the performance of the empirical Ze
algorithm that is based on chlorophyll concentration. It is found that the IOP-centered approach has higher accuracy
compared to the chlorophyll-a centered approach (e.g. in the South China Sea in winter, ε is 55.3 % and RMSE is 0.219).
The new algorithm is thus found not only worked well with waters of the Gulf of Mexico, Monterey Bay and the Arabian
Sea, but also worked well with waters of the China Sea.
SeaWiFS SeaWiFS Chl and AVHRR SST time-series in August, 1998 were used to evaluate short-term variability of Chl associated with upwelling events in the western Taiwan Strait. Extents of eutrophic waters (SeaWiFS Chlorophyll > 1mg/m3) and extents of colder than non-upwelling waters were calculated for the western strait and for the north and south portions, respectively. High extents of eutrophic waters were always accompanied by high extents of colder than nonupwelling waters, indicative of tight coupling of Chl with SST evolution and thus with upwelling activities. Only one-day lag of phytoplankton growth to upwelling was detected. The temporal patterns of upwelling events were found different in the northwestern and southwestern Taiwan Strait. In the north portion, a short relaxation of upwelling probably occurred between early and mid-August. One unique strong upwelling event was likely going from early through mid-August, peaking before Aug. 13th in the south portion. It resulted in chlorophyll enhancement developing and reaching peaks not concurrently in these two upwelling zones. The duration of one upwelling event in the western Taiwan Strait in August was estimated to be ca. 12 days. Two distinctive upwelling systems located in the northwestern and southwestern Taiwan Strait were further inferred.