Global models for investigation of phytoplankton blooms in the Gulf of Oman and the northwest of Arabian Sea
This study evaluates the performance of Earth system models for accurately simulating the phytoplankton productivity and bloom dynamics in the Oman Sea and the northwest of Arabian Sea. Satellite data (SeaWIFS ocean color) show two climatological blooms in this region, a wintertime bloom peaking in February and a summertime bloom peaking in September. On a regional scale, interannual variability of the wintertime bloom is dominated by cyclonic eddies which vary in location from year to year. During the wintertime, while both cooling in the winter and eddies control the blooms, variability in bloom location will arise from variability in the location of eddies, and so may not be predictable. In contrast, during the Southwest Monsoon, the dominant upwelling associated with the intense environmental forcing supersedes the effects of eddies, and the activity of the cold eddies is not pronounced. We consider numerical results from five different 3-D global Earth system models, which are denoted by CORE-TOPAZ, Coupled-TOPAZ, Coupled-BLING, Coupled-miniBLING, and the Geophysical Fluid Dynamics Laboratory (GFDL) Climate Model version 2.6 (CM2.6 miniBLING). Two coarse (1° grid resolution) models with a relatively complex biogeochemistry (TOPAZ: Tracers of Ocean Productivity with Allometric Zooplankton) capture the annual cycle but fail to capture both the eddies and the interannual variability. The results showed that the models differ from the observational data in terms of interannual variability. The low-resolution models (CORE- and coupled-TOPAZ) provide an almost uniform seasonal coefficient of variation, while both the data and eddy resolving CM2.6 models show higher interannual variability and seasonal changes. The coefficients of variabilities are particularly higher during the winter and summer blooms in the observations, while the low-resolution models do not see these signals. In other words, the low-resolution models fail to attain enough variability, while the high-resolution models (i.e. CM2.6) produce too much interannual variability. Accordingly, eddies are necessary to explain the variability in the data as opposed to the low-resolution models, but that the high-resolution model does not properly capture this variability. An eddy-resolving model (GFDL CM2.6) with a simpler biogeochemistry (miniBLING) displays larger interannual variability, but overestimates the wintertime bloom and captures eddy-bloom coupling in the south but not in the north. The models fail to capture both the magnitude of the wintertime bloom and its modulation by the eddies in part because of their failure to capture the observed sharp thermocline/nutricline in this region. In the wintertime, this leads to the excessive convective supply of nutrients and too strong of a bloom. However, for a few cases, eddies with blooms at the center are tracked in the southern part of the domain. For the model to simulate the observed wintertime blooms within cyclones, it will be necessary to represent this relatively unusual nutrient structure as well as the cyclonic or cold eddies. Both the temperature and mixed layer biases in the northern part of the Arabian Sea may result from having too much water from the Persian Gulf in this region. This is a challenge in the northern Arabian Sea as it requires capturing the details of the outflow from the Persian Gulf, something that is poorly done in global models.
Iranian Journal of Geophysics, Volume:10 Issue:4, 2017
62 - 74
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