Challenges in modelling spatiotemporally varying phytoplankton blooms in the 1 Northwestern Arabian Sea and Gulf of Oman 2 3

17 Recent years have shown an increase in harmful algal blooms in the Northwest Arabian Sea 18 and Gulf of Oman, raising the question of whether climate change will accelerate this trend. 19 This has led us to examine whether the Earth System Models used to simulate phytoplankton 20 productivity accurately capture bloom dynamics in this regionboth in terms of the annual 21 cycle and interannual variability. Satellite data (SeaWIFS ocean color) shows two 22 climatological blooms in this region, a wintertime bloom peaking in February and a 23 summertime bloom peaking in September. On a regional scale, interannual variability of the 24 wintertime bloom is dominated by cyclonic eddies which vary in location from one year to 25 another. Two coarse (1°) models with the relatively complex biogeochemistry (TOPAZ) 26 capture the annual cycle but neither eddies nor the interannual variability. An eddy-resolving 27 model (GFDL CM2.6) with a simpler biogeochemistry (miniBLING) displays larger 28 interannual variability, but overestimates the wintertime bloom and captures eddy-bloom 29

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