Abstract
Abstract. Recent years have shown an increase in harmful algal blooms in the Northwest Arabian Sea and Gulf of Oman, raising the question of whether climate change will accelerate this trend. This has led us to examine whether the Earth System Models used to simulate phytoplankton productivity accurately capture bloom dynamics in this region – both in terms of the annual cycle and interannual variability. 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 one year to another. Two coarse (1°) models with the relatively complex biogeochemistry (TOPAZ) capture the annual cycle but neither eddies nor the interannual 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 eddies in part because of their failure to capture the observed sharp thermocline and/or nutricline in this region. When CM2.6 is able to capture such features in the Southern part of the basin, eddies modulate diffusive nutrient supply to the surface (a mechanism not previously emphasized in the literature). 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 eddies. 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.
Highlights
The region of northwestern Arabian Sea and the Gulf of Oman (15–26◦ N, 56–66◦ E) is a highly productive region (Madhupratap et al, 1996; Tang et al, 2002), with satellite estimates of carbon export of 137 gC m−2 yr−1, much higher than the ∼ 80 gC m−2 yr−1 found in the Subpolar North Atlantic and Pacific (Dunne et al, 2007)
We examine the relationship of blooms and eddies using the GSM5 Maritorena et al (2002) product based on the SeaWIFS (Sea-viewing Wide Field-of-view Sensor) ocean color data and Sea Surface Height Anomaly (SSHA), based on altimeter data acquired from the Archiving, Validation and Interpretation of Satellite Oceanographic (AVISO) Data Center
Satellite analyses demonstrate the existence of two blooms, the stronger one associated with the Southwest Monsoon and the weaker one associated with the Northeast Monsoon as shown by Madhupratap et al (1996); Kawamiya and Oschlies (2003); Murtugudde et al (2007); and Al-Azri et al (2010)
Summary
The region of northwestern Arabian Sea and the Gulf of Oman (15–26◦ N, 56–66◦ E) is a highly productive region (Madhupratap et al, 1996; Tang et al, 2002), with satellite estimates of carbon export of 137 gC m−2 yr−1, much higher than the ∼ 80 gC m−2 yr−1 found in the Subpolar North Atlantic and Pacific (Dunne et al, 2007). Piontkovski et al (2012) suggested that the increased amplitude of the seasonal cycle of chlorophyll a might be associated with the increased variability of mesoscale eddy kinetic energy (EKE) per unit mass in the Gulf of Oman or in the western Arabian Sea. Gomes et al (2008) noted potential anticorrelation between sea surface height and chlorophyll, but did not find a consistent relationship over time. Since the model only simulates such a feature in the Southern Arabian Sea, it does not capture the observed relationship between SSH and biology Both the overestimation of the wintertime bloom and the failure to predict its modulation by eddies can be traced to difficulties in modeling the stratification of the Northwest Arabian Sea, most likely as a result of a failure to properly simulate overflows
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