Abstract
We investigate the effect of variable marine biogeochemical light absorption on Indian Ocean sea surface temperature (SST) and how this affects the South Asian climate. In twin experiments with a regional Earth System Model, we found that the average SST is lower over most of the domain when variable marine biogeochemical light absorption is taken into account, compared to the reference experiment with a constant light attenuation coefficient equal to 0.06 m-1. The most significant deviations (more than 1 °C) in SST are observed in the summer period. A considerable cooling of subsurface layers occurs, and the thermocline shifts upward in the experiment with the activated biogeochemical impact. Also, the phytoplankton primary production becomes higher, especially during periods of winter and summer phytoplankton blooms. The effect of altered SST variability on climate was investigated by coupling the ocean models to a regional atmosphere model. We find the largest effects on the amount of precipitation, particularly during the monsoon season. In the Arabian Sea, the reduction of the transport of humidity across the equator leads to a reduction of the large-scale precipitation in the eastern part of the basin, reinforcing the reduction of the convective precipitation. In the Bay of Bengal, it increases the large-scale precipitation, countering convective precipitation decline. Thus, the key impacts of including the full biogeochemical coupling with corresponding light attenuation, which in turn depends on variable chlorophyll-a concentration, include the enhanced phytoplankton primary production, a shallower thermocline, decreased SST and water temperature in subsurface layers, with cascading effects upon the model ocean physics which further translates into altered atmosphere dynamics.
Highlights
The vulnerability and the ability of society and natural systems to adapt to the impact of climate change vary significantly according to geographic regions and populations
The key impacts of including the full biogeochemical coupling with corresponding light attenuation, which in turn depends on variable chlorophyll-a concentration, include the enhanced 25 phytoplankton primary production, a shallower thermocline, decreased sea surface temperature (SST) and water temperature in subsurface layers, with cascading effects upon the model ocean physics which further translates into altered atmosphere dynamics
A regional Earth System Model based on the ROM model (Sein et al, 2015) has been implemented for the CORDEX South 605 Asia region
Summary
The vulnerability and the ability of society and natural systems to adapt to the impact of climate change vary significantly according to geographic regions and populations. The warming of the eastern equatorial Pacific due to the influence of biological productivity was reported by (Lengaigne et al, 2007), who 70 compared fully coupled ocean-atmosphere-biogeochemistry model experiment with fixed-chlorophyll model experiment They discussed the inconsistency between the results of forced ocean models and the fully coupled models, suggesting that the impact of marine biogeochemistry upon SST and corresponding cooling/warming is related to the way radiation is treated in the control experiments. These models do not well account for small scale dynamics and often yield a spatially smoothed picture of phytoplankton dynamics In this study, both the atmospheric as well as the oceanic components are in a resolution that fully provides the added value of regionalization compared to global models. This has been demonstrated previously for the North Atlantic as well as the NW European shelf seas (Sein et al, 2015; Sein et al, 2020)
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