Regional downscale of projection of sea level rise in the Indian Ocean based on CMIP6 simulations
SAJIDH CK*, ABHISEK CHATTERJEE
Indian National Centre for Ocean Information Services
Indian national centre for ocean information services, Pragathi Nagar, Nizampet, Hyderabad, Telangana, India Pincode: 500090 Mobile: 9633557826 E-mail: firstname.lastname@example.org
The unabated global sea-level change puts human lives at risk of coastal calamities around the globe. The impact is particularly devastating for the heavily populated south Asian countries which is the home of more than 1/3rd world population. The rise in global mean sea level is reported to be around 3.1 mm/yr in the last two decades and is expected to rise by more than 1 m by the end of this century. More importantly, the regional sea level change differs significantly from the global mean change. Therefore, at a local level, for the policymakers, these global mean assessments are of little use. In order to implement the mitigation policies against the climate change risks, the regional assessment of sea level rise and associated extremes are of primary importance.
This study aimed to estimate more representative dynamic sea level change estimates for the Indian Ocean by selecting a subset of best performing models among the available models from phase six of the coupled model intercomparison project (CMIP6) simulations. A total of 27 models are analysed using basic statistical metrics and a multi-variate skill score to judge their fidelity in simulating the Indian Ocean historical sea level and associated physical processes. The best performing models were then used to estimate the future projection of sea level change for the Indian Ocean under the SSP2-4.5 and SSP5-8.5 scenarios.
Our analysis suggests that the multi-model mean (MMM) of the dynamic sea level (DSL) compares well with the observation. However, there are few notable consistent biases across all models which are also reflected in the MMM. In the north Indian Ocean (NIO), all models produce positive DSL anomaly in the western basin and negative in the east similar to the SST bias observed earlier. The sharp change in the DSL from positive to negative driven by zero windstress curl in the south Indian Ocean is stronger in the MMM driven primarily by the consistent positive bias in the windstress curl in the south tropical Indian Ocean (STIO) owing to the stronger southwesterly tread winds in the subtropics and westerlies in the south. Overall, all the models show positive bias in the zonally averaged DSL across the basin. The skill score of the models in reproducing mean DSL is quite high for the entire Indian Ocean with a skill score of more than 0.95 as was earlier noted for the global ocean. However, it varies significantly within the basin with a large spread in the NIO and SIO between 0.75-0.98 and relatively better in the STIO. The large spread in the NIO and SIO is primarily due to a large bias in the standard deviation in the DSL of these models. The mean windsress curl shows reasonably good comparison with a skill score between 0.6-0.8 for the Indian Ocean with a much higher skill score in the SIO for most of the models. For the equatorial belt, all the models underestimate the west-east sea level gradient compared to the observation owing to the anomalous easterlies in majority of the models. This resulted in stronger IOD like features in the tropical belt of the Indian Ocean making them questionable for interannual/decadal and long-term projection in this equatorial regime and associated climatic internal variabilities of the Indian Ocean. This IOD like bias is also reflected in the much larger dominance of the first mode of the empirical orthogonal function.
In general, the high resolution models perform better in almost all the statistical parameters. Particularly, among all the 27 models, MPI-ESM1-2-HR, HadGEM-G3-MM, CMCC-CM2-HR4 and CNRM-CMR1-HR performance was found to be superior across the Indian Ocean. The projected change in the sea level suggests a consistent rising pattern in the western NIO along the coast of Somalia and Arabia and along the west and east coast of India. The overall pattern of sea level rise remains the same for the SSP2-4.5 and SSP5-8.5 scenarios, but with a higher magnitude under the high emission situation.