Future Impact of Weakening Atlantic Thermohaline Circulation on the Loop Current in the Gulf of Mexico

The IPCC-AR4 climate model simulations under SRESA1B scenario project that the upper ocean temperature in the Gulf of Mexico (GoM) may increase by approximately 2oC between 2000 and 2100. The projected warming of the GoM cannot be explained by the atmospheric forcing alone because the anomalous net surface flux derived from the IPCC-AR4 climate models slightly cools the sea surface in the GoM. Therefore, ocean dynamics must play an important role to account for the projected warming in the GoM. However, the IPCC-AR4 climate models have typical spatial resolution of about one degree. Thus, the inflow-outflow system of the GoM that connects the Yucatan Current, Loop Current, and Florida Current are poorly resolved. The IPCC-AR4 climate models project that the Atlantic thermohaline circulation may slow down about 25% between 2000 and 2100. Since the Atlantic western boundary current system, including the Loop Current, is an important pathway of the Atlantic thermohaline circulation, it is expected that the Loop Current be also reduced. However, it is unclear if and how the reduced Loop Current contributes to the projected warming of the GoM.

To have a detailed and reliable assessment of the regional impact of global climate change on the ocean circulation and water mass characteristics of GoM, dynamic downscaling of the IPCC-AR4 climate models to the GoM region is performed by using the Hybrid Coordinate Ocean Model coupled to Atmospheric Mixed Layer (HYCOM-AML). The model domain for the HYCOM-AML contains the Atlantic Ocean between 100E and 20E bounded north and south by 50N and 20S, respectively. A multi-level nested-domain system with about 10km resolution is used in the GoM and lower resolution elsewhere. The regional HYCOM-AML will be initialized and integrated for each decade-averaged period between 2000 and 2100 by using the atmospheric forcing and initial-boundary conditions derived from the IPCC-AR4 climate model simulations under three CO2 emission scenarios. The impact of the climate change on the strength, position and eddy characteristics of the Loop Current is assessed. Potential implications of the Loop Current changes, under a range of climate change scenarios, on bluefin tuna spawning patterns are also discussed.