Sensitivity of the Atlantic meridional overturning circulation (AMOC) and variability to Indian ocean temperature

Abstract:

We use an alternative approach to study the Atlantic meridional overturning circulation (AMOC) sensitivity wherein AMOC intensity is modulated remotely through the teleconnection from the tropical Indian Ocean (TIO). Using the latest coupled-model from Insitut Pierre Simon Laplace (IPSL-CM6), we have designed ensemble experiments nudging the surface temperatures of the TIO by -2°C, +1°C, and +2°C for 100 years, extending for several hundred years. We then investigate the AMOC response, including its variability, on decadal to centennial timescales. We find that the initial changes in AMOC (≤30 years) are largely atmospherically driven by changes in the North Atlantic forced by atmospheric planetary waves induced from the TIO, while on longer timescales AMOC changes are largely driven salinity changes arriving to high-latitudes from the tropical Atlantic. Ultimately, nudging the TIO surface temperature induces a roughly 8 Sv AMOC response per 1°C of warming. We also find that the strengthening of mean AMOC also increases AMOC decadal and longer variability. Further, we explore the changes in water mass transformation rates in the Labrador and Nordic seas that induce those AMOC changes. Water mass transformation in the Labrador Sea is largely driven by the surface heat fluxes and act as a key component to increasing the mean AMOC on shorter timescales. The Nordic Sea transformation responds after the Labrador Sea and is driven through both heat and water flux components of the buoyancy transformation. Exploring an asymmetric response in IPSL-CM6, increasing the mean AMOC state is largely driven through the Labrador Sea, while decreasing AMOC is attributed, in part, to the expansion of sea ice covering the Nordic Sea convective region.