Abstract
The Atlantic meridional overturning circulation (AMOC) is one of the main drivers of climate variability at decadal and longer time scales. As there are no direct multi-decadal observations of this key circulation, the reconstruction of past AMOC variations is essential. This work presents a step forward in reconstructing the AMOC using climate models and time-varying surface nudging of salinity and temperature data, for which independent multi-decadal observed series are available. A number of nudging protocols are explored in a perfect model framework to best reproduce the AMOC variability accommodating to the characteristics of SST and SSS available products. As reference SST products with sufficient space and time coverage are available, we here choose to focus on the limitations associated to SSS products with the goal of providing protocols using independent salinity products. We consider a global gridded dataset and, additionally, a coarser SSS dataset restricted to the Atlantic and with a quite low spatial resolution (order of 10 degrees vs. 2 for the model grid). We show how, using the latter, we can improve the efficiency of the nudging on the AMOC reconstruction by adding a high-resolution annual cycle to the coarse resolution SSS product as well as a spatial downscaling to account for SSS gradient. The final protocol retained for the coarse SSS data is able to reconstruct a 100-year long AMOC period (average of 10.18 Sv and a standard deviation of 1.39 Sv), with a correlation of 0.76 to the target and a RMSE of 0.99 Sv. These values can be respectively compared to 0.85 and 0.75 Sv when using the global salinity surface observations. This work provides a first step towards understanding the limitations and prospects of historical AMOC reconstructions using different sea surface salinity datasets for the surface nudging.
Highlights
The Atlantic meridional overturning circulation (AMOC), as an element of the ocean circulation, is a key component of the North Atlantic climate system (Buckley and Marshall 2016)
Salinity exerts a strong impact on vertical stability at these latitudes (Swingedouw et al 2008) and sea surface salinity (SSS) nudging plays a key role for sea surface temperature (SST) reconstruction in this region, causing a reduced correlation of the SST in the experiments with reduced SSS nudging (Atl-Full and FDS-Full)
The protocol is developed in a perfect model framework using sea surface salinity and temperature nudging within the IPSL-CM5A2 model
Summary
The Atlantic meridional overturning circulation (AMOC), as an element of the ocean circulation, is a key component of the North Atlantic climate system (Buckley and Marshall 2016). Reconstructing the climate system using surface-only air–sea data and a climate model has been previously attempted (Keenlyside et al 2008; Zhang et al 2010) Including such information, improved the reconstruction of ocean interior estimates such as Labrador Sea water, which is directly linked to AMOC variability. These studies were later improved in perfect model framework using information from the SST, and from the SSS of the target period. Servonnat et al (2014) and Ortega et al (2017) showed that this can be sufficient to reconstruct AMOC modulations, in particular large variations and peaks Following these studies, the sea–ice covered regions are excluded from the nudging to avoid spurious oceanic convection and physical disruption of the sea ice equilibrium. This additional term can be written on the right hand side of the equations describing the time evolution of SST and SSS respectively: SSTmodel t
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