CMIP5 Model Intercomparison of Freshwater Budget and Circulation in the North Atlantic

Abstract

The subpolar North Atlantic is a center of variability of ocean properties, wind stress curl, and air-sea exchanges. Observations and hindcast simulations suggest that from the early 1970s to the mid-1990s the subpolar gyre became fresher while the gyre and meridional circulations intensified. This is opposite to the relationship of freshening causing a weakened circulation, most often reproduced by climate models. The authors hypothesize that both these configurations exist but dominate on different time scales: a fresher subpolar gyre when the circulation is more intense, at interannual frequencies (configuration A), and a saltier subpolar gyre when the circulation is more intense, at longer periods (configuration B). Rather than going into the detail of the mechanisms sustaining each configuration, the authors' objective is to identify which configuration dominates and to test whether this depends on frequency, in preindustrial control runs of five climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). To this end, the authors have developed a novel intercomparison method that enables analysis of freshwater budget and circulation changes in a physical perspective that overcomes model specificities. Lag correlations and a crossspectral analysis between freshwater content changes and circulation indices validate the authors' hypothesis, as configuration A is only visible at interannual frequencies while configuration B is mostly visible at decadal and longer periods, suggesting that the driving role of salinity on the circulation depends on frequency. Overall, this analysis underscores the large differences among state-of-the-art climate models in their representations of the North Atlantic freshwater budget.

Predicting a Decadal Shift in North Atlantic Climate Variability Using the GFDL Forecast System

“…6). Importantly, for the mechanism described in this study, an increase in the SPG salinity tends to precede the intensification of the AMOC in CM2.1, as described by Deshayes et al (2014) for the GFDL CM3 model, where the ocean component is quite similar to that of CM2.1, with the same sensitivity to salinity (not shown). 8).…”

“…The evolution after initialization indicates an increase in salinity consistent with the increased transport from the subtropics, but in these predictions, the anomalies remain negative (Fig. Deshayes et al (2014) showed that the relationship between AMOC and salinity in climate models actually depends on the time scale of interest and on the model's internal variability, stressing the need for additional continuous observations of SPG salinity to fully address this issue. In contrast, in the 1995 hindcasts, the ocean is initialized with slightly positive salinity anomalies both at the surface and over the upper 600 m, which further increase during the hindcast period (Fig.…”

“…One would expect the magnitude and timing of the North Atlantic predicted warming to depend on the model used to run the predictions, as well as on the ocean initialization and details of the assimilation scheme (Bellucci et al 2013). The timing of the shift in the predictions will therefore likely depend on the adjustment response of the simulated AMOC to NAO forcing (Eden and Willebrand 2001;Deshayes and Frankignoul 2008). The timing of the shift in the predictions will therefore likely depend on the adjustment response of the simulated AMOC to NAO forcing (Eden and Willebrand 2001;Deshayes and Frankignoul 2008).…”

North Atlantic Multi-Decadal Variability — Mechanisms and Predictability

“…The relative importance of these processes on longer timescales is not clear, and differs among CGCMs (Ba et al, 2013b); in some models salinity variations are key to AMV (Ba et al, 2013a), while others suggest AMV is purely temperature driven (Dijkstra et al, 2006). Furthermore, the impact of salinity is likely timescale dependent (Deshayes et al, 2013). This sensitivity may arise because temperature and salinity anomalies affect density in opposite ways -salinity increasing density, temperature decreasing it -with temperature controlling their relative importance.…”

Influence des régimes de temps atmosphériques sur la circulation océanique de l'Atlantique Nord

“…Néanmoins, si l'effet des anomalies de salinité sur la densité est compensé par l'effet des anomalies de température, concomitant et de signe opposé, alors il est possible que la circulation thermohaline s'intensifie alors même que la salinité diminue dans l'Atlantique subpolaire. Deshayes et al (2014) La Météorologie -n°87 -novembre 2014 lement appelés flux de flottabilité ; Penduff et al, 2011). Mais les liens unissant variabilité atmosphérique et variabilité océanique sont difficiles à mettre en évidence.…”