Salinity Biases and the Variability of the Atlantic Meridional Overturning Circulation in GFDL-CM3
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Article pISSN 1738-5261 eISSN 2005-7172
Salinity Blases and the Variablity of the Atlantic Meridional Overturning Circulation in GFDL-CM3 Fernanda D. A. O. Matos1*, Janini Pereira2, and Marcus Dengler3 1
Graduate Program in Geophysics, Geosciences Institute, Federal University of Bahia, Salvador 40170-280, Brazil Department of Earth and Environmental Physics, Physics Institute, Federal University of Bahia, Salvador 40170-280, Brazil 3 GEOMAR Helmholtz Center for Ocean Research Kiel, Kiel D-24105, Germany 2
Received 26 May 2020; Revised 19 August 2020; Accepted 23 August 2020 © KSO, KIOST and Springer 2020
Abstract − The variability of the Atlantic Meridional Overturning Circulation (AMOC) and the Meridional Freshwater Transport (Mov) at 24°N, 11°S, and 30°S, simulated with the GFDL-CM3 model under the historical (1860–2005) and RCP 4.5 and 8.5 scenarios (2006–2100) are examined. The results are compared with the climatology and observations in the search for salinity biases that would compromise the accuracy of the state of the future climate predicted by this model. The AMOC wavelet analysis shows a predominant decadal variability at 24°N, whereas the annual signal is more prominent in the South Atlantic. The Mov magnitude varies considerably among the latitudes: increasing at 24°N, rather stable at 11°S and decreasing at 30°S. Such a pattern is followed by an AMOC weakening regardless of the latitude until the end of the 21st century, indicating that a reduction in Mov in the southern South Atlantic and an increase in the North Atlantic are potential drivers of the AMOC destabilization. Both model and observations agree on the stability of the circulation, even though the model projects a stronger AMOC than the latter. Besides the AMOC weakening, a shoaling of its upper limb is also observed. Salinity biases were found mostly in the South Atlantic in the upper 1000 m of the water column, with the model results significantly deviating from the observations and climatology in the mid-Atlantic, especially at 30°S. Possible causes for the abovementioned results, including potential drivers for salinity biases are discussed. Keywords − Atlantic meridional overturning circulation, meridional dreshwater transport, GFDL-CM3, salinity biases
1. Introduction The Atlantic Meridional Overturning Circulation (AMOC) consists of an upper limb of warm and saline waters moving *Corresponding author. E-mail: [email protected]
northward and a cold and dense lower limb returning southward, comprising the North Atlantic Deep Water (NADW) (Cheng et al. 2013; Buckley and Marshal 2016). The maximum AMOC transport is observed in its upper cell, at around 1200 m and within 30°N to 65°N in the Atlantic (McCarthy et al. 2017). The AMOC can be directly destabilized by fluctuations in heat and freshwater fluxes and is mainly driven by buoyancy gains and losses (Lozier 2012). Thus, during the process of buoyancy gain (loss), either due to an increase (reduction) in temperature or a r
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