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ATLAS Deliverable 1.5: Recent Mediterranean Outflow Water and Atlantic Meridional Overturning Circulation correlations

Mosquera Giménez, Ángela; Vélez-Belchí, Pedro

The ATLAS project works towards improving understanding of the North Atlantic Ocean, in particular, how it is connected, how it functions and how it will respond to future changes. The first step to achieve this goal is the study of the Atlantic Meridional Overturning Circulation (AMOC), since it connects deep-sea ecosystems with the transport of heat, salt, and energy in the whole Atlantic Ocean.
The AMOC is the main contributor to the heat interchange in the North Atlantic (Kanzow et al., 2010). It consists, from a simplified point of view, of an upper transport of warm water to the north and a deep transport of cold water to the equator. Thus, the AMOC carries an important amount of heat, approximately 1.5 PW at 25°N, from the equator to high latitudes (Huisman et al., 2010), helping to regulate the climate, especially in Europe (Bryden et al., 2005). Any slowdown in the AMOC, due to both the natural variability and the anthropogenic climate change, would significantly reduce temperature in the regions around the North Atlantic and even in other parts of the world (Vellinga et al., 2002).
The strength of the AMOC is affected by the input of freshwater in the North Atlantic. Light, fresh water entering in the North Atlantic would produce a reduction in the transport of the AMOC (Willis, 2010), while a supply of salty water, such as the Mediterranean Outflow Water (MOW), would enhance the formation of deep water (Rahmstorf, 1998), strengthening the AMOC (Reid, 1978, 1979; Artale et al., 2002; Cacho et al., 2000), and even stabilizing it (Ivanovic et al., 2014).
The MOW is an intermediate water mass, salty and warm, that is formed when dense Mediterranean water passes through the Strait of Gibraltar and mixes with the North Atlantic Central Water (Daniault et al., 1994). This water mass spreads into the North Atlantic at an average depth of about 1100 m, and its signal can be found as far as Bermuda in the west and Rockall Trough in the north (Bozec et al., 2011). The interchange of water between the Mediterranean and the Atlantic depends on two factors: the geometry of the Strait of Gibraltar, which regulates the total volume of water able to physically pass from one basin to the other, and the density gradient, which contributes to controlling salinity and temperature exchange (Ivanovic et al., 2014).
In this report, in order to determine if the variation of the volume of the MOW is having any effect on the fluctuation of the transport of the AMOC, we will use data from the Argo observing system and RAPID arrays to estimate if there is correlation between the inter-annual to decadal MOW distribution in the eastern Atlantic and the components of the AMOC, in particular with the upper mid-ocean transport (UMO).

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