North Atlantic Sub‐Polar Gyre Climate Index: A New Approach

Abstract

As decadal predictions become operational, the need to use, understand, and extract information from them becomes essential. A climate index is a simple diagnostic quantity that can be used to characterize integral aspects of a geophysical system such as circulation patterns, and thus can be used to evaluate decadal forecasts. One of the most studied and well documented regions of the World Ocean is the North Atlantic. The North Atlantic subpolar gyre is an important region for the modulation of European climate and where skillful predictions of up to a decade can be obtained. Ocean re‐analysis (ORA‐S4) data from 1959 to 2017 are used to introduce a new methodology to compute a climate index of the North Atlantic subpolar gyre that captures both the variability in its strength and shape, the latter was to our knowledge never investigated previously as part of the variability of the gyre on interannual to decadal time scales. The methodology reveals two states of the gyre (before and after 2000), the former is mainly driven by temperature and the latter by a combination of mechanisms that interact to sustain a relatively stable subpolar gyre in terms of strength.

North Atlantic warming over six decades drives decreases in krill abundance with no associated range shift

“…As ocean temperature rises, we generally expect species distributions to track towards historically cooler regions in line with their thermal affinities 28 . However, this study shows for the first time in the North Atlantic that marine populations may not simply just shift their distributions northward due to shifting isotherms and hence re-establish new geographic habitats (thereby maintaining their relative abundance) but may in fact be spatially constrained due to ocean currents and strong thermal boundaries such as the polar front limiting their northward expansions 29 . As a consequence, a species may decline in situ inside its historical core geographical ranges and thermal niche as its habitat becomes spatially squeezed.…”

The North Atlantic Current and its Volume and Freshwater Transports in the Subpolar North Atlantic, Time Period 1993–2016

“…This has pressed the scientific community to look for an additional index that could explain the gyre variability during this period. The NASPG from Biri and Klein (2019) is low in 1999 (reduced transport) in combination with a gyre expansion and a northward shift of the southernmost position of the gyre and an eastward shift of both the easternmost and westernmost positions of the gyre. This might allow the intrusion of subtropical water masses in the eastern basin, compatible with our observations of a northward shift of the southernmost NAC branch in 1999 and 2000 (Figure 5a) and the positive salinity anomaly also east of 30°W despite a reduced transport.…”

“…For the discussion of our results, we considered three indices: the subpolar gyre index (SPGI) from Berx and Payne (2017) and the one from Hátún and Chafik (2018) (Figure 1a) and the North Atlantic SPGI (NASPG) from Biri and Klein (2019). Before 1995, the SPGIs are also strongly positive (Figure 1a), denoting a strong subpolar gyre, in agreement with the NAO.…”

“…The year 2003 is characterized by a sudden and strong salinity decrease observed in the western study region (Figures 5a, 6, and 8) but not in the RT (Figure 7) or in the subtropical region (Stendardo et al, 2016); the NAO index is at its neutral state while the SPGIs still indicate a weakening of the gyre (Berx & Payne, 2017; Hátún & Chafik, 2018) with any outstanding feature. The NASPG, on the other hand, stays relatively strong but stable after 2000 (Biri & Klein, 2019). In Stendardo et al (2016), this particular year was discussed in relation to the SF identified with the horizontal salinity gradient at 300 m. A salinity gradient is indeed a unique feature of the SF separating the salty subtropical water from the fresh subpolar water.…”

Early-winter North Atlantic low-level jet latitude biases in climate models: implications for simulated regional atmosphere-ocean linkages

“…SST SPG and HF SPG . This box broadly spans the spatial extent of the SPG as identified by Biri and Klein (2019). The southern boundary of the box was chosen to account for annual-decadal variability in SPG location but also balanced against being too far south into other key regions of the NA, in particular the Gulf Stream.…”