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Figure 6. Hovmöller diagrams showing the temporal changes of potential vorticity [X107'* m-' s7'] (filled colors), potential density 69 [kg m”*] (black contour lines’ 
and winter mixed layer depth (JFM, black dashed lines) from 1994 to 2018 in the water column. The left panels show the temporal variability from 0 to 1,000 m along the 
section at 30°W divided into three subsections: 61°N-65°N, 59°N-61°N, and 56°N-59°N. The right panels show the temporal variability from 0 to 1,000 m along the 
section at 14°W divided into three subsections: 57°N-65°N, 53°N-57°N. and 45°N-53°N 
surface outcrop area reaches its minimum, as observed in 1998 within the isopycnal bin at 09 = 27.5 kg m. In 
ıhis case, a corresponding minimum in the outcrop area was observed in 1995 within the isopycnal bin at 
52=271 ke m}. 
Periods characterized by significant shoaling of the MLD, accompanied by reduced outcrop areas of the deeper 
ısopycnal layers (Figures 7a and 7b), such as in 1998, 2004, 2010, 2013 and 2017 (green bars in Figure 7), align 
with periods a weakened subpolar gyre and a westward retraction of its front, as shown by the subpolar gyre index 
proposed by Berx and Payne (2017). Their gyre index is defined as the first principal component of an empirical 
orthogonal function analysis of the sea level anomaly field in the North Atlantic Ocean. Conversely, instances of 
MILD deepening followed by increased outcrop areas in deeper isopycnal layers, such as in 1995, 2000, 2009, 
2015 and 2018 (purples bars in Figures 7a and 7b), correspond to periods with positive values of the gyre index. 
signifying a wider spread of the subpolar front (Berx & Payne, 2017). 
[he eastern subpolar North Atlantic experienced a large freshening between 2012 — 2016, as reported by Holliday 
et al. (2020). This freshening was attributed to an unusual winter wind pattern that influenced the ocean circu- 
lation. including a slowdown of the NAC. resulting in a reduced input of subtropical water into the eastern North 
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