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ADVANCING EARTH 
AND SPACE SCIENCES 
Journal of Geophysical Research: Oceans 
10.1029/2023JC019937 
noted that the GSA primarily affected the SPMW layers, which eroded and reduced in size over time. The 
decrease in SPMW formation was attributed to a reduced inflow of subtropical water, which prevented its 
ıransformation into SPMW by air-sea fluxes. Consistent with these recent finding (Bilö et al., 2022; Holliday 
et al., 2020), our results indicate an initial increase in surface salinity and temperature that continued until 2010 in 
‚he southern part of the subpolar gyre, specifically within the lightest density bins ranging from 69 = 27.1 to 
27.2 kg m (Figures 7c-7e). This was followed by a rapid cooling and freshening until 2016. While it may be 
surprising not to observe a similar trend in the densest isopycnal layers (Figures 7a and 7b), even with a certain 
delay, moving toward the core of the subpolar gyre introduces greater complexity in the gyre's dynamic. Previous 
studies (Fox et al., 2022; Fratantoni & McCartney, 2010; Stendardo et al., 2020) have shown that the gyre's 
dynamic is influenced not only by the advection of warm and saline subtropical water along the pathway of the 
NAC but also by the advection of fresh and cold subpolar water from the western boundary regions toward the 
sast. The strong high salinity and temperature observed in 2015 within the densest isopycnal bins (Figures 7a and 
7b) may be a result of the increased salinity and temperature observed in the lightest isopycnal bins in 2014 
(Figures 7e and 7d). Fox et al. (2022) attributed the freshening and cooling after the 2016 to the advection of 
Labrador Current in the eastern North Atlantic rather than to a decrease in the northward transport of warmer and 
saline surface subtropical water. This advection led to significant freshening in the subpolar gyre until at least 
2017. As suggested by Bil6 et al. (2022), SPMW formation starts to strengthen after 2015, which is consistent 
with our results. 
The formation rates of SPMW exhibit significant variability throughout the entire period, as indicated by both the 
kinematic and the thermodynamic estimates (Figures 8a and 8b). When examining the two time-series in 
Tigures 8a and 8b, substantial differences in formation rates can be expected between the two approaches, as 
water masses can form also below the surface ocean due to diapycnal mixing (Fröhle et al., 2022). In our study, 
ıhere is a substantial disparity between the thermodynamic and kinematic approaches regarding where net sub- 
duction starts to occur within the isopycnal layers. According to the thermodynamic approach, net subduction 
primarily occurs on the densest isopycnal bins, beginning at 27.4 kg m? (see Figure 4b). Yearly formation rates 
(Figure 8b) reveal that net subduction can also occur at lighter isopycnals, such as in 1998 and 2010. These are 
also the years highlighted in green in Figure 7 when lighter isopycnals occupy a larger area at the surface at the 
axpense of the denser isopycnals, which occupy a smaller area. On the other hand, in other years like 1994, 1995 
and 2015 net subduction only occurs at the isopycnal at 27.5 kg m (Figure 8b), which occupies the largest 
surface area (Figure 7). In contrast, the kinematic approach generally suggests that net subduction begins already 
at the isopycnal bin 27.1 kg m” (refer to Figure 4b). Here as well, the yearly mean formation rate exhibits some 
interesting variability compared to the general mean (Figure 8a). For instance, in 1994, 1995, and 2015 net 
subduction only occurs until reaches the isopycnal at 27.4 kg m”?, while the densest isopycnal indicates a net 
obduction. This obduction appears to be driven in these cases by the horizontal component (Figure 8d). These are 
the years that also demonstrate such large formation due to air-sea fluxes, implying that during those years, the 
denser SPMW formed due to air-sea fluxes is diluted and consumed due to mixing. These results are in agreement 
with Marsh et al. (2005), who found a negative mixing-driven formation rate indicating consumption of water 
mass within the region of formation. A study from Fröhle et al. (2022), also highlights the substantial role of 
diapycnal mixing in the export of NADW at 53°N. 
Conversely, years such as 1997 and 1999 show net obduction until the isopycnal bin at 27.3 kg m and sub- 
duction for the densest isopycnal bins similar to what is observed in the thermodynamic approach. Another 
‚undamental difference between the two approaches is the homogeneity of the horizontal distribution of the 
positive and negative formation rates within the isopycnal layers. The thermodynamic approach displays a more 
homogeneous distribution, with a clear separation between positive formation rate (subduction) primarily 
occurring in the southern part of the isopycnal layer and negative formation rate (obduction) happening in the 
northern part (see Figure 3). Consequently, when computing the net formation rate, the lightest densities, where 
negative formation rates occupy a larger area than positive formation rates, exhibit net obduction. On the other 
hand, the kinematic approach is less homogeneous due to large spatial variability driven by the horizonal currents. 
Che entrainment component resembles the distribution observed in the thermodynamic approach and displays a 
more homogeneous pattern. Nevertheless, the time-series of net formation rate obtained only from the entrain- 
ment component still disagree with the time-series from the thermodynamic approach (see Figures 8b and 8c). 
This is most likely because the positive values of the formation rate are still higher than the negative values (see 
Figure 5). 
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