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RESEARCH ARTICLE 
L0.1029/2023JC019937 
Key Points: 
Subduction of water below the mixed 
layer is driven by entrainment with 
advection modulating the small-scale 
dynamics 
Volume and formation of Subpolar 
Mode Water changes every year in size 
and location 
[he difference between the formation 
rate from the thermodynamic and 
kinematic approaches suggests a large 
role of diapycnal mixing 
Supporting Information: 
Supporting Information may be found in 
the online version of this article 
Correspondence to: 
. Stendardo, 
laria.stendardo @uni-bremen.de 
Citation: 
Stendardo, I., Buongiorno Nardelli, B., 
Durante, S., Iudicone, D., & Kieke, D. 
(2024). Interannual variability of Subpolar 
Mode Water in the Subpolar North 
Atlantic. Journal of Geophysical 
Research: Oceans, 129, e20231C019937 
https://doi.org/10.1029/20231C019937 
Author Contributions: 
Conceptualization: I. Stendardo, 
3. Buongiorno Nardelli, D. Iudicone, 
DJ. Kieke 
Data curation: B. Buongiorno Nardelli 
Formal analysis: I. Stendardo, 
3. Buongiorno Nardelli, S. Durante 
Funding acquisition: I. Stendardo, 
3. Buongiorno Nardelli 
[nvestigation: I. Stendardo 
Vlethodology: I. Stendardo, 
3. Buongiorno Nardelli, S. Durante, 
DD. Iudicone 
Project administration: I. Stendardo 
Visualization: I. Stendardo 
Writing — original draft: I. Stendardo 
Received 17 APR 2023 
Accepted 4 MAR 2024 
9 2024. The Authors. 
This is an open access article under the 
terms of the Creative Commons 
Attribution-NonCommercial-NoDerivs 
License, which permits use and 
distribution in any medium, provided the 
original work is properly cited, the use is 
non-commercial and no modifications or 
adaptations are made. 
STENDARTIMO ET 
Ay 
Kl 
Interannual Variability of Subpolar Mode Water in the 
Subpolar North Atlantic 
I. Stendardo! ®, B. Buongiorno Nardelli? ®, S. Durante”* ©, D. Iudicone* ®, and D. Kieke'“ © 
University of Bremen, Institute of Environmental Physics, Bremen, Germany, 2Istituto di Scienze Marine, Consiglio 
Nazionale delle Ricerche, Rome, Italy, “Istituto per lo studio degli impatti Antropici e Sostenibilitä in ambiente marino, 
Consiglio Nazionale delle Ricerche, Rome, Italy, *Stazione Zoologica Anton Dohrn di Napoli, Naples, Italy, °Bundesamt 
‚ür Seeschifffahrt und Hydrographie (BSH), Hamburg, Germany 
Abstract Subpolar Mode Water (SPMW) is an important water mass originating in the eastern North 
Atlantic. Its formation, subject to modification through oceanic interior mixing, can directly influence the volume 
of water contributing to the Atlantic meridional overturning circulation. Utilizing observation-based data sets 
spanning from 1993 to 2018, we estimated the formation rates and volume of SPMW within isopycnal layers and 
examined its temporal variability. Two complementary approaches were used to estimate the formation rate: a 
ıhermodynamic approach focusing on the air-sea interactions and a kinematic approach involving volume 
‚ransport from the mixed layer to the ocean's interior, including the entrainment/detrainment of the mixed layer 
itself. This is the first time that thermodynamic and kinematic approaches are applied to observation-based data in 
(he North Atlantic. Our results suggest a substantial role of diapycnal mixing in diluting the dense waters formed 
'y air-sea fluxes toward the range of SPMW densities. The study reveals a complex interplay of processes, with 
entrainment being the primary driver of subduction/obduction rates, while advection contributes to the overall 
small-scale dynamics. Variations in the volume and location of SPMW formation are observed from year to year. 
Notably, when SPMW forms extensively in lighter isopycnal layers, the volume occupied by denser isopycnals 
decreases and vice versa. We attributed this compensation effect to a propagation signal, where formation in the 
lightest isopycnal bins influences the formation in denser isopycnal bins with a delay of a few years, emphasizing 
ıhe circulation's role in shaping the SPMW distribution. 
Plain Language Summary We present an analysis of one important type of water mass, the Subpolar 
Mode Water (SPMW) located in the North Atlantic Ocean. We looked at data collected over a period of 26 years 
from 1993 to 2018, to understand how SPMW forms and how its volume changes over time. We used two 
different methods to calculate how much SPMW forms each year to have the best possible picture. One method 
ı10oked at air-water interactions, and the other at how the water moves from the top layer of the ocean to deeper 
parts. This is the first time these methods are applied to observation-based data in the North Atlantic. Our 
findings highlight the important role of water mixing in changing the density of SPMW. We also found that one 
of the main reasons how the SPMW leaves the top layer of the ocean is through the shoaling/deepening of the 
mixed layer itself over time, modulated horizontally by the small-scale dynamics of the ocean. Additionally, the 
volume of SPMW and location where it forms can vary from year to year. Notably, when SPMW forms in larger 
amounts in the upper part of the ocean. there is less SPMW in the denser water. 
1. Introduction 
Subpolar Mode Water (SPMW) is a large volume of water in the upper 1,000 m of the water column of the 
subpolar North Atlantic (Brambilla & Talley, 2008). It originates in the eastern and northeastern subpolar North 
Atlantic and represents a variety of near-surface waters of intermediate densities, occupying the layers between 
'he ocean surface and the permanent pycnocline (e.g., Brambilla et al., 2008). It is characterized by a thick layer of 
nearly uniform properties like temperature, salinity, oxygen and density (e.g., McCartney & Talley, 1982). 
SPMW is found along the pathways of the several branches of the North Atlantic Current (NAC, see the general 
circulation scheme in Figure 1a). Following the cyclonic pathway of the upper ocean circulation in the subpolar 
gyre, SPMW density increases downstream (Brambilla & Talley, 2008; Petit et al., 2021). Part of this water mass 
is advected toward the Irminger Sea, the Labrador Sea and the Nordic Seas where it contributes to the formation of 
the North Atlantic Deep Water (NADW) (de Boisseson et al., 2012; Petit et al.. 2021). 
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