North Atlantic Subtropical Mode Water properties: Intrinsic and atmospherically-forced interannual variability

Abstract. This study investigates the contributions of the ocean’s chaotic intrinsic variability (CIV) and atmospherically-forced variability on the interannual fluctuations of the North Atlantic Eighteen Degree Water (EDW) properties. Utilizing a 1/4° regional 50-member ocean/sea-ice ensemble simulation driven by an original surface forcing method and perturbed initially, the forced variability of EDW properties is estimated from ensemble mean fluctuations, while CIV is determined from deviations around the ensemble mean within each member. The model successfully captures the main features of EDW, showing good agreement with observation-based ARMOR3D data in terms of location, seasonality, mean temperature and volume, and interannual variance of its main properties. CIV significantly impacts EDW, explaining 10–13 and 28–44 % of the interannual variance of its geometric and thermohaline mean properties, respectively, with a maximum imprint on EDW temperature. Observed and simulated intrinsic-to-total variance ratios are mostly consistent, dispelling concerns about a signal-to-noise paradox. This study also illustrates the advantages of ensemble simulations over single simulations in understanding oceanic fluctuations and attributing them to external drivers, while also cautioning against overreliance on individual simulations assessments.