Sampling--Dependent Transition Paths of Iceland-Scotland Overflow Water

TL;DR

This paper uses Transition Path Theory applied to Markov chains to analyze the flow paths of Iceland-Scotland Overflow Water, revealing how sampling density affects the detection of deep boundary currents and providing guidance for future data collection.

Contribution

It introduces a Markov chain-based approach to study ISOW flow paths and demonstrates the impact of sampling density on identifying circulation features.

Findings

Dense sampling supports a well-defined deep boundary current

Heterogeneous sampling may bias flow path analysis

Recommendations for improving float data collection

Abstract

In this note, we apply Transition Path Theory (TPT) from Markov chains to shed light on the problem of Iceland-Scotland Overflow Water (ISOW) equatorward export. A recent analysis of observed trajectories of submerged floats demanded revision of the traditional abyssal circulation theory, which postulates that ISOW should steadily flow along a deep boundary current (DBC) around the subpolar North Atlantic prior to exiting it. The TPT analyses carried out here allow to focus the attention on the portions of flow from the origin of ISOW to the region where ISOW exits the subpolar North Atlantic and suggest that insufficient sampling may be biasing the aforementioned demand. The analyses, appropriately adapted to represent a continuous input of ISOW, are carried out on three time-homogeneous Markov chains modeling the ISOW flow. One is constructed using a high number of simulated trajectories homogeneously covering the flow domain. The other two use much fewer trajectories which heterogeneously cover the domain. The trajectories in the latter two chains are observed trajectories or simulated trajectories subsampled at the observed frequency. While the densely sampled chain supports a well-defined DBC, the more heterogeneously sampled chains do not, irrespective of whether observed or simulated trajectories are used. Studying the sampling sensitivity of the Markov chains, we can give recommendations for enlarging the existing float dataset to improve the significance of conclusions about time-asymptotic aspects of the ISOW circulation.