A detailed analysis of the origin of deep-decoupling oscillations

TL;DR

This paper investigates the origin and sensitivity of deep-decoupling oscillations in ocean models, using a refined Welander model with smooth switching to better understand their dynamics and implications for climate variability.

Contribution

It introduces a non-instantaneous switching approach in the Welander model, revealing new oscillation types and their dependence on mixing transition speeds and freshwater influx.

Findings

Deep-decoupling oscillations persist with smooth switching but require faster transitions.

Gradual freshwater influx can alter oscillation types and reduce deep water formation.

Multiple oscillation regimes are identified depending on model parameters.

Abstract

The variability of the strength of the Atlantic Meridional Overturning Circulation is influenced substantially by the formation of deep water in the North Atlantic. In many ocean models, so-called deep-decoupling oscillations have been found, whose timescale depends on the characteristics of convective vertical mixing processes. Their precise origin and sensitivity to the representation of mixing have remained unclear so far. To study this problem, we revisit a conceptual Welander model for the evolution of temperature and salinity in two vertically stacked boxes for surface and deep water, which interact through diffusion and/or convective adjustment. The model is known to exhibit several types of deep-decoupling oscillations, with phases of weak diffusive mixing interspersed with strong convective mixing, when the switching between them is assumed to be instantaneous. We present a comprehensive study of oscillations in Welander's model with non-instantaneous switching between mixing phases, as described by a smooth switching function. A dynamical systems approach allows us to distinguish four types of oscillations, in terms of their phases of diffusive versus convective mixing, and to identify the regions in the relevant parameter plane where they exist. The characteristic deep-decoupling oscillations still exist for non-instantaneous switching, but they require switching that is considerably faster than needed for sustaining oscillatory behaviour. Furthermore, we demonstrate how a gradual freshwater influx can lead to transitions between different vertical mixing oscillations. Notably, the convective mixing phase becomes shorter and even disappears, resulting in long periods of much reduced deep water formation. The results are relevant for the interpretation of ocean-climate variability in models and (proxy) observations.