An objective algorithm for estimating maximum oceanic mixed layer depth using seasonality indices derived from Argo temperature/salinity profiles

Abstract

AbstractIn this study, we propose a new algorithm for estimating the annual maximum mixed layer depth (M2LD) analogous to a full range of local “ventilation” depth, and corresponding to the deepest surface to which atmospheric influence can be “felt.” Two “seasonality indices” are defined, respectively, for temperature and salinity through Fourier analysis of their time series using Argo data, on the basis of which a significant local minimum of the index corresponding to a maximum penetration depth can be identified. A final M2LD is then determined by maximizing the thermal and haline effects. Unlike most of the previous schemes which use arbitrary thresholds or subjective criteria, the new algorithm is objective, robust, and property adaptive provided a significant periodic geophysical forcing such as annual cycle is available. The validity of our methodology is confirmed by the spatial correlation of the tropical dominance of saline effect (mainly related to rainfall cycle) and the extratropical dominance of thermal effect (mainly related to solar cycle). It is also recognized that the M2LD distribution is characterized by the coexistence of basin‐scale zonal structures and eddy‐scale local patches. In addition to the fundamental buoyancy forcing caused mainly by latitude‐dependent solar radiation, the impressive two‐scale pattern is found to be primarily attributable to (1) large‐wave climate due to extreme winds (large scale) and (2) systematic eddy shedding as a result of persistent winds (mesoscale). Moreover, a general geographical consistency and a good quantitative agreement are found between the new algorithm and those published in the literature. However, a major discrepancy in our result is the existence of a constantly deeper M2LD band compared with other results in the midlatitude oceans of both hemispheres. Given the better correspondence of our M2LDs with the depth of the oxygen saturation limit, it is argued that there might be a systematic underestimation with existing criteria in these regions. Our results demonstrate that the M2LD may serve as an integrated proxy for studying the coherent multidisciplinary variabilities of the coupled ocean–atmosphere system.