Recent Changes in Deep Ventilation of the Mediterranean Sea; Evidence From Long-Term Transient Tracer Observations

Pingyang Li,Toste Tanhua

doi:10.3389/fmars.2020.00594

Abstract

The Mediterranean Sea is a small region of the global ocean but with a very active overturning circulation that allows surface perturbations to be transported to the interior ocean. Understanding of ventilation is important for understanding and predicting climate change and its impact on ocean ecosystems. To quantify changes of deep ventilation, we investigated the spatiotemporal variability of transient tracers (i.e. CFC-12 and SF6) observations combined with temporal evolution of hydrographic and oxygen observations in the Mediterranean Sea from 13 cruises conducted during 1987-2018, with emphasize on the update from 2011 to 2018. Spatially, both the Eastern and Western Mediterranean Deep Water (EMDW and WMDW) show a general west-to-east gradient of increasing salinity and potential temperature but decreasing oxygen and transient tracer concentrations. Temporally, stagnant and weak ventilation is found in most areas of the EMDW during the last decade despite the prevailing ventilation in the Adriatic Deep Water between 2011 and 2016, which could be a result of the weakened Adriatic source intensity. The EMDW has been a mixture of the older Southern Aegean Sea dense waters formed during the Eastern Mediterranean Transient (EMT) event, and the more recent ventilated deep-water of the Adriatic origin. In the western Mediterranean basin, we found uplifting of old WMDW being replaced by the new deep-water from the Western Mediterranean Transition (WMT) event and uplifting of the new WMDW toward the Alboran Sea. The temporal variability revealed enhanced ventilation after the WMT event but slightly weakened ventilation after 2016, which could be a result of combined influences from the eastern (for the weakened Adriatic source intensity) and western (for the weakened influence from the WMT event) Mediterranean Sea. Additionally, the Mediterranean Sea is characterized by a Tracer Minimum Zone (TMZ) at mid-depth of the water column attributed to the rapid deep ventilation so that the TMZ is the slowest ventilated layer. This zone of weak ventilation stretches across the whole Mediterranean Sea from the Levantine basin into the western basin.

Highlights

Ocean ventilation is an important process in the Earth system that transports ocean surface properties, such as salinity, heat, CO2 and dissolved gases to the interior ocean (Luyten et al, 1983; Khatiwala et al, 2012)
An area of elevated CFC-12 concentration water was centered at ∼700 m depth in the northern Cretan Passage originated from the Aegean Sea, which implies that newly ventilated Cretan Intermediate Water (CIW)
The Mediterranean Sea is one of the best-ventilated bodies of water in the global ocean and is as such characterized by high transient tracer concentrations in the deep layer below a zone of lower tracer concentrations in the intermediate layer, the Tracer Minimum Zone (TMZ). This zone of weak ventilation stretches across the whole Mediterranean Sea from the Levantine basin into the western basin

Summary

Introduction

Ocean ventilation is an important process in the Earth system that transports ocean surface properties, such as salinity, heat, CO2 and dissolved gases to the interior ocean (Luyten et al, 1983; Khatiwala et al, 2012). As a microcosm of the global ocean where climate change can be observed on a shorter time-scale (Schroeder et al, 2016), the Mediterranean Sea (Figure 1a) is well ventilated but with highly variable ventilation patterns in both time and space. Such ventilation patterns can directly be illustrated by, for instance, long-term observations of transient tracers. Transient tracers are taken up by the ocean at the air-sea interface and transferred into the deep ocean by the Deep/Dense Water Formation (DWF) through convection or subduction processes. The deep water exchanges between the two basins are limited by the Strait of Sicily, which leads to a relative independent deep water circulation in each basin

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