Editorial: Impact of Deep Oceanic Processes on Circulation and Climate Variability: Examples From the Mediterranean Sea and the Global Ocean

Abstract

The ocean is a crucial component of the Earth’s climate system. Heat and CO2 are absorbed in the ocean’s surface and transported throughout the ocean depths through the overturning circulation. Exchange across the ocean’s turbulent surface boundary layer can happen rapidly, in hours or days, and significant exchange of water between the boundary layer and the stratified main thermocline occurs over timescales of years to decades. Deepwater takes many decades to millennia to return to the surface, acting as long-term storage for heat and CO2 and thereby lessening the near-term impacts of climate change. The understanding of mechanisms and rates that control the bottom flows is essential to quantify re-transfer towards the upper layers of the energy stored at the seafloor. These processes are significantly affecting the ocean system as a whole and could contribute to accelerating the rising climate trends (thermohaline circulation, sea-level rise, and ocean acidification).

The Mediterranean Sea, like the ocean, has its overturning circulation and it represents a suitable lab for investigating physical mechanisms such as deepwater formation, mixing processes, strait dynamics, advective-convective feedbacks that drive the ocean variability, and the internal exchange mechanisms. Also, the scale of variability is shorter compared to other ocean basins in time and space.

As mechanisms governing exchanges of heat and carbon in the ocean occur with long timescales, observational datasets over many decades are required to document, understand, and predict the climate system as a whole. This is also an essential requirement to detect and attribute changes driven by human activities and to predict how the climate system will likely behave in the future. The needs for and uses of deep ocean data extend well beyond closing the global heat budget. Deep ocean data are needed to initialize and constrain ocean models and improve their representation of mixing of heat downwards/upwards within the deep ocean.

In order to understand past and future climate changes, the characterization of the still unexplored deep dynamics aims to provide crucial results to support new interpretations of the paleo circulation and of those processes that have influenced ventilation and water masses overturning. These new insights will also be essential for leading, in the near future, new tailored parameterizations able to adequately represent the dynamics below 2000m of depth.

This article collection conceived in the framework of MedCliver (http://www.medclivar.eu/) community, aims to gather outcomes on deep ocean circulation and bottom mixing not only in the Mediterranean area but also through other important case studies relevant in the characterization of deep processes, considering three different points of view:

1- Theoretical (role of the bottom diffusion) 2- New assimilation and modeling 3- In situ observations of deep long time series

These will contribute to improving knowledge of the impacts of key deep processes on the climate system.