Abstract
Considerable advances in the global ocean observing system over the last two decades offers an opportunity to provide more quantitative information on changes in heat and freshwater storage. Variations in these storage terms can arise through internal variability and also the response of the ocean to anthropogenic climate change. Disentangling these competing influences on the regional patterns of change and elucidating their governing processes remains an outstanding scientific challenge. This challenge is compounded by instrumental and sampling uncertainties. The combined use of ocean observations and model simulations is the most viable method to assess the forced signal from noise and ascertain the primary drivers of variability and change. Moreover, this approach offers the potential for improved seasonal-to-decadal predictions and the possibility to develop powerful multi-variate constraints on climate model future projections. Regional heat storage changes dominate the steric contribution to sea level rise over most of the ocean and are vital to understanding both global and regional heat budgets. Variations in regional freshwater storage are particularly relevant to our understanding of changes in the hydrological cycle and can potentially be used to verify local ocean mass addition from terrestrial and cryospheric systems associated with contemporary sea level rise. This White Paper will examine the ability of the current ocean observing system to quantify changes in regional heat and freshwater storage. In particular we will seek to answer the question: What time and space scales are currently resolved in different regions of the global oceans? In light of some of the key scientific questions, we will discuss the requirements for measurement accuracy, sampling, and coverage as well as the synergies that can be leveraged by more comprehensively analyzing the multi-variable arrays provided by the integrated observing system.
Highlights
The global ocean plays a fundamental role in the climate system through its ability to store and redistribute large quantities of heat and freshwater
In section “Projected Changes in Ocean Heat and Freshwater Content,” we present a synopsis of what is known about the emergent patterns of heat content (HC) and freshwater content (FWC) change from climate model simulations
The regional North Atlantic cooling is likely linked to changes in the circulation such as anomalously weak LSW deep convection (Thornalley et al, 2018) and/or long-term Atlantic Meridional Overturning Circulation (AMOC) weakening (e.g., Caesar et al, 2018; Smeed et al, 2018) and to changes in the air–sea fluxes (e.g., Robson et al, 2014; Dubois et al, 2018; Josey et al, 2018). These results suggest that longterm changes in the AMOC will have a first-order impact on the future HC changes in the North Atlantic
Summary
The global ocean plays a fundamental role in the climate system through its ability to store and redistribute large quantities of heat and freshwater. These changes include: intensification of the hydrological cycle (with corresponding amplification of the spatial variations in surface and subsurface salinity); continued warming of the global oceans and sea level rise; a weakening of the AMOC (and associated northward heat transport); intensification of the Southern Ocean winds; and increased freshwater input to the high-latitudes (from both moisture transport and melting ice).
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