The observational record exhibits clear evidence of large amplitude, decadal to multi-decadal variability in sub-polar fresh water (FW) storage and gyre strength, but is barely long enough to have measured a single 50-year cycle. The Atlantic Meridional Overturning Circulation (AMOC) has been directly measured for even less time, and certainly not long enough to identify correlative behavior. A substantial number of observational and model studies have focused on various aspects of this topic, but have not yet produced a unified picture of how this part of the ocean system operates. The disparate range of interpretations for the causes and consequences of observed and modeled North Atlantic FW content and circulation variability has motivated an attempt to better sort them out. A model?data intercomparison study is proposed which will investigate the timing, amplitude and variability of FW content in relation to the full three-dimensional (gyre and overturning) circulation variability and property fluxes in the North Atlantic. A suite of analytical indices will be developed and consistently applied to each of four coupled model?s long control runs to characterize their oceanic FW and heat budgets and assess underlying mechanisms of variability. The working hypothesis is that the recently observed changes in North Atlantic FW content reflect natural variability in this part of the climate system. Moreover, there are times when FW content plays an indirect or ?passive? role in the North Atlantic (i.e. the atmospheric forcing controls the ocean FW content and circulation variability); while at other times, salinity directly influences the circulation (i.e. increased (decreased) high latitude FW content weakens (intensifies) the sub-polar ocean circulation).
Scientific Merit: By comparing several well-respected coupled climate model simulations and observations in a consistent manner, the proposed work will clarify connections between volumetric FW anomalies in the sub-polar North Atlantic and strength of the full three-dimensional gyre and overturning circulations. It will provide new perspectives regarding the timescales and mechanisms of internal variability in a part of the climate system known to exhibit large fluctuations. It will also update the time series of observed FW and heat content changes in the North Atlantic, provide meaningful estimates of their uncertainties, and elucidate the degree to which the recently observed fluctuations fall within the range of natural variability or alternatively reflect a consequence of some other forcing.
Broader Impacts: The proposed work aims to develop and promote a set of diagnostic tools which will consistently and efficiently evaluate property content and ocean circulation in various coupled model simulations. These tools may be extended to other parts of the global ocean. They may also help to identify strengths and weaknesses in models? representations of processes, parameterizations and feedbacks. Construction of climatology products with spatial error covariances will better constrain the uncertainty in estimates of heat and FW content variability with substantial impact on how these are interpreted. Development of software, which will apply Gauss-Markov techniques to estimating mean and error covariance fields in the proposed North Atlantic work, can then readily be applied to global hydrographic climatologies. Assimilation efforts such as ECCO (?Estimating the Circulation and Climate of the Ocean?) will benefit directly because, for the purpose of state estimation, knowledge of the spatial error covariance is essential; yet this remains incompletely (or not at all) addressed within most available climatology products.
This project is a contribution to the U.S. CLIVAR (CLImate VARiability and predictability) Program and the Ocean Research Priorities Plan (near-term priority on the Atlantic Meridional Overturning Circulation).
