The interplay between sea level and Atlantic Meridional Overturning Circulation: Cause and effect relationships, predictability, and coastal implications

The global mean sea level is rising as the result of ocean warming and melting of terrestrial glaciers and ice sheets. Regional sea level changes can deviate significantly from the global average change, which is largely the result of the spatial redistribution of heat and freshwater by ocean circulation. The latter is often simplified by a zonal integral of meridional velocities known as the Meridional Overturning Circulation (MOC). The MOC-modulated meridional divergence of heat and freshwater drives the large-scale steric (due to density changes) sea level changes. Near the coast these changes provide background conditions that, when combined with the effect of tides and with synoptic sea level fluctuations due to the variable atmospheric pressure and winds, can result in nuisance flooding and storm surge events that oftentimes affect densely populated urban areas.
The goal of this proposal is to establish the relationships between the MOC and the gyre-scale and coastal sea level changes throughout the Atlantic Ocean basin, and identify the key mechanisms responsible for these relationships. The main focus will be at understanding how the large-scale sea level patterns influenced by the MOC may affect coastal sea level in the Atlantic Ocean and the U.S. East Coast, in particular. The proposed research will utilize a suite of observational data collected by Atlantic MOC observing arrays (e.g., RAPID/MOCHA/WBTS, MOVE, SAMBA, OSNAP, and a combination of altimetry and hydrography, etc.); satellite measurements of sea surface height (SSH), temperature (SST), salinity (SSS), and winds; hydrographic data (Argo, XBT, CTD); coastal tide gauges; and atmospheric re-analyses. Statistical techniques (e.g. Empirical Orthogonal Functions (EOFs), including joint and complex EOFs, wavelet and cross-wavelet transforms, wavelet coherence, etc.) will be used to identify the leading modes of variability, how these modes evolve in space and time, and lag-lead relationships between the modes and other variables. The established relationships will be investigated in more detail using an eddy-permitting Estimating Circulation and Climate of the Ocean Version 5 (ECCOv5) state estimate. The use ECCOv5 will allow budget closures and better process understanding. In addition, the ECCOv5 adjoint sensitivities will be used to quantitatively evaluate the causal mechanisms of regional (including coastal) sea level and heat content variability, as well as changes in the Atlantic MOC.
The proposed research directly addresses the second priority area of the FY19 CVP Decadal Climate Variability and Predictability call: “Investigation of the relationship between the Atlantic Meridional Overturning Circulation (AMOC) and global and regional sea level (historical, current, and/or future), with a focus on understanding sea level extremes and coastal impacts in the United States, for the improved understanding of the ocean-climate system”. It also addresses NOAA’s goals by (i) contributing to understanding and predicting changes in climate, weather, oceans and coasts; and (ii) collecting and analyzing information critical for conservation and management of coastal and marine ecosystems and resources.