Causes for the Variability and Change of Physical Ocean Conditions over the Northeast U.S. Shelf: Impacts of ENSO and NAO in a Changing Climate

The marine ecosystems on the northeast United States (NEUS) shelf are particularly vulnerable to climate variability and change, because the physical ocean conditions (e.g., temperature, salinity, currents and sea level) in this region are strongly influenced by the cold Labrador Current from the north, warm Gulf Stream from the south, and open ocean variability from the east, with each being strongly affected by the changing climate. The NEUS shelf is also subject to strong local forcing (e.g., wind, surface buoyancy fluxes, river runoff), which dominates interannual variability and is linked to the North Atlantic Oscillation (NAO) during recent decades. The effect of remote (relative to local) forcing on interannual variability of NEUS coast, however, has never been quantified. On decadal and interdecadal (collectively referred to as “decadal” hereafter) timescales, modeling studies suggested that variability of the NEUS shelf is most susceptible to the variability and change in Atlantic Meridional Overturning Circulation (AMOC), including its weakening due to anthropogenic warming. The coarse resolutions of the models, however, cannot resolve the complex bathymetry of the continental shelf and slope; yet, the shelf dynamics differ from that of the open ocean, and the continental slope acts as a dynamical barrier for the exchange between coastal and open ocean. Inadequate resolution of the shelf and slope can cause artificially strong impacts from the open ocean, including the effect of the AMOC. Recent observational analyses suggest that decadal variability of the NEUS coast is linked to both the NAO and El Niño – Southern Oscillation (ENSO). The mechanisms for ENSO to affect the physical ocean conditions and therefore the large marine ecosystem (LME) of the NEUS shelf remain unknown.The overall goal of this proposal is to: quantify the remote and local forcing of coastal ocean conditions (e.g. temperature, salinity, current) on the NEUS shelf since the 1960s on interannual and decadal timescales; investigate the associated mechanisms, and assess the impacts of the NAO and ENSO on this region. We will carry out a hierarchy of high-resolution modeling experiments using the Regional Ocean Modeling System (ROMS) in a domain that covers the entire US east coast. By using a high-resolution model, we can resolve the bathymetry of the NEUS shelf and slope. The domain of the model is fairly large to more accurately depict signals coming from the open ocean, including properly representing the Gulf Stream and the impact of the Labrador Current. To extract the NAO and ENSO signals, we will apply the Bayesian Dynamical Linear Model (DLM), which can capture the non-stationary impacts of climate modes, as demonstrated by our recent studies. To confirm the DLM results, we will also use Linear Inverse Modeling (LIM) to extract the ENSO/nonENSO-related signals. The model results will be analyzed in conjunction with available satellite and in situ observations. The Mid-Atlantic Bight (MAB) and the Gulf of Maine (GOM) of the NEUS coast are quite densely observed compared to coastal oceans globally.