Warming waters of the Gulf of Maine: The role of Shelf, Slope and Gulf Stream Water masses

Abstract

We present an analysis of a 19-year time series of mooring data (T, S,) collected since 2003 in Jordan Basin in the eastern Gulf of Maine and beginning in 2016, in situ nitrate; we also include hydrographic station data collected by NOAA’s ECOMON Program. Analyses support, in part, conclusions drawn by others that the Gulf of Maine has warmed and continues to warm as a result of long-term and shorter term air-sea heat fluxes. We show that warming is to a large extent a result of changes in advective heat fluxes via lateral exchanges with water masses in neighboring shelf and slope regions (Warm Slope Water and Scotian Shelf Water). Water properties of the source waters and their inflows to the Gulf have been shown to have been changing over time, and now reveal a mix that includes warm, salty, low nutrient Gulf Stream Water. The result is a new baseline in the Gulf of Maine, of warmer temperatures and higher salinities, which appears to have become established after 2010. Annual cycles of spring-to summer water column warming and fall-to-winter cooling and convective mixing are presented and discussed, showing that the maximum depth of convective mixing in the eastern interior Gulf of Maine does not usually exceed 150 m, a depth constrained by the presence a strong vertical density gradient created by layer of high salinity bottom waters. Approximately-six months after the annual late-summer influx to the Gulf of warm and salty water masses at depth and along the bottom, a mass of cold, low salinity Scotian Shelf Water arrives in the upper water layers in winter – these two major water mass fluxes are a half year out of phase with one another, every year. We suggest that these two water mass intrusion events are linked – that the winter-spring inflow of Scotian Shelf Water creates a barotropic pressure gradient in the Gulf that persists into the summer, thereby impeding the inflow of warm and salty Slope, and/or Gulf Stream Water, until that Scotian Shelf Water mass has become sufficiently dispersed in late summer, thereby effectively opening a valve that allows warm and salty deep waters from outside the Gulf to penetrate.