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

•The Gulf of Maine has warmed and continues to warm, to a large extent as the result of changes in advective heat fluxes driven by way of lateral exchanges with water masses in neighboring shelf and slope regions;•Water properties of the source waters themselves are shown to have been changing over time; moreover, cross shelf volume fluxes of those water masses to the Gulf have changed over time, and have become more highly variable in recent years.•Late summer episodes of abrupt influxes to the interior Gulf of warm and salty waters carrying reduced nitrate concentrations, reveal a mix that includes warm, salty, low nutrient Gulf Stream Water, which in some years arrives as an abrupt, bore-like, high salinity wave that precedes a continuing inflow of warm and salty waters over a period of several months.•A new baseline in the Gulf of Maine, of warmer temperatures and higher salinities, became established after 2010.•It was on top of that new baseline that a greater surface heat flux in the winter to summer period of 2011–2012 made possible the ‘2012 heat wave’ in the Gulf of Maine, which was followed by two similar events in 2016 and 2021.•We suggest that these ‘heat waves’ are the result of 4–7 year oscillations between periods of warm and salty waters in the Gulf, and colder and fresher waters. Moreover, we suggest that this oscillation it is linked to an already-established multi-year variability, with frequency of 4–5 years, in the path of the Gulf Stream east of 60⁰W.•We present evidence 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.