Changing freshwater content: Insights from the subpolar North Atlantic and new oceanographic challenges

Observations and analyses of oceanic inventories of heat and freshwater have recently provided convincing evidence of systematic global-scale changes. Some recent studies aimed at observing, modelling and understanding these changes are collected together in this special issue of Progress in Oceanography. This introductory article provides some background on the procedures used to define these changes and their importance to climate change, with special reference to the North Atlantic basin. In particular, we show that significant changes in the properties and distributions of the major intermediate and deep water masses occurred in the North Atlantic’s subpolar gyre between the 1960s and 1990s. These changes are described using volumetric temperature-salinity censuses and other analyses based on compilations of observations from the warm salty 1964–1972 period and the cold fresh 1995–1997 period. This article and the others in this special issue are intended to provide an overview of recent advances in our knowledge of large-scale heat and freshwater changes in the ocean with the hope that some of the open questions will inspire future work.