Deep water trends and variability at the BATS site in the subtropical North Atlantic and consequences on local sea level budget

There is no robust evidence for warming of the global upper ocean and an associated sea-level rise based on a nearly global and continuous data coverage of the surface and upper ocean. The amount of sea level change contributed by the deep ocean is unclear, however, because of limited data availability below 2000 m. The Bermuda Atlantic Time-series Study (BATS) started monthly cruises in 1988 in the subtropical North Atlantic, and during each cruise at least one vertical profile is conducted to a depth > 4000 m . The BATS dataset provides a unique estimation of the seasonal to inter-annual hydrographic variability in the deep ocean as well as long-term trends at a fixed location. In this paper, we focus on the 2000–4000 m deep layer, and find (1) an isopycnal cooling and freshening trend in the deep subtropical North Atlantic, (2) an isobaric warming since ∼ year 2000, despite the continued isopycnal cooling, driven by a gradual deepening of neutral surfaces with unknown spatial extent, and (3) a previously undocumented high frequency variability ( > 1 yr − 1 ) of the deep ocean characterized by large (100 m) episodic vertical displacements of neutral surfaces. The observed water mass changes below 2000 m contribute ∼ 1 mm / yr to the 0–4000 m steric height budget, and is driven mainly by the halosteric contribution.