Abstract
Abstract. In January and February 2010, a sixth transatlantic hydrographic section was completed across 25° N, extending the hydrographic record at this latitude to over half a century. In combination with continuous transport measurements made since 2004 at 26.5° N by the Rapid-WATCH project, we reassess transport variability in the 25° N hydrographic record. Past studies of transport variability at this latitude have assumed transport estimates from each hydrographic section to represent annual average conditions. In this study the uncertainty in this assumption is assessed through use of Rapid-WATCH observations to quantify sub-seasonal and seasonal transport variability. Whilst in the upper-ocean no significant interannual or decadal transport variability are identified in the hydrographic record, in the deep ocean transport variability in both depth and potential temperature classes suggests some interannual or decadal variability may have occurred. This is particularly striking in the lower North Atlantic Deep Water where southward transports prior to 1998 were greater than recent transports by several Sverdrups. Whilst a cooling and freshening of Denmark Straits Overflow Water has occurred which is coincident with these transport changes, these water mass changes appear to be density compensated. Transport changes are the result of changing velocity shear in the vicinity of the Deep Western Boundary Current.
Highlights
In January and February 2010, a sixth hydrographic section was completed across 25◦ N in the Atlantic Ocean, extending the decadal timeseries of hydrographic measurements available at this latitude
We examine whether any significant interannual or longer term changes have been observed in the six hydrographic sections which is outside the range of shorter term variability in the Rapid-WATCH observations
Observations of deep water transport variability elsewhere in the North Atlantic that may help resolve this issue are sparse in both time and space and those observations that do exist at high latitudes are difficult to reconcile with transport variability in this study
Summary
In January and February 2010, a sixth hydrographic section was completed across 25◦ N in the Atlantic Ocean, extending the decadal timeseries of hydrographic measurements available at this latitude. Despite the seemingly sparse nature of the observations, the oceanographic section across 25◦ N represents one of the best observed regions of the world oceans and offers an excellent opportunity to investigate large-scale decadal changes in ocean state. Since April 2004 the Rapid-WATCH project (Rapid climate change – Will the Atlantic Thermohaline Circulation Halt) has provided continuous observations of Atlantic transport variability at 26.5◦ N. This has revealed surprisingly large transport variability on sub-seasonal and seasonal timescales (Cunningham et al, 2007; Kanzow et al, 2010) and has raised questions over the significance of longer term trends in Atlantic transport reported in earlier 25◦ N hydrographic sections that suggested a weakening of the Atlantic Meridional
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