Information from low-density expendable bathythermograph transects: North Atlantic mean temperature structure and quasi-decadal variability

Abstract

The 1999 Ocean Observing System conference recommended that the Low density (LD) expendable bathythermograph (XBT) network be discontinued contingent on the completion of studies showing that the global Argo network and/or satellite altimetry can provide equivalent information. Herein, information content in North Atlantic LD lines relative to quasi-decadal variability in upper layer temperature structure is addressed as the first step in achieving this recommendation. Two LD lines are located in the subpolar gyre and support results from previous studies of shorter length that ocean advection and not only air–sea fluxes plays an important role, particularly in the eastern gyre, in determining the characteristics of the water masses transported to the source regions of North Atlantic Deep Water. Several sections cross the Gulf Stream and Labrador Current. They provide evidence to support the hypothesis that changes in the intensity of Labrador Current properties cause meridional motions of the Gulf Stream. Decadal variability in the subtropical gyre of the North Atlantic has been attributed to westward propagation of temperature anomalies by the mean currents and planetary waves. Modeling studies suggest that these signals are dominant in the thermocline at the northern latitudes (order 30°) of the subtropical gyre. Similar calculations from a line crossing the subtropical gyre at these same latitudes shows no indication of westward propagation. Quasi-decadal signals in upper layer temperature are coincident across the entire gyre with some suggestion of eastward motion of temperature anomalies. One line on the southern boundary of the subtropical gyre does include westward signal movement from the eastern boundary. Based on the information content of individual lines, recommendations are made relative to the continuation of specific transects until it has been demonstrated that Argo and satellite altimetry can provide equivalent results.