Abstract:
Significant decadal variability in ventilation has been documented from a time series of eight hydrographic surveys with transient tracer measurements in the Eastern Tropical North Atlantic Oxygen Minimum Zone (ETNA-OMZ) between 2006 and 2016. The changes in ventilation were identified by using transient tracer (CFC-12 and SF6) measurements. Variability in transient tracer mean age calculated by the Transit Time Distribution (TTD) method with standard parameterization was used as metric.
The data was interpolated to density surfaces and sorted according to distinct salinity ranges to separate the southern and northern water masses, which mix within the OMZ.
The tracer distributions on the density surfaces show an overall trend of increasing tracer concentrations throughout the water column. However, the concentration increase is non monotonic and shows a clear temporal variability on all density surfaces. The highest spatial variability is shown in the OMZ core with a clear north-south and west-east gradient from high to low concentrations. The mean age appears to be relatively constant over time with highest variability in the NW area on all density surfaces. The spatial variability shows increasing ages from north to south and west to east. The oxygen distribution however is decoupled from the mean age distribution. A decrease in ventilation is not necessarily manifested as a decrease in the oxygen, and vice versa. The oxygen budget is thus not only related to the changes in ventilation. The variability in oxygen consumption within the OMZ or the oxygen utilization rates along the flow pathway modulates the oxygen budget independent of ventilation.
