Observations of temporal changes of tritium-3He age in the eastern North Atlantic thermocline: Evidence for changes in ventilation?
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A compilation of fifteen years of tritium and He-3 measurements is used to examine the ventilation of the eastern North Atlantic subtropical gyre with specific emphasis on the temporal character of the tracer age field. A multivariate regression analysis in the form of a spatiotemporal Taylor expansion is applied to observations interpolated onto isopycnal surfaces. The time-dependent component of the tracer age field is found to be statistically significant, explaining approximately 10% of the Variance of the tracer age observations in the upper thermocline (sigma(theta) = 26.5) and increasing to roughly 50% of the variance in the lower thermocline (sigma(theta) = 27.0). The observed transient tracer age increases over the 15 years of observations with the fractional rate of change of the age field varying between 2% and 5% per year. The largest observed changes occur on the deepest, most slowly ventilated isopycnal surfaces. The second derivative of the tritium-He-3 age with time suggests that the tracer age field may be approaching a steady state. If tritium-He-3 age is interpreted as a true measure of the advective ventilation age, the temporal changes in age would imply a slackening of the ventilation of the lower main thermocline of greater than 50% from the late 1970’s to the early 1990’s. However, consideration of the full advective-diffusive balance of tritium-He-3 age reveals that the changes in tracer age field represent a time-dependent adjustment of the transient tracer concentrations in conjunction with a steady local circulation field. integral approximations of the upstream evolution of the tracer field also fail to demonstrate evidence for decadal changes in ventilation. The integral balance along the path of subduction yields an improved estimate of the true ventilation age based on the temporal tendency of the age field along the path of ventilation. An approximation of this integral suggests that actual ventilation ages can be up to 40% larger than the measured tracer age in the deeper portions of the North Atlantic thermocline. Proper accounting of the time-dependent biases of the tracer age dating technique are a prerequisite for examining transient tracer measurements for evidence of changes in the physical ventilation of the upper ocean.