Showing papers by "David Kadko published in 2011"

Inferring upwelling rates in the equatorial Atlantic using 7Be measurements in the upper ocean

Abstract: Ocean upwelling rates are difficult to measure because of the relatively small velocities involved, and therefore are typically inferred from indirect methods such as heat budget estimates or tracer observations. Here we present the first results using a novel technique, based on the isotope 7 Be, to infer rates of upwelling along the equator. Beryllium-7 (half-life=53.3 d) is a cosmic-ray produced radioactive nuclide that is deposited by rainfall upon the ocean surface and subsequently enriched and homogenized within the mixed layer. Previous investigations have utilized the penetration of characteristically high mixed layer concentrations into the upper thermocline to trace ocean ventilation and subduction over seasonal timescales. Here, the tracer is used in a reverse sense; that is, the 7 Be concentration in the usually 7 Be-rich surface mixed layer will be diluted from penetration of 7 Be “dead” water upwelled from below. This dilution provides a means to infer upwelling rates. Furthermore, with knowledge of upwelling rates, 7 Be profiles can be used to constrain vertical diffusivity within the upper thermocline. These ideas were tested with samples collected during the Tropical Atlantic Climate Experiment (TACE) cruise (May 22–June 27, 2009). The observations indicated a nearly linear relationship between 7 Be inventory and mixed layer temperature, as with increased upwelling, lower mixed layer temperatures correspond to greater 7 Be dilution from depth. With this data, upwelling rates were estimated at a number of stations near the equator between 0°E and 30°W within and adjacent to the equatorial cold tongue. The derived upwelling rates ranged from 0 to 2.2 m/d, with maximum values found between the equator and 2°S. The corresponding K z values derived for the upper thermocline were in the range 1–4×10 −4 m 2 /s.

Deposition of 7Be to Bermuda and the regional ocean: Environmental factors affecting estimates of atmospheric flux to the ocean

Abstract: [1] The distribution of 7Be in ocean surface waters is used as tracer of upper ocean transport and atmospheric deposition processes. However, there is very little ocean deposition data available to characterize the temporal and spatial input of 7Be to the oceans and test model results. Here we measure the deposition of 7Be in bulk collectors at two sites at Bermuda over a span of nearly 2 years (April 2007 to January 2009) and compare these rates to the flux required to sustain the inventory of 7Be measured in the nearby Sargasso Sea. The Tudor Hill collector site undersampled (by ∼40%) both the rainfall compared to other Bermuda sites and the 7Be flux required for the ocean inventory. On the other hand, the 7Be flux captured at the Bermuda Institute of Ocean Sciences station site (0.048 dpm cm−2 d−1) matched that expected from the ocean observations. Previously measured long-term atmospheric concentration of 7Be in surface air at Bermuda was used to estimate deposition velocities and scavenging ratios, and our estimates in this marine environment were found to be similar to those measured in continental regions. The deposition of 7Be to the oceans is overwhelmingly determined by wet processes; dry deposition to the ocean surface accounts for only a few percent, at most, of the total deposition to the ocean. We place these measurements in a longer-term and large-scale spatial context by using climatological rainfall data on Bermuda and ocean rainfall estimates from the Global Precipitation Climatology Program and Tropical Rainfall Measuring Mission.