Abstract
Abstract This study describes how the hull temperature (Ttop) measurements from multisensor surface velocity program (SVP) drifters can be combined with other measurements to provide quantitative information on near-surface vertical temperature stratification during large daily cycles. First, Ttop is compared to the temperature measured at 17 -cm depth from a float tethered to the SVP drifter. These 2007–12 SVP drifters present a larger daily cycle by 1%–3% for 1°–2°C daily cycle amplitudes, with a maximum difference close to the local noon. The difference could result from flow around the SVP drifter in the presence of temperature stratification in the top 20 cm of the water column but also from a small influence of internal drifter temperature on Ttop. The largest differences were found for small drifters (Technocean) for very large daily cycles, as expected from their shallower measurements. The vertical stratification is estimated by comparing these hull data with the deeper T or conductivity C measurements from Sea-Bird sensors 25 (Pacific Gyre) to 45 cm (MetOcean) below the top temperature sensor. The largest stratification is usually found near local noon and early afternoon. For a daily cycle amplitude of 1°C, these differences with the upper level are in the range of 3%–5% of the daily cycle for the Pacific Gyre drifters and 6%–10% for MetOcean drifters with the largest values occurring when the midday sun elevation is lowest. The relative differences increase for larger daily cycles, and the vertical profiles become less linear. These estimated stratifications are well above the uncertainty on Ttop.
Highlights
Diurnal warm surface layers impact the estimates of air–sea flux exchanges (Fairall et al 1996; Ward 2006)
On the Self-Contained Autonomous Microprofiler (SCAMP) profiles collected during large daily cycles at the same time as the Surplas data in the Bay of Biscay in particular on 12 May 2009, we observed gradients of 0.18C over a distance of 1.5 cm, in the same range as what is implied by the Surpact C data. Based on these estimates and what we found in the difference with the Technocean Ttop, Ttop could be close on those days of large cycles to the temperature near 13–15 cm, which is roughly the sensor depth for that kind of ‘‘small sphere’’ surface velocity program (SVP) drifter
It would be interesting to separate the two categories, which could have different characteristics of the Ttop measurement, but the statistics were not sufficient for that here. We further investigated this difference in the daily cycle close to the surface and deeper by considering the Pacific Gyre drifters, where there is a slight shift in immersion between the conductivity cell and the Tsal measurement, which can be used to infer more quantitative information on stratification near 40 cm
Summary
Diurnal warm surface layers impact the estimates of air–sea flux exchanges (Fairall et al 1996; Ward 2006). These small daily cycles likely correspond to windy situations, for which we expect that the sphere plunges often in the waves as it is towed by its drogue, and the actual depth of the sensor might be deeper than the T sensor of the Surplas that floats at the sea surface Another contribution is from the thermal inertia and the 30-min averaging in the Ttop measurements for these drifters that will slightly reduce the amplitude of the daily cycle (and the 1000–1600 LST difference). This is but a small part of the daily cycle amplitude; and the Ttop data should be helpful in estimating near-surface daily temperature cycles as was done, for example, in Bellenger and Duvel (2009)
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