Solar warming of near-bottom water over a fringing reef

The Kilo Nalu Observatory is located on the foreslope of a fringing reef on the south shore of Oahu, Hawaii. A cabled node at 12-m depth has enabled continuous real-time temperature observations from a thermistor chain extending from 1 to 7 m above bottom. Data from a 27-month deployment in 2007–2009 reveal repeated instances of subsurface temperature inversions. The usual diurnal pattern shows increases in temperature throughout the water column after sunrise, peaking in the early afternoon. Bottom waters typically warm faster than those at mid-depth, driving an inversion in the thermal profile. The onset and evolution of the inversions are consistent with an analytical model of radiation absorption and the contribution to bottom temperature from solar warming of the seafloor. The maximum size, duration and seasonal distribution of the inversions indicate that salinity compensation is a major limiting factor. In the absence of salinity compensation, the implication is that bottom heating destabilizes the water column and convective transport results. In addition, recurring afternoon onshore bottom currents contribute to the termination of inversions. Although radiative heating may exacerbate coral heat stress, radiation-driven thermal convection and exposure to the open ocean modulate temperatures over the reef

Experimental characterization of starting jet dynamics

The dynamics of a laminar starting jet are explored in a series of laboratory experiments and numerical simulations. We identify new, objective methods for characterizing the leading vortex ring, enabling robust comparisons with results from a numerical model. Observations of circulation in the initial vortex ring and for the total jet are reported along with strain rate at the leading stagnation point. Growth and pairing of shear instabilities trailing the leading vortex ring is observed. Development of these secondary vortices and their subsequent interactions with the leading vortex has significant effects on the characteristics of the primary vortex ring. Strong fluctuations in strain rate at the leading edge are associated with the pairing of the initial vortex ring with a trailing secondary ringSupport for this research was provided by the Spanish MEC and European Union under Projects # ENE2005-08580-C02-01 and DPI2005-08654-C04-01Publicad

High frequency temperature variability reduces the risk of coral bleaching

Coral bleaching is the detrimental expulsion of algal symbionts from their cnidarian hosts, and predominantly occurs when corals are exposed to thermal stress. The incidence and severity of bleaching is often spatially heterogeneous within reef-scales (<1 km), and is therefore not predictable using conventional remote sensing products. Here, we systematically assess the relationship between in situ measurements of 20 environmental variables, along with seven remotely sensed SST thermal stress metrics, and 81 observed bleaching events at coral reef locations spanning five major reef regions globally. We find that high-frequency temperature variability (i.e., daily temperature range) was the most influential factor in predicting bleaching prevalence and had a mitigating effect, such that a 1 °C increase in daily temperature range would reduce the odds of more severe bleaching by a factor of 33. Our findings suggest that reefs with greater high-frequency temperature variability may represent particularly important opportunities to conserve coral ecosystems against the major threat posed by warming ocean temperatures

Miniature thermistor chain for determining surficial sediment porewater advection

A miniature thermistor chain (mTc) was developed to measure the subdiurnal variability of temperature in the upper layers of subtidal coastal permeable (sandy) sediments and across the sediment-water interface (SWI). The mTc has 15 precision thermistors (0.002°C accuracy) attached by narrow tines to a stainless steel backbone that connects to an electronics module, all of which is buried in the top 20 cm of the sediment. Instrument performance was tested by deploying the mTc in nearshore permeable sediment at the Kilo Nalu Observatory, Oahu, Hawaii over an 80-d period. The mTc reached thermal equilibrium with the adjoining sediment within a few days after deployment and then recorded the advective propagation of the sub-daily water-column temperature variation into the sediment. The data produced are consistent with predicted effects of surface waves on advective porewater transport: transport rate increased with wave height and decreased with depth below the SWI, and temperature time lag increased with depth below the SWI. Data from an independent, more deeply buried thermistor are in good agreement with the mTc time-series data, showing attenuated temperature variability and similar (but longer, as expected) thermal time lags. Because thermal variations in surficial sediments is dominated by advection in wavy environments, mTc subdiurnal temperature propagation data can be used to calculate advective transport across the SWI and as deep as 20 cm into the sediment (i.e., over depths where advection dominates over thermal diffusion).This is the publisher’s final pdf. The published article is copyrighted by the American Society of Limnology and Oceanography, Inc. and can be found at: http://aslo.org/lomethods/index.html

ADCP Bias and Stokes Drift in AUV-Based Velocity Measurements

AbstractA theoretical model is developed to describe how autonomous underwater vehicle (AUV)-based current measurements are influenced by a surface wave field. The model quantifies a quasi-Lagrangian, wave-induced velocity bias as a function of the local wave conditions, and the vehicle’s depth and velocity using a first-order expansion of the linear wave solution. The theoretical bias is verified via field experiments carried out off the coast of Oahu, Hawaii. Spatially averaged along- and cross-track AUV velocity measurements are calculated over one effective wavelength and compared with time-averaged, fixed ADCP measurements in a range of wave and current conditions. The wave-induced bias is calculated using wave directional spectra derived from fixed ADCP data. Ensemble-averaged velocity differences confirm the presence of the wave-induced bias O(1–5) cm s−1 and reveal an additional bias in the direction of the vehicle motion O(1) cm s−1. The analysis considers velocity measurements made using a Remote Environmental Monitoring Units (REMUS) 100 AUV, but the content applies to any small AUV (vehicle size wavelength) immersed in a wave field