Influence of natural surfactants on short wind waves in the coastal Peruvian waters

Results from measurements of wave slope statistics during the R/V Meteor M91 cruise in the coastal upwelling regions off the coast of Peru are reported. Wave slope probability distributions were measured with an instrument based on the reflection of light at the water surface and a method very similar to the Cox and Munk (1954b) sun glitter technique. During the cruise, the mean square slope (mss) of the waves was found to be very variable, despite the limited range of encountered wind speeds. The Cox and Munk (1954b) parameterization for clean water is found to overestimate mss, but most measurements fall in the range spanned by their clean water and slick parameterizations. The observed variability of mss is attributed to the wave damping effect of surface films, generated by increased biological production in the upwelling zones. The small footprint and high temporal resolution of the measurement allows for tracking abrupt changes in conditions caused by the often patchy structure of the surface films

Storyline description of Southern Hemisphere midlatitude circulation and precipitation response to greenhouse gas forcing

As evidence of climate change strengthens, knowledge of its regional implications becomes an urgent need for decision making. Current understanding of regional precipitation changes is substantially limited by our understanding of the atmospheric circulation response to climate change, which to a high degree remains uncertain. This uncertainty is reflected in the wide spread in atmospheric circulation changes projected in multimodel ensembles, which cannot be directly interpreted in a probabilistic sense. The uncertainty can instead be represented by studying a discrete set of physically plausible storylines of atmospheric circulation changes. By mining CMIP5 model output, here we take this broader perspective and develop storylines for Southern Hemisphere (SH) midlatitude circulation changes, conditioned on the degree of global-mean warming, based on the climate responses of two remote drivers: the enhanced warming of the tropical upper troposphere and the strengthening of the stratospheric polar vortex. For the three continental domains in the SH, we analyse the precipitation changes under each storyline. To allow comparison with previous studies, we also link both circulation and precipitation changes with those of the Southern Annular Mode. Our results show that the response to tropical warming leads to a strengthening of the midlatitude westerly winds, whilst the response to a delayed breakdown (for DJF) or strengthening (for JJA) of the stratospheric vortex leads to a poleward shift of the westerly winds and the storm tracks. However, the circulation response is not zonally symmetric and the regional precipitation storylines for South America, South Africa, South Australia and New Zealand exhibit quite specific dependencies on the two remote drivers, which are not well represented by changes in the Southern Annular Mode

The first joint ESGAR/ ESPR consensus statement on the technical performance of cross-sectional small bowel and colonic imaging

Objectives: To develop guidelines describing a standardised approach to patient preparation and acquisition protocols for magnetic resonance imaging (MRI), computed tomography (CT) and ultrasound (US) of the small bowel and colon, with an emphasis on imaging inflammatory bowel disease. Methods: An expert consensus committee of 13 members from the European Society of Gastrointestinal and Abdominal Radiology (ESGAR) and European Society of Paediatric Radiology (ESPR) undertook a six-stage modified Delphi process, including a detailed literature review, to create a series of consensus statements concerning patient preparation, imaging hardware and image acquisition protocols. Results: One hundred and fifty-seven statements were scored for agreement by the panel of which 129 statements (82 %) achieved immediate consensus with a further 19 (12 %) achieving consensus after appropriate modification. Nine (6 %) statements were rejected as consensus could not be reached. Conclusions: These expert consensus recommendations can be used to help guide cross-sectional radiological practice for imaging the small bowel and colon. Key points: • Cross-sectional imaging is increasingly used to evaluate the bowel • Image quality is paramount to achieving high diagnostic accuracy • Guidelines concerning patient preparation and image acquisition protocols are provided

A Parameter Model of Gas Exchange for the Seasonal Sea Ice Zone

Carbon budgets for the polar oceans require better constraint on air–sea gas exchange in the sea ice zone (SIZ). Here, we utilize advances in the theory of turbulence, mixing and air–sea flux in the ice–ocean boundary layer (IOBL) to formulate a simple model for gas exchange when the surface ocean is partially covered by sea ice. The gas transfer velocity (k) is related to shear-driven and convection-driven turbulence in the aqueous mass boundary layer, and to the mean-squared wave slope at the air–sea interface. We use the model to estimate k along the drift track of ice-tethered profilers (ITPs) in the Arctic. Individual estimates of daily-averaged k from ITP drifts ranged between 1.1 and 22 m d−1, and the fraction of open water (f) ranged from 0 to 0.83. Converted to area-weighted effective transfer velocities (keff), the minimum value of keff was 10−55 m d−1 near f = 0 with values exceeding keff = 5 m d−1 at f = 0.4. The model indicates that effects from shear and convection in the sea ice zone contribute an additional 40% to the magnitude of keff, beyond what would be predicted from an estimate of keff based solely upon a wind speed parameterization. Although the ultimate scaling relationship for gas exchange in the sea ice zone will require validation in laboratory and field studies, the basic parameter model described here demonstrates that it is feasible to formulate estimates of k based upon properties of the IOBL using data sources that presently exist

Analyzing the footprints of near-surface aqueous turbulence: An image processing-based approach

In this contribution, a detailed investigation of surface thermal patterns on the water surface is presented, with wind speeds ranging from 1 to 7 m s  − 1 and various surface conditions. Distinct structures can be observed on the surface—small-scale short-lived structures termed fish scales and larger-scale cold streaks that are consistent with the footprints of Langmuir circulations. The structure of the surface heat pattern depends strongly on wind-induced stress. Consistent behavior regarding the spacing of cold streaks can be observed in a range of laboratory facilities when expressed as a function of water-sided friction velocity, u * . This behavior systematically decreased until a point of saturation at u *  = 0.7 cm/s. We present a new image processing-based approach to the analysis of the spacing of cold streaks based on a machine learning approach to classify the thermal footprints of near-surface turbulence. Comparison is made with studies of Langmuir circulation and the following key points are found. Results suggest a saturation in the tangential stress, anticipating that similar behavior will be observed in the open ocean. A relation to Langmuir numbers shows that thermal footprints in infrared images are consistent with Langmuir circulations and depend strongly on wind wave conditions

Direct measurements of CO2 flux in the Greenland Sea

During summer 2006 eddy correlation CO2 fluxes were measured in the Greenland Sea using a novel system set-up with two shrouded LICOR-7500 detectors. One detector was used exclusively to determine, and allow the removal of, the bias on CO2 fluxes due to sensor motion. A recently published correction method for the CO2-H2O cross-correlation was applied to the data set. We show that even with shrouded sensors the data require significant correction due to this cross-correlation. This correction adjusts the average CO2 flux by an order of magnitude from -6.7 x 10⁻² mol m⁻² day⁻¹ to -0.61 x 10⁻² mol m⁻² day⁻¹, making the corrected fluxes comparable to those calculated using established parameterizations for transfer velocity

Lake-size dependency of wind shear and convection as controls on gas exchange

High-frequency physical observations from 40 temperate lakes were used to examine the relative contributions of wind shear (u*) and convection (w*) to turbulence in the surface mixed layer. Seasonal patterns of u* and w* were dissimilar; u* was often highest in the spring, while w * increased throughout the summer to a maximum in early fall. Convection was a larger mixed-layer turbulence source than wind shear (u */w*-1 for lakes* and w* differ in temporal pattern and magnitude across lakes, both convection and wind shear should be considered in future formulations of lake-air gas exchange, especially for small lakes. © 2012 by the American Geophysical Union.Jordan S. Read, David P. Hamilton, Ankur R. Desai, Kevin C. Rose, Sally MacIntyre, John D. Lenters, Robyn L. Smyth, Paul C. Hanson, Jonathan J. Cole, Peter A. Staehr, James A. Rusak, Donald C. Pierson, Justin D. Brookes, Alo Laas, and Chin H. W

Sea surface pCO2 and O2 in the Southern Ocean during the austral fall, 2008

The physical and biological processes controlling surface mixed layer pCO2 and O2 were evaluated using in situ sensors mounted on a Lagrangian drifter deployed in the Atlantic sector of the Southern Ocean (∼50°S, ∼37°W) during the austral fall of 2008. The drifter was deployed three times during different phases of the study. The surface ocean pCO2 was always less than atmospheric pCO2 (−50.4 to −76.1 μatm), and the ocean was a net sink for CO2 with fluxes averaging between 16.2 and 17.8 mmol C m−2 d−1. Vertical entrainment was the dominant process controlling mixed layer CO2, with fluxes that were 1.8 to 2.2 times greater than the gas exchange fluxes during the first two drifter deployments, and was 1.7 times greater during the third deployment. In contrast, during the first two deployments the surface mixed layer was always a source of O2 to the atmosphere, and air-sea gas exchange was the dominant process occurring, with fluxes that were 2.0 to 4.1 times greater than the vertical entrainment flux. During the third deployment O2 was near saturation the entire deployment and was a small source of O2 to the atmosphere. Net community production (NCP) was low during this study, with mean fluxes of 3.2 to 6.4 mmol C m−2 d−1 during the first deployment and nondetectable (within uncertainty) in the third. During the second deployment the NCP was not separable from lateral advection. Overall, this study indicates that in the early fall the area is a significant sink for atmospheric CO2

Microbreaking and the enhancement of air-water transfer velocity

The role of microscale wave breaking in controlling the air-water transfer of heat and gas is investigated in a laboratory wind-wave tank. The local heat transfer velocity, kH, is measured using an active infrared technique and the tank-averaged gas transfer velocity, kG, is measured using conservative mass balances. Simultaneous, colocated infrared and wave slope imagery show that wave-related areas of thermal boundary layer disruption and renewal are the turbulent wakes of microscale breaking waves, or microbreakers. The fractional area coverage of microbreakers, AB, is found to be 0.1–0.4 in the wind speed range 4.2–9.3 m s−1 for cleaned and surfactant-influenced surfaces, and kH and kG are correlated with AB. The correlation of kH with AB is independent of fetch and the presence of surfactants, while that for kG with AB depends on surfactants. Additionally, AB is correlated with the mean square wave slope, 〈S2〉, which has shown promise as a correlate for kG in previous studies. The ratio of kH measured inside and outside the microbreaker wakes is 3.4, demonstrating that at these wind speeds, up to 75% of the transfer is the direct result of microbreaking. These results provide quantitative evidence that microbreaking is the dominant mechanism contributing to air-water heat and gas transfer at low to moderate wind speeds