Diurnal changes in the bio-optical properties of the phytoplankton in the Alborán Sea (Mediterranean Sea)

The changes in the phytoplankton absorption properties during a diurnal cycle were investigated at one station located in the north-western area of the Alborán Sea. The experiment was performed in spring when the water column was strongly stratified. This hydrological situation permitted the establishment of a deep chlorophyll a (chl a) fluorescence maximum (DFM) which was located on average close to the lower limit of the mixed layer and the nutricline. The relative abundance of pico-phytoplankton (estimated as its contribution to the total chl a) was higher in the surface, however, micro-phytoplankton dominated the community at the DFM level. Chl a specific absorption coefficient (a*(λ)) also varied with optical depth, with a* (the spectrally average specific absorption coefficient) decreasing by 30% at the DFM depth with respect to the surface. A significant negative correlation between the contribution of the micro-phytoplankton to the total chl a and a* was obtained indicating that a* reduction was due to changes in the packaging effect. Below the euphotic layer, a* increased three-fold with respect to the DFM, which agrees with the expected accumulation of accessory pigments relative to chl a as an acclimation response to the low available irradiance. The most conspicuous change during the diurnal cycle was produced in the euphotic layer where the chl a concentration decreased significantly in the afternoon (from a mean concentration of 1.1 μg L⁻¹ to 0.7 μg L⁻¹) and increased at dusk when it averaged 1.4 μg L⁻¹. In addition, a* and the blue-to-red absorption band ratio increased in the afternoon. These results suggest that a*(λ) diurnal variability was due to increase in photo-protective and accessory pigments relative to chl a. The variation ranges of a*(λ) at 675 and 440 nm (the absorption peaks in the red and blue spectral bands, respectively) in the euphotic layer were 0.01–0.04 and 0.02–0.10 m² mg⁻¹ chl a, respectively. Approximately 30% out of this variability can be attributed to the diurnal cycle. This factor should therefore be taken into account in refining primary production models based on phytoplankton light absorption.     

SMOS reveals the signature of Indian Ocean Dipole events

The tropical Indian Ocean experiences an interannual mode of climatic variability, known as the Indian Ocean Dipole (IOD). The signature of this variability in ocean salinity is hypothesized based on modeling and assimilation studies, on account of scanty observations. Soil Moisture and Ocean Salinity (SMOS) satellite has been designed to take up the challenge of sea surface salinity remote sensing. We show that SMOS data can be used to infer the pattern of salinity variability linked with the IOD events. The core of maximum variability is located in the central tropical basin, south of the equator. This region is anomalously salty during the 2010 negative IOD event, and anomalously fresh during the 2011 positive IOD event. The peak-to-peak anomaly exceeds one salinity unit, between late 2010 and late 2011. In conjunction with other observational datasets, SMOS data allow us to draw the salt budget of the area. It turns out that the horizontal advection is the main driver of salinity anomalies. This finding is confirmed by the analysis of the outputs of a numerical model. This study shows that the advent of SMOS makes it feasible the quantitative assessment of the mechanisms of ocean surface salinity variability in the tropical basins, at interannual timescales.

     

Sea Surface Salinity Observations from Space with the SMOS Satellite: A New Means to Monitor the Marine Branch of the Water Cycle

While it is well known that the ocean is one of the most important component of the climate system, with a heat capacity 1,100 times greater than the atmosphere, the ocean is also the primary reservoir for freshwater transport to the atmosphere and largest component of the global water cycle. Two new satellite sensors, the ESA Soil Moisture and Ocean Salinity (SMOS) and the NASA Aquarius SAC-D missions, are now providing the first space-borne measurements of the sea surface salinity (SSS). In this paper, we present examples demonstrating how SMOS-derived SSS data are being used to better characterize key land–ocean and atmosphere–ocean interaction processes that occur within the marine hydrological cycle. In particular, SMOS with its ocean mapping capability provides observations across the world’s largest tropical ocean fresh pool regions, and we discuss from intraseasonal to interannual precipitation impacts as well as large-scale river runoff from the Amazon–Orinoco and Congo rivers and its offshore advection. Synergistic multi-satellite analyses of these new surface salinity data sets combined with sea surface temperature, dynamical height and currents from altimetry, surface wind, ocean color, rainfall estimates, and in situ observations are shown to yield new freshwater budget insight. Finally, SSS observations from the SMOS and Aquarius/SAC-D sensors are combined to examine the response of the upper ocean to tropical cyclone passage including the potential role that a freshwater-induced upper ocean barrier layer may play in modulating surface cooling and enthalpy flux in tropical cyclone track regions.     

SMOS reveals the signature of Indian Ocean Dipole events

The tropical Indian Ocean experiences an interannual mode of climatic variability, known as the Indian Ocean Dipole (IOD). The signature of this variability in ocean salinity is hypothesized based on modeling and assimilation studies, on account of scanty observations. Soil Moisture and Ocean Salinity (SMOS) satellite has been designed to take up the challenge of sea surface salinity remote sensing. We show that SMOS data can be used to infer the pattern of salinity variability linked with the IOD events. The core of maximum variability is located in the central tropical basin, south of the equator. This region is anomalously salty during the 2010 negative IOD event, and anomalously fresh during the 2011 positive IOD event. The peak-to-peak anomaly exceeds one salinity unit, between late 2010 and late 2011. In conjunction with other observational datasets, SMOS data allow us to draw the salt budget of the area. It turns out that the horizontal advection is the main driver of salinity anomalies. This finding is confirmed by the analysis of the outputs of a numerical model. This study shows that the advent of SMOS makes it feasible the quantitative assessment of the mechanisms of ocean surface salinity variability in the tropical basins, at interannual timescales.     

Air Flow Structure Over Short-gravity Breaking Water Waves

Despite its importance for momentum and mass transfer across the air–sea interface, the dynamics of airflow over breaking waves is largely unknown. To fill this gap, velocity and vorticity distributions above short-gravity breaking waves have been measured in a wind-wave tank. A Digital Particle Image velocimetry technique (DPIV) was developed to accomplish these measurements above single breaking waves, propagating in mechanically-generated wave groups and forced by the wind. By varying the wind speed and initial characteristics of the groups, the airflow structure was captured over waves at different stages of the breaking process, and breaking with various intensities. The instantaneous airflow that separates from a sharp breaking crest is very similar to that occurring over a backward facing step. The separation bubble is however strongly unsteady: the steeper the wave crest and the larger the Reynolds number based on the crest-height, the higher the separated layer and the farther downwind the reattachment point. Instantaneous flow topology displays specific features of three-dimensional separation patterns. The tangential stress above the wave profile does not exhibit spikes at reattachment but grows progressively downwind from zero at reattachment to a value at the next crest approximately that found at the upwind breaking crest. Static pressure measurements revealed that large pressure falls are generated by vortices in the separated layer, as found in separated flows over solids. This study may provide useful data for theoretical and numerical modelling of the flow and associated phenomena.