AlborEx: a multi-platform interdisciplinary view of Meso and Submesoscale processes

Ananda Pascual (IMEDEA(CSIC-UIB), Spain)

CoAuthors

Ruiz Simón (IMEDEA(CSIC-UIB), Spain); Troupin Charles (SOCIB, Spain); Antonio Olita (CNR, Italy); Mariona Claret (McGillU, Canada); Amala Mahadevan (WHOI, US); Baptiste Mourre (SOCIB, SPAIN); Pierre-Marie Poulain (OGS, Italy); Arthur Capet (OSG, Italy); Joaquín Tintoré (SOCIB and IMEDEA(CSIC-UIB), Spain); Antonio Tovar (IMEDEA(CSIC-UIB) and ICMAN, SPAIN)

Event: 2015 Ocean Surface Topography Science Team Meeting

Session: OSTST Opening Plenary Session

Presentation type: Type Keynote/invited

Vertical motion associated with mesoscale and sub-mesoscale features plays a major role in the exchanges of properties between the surface and the ocean interior. Modelling studies of frontal regions suggest that vertical exchange is enhanced at density fronts. Significant uncertainty still exists, however, in our understanding of the net effect of fine-scale variability on biochemical tracer redistribution and the consequent marine ecosystem response.

One of the key challenges in estimating vertical fluxes from observations is related to the availability of high-resolution data. To anticipate the 2D SSH fields that will be provided by SWOT, allowing first-time measurements in the 15-100 km band, multi-sensor synoptic observations need to be collected to contribute to the understanding of the vertical exchanges associated with (sub)-mesoscale structures. The observational approach must be integrated with numerical simulations, both realistic and process-oriented.

In this study, we present the results of ALBOREX, a multi-platform and multi-disciplinary experiment completed in May 2014 as a part of PERSEUS EU funded project. This unique process-oriented experiment in the Eastern Alboran Sea, aimed at studying submesoscale dynamics and interactions at the mesoscale. The field campaign, conducted during 8 days, included 25 drifters, 2 gliders, 3 Argo floats and one ship (66 CTDs and 500 biochemical samples). The drifters followed coherently an anticyclonic gyre. Near real time data from ADCP showed consistent patterns with currents up to 1m/s (2 knots) in the southern part of the sampled domain. This is almost a factor of 2 larger than the magnitude of derived surface currents from standard altimetry gridded fields. The Rossby number derived from ADCP data reaches a value of 1.5 suggesting significant ageostrophic motion. QG Omega equation is used to investigate the mesoscale contribution (structures larger than 20 km) to the upward/downward motion associated with the anticyclonic gyre in the frontal zone. Vertical velocities of about ±20 m/day have been diagnosed.

Quasi-synoptic glider observations revealed submesoscale structures associated with a frontal zone and a deep chlorophyll maximum (DCM), at a resolution that enabled us to investigate its relationship with mesoscale and submesoscale dynamics. The front was formed where Atlantic Waters (AW), entering from Gibraltar, encounter local Mediterranean waters, with a density difference of about 1.5 kg/m³ in less than 5 km. Simultaneous Argo profiler samplings, equipped with Photosynthetic Active Radiation (PAR) sensor, allowed to calibrate and empirical function to model instantaneous PAR profiles in the Glider temporal and spatial frame as a function of depth and Chlorophyll. Then, a standard bio-optical model was applied to assess the instantaneous primary production rates along the glider trip. Negative exponential relationships between Apparent Oxygen Utilization (AOU) and instantaneous primary production, within the high biomass patches, draw different slopes as function of the water masses. Integrated primary production maxima are detected in correspondence of the AW veins intercepted by the glider.

Observations are complemented by numerical simulations. A retrospective simulation has been carried out over the sea-trial period using data assimilation, with the aim to simulate the small scale oceanic fields as realistically as possible so to support the analysis and interpretation of the collected dataset. The outputs of this simulation in synergy with glider observations are used to initialize a Process Study Ocean Model that resolves vertical transport at (sub-) mesoscale fronts in the Eastern Alboran Sea. The aim is to explain subduction features (namely tongues of temperature, chlorophyll and oxygen observed from glider data and in good agreement with nutrient vertical distributions, CTD and Argo data), sampled during the ALBOREX experiment. From a physical point of view, frontal baroclinic instabilities at mesoscales and submesoscales are linked, since submesoscale instabilities at the upper layer seem to trigger deeper frontal mesoscale instabilities. From a biological viewpoint, remineralization and production are combined, but might be spatially separated. Large production occurs on the Western Alboran due to entrainment of nutrients through Gibraltar Strait . This production then is laterally advected by the Atlantic jet towards the Eastern Alboran Sea. On this side, Mediterranean and Atlantic origin waters form mesoscale fronts with large density gradients that trigger submesoscale instabilities. Vertical transport associated with these instabilities is typically of the order of tens of meters a day, which might explain the subducted features observed.
 

Keynote/invited presentation show times:

RoomStart DateEnd Date
Grand Ballroom 1 Tue, Oct 20 2015,11:20 Tue, Oct 20 2015,11:40
Ananda Pascual
IMEDEA(CSIC-UIB)
Spain
ananda.pascual@imedea.uib-csic.es