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Séminaires

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Jeudi 18 Juin - Deep Eddy Kinetic Energy in the tropical Pacific from Lagrangian Floats

by SEMSOU last modified Jun 18, 2020 03:45 PM
When Jun 18, 2020
from 10:00 AM to 11:00 AM
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Audrey Delpech

LEGOS, Toulouse, France

 

Title: Deep Eddy Kinetic Energy in the tropical Pacific from Lagrangian Floats



Abstract: At the ocean surface, satellite observations have shown evidence of a large spectrum of waves at low latitudes, responsible for a large part of the surface variability. However, very little is known on the existence and properties of the deep variability. Most of the subsurface observations rely on localized measurements, which do not allow for a global estimation. In this study, we use the velocity estimates, provided by the Argo floats drifts at 1000 m, to analyze the spatial and temporal distribution of the deep Eddy Kinetic Energy (EKE) and its spectral signature with an unprecedented time and space coverage. In the tropical Pacific, high EKE is found along the Equator, at the western boundary and poleward of 7°N. EKE meridional distribution is also found to vary at the scale of the meridionally-alternating mean zonal jets: it is higher inside eastward currents. We develop a statistical scale analysis to determine the temporal and spatial scale-dependence of this deep EKE footprint. We show the presence of periodic features whose characteristics are compatible with theoretical Equatorial waves dispersion relations. Annual and semi-annual Rossby waves are observed at the Equator, as well as 30-day Yanai waves, consistent with surface Tropical Instability Waves. The location and intensification of these waves match the downward energy propagation predicted by the ray tracing linear theory. Short-scale variability (with ~70-day periods and 500-km wavelength) has also been detected poleward of 7°N. The generation mechanisms of this variability is discussed, as well as its potential importance for the mean circulation.

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Jeudi 7 Mai - Why do Benguela Niños lead Atlantic Niños?

by SEMSOU last modified Jun 18, 2020 03:40 PM
When May 07, 2020
from 11:00 AM to 12:00 PM
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Serena Illig

LEGOS, Toulouse, France

 

Title: Why do Benguela Niños lead Atlantic Niños?



Abstract: We investigate the lag between warm interannual Sea Surface Temperature (SST) events in the eastern equatorial Atlantic, the Atlantic Niños, and the occurrence of Benguela Niños along the southwestern Angolan coast. It is commonly agreed that both events are associated with equatorial and subsequent coastal-trapped wave propagations driven remotely by a relaxation of the trade-winds. Yet, we observe that coastal SST anomalies off Angola tend to precede the ones in the equatorial cold tongue region by ~1 month. We explain this counter-intuitive behavior using experimentation with a tropical Atlantic Ocean model. Using idealized wind-stress perturbations from a composite analysis, we simulate warm equatorial and coastal events over a stationary and then, seasonally-varying ocean mean-state. Results show that when wind-stress perturbations are confined to the western central equatorial Atlantic, the model yields equatorial events leading the coastal variability, consistent with the propagation path of the waves. This implies that neither the differences in the ocean stratification between the two regions (thermocline depths or modal wave contributions) nor its seasonal variability controls the timing between events. Only if wind-stress anomalies are prescribed in the coastal fringe, the coastal warming precedes the eastern equatorial SST anomaly peak, emphasizing the role of the local forcing in the phenology of Benguela Niños. Both warmings originate from a reduction in the strength of the South-Atlantic Anticyclone. Nevertheless, local processes initiate the coastal warming before the remotely-forced equatorial waves impact the eastern equatorial SST. Then, equatorward coastal wind anomalies, driven by a convergent anomalous circulation located on the warm Atlantic Niño, stop the remotely-forced coastal warming prematurely. In conclusion, this study shows evidence that Atlantic and Benguela Niños are connected via an ocean teleconnection associated with equatorial and coastal wave propagations, but they are also tied by a large-scale atmospheric circulation and ocean-atmosphere interactions.

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Jeudi 30 Avril - Imprint of Intrinsic Oceanic Variability on Interannual Variability in the Southwest Tropical Pacific

by SEMSOU last modified Jun 18, 2020 03:34 PM
When Apr 30, 2020
from 11:00 AM to 12:00 PM
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Sophie Cravatte

LEGOS, Toulouse, France

 

Title: Imprint of Intrinsic Oceanic Variability on Interannual Variability in the Southwest Tropical Pacific

 

Abstract: The Southwest Pacific Ocean plays an important role in the Pacific Ocean. It is on the pathways of waters redistributed from the southern subtropical gyre to the equatorial region. The westward South Equatorial Current divides into intense zonal jets when encountering the islands of Fiji, Vanuatu or New Caledonia. These zonal jets enter the Coral Sea and flow to the Australian coast, where they bifurcate toward the Solomon Sea or the Tasman Sea. The variability of these currents, resulting in a variability of the recharge of the equatorial band, is thought to participate to the modulation of ENSO (El Nino Southern Oscillation) cycle. Yet, the processes governing this interannual variability are not completely understood. The dominant mode of interannual variability in the Tropical Pacific is ENSO, but its impacts on currents in the Southwest Pacific are not well understood. An ensemble of 50 global 1/4° ocean simulations, driven by the same realistic atmospheric forcing over 1960-2015 from slightly perturbed initial conditions, is analyzed over 1980-2015 to interpret the currents interannual variability. The "deterministic" variability driven by the atmosphere (e.g. ENSO) is estimated from the ensemble mean evolution. The intrinsic ocean variability is then quantified from the random dispersion of the simulations around the ensemble mean. We show that there are large parts of the region where interannual transport variability is firstly driven by stochastic processes, which probably arise from a rectification of the lower-frequency signal by mesoscale stochastic activity. This important role played by ocean-only stochastic dynamics may thus hamper our capability to predict the interannual variability of the oceanic circulation in the Southwest Pacific.


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