The main objective of the ECUME group is to understand the role of the Ocean in Tropical Climate. In the Indian Ocean, our project is based on the design of observational experiment, on the use of ocean and atmosphere forced simulations as well as ocean-atmosphere coupled simulations and ocean data assimilation. Our principal center of interests are summarized below :
- Role of salinity and the barrier layer in the Indian Ocean dynamics and thermodynamics
- Long equatorial waves in the Indian Ocean
- Variability of the Indonesian Throughflow (observations)
- Role of the Throughflow in the Indian Ocean variability
- CIRENE observational project (PDF french version or english summary)
Observation and model studies have shown the presence of a barrier layer in the western Pacific Ocean. Briefly, the barrier layer is a layer of water located between the upper thermocline and the base of the mixed-layer. The characteristics of this barrier layer is that the temperature is mostly identical to the mixed-layer temperature while the salinity is saltier than in the mixed-layer. The base of the mixed-layer is therefore only characterized by a salinity gradient. Such a phenomenon is likely to happen in regions of low winds and high precipitation favouring the development of a thin upper ocean fresh layer. Such regions are typically the western equatorial Pacific Ocean, but is also likely to be present in the eastern equatorial Indian Ocean. The lack of observations in the Indian Ocean does not allow at that time to describe the presence and variability of the barier layer, nor to understand the role that it can play in that region. In order to investigate its role in the Indian Ocean and thus to potentially contribute to the design of future programs of observations, model studies are presently conducted at LODYC in the ECUME group by Sebastien Masson.
His studies are focusing on the 1979-1998 period using the OPA model in its Three Tropical Ocean Configuration (TOTEM) forced either by the ERA15 forcing fields and by the ERS winds during the 1992-1998 period. His results (to be submitted soon) show the presence of a barrier layer in the eastern Indian Ocean, characterize its seasonal and interannual variability as well as the role it plays in the dynamics and thermodynamics of the Indian Ocean.
In a recent paper (in press in Climate Dynamics), Jean-Luc Le Blanc and Jean-Philippe Boulanger studied the "Propagation and Reflection of Long Equatorial Waves in the Indian Ocean from TOPEX/POSEIDON Data during the 1993 -1998 Period" (Download the paper: PDF format, 1Mo; or the figures, PDF format, 1Mo)
Their study offers a description of long equatorial wave propagation and reflection in the Indian Ocean as observed by TOPEX/POSEIDON satellite. The equatorial long wave amplitudes are computed from January 1993 until December 1998, focusing on Kelvin (K) and first Rossby (R1) modes. The K wave is observed to propagate with a baroclinic phase speed close to 2m/s while the R1 has a phase speed of 0.7m/s. Wave reflections at both eastern and western boundaries are clearly observed during the entire period. At the eastern boundary K reflects into R1 with a reflection coefficient of 1.04 (reflection efficiency of 85% of that of an infinite meridional wall), and at the western boundary R1 reflects into K with a coefficient of 0.50 on average although a large seasonal and interannual variability of the reflection efficiency of the African coast is observed. Reflected K waves strongly influence the R1 variability near the western boundary as far as 60°E. Reflected R waves in turn, influence the K variability near the eastern boundary as far as 80°E. Although long equatorial waves are mainly forced by the changes in the winds during the inter-monsoon periods, reflection have a strong influence on the ocean’s variability over a large part of the basin on both seasonal and interannual time scales.
Focusing on the seasonal cycle of long equatorial waves (computed over 1993-1996), it is found that the constructive/destructive interaction between the wind forcing - and wave reflection is responsible for the K (resp. R1) seasonal cycle to show a semi-annual (resp. annual) period near the western (resp. eastern) boundary. In 1994 and 1997-98, strong interannual easterlies coincident with El Niño onsets in the Pacific occured in the Indian Ocean. Long equatorial wave propagation and reflection play a major role in creating an east-west sea level gradient along the equator, suggesting that wave dynamics and wave reflection may play a significant role in the Indian Dipole Mode suggested by several authors.
In the continuity of the JADE Programme (LODYC, M. Fieux and R. Molcard) and studies by Morrow and Birol (LEGOS), the TIP 2000 campaign of observations has various objectives:
- observe and describe the vertical structures of meso-scale anomalies (Rossby waves) observed by altimetry in the Leeuwin Current and off the Lombok Straight. These anomalies which propagate westward may play an important role in the heat and salt transport toward the Central Southern Indian Ocean
- observe and quantify the intensity of the Indonesian Throughflow
- complete the entire WOCE I10 section between Indonesia and Australia
Such objectives can be reached through the following actions:
- deployement of two subsurface moorings north and south of the Timor Straight allowing to collect data on the intensity of the Indonesian Throughflow simulatenously with the deployement of american moorings in the Makassar Straight
- mesurement of temperature data by XBT profiles from Indonesia (Bali) to Australia (Perth) as well as by CTD profiles allowing to improve our knowledge on salinity in that part of the Indian Ocean
- measurement of upper ocean currents by ADCP
This project aims at understanding the role of heat and salt transports from the Pacific to the Indian Ocean on the modes of variability of the Tropical Oceans with a special focus on Intraseasonal Oscillation Activity. This project is conducted using ocean-atmosphere coupled general circulation models (ORCA-LMDZ).
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