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Neetu S.,National Institute of Oceanography of India | Lengaigne M.,National Institute of Oceanography of India | Lengaigne M.,Institute Pierre Simon Laplace | Vincent E.M.,Institute Pierre Simon Laplace | And 6 more authors.
Journal of Geophysical Research: Oceans | Year: 2012

Surface cooling induced by tropical cyclones (TCs) is about three times larger during premonsoon than during postmonsoon season in the Bay of Bengal. We investigate processes responsible for this seasonal contrast using an ocean general circulation model. The model is forced by TC winds prescribed from an analytic vortex using observed TC tracks and intensities during 1978-2007. The simulation accurately captures the seasonal cycle of salinity, temperature, and barrier layer in this region, with fresher waters, deeper upper-ocean stratification, and thicker barrier layers during postmonsoon season. It also reproduces the three times larger TC-induced cooling during premonsoon than during postmonsoon season. This difference is essentially related to seasonal changes in oceanic stratification rather than to differences in TC wind energy input. During the postmonsoon season, a deeper thermal stratification combined with a considerable upper-ocean freshening strongly inhibits surface cooling induced by vertical mixing underneath TCs. On average, thermal stratification accounts for ∼60% of this cooling reduction during postmonsoon season, while haline stratification accounts for the remaining 40%. Their respective contributions however strongly vary within the Bay: haline stratification explains a large part of the TC-induced cooling inhibition offshore of northern rim of the Bay (Bangladesh-Myanmar-east coast of India), where salinity seasonal changes are the strongest, while thermal stratification explains all the cooling inhibition in the southwestern Bay. This study hence advocates for an improved representation of upper-ocean salinity and temperature effects in statistical and dynamical TCs forecasts that could lead to significant improvements of TC intensity prediction skill. © 2012. American Geophysical Union. All Rights Reserved.

Jourdain N.C.,CIRAD - Agricultural Research for Development | Marchesiello P.,Institute Of Recherche Pour Le Developpement | Marchesiello P.,Laboratoire dEtudes en Geophysique et Oceanographie Spatiale | Menkes C.E.,Institute Of Recherche Pour Le Developpement | And 6 more authors.
Journal of Climate | Year: 2011

The Weather Research and Forecast model at 1/3̊ resolution is used to simulate the statistics of tropical cyclone (TC) activity in the present climate of the South Pacific. In addition to the large-scale conditions, the model is shown to reproduce a wide range of mesoscale convective systems. Tropical cyclones grow from the most intense of these systems formed along the South Pacific convergence zone (SPCZ) and sometimes develop into hurricanes. The three-dimensional structure of simulated tropical cyclones is in excellent agreement with dropsondes and satellite observations. The mean seasonal and spatial distributions of TC genesis and occurrence are also in good agreement with the Joint Typhoon Warning Center (JTWC) data. It is noted, however, that the spatial pattern of TC activity is shifted to the northeast because of a similar bias in the environmental forcing. Over the whole genesis area, 8.2 ± 3.5 cyclones are produced seasonally in the model, compared with 6.6 ± 3.0 in the JTWC data. Part of the interannual variability is associated with El Niño-Southern Oscillation (ENSO). ENSO-driven displacement of the SPCZ position produces a dipole pattern of correlation and results in a weaker correlation when the opposing correlations of the dipole are amalgamated over the entire South Pacific region. As a result, environmentally forced variability at the regional scale is relatively weak, that is, of comparable order to stochastic variability (±1.7 cyclones yr-1), which is estimated from a 10-yr climatological simulation. Stochastic variability appears essentially related to mesoscale interactions, which also affect TC tracks and the resulting occurrence. © 2011 American Meteorological Society.

Gushchina D.,Moscow State University | Dewitte B.,Laboratoire dEtudes en Geophysique et Oceanographie Spatiale
Central European Journal of Geosciences | Year: 2011

The intraseasonal tropical variability (ITV) patterns in the tropical troposphere are documented using double space-time Fourier analysis. Madden and Julian oscillations (MJO) as well as equatorial coupled waves (Kelvin and Rossby) are investigated based on the NCEP/NCAR Reanalysis data for the 1977-2006 period and the outputs of an intermediate ocean-atmosphere coupled model named LODCA-OTCM. A strong seasonal dependence of the ITV/ENSO relationship is evidenced. The leading relationship for equatorial Rossby waves (with the correlation of the same order than for the MJO) is documented; namely, it is shown that intensification of Rossby waves in the central Pacific during boreal summer precedes by half a year the peak of El Niño. The fact that MJO activity in spring-summer is associated to the strength of subsequent El Niño is confirmed. It is shown that LODCA-QTCM is capable of simulating the convectively coupled equatorial waves in outgoing long wave radiation and zonal wind at 850 hPa fields with skill comparable to other Coupled General Circulation Models. The ITV/ENSO relationship is modulated at low frequency. In particular the periods of low ENSO amplitude are associated with weaker MJO activity and a cancellation of MJO at the ENSO development phase. In opposition, during the decaying phase, MJO signal is strong. The periods of strong ENSO activity are associated with a marked coupling between MJO, Kelvin and equatorially Rossby waves and ENSO; the precursor signal of MJO (Rossby waves) in the western (central) Pacific is obvious. The results provide material for the observed change in ENSO characteristics in recent years and question whether the characteristics of the ITV/ENSO relationship may be sensitive to the observed warming in the central tropical Pacific. © 2011 © Versita Warsaw and Springer-Verlag Wien.

Meyssignac B.,Laboratoire dEtudes en Geophysique et Oceanographie Spatiale | Becker M.,Laboratoire dEtudes en Geophysique et Oceanographie Spatiale | Becker M.,IRD Montpellier | Llovel W.,Laboratoire dEtudes en Geophysique et Oceanographie Spatiale | And 2 more authors.
Surveys in Geophysics | Year: 2012

We compare different past sea level reconstructions over the 1950-2009 time span using the Empirical Orthogonal Function (EOF) approach. The reconstructions are based on 91 long (up to 60 years) but sparsely distributed tide-gauge records and gridded sea level data from two numerical ocean models over 1958-2007 (the DRAKKAR/NEMO model without data assimilation and the simple ocean data assimilation ocean reanalysis-SODA-) and satellite altimetry data over 1993-2009. We find that the reconstructed global mean sea level computed over the ~60-year-long time span little depends on the input spatial grids. This is unlike the regional variability maps that appear very sensitive to the considered input spatial grids. Using the DRAKKAR/NEMO model, we test the influence of the period covered by the input spatial grids and the number of EOFs modes used to reconstruct sea level. Comparing with tide-gauge records not used in the reconstruction, we determine optimal values for these two parameters. As suggested by previous studies, the longer the time span covered by the spatial grids, the better the fit with unused tide gauges. Comparison of the reconstructed regional trends over 1950-2009 based on the two ocean models and satellite altimetry grids shows good agreement in the tropics and substantial differences in the mid and high latitude regions, and in western boundary current areas as well. The reconstructed spatial variability seems very sensitive to the input spatial information. No clear best case emerges. Thus, using the longest available model-based spatial functions will not necessarily give the most realistic results as it will be much dependent on the quality of the model (and its associated forcing). Altimetry-based reconstructions (with 17-year long input grids) give results somewhat similar to cases with longer model grids. It is likely that better representation of the sea level regional variability by satellite altimetry compensates the shorter input grids length. While waiting for much longer altimetry records, improved past sea level reconstructions may be obtained by averaging an ensemble of different model-based reconstructions, as classically done in climate modelling. Here, we present such a 'mean' reconstruction (with associated uncertainty) based on averaging the three individual reconstructions discussed above. © 2012 Springer Science+Business Media B.V.

Dencausse G.,French National Center for Scientific Research | Dencausse G.,Laboratoire dEtudes en Geophysique et Oceanographie Spatiale | Arhan M.,French National Center for Scientific Research | Speich S.,French National Center for Scientific Research
Journal of Geophysical Research: Oceans | Year: 2011

A 14.3 year series of weekly absolute sea surface height (SSH) and associated geostrophic velocities is used for a study of the subtropical-to-subantarctic frontal system in the region south of Africa. Detecting the fronts from surface velocity maxima confirms a two-stepped transition in both the southeastern Atlantic Ocean (the Northern and Southern Subtropical fronts (NSTF, SSTF)) and southwestern Indian Ocean (the Agulhas Front and SSTF), as proposed previously from hydrographic data. An additional front associated with westward flow north of the NSTF is indicative of a partial eastern closure of the South Atlantic subtropical gyre between 11°E and 17.5°E. The role of northwestward propagating Agulhas rings in connecting the two fronts explains the varying location. The SSTF, which marks the southern limit of subtropical waters, is found continuous from the Atlantic to the Indian Ocean in the time-averaged SSH field. In the weekly fields, however, the velocity maximum criterion defining the front is often met at the southern flanks of Agulhas rings or Agulhas eddies at 12°E-23°E, indicating front disruptions. Such discontinuities suggest that no South Atlantic Central Water is directly advected into the Indian Ocean. Instead, the eastward limb of the Indo-Atlantic "super gyre" encompassing the subtropical gyres of both oceans would rest, at these longitudes, on diffusive processes at the upper levels, and possibly be enhanced at depth. A schematic diagram of the fronts and their relations to Agulhas rings and eddies is proposed. Copyright © 2011 by the American Geophysical Union.

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