Browsing by Author "Goubanova, Katerina"
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Item Restricted Assessing the impact of downscaled winds on a regional ocean model simulation of the Humboldt system(Elsevier, 2013-05) Cambon, Gildas; Goubanova, Katerina; Marchesiello, Patrick; Dewitte, Boris; Illig, Serena; Echevin, VincentSimulating the oceanic circulation in Eastern Boundary Upwelling Systems (EBUS) is a challenging issue due to the paucity of wind stress products of a sufficiently high spatial resolution to simulate the observed upwelling dynamics. In this study, we present the results of regional simulations of the Humboldt current system (Peru and Chile coasts) to assess the value of a statistical downscaling model of surface forcing. Twin experiments that differ only from the momentum flux forcing are carried out over the 1992–2000 period that encompasses the major 1997/98 El Niño/La Niña event. It is shown that the mean biases of the oceanic circulation can be drastically reduced simply substituting the mean wind field of NCEP reanalysis by a higher resolution mean product (QuikSCAT). The statistical downscaling model improves further the simulations allowing more realistic intraseasonal and interannual coastal undercurrent variability, which is notoriously strong off Central Peru and Central Chile. Despite some limitations, our results suggest that the statistical approach may be useful to regional oceanic studies of present and future climates.Item Restricted Change in El Niño flavours over 1958-2008: implications for the long-term trend of the upwelling off Peru(Elsevier, 2012-11-15) Dewitte, Boris; Vazquez-Cuervo, J.; Goubanova, Katerina; Illig, Serena; Takahashi, Ken; Cambon, G.; Purca, S.; Correa, D.; Gutierrez, D.; Sifeddine, A.; Ortlieb, L.The tropical Pacific variability has experienced changes in its characteristics over the last decades. In particular, there is some evidence of an increased occurrence of El Niño events in the central Pacific (a.k.a. ‘Central Pacific El Niño’ (CP El Niño) or ‘El Niño Modoki’), in contrast with the cold tongue or Eastern Pacific (EP) El Niño which develops in the eastern Pacific. Here we show that the different flavours of El Niño imply a contrasted Equatorial Kelvin Wave (EKW) characteristic and that their rectification on the mean upwelling condition off Peru through oceanic teleconnection is changed when the CP El Niño frequency of occurrence increases. The Simple Ocean Data Assimilation (SODA) reanalysis product is first used to document the seasonal evolution of the EKW during CP and EP El Niño. It is shown that the strong positive asymmetry of ENSO (El Niño Southern Oscillation) is mostly reflected into the EKW activity of the EP El Niño whereas during CP El Niño, the EKW is negatively skewed in the eastern Pacific. Along with slightly cooler conditions off Peru (shallow thermocline) during CP El Niño, this is favourable for the accumulation of cooler SST anomalies along the coast by the remotely forced coastal Kelvin wave. Such a process is observed in a high-resolution regional model of the Humboldt Current system using the SODA outputs as boundary conditions. In particular the model simulates a cooling trend of the SST off Peru although the wind stress forcing has no trend. The model is further used to document the vertical structure along the coast during the two types of El Niño. It is suggested that the increased occurrence of the CP El Niño may also lead to a reduction of mesoscale activity off Peru.Item Open Access ENSO regimes: reinterpreting the canonical and Modoki El Niño(American Geophysical Union (AGU), 2011-05) Takahashi, Ken; Montecinos, Aldo; Goubanova, Katerina; Dewitte, BorisWe propose that the first two empirical orthogonal function (EOF) modes of tropical Pacific sea surface temperature (SST) anomalies do not describe different phenomena (i.e., El Niño‐Southern Oscillation (ENSO) and “El Niño Modoki”) but rather the nonlinear evolution of ENSO. We introduce two new uncorrelated indices (E and C), based on the leading EOFs, that respectively account for extreme warm events in the eastern and cold/moderate warm events in the central equatorial Pacific, corresponding to regimes with different evolution. Recent trends in ENSO can be described as an increase in the central Pacific (C) variability that is associated with stronger cold events, as well as a reduction in the eastern Pacific (E) variability within the cold/moderate warm regime, consistent with model projections. However, little can be said observationally with respect to the extreme warm regime.Item Open Access Modes of covariability between sea surface temperature and wind stress intraseasonal anomalies along the coast of Peru from satellite observations (2000–2008)(American Geophysical Union (AGU), 2011-04) Dewitte, Boris; Illig, Serena; Renault, L.; Goubanova, Katerina; Takahashi, Ken; Gushchina, D.; Mosquera Vásquez, Kobi Alberto; Purca, S.The Tropical Rainfall Measuring Mission Microwave Imager sea surface temperature (SST) and QuikSCAT wind stress satellite data are used to investigate the intraseasonal upwelling variability along the coat of Peru over the period 2000–2008. Two regions of peak variance correspond to the central Peru region (Pisco region, 15°S) and the northern Peru region (Piura region, 5°S). A covariance analysis reveals a significant coherency between winds and SST anomalies off Pisco, consistent with Ekman pumping and transport dynamics. The upwelling cell consists in a meridionally extended fringe of colder (warmer) water extending as far as 250 km from the coast at 15°S. In the Piura region, the intraseasonal covariability pattern is represented by two modes, one relevant to the direct Ekman dynamics and the other one associated with the remote forcing of intraseasonal oceanic Kelvin wave. Two regimes of variability are evidenced. A low‐period regime (10–25 days) is the signature of Ekman transport/pumping dynamics and is remotely forced by the migratory atmospheric disturbances across the southeastern Pacific anticyclone. A high‐period regime (35–60 day band) is associated with the combined forcing of oceanic equatorial Kelvin waves and migratory atmospheric disturbances in the midlatitudes. In particular, the modes of covariability exhibit a prominent ∼50 day period energy peak. It is shown that this period arises from the impact of the first two baroclinic modes Kelvin wave, with the second baroclinic mode Kelvin wave being more influential on the Piura region.Item Restricted Statistical downscaling of sea-surface wind over the Peru–Chile upwelling region: diagnosing the impact of climate change from the IPSL-CM4 model(Springer, 2011-04) Goubanova, Katerina; Echevin, V.; Dewitte, Boris; Codron, F.; Takahashi, Ken; Terray, P.; Vrac, M.The key aspect of the ocean circulation off Peru–Chile is the wind-driven upwelling of deep, cold, nutrient-rich waters that promote a rich marine ecosystem. It has been suggested that global warming may be associated with an intensification of upwelling-favorable winds. However, the lack of high-resolution long-term observations has been a limitation for a quantitative analysis of this process. In this study, we use a statistical downscaling method to assess the regional impact of climate change on the sea-surface wind over the Peru–Chile upwelling region as simulated by the global coupled general circulation model IPSL-CM4. Taking advantage of the high-resolution QuikSCAT wind product and of the NCEP reanalysis data, a statistical model based on multiple linear regressions is built for the daily mean meridional and zonal wind at 10 m for the period 2000–2008. The large-scale 10 m wind components and sea level pressure are used as regional circulation predictors. The skill of the downscaling method is assessed by comparing with the surface wind derived from the ERS satellite measurements, with in situ wind observations collected by ICOADS and through crossvalidation. It is then applied to the outputs of the IPSLCM4 model over stabilized periods of the pre-industrial, 2 x CO₂ and 4 x CO₂ IPCC climate scenarios. The results indicate that surface along-shore winds off central Chile (off central Peru) experience a significant intensification (weakening) during Austral winter (summer) in warmer climates. This is associated with a general decrease in intra-seasonal variability.Item Open Access Variaciones intraestacionales de la Temperatura Superficial del Mar en la costa del Perú(Instituto Geofísico del Perú, 2015-09) Goubanova, Katerina; Illig, Serena; Dewitte, Boris; Takahashi, KenEn el presente artículo nos enfocaremos en las fluctuaciones de la TSM que se encuentran sobre la escala de tiempo intraestacional, la cual se refiere a las oscilaciones de las variables climáticas con un periodo que varía entre 30 y 120 días. Cabe mencionar que una parte de esta variabilidad intraestacional proviene de la variabilidad interna del océano y es asociada a la actividad de remolinos de mesoescala. El presente artículo no analiza la variabilidad interna, la cual es más marcada afuera de la zona de afloramiento costero, sino se centra sobre la variabilidad forzada que puede explicar los eventos cálidos o fríos asociados a una alteración del afloramiento promedio dentro de una estación. Es importante predecir tal tipo de evento pues este puede tener un gran impacto sobre el ecosistema marino y sobre el clima costero.