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Lee Y.-K.,Seoul National University | Yeh S.-W.,Hanyang University | Dewitte B.,Laboratoire dEtude en Geophysique et Oceanographie Spatiale | Moon B.-K.,Chonbuk National University | Jhun J.-G.,Seoul National University
Theoretical and Applied Climatology | Year: 2012

In order to understand the change in oceanic variability associated with the climate shift of the mid-1970s, we analyze the contribution of momentum forcing to the leading baroclinic modes over the tropical Pacific using Simple Ocean Data Assimilation (SODA, version 2. 0. 2) for the period of 1958-1997. Specifically, we look at the statistical relationship between the wind projection coefficients and climate indices and attempt to provide a physical explanation for the observed changes. It is found that the wind stress projection coefficients according to the oceanic baroclinic modes are different in terms of their magnitude and phase in the tropical Pacific, reflecting a specific forcing associated with each mode before and after the 1976 climate shift. Compared to that before the 1970s, the first baroclinic mode is had a greater effect on the interannual sea surface temperature due to equatorial wave dynamics, and there was an increased delayed response of the second baroclinic mode variability to the interannual atmospheric forcing after the late 1970s. This reflects changes in ENSO feedback processes associated with the climate shift. Our analysis further indicates that, after the late 1970s, there was a decrease in the wind stress forcing projecting onto the Ekman layer, which is associated with increased mixed-layer depth. This result suggests that the changes in the ENSO properties before and after the late 1970s are largely associated with the changes in the way in which the wind stress forcing is dynamically projected onto the surface layer of the tropical Pacific Ocean over interannual timescales. © 2011 Springer-Verlag. Source

Yeh S.-W.,Hanyang University | Dewitte B.,Laboratoire dEtude en Geophysique et Oceanographie Spatiale | Yim B.Y.,Yonsei University | Noh Y.,Yonsei University
Climate Dynamics | Year: 2010

The response of El Niño and Southern Oscillation (ENSO)-like variability to global warming varies comparatively between the two different climate system models, i.e., the Meteorological Research Institute (MRI) and Geophysical Fluid Dynamics Laboratory (GFDL) Coupled General Circulation Models (CGCMs). Here, we examine the role of the simulated upper ocean temperature structure in the different sensitivities of the simulated ENSO variability in the models based on the different level of CO2 concentrations. In the MRI model, the sea surface temperature (SST) undergoes a rather drastic modification, namely a tendency toward a permanent El Niño-like state. This is associated with an enhanced stratification which results in greater ENSO amplitude for the MRI model. On the other hand, the ENSO simulated by GFDL model is hardly modified although the mean temperature in the near surface layer increases. In order to understand the associated mechanisms we carry out a vertical mode decomposition of the mean equatorial stratification and a simplified heat balance analysis using an intermediate tropical Pacific model tuned from the CGCM outputs. It is found that in the MRI model the increased stratification is associated with an enhancement of the zonal advective feedback and the non-linear advection. In the GFDL model, on the other hand, the thermocline variability and associated anomalous vertical advection are reduced in the eastern equatorial Pacific under global warming, which erodes the thermocline feedback and explains why the ENSO amplitude is reduced in a warmer climate in this model. It is suggested that change in stratification associated with global warming impacts the equatorial wave dynamics in a way that enhances the second baroclinic mode over the gravest one, which leads to the change in feedback processes in the CGCMs. Our results illustrate that the upper ocean vertical structure simulated in the CGCMs is a key parameter of the sensitivity of ENSO-like SST variability to global warming. © 2010 Springer-Verlag. Source

Thual S.,Laboratoire dEtude en Geophysique et Oceanographie Spatiale | Thual O.,CNRS Institute of Fluid Mechanics of Toulouse | Dewitte B.,Laboratoire dEtude en Geophysique et Oceanographie Spatiale
Quarterly Journal of the Royal Meteorological Society | Year: 2013

The El Niño-Southern Oscillation (ENSO) is driven by ocean-atmosphere interactions in the equatorial Pacific, and this variability is often attributed to coupled modes that are evidenced by the temporal stability analysis of anomaly models. Here, the further diagnostic of absolute/convective instability is considered, which assesses whether small perturbations localized in space and time (e.g. random wind disturbances) lead to instabilities that develop in-place or propagate away from the perturbed region. It is shown that boundary conditions play a secondary role for this approach and that the development of large-scale wave packets in the equatorial Pacific basin is possible, as in the case of an infinite domain. As an illustration, two simple coupledmodels are diagnosed that rely either on thermocline processes or zonal advective processes. The model with thermocline processes is 'absolutely unstable' and therefore develops intrinsic oscillations, while the model with zonal advective processes is 'convectively unstable' and therefore acts as a noise amplifier. The identification of the two instability regimes may characterize different ENSO formation mechanisms as a response to random wind disturbances. For the absolutely unstable regime, a standing ENSO-like oscillation can develop in the equatorial Pacific without involving boundary reflections, while for the convectively unstable regime boundary reflections are essential. © 2012 Royal Meteorological Society. Source

Gushchina D.,Moscow State University | Dewitte B.,Laboratoire dEtude en Geophysique et Oceanographie Spatiale
Monthly Weather Review | Year: 2012

The characteristics of intraseasonal tropical variability (ITV) associated with the two flavors of El Niñ o [i. e., the canonical or eastern Pacific (EP) El Niño and the Modoki or central Pacific (CP) El Niñ o] are documented using composite and regression analysis. Double space-time Fourier analysis is applied to the NCEP-NCAR zonal wind at 850 hPa (U850) to separate the different components of the ITV in the tropical troposphere, which is then used to define indices of wave activity, and document the spatial pattern of the waves. It is shown that the ITV characteristics are altered during CP El Niño compared to the typical seasonal dependence of the ITV-ENSO relationship. In particular, while EP El Niño is characterized by enhanced MJO and equatorial Rossby (ER) wave activity during spring-summer prior to the ENSO peak, during CP El Niñ o, the ITV activity is increased during the mature and decaying phases. It is suggested that ITV is more propitious to the triggering of the EP event; while during the CP event, it contributes mostly to the persistence of positive SST anomalies. The oceanic response of these ITV anomalous patterns is further investigated in the Simple Ocean Data Assimilation (SODA) reanalysis by documenting the seasonal evolution of the intraseasonal equatorial oceanic Kelvin wave (IEKW) activity during the two flavors of El Niñ o. It is shown that anomalous westerlies associated with ITV may generate the corresponding response in the ocean in the form of anomalous IEKW activity. ©2012 American Meteorological Society. Source

Barruol G.,University Paris Diderot | Cordier E.,University Paris Diderot | Bascou J.,CNRS Magmas and Volcanoes Laboratory | Fontaine F.R.,University Paris Diderot | And 3 more authors.
Geophysical Research Letters | Year: 2013

The deployment of a seismic network along the Adélie and George V coasts in East Antarctica during the period 2009-2012 provides the opportunity to monitor cryoseismic activity and to obtain new insights on the relationship between tidal cycles and coastal glacier dynamics. Here we focus on records from a seismometer located on a rocky outcrop in the vicinity of the grounding line of the 35 km broad Mertz glacier, a major outflow of this region. We detect numerous icequakes (50,000 events within 10 months and up to 100 events/h) and demonstrate their clear tidal modulation. We suggest that they result from ice friction and fracturing around the rocky peak and from the glacier flexure in response to the falling and rising tides at its grounding area. We propose that such icequake monitoring could be used as a climate proxy since grounding lines are subject to migrate with sea level changes. Key Points Large microseismicity is recorded at the Mertz glacier grounding area Icequakes show clear evidence of tide modulation Ice fracturing is proposed to be largely controlled by glacier flexure ©2013. American Geophysical Union. All Rights Reserved. Source

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