Ocean response to wind perturbations: implications for ENSO and decadal climate variability

The El Nino Southern Oscillation (ENSO) is the dominant mode of climate variability in the Pacific which influences weather and climate patterns worldwide. Variations in equatorial zonal wind stress in the western Pacific Ocean, an important part of the ENSO cycle, impact the ocean thermocline in the Pacific both locally (in the equatorial region) and remotely (away from the equator along the western coasts of the Americas). Through atmospheric teleconnections, ENSO also induces variations in the meridional winds along the California coast which contribute to changes in the ocean thermocline and modulate the California Current System (CCS). One of the prevailing paradigms for the dynamics of ENSO is the recharge/discharge oscillator model (Jin 1997). Ocean heat recharge is a key element of this model, in which a net deepening of the equatorial thermocline, acting to increase the equatorial ocean heat content and recharge the ocean, must occur prior to the following El Nino event.;In this study we employ several idealized approaches to systematically explore the response of the tropical ocean to zonal and meridional wind variations associated with the ENSO cycle and to similar wind variations but on longer, decadal timescales. Firstly, a 1½-layer shallow-water model is used to examine variations in the thermocline depth in the North Pacific Ocean associated with simulated ENSO events and decadal climate variability. Remote and local forcing of thermocline depth anomalies along the coast of California is investigated. Subsurface thermal anomalies can be forced along the coast of North America either by propagation of thermocline anomalies initially forced in the eastern equatorial Pacific Ocean, or by variations in the local wind stress caused by atmospheric teleconnections from the equatorial region. These two types of thermocline anomalies are examined within the shallow-water model.;Second, a low-frequency approximation is used to obtain analytical solutions to the shallow-water equations in terms of a perturbation expansion which involves a parameter epsilon=epsilonm+io, where epsilon m, is the ocean damping rate and (a is the dominant frequency of the wind forcing applied to the ocean basin. The low-frequency approximation assumes that this parameter is small as compared to 1/Tk, where T k is the time necessary for equatorial Kelvin waves to cross the Pacific Ocean, i.e. |epsilon|<<1/Tk. The derived expressions allow us to examine in detail the same idealized cases relevant to ENSO and decadal climate variability explored in the shallow-water model. The results from the low-frequency expression and the shallow-water model are found to agree well in both magnitude and spatial scale of thermocline depth anomalies.;Further, the low-frequency limit is used to provide several predictions (in terms of simple analytical expressions) for the ocean recharge - a key component of the recharge oscillator paradigm of ENSO. A general expression for the phase difference between variations in equatorial warm water volume and eastern equatorial Pacific SST is obtained that depends only on three factors -- the typical oscillation frequency, oceanic damping rates and the meridional shape of the wind stress anomalies. Using data from the Coupled Model Intercomparison Project (CMIP3) this phase difference is investigated in various coupled GCMs and compared with the low-frequency prediction. We show that, even though this phase difference varies broadly from one model to the next, affecting a number of ENSO characteristics including its period and amplitude, in most cases the low-frequency approximation gives an accurate quantitative estimation of the phase lag in these models.