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Fei-Fei Jin

Abstract

Basinwide very low-frequency (VLF) modes with zonal uniformly deepening and shoaling of the equatorial thermocline are found as solutions of linear shallow-water equations with the meridional basin boundaries under an equatorial β plane. They can be understood as the free heat-content recharge oscillations. The interannual VLF modes had been recognized as the essential part of the coupled recharge oscillator theory for the El Niño–Southern Oscillation. It is suggested that the decadal VLF modes may also be transformed into coupled modes relevant to the Pacific decadal climate variability through the ocean–atmosphere interaction in the Tropics.

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Fei-Fei Jin

Abstract

A new conceptual model for ENSO has been constructed based upon the positive feedback of tropical ocean–atmosphere interaction proposed by Bjerknes as the growth mechanism and the recharge–discharge of the equatorial heat content as the phase-transition mechanism suggested by Cane and Zebiak and by Wyrtki. This model combines SST dynamics and ocean adjustment dynamics into a coupled basinwide recharge oscillator that relies on the nonequilibrium between the zonal mean equatorial thermocline depth and wind stress. Over a wide range of the relative coupling coefficient, this recharge oscillator can be either self-excited or stochastically sustained. Its period is robust in the range of 3–5 years. This recharge oscillator model clearly depicts the slow physics of ENSO and also embodies the delayed oscillator (Schopf and Suarez; Battisti and Hirst) without requiring an explicit wave delay. It can also be viewed as a mixed SST–ocean dynamics oscillator due to the fact that it arises from the merging of two uncoupled modes, a decaying SST mode and a basinwide ocean adjustment mode, through the tropical ocean–atmosphere coupling. The basic characteristics of this recharge oscillator, including the relationship between the equatorial western Pacific thermocline depth and the eastern Pacific SST anomalies, are in agreement with those of ENSO variability in the observations and simulations with the Zebiak–Cane model.

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Fei-Fei Jin

Abstract

A linear coupled model for the atmosphere–upper-ocean system is proposed to highlight the mechanisms of decadal to interdecadal climate variability in the North Pacific. In this model, wind stress anomalies over the North Pacific are related to anomalies in the meridional temperature gradient of the upper ocean. The latter depends upon air–sea thermodynamical feedbacks and meridional heat transport by upper-ocean currents. Slow adjustment of the oceanic gyre circulation to the change in wind stress is accomplished by the forced baroclinic oceanic Rossby waves, which carry out the meridional heat transport. Uncoupled ocean dynamic adjustment can produce a weak decadal to interdecadal peak in the power spectrum of the meridional transport under temporal white noise wind stress forcing with organized spatial structure. Coupled dynamics produce a basin-scale interdecadal oscillatory mode. This mode arises from the dynamic coupling and the memory of the system, residing in the slow gyre circulation adjustment. Its stability is heavily controlled by the ocean thermal damping, and its period is about one and one-half to three times the decadal ocean dynamic adjustment time. In the relevant parameter regime, this coupled mode produces a robust and pronounced interdecadal spectral peak in the upper-ocean temperature and the Sverdrup transport of the gyre circulation. The interdecadal oscillations reproduced in the simple model provide insights into main physical mechanisms of the North Pacific decadal–interdecadal variability observed in nature and simulated in coupled general circulation models.

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Fei-Fei Jin

Abstract

The conceptual recharge oscillator model intuitively established in Part I is derived from a dynamical framework of a Cane–Zebiak type model for tropical ocean–atmosphere interaction. A two-strip approximation to the equatorial ocean dynamics and one-strip approximation to the SST dynamics are employed to obtain a stripped-down coupled model that captures the main physics of the Cane–Zebiak type model. It is shown that the conceptual recharge oscillator model can be obtained from the stripped-down coupled model with a two-box approximation in the zonal direction or a low-frequency approximation to filter out high-frequency modes. Linear solutions of the stripped-down model are analytically solved and the dependence of coupled modes on various model parameters is delineated. In different parameter regimes, the stripped-down coupled model describes a coupled-wave mode and a mixed SST–ocean-dynamics mode that results from the merger of a nonoscillatory ocean adjustment mode with an SST mode. These two coupled oscillatory modes undergo a further merger. In the neighborhood of this merger, the leading mode of the system becomes a generalized mixed mode. It is suggested that the slow ENSO regime can be best characterized by this generalized mixed mode whose essential physics are described by the conceptual recharge oscillator model proposed in Part I.

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Jeff Drbohlav and Fei-Fei Jin

Abstract

A meridional-plane, hemispherical ocean model is developed to study interdecadal variability of the thermohaline circulation (THC). The model differs from previous formulations of zonally averaged ocean models by using a prognostic equation to calculate the meridional velocity. This allows the incorporation of an adjustment timescale comparable to the advective timescale of the meridional overturning. An interdecadal oscillation is documented for an idealized ocean of homogeneous salinity forced by a time-independent surface heat flux.

The governing equations are linearized about a basic state in order to isolate the effect of parameter changes on the oscillation. An eigenvalue analysis reveals that the frequency of the oscillation is independent of the advective timescale. Instead, the interdecadal timescale of the oscillation is set by the slow adjustment of the overturning intensity to a meridional pressure gradient anomaly. The oscillatory instability, on the other hand, is dependent on both advective and adjustment processes. Advection by the basic state acts locally to reinforce the meridional pressure gradient anomaly, whereas the delayed adjustment of the overturning intensity to this pressure gradient anomaly modifies the poleward transport of heat, thereby initiating the phase reversal of the pressure gradient anomaly. Thus, the advection of temperature anomalies by the basic state provides a positive feedback while the adjustment of the overturning intensity serves as the phase-switching mechanism.

The relevance of this “adjustment oscillator” to the interdecadal variability simulated in idealized ocean general circulation models is discussed. The results strongly suggest that internal, interdecadal variability of the THC is not an inherently three-dimensional or nonlinear phenomenon and that this type of variability cannot be conceptualized as a loop oscillator.

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Luis Bejarano and Fei-Fei Jin

Abstract

To study the regimes of leading ocean–atmosphere coupled modes of relevance to the El Niño–Southern Oscillation (ENSO) phenomenon, a comprehensive eigenmode analysis of an intermediate coupled model linearized with respect to an array of basic states is performed. Different kinds of leading modes are found to coexist and become unstable under wide ranges of basic states and parameter conditions. In particular, the two most important modes have periods of around 4 and 2 yr. They are referred to as the quasi-quadrennial (QQ) and the quasi-biennial (QB) modes, respectively. The positive coupled feedback destabilizes and quantizes the near-continuous spectrum for the low-frequency modes of the upper-ocean dynamics, giving rise to these leading modes with distinct periodicities. The primary mechanism for the phase transition of the QQ mode is due to the slow oceanic dynamic adjustment of equatorial heat content, which is consistent with the simple conceptual recharge oscillator, whereas anomalous advection of sea surface temperature by equatorial zonal current anomalies plays a dominant role in the phase transition of the QB mode. The coexistence of these ENSO-like coupled modes under the present climate conditions may provide an explanation for the observed rich variations in ENSO behaviors.

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Amy Solomon and Fei-Fei Jin

Abstract

Concurrent with most large El Niño events, cold sea surface temperature (SST) anomalies are observed over the western Pacific warm pool region (WPWP). Observational evidence that SST anomalies that form in the off-equatorial western Pacific during El Niño–Southern Oscillation (ENSO) cycles are forced by subsurface ocean processes equatorward of 12°N and air–sea fluxes poleward of 12°N is presented. It is demonstrated that diurnal mixing in the ocean equatorward of 12°N plays a significant role in bringing subsurface temperature anomalies to the sea surface during an El Niño event.

The role of SST anomalies equatorward of 12°N in ENSO cycles is tested in the Zebiak–Cane coupled model, modified to allow for the impact of subsurface temperatures on SSTs. This coupled model successfully simulates cold SST anomalies in the off-equatorial northwestern Pacific that are observed to occur during the warm phase of ENSO and the atmospheric response to these anomalies, which is composed of both westerlies in the central Pacific and easterlies in the far western equatorial Pacific. It is found that there is little net change in the zonal mean wind stress at the equator, suggesting that the westerlies cancel the impact of the easterlies on the basin-scale tilt of the equatorial zonal mean thermocline depth. The anomalous westerly winds in the central equatorial Pacific are found to increase the amplitude of an El Niño event directly by increasing anomalous warm zonal advection and reducing upwelling. Moreover, the off-equatorial anticyclonic wind stress associated with the cold SST anomalies during the warm phase of ENSO tends to reduce the discharge of the equatorial heat content. Thus, the coupled processes over the western Pacific warm pool can serve as a positive feedback to amplify ENSO cycles.

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Fei-Fei Jin and David Neelin

Abstract

The parameter-space dependence of the eigenmodes of the coupled tropical ocean-atmosphere system, linearized about a climatological basic state, is further examined in a stripped-down intermediate coupled model using the formulation derived in Part II of this study to permit analytical treatment for a finite ocean basin. Part II examined the limit of weak coupling and showed the rapid transition to the mixed SST/ocean dynamics modes of Part I, where it was argued that realistically coupled modes are best understood from strong coupling. Here cases with order unity and larger coupling are explored to provide analytical prototypes for the fully coupled case from a system that explicitly treats spatial structure in a finite basin. The coupled dynamics is explored for several regions of parameter space where simplifications are possible, as well as for the transition from the well-separated case to mixed modes.

The case of surface-layer processes only provides a simple example of westward-propagating SST modes. Extensive results are given for SST modes in the fast-wave limit. In addition to propagating SST modes, stationary, purely growing SST modes exist over a significant range of parameters; these are focused on because of their close relation to the mixed SST/ocean-dynamics modes with standing SST oscillations and subsurface memory. The latter can be thought of as stationary SST modes perturbed by wave dynamics. The east basin trapping exhibited by these modes can be produced oven in a zonally homogeneous basic state as the result of east-west asymmetry due to β in both atmosphere and ocean.

An important new case is the strong-coupling limit where strongly growing modes dominated by coupled processes are examined. These depend on both SST and ocean-dynamics time scales, but equatorial oceanic wave dynamics in the conventional sense is secondary to coupled processes in the basin interior. Because of this, these strongly growing modes are directly connected to SST modes in the fast-wave limit: extrapolating from the strong-coupling limit toward the fast-wave limit, and vice versa, permits this eigensurface to be pieced together qualitatively. Purely growing modes in the strong-coupling limit can be traced all the way from the fast-wave limit to its converse, the fast-SST limit. This, and the relation of the strongly coupled modes to the SST modes, serves to explain the connection of the eigensurfaces found in Part I and suggests that they must be a very robust feature of the coupled system.

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Ruihuang Xie and Fei-Fei Jin

Abstract

Modern instrumental records reveal that El Niño events differ in their spatial patterns and temporal evolutions. Attempts have been made to categorize them roughly into two main types: eastern Pacific (EP; or cold tongue) and central Pacific (CP; or warm pool) El Niño events. In this study, a modified version of the Zebiak–Cane (MZC) coupled model is used to examine the dynamics of these two types of El Niño events. Linear eigenanalysis of the model is conducted to show that there are two leading El Niño–Southern Oscillation (ENSO) modes with their SST patterns resembling those of two types of El Niño. Thus, they are referred to as the EP and CP ENSO modes. These two modes are sensitive to changes in the mean states. The heat budget analyses demonstrate that the EP (CP) mode is dominated by thermocline (zonal advective) feedback. Therefore, the weak (strong) mean wind stress and deep (shallow) mean thermocline prefer the EP (CP) ENSO mode because of the relative dominance of thermocline (zonal advective) feedback under such a mean state. Consistent with the linear stability analysis, the occurrence ratio of CP/EP El Niño events in the nonlinear simulations generally increases toward the regime where the linear CP ENSO mode has relatively higher growth rate. These analyses suggest that the coexistence of two leading ENSO modes is responsible for two types of El Niño simulated in the MZC model. This model result may provide a plausible scenario for the observed ENSO diversity.

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Masahiro Watanabe and Fei-Fei Jin

Abstract

A newly developed, linear baroclinic model (LBM) and its application to the tropical ENSO teleconnection is described. The model, based on primitive equations linearized about the observed, zonally varying basic state in northern winter, involves linear schemes for the cumulus convection, and surface sensible and latent heat fluxes, referred to as the moist LBM. This enables us to solve a steady-state response of the coupled dynamical–convective system to a given SST anomaly, which is fairly different from the conventional dry LBM. Linear representation of the convection is acceptable for a realistic range of SST anomalies, reproducing well the Rossby wave train computed in the conventional LBM forced by a tropical heating.

The moist LBM is used to examine the formation mechanisms of an anomalous low-level anticyclone near the Philippines that links El Niño with the Asian winter monsoon. Given that the conventional LBM simulates the Philippine Sea anticyclone as a Rossby response to the anomalous diabatic cooling associated with the weakened convection over the Maritime Continent, causes of the convective suppression are examined. Moist LBM experiments forced by observed El Niño SST anomalies indicate that a basinwide warming of the Indian Ocean, in addition to SST anomalies in the Pacific, has a considerable impact in weakening the convection over the Maritime Continent through a modulation of the Walker circulation. Observational analysis supports this idea and further suggests that the lagged Indian Ocean response to El Niño contributes to determining when the Philippine Sea anticyclone is developed. The moist LBM identified a positive wind–evaporation feedback at work between the Philippine anticyclone and the western Pacific SST anomaly, which might also contribute.

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