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Soon-Il An

Abstract

This study introduces the conditional maximum covariance analysis (CMCA). The normal maximum covariance analysis (MCA) is a method that isolates the most coherent pairs of spatial patterns and their associated time series by performing an eigenanalysis on the temporal covariance matrix between two geophysical fields. Different from the normal MCA, the CMCA not only isolates the most coherent patterns between two fields but also excludes the unwanted signal by subtracting the regressed value of each employed field that depends on the unwanted signal.

To evaluate the usefulness of the CMCA, it is applied to the tropical Indian Ocean sea surface temperature and surface wind stress anomalies, from which the El Niño–Southern Oscillation (ENSO) signal is removed. Results show that the first mode of the CMCA represents an east–west contrast pattern in SST and a monopole pattern in the zonal wind stress centered at the equatorial central Indian Ocean. The corresponding expansion coefficients are completely uncorrelated with the ENSO index. On the other hand, in the normal MCA, the expansion coefficients are correlated with both the ENSO index and the Indian Ocean east–west contrast pattern index. Thus, the CMCA method effectively detected the coherent patterns induced by the local air–sea interaction without the ENSO signal considered as an external factor, whereas the normal MCA detected the coherent patterns, but the effects of local and external factors cannot be separated.

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Soon-Il An

Abstract

The equatorial Pacific atmosphere responds differently to global warming in the Gill-type and Lindzen–Nigam models. Under an assumption of no change in the zonal sea surface temperature (SST) gradient in the Gill-type model, the Walker circulation is intensified in a warmer climate relative to current climatic conditions, while slightly weakened in the Lindzen–Nigam model. Furthermore, for more accurate derivation of the surface wind, the free atmosphere in the Gill-type model is combined with the atmospheric boundary layer. This modified Gill-type model actually produces weaker surface wind than the Gill-type model would, but the sensitivity of the Walker circulation to the warmer climate is similar to that obtained from the Gill-type model. These results may explain why the zonal gradient of equatorial Pacific SST during the twentieth century is observed to strengthen while the Walker circulation is not, even though they are dynamically linked.

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Soon-Il An

Abstract

Using ocean data assimilation products, variability of eastern Pacific Ocean tropical instability waves (TIWs) and their interaction with the El Niño–Southern Oscillation (ENSO) were analyzed. TIWs are known to heat the cold tongue through horizontal advection. Conversely, variability of the cold tongue influences TIW variability (TIWV). During La Niña, TIWs are more active and contribute to anomalous warming. During El Niño, TIWs are suppressed and induce an anomalous cooling. TIWV thus acts as negative feedback to ENSO. Interestingly, this feedback is stronger during La Niña than during El Niño. To investigate this negative/asymmetric feedback, a simple parameterization for the horizontal thermal flux convergence due to TIWs was incorporated into a simple ENSO model. The model results suggested that asymmetric thermal heating associated with TIWs can explain the El Niño–La Niña asymmetry (with larger-amplitude El Niños).

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Soon-Il An
and
Fei-Fei Jin

Abstract

El Niño events (warm) are often stronger than La Niña events (cold). This asymmetry is an intrinsic nonlinear characteristic of the El Niño–Southern Oscillation (ENSO) phenomenon. In order to measure the nonlinearity of ENSO, the maximum potential intensity (MPI) index and the nonlinear dynamic heating (NDH) of ENSO are proposed as qualitative and quantitative measures. The 1997/98 El Niño that was recorded as the strongest event in the past century and another strong El Niño event in 1982/83 nearly reached the MPI. During these superwarming events, the normal climatological conditions of the ocean and atmosphere were collapsed completely. The huge bursts of ENSO activity manifested in these events are attributable to the nonlinear dynamic processes.

Through a heat budget analysis of the ocean mixed layer it is found that throughout much of the ENSO episodes of 1982/83 and 1997/98, the NDH strengthened these warm events and weakened subsequent La Niña events. This led to the warm–cold asymmetry. It is also found that the eastward-propagating feature in these two El Niño events provided a favorable phase relationship between temperature and current that resulted in the strong nonlinear dynamical warming. For the westward-propagating El Niño events prior to the late 1970s (e.g., 1957/58 and 1972/73 ENSOs) the phase relationships between zonal temperature gradient and current and between the surface and subsurface temperature anomalies are unfavorable for nonlinear dynamic heating, and thereby the ENSO events are not strong.

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Soon-Il An
and
In-Sik Kang

Abstract

The recharge oscillator paradigm for ENSO is further investigated by using a simple coupled model, which externally includes the equatorial wave dynamics represented by the Kelvin and gravest symmetric Rossby waves. To investigate the role of eddies in the Pacific basin–wide adjustment to the wind forcing, particularly at the western and eastern boundaries, the zonal mean and eddy parts are treated separately in the current model.

It is clearly demonstrated that the basin-wide adjustment of the tropical ocean is accomplished by the net mass transport induced by the meridional transport over the tropical ocean interior and the zonal fluxes at the boundaries. With a reasonable choice of the reflection coefficient, particularly at the western boundary, the meridional transport plays a bigger role than the zonal boundary flux and determines the sign of zonal-mean thermocline depth tendency, in a way that the discharge of equatorial mass in the warm phase and recharge in the cold phase serve as a phase transition mechanism of the coupled system. The meridional mass transport is induced mainly by a geostrophic current associated with the east–west slope of thermocline depth, established quickly by the wind forcing. Also discussed in this paper is the difference between the recharge oscillator and the delayed oscillator in explaining the phase transition mechanism of ENSO.

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Soon-Il An
and
Fei-Fei Jin

Abstract

The vertical advection of anomalous subsurface temperature by the mean upwelling and the zonal advection of mean sea surface temperature (SST) by anomalous current are known to be essential for the equatorial SST anomaly associated with the El Niño–Southern Oscillation (ENSO). In the coupled model, these two processes are referred to as the thermocline feedback and the zonal advective feedback, respectively. Using a version of a recharge oscillator model for ENSO obtained from the stripped-down approximation of the Cane–Zebiak-type model, it is demonstrated that these two feedbacks, which are linked dynamically through the geostrophic approximation, tend constructively to contribute to the growth and phase transition of ENSO. However, these two feedbacks control the leading coupled mode in different ways. The thermocline feedback leads to a coupled mode through the merging of the damped SST mode and ocean adjustment mode, whereas the zonal advective feedback tends to destabilize the gravest ocean basin mode. With both of these feedbacks, the leading modes of the coupled model still can be traced back to these different origins under moderate changes in the model setup. The main consequence of these sensitivities is that the growth rate and frequency of the ENSO mode may be sensitive to slight changes in basic-state parameters, which control the strength of these feedbacks.

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Soon-Il An
and
Bin Wang

Abstract

The peaks of El Niño in the Cane–Zebiak (CZ) model tend to appear most frequently around November when the ocean Rossby waves, which were amplified during the previous unstable season (February–May), turn back to the eastern Pacific and when the local instability in the eastern Pacific is very weak. The peaks of La Niña in the CZ model occur most frequently in boreal summer, in contrast to the observed counterpart that usually occurs in boreal winter. Sensitivity experiments indicate that the phase locking of the La Niña to boreal summer is primarily caused by seasonal variations of the tropical convergence zone, which regulate convective heating through atmospheric convergence feedback. The observed thermocline and the wind anomalies in the western Pacific exhibit considerable seasonal variations. These were missed in the original CZ model. In a modified CZ model that includes the seasonal variations of the western Pacific wind anomalies and the basic-state thermocline depth, the peaks of La Niña preferably occur in boreal winter, suggesting that the seasonal variation of the western Pacific surface wind anomalies and the mean thermocline depth are critical factors for the phase locking of the mature La Niña to boreal winter. The mechanisms by which these factors affect ENSO phase locking are also discussed.

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Baek-Min Kim
and
Soon-Il An

Abstract

The regime behavior of the low-order El Niño–Southern Oscillation (ENSO) model, according to an increase in the radiative–convective equilibrium sea surface temperature (SST; Tr ), is studied to provide a possible explanation for the observed increase in ENSO irregularity characterized by decadal modulation. During recent decades, a clear increasing trend of the warm-pool SST has been observed. In this study, the increase in the warm-pool maximum SST is interpreted as an increase in Tr following previous studies. A bifurcation analysis with Tr as a control parameter is conducted to reveal that the degree of ENSO irregularity in the model is effectively controlled by the equilibrium states of the model. At a critical value of Tr , bifurcation analysis reveals that period-doubling bifurcation occurs and an amplitude-modulated ENSO emerges. At this point, a subcycle appears within the preexisting ENSO cycle, which initiates decadal modulation of ENSO. As Tr increases further, nested oscillations are successively generated, illustrating clear decadal modulation of ENSO. The qualitative regime changes revealed in this study are supported by the observation of regime shifts in the 1970s. With increasing Tr , the mean zonal SST gradient increases, and the model adjusts toward a “La Niña–like” mean state. Further constraint with shoaling of the mean thermocline depth and increasing stratification causes ENSO to exhibit stronger amplitude modulation. Furthermore, the timing of the period-doubling bifurcation advances with these two effects.

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Hyo-Jeong Kim
and
Soon-Il An

Abstract

The Pacific meridional overturning circulation (PMOC) is not well known compared to the Atlantic meridional overturning circulation (AMOC), due to its absence today. However, considering PMOC development under different climate conditions shown by proxy and modeling studies, a better understanding of PMOC is appropriate to properly assess the past and future climate change associated with global ocean circulation. Here, the PMOC response to freshwater forcing in the North Atlantic (NA) is investigated using an Earth system model of intermediate complexity under glacial (i.e., Last Glacial Maximum) and interglacial [i.e., preindustrial with/without inflow through Bering Strait (BS)] conditions. The water hosing over NA led to the shutdown of the AMOC, which accompanied an active PMOC except for the preindustrial condition with the opening BS, indicating that the emergence of the PMOC is constrained by the freshwater inflow through the BS, which hinders its destabilization through enhancing ocean stratification. However, the closure of the BS itself could not explain how the sinking motion is maintained in the North Pacific. Here we found that various atmospheric and oceanic processes are involved to sustain the active PMOC. First, an atmospheric teleconnection associated with the collapsed AMOC encouraged the evaporation in the sinking region, causing buoyancy loss at the surface of the North Pacific. Second, the strengthened subpolar gyre transported saltier water northward, enhancing dense water formation. Finally, the vigorous upwelling in the Southern Ocean enabled a consistent mass supply to the sinking region, with the aid of enhanced westerlies.

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Soong-Ki Kim
and
Soon-Il An

Abstract

The life cycle of El Niño–Southern Oscillation (ENSO) typically follows a seasonal march, with onset in spring, developing during summer, maturing in boreal winter, and decaying over the following spring. This feature is referred to as ENSO phase locking. Recent studies have noted that seasonal modulation of the ENSO growth rate is essential for this process. This study investigates the fundamental effect of a seasonally varying growth rate on ENSO phase locking using a modified seasonally dependent recharge oscillator model. There are two phase locking regimes associated with the strength of the seasonal modulation of growth rate: 1) a weak regime in which only a single peak occurs and 2) a strong regime in which two types of events occur either with a single peak or with a double peak. Notably, there is a seasonal gap in the strong regime, during which the ENSO peak cannot occur because of large-scale ocean–atmosphere coupled processes. We also retrieve a simple analytical solution of the seasonal variance of ENSO, revealing that the variance is governed by the time integral of seasonally varying growth rate. Based on this formulation, we propose a seasonal energy index (SEI) that explains the seasonal gap and provides an intuitive explanation for ENSO phase locking, potentially applicable to global climate model ENSO diagnostics.

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