Delayed Oscillator Mechanism Research Articles

A Simple Conceptual Model for the Self‐Sustained Multidecadal AMOC Variability

AbstractMultidecadal variability of Atlantic Meridional Overturning Circulation (AMOC) has been reconstructed by various proxies, simulated in climate models, and linked to multidecadal Arctic salinity variability. Here we construct a simple conceptual model to understand the two‐way interactions of the Arctic with multidecadal AMOC variability through a delayed oscillator mechanism. We revise Stommel's Two‐Box Model by including an advective time delay for the Arctic density/salinity anomalies to reach the subpolar North Atlantic and a coupled negative feedback between the AMOC and the freshwater flux entering the Arctic through atmosphere and/or sea ice responses. Self‐sustained multidecadal AMOC oscillations exist in the revised Stommel's Two‐Box model if the oceanic advective time delay is longer than the oscillation threshold, and the periods of the AMOC delayed oscillator depend crucially on this advective time delay. The coupled freshwater feedback provides additional delayed negative feedback and reduces the advective time delay threshold required for the oscillations.

Assessing the Quality of Regional Ocean Reanalysis Data from ENSO Signals

AbstractThe quality of regional ocean reanalysis data for “the joining area of Asia and the Indian-Pacific Ocean (AIPO)” has been assessed from the perspective of ENSO-related ocean signals. The results derived from the AIPO reanalysis, including SST, sea surface height (SSH), and subsurface ocean temperature and currents, are compared with those of Hadley Center Sea Ice and Sea Surface Temperature (HadISST) data set and Simple Ocean Data Assimilation (SODA) reanalysis data. Both the spatial pattern and the characteristics of evolution of the ENSO-related ocean temperature anomalies are well reproduced by the AIPO reanalysis data. The physical processes proposed to explain the life cycle of ENSO, including the delayed oscillator mechanism, rechargedischarge mechanism, and the zonal advection feedback, are reasonably represented in this dataset. However, the westward Rossby wave signal in 1992 is not obvious in the AIPO data, and the magnitude of the heat content anomalies is different from that of the SOD...

Vertical structure variability and equatorial waves during central Pacific and eastern Pacific El Niños in a coupled general circulation model

Recent studies report that two types of El Nino events have been observed. One is the cold tongue El Nino or Eastern Pacific El Nino (EP El Nino), which is characterized by relatively large sea surface temperature (SST) anomalies in the eastern Pacific, and the other is the warm pool El Nino (a.k.a. ‘Central Pacific El Nino’ (CP El Nino) or ‘El Nino Modoki’), in which SST anomalies are confined to the central Pacific. Here the vertical structure variability of the periods during EP and CP is investigated based on the GFDL_CM2.1 model in order to explain the difference in equatorial wave dynamics and associated negative feedback mechanisms. It is shown that the mean stratification in the vicinity of the thermocline of the central Pacific is reduced during CP El Nino, which favours the contribution of the gravest baroclinic mode relatively to the higher-order slower baroclinic mode. Energetic Kelvin and first-meridional Rossby wave are evidenced during the CP El Nino with distinctive amplitude and propagating characteristics according to their vertical structure (mostly first and second baroclinic modes). In particular, the first baroclinic mode during CP El Nino is associated to the ocean basin mode and participates to the recharge process during the whole El Nino cycle, whereas the second baroclinic mode is mostly driving the discharge process through the delayed oscillator mechanism. This may explain that the phase transition from warm to neutral/cold conditions during the CP El Nino is delayed and/or disrupted compared to the EP El Nino. Our results have implications for the interpretation of the variability during periods of high CP El Nino occurrence like the last decade.

A dynamical ocean feedback mechanism for the Madden–Julian Oscillation

AbstractComposite analysis is applied to study the dynamical ocean response to Madden–Julian (MJ) events, measured by anomalies in sea surface height from the merged TOPEX/Poseidon–European Remote Sensing satellite altimetry dataset. In each of the tropical ocean basins, significant equatorial waves are forced, which are shown to modulate the sea surface temperature (SST) by 0.2–0.3°C in the absence of strong surface heat fluxes. In the Indian Ocean there is a clear dynamical response which may play a significant role in generating later MJ events. Surface westerly winds, associated with the active phase of the Madden–Julian Oscillation (MJO), force an eastward‐propagating oceanic downwelling equatorial Kelvin wave, which, on reaching the eastern boundary at Sumatra, forces reflected downwelling equatorial Rossby waves and coastal Kelvin waves. The coastal Kelvin waves propagate southwards towards northern Australia and northwards into the Bay of Bengal, and will be important for local physical, chemical and biological processes. The equatorial Rossby waves propagate westward across the Indian Ocean, arriving in the western Indian Ocean approximately 80–100 days after the initial Kelvin wave was generated. The arrival of these waves generates positive SST anomalies which leads to convection and may trigger the next‐but‐one MJ event, or amplify the low‐frequency tail of the MJO. This constitutes a coupled feedback mechanism from the ocean dynamics onto the MJO, somewhat similar to the delayed oscillator mechanism for the El Niño Southern Oscillation. Copyright © 2010 Royal Meteorological Society

The Energetics of El Niño and La Niña

Data from a realistic model of the ocean, forced with observed atmospheric conditions for the period 1953–92, are analyzed to determine the energetics of interannual variability in the tropical Pacific. The work done by the winds on the ocean, rather than generating kinetic energy, does work against pressure gradients and generates buoyancy power, which in turn is responsible for the rate of change of available potential energy (APE). This means interannual fluctuations in work done by the wind have a phase that leads variations in APE. Variations in the sea surface temperature (SST) of the eastern equatorial Pacific and in APE are highly correlated and in phase so that changes in the work done by the wind are precursors of El Niño. The wind does positive work on the ocean during the half cycle that starts with the peak of El Niño and continues into La Niña; it does negative work during the remaining half cycle. The results corroborate the delayed oscillator mechanism that qualitatively describes the deterministic behavior of ENSO. In that paradigm, a thermocline perturbation appearing in the western equatorial Pacific affects the transition from one phase of ENSO to the next when that perturbation arrives in the eastern equatorial Pacific where it influences SST. The analysis of energetics indicates that the transition starts earlier, during La Niña, when the perturbation is still in the far western equatorial Pacific. Although the perturbation at that stage affects the thermal structure mainly in the thermocline, at depth, the associated currents are manifest at the surface and immediately affect work done by the wind. For the simulation presented here, the change in energy resulting from adjustment processes far outweighs that due to stochastic processes, such as intraseasonal wind bursts, at least during periods of successive El Niño and La Niña events.

Tracking Down the ENSO Delayed Oscillator with an Adjoint OGCM

Abstract According to the delayed-oscillator picture of ENSO, a positive SST anomaly in the eastern tropical Pacific will cause westerly wind anomalies closer to the date line to first give a positive feedback, and later, via planetary wave reflection at the western boundary, a negative feedback. The aim of this study is to follow a chain of sensitivities that lead to a delayed-oscillator mechanism in a general circulation model. To this end, the adjoint of such an ocean model is used for studying sensitivities of ENSO indices. The ocean model used in this study is the Hamburg Ocean Primitive Equation (HOPE) ocean general circulation model. Its adjoint has been constructed using the Adjoint Model Compiler. Applied to a scalar function computed with a forward model run, an adjoint run goes back in time and calculates sensitivities as the derivatives of this function to forcing fields or ocean state variables at earlier times. Results from six adjoint runs are reported, tracing the sensitivities of the NINO...

Decadal Variability in ENSO Predictability and Prediction

A simple coupled model is used to examine decadal variations in El Niño–Southern Oscillation (ENSO) prediction skill and predictability. Without any external forcing, the coupled model produces regular ENSO-like variability with a 5-yr period. Superimposed on the 5-yr oscillation is a relatively weak decadal amplitude modulation with a 20-yr period. External uncoupled atmospheric “weather noise” that is determined from observations is introduced into the coupled model. Including the weather noise leads to irregularity in the ENSO events, shifts the dominant period to 4 yr, and amplifies the decadal signal. The decadal signal results without any external prescribed changes to the mean climate of the model. Using the coupled simulation with weather noise as initial conditions and for verification, a large ensemble of prediction experiments were made. The forecast skill and predictability were examined and shown to have a strong decadal dependence. During decades when the amplitude of the interannual variability is large, the forecast skill is relatively high and the limit of predictability is relatively long. Conversely, during decades when the amplitude of the interannual variability is low, the forecast skill is relatively low and the limit of predictability is relatively short. During decades when the predictability is high, the delayed oscillator mechanism drives the sea surface temperature anomaly (SSTA), and during decades when the predictability is low, the atmospheric noise strongly influences the SSTA. Additional experiments indicate that the relative effectiveness of the delayed oscillator mechanism versus the external noise forcing in determining interannual SSTA variability is strongly influenced by much slower timescale (decadal) variations in the state of the coupled model.

Locking of El Niño’s Peak Time to the End of the Calendar Year in the Delayed Oscillator Picture of ENSO

El Niño events owe their name to their tendency to be locked to the seasonal cycle. A simple explanation is proposed here for the locking of the peak of ENSO’s basin-scale warming to the end of the calendar year. The explanation is based on incorporating a seasonally varying coupled ocean–atmosphere instability strength into the delayed oscillator mechanism for the ENSO cycle. It is shown that the seasonally varying amplification of the Rossby and Kelvin ocean waves by the coupled instability forces the events to peak when this amplification is at its minimum strength, at the end of the calendar year. The mechanism is demonstrated using a simple delayed oscillator model and is further analyzed using the Cane–Zebiak model. Being based on the oversimplified delayed oscillator paradigm of ENSO, the proposed mechanism cannot be expected to fully explain the locking of observed events to the end of the year. However, the wave dynamics perspective it offers to approaching the ENSO phase-locking problem may serve as a first step toward a fuller explanation based on more realistic models and additional data analysis.

Coupled processes for equatorial Pacific interannual variability

AbstractDiagnostics for coupled processes relevant to equatorial Pacific interannual variability are defined, using a simplified analytical model, and applied to general circulation model simulations. The analytical model is a simplification of the Neelin model (Neelin 1991). The atmospheric component is represented by a direct zonal thermal circulation. The oceanic dynamics take into account a shear layer and a Kelvin wave. Analytical calculations lead to an unstable slow‐propagating coupled mode, referred to as the TK‐mode. All the processes of this mode are explicitly identified, as are their contributions to the resulting total growth rate. These contributions are computed in forced oceanic (covering the 1979–93 period) and in coupled ocean/atmosphere general circulation model simulations. The results are sensitive to the parameters but allow relative comparisons between general circulation model simulations.The forced oceanic simulation suggests that the total instability is nearly constant all over the equatorial Pacific basin, but the central part is dominated by the zonal advective coupled process, while the eastern part is controlled by both zonal and vertical advective coupled processes. The thermocline coupled process appears largest in the eastern part of the basin, but its contribution is still weaker than the other two.The coupled simulation appears globally more unstable than the forced one. The total instability is larger in the eastern part of the Pacific basin. This result is consistent with the location and the frequent occurrence of the simulated interannual signals in the eastern part of the basin.The seasonal cycle of the forced simulation shows maximum instability between July and November. This could suggest an approach towards the understanding of the seasonal phase lock of the onset of warm El Niño/Southern Oscillation events. Possible interactions between the delayed oscillator mechanism or the westerly wind bursts with the TK‐mode processes are discussed. In order to understand the real system, these diagnostics should be applied to the observations and not just models. Nonetheless, analysis of coupled processes allows objective comparisons between different general circulation model simulations, and the results presented in this paper should be considered as a first step in this direction.

El Niño in the 1990s

The evolution of the unusual warming events of the first half of the 1990s is discussed based on the National Center for Environmental Prediction ocean re‐analysis data, covering 1980–1995. The warmings occurring in 1991–1992, 1993, and 1994 are viewed as individual episodes, rather than as one long El Niño. It is shown that the warm episodes of the 1990s differ from previous El Niños in two important ways. First, subsurface anomalies are more strongly coupled to the mixed layer across the equatorial Pacific in the 1990s than in earlier years. Second, a persistent warm SST anomaly in the central Pacific has been instrumental in initiating warm events. Both of these factors interfere with the dominant mode of variability in the tropical Pacific air‐sea system, namely the delayed oscillator mechanism. We postulate that the low skill of dynamical atmosphere‐ocean models in predicting the recent warmings is related to a weakened delayed oscillator mechanism.

Annual Cycle and ENSO in a Coupled Ocean–Atmosphere General Circulation Model

Results from multiyear integrations of a coupled ocean–atmosphere general circulation model are described. The atmospheric component is a rhomboidal 15, 18-level version of the Center for Ocean–Land–Atmosphere Studies atmospheric general circulation model. The oceanic component is the Geophysical Fluid Dynamics Laboratory ocean model with a horizontal domain extending from 70°S to 65°N. The ocean model uses 1.5° horizontal resolution, with meridional resolution increasing to 0.5° near the equator, and 20 vertical levels, most in the upper 300 m. No flux adjustments are employed. An initial multiyear integration showed significant climate drift in the tropical Pacific sea surface temperatures. Several modifications were made in the coupled model to reduce these errors. Changes were made to the atmospheric model cloudiness parameterizations, increasing solar radiation at the surface in the western equatorial Pacific and decreasing it in the eastern Pacific, that improved the simulation of the time-mean sea surface temperature. Large errors in the wind direction near the western coast of South America resulted in large mean SST errors in that region. A procedure to reduce these errors by extrapolating wind stress values away from the coast to coastal points was devised and implemented. Results from the last 17 years of a 62-yr simulation are described. The model produces a reasonably realistic annual cycle of equatorial Pacific sea surface temperature. However, the upper-ocean thermal structure has serious errors. Interannual variability for tropical Pacific sea surface temperatures, precipitation, and sea level pressure that resemble the observed El Niño–Southern Oscillation (ENSO) in structure and evolution is found. However, differences from observed behavior are also evident. The mechanism responsible for the interannual variability appears to be similar to the delayed oscillator mechanism that occurs in the real climate system. The structure of precipitation, sea level pressure, and geopotential anomalies associated with the tropical Pacific sea surface temperature interannual variability are isolated and described. The coupled model is capable of producing structures that are similar to those observed. It is concluded that atmosphere–ocean general circulation models are beginning to capture some of the observed characteristics of the climatology of the tropical Pacific and the interannual variability associated with the El Niño–Southern Oscillation. Remaining obstacles to realistic simulations appear to include ocean model errors in the eastern equatorial Pacific, errors associated with cloud–radiation interactions, and perhaps errors associated with inadequate meridional resolution in the atmospheric model equatorial Pacific.

Evidence for the Delayed Oscillator Mechanism for ENSO: The “Observed” Oceanic Kelvin Mode in the Far Western Pacific

Observed surface winds from 1961 through September 1992 are used to force a reduced gravity shallow water ocean model. Results from the hindcast of ocean variability are found to be consistent with the results Li and Clarke presented in a previous study of sea level and zonal wind variability in the tropical Pacific. In this note the apparent discrepancies are reconciled between “delayed oscillator” theory and calculated lag correlations between the observationally based records of western boundary Kelvin mode amplitude (ηW) and zonal wind forcing. Evidence for sensitivity in these “delayed oscillator” lag correlations is presented from a variety of sources, including: the hindcast data, output from the standard version of the Zebiak and Cane coupled ocean-atmosphere model, and through three cases with idealized time series of ENSO variability. The authors demonstrate that the low lag correlations for ηW leading the zonal wind forcing by 1 to 1½ years is not inconsistent with the hypothesized role of western boundary reflections as the ultimate termination mechanism for ENSO anomalies. The lack of regularity in the system studied guarantees a degraded correlation for ηW leading the zonal wind by 1 to 1½ years. This lack of regularity is not contained in, nor explained by, delayed oscillator theory. A robust feature in all of the records examined in this work is the existence of upwelling Kelvin signals in the far western equatorial Pacific Ocean due to the developing warm (ENSO) events. The amplitude of the observed Kelvin signals in the western Pacific is sufficient to terminate the developing ENSO events via the delayed oscillator mechanism.