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Fengpeng Sun and Jin-Yi Yu

Abstract

This study examines the slow modulation of El Niño–Southern Oscillation (ENSO) intensity and its underlying mechanism. A 10–15-yr ENSO intensity modulation cycle is identified from historical and paleoclimate data by calculating the envelope function of boreal winter Niño-3.4 and Niño-3 sea surface temperature (SST) indices. Composite analyses reveal interesting spatial asymmetries between El Niño and La Niña events within the modulation cycle. In the enhanced intensity periods of the cycle, El Niño is located in the eastern tropical Pacific and La Niña in the central tropical Pacific. The asymmetry is reversed in the weakened intensity periods: El Niño centers in the central Pacific and La Niña in the eastern Pacific. El Niño and La Niña centered in the eastern Pacific are accompanied with basin-scale surface wind and thermocline anomalies, whereas those centered in the central Pacific are accompanied with local wind and thermocline anomalies. The El Niño–La Niña asymmetries provide a possible mechanism for ENSO to exert a nonzero residual effect that could lead to slow changes in the Pacific mean state. The mean state changes are characterized by an SST dipole pattern between the eastern and central tropical Pacific, which appears as one leading EOF mode of tropical Pacific decadal variability. The Pacific Walker circulation migrates zonally in association with this decadal mode and also changes the mean surface wind and thermocline patterns along the equator. Although the causality has not been established, it is speculated that the mean state changes in turn favor the alternative spatial patterns of El Niño and La Niña that manifest as the reversed ENSO asymmetries. Using these findings, an ENSO–Pacific climate interaction mechanism is hypothesized to explain the decadal ENSO intensity modulation cycle.

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Lei Wang and Jin-Yi Yu

Abstract

The tropospheric biennial oscillation (TBO) is conventionally considered to involve transitions between the Indian and Australian summer monsoons and the interactions between these two monsoons and the underlying Indo-Pacific Oceans. Here it is shown that, since the early 1990s, the TBO has evolved to mainly involve the transitions between the western North Pacific (WNP) and Australian monsoons. In this framework, the WNP monsoon replaces the Indian monsoon as the active Northern Hemisphere TBO monsoon center during recent decades. This change is found to be caused by stronger Pacific–Atlantic coupling and an increased influence of the tropical Atlantic Ocean on the Indian and WNP monsoons. The increased Atlantic Ocean influence damps the Pacific Ocean influence on the Indian summer monsoon (leading to a decrease in its variability) but amplifies the Pacific Ocean influence on the WNP summer monsoon (leading to an increase in its variability). These results suggest that the Pacific–Atlantic interactions have become more important to the TBO dynamics during recent decades.

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Jian Ma and Jin-Yi Yu

Abstract

This study analyzes representative concentration pathway 4.5 projections by 18 models from phase 5 of the Coupled Model Intercomparison Project to show that surface warming patterns in the equatorial Pacific during the twenty-first century (centennial warming) are influenced by the relative strengths of the Walker and Hadley circulations. The stronger the Hadley (Walker) circulation is, the greater the surface warming in the central Pacific (CP) [eastern Pacific (EP)]. The EP warming is associated with the Bjerknes feedback, while the CP warming is associated with the wind–evaporation–sea surface temperature feedback. This atmospheric circulation influence on the centennial warming is similar to that found for the EP and CP El Niño. This suggests a methodology to constrain the estimate of the projected surface warming patterns in the equatorial Pacific using recent El Niño activity. The constraint indicates that the “most likely” centennial warming patterns have a maximum in the EP and are 39% weaker than the warming projected by the 18-model mean. The most-likely projection also shows alternating stronger and weaker warming in the subtropical North Pacific, which is not predicted by the 18-model mean projection. Nevertheless, the two projections agree on the minimum warming in the southeastern subtropical Pacific.

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John D. Farrara and Jin-Yi Yu

Abstract

The interannual variability in the southwest U.S. monsoon and its relationship to sea surface temperature (SST) anomalies is investigated via experiments conducted with the University of California, Los Angeles, atmospheric general circulation model (AGCM). When the model is run without interannual variations in SSTs at the lower boundary, the simulation of the climatological mean monsoon is quite similar to the observed. In addition, the interannual precipitation variance and wet minus dry monsoon composite differences in the precipitation and monsoon circulation are largely realistic.

When interannual variations in SSTs are introduced, the simulated interannual precipitation variance over the southwest U.S. monsoon region does not increase. Nor do SSTs seem to be important in selecting for wet or dry monsoons in this simulation, as there is little correspondence between observed wet and dry monsoon years and simulated wet and dry years. These results were confirmed through a 20-member ensemble of shorter seasonal simulations forced by an SST anomaly field corresponding to that observed for a wet minus dry southwest U.S. monsoon composite.

When the AGCM is coupled to a mixed-layer ocean model, the pattern of SST anomalies generated in association with wet and dry monsoons is remarkably similar to that observed: there is a large area of positive SST anomalies in the subtropical eastern Pacific Ocean and weaker negative anomalies in the midlatitude North Pacific and Gulf of Mexico. It is demonstrated that the SST anomalies in the Pacific Ocean are forced by anomalies in the net surface solar radiative flux from the atmosphere associated with variations in planetary boundary layer stratus clouds; these variations are enhanced by a positive feedback between SST and stratus cloud variations. The anomalies in the Gulf of Mexico are associated with anomalous latent heat fluxes there. It is concluded that internal atmospheric variations are capable of 1) producing interannual variations in the southwest U.S. monsoon that are comparable to those observed, and 2) thermodynamically forcing the SST anomalies in the adjacent Pacific Ocean and Gulf of Mexico that are observed to accompany these variations. The implications of these results for seasonal forecasting are rather pessimistic since variations associated with internal atmospheric processes cannot be predicted on seasonal timescales.

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Jin-Yi Yu and Dennis L. Hartmann

Abstract

The effect of large-scale mountains on atmospheric variability is studied in a series of GCM experiments in which a single mountain is varied in height from 0 to 4 km. High-frequency (τ < 7 days) and low-frequency (τ > 30 days) variability are largest in the jet exit region, while the intermediate-frequency (7 < τ < 30 days) variability has its maximum upstream of the mountain where it exhibits enhanced equatorward propagation. High and intermediate frequencies change from zonal wave trains to localized wave packets as orographic forcing is increased, but they retain their characteristic scale and frequency. The dominant pattern of low-frequency is variability changes from a zonally symmetric oscillation, for which transient eddy-zonal flow interaction is the dominant mechanism, to a more localized oscillation of the jet downstream of the mountain. The transient eddy forcing still plays a significant role in maintaining the variations of this more localized jet, however.

The total amount of wave energy remains almost constant as the mountain height is increased, but the distribution of wave energy shifts from transient to stationary and from high frequencies to low frequencies. Low-frequency variability shows a step function response to orographic forcing in that it shows no response to a 1-km mountain, increases substantially in response to a 2-km mountain, and then shows little further increase as the mountain is raised to 3 and 4 km. This behavior suggests that the mechanism that generates the additional low-frequency variability in the mountain-forced experiments becomes effective after the zonal asymmetry reaches a critical value and then does not respond much to further increases in asymmetry.

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Jin-Yi Yu and Carlos R. Mechoso

Abstract

The hypothesis that Peruvian stratocumulus play an important role on both the annual mean and annual variations of sea surface temperature (SST) in the eastern equatorial Pacific is examined. The problem is addressed by performing sensitivity experiments using the University of California, Los Angeles, coupled atmosphere–ocean GCM with different idealized temporal variations of stratocumulus in a region along the coast of Peru.

The results obtained are consistent with the notion that Peruvian stratocumulus are a key component of the interhemispherically asymmetric features that characterize the annual mean climate of the eastern equatorial Pacific, including the cold SSTs off Peru and the absence of a southern ITCZ. The principal new finding of this study is that the annual variations (i.e., deviations from the annual mean) of Peruvian stratocumulus are linked to the differences between the amplitude, duration, and westward propagation of the warm and cold phases of the equatorial cold tongue. In the model’s context, only if the prescribed annual variations of Peruvian stratocumulus have the same phase as the observed variations are those differences successfully captured.

The impact of Peruvian stratocumulus on equatorial SST involves “dynamical” and “thermal” effects. The former develop through an enhancement of the northerly component of the surface wind from the Peruvian coast to the equator. The thermal effects develop through the special relationships between SST and surface evaporation over the equatorial cold tongue, which contributes to extend the cold phase until the end of the year. A successful portrayal of this behavior requires a realistic simulation of the annual variations of surface wind over the equatorial cold tongue.

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Jin-Yi Yu and Seon Tae Kim

Abstract

This study examines the linkages between leading patterns of interannual sea level pressure (SLP) variability over the extratropical Pacific (20°–60°N) and the eastern Pacific (EP) and central Pacific (CP) types of El Niño–Southern Oscillation (ENSO). The first empirical orthogonal function (EOF) mode of the extratropical SLP anomalies represents variations of the Aleutian low, and the second EOF mode represents the North Pacific Oscillation (NPO) and is characterized by a meridional SLP anomaly dipole with a nodal point near 50°N. It is shown that a fraction of the first SLP mode can be excited by both the EP and CP types of ENSO. The SLP response to the EP type is stronger and more immediate. The tropical–extratropical teleconnection appears to act more slowly for the CP ENSO. During the decay phase of EP events, the associated extratropical SLP anomalies shift from the first SLP mode to the second SLP mode. As the second SLP mode grows, subtropical SST anomalies are induced beneath via surface heat flux anomalies. The SST anomalies persist after the peak in strength of the second SLP mode, likely because of the seasonal footprinting mechanism, and lead to the development of the CP type of ENSO. This study shows that the CP ENSO is an extratropically excited mode of tropical Pacific variability and also suggests that the decay of an EP type of ENSO can lead to the onset of a CP type of ENSO with the aid of the NPO. This extratropical linking mechanism appears to be at work during the 1972, 1982, and 1997 strong El Niño events, which were all EP events and were all followed by strong CP La Niña events after the NPO was excited in the extratropics. This study concludes that extratropical SLP variations play an important role in exciting the CP type of ENSO and in linking the transitions from the EP to CP events.

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Jin-Yi Yu and Dennis L. Hartmann

Abstract

Zonal flow vacillation with very long time scales is observed in a 3070-day simple GCM simulation with zonally symmetric forcing. The long lasting zonal wind anomalies suggest that zonal flow vacillation is self-maintained. Wave-mean flow interactions are investigated by composite analysis and transform Eulerian momentum budget analysis. Nonlinear life-cycle simulations are conducted to demonstrate that each extreme phase of the zonal flow vacillation is a quasi stable state and is self-maintained by the embedded synoptic eddies.

The firm EOF mode of zonal-mean wind shows an out of phase relation between anomalies at 60°S and at 40°S with a barotropic structure. This structure is similar to the dominant vacillation pattern observed in the Southern Hemisphere. The composite jet stream in the high (low) index phase of zonal flow vacillation shifts poleward (equatorward) from the time-mean location and becomes broader (narrower) and weaker (stronger). Composite eddies in the high index Phase tilt NW-SE and show mostly equatorward propagation, while eddies in the low index phase have “banana” shapes and propagate both equatorward and poleward. Transformed Eulerian momentum budget analyses show that the differences of wave propagation between two extreme phases result in the anomalous eddy forcing needed to maintain zonal wind anomalies against frictional damping.

Budget analyses also indicate that eddy momentum flux convergence is the major positive forcing in both the extreme and transition phases. Eddy baroclinic forcing exerts weak damping on the wind anomalies in the upper troposphere but acts together with residual circulation forcing to counteract frictional damping near the surface. The major balance during the index cycle is between eddy barotropic forcing and residual circulation forcing in the upper troposphere and between residual circulation forcing and frictional damping in the lower troposphere. Further comparisons of eddy forcing from various time-scale eddies show that the anomalous eddy forcing is primarily provided by synoptic time scales. Two nonlinear life-cycle simulations, started separately from the composite zonal flows of the two extreme phases and small-amplitude wavenumber 6 perturbations, display the intensification of initial wind anomalies by the growing eddies. A dual-jet stream structure appears in the life-cycle simulation started from the high index composite, and a more intense single jet stream structure evolves from the low index initial state.

It is noticed that maximum wind anomalies are established earlier at higher latitudes than at lower latitudes. This suggests that the mechanisms triggering transitions from one self-maintained phase to the other operate at higher latitudes. It is suspected that barotropic instability/stability is a possible triggering mechanism for transition from one state to another.

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Jin-Yi Yu and Seon Tae Kim

Abstract

This study examines preindustrial simulations from Coupled Model Intercomparison Project, phase 3 (CMIP3), models to show that a tendency exists for El Niño sea surface temperature anomalies to be located farther eastward than La Niña anomalies during strong El Niño–Southern Oscillation (ENSO) events but farther westward than La Niña anomalies during weak ENSO events. Such reversed spatial asymmetries are shown to force a slow change in the tropical Pacific Ocean mean state that in return modulates ENSO amplitude. CMIP3 models that produce strong reversed asymmetries experience cyclic modulations of ENSO intensity, in which strong and weak events occur during opposite phases of a decadal variability mode associated with the residual effects of the reversed asymmetries. It is concluded that the reversed spatial asymmetries enable an ENSO–tropical Pacific mean state interaction mechanism that gives rise to a decadal modulation of ENSO intensity and that at least three CMIP3 models realistically simulate this interaction mechanism.

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Jin-Yi Yu and Carlos R. Mechoso

Abstract

This study examines interannual variability produced by a recent version of the University of California, Los Angeles, coupled atmosphere–ocean general circulation model (CGCM). The CGCM is shown to produce ENSO-like climate variability with reasonable frequency and amplitude. A multichannel singular spectrum analysis identifies the simulated ENSO cycle and permits examination of the associated evolution of atmospheric and oceanic states. During the cycle, the evolution of upper-ocean heat content in the tropical Pacific is characterized by a zonal oscillation between the western and eastern equatorial Pacific and a meridional oscillation between the equator and 10°N. The zonal oscillation is related to the amplification of the cycle, and the meridional oscillation is related to the transition between phases of the cycle. It is found that the north–south ocean heat content difference always reaches a threshold near the onset of a warm/cold event.

The three-dimensional evolution of ocean temperature anomalies in the tropical Pacific during the simulated ENSO cycle is characterized by four major features: 1) a build up in the subsurface of the western equatorial sector during the pre-onset stage, 2) a fast spread from the western subsurface to the eastern surface along the equator during the onset stage, 3) a zonal extension and amplification at the surface during the growth stage, and 4) a northward and downward spread during the decay stage.

Ocean temperature budget analyses show that the buildup of subsurface temperature anomalies is dominated by the vertical advection process in the western sector and the meridional advection process in the central sector. The former process is associated with vertical displacements of the thermocline, which is an important element of the delayed oscillator theory. The latter process is associated with a Sverdrup imbalance between trade wind and thermocline anomalies and is emphasized as the primary charge–discharge process by the recharge oscillator theory. It is argued that both processes play key roles in producing subsurface ocean memory for the phase transitions of the ENSO cycle.

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