Amplitude Of El Research Articles

Improving the Forecasting of El Niño Amplitude Based on an Ensemble Forecast Strategy for Westerly Wind Bursts

Abstract The forecasting of El Niño amplitude remains uncertain, with false alarms or underestimations often occurring. It has been suggested that westerly wind bursts (WWBs) are crucial atmospheric stochastic forcing that affects the development, amplitude, and predictability of El Niño. Effectively capturing the influence of WWBs in El Niño forecasting systems to mitigate El Niño forecast uncertainties continues to be a widely discussed topic. In this work, two El Niño ensemble forecast scenarios incorporating WWBs were devised utilizing an intermediate coupled model capable of simulating the rational features of El Niño–Southern Oscillation and a refined WWB parameterization scheme. To start with, both the seasonal variations and the dependence on the oceanic background state were considered to improve the parameterization of WWBs. Furthermore, considering the short intrinsic predictability limit of WWBs with respect to the interannual scale of El Niño evolution, a WWB ensemble forecast strategy was designed to obtain their occurrence probability and the statistical features of their magnitude and central location. With this in mind, an ensemble forecast of El Niño events occurring during 1979–2021 based on a WWB ensemble forecast was established (termed WWB-based). For comparison, a conventional El Niño ensemble forecast based on initial condition (IC) perturbations was also conducted (termed IC-based). Results indicated that the WWB-based approach shows better performance in forecasting the El Niño amplitude than the IC-based one. The present approach suggests that an ensemble forecast with proper consideration of the scale interaction between the fast WWBs and interannual variations is more physically consistent. Significance Statement Westerly wind bursts (WWBs) are believed to influence the growth and amplitude of El Niño events. Unfortunately, the short intrinsic predictability limit of WWBs brings considerable uncertainty to the forecasting of El Niño amplitude. In this study, a refined parameterization scheme for WWBs, along with an ensemble forecast strategy designed to obtain the occurrence probability of WWBs and the statistical features of their magnitude and central location, was employed in an intermediate coupled model to produce an ensemble forecast for the El Niño events occurring during 1979–2021. The results showed that an ensemble forecast of El Niño based on WWB ensemble forecast can fully consider WWBs’ potential impact on the El Niño system and improve El Niño forecasts.

Interdecadal Variability of the Meridional Wind across the Eastern Equatorial Pacific and Its Relationship with ENSO

Abstract The interdecadal variability of cross-equatorial meridional winds in the eastern Pacific (VEP), which has been shown to correlate with the amplitude of El Niño–Southern Oscillation (ENSO) on the interdecadal time scale, is investigated using long-term observations and 22 models from phase 6 of the Coupled Model Intercomparison Project (CMIP6). Both observations and models exhibit a tight negative correlation between interannual variations of VEP and ENSO, and this relationship is synchronized with the interdecadal variability of VEP. In long-term observations, ENSO amplitude modulation is still seen to be out-of-phase with interdecadal variability of VEP. This relationship, however, is substantially underestimated among CMIP6 models, particularly in the historical simulations. The interdecadal variability of VEP is associated with both the interdecadal Pacific oscillation (IPO) and the Atlantic multidecadal oscillation (AMO) in observations. However, most CMIP6 preindustrial control (PI-control) experiments have no link between the VEP and AMO. In contrast, in historical simulations, the multimodel mean of interdecadal VEP shows a significant fluctuation with AMO around the 1980s, which might be caused by the anthropogenic aerosol forcing. Consequently, the interdecadal variation of the AMO–VEP relationship is likely a response to external forcing while the IPO–VEP relationship is mainly modulated by internal climate variability. Another plausible factor causing the weak AMO–VEP relationship in PI-control runs is the unrealistic relationship between modeled SSTs in the tropical Pacific and North Atlantic. Furthermore, model biases in the tropical Pacific may account for the weak relationship between interdecadal VEP and ENSO amplitude in CMIP6 models.

Revisiting the Influence of ENSO on the Arctic Stratosphere in CMIP5 and CMIP6 Models

The Arctic stratospheric response to El Niño–Southern Oscillation (ENSO) is assessed using the historical simulations provided by the Coupled Model Intercomparison Project Phases 5 and 6 (CMIP5 and CMIP6, respectively). CMIP6 models can well reproduce the ENSO signals in the Arctic stratosphere and have an ameliorated performance compared to CMIP5 models. Specifically, El Niño is associated with an intensified Pacific–North American pattern that leads to a considerable enhancement of planetary wavenumber 1 but a small reduction of planetary wavenumber 2, and thus, a warm and weakened stratospheric polar vortex. The case for La Niña is nearly the opposite, with a cool and strengthened stratospheric polar vortex. In CMIP6, the ENSO-related stratospheric signal matures in the February–March–April season and increases with ENSO magnitude, regardless of the ENSO phase. However, the stratospheric response to strong El Niño (La Niña) is weaker (stronger) than that which should be achieved if the response changes linearly with the amplitude of El Niño (La Niña). An asymmetric time evolution of stratospheric signals exists between strong El Niño and La Niña events. The stratospheric response caused by strong El Niño is weaker from late winter to early spring but stronger in middle and late spring compared to that caused by strong La Niña. By contrast, the Arctic stratospheric signal in moderate El Niño events is larger than that in moderate La Niña. Compared to ENSO-neutral winters, stratospheric sudden warming occurs more (less) frequently in El Niño (La Niña), as simulated by CMIP6 high-top models.

Impact of the Deep Chlorophyll Maximum in the Equatorial Pacific as Revealed in a Coupled Ocean GCM‐Ecosystem Model

AbstractA coupled ocean general circulation model (OGCM)‐ocean ecosystem model is used to investigate the effects of the deep chlorophyll maximum (DCM) on the ocean state in the equatorial Pacific Ocean. Climatological and interannual three‐dimensional (3‐D) chlorophyll (CHL) fields are captured well in control runs using the coupled ocean physics‐ecosystem model forced by prescribed atmospheric fields. Further sensitivity experiments are performed to assess the effects of 3‐D CHL structure using the OGCM with the prescribed CHL fields taken from the control runs: CHLclim and are runs in which climatological DCM effect is included or only surface CHL effect is included; CHLinter and are runs with interannually varying DCM effects included or not. The differences in the simulated ocean conditions are analyzed to explore DCM effects on sea surface temperature (SST) and amplitude of El Niño‐Southern Oscillation (ENSO). Two competing mechanisms responsible for the DCM effects are revealed: an ocean biology‐induced direct heating (OBH) effect, and an indirect cooling effect due to dynamic processes associated with vertical mixing and shallow meridional overturning circulation. There are three major findings: (a) DCM acts to reduce mean SST by around 0.2°C in the eastern equatorial Pacific, being larger than the surface CHL effects. (b) DCM interannual variability increases the ENSO amplitude to a comparable degree as the surface CHL effects. (c) The total net impact of vertical mixing, currents, and net surface heat flux makes SST drop more under the DCM effect than the surface CHL effect in the eastern Pacific. These findings provide a new insight into the feedback mechanisms for the bioclimate interactions.

How Does El Niño–Southern Oscillation Change Under Global Warming—A First Look at CMIP6

AbstractThe latest generation of coupled models, the sixth Coupled Models Intercomparison Project (CMIP6), is used to study the changes in the El Niño–Southern Oscillation (ENSO) in a warming climate. For the four future scenarios studied, the sea surface temperature variability increases in most CMIP6 models, but to varying degrees. This increase is linked to a weakening of the east‐west temperature gradient in the tropical Pacific Ocean, which is evident across all models. Just as in previous generations of climate models, we find that many characteristics of future ENSO remain uncertain. This includes changes in dominant time scale, extratropical teleconnection patterns, and amplitude of El Niño and La Niña events. For models with the strongest increase in future variability, the majority of the increase happens in the Eastern Pacific, where the strongest El Niño events usually occur.

Northern Tropical Atlantic Warming in El Niño Decaying Spring: Impacts of El Niño Amplitude

AbstractThe decay time of El Niño events is considered to be an important factor in determining whether or not the northern tropical Atlantic (NTA) warming occurs in boreal spring of El Niño decaying years. Through coupled model experiments with different types of sea surface temperature anomalies specified in the tropical Pacific domain, the present study illustrates that the NTA warming is larger and more persistent during slow than fast decaying El Niño events. The amplitude of El Niño events is shown to be another important factor in the difference of the spring NTA warming between slow and fast decaying El Niño events. The NTA warming difference is mainly attributed to the wind speed‐related latent heat flux difference in preceding winter according to the model simulations and the observations. The differences of NTA wind and SST variations between slow and fast decay El Niño simulations are mostly consistent with the observations.

Increased influence of ENSO on Antarctic temperature since the Industrial Era

Under the influence of recent global warming, modulation of frequencies and amplitude of El Niño-Southern Oscillation (ENSO) and its impacts on global climate have become great concerns to the global community. Antarctic climate is sensitive to these changes owing to tropical and Southern Hemispheric (SH) teleconnections. Antarctic surface air temperature (SAT) reconstructed approximately for the past five centuries (~1533 to 1993 CE) based on multiple oxygen isotope (δ18O) records of ice cores from East and West Antarctica show dominant oscillations in ENSO and Pacific Decadal Oscillation (PDO) frequency bands. Further, variance of the East Antarctica (EA) temperature record shows significant increasing trend at ENSO band and decreasing trend at PDO band since the industrial era (~1850 CE). This observation is consistent with the earlier report of increasing ENSO activity, reconstructed based on tropical-subtropical tree ring records. ENSO influence in the SH high-latitude is known to be characterized by Pacific South American (PSA) pattern reflected in the atmospheric pressure fields. Our investigation of greenhouse gas (GHG) forced model simulation results show an increasing trend in PSA activity since the industrial era. Thus, we suggest ENSO activity and its influence on Antarctic temperature are increasing in response to increasing radiative GHG forcing since the industrial era.

Weakening of the El Niño amplitude since the late 1990s and its link to decadal change in the North Pacific climate

The amplitude of El Niño, measured by the Nino3.4 sea surface temperature anomaly (SSTA) index, has exhibited an interdecadal change with a weakening trend since the late 1990s, characterized by the distinct Bjerknes stability index between 1980–1998 and 1999–2014. Statistical results suggest that this was primarily induced by the attenuation of the zonal wind stress and low‐level wind anomalies in response to the zonal equatorial SSTA gradient. The weakened atmospheric responses to the zonal equatorial SSTA gradient were associated with the obvious westward extension of the negative sea level pressure anomalies (SLPA) over the tropics. This was mainly attributed to the transition of the dominant atmospheric circulation over the North Pacific, where the Aleutian Low (AL) mode was replaced by the North Pacific Oscillation (NPO) mode after the late 1990s. Numerical experiments from the long‐term historical simulations of the GFDL‐CM3 model indicate that both of the AL and NPO were dominant over the North Pacific and alternated on the interdecadal time‐scale. When the NPO mode was dominant, it became more effective at playing a role in triggering an El Niño in the central Pacific via the seasonal footprinting mechanism over the subtropical northeastern Pacific. Such a change might cause the westward shift of the tropical SLPA and the attenuated atmospheric responses to the zonal SSTA gradient, ultimately resulting in the weakening of the El Niño amplitude.

Sensitivity of large dengue epidemics in Ecuador to long-lead predictions of El Niño

Long-lead forecasts of El Niño events are lacking despite their enormous societal and economic impacts. These climatic events lead to floods and droughts in many tropical regions, and damage agriculture and the economy in poor countries. Due to their impact on local climate, they can also affect human health by increasing the risk of vector-borne diseases, such as dengue fever. Physical processes at the origin of this complex coupled ocean-atmosphere phenomenon are just beginning to be better understood, with subsurface processes and stored heat as two of the main driving forces leading to the development of El Niño in a quasi-periodic manner. Taking advantage of this new knowledge, a statistical dynamic components model, using a state space approach and predictors relevant to the El Niño evolution, was specifically tailored to forecast warm events at lead times of about 2 years (well beyond the traditional spring barrier limit in El Niño predictability). This forecasting scheme provides skilful information on the amplitude of El Niño events, their duration, and the peak time of the sea surface temperature anomalies at a sufficient lead time as to efficiently serve preventive public health actions. The long-lead El Niño predictions were coupled to a statistical dengue model to estimate dengue cases during the 1998 and the 2010 epidemics in El Oro Province in Ecuador, where dengue is hyper-endemic. The dengue model correctly estimated these two largest dengue epidemics even at a 2-year simulation lead time. Thus, information is successfully passed from the El Niño forecast domain to the dengue estimation domain, and the long-lead El Niño predictions are shown to potentially anticipate the magnitude of dengue epidemics in the peak season. The results validate the sensitivity of large dengue epidemics in the region to the El Niño forecasts within the proposed model coupling set-up and imply a potential for increasing lead-time in dengue prediction. This coupled model framework and exploratory analysis, based on El Niño predictions, could be easily extended to other similarly transmitted diseases in tropical and subtropical countries, which are directly and severely affected by the large-scale temperature and precipitation teleconnections occurring before, during and after El Niño events.

Causes of ENSO Weakening during the Mid-Holocene

The causes of the change in amplitude of El Niño–Southern Oscillation (ENSO) during the mid-Holocene were investigated by diagnosing the model simulations that participated in the Paleoclimate Modelling Intercomparison Project phases 2 and 3. Consistent with paleoclimate records, 20 out of the 28 models reproduced weaker-than-preindustrial ENSO amplitude during the mid-Holocene. Two representative models were then selected to explore the underlying mechanisms of air–sea feedback processes. A mixed layer heat budget diagnosis indicated that the weakened ENSO amplitude was primarily attributed to the decrease in the Bjerknes thermocline feedback, while the meridional advective feedback also played a role. During the mid-Holocene, the thermocline response to a unit anomalous zonal wind stress forcing in the equatorial Pacific weakened in both models because of the increased ENSO meridional scale. A further investigation revealed that the greater ENSO meridional width was caused by the strengthening of the Pacific subtropical cell, which was attributed to the enhanced mean trade wind that resulted from the intensified Asian and African monsoon rainfall and associated large-scale east–west circulation in response to the mid-Holocene orbital forcing.

Asymmetric Modulation of El Niño and La Niña and the Linkage to Tropical Pacific Decadal Variability

El Niño–Southern Oscillation (ENSO) in a 1300-yr preindustrial control simulation of the Community Climate System Model, version 4 (CCSM4), exhibits distinct modulation in association with tropical Pacific decadal variability (TPDV). The frequency and duration of El Niño events modulate with changes in the interbasin sea surface temperature (SST) gradient related to the leading mode of TPDV, which resembles the interdecadal Pacific oscillation (IPO). La Niña shows similar changes with the IPO but is also controlled by changes in El Niño that often precedes La Niña, and these effects tend to cancel each other. The amplitude of ENSO, on the other hand, is closely related to the second leading mode of TPDV that affects the zonal and meridional contrast of tropical Pacific climate. Significant changes in the pattern and seasonal evolution related to this TPDV mode are found mainly for El Niño because of the nonlinear relation between the atmospheric deep convection and SSTs. The resultant changes in the amplitude of El Niño, in turn, affect the amplitude and duration of the following La Niña, as well as the asymmetry in their patterns and duration. The decadal ENSO modulation associated with both TPDV modes is not symmetrical between El Niño and La Niña and thus is not likely to occur solely as a result of random variability. The patterns of TPDV in CCSM4 have resemblance to those simulated by its atmospheric component coupled to a slab ocean model, suggesting that TPDV induced by stochastic atmospheric variability interacts with the ENSO dynamics.

La Niña–like Mean-State Response to Global Warming and Potential Oceanic Roles

Abstract The majority of the models that participated in phase 5 of the Coupled Model Intercomparison Project global warming experiments warm faster in the eastern equatorial Pacific Ocean than in the west. GFDL-ESM2M is an exception among the state-of-the-art global climate models in that the equatorial Pacific sea surface temperature (SST) in the west warms faster than in the east, and the Walker circulation strengthens in response to warming. This study shows that this “La Niña–like” trend simulated by GFDL-ESM2M could be a physically consistent response to warming, and that the forced response could have been detectable since the late twentieth century. Two additional models are examined: GFDL-ESM2G, which differs from GFDL-ESM2M only in the oceanic components, warms without a clear zonal SST gradient; and HadGEM2-CC exhibits a warming pattern that resembles the multimodel mean. A fundamental observed constraint between the amplitude of El Niño–Southern Oscillation (ENSO) and the mean-state zonal SST gradient is reproduced well by GFDL-ESM2M but not by the other two models, which display substantially weaker ENSO nonlinearity than is observed. Under this constraint, the weakening nonlinear ENSO amplitude in GFDL-ESM2M rectifies the mean state to be La Niña–like. GFDL-ESM2M exhibits more realistic equatorial thermal stratification than GFDL-ESM2G, which appears to be the most important difference for the ENSO nonlinearity. On longer time scales, the weaker polar amplification in GFDL-ESM2M may also explain the origin of the colder equatorial upwelling water, which could in turn weaken the ENSO amplitude.

El Niño phases embedded in Asian and North American drought reconstructions

The amplitude of El Niño-Southern Oscillation (ENSO) varies substantially at each phase of its evolution, affecting the timing and patterns of atmospheric teleconnections around the globe. Instrumental records are too short to capture the full behavior of ENSO variability. Here we use the well-validated Monsoon Asia Drought Atlas (MADA) and North America Drought Atlas (NADA) for the past 700 years, and show that tree-ring records from different regions represent tropical sea surface temperature (SST) conditions at various phases of ENSO. Three modes of tree-ring based summer drought variability are found to be correlated with ENSO: summer droughts over the Maritime Continent and Southwest North America (NA), and a dipole mode between Central and South Asia. A lagged correlation analysis is performed to determine the time when precipitation and temperature anomaly imprints on summer droughts as recorded in tree-rings. Drought anomalies in the Maritime Continent and Southwest NA represent ENSO at the developing and peak phases respectively, while those over Central/South Asia are associated with tropical-wide SST anomalies (including the Indian Ocean) at the decay phase of ENSO. Thus proxy records from different regions can provide valuable information on long-term behavior of ENSO at different phases.

Excitation of equatorial Kelvin and Yanai waves by tropical cyclones in an ocean general circulation model

Abstract. Tropical cyclones (TCs) actively contribute to the dynamics of Earth's coupled climate system. They influence oceanic mixing rates, upper-ocean heat content, and air–sea fluxes, with implications for atmosphere and ocean dynamics on multiple spatial and temporal scales. Using an ocean general circulation model with modified surface wind forcing, we explore how TC winds can excite equatorial ocean waves in the tropical Pacific. We highlight a situation where three successive TCs in the western North Pacific region, corresponding to events in 2003, excite a combination of Kelvin and Yanai waves in the equatorial Pacific. The resultant thermocline adjustment significantly modifies the thermal structure of the upper equatorial Pacific and leads to eastward zonal heat transport. Observations of upper-ocean temperature by the Tropical Atmosphere Ocean (TAO) buoy array and sea-level height anomalies using altimetry reveal wave passage during the same time period with similar properties to the modeled wave, although our idealized model methodology disallows precise identification of the TC forcing with the observed waves. Results indicate that direct oceanographic forcing by TCs may be important for understanding the spectrum of equatorial ocean waves, thus remotely influencing tropical mixing and surface energy budgets. Because equatorial Kelvin waves are closely linked to interannual variability in the tropical Pacific, these findings also suggest TC wind forcing may influence the timing and amplitude of El Niño events.

The CSIRO Mk3L climate system model version 1.0 – Part 2: Response to external forcings

Abstract. The CSIRO Mk3L climate system model is a coupled general circulation model, designed primarily for millennial-scale climate simulation and palaeoclimate research. Mk3L includes components which describe the atmosphere, ocean, sea ice and land surface, and combines computational efficiency with a stable and realistic control climatology. It is freely available to the research community. This paper evaluates the response of the model to external forcings which correspond to past and future changes in the climate system. A simulation of the mid-Holocene climate is performed, in which changes in the seasonal and meridional distribution of incoming solar radiation are imposed. Mk3L correctly simulates increased summer temperatures at northern mid-latitudes and cooling in the tropics. However, it is unable to capture some of the regional-scale features of the mid-Holocene climate, with the precipitation over Northern Africa being deficient. The model simulates a reduction of between 7 and 15% in the amplitude of El Niño-Southern Oscillation, a smaller decrease than that implied by the palaeoclimate record. However, the realism of the simulated ENSO is limited by the model's relatively coarse spatial resolution. Transient simulations of the late Holocene climate are then performed. The evolving distribution of insolation is imposed, and an acceleration technique is applied and assessed. The model successfully captures the temperature changes in each hemisphere and the upward trend in ENSO variability. However, the lack of a dynamic vegetation scheme does not allow it to simulate an abrupt desertification of the Sahara. To assess the response of Mk3L to other forcings, transient simulations of the last millennium are performed. Changes in solar irradiance, atmospheric greenhouse gas concentrations and volcanic emissions are applied to the model. The model is again broadly successful at simulating larger-scale changes in the climate system. Both the magnitude and the spatial pattern of the simulated 20th century warming are consistent with observations. However, the model underestimates the magnitude of the relative warmth associated with the Mediaeval Climate Anomaly. Finally, three transient simulations are performed, in which the atmospheric CO2 concentration is stabilised at two, three and four times the pre-industrial value. All three simulations exhibit ongoing surface warming, reduced sea ice cover, and a reduction in the rate of North Atlantic Deep Water formation followed by its gradual recovery. Antarctic Bottom Water formation ceases, with the shutdown being permanent for a trebling and quadrupling of the CO2 concentration. The transient and equilibrium climate sensitivities of the model are determined. The short-term transient response to a doubling of the CO2 concentration at 1% per year is a warming of 1.59 ± 0.08 K, while the long-term equilibrium response is a warming of at least 3.85 ± 0.02 K.

Diversity and functional groups dynamics affected by drought and fire in Patagonian grasslands

During 1998–1999 a severe drought occurred in northwestern Patagonia that provoked an extensive wildfire. We monitored vegetation cover and the soil seed bank to study the diversity and functional group gap dynamics in burned and unburned sites. Species were grouped into 3 functional groups: forbs, fugitive species, and annual grasses. Post-drought vegetation recovered quickly due to a rainy spring in the second year but decreased after a dry and warm spring in the third year. These patterns underline the close relationship that exists between phenological phases and meteorological variables. Drought decreased richness but did not affect the presence of stress-tolerant species, whereas fire increased richness by allowing the establishment of fugitive species. Species in the fugitive functional group may be fire adapted and depend on seed accumulation in the seed bank (storage effect) to coexist with other gap species. Forbs exhibited their highest vegetation cover and seed bank density in the unburned site. Global climate change suggests an increase in the frequency and amplitude of El Niño/Southern Oscillation phenomena that, in northwestern Patagonia, are related to the occurrence of drought, fire, and changes in vegetation dynamics.

Decadal Modulation of ENSO in a Hybrid Coupled Model

Abstract Decadal variations in the amplitude of El Niño and the Southern Oscillation have been the subject of great interest in the literature for the past decade. One theory suggests that ENSO is best described as a stable system driven by linear dynamics and that stochastic atmospheric forcing is responsible for the development and modulation of ENSO on interannual as well as decadal time scales. Another theory suggests that ENSO is driven by strong nonlinear coupled feedbacks between the ocean and atmosphere and low frequency changes in ENSO amplitude are driven by decadal changes in the tropical Pacific mean state. Unfortunately, the observed record is too short to collect reliable statistics for such low frequency behavior. A hybrid coupled model composed of a simple statistical atmosphere coupled to the Poseidon isopycnal ocean model has been developed for the study of ENSO decadal variability. The model simulates realistic ENSO variability on interannual and decadal time scales with negligible climate drift over 1000 years. Through analysis and experimentation the authors show that low frequency changes in the atmospheric “weather noise” drive changes in the tropical Pacific mean state leading to changes in the amplitude of ENSO on decadal time scales. Additional model simulations suggest that, while predictability is limited by the presence of atmospheric noise, there are extended periods when the coupled instability, strengthened by changes in the mean state, is insensitive to noise on interannual time scales. The relationship between decadal modulation of ENSO and mean state changes resides somewhere between the linear damped stochastically forced theory and the strongly unstable theory. Unlike the strongly unstable system, changes in ENSO amplitude on longer time scales are determined by the stochastic forcing. The stochastic forcing is not necessary in this model to sustain ENSO; however, its presence is crucial for low frequency changes in the mean state of the tropical Pacific. The strong relationship between the mean state and ENSO amplitude modulation in the model is in opposition to the linear damped stochastically forced theory. The fact that changes in the tropical Pacific mean state lead directly to changes in ENSO amplitude and predictability has positive implications for predictability.

Secular Changes of Annual and Interannual Variability in the Tropics during the Past Century

Abstract Wavelet transforms, which can unfold signals in both time and frequency domains, are used to analyze the Comprehensive Ocean and Atmospheric Data Sets for the period 1870–1988. The focus is on secular changes in the interannual variability and the annual cycle of selected equatorial regions. The amplitude of El Nino/Southern Oscillation (ENSO) is found to be large from 1885 to 1915, to be small during the period 1915–1950, and to increase rapidly after about 1960. Surprisingly, the decadal variations in the amplitude of ENSO are not matched by similar decadal variations in the amplitude of the annual cycle. On short timescales of 2–5 years, ENSO strongly influences the annual cycle in certain parts of the central and eastern tropical Pacific where the thermocline is shallow. The annual cycle is weak in warm El Nino years and is strong in cold La Nina years. This result suggests that the amplitude of the seasonal cycle is affected by interannual variations in the depth of the thermocline and in th...