Biennial Mode Research Articles

Analysis of gap-free chlorophyll-a data from MODIS in Arabian Sea, reconstructed using DINEOF

ABSTRACTThe chlorophyll-a concentration (chl-a), which is an index of phytoplankton pigment present in the oceans, is considered as a key indicator of health of marine ecosystems that could have direct effect on the human life. In this study, spatial and temporal variability of chl-a in the Arabian Sea (AS) is examined using reconstructed cloud-free ocean colour data for the period 2002–2015. Data Interpolating Empirical Orthogonal Function method is used to reconstruct the missing data. Subsequently, wavelet analysis is applied on the reconstructed data to assess the temporal variability in terms of seasonal, intra-seasonal, and interannual variability of chl-a in the AS. Wavelet analysis clearly depicted the low-frequency, stationary modes or approximation levels inferring the monthly, seasonal, and annual mean of the signal, while the high-frequency, non-stationary modes indicated the local abnormalities. From the analysis of gap-free data, the presence of biennial mode of variability in the northern AS chl-a is observed. The analysis further showed the existence of intra-seasonal oscillations in the northern AS during summer monsoon and single dominant peak during winter monsoon. Chl-a appeared to decline slightly during the entire study period across all the selected regions of the AS. Also, it is observed that chl-a in the northwestern region is highly dynamic than in the other regions of the AS.

The tropospheric biennial oscillation defined by a biennial mode of sea surface temperature and its impact on the atmospheric circulation and precipitation in the tropical eastern Indo-western Pacific region

Temporal and spatial patterns of anomalous atmospheric circulation and precipitation over the Indo-Pacific region are analyzed in conjunction with the Tropospheric Biennial Oscillation as represented by the biennial mode of sea surface temperature anomalies (SSTA). The biennial components of key variables are identified independently of other variability via CSEOF analysis. Then, its impact on the Asian-Australian monsoon is examined. The biennial mode exhibits a seasonally distinctive atmospheric response over the tropical eastern Indo-western Pacific (EIWP) region (90°–150°E, 20°S–20°N). In boreal summer, local meridional circulation is a distinguishing characteristic over the tropical EIWP region, whereas a meridionally expanded branch of intensified zonal circulation develops in austral summer. Temporally varying evolution and distinct timing of SSTA phase transition in the Indian and Pacific Oceans is considered a main factor for this variation of circulation in the tropical EIWP region. The impact of the biennial mode is not the same between the two seasons, with different impacts over ocean areas in Asian monsoon and Australian monsoon regions.

The influence of Indian Ocean sea surface temperature on the variability of monsoon rainfall over India

This study focuses on the spatial and temporal variability of southwest monsoon (SWM) and northeast monsoon (NEM) rainfall over India, after the 1976–1977 regime shift. Through wavelet analysis the dominant mode of variability in the SWM and NEM rainfall is found to be in the 2–8-year scale (ENSO band). Therefore, scale-averaged wavelet power (SAP) of SWM (SAPSWM) and NEM (SAPNEM) rainfall obtained from a high-resolution dataset for the period 1977–2001 was subjected to empirical orthogonal function analysis, in order to understand the homogenous regions of rainfall variability. As the study utilizes SAP, the decadal variability is observed in the spatial and temporal patterns of rainfall. The maximum variability of SAPSWM rainfall is found to be in the 2–4-year period, whereas, for the SAPNEM rainfall, the biennial mode explains the variability over the core region of NEM rainfall. Significant power is also observed in the 4-year period. The evolution of the principal components (PCs) is consistent with the above/below normal rainfall received over the region. Similar analysis has been carried out also for the SAP of monthly sea surface temperature (SAPSST) over Indian Ocean for the months from January to September. Compared with any other month, the decadal changes of the PCs of SAPSWM and SAPNEM rainfall are highly correlated with the decadal variations of PC1 of SAPSST for the month of March. Therefore, the relation between the rainfall and SST was explained using SAPSST for the month of March. Significant biennial power is also observed in March SST over southeast Indian Ocean (85–110°E and 5–40°S) after the climate shift of 1976. In order to identify any change in the relation between NEM rainfall and SAPSST (March) before and after 1976, a correlation between NEM rainfall and PC1 of SAPSST (March) was found for the period 1951–1975 and 1977–2001. Results show that correlation has increased after 1976. Therefore, this PC can be made use of in the prediction of NEM rainfall. Copyright © 2012 Royal Meteorological Society

Nesting ecology of the critically endangered Fijian Crested Iguana Brachylophus vitiensis in a Pacific tropical dry forest

Tropical dry forest (TDF) ecosystems occur throughout tropical and subtropical zones and one of their primary characteristics is that they are structurally and functionally constrained by seasonal fluctuations in moisture levels. Organisms within them synchronize growth and reproduction with water availability and many exhibit adaptations to drought conditions. We examined the previously unknown nesting ecology of the Critically Endangered Fijian Crested Iguana Brachylophus vitiensis a TDF specialist, on the Fijian island of Yadua Taba over two field seasons. In Fiji, the TDF and the endemic Crested Iguanas are threatened on a national level by anthropogenic factors and Yadua Taba Island remains the only protected site. Evaluation of adult female reproductive status revealed that only 52% of adult females reproduced in a given year. We suggest that female Crested Iguanas have adopted a biennial mode of reproduction in response to resource limitation.We found no evidence for communal nesting or nest site defence by females. All nest burrows were constructed on the forest floor in shaded areas. Mean nest temperatures (24.5�C), levels of site openness (10.9%) and total incident radiation (2.57 MJ m2 day) recorded at nest sites were less than those reported for other Iguaninae. Mean egg size (17.9 g) was large relative to most Iguaninae and was independent of the small mean clutch size (2.9 eggs). Mean incubation period was extremely long (256 days) and spanned the eight-month dry season. Mean hatching success also was high (87%). We examine reproductive strategy in relation to the environmental conditions of the TDF habitat on Yadua Taba. This research provides a broader understanding of Crested Iguana nesting ecology and is a step towards the successful implementation of the Species Recovery Plan for this species.

Near-Annual SST Variability in the Equatorial Pacific in a Coupled General Circulation Model

Abstract Equatorial Pacific sea surface temperature (SST) anomalies in the Center for Ocean–Land–Atmosphere Studies (COLA) interactive ensemble coupled general circulation model show near-annual variability as well as biennial El Niño–Southern Oscillation (ENSO) variability. There are two types of near-annual modes: a westward propagating mode and a stationary mode. For the westward propagating near-annual mode, warm SST anomalies are generated in the eastern equatorial Pacific in boreal spring and propagate westward in boreal summer. Consistent westward propagation is seen in precipitation, surface wind, and ocean current. For the stationary near-annual mode, warm SST anomalies develop near the date line in boreal winter and decay locally in boreal spring. Westward propagation of warm SST anomalies also appears in the developing year of the biennial ENSO mode. However, warm SST anomalies for the westward propagating near-annual mode occur about two months earlier than those for the biennial ENSO mode and are quickly replaced by cold SST anomalies, whereas warm SST anomalies for the biennial ENSO mode only experience moderate weakening. Anomalous zonal advection contributes to the generation and westward propagation of warm SST anomalies for both the westward propagating near-annual mode and the biennial ENSO mode. However, the role of mean upwelling is markedly different. The mean upwelling term contributes to the generation of warm SST anomalies for the biennial ENSO mode, but is mainly a damping term for the westward propagating near-annual mode. The development of warm SST anomalies for the stationary near-annual mode is partially due to anomalous zonal advection and upwelling, similar to the amplification of warm SST anomalies in the equatorial central Pacific for the biennial ENSO mode. The mean upwelling term is negative in the eastern equatorial Pacific for the stationary near-annual mode, which is opposite to the ENSO mode. The development of cold SST anomalies in the aftermath of warm SST anomalies for the westward propagating near-annual mode is coupled to large easterly wind anomalies, which occur between the warm and cold SST anomalies. The easterly anomalies contribute to the cold SST anomalies through anomalous zonal, meridional, and vertical advection and surface evaporation. The cold SST anomalies, in turn, enhance the easterly anomalies through a Rossby-wave-type response. The above processes are most effective during boreal spring when the mean near-surface-layer ocean temperature gradient is the largest. It is suggested that the westward propagating near-annual mode is related to air–sea interaction processes that are limited to the near-surface layers.

Investigation of tropical Pacific upper-ocean variability using cyclostationary EOFs of assimilated data

Variability of the subsurface temperature, current, and heat content in the tropical Pacific Ocean has been extracted in association with the two dominant modes of the sea surface temperature anomaly (SSTA): the low-frequency mode and the biennial mode. In a recent paper, these two modes were identified as the major modes of El Nino-Southern Oscillation (ENSO). The low-frequency mode, which explains about 36% of the total SSTA variability, represents the dominant component of SSTA variability in the tropical Pacific, and is associated not with a fast physical evolution but with a slow stochastic undulation. The biennial mode, which is the second dominant component and explains about 12% of the total variability exhibits, on the other hand, a strong physical evolution. The space–time patterns of the subsurface variability were derived from an assimilated data set via a cyclostationary empirical orthogonal functions (CSEOF) analysis and the regression of the resulting principal component (PC) time series on the target PC time series of the surface modes. Extracted space–time patterns describe the detailed evolution of the physical changes in the upper ocean of the tropical Pacific that are associated with the corresponding surface modes. Specifically, they clearly show the surface and subsurface connection of the physical changes during ENSO events, and the role of equatorial waves in the manifestation of physical changes at the surface. The derived patterns of heat content, subsurface temperature, and zonal current anomalies realistically depict the detailed temporal changes of those variables and are consistent with our understanding of the physics in the tropical Pacific Ocean. The biennial mode appears to depict faithfully the phase progression of El Nino and La Nina. The propagation of equatorial Kelvin waves along the thermocline is clearly visible during El Nino and La Nina events in the cyclostationary representation of the physical modes in the tropical Pacific Ocean. Although the low-frequency mode explains three times more SSTA variability than the biennial mode, the former does not induce strong equatorial wave activity. This observation is significant considering that both El Nino or La Nina are often viewed simply in terms of a significant SST change in the tropical Pacific. The results of the present study indicate: (1) that the two ENSO modes represent significantly different physical evolutions; (2) that the amount of SST warming or cooling does not dictate the physical evolution of ENSO; and (3) that the two modes play essentially different dynamical roles including the generation of equatorial waves.

The Principal Physical Modes of Variability over the Tropical Pacific

Abstract The structures of the two principal modes of sea surface temperature (SST) variability were extracted by conducting cyclostationary EOF (CSEOF) analysis and regression analysis on several key variables. The CSEOF analysis extracts two dominant modes of SST variability that are distinct in nature. The first CSEOF is stochastic in nature and represents a standing mode of SST variability associated with a basinwide change in the surface wind. The second CSEOF exhibits a strong deterministic component describing a biennial oscillation between a warm phase and a cold phase. The surface wind directional change in the far-western Pacific appears to be instrumental for the oscillation between the two phases. Because of the distinct nature of evolution, dynamical and thermodynamical responses of the two modes are different. Further, the predictability of the two modes is different. Specifically, the biennial mode is more predictable because of the strong deterministic component associated with its evoluti...

Investigation of ENSO variability using cyclostationary EOFs of observational data

¶The variability of El Nino-Southern Oscillation (ENSO) on all time scales, ranging from months to decades, has been studied from the perspective of sea-surface temperature anomaly (SSTA), sea-level pressure anomaly (SLPA), and surface wind anomaly (SWA) fields using a statistical tool called cyclostationary empirical orthogonal function (CSEOF) analysis. The analysis indicates that a significant fraction of variability in the tropical Pacific can be explained in terms of an irregular interplay of two dominant modes: the low-frequency mode and the biennial mode. These modes, and in particular the biennial mode, are well defined coupled modes of the tropical ocean-atmospheric circulation system, as suggested by strong correlation (> 0.9) in the evolution of the different physical variables. The low-frequency mode and its PC time series are very similar to the so-called “interdecadal” mode identified by earlier investigators. The corresponding PC time series of the low-frequency mode has a broad spectral peak at 5.3 yrs. The biennial mode represents the oscillation pattern of El Nino and La Nina and has a clear biennial cycle, the amplitude of which varies on a longer time scale. There is a broad spectral peak also at 5.3 yrs in the principal component (PC) time series of this mode. Unlike in earlier studies, this biennial mode is clearly separable from the low-frequency mode, since these two modes have different physical evolution patterns. Results of the present study also indicate that the biennial component, having a clear phase-locking tendency, is not as variable as has been suggested in some previous investigations. It is shown that much of the inter-El Nino variability can be explained in terms of an irregular interplay between the biennial mode and the low-frequency mode. Finally, the role of the low-frequency mode in the tropical Pacific seems to be more important than has been suggested previously. Specifically, recent warming events since 1975 are shown to be associated with the low-frequency mode rather than with the biennial mode.

Mechanism of Kelvin and Rossby waves during ENSO events

¶The fields of sea-level height anomaly (SLHA) and surface zonal wind anomaly (SZWA) have been analyzed to investigate the typical evolution of spatial patterns during El Nino-Southern Oscillation (ENSO) events. Sea surface temperature (SST) changes during ENSO events are represented as an irregular interplay of two dominant modes, low-frequency mode and biennial mode. Cyclostationary principal component (PC) time series of the former variables are regressed onto the PC time series of the two dominant SSTA modes to find the spatial patterns of SLHA and SZWA consistent with the two SSTA modes. The two regressed patterns of SLHA explain a large portion of SLHA total variability. The reconstruction of SLHA using only the two components reasonably depicts major ENSO events. Although the low-frequency component of SST variability is much larger than the biennial component, the former does not induce strong Kelvin and Rossby waves. The biennial mode induces much stronger dynamical ocean response than the low-frequency mode. Further decomposition of the SLHA modes into Kelvin and Rossby components shows how these two types of equatorial waves evolve during typical ENSO events. The propagation and reflection of these waves are clearly portrayed in the regressed patterns leading to a better understanding of the wave mechanism in the tropical Pacific associated with ENSO. A close examination suggests that the delayed action oscillator hypothesis is generally consistent with the analysis results reported here. Rossby wave development in the central Pacific in the initiation stage of ENSO and the subsequent reflection of Kelvin waves at the western boundary seems to be an important mechanism for further development of ENSO. The development of Kelvin waves forced by the surface wind in the far-western Pacific cannot be ruled out as a possible mechanism for the growth of ENSO. While Kelvin waves in the far-western Pacific serve as an intiation mechanism of ENSO, they also cause the termination of existing ENSO condition in the central and eastern Pacific, thereby leading to a biennial oscillation over the tropical Pacific. The Kelvin waves from the western Pacific erode the thermocline structure in the central Pacific preventing further devlopment of ENSO and ultimately terminating it. It should be emphasized that this wave mechanism is clear and active only in the biennial mode.

RETENTION OF OFFSPRING IN A WILD POPULATION OF UNGULATES

A thirty-three year study of mountain goats (Oreamnos americanus) in their natural habitat revealed an unexpected plasticity in their reproductive pattern that was correlated with herd size. Females in this study population retained their offspring as yearlings (biennial reproduction) for the first sixteen years of the study while the population was stable or declining, whereas they did not retain yearlings (annual reproduction) for the next ten years while the population was increasing. Females again retained their offspring during the last seven years of the study when the population was either stable or decreasing. Offspring who are considered as retained are those who are allowed to remain within a few meters of their mothers at all times, who share food resources with their mothers, and who are allowed to nurse. Yearlings who are not retained (their mothers reproduce annually) are forced to remain many meters distant from their mothers, are unable to share food, and are never allowed to nurse. Individual differences were evident in females with respect to whether they reproduced annually or biennially. In some cases females reproduced annually for several years and subsequently shifted to a biennial mode of reproduction. In other cases, however, known individuals maintained a biennial pattern when the majority of the females were reproducing annually. Surprisingly, the rate of survival of both young of the year and yearlings was not affected by the mother's mode of reproduction. The data presented here support the idea that members of this population commonly reproduced every other year rather than that they reproduced every year, but often lost their offspring.

Answer to Comments on “Space–Time Structure of Monsoon Interannual Variability”

The conceptual model proposed by Khandekar aims at explaining the occurrence of good or bad Indian summer monsoons (ISM). He argues that good or bad monsoon years can be explained by three independent factors: El Nino–Southern Oscillation (ENSO), Eurasian snow cover, and the stratospheric wind quasi-biennial oscillation (QBO). On the other hand, the results in my paper (Terray 1995) suggest that ISM interannual variability is dominated by two low-frequency modes, the biennial (B) mode with a period very close to 2 yr and the lowfrequency (LF) mode with a period close to 4–6 yr. The LF mode contains more energy than the B mode, but the ocean–atmosphere coupling seems to be stronger at the B timescale over the Indian Ocean. Finally, the occurrence and intensity of exceptional ISMs depend upon the coupling or the decoupling of the B and LF modes during recent decades. It is true that Khandekar’s model for drought and flood in the Indian monsoon has some connections with my results and can be, among others, a working hypothesis for explaining them. However, there are still some issues that need to be clarified in Khandekar’s model from my point of view.

Observed lead-lag relationships between Indian summer monsoon and some meteorological variables

Lagged relationships between the Indian summer monsoon and several climate variables are investigated. The variables examined are gridded fields of snow cover (14 years), sea surface temperature (41 years) and 500 hPa geopotential height north of 20 °N (42 years). We also used series of global air temperature (108 years) and Southern Oscillation index (112 years). Precipitation over all India during June–September over a 112 year period are used as Indian monsoon index. Emphasis is put on early monsoon precursors. In agreement with the tendency for a low frequency oscillation in the ocean-atmosphere system, several precursor patterns are identified as early as the year preceding the monsoon. The most important key regions and seasons of largest correlations are selected and the corresponding series are used to perform a monsoon prediction. The prediction shows however a relatively moderate score mainly due to the not highly significant correlations. To improve the predictions we filtered the variables into their biennial (1.5–3.5 years) and low frequency (3.5–7.5 years) modes. Correlations between the monsoon and the filtered variables are higher than those obtained without filtering especially for the biennial mode. The two modes are out-of-phase before the monsoon and in-phase during and after. This phasing is found in all variables except for snow cover for which the two modes are in-phase before the monsoon and out-of-phase during and after. It is suggested that such phasing may be important for the formation of snow and could explain the higher correlations when variables are concomitant or are lagging the monsoon. Early predictions of the monsoon based on those two modes show improved scores with highly significant correlations with the actual monsoon.

Space-Time Structure of Monsoon Interannual Variability

The analysis of corrected ship reports [sea level pressure (SLP), sea surface temperature (SST), air temperature (AT)] and corrected land data (SLP, AT, rainfall) in the Indian sector reveals the existence of two low-frequency modes of monsoon variability during the 1900-1970 period. - A definite biennial (B) mode exists on the SLP fields. This B oscillation is unambiguously linked with a southwest-northeast SLP anomaly gradient. During the summer monsoon, the B SLP pattern can be interpreted as an expansion/contraction of the monsoon activity since this mode is strongly coupled with rainfall variations over peninsular India. - A strong low-frequency (LF) mode with period spanning 4-6 years is also seen on SLP fields over the Indian Ocean and subcontinent. The variance associated with this band is typically more important than the one observed for the B mode, and its spatial mark is also strikingly different since it is linked with a global pattern of variation. This mode has also a strong influence on the Indian summer rainfall fluctuations, particularly on the Ghats and in the Indo-Gangetic plains. The amplitude of these oscillations varies widely during the 1900-1970 period. The LF mode is well defined during 1900-1923 and 1947-1970. There is a tendency for the energy associated with the B mode to decrease on the land while it increases over the Indian Ocean during the whole 1900-1970 interval. Although these two timescales exist also on SST fields, cross-spectral analysis shows that ocean-atmosphere interactions are much stronger at the B timescale. This result stresses the B nature of the monsoon system. The existence of these interannual signals in the Indian areas where the annual cycle is so strong raises difficult problems : How can climatic anomalies persist for several years in spite of strong seasonality ? Or, still more intriguing, how can be explained the persistence of climatic anomalies during one year and the appearance of opposite sign climatic anomalies in the following year for the B mode ? A composite analysis has suggested an explanation for this last problem : SST anomalies produced by exceptional summer monsoons can indeed persist during the whole winter and exert an influence on the next summer monsoon.

Interannual and interdecadal oscillation patterns in sea level

Relative sea-level height (RSLH) data at 213 tide-gauge stations have been analyzed on a monthly and an annual basis to study interannual and interdecadal oscillations, respectively. The main tools of the study are singular spectrum analysis (SSA) and multi-channel SSA (M-SSA). Very-low-frequency variability of RSLH was filtered by SSA to estimate the linear trend at each station. Global sea-level rise, after postglacial rebound corrections, has been found to equal 1.62±0.38 mm/y, by averaging over 175 stations which have a trend consistent with the neighboring ones. We have identified two dominant time scales of El Nino-Southern Oscillation (ENSO) variability, quasi-biennial and low-frequency, in the RSLH data at almost all stations. However, the amplitudes of both ENSO signals are higher in the equatorial Pacific and along the west coast of North America. RSLH data were interpolated along ocean coasts by latitudinal intervals of 5 or 10 degrees, depending on station density. Interannual variability was then examined by M-SSA in five regions: eastern Pacific (25°S–55°N at 10° resolution), western Pacific (35°S–45°N at 10°), equatorial Pacific (123°E–169°W, 6 stations), eastern Atlantic (30°S, 0°, and 30°N–70°N at 5°) and western Atlantic (50°S–50°N at 10°). Throughout the Pacific, we have found three dominant spatio-temporal oscillatory patterns, associated with time scales of ENSO variability; their periods are 2, 2.5–3 and 4–6 y. In the eastern Pacific, the biennial mode and the 6-y low-frequency mode propagate poleward. There is a southward propagation of low-frequency modes in the western Pacific RSLH, between 35°N and 5°S, but no clear propagation in the latitudes further south. However, equatorward propagation of the biennial signal is very clear in the Southern Hemisphere. In the equatorial Pacific, both the quasi-quadrennial and quasi-biennial modes at 10°N propagate westward. Strong and weak El Nino years are evident in the sea-level time series reconstructed from the quasi-biennial and low-frequency modes. Interannual variability with periods of 3 and 4–8 y is detected in the Atlantic RSLH data. In the eastern Atlantic region, we have found slow propagation of both modes northward and southward, away from 40–45°N. Interdecadal oscillations were studied using 81 stations with sufficiently long and continuous records. Most of these have variability at 9–13 and some at 18 y. Two significant eigenmode pairs, corresponding to periods of 11.6 and 12.8 y, are found in the eastern and western Atlantic ocean at latitudes 40°N–70°N and 10°N–50°N, respectively.

A Coupled Air-Sea Biennial Mechanism in the Tropical Indian and Pacific Regions: Role of the Ocean

Variations in the upper-ocean beat content are part of a mechanism to explain biennial signals in the tropical Indian and Pacific ocean regions. The mechanism involves modulations of the annual cycle of convection and processes of the dynamically coupled ocean and atmosphere system in the tropics. A critical element of that mechanism is persistent sea surface temperature anomalies on the time scale of one seasonal cycle. Analyses of composite vertical temperature profiles from hydrographic station data for various near-equatorial areas in the Indian and Pacific oceans show that variations in the ocean heat content depend on the depth of the thermocline in the warm-pool region (both eastern Indian and western Pacific) and temperatures in the upper-ocean mixed layer away from the warm pool (western Indian and eastern Pacific). The variations in thermocline depth in the Pacific are similar to those for El Niño-Southern Oscillation (ENSO) events and are present in the biennial mode as well. Apparently, similar sets of mechanisms operate on both ENSO and biennial time scales. These results suggest that changes in upper-ocean heat content contribute to the persistence of sea surface temperature anomalies important to the biennial mechanism, that both the Indian and Pacific oceans are actively involved in ENSO, and that ENSO could be an amplification of the biennial cycle.

The biennial component of ENSO variability

Using surface marine wind and sea surface temperature data from the period 1950–1987, together with sea surface temperature and sea level pressure data from several stations in the Pacific, we have identified two dominant time scales of El Niño-Southern Oscillation (ENSO) variability. One is a biennial mode, with periods near 24 months, the other a lower frequency concentration of variance in periods of four to five years. A particularly well defined standing biennial component of ENSO variability, tightly phase-locked with the annual cycle, appears in the surface wind field of the equatorial eastern Indian Ocean. This is part of a larger scale biennial circulation over the low latitude eastern Indian Ocean-western Pacific sector. This biennial circulation is a fundamental element of ENSO variability. It exhibits a strong tendency for phase locking with the annual cycle and introduces a degree of regularity into the ENSO cycle. However, it varies in amplitude from cycle to cycle and sometimes changes phase. “Warm” and “cold” episodes are not unrelated events, but reflect opposite phases of the biennial cycle, modified by the low-frequency mode. Variations in ENSO swings can be formally described in terms of relative phasing and amplitude of the biennial and low frequency components. While the low-frequency mode typically exhibits about the same amplitude as the biennial fluctuations, it does not seem to exhibit the features associated with “ENSO episodes”, which are captured by the biennial mode. The problem of understanding biennial variability seems to be intimately linked with an understanding of the mean annual cycle. The observational results point to the annual cycle as a fundamental pacemaker of the ENSO cycle. They strongly suggest that coupled mode theory must be developed in the context of the annual cycle. They also indicate that the ENSO cycle cannot be fully described and understood without consideration of the entire Indian Ocean-Pacific Ocean sector and thus raise questions regarding the limitations of modeling the tropical Pacific as an isolated coupled ocean/atmospheric system.