Tropical Instability Waves Research Articles

Spatial and temporal variability in nutrients and carbon uptake during 2004 and 2005 in the eastern equatorial Pacific Ocean

Abstract. The eastern equatorial Pacific plays a~great role in the global carbon budget due to its enhanced biological productivity linked to the equatorial upwelling. However, as confirmed by the Equatorial Biocomplexity cruises in 2004 and 2005, nutrient upwelling supply varies strongly, partly due to the tropical instability waves (TIWs). The aim of this study was to examine patterns of spatial and temporal variability in the biological uptake of NO3, Si(OH)4 and carbon in this region, and to evaluate the role of biological and physical interactions controlling this variability over seasonal and intraseasonal time scales. Here, high resolution Pacific ROMS–CoSiNE (Regional Ocean Modeling System–Carbon, Silicon, Nitrogen Ecosystem) model results were evaluated with in situ and remote sensing data. The results of model–data comparison revealed a good agreement in domain-average hydrographic and biological rate estimates, and patterns of spatio-temporal variability in primary productivity. We confirmed that TIWs have the potential to enhance phytoplankton biomass through an increased supply of nutrients and elevated local and instantaneous phytoplankton nutrient uptake as opposed to only advecting biomass. Furthermore, we concluded that initial biological conditions (e.g., zooplankton biomass) may play an important additional constraint on biological responses, in particular of large phytoplankton such as diatoms, to TIW-induced perturbations in the physical and biogeochemical fields and fluxes. In order to fully resolve the complexity of biological and physical interactions in the eastern equatorial Pacific, we recommended improving CoSiNE and other models by introducing more phytoplankton groups, variable Redfield and carbon to chlorophyll ratios, as well as resolving the Fe–Si co-limitation of phytoplankton growth.

Modulation of equatorial turbulence by a tropical instability wave

Strong modulation of turbulent mixing by a westward‐propagating tropical instability wave (TIW) was observed in the stratified shear layer between the equatorial undercurrent (EUC) and the surface mixed layer during October and November 2008 at 0°N 140°W. The unique deep diurnal‐cycle mixing in the stratified layer beneath the equatorial cold tongue was observed where nighttime turbulent mixing was a factor of 10 greater than during daytime. The turbulent kinetic energy dissipation rate, ε, was Ο(10−6) W kg−1, and the turbulent heat flux was ∼−500 W m−2, at least 5–10 times greater than observed previously in the central equatorial Pacific. Turbulence mixing varied significantly during the four distinct phases of the meridional flow associated with the TIW. Observations during the northward‐to‐southward transition recorded the largest values of reduced shear squared, the thickest nighttime surface mixed layer, the deepest penetration of the deep‐cycle turbulence, and the largest turbulent heat flux and largest integrated ε in the deep‐cycle layer (DCL). During steady southward flow, the depth of the bases of the nighttime surface mixed layer and of the DCL were the shallowest. A 50‐m‐thick layer of strong turbulence was observed immediately above the EUC core during the northward‐to‐southward and steady southward phases. Here, the average ε exceeded 10−6 W kg−1, the eddy diffusivity exceeded 10−3 m2 s−1, and the turbulent heat flux was ∼−500 W m−2. To parameterize mixing in the central equatorial Pacific accurately, numerical models must simulate the enhancement of mixing associated with TIWs and also the variability of mixing in different TIW phases.

High frequency fluctuations in the heat content of an ocean general circulation model

Abstract. The transport and storage of heat by the ocean is of crucial importance because of its effect on ocean dynamics and its impact on the atmosphere, climate and climate change. Unfortunately, limits to the amount of data that can be collected and stored mean that many experimental and modelling studies of the heat budget have to make use of mean datasets where the effects of short term fluctuations are lost. In this paper we investigate the magnitude of the resulting errors by making use of data from OCCAM, a high resolution global ocean model. The model carries out a proper heat balance every time step so any imbalances that are found in the analysis must result from the use of mean fields. The study concentrates on two areas of the ocean affecting the El Nino. The first is the region of tropical instability waves north of the Equator. The second is in the upwelling region along the Equator. It is shown that in both cases, processes with a period of less than five days can have a significant impact on the heat budget. Thus, analyses using data averaged over five days or more are likely to have significant errors. It is also shown that if a series of instantaneous values is available, reasonable estimates can be made of the size of the errors. In model studies, such values are available in the form of the datasets used to restart the model. In experimental studies they may be in the form of individual unaveraged observations.

Role of vertical mixing originating from small vertical scale structures above and within the equatorial thermocline in an OGCM

Recent high vertical resolution measurements show small vertical scale structures (SVSs) are present in the flow above and within the equatorial thermocline and that these structures contribute significantly to ocean mixing. The SVSs are typically unresolved in OGCMs and thus their impact needs to be parameterized. We investigate the impact of the mixing induced by the SVSs on the state of the equatorial Pacific in an ocean general circulation model (OGCM). As a first step to determine the importance of the SVS induced mixing we introduce an enhanced mixing within and above the equatorial thermocline. It is found that this enhanced mixing reduces the stratification above the thermocline, and sharpens the thermocline through the Phillips effect. The sharpened thermocline limits the exchange of heat across the thermocline and traps the surface heating above the thermocline. The reduced stratification leads to less cooling of the mixed layer through entrainment, a reduced annual cycle and an increase in the annual mean of the sea surface temperature (SST) in the eastern equatorial cold tongue. The depth dependency in enhanced SVS mixing is crucial to its impact; when the enhanced mixing is applied throughout the depth of the ocean (as has been done usually in previous studies,) the cold tongue SST is cooled further. In the western equatorial Pacific, where the thermocline is deeper, SVS enhanced mixing induces a colder SST. We also find that the SVS mixing reduces the eddy kinetic energy associated with the tropical instability waves through a reduction of the meridional and vertical shear of the equatorial currents and temperature gradient.

The role of atmospheric internal variability on the tropical instability wave dynamics

[1] The impact of atmospheric internal variability on tropical instability wave (TIW) activity in the eastern equatorial Pacific is examined. To diagnose the atmospheric internal variability, two simulations were performed with a state-of-the-art coupled general circulation model that uses an eddy permitting ocean component model. Standard coupling procedures are implemented in the control simulation. In the experimental simulation, the so-called interactive ensemble coupling is used, which systematically reduces the contribution of internal atmospheric dynamics to the air-sea fluxes of heat, momentum and fresh water. In the eastern equatorial Pacific, the reduction of the atmospheric internal variability leads to an enhancement of the available potential energy and higher exchanges from mean to eddy potential energy. The perturbations in the available potential energy and the eddy potential energy contribute to the enhancement in the TIW activity through the increased eddy kinetic energy. Due to the negative correlation between the atmospheric internal variability and TIW activity, the covariance between the momentum flux at the air-sea interface and the ocean surface currents as well as heat flux at the air-sea interface and the sea surface temperatures were nearly conserved west of 120°W between the control and the experimental simulations.

Tropical instability waves linked to sea surface salinity anomalies

Stretching westward from the South American coast, a tongue of cold water straddles the equator, reaching at times clear across the Pacific Ocean. The behavior of this cold tongue affects the marine ecosystem within its reach and influences the global climate through the El Niño–Southern Oscillation. The Pacific cold tongue is mirrored in the Atlantic, though that version is much weaker and less well defined. In the Pacific, westward propagating waves can periodically radiate outward from the South American coast. Known as tropical instability waves (TIWs), they give the cold tongue's edges a ribbed appearance when observed from space. TIWs are already associated with anomalous sea surface heights and temperatures, surface winds, chlorophyll concentrations, and ocean circulation patterns. Research by Lee et al. provides the first observational evidence of sea surface salinity anomalies associated with TIWs.

Parameterization of Tropical Instability Waves and Examination of Their Impact on ENSO Characteristics

Abstract The impact of tropical instability waves (TIWs) on El Niño–Southern Oscillation (ENSO) characteristics is investigated by introducing a new parameterization of TIWs into an atmosphere–ocean general circulation model (AOGCM), the Model for Interdisciplinary Research on Climate (MIROC), with a medium-resolution (~1.4°) ocean model (known as MIROCmedres). Because this resolution is not sufficient to reproduce eddies at the spatial scale of TIWs, this approach isolates TIW effects from other factors that can affect ENSO characteristics. The parameterization scheme represents the effect of baroclinic eddy heat transport by TIWs. A 100-yr integration reveals a significant role of TIWs in observed ENSO asymmetry. Asymmetric heat transport associated with TIWs that are active (inactive) during La Niña (El Niño) generates a significant asymmetric negative feedback to ENSO and explains the observed asymmetric feature of a stronger-amplitude El Niño and weaker-amplitude La Niña. Furthermore, the parameterized eddy heat flux also affects the mean subsurface heat balance via the shallowing and steepening thermocline. This change in subsurface stratification induces a stronger thermocline feedback and a longer ENSO period.

Aquarius reveals salinity structure of tropical instability waves

Sea surface salinity (SSS) measurements from the Aquarius/SAC‐D satellite during September–December 2011 provide the first satellite observations of the salinity structure of tropical instability waves (TIWs) in the Pacific. The related SSS anomaly has a magnitude of approximately ±0.5 PSU. Different from sea surface temperature (SST) and sea surface height anomaly (SSHA) where TIW‐related propagating signals are stronger a few degrees away from the equator, the SSS signature of TIWs is largest near the equator in the eastern equatorial Pacific where salty South Pacific water meets the fresher Inter‐tropical Convergence Zone water. The dominant westward propagation speed of SSS near the equator is approximately 1 m/s. This is twice as fast as the 0.5 m/s TIW speed widely reported in the literature, typically from SST and SSHA away from the equator. This difference is attributed to the more dominant 17‐day TIWs near the equator that have a 1 m/s dominant phase speed and the stronger 33‐day TIWs away from the equator that have a 0.5 m/s dominant phase speed. The results demonstrate the important value of Aquarius in studying TIWs.

Ocean–Atmosphere Characteristics of Tropical Instability Waves Simulated in the NCEP Climate Forecast System Reanalysis

Abstract The NCEP Climate Forecast System Reanalysis (CFSR) represents a new effort with the first guess from a high-resolution coupled system and offers prospects for improved simulation of mesoscale air–sea coupled variability. This study aims to describe the characteristics of ocean–atmosphere covariability associated with tropical instability waves (TIWs) in the Pacific for the CFSR, and to assess how well they agree with in situ and satellite observations. Multiyear daily high-resolution CFSR data are used to describe variability associated with TIWs. Results show that TIW-induced SST variations exhibit pronounced seasonal and interannual variability that are tightly connected with cold tongue variations. The analysis illustrates coherent patterns associated with TIWs, both in the ocean and the atmosphere. Moisture and air temperature maximums are located west of SST maximums, leading to downstream displacement of surface pressure minimums relative to SST maximums. Surface winds accelerate (decelerate) over warm (cold) water, and a thermally direct circulation is created. Significant signals are observed in low-level cloud cover, which are closely in phase with surface wind convergences. The magnitudes of TIW-induced surface wind, surface pressure, and cloud cover perturbations agree well with in situ and satellite observations. Further analysis shows that surface net heat flux perturbations are dominated by latent heat fluxes and have a large negative feedback on TIW SST variability (~40 W m−2 °C−1). Water vapor perturbation is the primary factor contributing to changes in latent heat fluxes, while SST-induced wind perturbation plays a secondary role. The analysis presented here highlights that the CFSR provides an unprecedented opportunity to study the physical mechanisms for the TIWs, as well as their influences on climate variability.

Assimilating altimetric data to control the tropical instability waves: an observing system simulation experiment study

Tropical instability waves (TIWs) are not easily simulated by ocean circulation models primarily because such waves are very sensitive to wind forcing. In this study, we investigate the impact of assimilating sea surface height (SSH) observations on the control of TIWs in an observing system simulation experiment (OSSE) context based on a regional model configuration of the tropical Atlantic. A Kalman filtering method with suitable adaptations is found to be successful when altimetric data are assimilated in conjunction with sea surface temperature and some in situ temperature/salinity profiles. In this rather realistic system, the TIW phase is roughly controlled with a single nadir observing satellite. However, a right correction of the TIW structure and amplitude requires at least two nadir observing satellites or a wide swath observing satellite. The significant impact of orbital parameters is also demonstrated: in particular, the Jason or GFO satellite orbits are found to be more suitable than the ENVISAT orbit. More generally, it is found that as soon as adequate sub-sampling exists (with periods of 5–10 days), the length of the repetitivity cycle of orbits does not have a significant impact.

Impact of the Atmospheric Mean State on Tropical Instability Wave Activity

Abstract The features of simulated tropical instability waves (TIWs) in the Pacific Ocean are compared between atmospheric models of two different resolutions coupled with a uniform oceanic model. Results show that TIWs are more active in the high-resolution model, even though it includes atmospheric negative feedback. Such negative feedback is not identified in the low-resolution atmospheric model because of the absence of atmospheric responses. Comparison of the energetics between the two models shows that the large TIW activity in the higher-resolution model is due to the difference in barotropic energy sources near the surface. A high-resolution atmosphere results in a tighter intertropical convergence zone and associated stronger wind curl and shear. This causes a stronger surface current shear between the South Equatorial Current (SEC) and North Equatorial Counter Current (NECC), which is one of the main sources of TIW kinetic energy. These results indicate the important role of the atmospheric mean field on TIW activity and the advantage of using high-resolution models to represent coupling among multiscale phenomena.

Interannual variations of Atlantic tropical instability waves

Observations are used to develop metrics for interannual tropical instability wave (TIW) variability in the Atlantic and to relate that variability to larger scale processes. The analysis is partitioned into different latitude bands to distinguish between off‐equatorial (5°S, 2°N, and 5°N) and near‐equatorial (2°S and 0°) TIWs. TIW metrics based on sea surface temperature (SST) and sea level anomaly (SLA) fluctuations are compared against interannual anomalies of SST in the cold tongue region. To examine the role of barotropic shear instabilities in modulating the intensity of a TIW season, wind stress and near‐surface current indices are developed in regions where the shear between the Equatorial Undercurrent (EUC) and the northern branch of the South Equatorial Current (nSEC) and between the nSEC and the North Equatorial Countercurrent (NECC) are expected to be largest. Good agreement is found between the SST and SLA TIW metrics along the off‐equatorial latitude bands, and interannual variations of both metrics can largely be attributed to barotropic shear instabilities. In particular, years with low (high) TIW variance along the off‐equatorial latitude bands are associated with anomalously warm (cold) SSTs in the cold tongue region, weak (strong) wind stress divergence and curl in the EUC‐nSEC region, and weak (strong) zonal current shear in the nSEC‐NECC region. In contrast, in the near‐equatorial latitude bands, poor agreement is found between interannual TIW activity based on the SST and SLA metrics, and near‐equatorial TIW variability cannot be explained by the large‐scale SST, wind stress divergence and curl, and current shear indices.

Seasonality of Tropical Instability Waves and Its Feedback to the Seasonal Cycle in the Tropical Eastern Pacific

This study investigated the seasonality of tropical instability waves (TIWs) and its feedback to the seasonal cycle in the tropical eastern Pacific using a high-resolution ocean model covering 1958–2007. The climatological mean of the TIWs featured intraseasonal fluctuations, implying that TIWs are not occurring randomly, but their amplitude is partly in phase from one year to another. This seasonality of TIW activity is attributed to their dependency on the seasonal mean variation of current and temperature. Energy conversion analysis confirmed that the strong variability of TIWs near 4°N was due to the barotropic energy conversion associated with the large meridional shear of NECC and SEC and that at another pole near 2°N was due to the baroclinic energy conversion associated with the temperature front in the mixed layer. The former and latter poles are somehow largely responsible for amplifying the dynamic and thermal eddies of TIWs, respectively. The intensified TIWs during a boreal fall increase the tropical eastern Pacific SST by associating the warm thermal advection by anomalous currents, with a rate of up to 1°C/month in September. Therefore, this leads to interactive feedback between seasonal and intraseasonal variations, that is, TIWs in the tropical eastern Pacific.

Cyclonic eddies formed at the Pacific tropical instability wave fronts

[1] Sea surface temperature images and surface drifter observations are compared to the results from a high-resolution numerical simulation to study the properties of cyclonic eddies generated at the density front of the tropical instability waves in the tropical Pacific Ocean. These cyclonic eddies, of which the diameter is about 30–100 km and the vertical extent is limited to the upper 100 m in depth, have physical characteristics similar to those of smaller submesoscale eddies at the midlatitudes according to the model. They have highly coherent structures below the surface, carrying cold and salty upwelled equatorial water probably rich in marine life. The stretching and tilting of the upper layer of the ocean provides the main mechanism responsible for the intense cyclonic vorticity of the eddies, involving complex evolution of the density field into occluded fronts.

Observation of first and second baroclinic mode Yanai waves in the ocean

AbstractThe structure and propagation of oceanic Yanai waves (also known as mixed Rossby–gravity waves) are investigated by analysis of satellite‐derived sea‐surface‐height (SSH) data. Significant spectral peaks along the dispersion curves of first and second baroclinic mode Yanai waves are identified by the wavenumber–frequency spectral analysis of SSH for the period 10–17 days. The spatial structure and propagation of these modes are described by an analysis of SSH time series filtered in the frequency–wavenumber domain that includes strong signals along the dispersion curves but excludes those of 17‐day tropical instability waves. The difference in meridional structure between the first and second baroclinic modes, which is consistent with theory, is evident. The slow eastward propagation of first and second baroclinic mode Yanai wave packets, which is consistent with the group velocity derived from theory, is also isolated from the SSH data. Year‐to‐year variability of Yanai wave activity is compared with that of the El Niño–Southern Oscillation (ENSO), showing a moderate correlation with ENSO Modoki. © Published in 2011 by John Wiley & Sons Ltd.

Barotropic Rossby Waves Radiating from Tropical Instability Waves in the Pacific Ocean

Abstract Tropical instability waves are triggered by instabilities of the equatorial current systems, and their sea level signal, with peak amplitude near 5°N, is one of the most prominent features of the dynamic topography of the tropics. Cross-spectral analysis of satellite altimetry observations shows that there is sea level variability in the Pacific Ocean as far north as Hawaii (i.e., 20°N) that is coherent with the sea level variability near 5°N associated with tropical instability waves. Within the uncertainty of the analysis, this off-equatorial variability obeys the dispersion relation for nondivergent, barotropic Rossby waves over a fairly broad range of periods (26–38 days) and zonal wavelengths (9°–23° of longitude) that are associated with tropical instability waves. The dispersion relation and observed wave properties further suggest that the waves are carrying energy away from the instabilities toward the North Pacific subtropical gyre, which, together with the observed coherence of the sea level signal of the barotropic waves with that of the tropical instability waves, suggests that the barotropic Rossby waves are being radiated from the tropical instability waves. The poleward transport of kinetic energy and westward momentum by these barotropic Rossby waves may influence the circulation in the subtropics.

Response and impact of equatorial ocean dynamics and tropical instability waves in the tropical Atlantic under global warming: A regional coupled downscaling study

[1] A regional coupled model is used for a dynamic downscaling over the tropical Atlantic based on a global warming simulation carried out with the Geophysical Fluid Dynamics Laboratory CM2.1. The regional coupled model features a realistic representation of equatorial ocean dynamical processes such as the tropical instability waves (TIWs) that are not adequately simulated in many global coupled climate models. The coupled downscaling hence provides a unique opportunity to assess their response and impact in a changing climate. Under global warming, both global and regional models exhibit an increased (decreased) rainfall in the tropical northeast (South) Atlantic. Given this asymmetric change in mean state, the regional model produces the intensified near-surface cross-equatorial southerly wind and zonal currents. The equatorial cold tongue exhibits a reduced surface warming due to the enhanced upwelling. It is mainly associated with the increased vertical velocities driven by cross-equatorial wind, in contrast to the equatorial Pacific, where thermal stratification is suggested to be more important under global warming. The strengthened upwelling and zonal currents in turn amplify the dynamic instability of the equatorial ocean, thereby intensifying TIWs. The increased eddy heat flux significantly warms the equator and counters the effect of enhanced upwelling. Zonal eddy heat flux makes the largest contribution, suggesting a need for sustained monitoring of TIWs with spatially denser observational arrays in the equatorial oceans. Overall, results suggest that eddy heat flux is an important factor that may impact the mean state warming of equatorial oceans, as it does in the current climate.

Submesoscale dynamics in tropical instability waves

Submesoscale dynamics in tropical instability waves (TIWs) of the Pacific ocean are analyzed from nested numerical simulations of increasing resolution (36 km, 12 km, and 4 km). It is shown that numerical convergence, as determined by an invariance of the kinetic energy (KE) spectrum with resolution, can be obtained for grid spacing around 10 km. This finding contrasts with mid-latitude simulations of submesoscale processes that do not converge even for grid spacing less than 1 km. The difference is associated with the larger Rossby radius of deformation at low latitudes due to the weaker Coriolis parameter. For the same reason, the mixed layer Rossby radius is larger as well, resulting in submesoscale mixed layer eddies (MLEs) with scales of 50–200 km. Similar to MLEs at mid-latitudes, those eddies that form at TIW fronts restratify the mixed layer by releasing available potential energy. They lead to an additional source of KE at submesoscale that is larger in the finer resolution simulations. At wavelengths smaller than the mesoscale peak of KE injection, a forward cascade of KE is evident in the simulations where three dynamical ranges are observed: a quasi-inertial range of slope k −2, a pre-dissipation range and a far-dissipation range. Numerical dissipation is evaluated and significant dissipation is found to occur in a pre-dissipation range, i.e., for wavenumbers well below the range where the KE spectrum tails off exponentially.

An assessment of oceanic variability in the NCEP climate forecast system reanalysis

At the National Centers for Environmental Prediction (NCEP), a reanalysis of the atmosphere, ocean, sea ice and land over the period 1979–2009, referred to as the climate forecast system reanalysis (CFSR), was recently completed. The oceanic component of CFSR includes many advances: (a) the MOM4 ocean model with an interactive sea-ice, (b) the 6 h coupled model forecast as the first guess, (c) inclusion of the mean climatological river runoff, and (d) high spatial (0.5° × 0.5°) and temporal (hourly) model outputs. Since the CFSR will be used by many in initializing/validating ocean models and climate research, the primary motivation of the paper is to inform the user community about the saline features in the CFSR ocean component, and how the ocean reanalysis compares with in situ observations and previous reanalysis. The net ocean surface heat flux of the CFSR has smaller biases compared to the sum of the latent and sensible heat fluxes from the objectively analyzed air-sea fluxes (OAFlux) and the shortwave and longwave radiation fluxes from the International Satellite Cloud Climatology Project (ISCCP-FD) than the NCEP/NCAR reanalysis (R1) and NCEP/DOE reanalysis (R2) in both the tropics and extratropics. The ocean surface wind stress of the CFSR has smaller biases and higher correlation with the ERA40 produced by the European Centre for Medium-Range Weather Forecasts than the R1 and R2, particularly in the tropical Indian and Pacific Ocean. The CFSR also has smaller errors compared to the QuickSCAT climatology for September 1999 to October 2009 than the R1 and R2. However, the trade winds of the CFSR in the central equatorial Pacific are too strong prior to 1999, and become close to observations once the ATOVS radiance data are assimilated in late 1998. A sudden reduction of easterly wind bias is related to the sudden onset of a warm bias in the eastern equatorial Pacific temperature around 1998/1999. The sea surface height and top 300 m heat content (HC300) of the CFSR compare with observations better than the GODAS in the tropical Indian Ocean and extratropics, but much worse in the tropical Atlantic, probably due to discontinuity in the deep ocean temperature and salinity caused by the six data streams of the CFSR. In terms of climate variability, the CFSR provides a good simulation of tropical instability waves and oceanic Kelvin waves in the tropical Pacific, and the dominant modes of HC300 that are associated with El Nino and Southern Oscillation, Indian Ocean Dipole, Pacific Decadal Oscillation and Atlantic Meridional Overturning Circulation.

The NCEP GODAS Ocean Analysis of the Tropical Pacific Mixed Layer Heat Budget on Seasonal to Interannual Time Scales

Abstract The mixed layer heat budget in the tropical Pacific is diagnosed using pentad (5 day) averaged outputs from the Global Ocean Data Assimilation System (GODAS), which is operational at the National Centers for Environmental Prediction (NCEP). The GODAS is currently used by the NCEP Climate Prediction Center (CPC) to monitor and to understand El Niño and La Niña in near real time. The purpose of this study is to assess the feasibility of using an operational ocean data assimilation system to understand SST variability. The climatological mean and seasonal cycle of mixed layer heat budgets derived from GODAS agree reasonably well with previous observational and model-based estimates. However, significant differences and biases were noticed. Large biases were found in GODAS zonal and meridional currents, which contributed to biases in the annual cycle of zonal and meridional advective heat fluxes. The warming due to tropical instability waves in boreal fall is severely underestimated owing to use of a 4-week data assimilation window. On interannual time scales, the GODAS heat budget closure is good for weak-to-moderate El Niños. A composite for weak-to-moderate El Niños suggests that zonal and meridional temperature advection and vertical entrainment/diffusion all contributed to the onset of the event and that zonal advection played the dominant role during decay of the event and the transition to La Niña. The net surface heat flux acts as a damping during the development stage, but plays a critical role in the decay of El Niño and the transition to the following La Niña. The GODAS heat budget closure is generally poor for strong La Niñas. Despite the biases, the GODAS heat budget analysis tool is useful in monitoring and understanding the physical processes controlling SST variability associated with ENSO. Therefore, it has been implemented operationally at CPC in support of NOAA’s ENSO forecasting.