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The tropical instability waves (TIWs) in the eastern tropical Pacific have generally been considered as surface-intensified structures resembling the first baroclinic mode. Here, we report on the existence of subsurface-intensified TIWs on the equator. These TIWs are primarily manifested in zonal velocities, inducing maximum velocity oscillations at 70-90 m depth with amplitudes of 0.1-0.2 m/s and periods of 5-20 days. They account for similar to 20% of the variance at 5- to 30-day periods, with another similar to 50% being contributed by the surface-intensified TIWs. These waves are most significant during the TIW seasons; they are energized in part by barotropic instabilities and usually last for 3-7 months. Via interacting with the mean flow, they can induce strong out-of-phase shear changes between similar to 50-m depth and just above the Equatorial Undercurrent core and may lead to complex diapycnal mixing structures. Their horizontal structures, generation mechanism(s), and large-scale impacts remain to be disclosed.

Tropical instability waves (TIWs) induce large physical and biological perturbations, which have a feedback onto the tropical Pacific climate and ecosystem. However, the extent to which TIW-induced chlorophyll perturbations (Chl(TIW)) can influence El Nirio-Southern Oscillation (ENSO) remains unknown. Here we used a hybrid-coupled model to investigate the Chl(TIW) effect on ENSO. Two experiments are conducted, one with the Chl(TIW) effect being represented in the control run (CTRL) and other with the Chl(TIW) effect being purposely excluded by filtering out Chl(TIW) signals (FILT). Results show that the amplitude of ENSO is increased by 27% in CTRL compared to FILT. Chl(TIW) tends to modulate the penetrative solar radiation in the upper ocean, acting to weaken the intensity of TIWs. Then, the weakened TIWs lead to a reduction in the equatorward meridional heat transport and consequently less warming effect on the cold tongue. Therefore, La Nina conditions tend to be intensified, and ENSO amplitude is increased. Plain Language Summary Tropical instability waves (TIWs) are prominent cusp-like waves in the tropical Pacific and Atlantic, with the period of 20-40 days and wavelength of 600-1,000 km. TIWs can induce large perturbations in chlorophyll (Chl(TIW)) and sea surface temperature. Given that chlorophyll absorbs solar radiation in the upper ocean and alters the upper-ocean temperature structure, it is natural to ask the following question: Can TIW-related mesoscale biology processes likeChl(TIW) influence the tropical climate? Here we examine the Chl(TIW) effect on El Nino-Southern Oscillation (ENSO) using a coupled physics-biogeochemistry model, with the Chl(TIW) effect taken into account or not. Results suggest that the Chl(TIW) effects tend to increase ENSO amplitude by 27%. This is because the Chl(TIW) effect weakens the intensity of TIW itself, which inevitably decreases the meridional heat transport onto the equator and consequently less warming effect on the cold tongue in the eastern equatorial Pacific. As a result, La Nina conditions tend to be intensified and ENSO amplitude is increased. Our study contributes to resolving the ongoing debate on the role of biological processes in modulating ENSO amplitude (i.e., increase or decrease), and improves current understanding of ENSO modulation.

The tropical instability waves (TIWs) in the eastern Pacific consist of waves with central periods of about 33 and 17days. While the former manifest as vortices north of the equator, and are known to modulate diapycnal mixing at their southern edge, the latter remain largely unexplored. Here the structure of the 17-day TIWs and the mechanism through which they may influence equatorial mixing are investigated based on long term in situ measurements and reanalysis data. Different from the well-recognized meridional velocity oscillation, the 17-day TIWs are found to induce northeast-southwest (NE-SW) velocity oscillations. They are confined within but asymmetric about the equator, differing from free Yanai waves, which they were commonly assumed to resemble. The vertical shear associated with the westward anomalous velocity superimposes on the shear of the mean flow, resulting in the strongest shear in the upper thermocline that is expected to facilitate diapycnal mixing therein. Plain Language Summary Observed from satellites, the tropical instability wave (TIW) in the eastern tropical Pacific Ocean is a 1,000-km-long gigantic combination of waves and vortexes. It emerges between the energetic zonally interleaving equatorial currents, impacts the atmosphere, and transfers enormous energy to the western Pacific Ocean and to the deep ocean as wellone of the research foci for both physical oceanographers and meteorologists. However, the structures of its huge body, and the associated complicated smaller-scale processes, remain unrevealed, primarily because the measurements that have been conducted, usually by research vessels, are too limited to derive a full picture. Here based on long-term observations at a hotspot of TIW and 4-D numerical model outputs, we identify the structure of TIW at the equator, which manifests as a pair of slanted clockwise and anticlockwise vortexes, and induces strong east-west oscillations. We also find the most efficient mechanism for the TIW to strengthen vertical velocity shear, shear instability, and ocean turbulent mixing. The findings therefore are important for understanding the TIW dynamics, TIWs' signatures in atmosphere-ocean interactions, and their impact on vertical heat transport from warmer sea surface to colder subsurface layers of the ocean.

International audience

International audience; Tropical instability waves (TIWs), the dominant form of eddy variability in the tropics, have a peak period at about 5 weeks and are strongly modulated by both the seasonal cycle and El Niño-Southern Oscillation (ENSO). In this study, we first demonstrated that TIW-induced nonlinear dynamical heating (NDH) is basically proportional to the TIW amplitude depicted by a complex index for TIW. We further delineated that this NDH, capturing the seasonally modulated nonlinear feedback of TIW activity onto ENSO, is well approximated by a theoretical formulation derived analytically from a simple linear stochastic model for the TIW index. The results of this study may be useful for the climate community to evaluate and understand the TIW-ENSO multiscale interaction.

Tropical instability waves (TIWs) play an important role in modulating the physical variability (e.g., sea surface temperature [SST] and heat flux) and biological activity (e.g., phytoplankton and chlorophyll (Chl)) in the eastern tropical Pacific. The impacts of TIW-induced Chl perturbations (Chl(TIW)) are examined using a coupled ocean physics-biology model in the tropical Pacific. TIW-scale biological and physical perturbations are extracted using a zonal high-passed filter. The TIW-induced perturbations of biological and physical fields (e.g., Chl, nitrate, iron, and SST.) exhibit their close relationships, with negative correlation between SST and Chl at TIW scale. Processes responsible for the impacts of Chl(TIW) are diagnosed. Chl(TIW) acts to modulate the incoming solar radiation that penetrates the bottom of the mixed layer (Q(pen)). In addition, two experiments are conducted with the impacts of Chl(TIW) on the ocean included or not. The results show that the Chl(TIW) effects tend to substantially increase seasonal variability of eddy kinetic energy (EKE) and slightly decrease SST in the eastern equatorial Pacific. At large scale, Chl(TIW) leads to an increase in mean Chl concentration, which induces a SST cooling and an increase in EKE in the boreal winter. At TIW scale, Chl(TIW) leads to the change in Q(pen) and a decrease in baroclinic conversion term and eventually decreases the EKE in the boreal spring and summer. So the Chl(TIW) acts to have influences both on the intensity of TIW and large-scale SST in the tropical Pacific, involving multiscale interactions and feedbacks between ocean physics and biology.

We examine the dependence of the amplitude of the El Nino-Southern Oscillation (ENSO) on the mixing coefficient parameterizing the lateral mixing of tracers (ARedi) The value of this coefficient is very uncertain, ranging in Earth System Models between a few hundred and a few thousand m2s−1, with some observational estimates showing even higher values. A suite of simulations is made with two spatially varying distributions of ARedi derived from satellite observations as well as four simulations where a spatially constant ARedi is varied over a factor of six. Surprisingly, larger values of ARedi result in stronger ENSO variability despite the higher mixing coefficients producing more efficient lateral diffusive damping of anomalies. This is because lateral mixing also warms the cold tongue, increasing vertical temperature gradients and decreasing horizontal temperature gradients. Larger vertical temperature gradients make sea surface temperatures more responsive to atmospheric forcing, while smaller horizontal temperature gradients shift the location of convection and make the atmosphere more responsive to sea surface temperature anomalies. The last effect holds across simulations as well as within individual simulations and thus also helps to explain interdecadal variability in ENSO amplitude. By contrast, a previously proposed anticorrelation between the amplitude interannual and annual variability does not hold across simulations, although it does hold within simulations. The propagation of thermocline depth anomalies is relatively insensitive to the value of ARedi. Properly specifying the lateral mixing along the equator (including distinguishing the impacts of subgridscale turbulence and tropical instability waves) appears essential to simulating ENSO.

The jets in the equatorial Pacific Ocean of a realistically forced global circulation model with a horizontal resolution of 1/12.5° cause a strong loss of phase coherence in semidiurnal internal tides that propagate equatorward from the French Polynesian Islands and Hawaii. This loss of coherence is quantified with a baroclinic energy analysis, in which the semidiurnal-band terms are separated into coherent, incoherent, and cross terms. For time scales longer than a year, the coherent energy flux approaches zero values at the equator, while the total flux is ∼500 W/m. The time variability of the incoherent energy flux is compared with the internal-tide travel-time variability, which is based on along-beam integrated phase speeds computed with the Taylor-Goldstein equation. The variability of monthly mean Taylor-Goldstein phase speeds agrees well with the phase speed variability inferred from steric sea surface height phases extracted with a plane-wave fit technique. On monthly time scales, the loss of phase coherence in the equatorward beams from the French Polynesian Islands is attributed to the time variability in the vertically sheared background flow associated with the jets and tropical instability waves. On an annual time scale, the effect of stratification variability is of equal or greater importance than the shear variability is to the loss of coherence. In the model simulations, low-frequency equatorial jets do not noticeably enhance the dissipation of the internal tide, but merely decohere and scatter it.

Enhanced mesoscale eddy activities or tropical instability waves (TIWs) exist along the northern front of the cold tongue in the eastern equatorial Pacific Ocean. In this study, we investigate seasonal variability of eddy kinetic energy (EKE) over this region and its associated dynamic mechanism using a global, eddy-resolving ocean general circulation model (OGCM) simulation, the equatorial mooring data and satellite altimeter observations. The seasonal-varying enhanced EKE signals are found to expand westward from 100°W in June to 180°W in December between 0°−6°N. This westward expansion in EKE is closely connected to the barotropically-baroclinically unstable zonal flows that are in thermal-wind balance with the seasonal-varying thermocline trough along 4°N. By adopting an 1½-layer reduced-gravity model, we confirm that the seasonal perturbation of the thermocline trough is dominated by the anti-cyclonic wind stress curl forcing, which develops due to southerly winds along 4°N from June to December.

Tropical Instability Waves (TIWs) and the El Nino-Southern Oscillation (ENSO) are two air-sea coupling phenomena that are prominent in the tropical Pacific, occurring at vastly different space-time scales. It has been challenging to adequately represent both of these processes within a large-scale coupled climate model, which has led to a poor understanding of the interactions between TIW-induced feedback and ENSO. In this study, a novel modeling system was developed that allows representation of TIW-scale air-sea coupling and its interaction with ENSO. Satellite data were first used to derive an empirical model for TIW-induced sea surface wind stress perturbations (τTIW). The model was then embedded in a basin-wide hybrid-coupled model (HCM) of the tropical Pacific. Because τTIW were internally determined from TIW-scale sea surface temperatures (SSTTIW) simulated in the ocean model, the wind-SST coupling at TIW scales was interactively represented within the large-scale coupled model. Because the τTIW–SSTTIW coupling part of the model can be turned on or off in the HCM simulations, the related TIW wind feedback effects can be isolated and examined in a straightforward way. Then, the TIW-scale wind feedback effects on the large-scale mean ocean state and interannual variability in the tropical Pacific were investigated based on this embedded system. The interactively represented TIW-scale wind forcing exerted an asymmetric influence on SSTs in the HCM, characterized by a mean-state cooling and by a positive feedback on interannual variability, acting to enhance ENSO amplitude. Roughly speaking, the feedback tends to increase interannual SST variability by approximately 9%. Additionally, there is a tendency for TIW wind to have an effect on the phase transition during ENSO evolution, with slightly shortened interannual oscillation periods. Additional sensitivity experiments were performed to elucidate the details of TIW wind effects on SST evolution during ENSO cycles. This article is protected by copyright. All rights reserved.

Reconstructions of the El Nino–Southern Oscillation (ENSO) are often created using the oxygen isotopic ratio in tropical coral skeletons (δ18O). However, coral δ18O can be difficult to interpret quantitatively, as it reflects changes in both temperature and the δ18O value of seawater. Small-scale (10–100 km) processes affecting local temperature and seawater δ18O are also poorly quantified and contribute an unknown amount to intercoral δ18O offsets. A new version of the Regional Ocean Modeling System capable of directly simulating seawater δ18O (isoROMS) is therefore presented to address these issues. The model is used to simulate δ18O variations over the 1979–2009 period throughout the Pacific at coarse (O(50 km)) resolution, in addition to 10 km downscaling experiments covering the central equatorial Pacific Line Islands, a preferred site for paleo-ENSO reconstruction from corals. A major impact of downscaling at the Line Islands is the ability to resolve fronts associated with tropical instability waves (TIWs), which generate large excursions in both temperature and seawater δ18O at Palmyra Atoll (5.9°N, 162.1°W). TIW-related sea surface temperature gradients are smaller at neighboring Christmas Island (1.9°N, 157.5°W), but the interaction of mesoscale features with the steep island topography nonetheless generates cross-island temperature differences of up to 1°C. These nonlinear processes alter the slope of the salinity:seawater δ18O relationship at Palmyra and Christmas, as well as affect the relation between coral δ18O and indices of ENSO variability. Consideration of the full physical oceanographic context of reef environments is therefore crucial for improving δ18O-based ENSO reconstructions.

Turbulent mixing in the equatorial Pacific Ocean is an important process that controls diapycnal heat transport and hence affects the air-sea interactions and global climate. It is recently shown that, in the eastern equatorial Pacific, strong mixing is induced in the thermocline by enhanced vertical shear associated with tropical instability waves (TIWs), which propagate westward along the equator at a speed of ∼0.5 m s−1 with a wavelength of ∼1000 km. In this study, using a high-resolution ocean general circulation model, we show that the TIWs can play an important role in inducing turbulent mixing in the thermocline also in the central equatorial Pacific, although the thermocline is too deep to be directly affected by the vertical shear of the TIWs. The front of the TIW is clearly manifested as a narrow strip of strong convergence of horizontal surface flow, from which area downward and westward propagating internal waves are intermittently emanated. These internal waves can be interpreted as lee waves generated by the surface-flow convergence zone, which acts like an inverted obstacle moving along the stratified ocean surface by inducing downward flow. The associated downward energy flux below the surface mixed layer increases as the TIW structure becomes deeper toward the central equatorial Pacific, so that the energy pathway to turbulent mixing in the thermocline can be created. The downward energy flux integrated over the entire equatorial Pacific and averaged during January 2011 amounts to ∼8.1 GW, occupying a significant fraction of the energy input to the TIWs.

This study evaluates capability of the Argo observation network for monitoring ocean variation, especially for eddy-scale variation, by using an optimum interpolation (OI) procedure. Sea surface dynamic height anomalies (DHAs) are derived from Argo temperature and salinity profile data, and DHA fields are obtained by the OI based on the space-time correlation scales estimated from along-track sea level anomaly (SLA) data by satellite altimetry. The DHA fields are compared with the SLA fields derived from the same OI applied to the along-track SLA data. The results show that the equatorial Kelvin waves and tropical instability waves are well captured by Argo floats. Eddies are also monitored effectively in the subtropical western North Pacific. The OI results of DHA do not agree well with those of SLA in the high latitudes. A simple test of the space-time OI analysis shows that more than six data in the e-folding domain, where the correlation coefficient of ocean variation is above e−1, are required for the reliable analysis with 99% confidence level. Argo floats provide sufficient number of observations for the reliable analysis in the low latitudes and some areas in the North Pacific. Two to three times more Argo data would be required in most of midlatitudes and much more in high latitudes for capturing eddy-scale variation.

A 15 year time series (1999–2014) from the 0°, 23°W Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) mooring, which includes an 8 month record (October 2008 to June 2009) of high-resolution near-surface velocity data, is used to analyze the diurnal variability of sea surface temperature, shear, and stratification in the central equatorial Atlantic. The ocean diurnal cycle exhibits pronounced seasonality that is linked to seasonal variations in the surface wind field. In boreal summer and fall, steady trade winds and clear skies dominate, with limited diurnal variability in sea surface temperature. Diurnal shear layers, with reduced Richardson numbers, are regularly observed descending into the marginally unstable equatorial undercurrent below the mixed layer, conditions favorable for the generation of deep-cycle turbulence. In contrast, in boreal winter and spring, winds are lighter and more variable, mixed layers are shallow, and diurnal variability of sea surface temperature is large. During these conditions, diurnal shear layers are less prominent, and the stability of the undercurrent increases, suggesting seasonal covariance between diurnal near-surface shear and deep-cycle turbulence. Modulation of the ocean diurnal cycle by tropical instability waves is also identified. This work provides the first observational assessment of the diurnal cycle of near-surface shear, stratification, and marginal instability in the equatorial Atlantic, confirming previous modeling results and offering a complementary perspective on similar work in the equatorial Pacific.

Sea surface salinity (SSS) data derived from the Aquarius/SAC-D satellite mission are analyzed along with other satellite and in situ data to assess Aquarius' capability to detect tropical instability waves (TIWs) and eddies in the tropical Atlantic Ocean and to investigate the influence that SSS has on the variability. Aquarius data show that the magnitude of SSS anomalies associated with the Atlantic TIWs is ±0.25 practical salinity unit, which is weaker than those in the Pacific by 50%. In the central equatorial Atlantic, SSS contribution to the mean meridional density gradient is similar to sea surface temperature (SST) contribution. Consequently, SSS is important to TIW-related surface density anomalies and perturbation potential energy (PPE). In this region, SSS influences surface PPE significantly through the direct effect and the indirect effect associated with SSS-SST covariability. Ignoring SSS effects would underestimate TIW-related PPE by approximately three times in the surface layer. SSS also regulates the seasonality of the TIWs. The boreal-spring peak of the PPE due to SSS leads that due to SST by about one month. Therefore, SSS not only affects the spatial structure, but the seasonal variability of the TIWs in the equatorial Atlantic. In the northeast Atlantic near the Amazon outflow and the North Brazil Current retroflection region and in the southeast Atlantic near the Congo River outflow, SSS accounts for 80–90% of the contribution to mean meridional density gradient. Not accounting for SSS effect would underestimate surface PPE in these regions by a factor of 10 and 4, respectively.

[1] Estimating the mixed-layer heat budget is a key issue for understanding the cold tongue development in the eastern equatorial Atlantic. A high-resolution ocean regional model is used to diagnose the mixed-layer heat budget online during the EGEE-3 experiment from May to August 2006. The heat budget shows the major role of the horizontal advection and turbulent mixing in the mixed-layer temperature balance in the cold tongue. The surface net heat flux and entrainment processes play a minor role. The equatorial cooling is mainly induced by low-frequency advection, which is balanced by high-frequency zonal and meridional advections. The high-frequency advections are organized in patterns along the northern edge of the cold tongue, where they are associated with strong sea surface temperature gradients and well-developed tropical instability waves in the western Atlantic. Special attention is paid to the wind energy flux, which controls horizontal advection and turbulent mixing. We suggest that the wind energy flux drives the vertical velocity, which in turn adjusts the mixed-layer depth, its stratification, and the vertical shear of the horizontal current. Although vertical advection is not essential in providing cold water in the Atlantic cold tongue, it is shown that the vertical velocity plays a central role in preconditioning the mixed layer and maximizes the turbulent mixing.

Observations of surface ozone (O3) mixing ratios carried out during two ground-based field campaigns in the Galapagos Islands are reported. The first campaign, Primera Investigacion sobre la Quimica, Evolucion y Reparto de Ozono, was carried out from September 2000 to July 2002. The second study, Climate and HAlogen Reactivity tropicaL EXperiment, was conducted from September 2010 to March 2012. These measurements complement the Southern Hemisphere ADditional OZonesonde observations made with weekly to monthly frequency at Galapagos. In this work, the daily, intraseasonal, seasonal and interannual variability of O3 in the marine boundary layer are described and compared to those observed in other tropical locations. The O3 diurnal cycle shows two regimes: (i) photochemical destruction followed by nighttime recovery in the cold season (July to November) and (ii) daytime advection and photochemical loss followed by nighttime depositional loss associated to windless conditions in the warm season (February to April). Wavelet spectral analysis of the intraseasonal variability of O3 reveals components with periods characteristic of tropical instability waves. The O3 seasonal variation in Galapagos is typical of the Southern Hemisphere, with a maximum in August and a minimum in February–March. Comparison with other measurements in remote tropical ocean locations shows that the change of the surface O3 seasonal cycle across the equator is explained by the position of the Intertropical Convergence Zone and the O3 levels upwind.

Velocity data from an array of acoustic Doppler current profilers moored about 0°, 140°W from May 1990 through June 1991 during the Tropical Instability Wave Experiment are used in conjunction with Tropical Atmosphere-Ocean array data and a blended sea surface temperature (SST) product to study the processes that control SST variations. The horizontal velocity data allow us to calculate the vertical velocity component by vertically integrating the continuity equation. Given the three-dimensional temperature flux divergence, we examine the role of the ocean circulation in SST. Upwelling and downwelling are found to be associated with cooling and warming, respectively, suggesting that a vertical velocity component of either sign affects SST. Both the temperature flux divergence and advective formulations for the ocean circulation's influence on the temperature budget show times when the ocean circulation appears to provide the primary control on SST and times when this is not the case, with the flux divergence formulation performing better than the advective formulation. Statistically, within a bandwidth encompassing the tropical instability waves and the intraseasonal variations, roughly half of the SST variation is accounted for by the ocean circulation. These results are encouraging, given that data sets with different spatial and temporal scales have been used. They suggest that future field experimentation which utilizes a flux divergence array with velocity and temperature data sampled at the same spatial and temporal scales will yield quantitatively improved results. The analyses also show that the ocean circulation, on average, provides a cooling effect requiring the net surface heat flux to be positive to maintain the mean background state. The cooling effect is mainly controlled by mean ocean circulation and temperature fields.

Tropical instability waves (TIWs) are detected in remotely-sensed sea surface height (SSH), temperature (SST), and wind records of the eastern equatorial Pacific. Analyses of TIW anomaly relationships reveal strong dynamical influence of TIWs within approximately 5° of the equator. The first influence is advective heat flux. The primary forcing of TIW SST anomalies is advection of the meridional temperature gradient by TIW currents. The second influence is modification of the wind stress and Ekman pumping fields by TIW surface ocean currents. By affecting surface stress and hence roughness, TIW currents in this low-wind region introduce a significant bias in scatterometer vector wind measurement. This bias is evident in both NSCAT and QuikSCAT winds. The difference between wind measurements from TAO moorings and scatterometers is phase-locked with TIW SST oscillations. These results have important implications for scatterometry and for understanding tropical dynamics, thermodynamics and biogeochemistry.

Tropical Instability Waves are a prominent feature in analyses of tropical Pacific sea surface temperature and sea level height variability, and may play a significant role in the heat, salt and momentum budgets of the equatorial oceans. Here we show that they may be strongly influenced by forcing from the atmosphere. In experiments with an ocean GCM we find that a phase shift in the subseasonal wind forcing gives rise to a corresponding phase shift in the TIWs, indicating a systematic influence of the high frequency wind forcing.

High resolution mooring time series are used to quantify significant chlorophyll anomalies associated with tropical instability waves (TIWs) in the equatorial Pacific. Distinct peaks characterized by very high chlorophyll (up to 3.5 mg m -3 ) are observed in association with TIW cold cusps. These high-chlorophyll peaks appear to differ with respect to scale and intensity from those previously observed at subductive fronts. The physical processes responsible for the observed chlorophyll distributions are not mutually exclusive, and include advection, horizontal mixing, enhanced upwelling and concentration of biomass at fronts. Given the potentially large spatial extent of these high chlorophyll bands, their importance as regions of increased productivity and CO 2 uptake is discussed.

A La Nina took place in the equatorial Pacific in 1999, and strong tropical instability waves (TIWs) developed, causing large meanders of a sea surface temperature (SST) front between the equator and 3°N. High-resolution satellite measurements are used to describe the variability of SST, surface wind velocity, column-integrated water vapor, cloud liquid water, and precipitation associated with these strong TIWs in 1999. Coherent ocean-atmosphere patterns emerge in both the Pacific and Atlantic, revealing rich regional characteristics. In the far eastern Pacific, southeasterly trades strengthen (weaken) over positive (negative) SST anomalies, apparently due to enhanced (reduced) mixing with high-speed winds aloft. In the central Pacific we find evidence that SST-induced sea level pressure changes also contribute substantially to wind fluctuations. Similar SST-wind covariability also exists in the Southern Hemisphere along 2°S, but the wind variability induced by unit SST anomaly is much larger than that north of the equator. In the central Pacific where the equatorial front is broad, the northern SST pattern has a large meridional scale and reaches as far north as 6°N. Further to the north in the Intertropical Convergence Zone (ITCZ) where local SST anomalies are small, significant variability is found in clouds and precipitation, which is further correlated with surface wind convergence. In the Atlantic, TIW signals in SST are strongly trapped near the equator, but they induce significant remote response in the ITCZ, which takes a more southern position than its Pacific counterpart and thus more susceptible to TIW influence.

Source waters of the Indo-Pacific equatorial thermocline are studied with a high-resolution ocean model. In the annual mean fields, tropical and subtropical overturning cells are found that upwell at the equator and downwell at 5 degrees and 20 degrees poleward of the equator respectively. Tropical cells are common in ocean models, but their role in ventilating the equatorial thermocline is obscure because the downwelled water is too warm to match the subsurface equatorial waters. The tropical cells are much weaker when the overturning is considered in density coordinates. When high-frequency mass fluxes are included tropical cells are compensated by an eddy-induced overturning. Seasonal variations and tropical instability waves are responsible for the compensation. It follows that only subtropical cells transfer surface water to the equatorial thermocline. Strong tropical cells are shown to be an artefact.

Observations from two new spaceborne microwave instruments in 1999 clearly reveal the atmospheric manifestation of tropical instability waves north of the Pacific equatorial cold tongue.

Tropical instability waves north and south of the equator play an important role in the transport of mass and heat in the tropical Pacific ocean. Using data from the TOPEX/Poseidon satellite and the Tropical Atmosphere-Ocean buoy array, we present important observational evidence that Rossby waves modify the phase of tropical instability waves, supporting a recent hypothesis on interactions between these waves. A marked increase in the correlation coefficient between the northern and southern instability wave fields when Rossby waves are present indicates an increased phase correspondence between the fields. There is also an associated amplification of the southern instability wave field. A comparison with buoy data indicates that the satellite is able to determine the phase of instability waves accurately. This application of satellite and in situ data marks a new approach in the identification and analysis of coupling mechanisms between important dynamical processes in the equatorial Pacific.

The Pathfinder AVHRR data in Plate I were provided by PODAAC at the Jet Propulsion Laboratory (JPL). This work was supported by NASA/JPL contract 1206715, NASA TRMM contract NAS5-9919 and NASA's Earth Science Information Partnership through contract SUB1998-101 from the University of Alabama at Huntsville.

This work was supported by the Department of Commerce/NOAA grant NA56GP0026. One of the authors (SM) was partially supported by a NASA Global Change Fellowship NGT-30288.

[1] Eddy diffusivity of the surface velocity field in the tropical Pacific Ocean was estimated using satellite-tracked drifting buoys (1979 through mid-1996). The tropical Pacific surface current system is characterized by nonstationarity, strong meridional shear, and an energetic mesoscale velocity field. Eddy diffusivity may be defined as the integral of the autocovariance of Lagrangian eddy velocities, requiring both stationary and homogeneous statistics of the eddy field. Eddy velocities were obtained by removing a splined mean field to eliminate mean shear from observations binned (1) spatially to group data that have similar dispersion characteristics and (2) temporally to create stationary eddy statistics. Zonal diffusivity estimates are up to ≈7 times larger than meridional diffusivity estimates in the high eddy energy regions. This anisotropy is associated with the meridional mesoscale wave motion (i.e., by equatorial and tropical instability waves) that increases eddy variance but does not lead to a proportional increase in water parcel diffusion because of the coherent character of the trajectory motion, at least for initial time lags. Simple autoregressive models of first and second order are used to describe and classify the resulting eddy statistics. An independent confirmation of the diffusivity estimate in the central/eastern Pacific was obtained by comparing tracer flux divergence computed from a parameterization using diffusivity estimates of our analysis with that from direct eddy Reynolds stress flux divergence. Our results show that diffusivity can be estimated for regions not considered previously either because of sparse data or the complexities of the velocity field.

Striking 20–30-day sea surface temperature waves observed along the equatorial front in the later half of the year are generally believed to be of an oceanic origin. Here we report the detection of atmospheric waves that are unambiguously tied to these oceanic waves, using new satellite measurements of surface winds. A general circulation model simulation reveals that these atmospheric waves have a shallow vertical structure trapped in the planetary boundary layer (PBL), unlike El Nino/Southern Oscillation where changes in deep convection are the cause of anomalous winds. Vertical wave motion penetrates well above the PBL and is likely to impact the distribution and transport of climatically important gas species such as ozone and dimethyl sulfide.

Data from more than 1900 Lagrangian drifters in the equatorial Pacific Ocean from 1980 to 1994 together with velocity records from two Tropical Ocean-Global Atmosphere - Tropical Atmosphere Ocean (TOGA-TAO) equatorial moorings at 11° and 140°W, advanced very high resolution radiometer (AVHRR) sea surface temperature (SST) product, and European Centre for Medium-Range Weather Forecasts (ECMWF) winds were used to investigate the effects and energetics of currents associated with the tropical instability waves (TIWs). Adaptive multitaper spectral analysis was used to estimate how spectral energy varied in the 15-to-30-day period TIW band. The drifter data was analyzed separately for high and low values of the TIW energy in regions of 20° longitude by 20° latitude centered at 0°N, 110°W and 0°N, 140°W to construct meridional profiles of energetics of the TIW region. High TIW energy values typically occurred around October when the South Equatorial Current (SEC) and the North Equatorial Countercurrent (NECC) both became stronger and the eddy kinetic and potential energy production at 140°W was noticeably larger. At 110°W the eddy kinetic and potential energy production existed all the time without large differences between the periods of high and low TIW activity. The meridional kinetic energy was enhanced in the region between the equator and 10°N from 150° to 100°W, with the largest values occurring between 110° and 140°W in longitude and around 5°N in latitude. The largest terms in the horizontal kinetic energy production equation were u′υ′¯U¯y and υ′υ′¯V¯y with maxima in the region of anticyclonic shear between SEC and NECC, from 2° to 6°N. The temperature variance, or the potential energy production, peaked closer to the equator at 3°N. The linear growth timescale of the instability was about 10 days. The time-variable wind supplied energy to the current fluctuations during the TIW off period, but for the TIW on period the wind energy input was reduced (at 110W) or even reversed (at 140°W, between 1°S and 7°N), suggesting that air-sea interaction was important in the total energy balance of the waves. The effect of instability was to reduce the shear of the mean current and to warm the equatorial cold tongue. These calculations suggest that there exists a balance between energy production and dissipation in the TIWs.

This study explores the relative impact of three major components of the tropical Pacific Ocean observing system on data assimilation analyses. The extensive observing components that are available during the 2-year period from October 1992 through September 1994 include sea level derived from TOPEX/POSEIDON altimetry, thermistor data from the Tropical Ocean/Global Atmosphere Tropical Atmosphere-Ocean (TOGA TAO) mooring array, and expendable bathythermographs (XBTs) from the Volunteer Observing Ship program. In the first part of this study, methods are introduced to assimilate these data into a primitive equation model using optimal interpolation. The resulting velocity and thermal analyses are then compared with a numerical simulation that has no data assimilation. The most innovative aspect of the assimilation procedure is the development of a statistical model that uses satellite altimetry to update the subsurface thermal field. To determine the impact of individual components of the observing system, three additional experiments are conducted. In each experiment, one component is withheld from the assimilation. The resulting analysis is then compared with the missing data set. Our results show (1) To resolve major features of the seasonal cycle, it is necessary to have either the altimeter or the XBT data. In the absence of XBTs, the analysis appears to develop a significant temporal drift in its thermal structure. (2) Intraseasonal variability, such as tropical instability waves, is best resolved using altimetry because of that data set's superior spatial resolution. (3) Thermistor data from the TOGA TAO mooring array are helpful, but not as crucial, in resolving the seasonal cycle within ±8° latitude as the XBTs. The mooring array is too coarsely spaced to allow this analysis system to resolve tropical instability waves.

Monthly period oscillations in temperature and velocity in the vicinity of the North Equatorial Countercurrent along 140°W are examined using moored time series data collected during 1988–1989. These oscillations, which presumably derive their energy from instability of the large-scale shear of the zonal equatorial current system, are found to be energetic across a broad band of periods of about 15–50 days. Along 7°N, wavelike motions at these periods propagate westward with a phase speed of about 30–40 cm s−1 and a zonal wavelength of 750–1150 km. The oscillations are approximately in geostrophic balance with the thermal field, with the velocity vector rotating anticyclonically around westward propagating high sea level centers. Zonal volume transport variations associated with these waves between 5° and 9°N at 20–180 m depth have a standard deviation of about 4.1 Sv, with peak-to-peak changes in some instances approaching 20 Sv. Maximum temperature variance along 140°W occurs between 5° and 7°N at 100–150 m depth in a region of strong mean vertical and meridional temperature gradient associated with a thermocline sloping upward to the north. At the surface, maximum temperature variability occurs from 2° to 5°N in the vicinity of the strong sea surface temperature front north of the equator. Analysis of the upper ocean temperature balance at 7°N, 140°W reveals that meridional advection is important both in the surface layer and in the thermocline. Vertical advection, estimated using vertical velocity calculated from a simplified linear vorticity balance, is also important in the thermocline, where it contributes a variance comparable to that of meridional advection. Zonal advection is likely to be significant in the upper 80 m, below which it is probably of secondary importance compared to meridional and vertical advection. One consequence of advection in the temperature balance is that variance is shifted toward lower frequencies in temperature vis-a-vis velocity spectra. This “red shift” may account for the apparent difference between periods for instability waves inferred from near-equatorial meridional velocity spectra, and slightly longer periods reported for oscillations of the sea surface temperature front north of the equator based on satellite imagery.

Westward-propagating waves with periods of 20–30 days and wavelengths of ∼ 1,100km are a prominent feature of sea-surface temperatures (SSTs) in the equatorial Pacific and Atlantic Oceans. They have been attributed to instabilities due to current shear. We compare SST observations from the spaceborne Along Track Scanning Radiometer (ATSR) and TOGA-TAO moored buoys with SSTs from a model of the tropical Pacific forced with observed daily windstress data. The phases of the strongest “Tropical Instability Waves” (TIWs) in the model are in closer correspondence with those observed than we would expect if these waves simply developed from infinitesimal disturbances (in which case their phases would be arbitrary). If we filter out the intraseasonal component of the windstress, all phase-correspondence is lost. We conclude that the phases of these waves are not arbitrary, but partially determined by the intraseasonal winds. The subsurface evolution of the model suggests a possible control mechanism is through interaction with remotely-forced subsurface Kelvin and Rossby waves. This is supported by an experiment which shows how zonal wind bursts in the west Pacific can modify the TIW field, but other mechanisms, such as local feedbacks, are also possible.

[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.

[1] 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.

[1] 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.

Tropical instability waves have been shown to have a major impact on the variability of temperature and nutrients along the equatorial wave guide. In order to assess the impact of these features on carbon species distributions during an ENSO event, sea surface temperature, salinity, sigma-t, nitrate, CO[sub 2] fugacity, total inorganic carbon, total alkalinity, and pH along the equator were measured from 130[degrees]W to 100[degrees]W during 8-15 May 1992. Concurrent moored measurements of surface currents and temperature were also made at 0[degrees], 110[degrees]W. Results indicate that tropical instability waves, with periods of 15 to 20 days and zonal wavelengths of 700-800 km, controlled the observed spatial variability of the CO[sub 2] species, nitrate and hydrographic parameters at the equator. 37 refs., 3 figs.

[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.

[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.

(1) This study documents the variability of noise in the equatorial Pacific associated with the El Nino and La Nina events based on the ensemble retrospective forecasts of the Climate Forecast System. It is found that the noise (measured by the ensemble spread) in the western equatorial Pacific zonal wind stress is enhanced around and before the peak of El Nino events and weakened around 2-3 months after the peak of El Nino events. The change in the wind stress noise is communicated to the thermocline depth in the eastern equatorial Pacific with about 1 month time lag. The eastern equatorial Pacific SST noise, however, decreases during the decay stages of El Nino events while the corresponding noise in the surface zonal wind and heat flux increases. This decrease in the SST noise is related to the weakening of the SST front and the suppression of the tropical instability waves along the flanks of the equatorial Pacific cold tongue that is associated with the SST warming due to El Nino events. Similar relations are seen during La Nina events except that the changes are in opposite sense. As such, the signal-to-noise ratio and, thus, the predictability for the eastern equatorial Pacific SST is relatively high during warm events compared to cold events.

[1] The nonlinearity of Tropical Instability Waves (TIWs) is studied using shallow-water equations with various mean states from the equatorial Pacific Ocean. In the early linear stage, unstable TIWs, centered near 5°N with a wavelength about 1000 km and a period about one month, dominate the whole domain. However neutral Yanai waves with periods about 15―22 days emerge near the equator when the unstable TIWs grow into fully nonlinear vortices and begin to rotate, which stabilizes the mean states substantially. The strength of these Yanai waves are sensitive to the instability of the initial mean flow. Meanwhile the TIWs centered near 5°N are slowed down and weakened. The external forcing terms are found to be important for them to retain their dominance from 3°N to 7°N and also be able to suppress the late emerging Yanai waves if strong enough.

[1] The breeding method of Toth and Kalnay finds the perturbations that grow naturally in a dynamical system like the atmosphere or the ocean. Here breeding is applied to a global ocean model forced by reanalysis winds in order to identify instabilities of weekly and monthly timescales. This study extends the method to show how the energy equations for the bred vectors can be derived with only very minimal approximations and used to assess the physical mechanisms that give rise to the instabilities. Tropical Instability Waves in the tropical Pacific are diagnosed, confirming the existence of bands of both baroclinic and barotropic energy conversions indicated earlier by Masina et al. and others. In the South Atlantic Convergence Zone, the bred vector energy analysis shows that there is kinetic to potential ocean eddy energy conversion, suggesting that the growing instabilities found in this area are forced by the wind.

On the basis of integrations of an eddy-permitting coupled physical-biological model of the tropical Pacific we explore changes in the simulated mean circulation as well as its intraseasonal to interannual variability driven by the biologically modulated vertical absorption profiles of solar radiation. Three sensitivity ocean hind-cast experiments, covering the period from 1948 to 2003, are performed. In the first one, simulated chlorophyll affects the attenuation of light in the water column, while in the second experiment, the chlorophyll concentration is kept constant in time by prescribing an empirically derived spatial pattern. The third experiment uses a spatially and temporally constant value for the attenuation depth. The biotically induced differential heating is generated by increased absorption of light in the surface layers, leading to a surface warming and subsurface cooling. The effect is largest in the eastern equatorial Pacific. However, the initial vertical redistribution of heat leads to considerable changes of the near-surface ocean circulation subsequently influencing the near-surface temperature structure. In general, including biophysical coupling improves the model performance in terms of temperature and ocean circulation patterns. In particular, the upwelling in the eastern equatorial Pacific is enhanced, the mixed layer becomes shallower, the warm bias in the eastern Pacific is reduced, and the zonal temperature gradient increases. This leads to stronger La Niña events and an associated increase in the variability of the Niño3 SSTA time series. Furthermore, the eddy kinetic energy (EKE) associated with mesoscale eddies in the eastern equatorial Pacific increases by almost 100% because of enhanced EKE production due to enhanced horizontal and vertical shear of the mean currents.

[1] Ocean eddies and fronts affect surface stress via two mechanisms: (1) ocean surface currents altering the relative motion between air and sea and, hence, the stress fields and (2) ocean sea surface temperature (SST) gradients forcing changes in stability and near-surface winds. In this paper, we quantify the first effect and how it impacts Tropical Instability Waves (TIW) in the eastern Pacific. High-resolution satellite data and a regional coupled model are used to distinguish between stress changes due to the surface currents and those due to the changes in stability and near-surface winds. It is found that both mechanisms affect the surface stress curl, but they do so at different latitudes, allowing for their effect on Ekman pumping to be distinguished. The Ekman pumping due to the surface current effect alone, leads to significant damping of the TIWs. In terms of the eddy kinetic energy, the inclusion of surface current in the stress leads to decay with an e-folding time comparable with the period of the TIWs. It is, thus, an important damping mechanism to be included in ocean and coupled ocean-atmosphere models.

[1] Tropical instability waves (TIWs) are 20- to 40-day waves that appear along the sea surface temperature (SST) front of the equatorial cold tongues. In this study, we investigate the atmospheric planetary boundary layer response to TIW-induced SST variations in the tropical Atlantic Ocean using Community Atmosphere Model version 3 (CAM3) developed at the National Center for Atmospheric Research and compare the model simulation with available satellite measurements. CAM3 is used to simulate the response to Atlantic TIWs with T85 and T42 horizontal resolutions forced with daily Tropical Rainfall Measuring Mission microwave imager (TMI) SSTs and T85 horizontal resolution forced with weekly TMI and Reynolds SSTs. All four runs exhibit an atmospheric response to the SST anomalies, but the amplitude of the response is different in each case. The model simulates winds, convergence, column-integrated water vapor, and precipitation perturbations associated with TIWs that are similar in many respects to the satellite observations. However, the wind convergence and divergence anomalies in the satellite observations are located west of the SST anomalies. In the CAM3 simulations the wind convergence is more in phase with SST anomalies. Differences between the observations and the model simulations appear to be due to both differences in the boundary layer response and differences in the horizontal advection process.

[1] The impacts of parameterized lateral ocean viscosity on climate are explored using three 120-year integrations of a fully coupled climate model. Reducing viscosity leads to a generally improved ocean circulation at the expense of increased numerical noise. Five domains are discussed in detail: the equatorial Pacific, where the emergence of tropical instability waves reduces the cold tongue bias; the Southern Ocean, where the Antarctic Circumpolar Current increases its kinetic energy but reduces its transport; the Arctic Ocean, where an improved representation of the Atlantic inflow leads to a better sea-ice distribution; the North Pacific, where the more realistic path of the Kuroshio leads to more realistic temperatures across the midlatitude Pacific; and the northern marginal seas, where stronger boundary currents lead to significantly less sea-ice. Although the ocean circulation and sea-ice distribution improve, the oceanic heat uptake, the poleward heat transport, and the large scale atmospheric circulation are not changed significantly. In particular, the improvements to the equatorial cold tongue did not lead to better representation of tropical precipitation or El Nino.

[1] Using a database of roughly 300 basin-wide conductivity-temperature-depth (CTD) stations per year from 1997 to 2003, we examined interannual and seasonal variability of nutrients and chlorophyll across the equatorial Pacific. During this period, chlorophyll concentrations exceeded the range of previous measurements for this region. Nitrate, silicic acid, and phosphate also varied widely but not necessarily coherently with each other or with chlorophyll. Across the La Nina to El Nino continuum there was nonmonotonic variability in chlorophyll, particulate backscatter (a proxy for phytoplankton carbon), and large size fraction (diatom) chlorophyll. In general, while El Nino was associated with decreased phytoplankton biomass, there was no corresponding increase in phytoplankton or diatoms during La Nina. However, there were increases in macronutrients in response to La Nina. We suggest that the lack of a biological response to these nutrient increases is due to their decoupling from iron supply. Multiple linear regression analysis of the physical factors responsible for vertical nutrient fluxes emphasized the importance of winds in the central and western Pacific, and thermocline depth in the east. We suggest that the role of the winds is not limited to enhancement of upwelling, but perhaps more importantly to increased vertical mixing of nutrients. Seasonal patterns were weak but consistent with previous work which has suggested that enhanced productivity in the second half of the calendar year causes reduced surface pCO2. Enhanced coverage of nutrient sensors on the tropical atmosphere ocean (TAO) array would help to quantify seasonal signals associated with processes such as tropical instability waves and lead to a better understanding of the links between productivity, carbon export, and air-sea CO2 exchange.

n n Recent high-resolution space-based observations reveal significant two-way air-sea interactions associated with tropical instability waves (TIWs) in the tropical oceans. But in most large-scale climate modeling studies, the atmospheric wind response to TIW-induced sea surface temperature (SSTTIW) anomalies has not been taken into account realistically and thus the corresponding feedbacks to the ocean are not known. Here we apply a singular value decomposition (SVD) analysis to satellite data to derive an empirical model for the SSTTIW induced wind stress (tau(TIW)) variability in the eastern tropical Pacific. The derived SSTTIW-tau(TIW) relationship is then nested into a basin-scale ocean and a hybrid coupled ocean-atmosphere model of the tropical Pacific to take into account the TIW-induced wind SST coupling. It is demonstrated that TIW-induced wind feedback to the ocean can have a rectified effect on large-scale mean ocean state and interannual variability, with an asymmetric difference in SST (a cooling) and a significant modulation of ENSO amplitude.