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Large perturbations in chlorophyll (Chl) are observed to coexist at interannual and tropical instability wave (TIW) scales in the tropical Pacific; at present, their combined effects on El Niño-Southern Oscillation (ENSO) through ocean biology-induced heating (OBH) feedbacks are not understood well. Here, a hybrid coupled model (HCM) for the atmosphere and ocean physics-biogeochemistry (AOPB) in the tropical Pacific is adopted to quantify how ENSO can be modulated by Chl perturbations at interannual and TIW scales, individually or collectively, respectively. The HCM-based sensitivity experiments demonstrate a counteracting effect on ENSO: the bio-climate feedback due to large-scale Chl interannual variability acts to damp ENSO through its impact on upper-ocean stratification and vertical mixing, whereas that due to TIW-scale Chl perturbations tends to amplify ENSO. Because ENSO simulations are sensitively dependent on the ways Chl effects are represented at these different scales, it is necessary to adequately take into account these related differential Chl effects in climate modeling. A bias source for ENSO simulations is illustrated that is related with the Chl effects in the tropical Pacific, adding in a new insight into interactions between the climate system and ocean ecosystem on different scales in the region. These results reveal a level of complexity of ENSO modulations resulting from Chl effects at interannual and TIW scales, which are associated with ocean biogeochemical processes and their interactions with physical processes in the tropical Pacific. [ABSTRACT FROM AUTHOR]

In the boreal summer of 2021, the equatorial Atlantic experienced the strongest warm event, that is, Atlantic Niño, since the beginning of satellite observations in the 1970s. Such events have far‐reaching impacts on large‐scale wind patterns and rainfall over the surrounding continents. Yet, developing a paradigm of how Atlantic Niño interacts with the upper‐ocean currents and intraseasonal waves remains elusive. Here we show that the equatorial Kelvin wave associated with the onset of the 2021 Atlantic Niño modulated both the background flow and the eddy flux of the equatorial upper‐ocean circulation, causing an extremely weak and delayed tropical instability wave (TIW) season. TIW‐induced variations of sea surface temperature (SST), sea surface salinity, sea surface height, and eddy temperature advection were exceptionally weak during May to July, the climatological peak of TIW activity, but rebounded in August when higher than normal variability was observed. Moored velocity data at 23°W show that during the peak of the 2021 Atlantic Niño from June to August, the Equatorial Undercurrent was deeper and stronger than usual. An anomalously weak eddy momentum flux strongly suppressed barotropic energy conversion north of the equator from May to July, likely contributing to low TIW activity. Reduced baroclinic energy conversion also might have played a role, as the meridional gradient of SST was sharply reduced during the Atlantic Niño. Despite extremely weak TIW velocities, modest intraseasonal variability of chlorophyll‐a (Chl‐a) was observed during the Atlantic Niño, due to pronounced meridional Chl‐a gradients that partly compensated for the weak TIWs. Plain Language Summary: Every few years the eastern equatorial Atlantic Ocean is significantly warmer than usual during boreal summer. Such warm events are referred to as Atlantic Niño events, and share similarities with El Niño events in the Pacific. In 2021, the strongest Atlantic Niño in at least four decades was observed in the equatorial Atlantic. This study is the first that investigates the complex interaction between Atlantic Niño, tropical Atlantic upper‐ocean currents, and equatorial waves based on various observational data sets. We show that the developing 2021 Atlantic Niño weakened both the background flow and the variability of near‐surface currents in May, which in turn largely reduced the strength of intraseasonal (20–50 days) waves that are usually generated by instability of the upper‐ocean zonal currents. As a consequence, the cooling effect that these waves usually have north of the equator and the warming effect along the equator vanished from May to July 2021. Interestingly, variability of chlorophyll concentration was enhanced, suggesting that enhanced meridional chlorophyll gradients compensated for reduced wave activity. Key Points: The developing 2021 Atlantic Niño led to weaker equatorial surface currents and reduced vertical shear of upper‐ocean horizontal velocityStrong reduction of the surface flow, eddy flux, and meridional temperature gradient in May caused extremely weak and delayed tropical instability wave (TIW) seasonReduced meridional TIW advection contributed to sharpen the north equatorial Chl‐a front resulting in modest intraseasonal Chl‐a variability [ABSTRACT FROM AUTHOR]

Feedbacks from tropical instability waves (TIWs) on the seasonal cycle of the eastern Pacific Ocean are studied using two eddy‐rich ocean simulations, with and without TIWs. By warming the equatorial waters by up to 0.4°C through nonlinear advection in boreal summer and fall, TIWs reduce the amplitude of the seasonal cycle in upper ocean temperatures. In addition, TIWs stabilize the upper part of the Equatorial Undercurrent (EUC) through enhanced barotropic energy conversion, leading to a year‐round weakening by −0.15 m s−1 and preventing an unrealistic re‐intensification in boreal fall usually found in non‐eddy resolving models. A coarser simulation at 1‐degree horizontal resolution fails to reproduce the TIW‐induced nonlinear warming of equatorial waters, but succeeds in inhibiting the EUC re‐intensification. This suggests a threshold effect in TIW strength, associated with the model's ability to simulate eddies, which may be responsible for long‐standing biases displayed by global climate models in this region. Plain Language Summary: Tropical instability waves (TIWs) are meandering ocean waves that originate off the coast of Ecuador between June and December and propagate in the eastern equatorial Pacific Ocean. By mixing cold equatorial with warm off‐equatorial waters, TIWs influence the climate of this key region for the global carbon cycle. In this study, we use a regional ocean model to quantify the impact of these waves on the upper‐ocean seasonal cycle. We find that TIWs do not interact in the same way with ocean surface temperatures and currents. TIWs warm surface equatorial waters during the boreal summer and fall, reducing the amplitude of the seasonal temperature cycle. In contrast, TIWs reduce the strength of one of the strongest Pacific equatorial currents—the Equatorial Undercurrent—all year long. Moreover, TIWs prevent an unrealistic re‐intensification of this current in boreal fall often found in models unable to simulate these waves accurately. In particular, we provide evidence that the horizontal resolution of most climate models is not sufficient to simulate accurately TIW impacts on the Pacific Ocean, particularly its temperatures. These findings highlight the necessity to incorporate TIW effects in the future generation of global climate models, via enhanced parameterization or increased horizontal resolution. Key Points: Feedbacks from tropical instability waves (TIWs) on the eastern equatorial Pacific Ocean seasonal cycle are quantifiedTIWs reduce the amplitude of the upper equatorial ocean temperature seasonal cycle by warming equatorial waters in boreal summer and fallTIWs stabilize and weaken the upper Equatorial Undercurrent year‐round and prevent its re‐intensification in boreal fall [ABSTRACT FROM AUTHOR]

Tropical Instability Waves (TIWs) are the dominant source of intraseasonal variability in the central equatorial Atlantic and play an important role in the redistribution of heat in the upper ocean. Here we use multidecadal records of sea surface temperature, sea level anomaly, sea surface salinity, and near‐surface currents constructed from in situ and satellite observations to reveal a long‐term intensification of the intraseasonal variability of these variables due to an increase of TIW activity. Enhanced barotropic energy conversion from increased covariance of horizontal current fluctuations, rather than low‐frequency changes of the mean zonal currents, drives the TIW intensification. As a consequence, boreal summer cooling of tropical North Atlantic surface waters through horizontal eddy temperature advection increased by 0.03°C month−1 decade−1 during 1993–2021, a change of 74% ± 53% relative to the long‐term mean. The presented multidecadal TIW trends are strongly modulated by interannual variations like the 2021 Atlantic Niño. Plain Language Summary: In the equatorial Atlantic, temperature, salinity, sea level anomaly, and ocean velocity variations on time scales of tens of days are dominated by the presence and westward passage of large‐scale Tropical Instability Waves (TIWs). Several decades of satellite and surface drifter data as well as moored velocity observations show a long‐term intensification of TIW activity in all of these variables in the tropical North Atlantic where TIWs are most pronounced. We find that increased high‐frequency flow variability, and not long‐term changes of the mean zonal current system, drives the TIW intensification. One consequence of increased Atlantic Ocean TIW activity is the corresponding intensification of the horizontal eddy temperature advection pattern in boreal summer leading to stronger cooling of surface waters north of the equator. This equates to an increase in TIW‐driven sea surface temperature cooling of 74% ± 53% in the tropical North Atlantic during the last 3 decades. The presented multidecadal TIW trends are strongly modulated by interannual variations such as the 2021 Atlantic Niño. We further explore potential large‐scale drivers of the TIW intensification, including changes in high‐frequency wind variability. Key Points: In situ and satellite observations show a long‐term intensification of Tropical Instability Waves (TIWs) in the tropical North AtlanticEnhanced TIW activity is mainly due to increased barotropic instability associated with increased covariance of velocity fluctuationsAs a result, TIW‐driven sea surface cooling north of the equator due to eddy temperature advection has increased by 74% from 1993 to 2021 [ABSTRACT FROM AUTHOR]

In this paper, the intra-seasonal variability of the abyssal currents in the China Ocean Mineral Resources Association (COMRA) polymetallic nodule contact area, located in the western part of the Clarion and Clipperton Fraction Zone in the tropical East Pacific, is investigated using direct observations from subsurface mooring instruments as well as sea-surface height data and reanalysis products. Mooring observations were conducted from September 13, 2017 to August 15, 2018 in the COMRA contact area (10°N, 154°W). The results were as follows: (1) At depths below 200 m, the kinetic energy of intra-seasonal variability (20–100 d) accounts for more than 40% of the overall low-frequency variability, while the ratio reaches more than 50% below 2 000 m. (2) At depths below 200 m, currents show a synchronous oscillation with a characteristic time scale of 30 d, lasting from October to the following January; the energy of the 30-d oscillation increases with depth until the layer of approximately 4 616 m, and the maximum velocity is approximately 10 cm/s. (3) The 30-d oscillation of deep currents is correlated with the tropical instability waves in the upper ocean. [ABSTRACT FROM AUTHOR]

Based on velocity data from a long‐term moored observatory located at 0°N, 23°W we present evidence of a vertical asymmetry during the intraseasonal maxima of northward and southward upper‐ocean flow in the equatorial Atlantic Ocean. Periods of northward flow are characterized by a meridional velocity maximum close to the surface, while southward phases show a subsurface velocity maximum at about 40 m. We show that the observed asymmetry is caused by the local winds. Southerly wind stress at the equator drives northward flow near the surface and southward flow below that is superimposed on the Tropical Instability Wave (TIW) velocity field. This wind‐driven overturning cell, known as the Equatorial Roll, shows a distinct seasonal cycle linked to the seasonality of the meridional component of the south‐easterly trade winds. The superposition of vertical shear of the Equatorial Roll and TIWs causes asymmetric mixing during northward and southward TIW phases. Plain Language Summary: Tropical Instability Waves (TIWs) are clear in satellite measurements of sea surface temperature as horizontal undulations with wavelength of the order of 1,000 km in equatorial regions of both Atlantic and Pacific Oceans. TIWs are characterized by their distinctive upper‐ocean meridional velocity structure. TIWs amplify vertical shear and thus contribute to the generation of turbulence which in turn leads to the mixing of heat and freshwater downward into the deeper ocean. In this study we show that the prevailing southerly winds in the central equatorial Atlantic drive near‐surface northward and subsurface southward flows, which are superposed on the meridional TIW velocity field. The strength of this wind driven cell is linked to the seasonal cycle of the northward component of the trade winds, peaking in boreal fall when TIWs reach their maximum amplitude. The overturning cell affects the vertical structure of the meridional velocity field and thus has impact on the generation of current shear and turbulence. We show that the overturning reduces/enhances shear during northward/southward TIW flow, an asymmetry that is consistent with independent measurements showing asymmetric mixing. Key Points: Composites of Tropical Instability Waves at 0°N, 23°W show a surface (subsurface) velocity maximum during northward (southward) phasesMeridional wind stress forces a seasonally‐varying, shallow cross‐equatorial overturning cell‐the Equatorial RollThe superposition of Tropical Instability Waves and Equatorial Roll causes asymmetric mixing during north‐ and southward phases [ABSTRACT FROM AUTHOR]

Tropical instability waves (TIWs) in the equatorial eastern Pacific exhibit 25–40‐day westward‐propagating fluctuations with seasonal and interannual variations, which are stronger during July–December and La Niña periods. There is evidence that the instability of the equatorial currents that generates TIWs also induce radiation of barotropic Rossby waves (BTRWTIW). Long‐term near‐bottom current measurements at 10.5°N and 131.3°W during 2004–2013 revealed a spectral peak at 25–40 days, where significant coherences were found with satellite‐measured sea surface height with maxima approximately 5°N. Simulated deep currents from a data‐assimilated ocean model concur with the observed near‐bottom currents, and both currents vary seasonally and interannually, consistent with the typical characteristics of TIW. Further analyses using 25–40‐day bandpass‐filtered barotropic velocity data from the model revealed that they reasonably satisfied the theoretical dispersion relation of BTRWTIW. The in situ and model analysis support the conclusion that BTRWTIW propagate north of 10°N in the northeastern Pacific. Plain Language Summary: Tropical instability waves (TIWs), which are located at the boundary between the warm pool and the cold tongue in the eastern Pacific, propagate westward with 25–40‐day periods and vary seasonally and interannually, which are stronger during July‐December and La Niña periods. Near‐bottom velocity measured over a 10‐year period at 10.5°N, 131.3°W just above the northern boundary of the waves fluctuates with 25–40‐day periods, coinciding with that of sea surface height (SSH) in the equatorial eastern Pacific, especially around 5°N. We find that the wavelike pattern has wave crests oriented southeast‐northwest from the model, and that this pattern appears across the study area and has characteristics consistent with TIWs including seasonal and interannual variations with the typical wavenumber and frequency. This pattern was verified to be a barotropic Rossby wave (BTRW) through a model result analysis. Thus, TIWs induce BTRWs that transfer their energy to the abyssal ocean above 10°N in the northeastern Pacific. This study provides a new‐observational evidence that near‐bottom currents vary with BTRWs induced by TIW. Key Points: In situ near‐bottom current velocity records are coherent with satellite‐measured sea surface height related to tropical instability wavesThe near‐bottom current variations are likely caused by northward propagating tropical instability wave‐induced barotropic Rossby wavesTropical instability wave‐induced barotropic Rossby waves vary interannually with maxima during the La Niña periods [ABSTRACT FROM AUTHOR]

Intraseasonal Kelvin waves (IKWs) and Tropical Instability Waves (TIWs) are essential components of the tropical Pacific coupled climate variability. While downwelling IKWs are precursors of ENSO (e.g., the El Niño Southern Oscillation), TIWs contribute to its asymmetry by mixing more/less warm off-equatorial and cold tongue waters during La Niña/El Niño. Theoretical studies and a few observational case studies also suggest that TIWs and IKWs can interact non-linearly. However, owing to the chaotic nature of TIWs, observational evidence that such a process occurs consistently has not been established thus far. Here, we document for the first time their interaction from satellite observations over a period spanning almost 30 years (1993–2018). From complex empirical orthogonal functions analysis and sea level decomposition into meridional modes, we evidence that a substantial share (∼42%) of the variance of TIWs-induced intraseasonal sea level anomalies are associated with IKWs activity. We show that non-linear dynamical heating (NDH) in the Eastern equatorial Pacific associated with this intraseasonal mode can be as large as that for interannual time scales. Implications for understanding the eastern tropical Pacific heat budget and ENSO variability are discussed.

Motivated by a recent active period of Tropical Instability Waves (TIWs) that followed the extreme 2015/2016 El Niño, we developed a stochastically forced linear model for TIWs with its damping rate modulated by the annual cycle and El Niño Southern Oscillation (ENSO). The model’s analytical and numerical solutions capture relatively well the observed Pacific TIWs amplitude variability dominated by annual and ENSO timescales. In particular, our model reproduces the seasonal increase in TIWs variance during summers and falls and the nonlinear relationship with the ENSO phase characterised by a suppression, respectively increase of TIW activity during El Niño, respectively La Niña. A substantial fraction of TIWs amplitude modulation emerges from the deterministic nonlinear interaction between ENSO and the annual cycle. This simple mathematical formulation allows capturing the nonlinear rectifications of TIWs activity onto the annual cycle and ENSO through, for instance, TIWs-induced ocean heat transport. Moreover, our approach serves as a general theoretical framework to quantify the deterministic variability in the covariance of climate transients owing to the combined modulation of the annual cycle and ENSO.

Motivated by a recent active period of Tropical Instability Waves (TIWs) that followed the extreme 2015/2016 El Niño, we developed a stochastically forced linear model for TIWs with its damping rate modulated by the annual cycle and El Niño Southern Oscillation (ENSO). The model's analytical and numerical solutions capture relatively well the observed Pacific TIWs amplitude variability dominated by annual and ENSO timescales. In particular, our model reproduces the seasonal increase in TIWs variance during summers and falls and the nonlinear relationship with the ENSO phase characterised by a suppression, respectively increase of TIW activity during El Niño, respectively La Niña. A substantial fraction of TIWs amplitude modulation emerges from the deterministic nonlinear interaction between ENSO and the annual cycle. This simple mathematical formulation allows capturing the nonlinear rectifications of TIWs activity onto the annual cycle and ENSO through, for instance, TIWs-induced ocean heat transport. Moreover, our approach serves as a general theoretical framework to quantify the deterministic variability in the covariance of climate transients owing to the combined modulation of the annual cycle and ENSO. [ABSTRACT FROM AUTHOR]

We identify and analyze tropical instability waves (TIWs) in the equatorial Atlantic Ocean during 2010–2018 using satellite derived observations of sea surface salinity (SSS), sea surface temperature (SST), sea level anomaly, and Argo profiles. In particular, the weekly 50‐km resolution SSS time series from the climate change initiative project provides an unprecedented opportunity to observe the salinity structure at a scale closer to the SST scale. We examine the relative contributions of SSS and SST to the horizontal surface density gradient on seasonal and interannual time scales and how they contribute to the TIW properties and energetics. For the central Atlantic TIWs, the maximum of the SST contribution to the density anomaly lags the SSS one by approximately one month. Argo vertical profiles indicate that temperature and salinity both significantly contribute to TIW‐related density anomalies. In May–June, salinity contributes to 50% of the perturbation potential energy in the top 60 m, and between 30% and 45% from July to September. While variations in SST appear to be related to dynamic processes, the interannual variability of SSS is also influenced by precipitations. However, the two leading modes of variability in the region (Atlantic Meridional and Zonal modes) do not well explain at 1°N these interannual variations. Plain Language Summary: Density is a key variable, influenced by both salinity and temperature and essential to understand the equatorial Atlantic dynamics. Each spring and summer, the horizontal density gradient generated by the equatorial upwelling is subject to undulations associated with tropical instability waves (TIWs). These waves are important features that influence both the ocean dynamics and air‐sea interactions. The scarcity of historical salinity observations has been a limiting factor in the study of the relative role of salinity and temperature (and therefore density) in the TIWs dynamics. To do so, we use here an unprecedented 9‐year satellite‐based dataset together with sea surface temperature and in situ Argo floats data. We show that the Atlantic TIWs seasonal and interannual variations are different in salinity and temperature. The TIWs surface salinity seasonal cycle leads the temperature one by one month. Concerning the wave energetics, salinity is responsible for almost half of the potential energy generated by the density gradient. Contributions of temperature and salinity are similar in May and June, while temperature dominates in July to September. Further characterization of the seasonal cycle, interannual variations, and energetics of TIWs will be necessary to better understand their role in climate. Key Points: Satellite and in situ data highlight large seasonal and interannual variability in the respective roles of salinity and temperature on tropical instability waves (TIWs)TIWs maximum surface salinity signal leads the temperature one by one monthIn the top 60‐m of the ocean, salinity and temperature each contribute to about 50% of the TIWs perturbation potential energy [ABSTRACT FROM AUTHOR]

Deep cycle turbulence (DCT) is a diurnally oscillating turbulence that penetrates into a stratified shear layer below the surface mixed layer, which is often observed in the eastern Pacific and Atlantic above the Equatorial Undercurrent (EUC). Here we present the simulation of DCT by a global ocean general circulation model (OGCM) for the first time. As the k‐ε vertical mixing scheme is used in the OGCM, the simulation of observed DCT structure based on in situ microstructure measurements can be explicitly demonstrated. The simulated DCT is found in all equatorial ocean basins, and its characteristics agree very well with observations. Zonal and meridional variations of DCT in the entire equatorial Pacific and Atlantic are described through constructing the composite diurnal cycle. In the central Pacific where the maximum shear associated with EUC is deep, the separation of DCT from the surface mixed layer is much more prominent than other areas. Plain Language Summary: Deep cycle turbulence (DCT) is a nighttime intensified turbulence that develops in the stratified layer below the base of the surface mixed layer. It is often observed below the equatorial Pacific and Atlantic cold tongue regions above the Equatorial Undercurrent (EUC). Mixing caused by DCT is essential in modulating sea surface temperature (SST), which could have a large impact on air‐sea interaction and thus global climate variability. However, simulations of DCT in global ocean models have not been demonstrated so far, and the spatial variation of DCT characteristics in the entire equatorial oceans is not well known. This study presents the first global ocean general circulation model simulation of DCT, demonstrated by the comparison of simulated turbulence with that derived from in situ observations. The simulated DCT is found in all equatorial ocean basins, and its characteristics agree very well with observations. Large‐scale spatial variability of DCT in the equatorial Pacific and Atlantic is described through the analysis of model output. The DCT layer completely separated from surface mixed layer is found at locations where the EUC is deep, such as the central equatorial Pacific near the dateline. Key Points: Deep cycle turbulence is well simulated for the first time by a global ocean general circulation model in all equatorial ocean basinsLarge‐scale zonal and meridional variations of the deep cycle turbulence in equatorial Pacific and Atlantic are fully describedSeparation of deep cycle turbulence from the surface mixed layer is most prominent in the central Pacific near the dateline [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: Tropical instability waves (TIWs) are westward propagating high‐frequency waves having a main period about 5 weeks. Their activity is strongly modulated by the cold tongue annual cycle and El Niño–Southern Oscillation (ENSO). At the same time, TIW activity as a whole systematically transports heat meridionally from warm to cold regions, and thus when they are modulated by ENSO, they can have a nonlinear rectification effect on ENSO in return. We find that the TIW‐induced rectification effect on ENSO can be related to the amplitude of a simple index that captures the main propagative wavy feature of TIW. This feedback effect prevents the growth of La Niña (El Niño) events by promoting a warming (cooling) through meridional convergence of TIW heat transport. Finally, we introduce a theoretical formulation for TIW‐induced effect by adopting a simple linear stochastic model for TIW focused on the complex index for TIW. This validated formulation shall be useful, for instance, for evaluating and understanding the climate model's ability in simulating the TIW‐ENSO multiscale interaction. Key Points: Nonlinear dynamical heating (NDH) due to tropical instability waves (TIWs) is largely proportional to the amplitude of a simple TIW indexTIW feedback onto El Niño–Southern Oscillation through TIW‐induced NDH is nonlinear and strongly seasonal dependentA theoretical derived simple expression for this feedback is in agreement with the deduced result from the reanalysis data [ABSTRACT FROM AUTHOR]

The causes of the seasonal cycle of vertical turbulent cooling at the base of the mixed layer are assessed using observations from moored buoys in the tropical Atlantic Intertropical Convergence Zone (ITCZ) (4°N, 23°W) and trade wind (15°N, 38°W) regions together with mixing parameterizations and a one‐dimensional model. At 4°N the parameterized turbulent cooling rates during 2017–2018 and 2019 agree with indirect estimates from the climatological mooring heat budget residual: both show mean cooling of 25–30 W m −2 during November–July, when winds are weakest and the mixed layer is thinnest, and 0–10 W m −2 during August–October. Mixing during November–July is driven by variability on multiple time scales, including subdiurnal, near‐inertial, and intraseasonal. Shear associated with tropical instability waves (TIWs) is found to generate mixing and monthly mean cooling of 15–30 W m −2 during May–July in 2017 and 2019. At 15°N the seasonal cycle of turbulent cooling is out of phase compared to 4°N, with largest cooling of up to 60 W m −2 during boreal fall. However, the relationships between wind speed, mixed layer depth, and turbulent mixing are similar: weaker mean winds and a thinner mixed layer in the fall are associated with stronger mixing and turbulent cooling of SST. These results emphasize the importance of seasonal modulations of mixed layer depth at both locations and shear from TIWs at 4°N. Plain Language Summary: In the tropics, vertical turbulent mixing transports heat from the surface waters to the deeper ocean, regulating sea surface temperature (SST). Turbulent mixing has been shown to cause pronounced cooling of SST in the eastern equatorial Pacific and Atlantic Oceans. This cooling is driven to a large extent by strong vertical shear of the zonal equatorial currents, which give rise to instability and mixing. In this study the seasonal cycles of turbulent cooling are assessed at two off‐equatorial locations in the tropical North Atlantic. Despite the different conditions at these two locations, it is found that turbulent cooling at both is strongest during the time of year when the mean winds are weakest and the mixed layer is thinnest. This is surprising, since it might be expected that stronger mean winds and associated deepening of the mixed layer would directly force much of the mixing and turbulent cooling at off‐equatorial locations. Instead, the shallow mean mixed layer, which is more responsive to surface forcing, sets the stage for stronger bursts of cooling. In addition, intraseasonal variations in shear and mixing from tropical instability waves are important at 4°N. These results will be useful for evaluating the mixing parameterizations used in numerical models. Key Points: The seasonal cycle of vertical turbulent cooling was assessed at 4°N, 23°W and 15°N, 38°WTurbulent cooling of sea surface temperature is strongest during the seasons when mean winds are weakest and the mixed layer is thinnestTropical instability waves contributed 15–30 W m −2 of monthly mean cooling at 4°N, 23°W during May–July 2017 and May–July 2019 [ABSTRACT FROM AUTHOR]

This study introduces a new flow‐dependent distribution sampling (FDDS) scheme for air–sea coupling. The FDDS scheme is implemented in a climate model and used to improve the simulated mean and variability of atmospheric and oceanic surface fields and thus air–sea fluxes. Most coupled circulation models use higher resolutions in the sea ice and ocean compared to the atmospheric model component, thereby explicitly simulating the atmospheric subgrid‐scale at the interface. However, the commonly applied averaging of surface fields and air–sea fluxes tends to smooth fine‐scale structures, such as oceanic fronts. The stochastic FDDS scheme samples the resolved spatial ocean (and sea ice) subgrid distribution that is usually not visible to a coarser‐resolution atmospheric model. Randomly drawn nodal ocean values are passed to the corresponding atmospheric boxes for the calculation of surface fluxes, aiming to enhance surface flux variability. The resulting surface field perturbations of the FDDS scheme are based on resolved dynamics, displaying pronounced seasonality with realistic magnitude. The AWI Climate Model is used to test the scheme on interannual time‐scales. Our set‐up features a high ocean‐to‐atmosphere resolution ratio in the Tropics, with grid‐point ratios of about 60:1. Compared to the default deterministic averaging, changes are largest in the Tropics leading to an improved spatial distribution of precipitation with bias reductions of up to 50%. Enhanced sea‐surface temperature variability in boreal winter further improves the seasonal phase locking of temperature anomalies associated with the El Niño–Southern Oscillation. Mean 2m temperature, sea ice thickness and concentration react with a contrasting dipole pattern between hemispheres but a joint increase of monthly and interannual variability. This first approach to implement a flow‐dependent stochastic coupling scheme shows considerable benefits for simulations of global climate, and various extensions and modifications of the scheme are possible. [ABSTRACT FROM AUTHOR]

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 ~20% of the variance at 5‐ to 30‐day periods, with another ~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 ~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. Plain Language Summary: The eastern equatorial Pacific is an area of intense ocean heat uptake due to the strong equatorial Sun. The fate of this heat depends on turbulent mixing processes in the upper ocean: heat may be returned quickly to the atmosphere or be carried into the deeper ocean and stored for many years, with obvious consequences for climate. The same is true of other important water properties such as CO2 content. Equatorial turbulence originates largely with vertically sheared currents. One important mechanism for this is tropical instability waves (TIWs), 1,000‐km waves that generate and amplify vertical shear (i.e., vertical differences in the horizontal current), which in turn generates the turbulence that mixes heat downward. Previous studies have shown that TIWs concentrated at the ocean surface carry the most energy. Here, we show that TIWs focused below the surface, while less energetic, can carry a disproportionate fraction of the shear, and it is of course that shear that drives turbulence and the resulting fluxes of heat, CO2 and other properties. We describe the times of the year when these subsurface TIWs are strongest, the periods at which they oscillate, and the depths at which they generate enhanced shear. Key Points: Subsurface tropical instability waves, with zonal velocity oscillation peaking at 70–90 m, are identified in the eastern equatorial PacificThe waves have periods of 5–20 days and amplitudes of 0.1–0.2 m/s and can persist for 3–7 months from July to the following FebruaryThe waves can induce periodically enhanced and reduced shear and hence mixing at ~50 m and above the core of the Equatorial Undercurrent [ABSTRACT FROM AUTHOR]

A close relationship between tropical instability wave (TIW)‐induced sea surface temperature (SST) and wind stress (τ) perturbations (SSTTIW and τTIW, respectively) has been identified from satellite observations. In this study, we applied an empirical TIW‐induced wind stress model (τx, τy)TIW=F(SST) to assess τTIW feedback. This empirical model solves (τx, τy)TIW from TIW‐induced wind stress divergence and curl estimated from time‐evolving downwind and crosswind SST gradients. The empirical model is incorporated into a high‐resolution ocean model based on the Regional Ocean Modeling System, and the derived (τx, τy)TIW can be added onto the climatological component to force the ocean model, by which the effect of τTIW can be taken into account adequately. From a comparison of two experiments with and without the τTIW effect, the τTIW had a substantial effect on the equatorial Pacific heat budget, which affected the long‐term mean SST, representing an approximate 0.2°C difference. An SST budget analysis suggested that the τTIW influenced SST primarily via thermal advection and vertical mixing, the effects of which were both amplified with additional τTIW feedback and almost counteracted each other. Analysis of two additional sensitivity experiments suggested that both TIW‐induced wind stress divergence and curl components could induce comparable SST differences, with a little difference on that the latter tended to induce more salient SST cooling at the equator. TIW‐induced wind stress feedback was assessed using a high‐resolution ocean circulation model. Thermal advection and vertical mixing were the two terms primarily affected by τTIW, with nearly counteracting effects. Both TIW‐induced wind stress divergence and curl components affected the ocean, showing comparable effects. [ABSTRACT FROM AUTHOR]

Equatorial Internal Wave Experiment observations at 0°, 140°W from October 2008 to February 2009 captured modulations of shear, stratification, and turbulence above the Equatorial Undercurrent by a series of tropical instability waves (TIWs). Analyzing these observations in terms of a four‐phase TIW cycle, we found that shear and stratification within the deep‐cycle layer being weakest in the middle of the N‐S phase (transition from northward to southward flow) and strongest in the late S phase (southward flow) and the early S‐N phase (transition from southward to northward flow). Turbulence was modulated but showed less dependence on the TIW cycle. The vertical diffusivity (KT) was largest during the N (northward flow) and N‐S phases, when stratification was weak, despite weak shear, and was smallest from the late S phase to the S‐N phase, when stratification was strong, despite strong shear. This tendency was less clear in turbulent heat flux because vertical temperature gradients were small at times when KT was large, and large when KT was small. We investigated the dynamics of shear and stratification variations within the TIW cycle by using an ocean general circulation model forced with observed winds. The model successfully reproduced the observed strong shear and stratification in the S phase, except for a small phase difference. The strong shear is explained by vortex stretching by TIWs. The strong stratification is explained by meridional and vertical advection. Plain Language Summary: Moored observations in the central equatorial Pacific during 2008 La Niña captured a series of tropical instability waves (TIWs). Vertical shear of horizontal velocity, stratification, and turbulent mixing above the Equatorial Undercurrent varied strongly in different phases of each TIW cycle. The dynamics of modulations of stratification and shear by the TIW cycle were identified using results from an oceanic model. Key Points: Moored observations capture variations of shear, stratification, and turbulence during equatorial Pacific tropical instability waves (TIWs)Shear and stratification within the deep‐cycle layer were modulated by the phase of each TIW, whereas turbulence exhibited less modulationAn ocean model forced with observed winds gave dynamical insight on the dependence of shear and stratification on the phase of the TIW cycle [ABSTRACT FROM AUTHOR]

Tropical instability waves (TIWs) are a major source of internally-generated oceanic variability in the equatorial Pacific Ocean. These non-linear phenomena play an important role in the sea surface temperature (SST) budget in a region critical for low-frequency modes of variability such as the El Niño-Southern Oscillation (ENSO). However, the direct contribution of TIW-driven stochastic variability to ENSO has received little attention. Here, we investigate the influence of TIWs on ENSO using a 1/4∘ ocean model coupled to a simple atmosphere. The use of a simple atmosphere removes complex intrinsic atmospheric variability while allowing the dominant mode of air−sea coupling to be represented as a statistical relationship between SST and wind stress anomalies. Using this hybrid coupled model, we perform a suite of coupled ensemble forecast experiments initiated with wind bursts in the western Pacific, where individual ensemble members differ only due to internal oceanic variability. We find that TIWs can induce a spread in the forecast amplitude of the Niño 3 SST anomaly 6-months after a given sequence of WWBs of approximately ±45% the size of the ensemble mean anomaly. Further, when various estimates of stochastic atmospheric forcing are added, oceanic internal variability is found to contribute between about 20% and 70% of the ensemble forecast spread, with the remainder attributable to the atmospheric variability. While the oceanic contribution to ENSO stochastic forcing requires further quantification beyond the idealized approach used here, our results nevertheless suggest that TIWs may impact ENSO irregularity and predictability. This has implications for ENSO representation in low-resolution coupled models. [ABSTRACT FROM AUTHOR]

Satellite observations revealed that there is a close relationship between perturbations of sea surface temperature (SST) and wind stress (τ) induced by tropical instability waves (TIWs; SSTTIW and τ TIW). Using the empirical relationship observed between TIW-induced wind stress divergence (curl) and downwind (crosswind) SST gradients, this study establishes a TIW-induced wind stress field perturbation model τ TIW = F(SST). This empirical model solves τ TIW from the TIW-induced wind stress divergence and curl, which are estimated from the downwind and crosswind SST gradients. This empirical τ TIW = F(SST) model can be incorporated into the ocean model to take into account the effect of τ TIW. By comparing two experiments with and without the τ TIW effect, this study demonstrates that τ TIW has a substantial effect on the equatorial Pacific heat budget and induces the long-term mean SST to exhibit a 0.2°C difference, which is consistent with previous studies. [ABSTRACT FROM AUTHOR]

The characteristics and formation conditions of eddy meridional heat transport (MHT) in the eastern equatorial Pacific are studied using the results of eddy-resolving numerical modeling as compared with observational and reanalysis data. Calculations of the eddy MHT convergence at the equator are performed and realistic results in the analyzed region except for its easternmost part are obtained. The interannual variability and velocity of propagation of tropical instability waves are estimated. The errors of the time-averaged model solution are analyzed, and the assumptions on the mechanisms of their occurrence are made. [ABSTRACT FROM AUTHOR]

Abstract: 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 (ChlTIW) 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 ChlTIW are diagnosed. ChlTIW acts to modulate the incoming solar radiation that penetrates the bottom of the mixed layer (Qpen). In addition, two experiments are conducted with the impacts of ChlTIW on the ocean included or not. The results show that the ChlTIW effects tend to substantially increase seasonal variability of eddy kinetic energy (EKE) and slightly decrease SST in the eastern equatorial Pacific. At large scale, ChlTIW 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, ChlTIW leads to the change in Qpen and a decrease in baroclinic conversion term and eventually decreases the EKE in the boreal spring and summer. So the ChlTIW 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. [ABSTRACT FROM AUTHOR]

Abstract: Recent evidence from mooring data in the equatorial Atlantic reveals that semiannual and longer time scale ocean current variability is close to being resonant with equatorial basin modes. Here we show that intraseasonal variability, with time scales of tens of days, provides the energy to maintain these resonant basin modes against dissipation. The mechanism is analogous to that by which storm systems in the atmosphere act to maintain the atmospheric jet stream. We demonstrate the mechanism using an idealized model setup that exhibits equatorial deep jets. The results are supported by direct analysis of available mooring data from the equatorial Atlantic Ocean covering a depth range of several thousand meters. The analysis of the mooring data suggests that the same mechanism also helps maintain the seasonal variability. [ABSTRACT FROM AUTHOR]

It is generally believed that both barotropic and baroclinic instabilities may account for the tropical instability wave (TIW) variabilities in the equatorial Pacific Ocean. However, the coupling mechanisms between the two different meridional TIW modes, i.e., the Yanai-mode at the equator and the Rossby-mode around 5°N, and the associated vertical connections, are yet to be clarified. This is done here using a recently developed multiscale decomposition tool, multiscale window transform (MWT), and the MWT-based theory of canonical transfer and TIW-scale eddy kinetic energy (EKE) budget analysis, based on the (1 / 12. 5) ∘ HYCOM reanalysis data. In the subsurface layer (30–200 m), barotropic instability is the primary energy source for the Yanai-mode TIWs, while baroclinic instability dominates the EKE generation of the Rossby-mode TIWs. In contrast, the two TIW modes in the surface layer (0–30 m) are largely modulated by nonlocal mechanisms through pressure work, which functions to transport EKE upward from the subsurface to the surface layer, and southward from the Rossby-mode region to the Yanai-mode region. The pressure fluxes substantially couple the two TIW modes in both the meridional and vertical directions, leading to significant coherence between EKE in different vertical layers and regions of the TIWs. In addition, the prevailing vertical pressure flux identified north of the equator explains why the EKE signal is more vertically coherent in the Rossby-mode region than the Yanai-mode region. This nonlocal energy pathway, as well as the well-known instability processes, undergoes strong seasonal variations that determine the seasonal cycles of the TIWs in the equatorial Pacific Ocean. • Barotropic (baroclinic) instability is the dominant energy source for the subsurface Yanai-mode (Rossby-mode) TIWs. • The surface TIW variability is largely driven by upward energy flux through pressure work from the subsurface layer. • The surface Yanai-mode TIWs are coupled with the Rossby-mode TIWs via southward energy flux to the equator. [ABSTRACT FROM AUTHOR]

Tropical instability waves (TIWs) are the dominant mesoscale variability in the eastern equatorial Pacific Ocean. TIWs have direct impacts on the local hydrology, biochemistry and atmospheric boundary layer, and feedback on ocean circulations and climate variability. In this study, the basic characteristics of Pacific Ocean TIWs simulated by an eddy-resolving ocean model and a coupled general circulation model are evaluated. The simulated TIW biases mainly result from the mean climatology state, as TIWs extract eddy energy from the mean potential and kinetic energy. Both the oceanic and coupled models reproduce the observed westward propagating large-scale Rossby waves between approximately 2°–8°N, but the simulated TIWs have shorter wavelengths than the observed waves due to the shallower thermocline. Meanwhile, the weak meridional shears of background zonal currents and the less-tilted pycnocline in these two models compared to the observations causes weak barotropic and baroclinic instability, which decreases the intensity of the simulated TIWs. We then contrast the TIWs from these two models and identify the roles of atmospheric feedback in modulating TIWs. The latent heat flux feedback is similar to observation in the coupled model but absent in the ocean model, contributing to the stronger standard deviation (STD) of the TIW SST in the ocean model. The ocean model is not able to capture realistic air–sea interaction processes when forced with prescribed atmospheric forcing. However, the misrepresented atmospheric feedback in the ocean model tends to decrease the sea surface height (SSH) variability, and the current feedback damping effect is stronger in the ocean model than in the coupled model. Combined with weaker barotropic conversion rate and baroclinic conversion rate in the ocean model than in the coupled model, the STD of the TIW SSH in the ocean model is weaker. • Mesoscale feedbacks of simulated tropical instability waves (TIWs) are evaluated. • Weak zonal current shear and shallow thermocline can damp TIWs and shorten wavelength. • Thermal feedback on SST is reproduced in coupled model but absent in oceanic model. • Wind stress curl response to SSH is stronger in oceanic model than observation. [ABSTRACT FROM AUTHOR]

The Marquesas form an isolated group of small islands in the Central South Pacific where quasi-permanent biological activity is observed. During La Niña events, this biological activity, shown by a net increase of chlorophyll-a concentration (Chl, a proxy of phytoplankton biomass), is particularly strong. It has been hypothesized that this strong activity is due to iron-rich waters advected from the equatorial region to the Marquesas by tropical instability waves (TIWs). Here we investigate this hypothesis over 18 years by combining satellite observations, re-analyses of ocean data, and Lagrangian diagnostics. Four La Niña events ranging from moderate to strong intensity occurred during this period, and our results show that the Chl plume within the archipelago can be indeed influenced by such equatorial advection, but this was observed during the strong 1998 and 2010 La Niña conditions only. Chl spatio-temporal patterns during the occurrence of other TIWs rather suggest the interaction of large-scale forcing events such as an uplift of the thermocline or the enhancement of coastal upwelling induced by the tropical strengthening of the trades with the islands leading to enhancement of phytoplankton biomass within the surface waters. Overall, whatever the conditions, our analyses suggest that the influence of the TIWs is to disperse, stir, and, therefore, modulate the shape of the existing phytoplankton plume.

Highlights: [•] Climate models often have a cold tongue bias in the tropical Pacific Ocean. [•] Poor simulation of tropical instability waves may contribute to this. [•] Heat budget calculation carried out in 1° and 1/4° models. [•] Meridional heat advection by tropical instability waves is increased with higher resolution. [•] Improved SST simulation during neutral and La Niña conditions. [ABSTRACT FROM AUTHOR]

A new version of the Community Atmospheric Model, CAM5.1, is used to quantify the impacts of tropical instability waves ( TIWs)-induced sea surface temperature ( SSTTIW) forcing on the atmosphere over the central-eastern tropical Pacific. Two CAM5.1-based simulations are performed, a control run in which monthly mean climatological SST ( SSTclim) field is prescribed as a surface boundary condition, and a perturbation run in which daily varying SSTTIW field explicitly extracted from satellite data is superimposed onto the SSTclim field to force the CAM5.1. It is illustrated that the additionally imposed SSTTIW forcing acts to have a significant effect on TIW-scale variability and mean atmospheric circulation structure over the central-eastern tropical Pacific. [ABSTRACT FROM AUTHOR]

A set of atmospheric general circulation model experiments were performed where the Community Atmospheric Model version 3.1 is forced with sea-surface temperatures (SSTs) that have the imprint of tropical instability waves (TIWs) of varying strengths. The presence of TIWs in the SSTs increased the variance in the large-scale circulation in the atmospheric boundary layer (ABL). The impact of TIWs in the SST anomalies is directly seen in enhanced mixing of the air temperature anomalies, thereby diffusing the air temperature gradients setup by the oceanic fronts throughout the ABL. The presence of TIWs transformed the diffusive-natured ABL to advection-dominated regime. This implies that within the ABL, the presence of TIWs leads to the enhanced interactions among the neighboring grid cells and prompts greater horizontal communications among atmospheric variables. However, the advection in the ABL due to the TIWs is not a linear function of increasing TIW strength. Unlike air temperature, zonal, and vertical velocity, the variance in the meridional velocity changes at the top of the ABL due to the momentum mixing across the ABL in the vertical direction. This causes the ABL to be more turbulent beyond seasonal time scales. This analysis also suggests that a simple parameterization is not sufficient to take the account of rectification effects on the atmospheric variables that are missing due to the lack of TIW representation in the coarse-resolution coupled general circulation models. [ABSTRACT FROM AUTHOR]