Tropical Instability Waves Research Articles

Tropical Instability Wave Variability in the Pacific and Its Relation to Large-Scale Currents

Abstract Shipboard acoustic Doppler current profiler (ADCP)-derived zonal currents from 170° to 110°W are assembled into composite seasonal and ENSO cycles to produce detailed representations of large-scale ocean flow regimes that favor tropical instability waves (TIWs). The instability-favorable portion of these cycles, namely, the August– October period of the seasonal cycle and the pre-December period of the ENSO cold phase, both have intense westward flow in the South Equatorial Current, most particularly the branch north of the equator (SECN), and strengthened eastward flows in the North Equatorial Countercurrent (NECC) and the Equatorial Undercurrent (EUC). Taken together these flows enhance current shear in the two regions generally associated with TIW activity, namely, the cyclonic and anticyclonic shear regions located to the south and north of the SECN, respectively. Direct correlation of ADCP currents and CTD densities to an instability index derived from equatorial 13–30-day meridional velocit...

Influence of ocean‐atmosphere coupling on the properties of tropical instability waves

In this study we investigate how the modulation of surface wind‐stress by tropical instability waves (TIWs) feeds back onto TIWs and plays a role in their fundamental properties. An ocean general circulation model is used, that reproduces qualitatively well the properties of TIWs when forced by climatological winds, although with a 30% underestimated amplitude. The ocean model is coupled to the atmosphere through a simple parameterization of the wind stress response to SST. The properties of the TIWs in the coupled simulations are compared with those without active coupling. Active coupling results in a negative feedback on TIWs, slightly reducing their temperature and meridional current variability, both at the surface and sub‐surface. This reduced activity modulates the meridional heat and momentum transport, resulting in modest changes to the mean state, with a cooler cold tongue and stronger equatorial currents.

The influence of the Galápagos Islands on tropical temperatures, currents and the generation of tropical instability waves

The Galápagos Islands provide a topographic barrier for the Southern Equatorial Current (SEC) and the Equatorial Undercurrent (EUC). An island wake effect can be diagnosed from the difference of an ocean general circulation model simulation which includes the Galápagos Islands and one which ignores their presence. Cold thermocline water upwells on the western side of the islands, and only during boreal winter season these cold waters can linger around the Islands at a depth of about 80 m and affect the far eastern equatorial Pacific surface waters. This effect is partly offset by the westward transport of cold surface waters by the SEC which creates a wake on the western side of the Islands. It is furthermore shown that changes in horizontal current shear, induced by the presence of the Galápagos Islands modify the generation of tropical instability waves and lead to a basin scale SST anomaly pattern.

Physical oceanographic conditions during GasEx‐2001

GasEx‐2001 is a study of air‐sea gas exchange in a region of CO2 outgassing. The bulk of the experiment followed a drifting array of near‐surface instruments deployed during the second half of February 2001 just south of the equator in the central Pacific Ocean. Physical oceanographic conditions including local currents, the seasonal cycle, Kelvin waves, and tropical instability waves are described using shipboard data and a variety of other data sources to set the large‐scale oceanographic context for GasEx‐2001. Local physical oceanographic conditions during GasEx‐2001 are then analyzed using shipboard data and a simple one‐dimensional mixed layer model. The thermocline shoals about 13 m over the 15‐day experiment, implying an upwelling rate of 1 × 10−5 m s−1. Zonal velocity is surface‐intensified and westward, with vertical shear mostly through the thermocline. Meridional velocity is also strongly sheared with a maximum equatorward flow in the thermocline that is much reduced by 17‐m depth. The mixed layer model exhibits more near‐surface warming over the course of the experiment than is observed. Prescribing upwelling in the model closes the heat budget within error estimates. Entrainment at the base of the mixed layer plays a limited role in the mixed layer budgets of carbon and other water properties. Vertical shear of horizontal velocity within the mixed layer and slippage of the array through the surface water also have small (but uncertain) roles in these budgets.

A New Theory for the Generation of the Equatorial Subsurface Countercurrents

Abstract An idealized numerical simulation of the tropical Atlantic Ocean is used to study the dynamics of an Atlantic subsurface countercurrent, the South Equatorial Undercurrent (SEUC). The particular structure of the SEUC between 28° and 10°W allows for a reformulation of the transformed Eulerian mean (TEM) equations with which the momentum balance of the SEUC can be explored. With this modified TEM framework, it is shown that between 28° and 10°W the SEUC is maintained against dissipation by the convergence of the Eliassen–Palm flux. The source of this Eliassen–Palm flux is the tropical instability waves that are generated along the shear between the Equatorial Undercurrent and the South Equatorial Current.

Ocean bottom pressure waves predicted in the tropical Pacific

Using a mass‐conserving (non‐Boussinesq) Pacific Ocean model and TOPEX/POSEIDON sea‐surface height measurements, we document previously unreported bottom pressure waves (BPWs), associated with large‐amplitude eddies (vortices) both north and south of the Equator. The bottom pressure signals, defined as the vertically integrated water‐mass anomalies, are found to propagate westward across the Pacific Ocean with an approximate 4‐mbar amplitude (1 mbar = 100 Newton/m2, approximately equivalent to 1 cm of sea surface height), continuously interacting with topographic features. Their wavelength is about 1100 kilometers, phase speed is about 0.5 m/s, and period is about 30‐days, all consistent with the known properties of tropical instability waves (TIWs). These results suggest that the constantly evolving tropical instability eddies might have strong bottom pressure signals, therefore, likely alias the U.S.‐German Gravity Recovery and Climate Experiment (GRACE) measurements, which have a monthly‐sampling period.

Biomass, feeding and metabolism of mesozooplankton in the equatorial Pacific along 180°

Abstract Mesozooplankton biomasses, grazing rates, and metabolism (respiration, ammonium and phosphate excretion) were measured in the central Pacific (180°; 8°S–8°N) during the EBENE cruise, in October–November 1996. In the HNLC zone, the highest mesozooplankton biomass occurred between 6°S and the equator. The decrease of biomass at the northern part of the HNLC zone was accounted to the passage of a tropical instability wave, carrying poorer zooplankton concentrations from the north. Within the HNLC zone, most of the biomass (60–96%) was found above 100 m in accordance with the upper distribution of phytoplankton and heterotrophic protists. No significant day–night differences were observed in mesozooplankton biomass. These results suggest a strong and permanent coupling between mesozooplankton and the first links of the food chain. Respiration and phosphate excretion rates were steady at the different latitudes, but at the equator, grazing rates were the lowest, and ammonium excretion the highest. These variations were accounted to latitudinal differences in microzooplankton density and in particles C/N ratios, suggesting that more nitrogen was available for food at the equator. Moreover, among the dominant copepod species, higher proportions of carnivorous were found at the equator. Higher particle grazing and ammonium excretion rates occurred at the equator during the day, while the chlorophyll gut content of copepods did not show any significant diel cycle. These results suggest a strong trophic link between mesozooplankton feeding activity, on the one hand, and cyclic variations of phytoplankton production and of of their main predators (microzooplankton protists), on the other hand. From biomass estimations of phytoplankton and protists in the euphotic layer and results of dilution experiments, the production and grazing mortality of the main autotrophic taxa (from picoplankton to microplankton) were computed at the equator. Mesozooplankton, which was mainly involved in the catch of the largest (>8 μm) food particles, such as diatoms or dinoflagellates, accounted for only a minor part (22%) in total phytoplankton grazing, compared to microzooplankton. As the chlorophyll ration was insufficient to balance its metabolism expenditures, mesozooplankton had to to complete its diet with other sources of food such as microzooplankton. The low level of zooplankton metabolic O:N ratios, suggesting the use of a food with a large protein content, supports of this hypothesis.

Ocean–Atmosphere Covariability in the Western Arabian Sea*

The western Arabian Sea exhibits strong spatial variability in sea surface temperature (SST) during the southwest monsoon, with changes in SST that can exceed 5°C over 200 km. Exploration of satellite-based and in situ data shows a strong connection between mesoscale SST features and changes in the atmospheric boundary layer. The fundamental relationship is that of weak (strong) wind velocities overlying cold (warm) SST features. There are also coherent changes in other near-surface meteorological parameters, such as the air–sea temperature difference and relative humidity—indicating changes in the stability of the planetary boundary layer over the mesoscale SST features. These relationships are similar to those recently reported over the equatorial Pacific tropical instability wave region. This observed covariability of atmospheric boundary layer structure and SST results in variations of the surface heat and moisture fluxes; latent heat flux is modified by changes in relative humidity (principally through the temperature dependence of saturation specific humidity), wind speed, and boundary layer stability over the cold filaments. The nonlinear dependence of latent heat flux on the three parameters leads to a net enhancement of latent heat flux from the mesoscale features, as compared to that computed using spatially averaged parameters. Additionally, the spatial structure of the heat-flux variability will tend to dampen the mesoscale SST features. The mesoscale wind variability results in strong wind stress curl patterns on the same spatial scales as the oceanic features. The resulting Ekman pumping variations may play an important role in the evolution of the ocean eddy fields in this region. Further examination of the processes controlling the observed covariability, and the oceanic and atmospheric response to the coupling should therefore be undertaken.

Tropical instability waves in the Atlantic Ocean

An idealized numerical model of the tropical Atlantic Ocean is used to study the structure, energetics and heat flux of the Atlantic tropical instability waves (TIWs). The model results compare well with the observations, and they both show that, unlike commonly assumed, the TIWs in the Atlantic exist on both sides of the equator and are generated not only in the summer but from May to January. Furthermore it is demonstrated that the Atlantic TIWs are generated by barotropic instability of the shear between the equatorial undercurrent and the northern south equatorial current and make a surprisingly small contribution to the heat budget of the equatorial mixed layer. The model results reveal that the often published strong meridional heat flux divergence of the TIWs is largely compensated for by their vertical heat flux divergence.

Effects of lateral mixing on the mean state and eddy activity of an equatorial ocean

Numerical experiments are performed using an ocean general circulation model (OGCM) to assess the impact of the form of lateral mixing of momentum and tracers on the state of an equatorial ocean. It is found that the large‐scale structure of both the velocity and temperature fields are very sensitive to the imposed parameterization of lateral mixing in the model. With uniform values for the viscosity and diffusion coefficient across the domain, a decrease in these coefficients increases the activity of tropical instability waves (TIW), resulting in an increase in the sea surface temperature of the cold tongue, and thus overcoming the cold bias often found in ocean models of the equatorial Pacific. However, there is an associated increase in the strength of the Equatorial Under‐Current (EUC) to unrealistic levels. It is found that the strength of the EUC can be limited, while little affecting the TIW activity, by applying an enhanced level of mixing in the vicinity of the equator. This enhanced mixing is used to model the effect of the observed interleaving of water masses across the equator. Both the level and distribution of eddy kinetic energy (associated with TIWs) varies with the level and orientation of the lateral mixing. An examination of the main energy conversion terms of mean to eddy energy reveals that both the barotropic and baroclinic energy conversion terms are important for the production of TIWs. The relative role of each of these conversion terms changes as the lateral mixing is decreased. At low levels of lateral mixing, barotropic instability is found to dominate the production of total eddy energy. Including enhanced mixing at the equator decreases this barotropic production, but surprisingly increases the baroclinic production away from the equator.

Microbial community abundance and biomass along a 180° transect in the equatorial Pacific during an El Niño‐Southern Oscillation cold phase

As part of the French Joint Global Ocean Flux Study Etude du Broutage en Zone Equatoriale program, we investigated the distributions of microorganisms (bacteria and protists <200 μm) in the upper 120 m of the equatorial Pacific from 8°S to 8°N, along 180°. Population distributions, determined by a combination of flow cytometry, microscopy and spectrofluorometry, were closely related to physical features across the study site. Phytoplankton biomass, ranging from 1.2 to 34.2 μg C L−1 and averaging 15.5 μg C L−1, was most enhanced in the divergence zone. Carbon to chlorophyll ratios were also enhanced in the divergence zone and showed distinct latitudinal variations. Heterotrophic biomass, excluding ciliates, was patchy across the area, ranging from 5 to 36 μg C L−1 and averaging 13 μg C L−1. Prokaryotic species (Prochlorococcus spp., Synechococcus spp., and heterotrophic bacteria) showed similar patterns of abundance, with the main feature being their distributional asymmetry to the south of the equator. Both autotrophic and heterotrophic biomass were enriched in the convergent zone at 4°–5°N between the South Equatorial Current and the North Equatorial Counter Current. Heterotrophic biomass exceeded phytoplankton biomass in the more nutrient‐impoverished waters to the north and in the branch of a tropical instability wave eddy. Microplankton represented only a small portion of the total autotrophic carbon and was comprised mostly of dinoflagellates. Large species dominated the relatively modest diatom biomass. Food web interactions and biogeochemical fluxes in the central equatorial Pacific may be significantly influenced by temporal and spatial variability of the microbial community associated with physical features of the region.

Numerical Simulation of Atmospheric Response to Pacific Tropical Instability Waves*

Tropical instability waves (TIWs) are 1000-km-long waves that appear along the sea surface temperature (SST) front of the equatorial cold tongue in the eastern Pacific. The study investigates the atmospheric planetary boundary layer (PBL) response to TIW-induced SST variations using a high-resolution regional climate model. An investigation is made of the importance of pressure gradients induced by changes in air temperature and moisture, and vertical mixing, which is parameterized in the model by a 1.5-level turbulence closure scheme. Significant turbulent flux anomalies of sensible and latent heat are caused by changes in the air‐sea temperature and moisture differences induced by the TIWs. Horizontal advection leads to the occurrence of the air temperature and moisture extrema downwind of the SST extrema. High and low hydrostatic surface pressures are then located downwind of the cold and warm SST patches, respectively. The maximum and minimum wind speeds occur in phase with SST, and a thermally direct circulation is created. The momentum budget indicates that pressure gradient, vertical mixing, and horizontal advection dominate. In the PBL the vertical mixing acts as a frictional drag on the pressure-gradient-driven winds. Over warm SST the mixed layer deepens relative to over cold SST. The model simulations of the phase and amplitude of wind velocity, wind convergence, and column-integrated water vapor perturbations due to TIWs are similar to those observed from satellite and in situ data.

Ocean physics and nutrient fields along 180° during an El Niño–Southern Oscillation cold phase

This paper presents physical and nutrient results from the Etude du Broutage en Zone Equatoriale cruise, conducted in the equatorial Pacific along the 180° meridian from 8°S to 8°N. Cold conditions of the El Niño–Southern Oscillation cycle were evident during the cruise (October–November 1996), and the equatorial upwelling was in its far western extension. Along the sampled section the nutrient‐enriched area was asymmetric around the equator, and a zone of high remineralization was found from 6° to 3°S. Intensive sampling at two 5‐day time series stations (3°S and 0°) provided some insight on high‐frequency variability. At 3°S a deep density mixed layer showed only small fluctuations in nutrients. On the equator a rapid reduction of surface nutrients during the time series station was attributed to advection of a different water mass from the northeast, in the southward branch of a Tropical Instability Wave (TIW). Measurements from the Tropical Atmosphere‐Ocean (TAO) array in the area confirmed significant contemporaneous TIW activity, which was linked to the cold conditions. Thus, in contrast to previous observations, it is shown that TIWs can contribute to relative decrease of nutrients at the equator. During the two time series sampling efforts, variability at diurnal and semidiurnal periods were found in physical parameters, originating from surface atmospheric forcing and internal wave activity at tidal frequency, respectively. In the 0–150 m layer, where intensive sampling of nutrients was performed, high‐frequency variability did not seem to modify nutrient distribution significantly and physical influences dominated over biology.

EBENE: A JGOFS investigation of plankton variability and trophic interactions in the equatorial Pacific (180°)

The Etude du Broutage en Zone Equatoriale (EBENE) transect (8°S–8°N) explored the equatorial high‐nutrient, low‐chlorophyll (HNLC) zone and adjacent oligotrophic areas during a La Niña period (October–November 1996). During this time the passage of a tropical instability wave also influenced the region north of the equator. We present a brief summary of EBENE findings, with an emphasis on phytoplankton utilization by the assemblage of protistan and animal consumers. Despite significant variability over the diel cycle, phytoplankton biomass at the equator was relatively constant on a 24‐hour timescale, denoting a dynamic balance between growth and losses. The magnitude of the daily cycle in phytoplankton biomass was well constrained by in situ observations of the diel variability in pigments and suspended particulates, by 14C uptake rates from in situ incubations, and from experimental determinations of specific growth and grazing rates. The general equilibrium of production and grazing processes is illustrated by applying biomass‐specific grazing rates from the equatorial station to measured planktonic biomass along the EBENE transect and comparing them to measured 14C uptake. Most of the grazing turnover is supported by the production of Prochloroccus (31%) and picoeukaryotic algae (34%). Among the consumers, microzooplankton (<200 μm) account for 59–98% of the grazing losses. The coherence of the results obtained by independent methods suggests that the essential features of the system have been adequately represented by rate and standing stock assessments from the EBENE study.

Mesozooplankton biomass and composition in the equatorial Pacific along 180°

Latitudinal variations in mesozooplankton biomass and composition were investigated along an equatorial transect (8°S–8°N, 180°) in October–November 1996. This study also included intensive sampling (3‐hour intervals for 48 hours) for diel variations in mesozooplankton vertical distributions at 3°S and the equator. Most of the study took place in the high‐nutrient, low‐chlorophyll (HNLC) area that stretched between 7°S and 5°N. Mesozooplankton latitudinal distributions were influenced by the passage of a tropical instability wave during the equatorial time series station, which brought lower mesozooplankton biomass from the northeast, contrasting with the lack of a similar effect on concentrations of phytoplankton and particulates. South of the equator, the distributions of the mesozooplankton showed variable patterns with respect to chlorophyll and surface nitrate concentrations that could be ascribed to different states of the HNLC ecosystem. Very low diel variations of mesozooplankton biomass in the 0–50 and 0–100 m depth strata, a shallow vertical distribution, and the dominance of the larger size fraction (500–2000 μm) appear to be typical of the equatorial Pacific HNLC mesozooplankton and contrast with tropical oligotrophic ecosystems. Effects of such characteristics are a low active carbon export and a continuous predatory pressure.

Barometric Pressure Variations Associated with Eastern Pacific Tropical Instability Waves*

Abstract Barometric pressure, surface temperature, and wind time series in the eastern equatorial Pacific are analyzed to determine if oceanic tropical instability wave (TIW) sea surface temperature variations cause barometric pressure gradients large enough to influence the atmospheric boundary layer. During the study period from April 2001 to September 2002, 11 TIWs propagated westward past 110°W, causing a spectral peak at 20–30 days in the sea surface temperature (SST) meridional difference between 2°N, 110°W and 0°, 110°W. Likewise, the meridional pressure difference also had a spectral peak in the 20–30-day TIW band. Cross-spectral analysis shows that within the TIW band, SST-induced pressure variations were roughly −0.1 hPa °C−1 in magnitude. The resulting pressure gradient force is comparable in magnitude to other terms in the meridional momentum balance. Implications about the role of the boundary layer capping in the adjustment to SST forcing are discussed.

A linear relationship between ENSO intensity and tropical instability wave activity in the eastern Pacific Ocean

The interannual variations of tropical instability waves (TIWs) in the eastern Pacific Ocean and their relationships with ENSO intensity are studied using a 28‐year long coupled atmosphere‐ocean general circulation model (CGCM) simulation. The activity of TIWs is measured by the root‐mean‐square value of the 50‐day high‐pass filtered sea surface temperature (SST) perturbations produced in the simulation. The CGCM realistically produces two branches of large TIW activity near the equator: one at 2°N and the other at 2°S. It is found that along both branches there are linear relationships between the year‐to‐year variations of TIW activity and those of ENSO intensity. TIW activity is enhanced or reduced in proportion to the NINO3 SST anomalies, with larger activity in La Niña years and smaller activity in El Niño years. It is found that ENSO modulates TIW activity in both branches primarily by changing the latitudinal SST gradient associated with the SST front immediately north of the equator. Weaker correlations are found between the year‐to‐year variations of TIW activity and those of the ocean current shears between the south equatorial current and the equatorial undercurrent or the north equatorial countercurrent. This CGCM study suggests that the baroclinic instability associated with the northern SST front is a major generation mechanism for both the northern and southern TIW branches in the eastern Pacific Ocean.

Seasonal mixed layer heat budget of the tropical Atlantic Ocean

This paper addresses the atmospheric and oceanic causes of the seasonal cycle of sea surface temperature (SST) in the tropical Atlantic on the basis of direct observations. Data sets include up to 4 years (September 1997 to February 2002) of measurements from moored buoys of the Pilot Research Array in the Tropical Atlantic (PIRATA), near‐surface drifting buoys, and a blended satellite in situ SST product. We analyze the mixed layer heat balance at eight PIRATA mooring locations and find that the seasonal cycles of latent heat loss and absorbed shortwave radiation are responsible for seasonal SST variability in the northwest basin (8°N–15°N along 38°W). Along the equator (10°W–35°W), contributions from latent heat loss are diminished, while horizontal temperature advection and vertical entrainment contribute significantly. Zonal temperature advection is especially important during boreal summer near the western edge of the cold tongue, while horizontal eddy temperature advection, which most likely results from tropical instability waves, opposes temperature advection by the mean flow. The dominant balance in the southeast (6°S–10°S along 10°W) is similar to that in the northwest, with both latent heat loss and absorbed solar radiation playing important roles.

Time scales of upper ocean temperature variability inferred from the PIRATA data (1997–2000)

Time series from the PIRATA 10‐minute measurements are investigated in order to study the upper‐ocean thermal field time‐scales of variability using the serial covariance power spectra method and wavelet analysis. This is one of the few studies that analyze in time and space the PIRATA buoy data in the region. Analysis of the depth of the 20°C isotherm show distinct patterns for the northwestern Atlantic at 8N–38W and the southeastern Atlantic, at 10S–10W. In the west, warm surface waters are accompanied by a shallower thermocline while in the east warm surface waters are associated with its deepening, consistent with Houghton [1991]. At higher frequencies the dominant signals in sea surface temperature are the diurnal and semi‐diurnal cycles. Spectral analysis of the depth of the 20°C isotherm show a predominance of the semi‐diurnal period. In the eastern Atlantic there are two additional dominant bands in the spectra: 10–14 days, 25–27 days and for the depth of the 20°C isotherm there is also a lower frequency band at 20–40 days associated with tropical instability waves at the equator. The first band can be associated with atmospherically forced motions and the lower frequency band with barotropic instability waves.

Sensitivity of Pacific Ocean Tropical Instability Waves to Initial Conditions

Abstract Tropical instability waves (TIWs) appear as monthly oscillations of the currents, sea level, and sea surface temperature of the eastern equatorial Pacific. They are understood as unstable waves feeding on the kinetic and potential energy of the mean currents. A general circulation model is shown to reproduce the main features associated with TIWs. It is then used to investigate the dynamical regime of TIWs, by assessing their sensitivity to oceanic initial conditions. Locally in space and time, small perturbations can grow enough to modify significantly the phase of the TIW field, suggesting some chaotic behavior. When considered over the whole active TIW region, however, the phases of the perturbed and unperturbed experiments remain in agreement. This suggests that TIW activity in this model is more consistent with a limit cycle behavior than with fully developed turbulence and that irregular behavior of TIWs mostly stems from external forcing by the wind. A stronger result is that TIWs in exper...