scholarly journals Long-Term Observations of Tropical Instability Waves

2002 ◽  
Vol 32 (9) ◽  
pp. 2715-2722 ◽  
Author(s):  
Robert F. Contreras
Keyword(s):  
Phase Speed ◽  
Warm Season ◽  
Equatorial Pacific ◽  
Enso Cycle ◽  
Cold Tongue ◽  
Near Surface ◽  
Instability Waves ◽  
Sst Front ◽  
Tropical Instability Waves ◽  
The Waves

Abstract Reynolds sea surface temperature (SST) data showing tropical instability waves (TIWs) in the tropical Pacific are analyzed along with current measurements from the Tropical Atmosphere–Ocean (TAO) buoy array and wind speeds from the European Remote Sensing Satellite (ERS) -1 and -2 scatterometers. TIWs are visible as undulations in the SST cold fronts that delineate the northern and southern boundaries of the cold tongue in the equatorial Pacific. The SST pattern results from advection of the SST front by instabilities in the near-surface equatorial currents. Although the waves are seen on both sides of the Pacific cold tongue and north of the equator in the Atlantic, they are most intense, and thereby most observable, in the north equatorial Pacific. The combination of data used in this analysis provides information about these waves, the factors controlling them, and their coupling to the atmosphere on annual and interannual timescales. On annual timescales, the TIWs generally establish a strong signal in July east of about 140°W with a westward phase speed of about 0.5 m s−1. By August, the waves usually occupy the longitudes between 160° and 100°W and continue to propagate west at roughly the same speed. With the onset of the warm season in the equatorial cold tongue (spring), the signal typically weakens and the propagation speeds show large variations. On interannual timescales, activity is strongest during the cold phase of the ENSO cycle (La Niña) when the cold tongue is most pronounced; the waves are weak or nonexistent during the warm phase of ENSO (El Niño) when the SST front is weak. The TIW signature in SST is noticeable throughout all seasons of the year provided that the gradient in SST at 140°W is greater than about 0.25°C (100 km)−1. In addition, analysis of the currents underlines the importance of the background currents to the zonal propagation speeds.

Tropical Cells and a Secondary Circulation near the Northern Front of the Equatorial Pacific Cold Tongue*

2010 ◽  
Vol 40 (9) ◽  
pp. 2091-2106 ◽  
Author(s):  
Renellys C. Perez ◽  
Meghan F. Cronin ◽  
William S. Kessler
Keyword(s):  
Mixed Layer ◽  
Equatorial Pacific ◽  
Secondary Circulation ◽  
Subsurface Water ◽  
Surface Mixed Layer ◽  
Cold Tongue ◽  
Near Surface ◽  
Instability Waves ◽  
Tropical Instability Waves ◽  
The Mean

Abstract Shipboard measurements and a model are used to describe the mean structure of meridional–vertical tropical cells (TCs) in the central equatorial Pacific and a secondary circulation associated with the northern front of the cold tongue. The shape of the front is convoluted by the passage of tropical instability waves (TIWs). When velocities are averaged in a coordinate system centered on the instantaneous position of the northern front, the measurements show a near-surface minimum in northward flow north of the surface front (convergent flow near the front). This convergence and inferred downwelling extend below the surface mixed layer, tilt poleward with depth, and are meridionally bounded by regions of divergence and upwelling. Similarly, the model shows that, on average, surface cold tongue water moves northward toward the frontal region and dives below tilted front, whereas subsurface water north of the front moves southward toward the front, upwells, and then moves northward in the surface mixed layer. The model is used to demonstrate that this mean quasi-adiabatic secondary circulation is not a frozen field that migrates with the front but is instead highly dependent on the phase of the TIWs: southward-upwelling flow on the warm side of the front tends to occur when the front is displaced southward, whereas northward-downwelling flow on the cold side of the front occurs when the front is displaced northward. Consequently, when averaged in geographic coordinates, the observed and simulated TCs appear to be equatorially asymmetric and show little trace of a secondary circulation near the mean front.

scholarly journals Nonlinear Effects of Tropical Instability Waves on the Equatorial Pacific Circulation

2010 ◽  
Vol 40 (2) ◽  
pp. 381-393 ◽  
Author(s):  
Jaclyn N. Brown ◽  
J. Stuart Godfrey ◽  
Susan E. Wijffels
Keyword(s):  
Low Frequency ◽  
Nonlinear Effects ◽  
Equatorial Pacific ◽  
Momentum Balance ◽  
Equatorial Pacific Ocean ◽  
Instability Waves ◽  
Tropical Instability Waves ◽  
The North ◽  
The Waves ◽  
Frequency State

Abstract In a numerical model of the equatorial Pacific Ocean, the ∼20-day period tropical instability waves, excited in the eastern half of the domain, are found to damp the strong zonal mean currents. The waves generate large, nonlinear, advection terms in the momentum balance, change the vorticity balance, and thus modulate the low-frequency state. The authors explore whether the effect of tropical instability waves on the background flow can instead be adequately parameterized by a constant-coefficient Laplacian friction scheme. On annual mean, a Laplacian friction coefficient that varies in space is required, for the coefficient is twice as large along the equator and a few degrees more to the north than elsewhere. In addition, wave activity varies in time. During active phases, such as the second half of the year and during La Niñas, the activity increases, which would require the Laplacian coefficient of friction to be at least twice as strong as during the inactive phases. Thus, a more sophisticated damping parameterization than simple Laplacian friction is required in ocean models that do not explicitly resolve tropical instability waves.

Off-Equatorial Deep-Cycle Turbulence Forced by Tropical Instability Waves in the Equatorial Pacific

2021 ◽  
Vol 51 (5) ◽  
pp. 1575-1593
Author(s):  
D. A. Cherian ◽  
D. B. Whitt ◽  
R. M. Holmes ◽  
R.-C. Lien ◽  
S. D. Bachman ◽  
...  
Keyword(s):  
Mixing Layer ◽  
Equatorial Pacific ◽  
Cold Tongue ◽  
Equatorial Undercurrent ◽  
Instability Waves ◽  
Tropical Instability Waves ◽  
Surface Mixing ◽  
Large Heat ◽  
Surface Buoyancy ◽  
A Site

AbstractThe equatorial Pacific cold tongue is a site of large heat absorption by the ocean. This heat uptake is enhanced by a daily cycle of shear turbulence beneath the mixed layer—“deep-cycle turbulence”—that removes heat from the sea surface and deposits it in the upper flank of the Equatorial Undercurrent. Deep-cycle turbulence results when turbulence is triggered daily in sheared and stratified flow that is marginally stable (gradient Richardson number Ri ≈ 0.25). Deep-cycle turbulence has been observed on numerous occasions in the cold tongue at 0°, 140°W, and may be modulated by tropical instability waves (TIWs). Here we use a primitive equation regional simulation of the cold tongue to show that deep-cycle turbulence may also occur off the equator within TIW cold cusps where the flow is marginally stable. In the cold cusp, preexisting equatorial zonal shear uz is enhanced by horizontal vortex stretching near the equator, and subsequently modified by horizontal vortex tilting terms to generate meridional shear υz off of the equator. Parameterized turbulence in the sheared flow of the cold cusp is triggered daily by the descent of the surface mixing layer associated with the weakening of the stabilizing surface buoyancy flux in the afternoon. Observational evidence for off-equatorial deep-cycle turbulence is restricted to a few CTD casts, which, when combined with shear from shipboard ADCP data, suggest the presence of marginally stable flow in TIW cold cusps. This study motivates further observational campaigns to characterize the modulation of deep-cycle turbulence by TIWs both on and off the equator.

scholarly journals Deep Intraseasonal Variability in the Central Equatorial Atlantic

2018 ◽  
Vol 48 (12) ◽  
pp. 2851-2865 ◽  
Author(s):  
Franz Philip Tuchen ◽  
Peter Brandt ◽  
Martin Claus ◽  
Rebecca Hummels
Keyword(s):  
Intraseasonal Variability ◽  
Deep Ocean ◽  
Minor Role ◽  
Equatorial Atlantic ◽  
Near Surface ◽  
Velocity Fluctuations ◽  
Instability Waves ◽  
Vertical Mode ◽  
Tropical Instability Waves ◽  
Meridional Velocity

AbstractBesides the zonal flow that dominates the seasonal and long-term variability in the equatorial Atlantic, energetic intraseasonal meridional velocity fluctuations are observed in large parts of the water column. We use 15 years of partly full-depth velocity data from an equatorial mooring at 23°W to investigate intraseasonal variability and specifically the downward propagation of intraseasonal energy from the near-surface into the deep ocean. Between 20 and 50 m, intraseasonal variability at 23°W peaks at periods between 30 and 40 days. It is associated with westward-propagating tropical instability waves, which undergo an annual intensification in August. At deeper levels down to about 2000 m considerable intraseasonal energy is still observed. A frequency–vertical mode decomposition reveals that meridional velocity fluctuations are more energetic than the zonal ones for periods < 50 days. The energy peak at 30–40 days and at vertical modes 2–5 excludes equatorial Rossby waves and suggests Yanai waves to be associated with the observed intraseasonal energy. Yanai waves that are considered to be generated by tropical instability waves propagate their energy from the near-surface west of 23°W downward and eastward to eventually reach the mooring location. The distribution of intraseasonal energy at the mooring position depends largely on the dominant frequency and the time, depth, and longitude of excitation, while the dominant vertical mode of the Yanai waves plays only a minor role. Observations also show the presence of weaker intraseasonal variability at 23°W below 2000 m that cannot be associated with tropical instability waves.

scholarly journals Multi-year satellite observations of instability waves in the Tropical Atlantic Ocean

Ocean Science ◽  
2005 ◽  
Vol 1 (2) ◽  
pp. 97-112 ◽  
Author(s):  
A. C. V. Caltabiano ◽  
I. S. Robinson ◽  
L. P. Pezzi
Keyword(s):  
Atlantic Ocean ◽  
Spectral Characteristics ◽  
Coupling Mechanism ◽  
Tropical Atlantic ◽  
Instability Waves ◽  
Tropical Atlantic Ocean ◽  
Tropical Instability Waves ◽  
Zonal Winds ◽  
Meridional Winds ◽  
The Waves

Abstract. Instability waves in the tropical Atlantic Ocean are analysed by microwave satellite-based data spanning from 1998 to 2001. This is the first multi-year observational study of the sea surface temperature (SST) signature of the Tropical Instability Waves (TIW) in the region. SST data were used to show that the waves spectral characteristics vary from year-to-year. They also vary on each latitude north of the equator, with the region of 1° N, 15° W concentrating the largest variability when the time series is averaged along the years. Analyses of wind components show that meridional winds are more affected near the equator and 1° N, while zonal winds are more affected further north at around 3° N and 4° N. Concurrent observations of SST, wind, atmospheric water vapour, liquid cloud water, precipitation rates and wind were used to suggest the possible influence of these waves on the Intertropical Convergence Zone (ITCZ). It seems that these instabilities have a large impact on the ITCZ due to its proximity of the equator, compared to its Pacific counterpart, and the geography of the tropical Atlantic basin. These analyses also suggest that the air-sea coupling mechanism suggested by Wallace can also be applied to the tropical Atlantic region.

scholarly journals A Generalized Method for Estimating the Structure of the Equatorial Atlantic Cold Tongue: Application to Drifter Observations

2013 ◽  
Vol 30 (8) ◽  
pp. 1884-1895 ◽  
Author(s):  
Verena Hormann ◽  
Rick Lumpkin ◽  
Renellys C. Perez
Keyword(s):  
Complete Analysis ◽  
Cold Tongue ◽  
Equatorial Atlantic ◽  
Instability Waves ◽  
Tropical Instability Waves ◽  
Large Application ◽  
Coordinate Mapping ◽  
Equatorial Current ◽  
Atlantic Cold Tongue ◽  
Northern Branch

Abstract A generalized method is developed to determine the position of the Atlantic northern cold tongue front across its zonal extent from satellite sea surface temperature (SST) data. Previous approaches estimated the frontal position subjectively or individually, calling for a more objective technique that is suitable for large datasets. The developed methodology is based on a median frontal SST, and associated positional uncertainties are on the order of 0.3° latitude for the period 1998–2011. Frontal characteristics are generally consistent with tropical instability waves (TIWs) and interannual variations are large. Application to drifter observations shows how the new methodology can be used to better understand circulation features near the northern cold tongue front. A drifter pair deployed on the eastern side of a passing TIW crest north of the front revealed that the trajectories of the drifters were clearly influenced by the shape of the front and they did not cross the front, but rather stayed close together about 2.5° north of the front. In a more complete analysis using all available drifters near the Atlantic northern cold tongue front, only about 12% of the trajectories crossed the front. Analyses in an along- and cross-frontal frame of reference complement isopycnal coordinate mapping, and tropical Atlantic drifter velocities averaged in frontal coordinates indicate a broadened shear zone between the northern branch of the South Equatorial Current and North Equatorial Countercurrent as well as meridional convergence near the front.

scholarly journals Biogeochemical impact of tropical instability waves in the equatorial Pacific

2005 ◽  
Vol 32 (24) ◽  
Author(s):  
T. Gorgues ◽  
C. Menkes ◽  
O. Aumont ◽  
J. Vialard ◽  
Y. Dandonneau ◽  
...  
Keyword(s):  
Equatorial Pacific ◽  
Instability Waves ◽  
Tropical Instability Waves
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