Biogeochemical impact of tropical instability waves in the equatorial Pacific

AbstractMesoscale activities over the equatorial Pacific Ocean are dominated by the Rossby and Yanai modes of tropical instability waves (TIWs). The TIW-induced surface velocity has not been accurately estimated in previous diagnostic models, especially for the meridional component across the equator. This study develops a diagnostic model that retains the acceleration terms to estimate the TIW surface velocity from the satellite-observed sea surface height. Validated against moored observations, the velocity across the equator is accurately estimated for the first time, much improved from existing products. The results identify the Rossby- and Yanai-mode TIWs as the northwest–southeastward (NW–SE) velocity oscillations north of the equator and the northeast–southwestward (NE–SW) velocity oscillations on the equator, respectively. Barotropic instability is the dominant energy source of the two TIW modes. The NE–SW velocity oscillation of the Yanai mode is associated with the counterclockwise shear of the South Equatorial Current on the equator. The two TIW modes induce different sea surface temperature patterns and vertical motions. Accurate estimates of TIW velocity are important for studying equatorial ocean dynamics and climate variability in the tropical Pacific Ocean.

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Generation Mechanism of Tropical Instability Waves in the Equatorial Pacific Ocean

Journal of Physical Oceanography ◽

10.1175/jpo-d-19-0094.1 ◽

2019 ◽

Vol 49 (11) ◽

pp. 2901-2915

Author(s):

Yuki Tanaka ◽

Toshiyuki Hibiya

Keyword(s):

Growth Rate ◽

General Circulation ◽

Rossby Waves ◽

Unstable Mode ◽

Equatorial Pacific ◽

Water Model ◽

Instability Waves ◽

Tropical Instability Waves ◽

Zonal Wavenumber ◽

Pv Gradient

AbstractTropical instability waves (TIWs) are prominent features in the equatorial Pacific, propagating westward at a speed of ~0.5 m s−1 with a wavelength of ~1000 km. In this study, we show that a linear stability analysis using a 1.5-layer shallow water model can predict successfully an unstable mode whose wavelength, phase speed, growth rate, and meridional structure are all consistent with those of the TIWs simulated by an eddy-resolving ocean general circulation model (OGCM). This unstable mode can be interpreted as resulting from the coupling of two Rossby waves, namely, one trapped just north of the equator (~1°–3.5°N) and the other trapped farther north (~3.5°–8°N). Although these two Rossby waves have opposite intrinsic phase propagation directions reflecting the negative and positive local meridional potential vorticity (PV) gradients, respectively, their actual propagation direction can be adjusted through the advection by the South Equatorial Current and the North Equatorial Countercurrent such that they might propagate westward at the same speed and with the same zonal wavenumber yielding the largest growth rate of TIWs. The unstable mode does not appear during the period in which the negative PV gradient is absent, which demonstrates its essential role in generating TIWs. Indeed, the seasonal and interannual variability of the TIWs simulated by the OGCM is shown to be significantly controlled by the strength of the negative PV gradient just north of the equator, suggesting that it could be a key parameter toward a dynamically based parameterization of the heat and momentum transfer associated with TIWs in coarse-resolution OGCMs.

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Direct Observations of Atmospheric Boundary Layer Response to SST Variations Associated with Tropical Instability Waves over the Eastern Equatorial Pacific*

Journal of Climate ◽

10.1175/1520-0442(2002)015<3379:dooabl>2.0.co;2 ◽

2002 ◽

Vol 15 (23) ◽

pp. 3379-3393 ◽

Cited By ~ 86

Author(s):

Hiroshi Hashizume ◽

Shang-Ping Xie ◽

Masatomo Fujiwara ◽

Masato Shiotani ◽

Tomowo Watanabe ◽

...

Keyword(s):

Boundary Layer ◽

Atmospheric Boundary Layer ◽

Equatorial Pacific ◽

Instability Waves ◽

Tropical Instability Waves ◽

Direct Observations ◽

Eastern Equatorial Pacific

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Long-Term Observations of Tropical Instability Waves

Journal of Physical Oceanography ◽

10.1175/1520-0485-32.9.2715 ◽

2002 ◽

Vol 32 (9) ◽

pp. 2715-2722 ◽

Cited By ~ 66

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.

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Nonlinear Effects of Tropical Instability Waves on the Equatorial Pacific Circulation

Journal of Physical Oceanography ◽

10.1175/2009jpo3963.1 ◽

2010 ◽

Vol 40 (2) ◽

pp. 381-393 ◽

Cited By ~ 7

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.

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

Journal of Physical Oceanography ◽

10.1175/2010jpo4366.1 ◽

2010 ◽

Vol 40 (9) ◽

pp. 2091-2106 ◽

Cited By ~ 12

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.

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