scholarly journals Long-distance radiation of Rossby waves from the equatorial current system

2021 ◽  
Author(s):  
J. Thomas Farrar ◽  
Theodore Durland ◽  
Steven R. Jayne ◽  
James F. Price
Keyword(s):  
Rossby Waves ◽  
Current System ◽  
Kamchatka Peninsula ◽  
Equatorial Region ◽  
Wave Radiation ◽  
Long Distance ◽  
Instability Waves ◽  
Tropical Instability Waves ◽  
The North ◽  
Equatorial Current

AbstractMeasurements from satellite altimetry are used to show that sea-surface height (SSH) variability throughout much of the North Pacific is coherent with the SSH signal of the tropical instability waves (TIWs) that result from instabilities of the equatorial currents. This variability has regular phase patterns consistent with freely propagating barotropic Rossby waves radiating energy away from the unstable equatorial currents, and the waves clearly propagate from the equatorial region to at least 30°N. The pattern of SSH variance at TIW frequencies exhibits remarkable patchiness on scales of hundreds of kilometers, which we interpret as being due to the combined effects of wave reflection, refraction, and interference. North of 40°N, more than 6000 km from the unstable equatorial currents, the SSH field remains coherent with the near-equatorial SSH variability, but it is not as clear whether the variability at the higher latitudes is a simple result of barotropic wave radiation from the tropical instability waves. Even more distant regions, as far north as the Aleutian Islands off of Alaska and the Kamchatka Peninsula of eastern Russia, have SSH variability that is significantly coherent with the near-equatorial instabilities. The variability is not well represented in the widely used gridded SSH data product commonly referred to as the AVISO or DUACS product, and this appears to be a result of spatial variations in the filtering properties of the objective mapping scheme.

scholarly journals Enhanced 2-h–8-day Oscillations Associated with Tropical Instability Waves

2014 ◽  
Vol 44 (7) ◽  
pp. 1908-1918 ◽  
Author(s):  
Zhao Jing ◽  
Lixin Wu ◽  
Dexing Wu ◽  
Bo Qiu
Keyword(s):  
Current System ◽  
Surface Wind ◽  
Surface Wind Speed ◽  
Daily Basis ◽  
Vertical Shear ◽  
Equatorial Undercurrent ◽  
Instability Waves ◽  
Tropical Instability Waves ◽  
Equatorial Current ◽  
Stable Current

Abstract The westward South Equatorial Current (SEC) and eastward Equatorial Undercurrent (EUC) is a marginally stable current system due to the strong vertical shear. The existence of wavelike motions may locally reduce the Richardson number enough to trigger instabilities. Here, velocity measurements from the Tropical Atmosphere Ocean (TAO) array are used to examine the variability of oscillations within 0.125–12 cycles per day (cpd). It is found that the 0.125–12-cpd oscillations become more energetic in the presence of strong tropical instability waves (TIWs). The enhancement of shear variance is most pronounced around the EUC core (115 m), while prominent elevation of kinetic energy occurs around 85 m, where the EUC shear is strongest. Particularly, the energetic 0.125–12-cpd oscillations during strong TIW seasons do not cycle on a daily basis and are more evident during the southward phase of TIWs. The enhanced 0.125–12-cpd oscillations during strong TIW seasons can be ascribed neither to the changing background stratification nor to the vertical migration of EUC core at the corresponding time scales. Its strength is tightly correlated with the EUC shear and, to a lesser extent, the TIW shear. A partial correlation analysis suggests that the correlation to the TIW shear is mainly due to the association between EUC and TIW shear. The strength of the 0.125–12-cpd oscillations does not follow the variation of surface wind speed and wind curl, implying that they are not directly generated by surface wind forcing.

The Role of the NECC in a Strong El Niño.

2020 ◽  
Author(s):  
David Webb
Keyword(s):  
Ocean Model ◽  
Warm Water ◽  
Ekman Transport ◽  
Global Ocean ◽  
Instability Waves ◽  
Tropical Instability Waves ◽  
The North ◽  
Ocean Science ◽  
The Pacific ◽  
North Equatorial Counter Current

<p>An analysis of archived data from the NEMO 1/12th degree global ocean model shows the importance of the North Equatorial Counter Current (NECC) in the development of the strong 1982–1983 and 1997–1998 El Niños.  The model results indicate that in a normal year the coreof warm water in the NECC is diluted by the surface Ekman transport, by geostrophic inflow and by tropical instability waves. During the development of the 1982–1983 and 1997–1998 El Niños, these processes had reduced effect at the longitudes of warmest equatorial temperatures. During the autumns of 1982 and 1997, the speed of the NECC was also increased by a stronger-than-normal annual Rossby wave and other changes in sea level in the western Pacific.  The resulting increased transport of warm water by the NECC resulted in water with temperatures above 28C reaching the eastern Pacific.  This appears to have been a major factor in moving the centre of deep atmospheric convection eastwards across the Pacific.</p><p>Note:  This is based on the paper published in Ocean Science.  An oral presentation is possible.</p>

scholarly journals Spectral signatures of the tropical Pacific dynamics from model and altimetry: A focus on the meso/submesoscale range

2018 ◽  
Author(s):  
Michel Tchilibou ◽  
Lionel Gourdeau ◽  
Rosemary Morrow ◽  
Guillaume Serazin ◽  
Bughsin Djath ◽  
...  
Keyword(s):  
Internal Waves ◽  
Tropical Pacific ◽  
Equatorial Region ◽  
Internal Tides ◽  
Spectral Energy ◽  
Instability Waves ◽  
Tropical Instability Waves ◽  
Geostrophic Turbulence ◽  
The Tropics ◽  
The Tropical Pacific

Abstract. The processes that contribute to the flat Sea Surface Height (SSH) wavenumber spectral slopes observed in the tropics by satellite altimetry are examined in the tropical Pacific. The tropical dynamics are first investigated with a 1/12° global model. The equatorial region from 10° N–10° S is dominated by Tropical Instability Waves with a peak of energy at 1000 km wavelength, strong anisotropy, and a cascade of energy from 600 km down to smaller scales. The off-equatorial regions from 10–20° latitude are characterized by a narrower mesoscale range, typical of mid latitudes. In the tropics, the spectral taper window and segment lengths need to be adjusted to include these larger energetic scales. The equatorial and off-equatorial regions of the 1/12° model have surface kinetic energy spectra consistent with quasi-geostrophic turbulence. The balanced component of the dynamics slightly flatten the EKE spectra, but modeled SSH wavenumber spectra maintain a steep slope that does not match the observed altimetric spectra. A second analysis is based on 1/36° high-frequency regional simulations in the western tropical Pacific, with and without explicit tides, where we find a strong signature of internal waves and internal tides that act to increase the smaller-scale SSH spectral energy power and flattening the SSH wavenumber spectra, in agreement with the altimetric spectra. The coherent M2 baroclinic tide is the dominant signal at ~ 140 km wavelength. At short scales, wavenumber SSH spectra are dominated by incoherent internal tides and internal waves which extend up to 200 km in wavelength. These incoherent internal waves impact on space scales observed by today's alongtrack altimetric SSH, and also on the future SWOT 2D swath observations, raising the question of altimetric observability of the shorter mesoscale structures in the tropics.

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 On the role of the North Equatorial Counter Current during a strong El Niño

Ocean Science ◽  
2018 ◽  
Vol 14 (4) ◽  
pp. 633-660 ◽  
Author(s):  
David John Webb
Keyword(s):  
Ocean Model ◽  
Warm Water ◽  
Ekman Transport ◽  
Global Ocean ◽  
Instability Waves ◽  
Tropical Instability Waves ◽  
The North ◽  
The Pacific ◽  
North Equatorial Counter Current ◽  
Counter Current

Abstract. An analysis of archived data from the NEMO 1∕12th degree global ocean model shows the importance of the North Equatorial Counter Current (NECC) in the development of the strong 1982–1983 and 1997–1998 El Niños. The model results indicate that in a normal year the core of warm water in the NECC is diluted by the surface Ekman transport, by geostrophic inflow and by tropical instability waves. During the development of the 1982–1983 and 1997–1998 El Niños, these processes had reduced effect at the longitudes of warmest equatorial temperatures and to the west. During the autumns of 1982 and 1997, the speed of the NECC was also increased by a stronger-than-normal annual Rossby wave. The increased transport of warm water by the NECC due to these changes resulted in warm water reaching the far eastern Pacific and appears to have been a major factor in moving the centre of deep atmospheric convection eastwards across the Pacific.

Rossby and Yanai Modes of Tropical Instability Waves in the Equatorial Pacific Ocean and a Diagnostic Model for Surface Currents

2020 ◽  
Vol 50 (10) ◽  
pp. 3009-3024
Author(s):  
Minyang Wang ◽  
Shang-Ping Xie ◽  
Samuel S. P. Shen ◽  
Yan Du
Keyword(s):  
Pacific Ocean ◽  
Equatorial Pacific ◽  
Ocean Dynamics ◽  
Sea Surface ◽  
Surface Velocity ◽  
Diagnostic Model ◽  
Equatorial Pacific Ocean ◽  
Instability Waves ◽  
Tropical Instability Waves ◽  
Equatorial Current

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.

Observations of the North Equatorial Current, Mindanao Current, and Kuroshio current system during the 2006/07 El Niño and 2007/08 La Niña

2009 ◽  
Vol 65 (3) ◽  
pp. 325-333 ◽  
Author(s):  
Yuji Kashino ◽  
Norievill España ◽  
Fadli Syamsudin ◽  
Kelvin J. Richards ◽  
Tommy Jensen ◽  
...  
Keyword(s):  
El Niño ◽  
El Nino ◽  
Current System ◽  
Kuroshio Current ◽  
La Niña ◽  
North Equatorial Current ◽  
The North ◽  
La Nina ◽  
Equatorial Current ◽  
Mindanao Current

scholarly journals Generation Mechanism of Tropical Instability Waves in the Equatorial Pacific Ocean

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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