tropical instability waves
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2022 ◽  
Vol 8 ◽  
Author(s):  
Jonathan Sherman ◽  
Ajit Subramaniam ◽  
Maxim Y. Gorbunov ◽  
Ana Fernández-Carrera ◽  
Rainer Kiko ◽  
...  

In the Equatorial Atlantic nitrogen availability is assumed to control phytoplankton dynamics. However, in situ measurements of phytoplankton physiology and productivity are surprisingly sparse in comparison with the North Atlantic. In addition to the formation of the Equatorial cold tongue in the boreal summer, tropical instability waves (TIWs) and related short-term processes may locally cause episodic events of enhanced nutrient supply to the euphotic layer. Here, we assess changes in phytoplankton photophysiology in response to such episodic events as well as short-term nutrient addition experiments using a pair of custom-built fluorometers that measure chlorophyll a (Chl a) variable fluorescence and fluorescence lifetimes. The fluorometers were deployed during a transatlantic cruise along the Equator in the fall of 2019. We hypothesized that the Equatorial Atlantic is nitrogen-limited, with an increasing degree of limitation to the west where the cold tongue is not prominent, and that infrequent nitrate injection by TIW related processes are the primary source alleviating this limitation. We further hypothesized phytoplankton are well acclimated to the low levels of nitrogen, and once nitrogen is supplied, they can rapidly utilize it to stimulate growth and productivity. Across three TIW events encountered, we observed increased productivity and chlorophyll a concentration concurrent with a decreased photochemical conversion efficiency and overall photophysiological competency. Moreover, the observed decrease in photosynthetic turnover rates toward the western section suggested a 70% decrease in growth rates compared to their maximum values under nutrient-replete conditions. This decrease aligned with the increased growth rates observed following 24 h incubation with added nitrate in the western section. These results support our hypotheses that nitrogen is the limiting factor in the region and that phytoplankton are in a state of balanced growth, waiting to “body surf” waves of nutrients which fuel growth and productivity.


2021 ◽  
Vol 51 (5) ◽  
pp. 1575-1593
Author(s):  
D. A. Cherian ◽  
D. B. Whitt ◽  
R. M. Holmes ◽  
R.-C. Lien ◽  
S. D. Bachman ◽  
...  

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.


Author(s):  
J. Thomas Farrar ◽  
Theodore Durland ◽  
Steven R. Jayne ◽  
James F. Price

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.


2021 ◽  
Author(s):  
Lisa Maillard ◽  
Julien Boucharel ◽  
Lionel Renault

<p>Tropical instability waves (TIWs) are oceanic cusp-like features propagating westward along the northern front of the tropical pacific cold tongue. Observational and modelling studies suggest that TIWs may have a large impact on the eastern tropical Pacific background state from seasonal to interannual time-scales, through heat advection and mixing. However, observations are coarse or limited to surface data, and modelling studies are often based on the comparison of low- vs. high-resolution simulations. In this study, we perform a set of regional high-resolution ocean simulations (CROCO 1/12°) in which we strongly damp (NUDG-RUN) or not (CTRL-RUN) TIWs propagation, by nudging the mixed layer meridional current velocities in the TIWs active region toward their climatological values. We first show that this approach do not alter the model internal physics, in particular related to the equatorial wave dynamics. The impact of TIWs on the oceanic mean state (zonal current and heat budget) is then assessed by comparing CTRL-RUN to NUDG-RUN. This approach allows quantifying for the first time the rectified effect of TIWs without degrading the model horizontal resolution, and may lead to a better understanding of ENSO asymmetry and the development of accurate TIWs parameterizations in Earth system models.</p>


2021 ◽  
Author(s):  
Mia Sophie Specht ◽  
Johann Jungclaus ◽  
Jürgen Bader

<div> <p><span>Tropical instability waves (TIWs) near the ocean surface are present in all tropical oceans and are known to be important for air-sea interactions and regional climate variability. Recent studies based on observations in the Pacific Ocean found that apart from TIWs at the surface, there also exist subsurface TIWs (subTIWs) which can alter vertical mixing. To date, most studies have focused on TIW related dynamics near the ocean surface. However, to properly assess vertical mixing in the upper ocean, improved understanding of the vertical structure of TIWs and the influence of subTIWs is needed. In this study, </span>we show subTIW<span> presence</span> in the Atlantic Ocean for the first time using mooring observations.<span>Further, we characterize subTIWs in the tropical Atlantic Ocean with a special focus on subTIW spatial and temporal variability and their effect on mixing. For this, data covering almost two decades are used that were generated from a comprehensive, global, high-resolution ocean model forced by the reanalysis ERA5. We find subTIWs between 40 m depth and the thermocline in both model and observations and unlike TIWs, subTIWs are frequently active both north and south of the Equator. The results of our study suggest that subTIWs induce a multi-layer shear structure which has the potential to destabilize the mean flow and thereby cause mixing. These effects are strongest north of the Equator where TIWs and subTIWs act simultaneously, implying possible TIW/subTIW interactions. </span>We conclude that subTIWs are a feature of the tropical Atlantic Ocean with regionally varying implications for vertical mixing and heat fluxes. <span>In addition, subTIWs differ from TIWs in their temporal and regional occurrences Therefore, subTIWs should be considered in </span>f<span>u</span>ture assessments of upper ocean dynamics, particularly in subTIW dominated regions.</p> </div>


2020 ◽  
Vol 50 (10) ◽  
pp. 3009-3024
Author(s):  
Minyang Wang ◽  
Shang-Ping Xie ◽  
Samuel S. P. Shen ◽  
Yan Du

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.


2020 ◽  
Vol 47 (7) ◽  
Author(s):  
Aoyun Xue ◽  
Fei‐Fei Jin ◽  
Wenjun Zhang ◽  
Julien Boucharel ◽  
Sen Zhao ◽  
...  

2020 ◽  
Author(s):  
David Webb

<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>


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