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 The Photophysiological Response of Nitrogen-Limited Phytoplankton to Episodic Nitrogen Supply Associated With Tropical Instability Waves in the Equatorial Atlantic

2022 ◽  
Vol 8 ◽  
Author(s):  
Jonathan Sherman ◽  
Ajit Subramaniam ◽  
Maxim Y. Gorbunov ◽  
Ana Fernández-Carrera ◽  
Rainer Kiko ◽  
...  
Keyword(s):  
Growth Rates ◽  
Nitrogen Availability ◽  
Boreal Summer ◽  
Limiting Factor ◽  
Cold Tongue ◽  
Equatorial Atlantic ◽  
Short Term ◽  
Instability Waves ◽  
Tropical Instability Waves ◽  
Episodic Events

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.

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 Halocarbon emissions and sources in the equatorial Atlantic Cold Tongue

2015 ◽  
Vol 12 (7) ◽  
pp. 5559-5608 ◽  
Author(s):  
H. Hepach ◽  
B. Quack ◽  
S. Raimund ◽  
T. Fischer ◽  
E. L. Atlas ◽  
...  
Keyword(s):  
Surface Water ◽  
Mixed Layer ◽  
Global Radiation ◽  
Surface Mixed Layer ◽  
Cold Tongue ◽  
Equatorial Atlantic ◽  
Tropical Ocean ◽  
Near Surface ◽  
Biological Source ◽  
Atlantic Cold Tongue

Abstract. Halocarbons from oceanic sources contribute to halogens in the troposphere, and can be transported into the stratosphere where they take part in ozone depletion. This paper presents distribution and sources in the equatorial Atlantic from June and July 2011 of the four compounds bromoform (CHBr3), dibromomethane (CH2Br2), methyl iodide (CH3I) and diiodomethane (CH2I2). Enhanced biological production during the Atlantic Cold Tongue (ACT) season, indicated by phytoplankton pigment concentrations, led to elevated concentrations of CHBr3 of up to 44.7 pmol L−1 and up to 9.2 pmol L−1 for CH2Br2 in surface water, which is comparable to other tropical upwelling systems. While both compounds correlated very well with each other in the surface water,CH2Br2 was often more elevated in greater depth than CHBr3, which showed maxima in the vicinity of the deep chlorophyll maximum. The deeper maximum of CH2Br2 indicates an additional source in comparison to CHBr3 or a slower degradation of CH2Br2. Concentrations of CH3I of up to 12.8 pmol L−1 in the surface water were measured. In contrary to expectations of a predominantly photochemical source in the tropical ocean, its distribution was mostly in agreement with biological parameters, indicating a~biological source. CH2I2 was very low in the near surface water with maximum concentrations of only 3.7 pmol L−1, and the observed anticorrelation with global radiation was likely due to its strong photolysis. CH2I2 showed distinct maxima in deeper waters similar to CH2Br2. For the first time, diapycnal fluxes of the four halocarbons from the upper thermocline into and out of the mixed layer were determined. These fluxes were low in comparison to the halocarbon sea-to-air fluxes. This indicates that despite the observed maximum concentrations at depth, production in the surface mixed layer is the main oceanic source for all four compounds and has an influence on emissions into the atmosphere. The calculated production rates of the compounds yield 34 (CHBr3), 10 (CH2Br2), 21 (CH3I) and 384 (CH2I2) pmol m−3 h−1 in the whole mixed layer.

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 Halocarbon emissions and sources in the equatorial Atlantic Cold Tongue

2015 ◽  
Vol 12 (21) ◽  
pp. 6369-6387 ◽  
Author(s):  
H. Hepach ◽  
B. Quack ◽  
S. Raimund ◽  
T. Fischer ◽  
E. L. Atlas ◽  
...  
Keyword(s):  
Surface Water ◽  
Mixed Layer ◽  
Surface Mixed Layer ◽  
Cold Tongue ◽  
Equatorial Atlantic ◽  
Tropical Ocean ◽  
Near Surface ◽  
Biological Source ◽  
First Time ◽  
Atlantic Cold Tongue

Abstract. Halocarbons from oceanic sources contribute to halogens in the troposphere, and can be transported into the stratosphere where they take part in ozone depletion. This paper presents distribution and sources in the equatorial Atlantic from June and July 2011 of the four compounds bromoform (CHBr3), dibromomethane (CH2Br2), methyl iodide (CH3I) and diiodomethane (CH2I2). Enhanced biological production during the Atlantic Cold Tongue (ACT) season, indicated by phytoplankton pigment concentrations, led to elevated concentrations of CHBr3 of up to 44.7 and up to 9.2 pmol L−1 for CH2Br2 in surface water, which is comparable to other tropical upwelling systems. While both compounds correlated very well with each other in the surface water, CH2Br2 was often more elevated in greater depth than CHBr3, which showed maxima in the vicinity of the deep chlorophyll maximum. The deeper maximum of CH2Br2 indicates an additional source in comparison to CHBr3 or a slower degradation of CH2Br2. Concentrations of CH3I of up to 12.8 pmol L−1 in the surface water were measured. In contrary to expectations of a predominantly photochemical source in the tropical ocean, its distribution was mostly in agreement with biological parameters, indicating a biological source. CH2I2 was very low in the near surface water with maximum concentrations of only 3.7 pmol L−1. CH2I2 showed distinct maxima in deeper waters similar to CH2Br2. For the first time, diapycnal fluxes of the four halocarbons from the upper thermocline into and out of the mixed layer were determined. These fluxes were low in comparison to the halocarbon sea-to-air fluxes. This indicates that despite the observed maximum concentrations at depth, production in the surface mixed layer is the main oceanic source for all four compounds and one of the main driving factors of their emissions into the atmosphere in the ACT-region. The calculated production rates of the compounds in the mixed layer are 34 ± 65 pmol m−3 h−1 for CHBr3, 10 ± 12 pmol m−3 h−1 for CH2Br2, 21 ± 24 pmol m−3 h−1 for CH3I and 384 ± 318 pmol m−3 h−1 for CH2I2 determined from 13 depth profiles.

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.

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.

A Modeling Study of the Impact of Tropical Instability Waves on the Heat Budget of the Eastern Equatorial Pacific

2006 ◽  
Vol 36 (5) ◽  
pp. 847-865 ◽  
Author(s):  
Christophe E. R. Menkes ◽  
Jérôme G. Vialard ◽  
Sean C. Kennan ◽  
Jean-Philippe Boulanger ◽  
Gurvan V. Madec
Keyword(s):  
Mixed Layer ◽  
Heat Budget ◽  
Vertical Mixing ◽  
Leading Edge ◽  
Cold Tongue ◽  
Instability Waves ◽  
Horizontal Advection ◽  
Vertical Advection ◽  
Tropical Instability Waves ◽  
The Impact

Abstract A numerical simulation is used to investigate the mixed layer heat balance of the tropical Pacific Ocean including the equatorial cold tongue and the region of vortices associated with tropical instability waves (TIWs). The study is motivated by a need to quantify the effects that TIWs have on the climatological heat budget of the cold tongue mixed layer; there has been some discrepancy between observations indicating very large equatorward heat transport by TIWs and models that disagree on the full three-dimensional budget. Validation of the model reveals that the TIW-induced circulation patterns are realistic but may have amplitudes about 15% weaker than those in the observations. The SST budget within tropical instabilities is first examined in a frame of reference moving with the associated tropical instability vortices (TIVs). Zonal advection of temperature anomalies and meridional advection of temperature by current anomalies dominate horizontal advection. These effects strongly heat the cold cusps and slightly cool the downwelling areas located at the leading edge of the vortices. Cooling by vertical mixing is structured at the vortex scale and almost compensates for horizontal advective heating in the cold cusps. In contrast to some previous studies, TIW-induced vertical advection is found to be negligible in the SST budget. Cooling by this term is only significant below the mixed layer. The effect of TIWs on the climatological heat budget is then investigated for the region bounded by 2°S–6°N, 160°–90°W, where instabilities are most active. TIW-induced horizontal advection leads to a warming of 0.84°C month−1, which is of the same order as the 0.77°C month−1 warming effect of atmospheric fluxes, while the mean currents and vertical mixing cool the upper ocean by −0.59°C month−1 and −1.06°C month−1, respectively. The cooling effect of TIW-induced vertical advection is also negligible in the long-term surface layer heat budget and only becomes significant below the mixed layer. The results above, and in particular the absence of cancellation between horizontal and vertical TIW-induced eddy advection, are robust in three other sensitivity experiments involving different mixing parameterizations and increased vertical resolution.

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