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Janna Köhler, Georg S. Völker, and Maren Walter

Abstract

In the tropical North Atlantic, mean winds introduce relatively little energy into the internal wave field, but hurricanes act as very energetic sources for near-inertial waves. In addition to the eventlike passage of such tropical cyclones, changes in the wind speed north of the trade wind system induce a seasonal cycle in surface swell, with potential implications for the high-frequency part of the internal wave field. Using a 5-yr mooring time series in the interior of the tropical North Atlantic at 16°N, the temporal variability of internal wave energy south of the main hurricane track in different frequency bands is studied, and the magnitude of its variability, along with possible energy transfer mechanisms, is analyzed. The results show that changes in near-inertial energy are dominated by the passage of internal waves generated by hurricanes centered several hundred kilometers north of the mooring. The major role of hurricanes in the generation of near-inertial waves is also seen in an extended slab model that takes the horizontal divergence of the near-inertial current field at the mixed layer base into account. A seasonal cycle is observed in the energy at the high-frequency end (frequencies above 6 cpd) of the internal wave spectrum. It is not in phase with the near-inertial energy variability but covaries with changes in the local surface waves. These high-frequency internal waves are most energetic at times when large-amplitude surface swell with long periods and correspondingly long wavelengths is observed.

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Edward D. Zaron and James N. Moum

Abstract

A reexamination of turbulence dissipation measurements from the equatorial Pacific shows that the turbulence diffusivities are not a simple function of the gradient Richardson number. A widely used mixing scheme, the K-profile parameterization, overpredicts the turbulent vertical heat flux by roughly a factor of 4 in the stably stratified region between the surface mixed layer and the Equatorial Undercurrent (EUC). Additionally, the heat flux divergence is of the incorrect sign in the upper 80 m. An alternative class of parameterizations is examined that expresses the mixing coefficients in terms of the large-scale kinetic energy, shear, and Richardson number. These representations collapse the turbulence diffusivities above and below the Equatorial Undercurrent, and a tuned version is able to reproduce the vertical turbulence heat flux within the 50–180-m depth range. Kinetic energy is not Galilean invariant, so the collapse of the data with the new parameterization suggests that oceanic turbulence responds to boundary forcing at depths well below the surface mixed layer.

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Tomomichi Ogata, Motoki Nagura, and Yukio Masumoto

Abstract

A possible formation mechanism of mean subsurface upwelling along the equator in the Indian Ocean is investigated using a series of hierarchical ocean general circulation model (OGCM) integrations and analytical considerations. In an eddy-resolving OGCM with realistic forcing, mean vertical velocity in the tropical Indian Ocean shows rather strong upwelling, with its maximum on the equator in the subsurface layer below the thermocline. Heat budget analysis exhibits that horizontal and vertical heat advection by deviations (i.e., due to deviations of velocity and temperature from the mean) balances with vertical advection caused by mean equatorial upwelling. Horizontal heat advection is mostly associated with intraseasonal variability with periods of 3–91 days, while contributions from longer periods (>91 days) are small. Sensitivity experiments with a coarse-resolution OGCM further demonstrate that such mean equatorial upwelling cannot be reproduced by seasonal forcing only. Adding the intraseasonal wind forcing, especially meridional wind variability with a period of 15 days, generates significant mean subsurface upwelling on the equator. Further experiments with idealized settings confirm the importance of intraseasonal mixed Rossby–gravity (MRG) waves to generate mean upwelling, which appears along the energy “beam” of the MRG wave. An analytical solution of the MRG waves indicates that wave-induced temperature advection caused by the MRG waves with upward (downward) phase propagation results in warming (cooling) on the equator. This wave-induced warming (cooling) is shown to balance with the mean equatorial upwelling (downwelling), which is consistent with simulated characteristics in the OGCM experiments.

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Pierre Dutrieux, Christophe E. Menkes, Jerome Vialard, Pierre Flament, and Bruno Blanke

Abstract

Tropical instability waves and tropical instability vortices (TIVs) exert major controls on ocean dynamics, thermodynamics, and biology on intraseasonal to seasonal time scales. To understand the fundamental mechanisms at play, a Lagrangian analysis of the 3D circulation of westward-propagating TIVs was performed in a high-resolution Atlantic Ocean simulation. The model reproduces the main temperature and velocity features of the tropical Atlantic mean state and the TIVs. Lagrangian diagnostics were used to track the water masses transported in vortices and exchanged with surrounding waters. The 3D circulation within vortices is consistent with previous observations and dominated by anticyclonic rotation with downwelling and upwelling near the leading and trailing edges of the vortex, respectively. This convergent flow creates sharp gradients at the TIV southwestern edge, where vertical mixing is most efficient. While TIVs remain highly dynamically coherent throughout their lifetime, significant exchanges occur with their surroundings, with 50% of their water being renewed over one rotation cycle. A detailed investigation of the eddies’ sources and sinks reveals that they mostly transport southern water zonally, while northern waters are mostly passing through or fluxed southward in their lee. A notable source of entrained water is the Equatorial Undercurrent.

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Yalin Fan, Isaac Ginis, and Tetsu Hara

Abstract

In coupled ocean–atmosphere models, it is usually assumed that the momentum flux into ocean currents is equal to the flux from air (wind stress). However, when the surface wave field grows (decays) in space or time, it gains (loses) momentum and reduces (increases) the momentum flux into subsurface currents compared to the flux from the wind. In particular, under tropical cyclone (TC) conditions the surface wave field is complex and fast varying in space and time and may significantly affect the momentum flux from wind into ocean. In this paper, numerical experiments are performed to investigate the momentum flux budget across the air–sea interface under both uniform and idealized TC winds. The wave fields are simulated using the WAVEWATCH III model. The difference between the momentum flux from wind and the flux into currents is estimated using an air–sea momentum flux budget model. In many of the experiments, the momentum flux into currents is significantly reduced relative to the flux from the wind. The percentage of this reduction depends on the choice of the drag coefficient parameterization and can be as large as 25%. For the TC cases, the reduction is mainly in the right-rear quadrant of the hurricane, and the percentage of the flux reduction is insensitive to the changes of the storm size and the asymmetry in the wind field but varies with the TC translation speed and the storm intensity. The results of this study suggest that it is important to explicitly resolve the effect of surface waves for accurate estimations of the momentum flux into currents under TCs.

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Martin Claus, Richard J. Greatbatch, Peter Brandt, and John M. Toole

Abstract

The equatorial deep jets (EDJs) are a ubiquitous feature of the equatorial oceans; in the Atlantic Ocean, they are the dominant mode of interannual variability of the zonal flow at intermediate depth. On the basis of more than 10 years of moored observations of zonal velocity at 23°W, the vertically propagating EDJs are best described as superimposed oscillations of the 13th to the 23rd baroclinic modes with a dominant oscillation period for all modes of 1650 days. This period is close to the resonance period of the respective gravest equatorial basin mode for the dominant vertical modes 16 and 17. It is argued that since the equatorial basin mode is composed of linear equatorial waves, a linear reduced-gravity model can be employed for each baroclinic mode, driven by spatially homogeneous zonal forcing oscillating with the EDJ period. The fit of the model solutions to observations at 23°W yields a basinwide reconstruction of the EDJs and the associated vertical structure of their forcing. From the resulting vertical profile of mean power input and vertical energy flux on the equator, it follows that the EDJs are locally maintained over a considerable depth range, from 500 to 2500 m, with the maximum power input and vertical energy flux at 1300 m. The strong dissipation closely ties the apparent vertical propagation of energy to the vertical distribution of power input and, together with the EDJs’ prevailing downward phase propagation, requires the phase of the forcing of the EDJs to propagate downward.

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Magdalena Andres, Ruth C. Musgrave, Daniel L. Rudnick, Kristin L. Zeiden, Thomas Peacock, and Jae-Hun Park

Abstract

As part of the Flow Encountering Abrupt Topography (FLEAT) program, an array of pressure-sensor equipped inverted echo sounders (PIESs) was deployed north of Palau where the westward-flowing North Equatorial Current encounters the southern end of the Kyushu–Palau Ridge in the tropical North Pacific. Capitalizing on concurrent observations from satellite altimetry, FLEAT Spray gliders, and shipboard hydrography, the PIESs’ 10-month duration hourly bottom pressure p and round-trip acoustic travel time τ records are used to examine the magnitude and predictability of sea level and pycnocline depth changes and to track signal propagations through the array. Sea level and pycnocline depth are found to vary in response to a range of ocean processes, with their magnitude and predictability strongly process dependent. Signals characterized here comprise the barotropic tides, semidiurnal and diurnal internal tides, southeastward-propagating superinertial waves, westward-propagating mesoscale eddies, and a strong signature of sea level increase and pycnocline deepening associated with the region’s relaxation from El Niño to La Niña conditions. The presence of a broad band of superinertial waves just above the inertial frequency was unexpected and the FLEAT observations and output from a numerical model suggest that these waves detected near Palau are forced by remote winds east of the Philippines. The PIES-based estimates of pycnocline displacement are found to have large uncertainties relative to overall variability in pycnocline depth, as localized deep current variations arising from interactions of the large-scale currents with the abrupt topography around Palau have significant travel time variability.

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Julien Jouanno, Frédéric Marin, Yves du Penhoat, and Jean-Marc Molines

Abstract

A regional numerical model of the tropical Atlantic Ocean and observations are analyzed to investigate the intraseasonal fluctuations of the sea surface temperature at the equator in the Gulf of Guinea. Results indicate that the seasonal cooling in this region is significantly shaped by short-duration cooling events caused by wind-forced equatorial waves: mixed Rossby–gravity waves within the 12–20-day period band, inertia–gravity waves with periods below 11 days, and equatorially trapped Kelvin waves with periods between 25 and 40 days. In these different ranges of frequencies, it is shown that the wave-induced horizontal oscillations of the northern front of the mean cold tongue dominate the variations of mixed layer temperature near the equator. But the model mixed layer heat budget also shows that the equatorial waves make a significant contribution to the mixed layer heat budget through modulation of the turbulent cooling, especially above the core of the Equatorial Undercurrent (EUC). The turbulent cooling variability is found to be mainly controlled by the intraseasonal modulation of the vertical shear in the upper ocean. This mechanism is maximum during periods of seasonal cooling, especially in boreal summer, when the surface South Equatorial Current is strongest and between 2°S and the equator, where the presence of the EUC provides a background vertical shear in the upper ocean. It applies for the three types of intraseasonal waves. Inertia–gravity waves also modulate the turbulent heat flux at the equator through vertical displacement of the core of the EUC in response to equatorial divergence and convergence.

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Kristopher B. Karnauskas, Raghu Murtugudde, and Antonio J. Busalacchi

Abstract

Although sustained observations yield a description of the mean equatorial current system from the western Pacific to the eastern terminus of the Tropical Atmosphere Ocean (TAO) array, a comprehensive observational dataset suitable for describing the structure and pathways of the Equatorial Undercurrent (EUC) east of 95°W does not exist and therefore climate models are unconstrained in a region that plays a critical role in ocean–atmosphere coupling. Furthermore, ocean models suggest that the interaction between the EUC and the Galápagos Islands (∼92°W) has a striking effect on the basic state and coupled variability of the tropical Pacific. To this end, the authors interpret historical measurements beginning with those made in conjunction with the discovery of the Pacific EUC in the 1950s, analyze velocity measurements from an equatorial TAO mooring at 85°W, and analyze a new dataset from archived shipboard ADCP measurements. Together, the observations yield a possible composite description of the EUC structure and pathways in the eastern equatorial Pacific that may be useful for model validation and guiding future observation.

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Sophie Cravatte, William S. Kessler, and Frédéric Marin

Abstract

Argo float data in the tropical Pacific Ocean during January 2003–August 2011 are analyzed to obtain Lagrangian subsurface velocities at their parking depths. Maps of mean zonal velocities at 1000 and 1500 m are presented. At both depths, a series of alternating westward and eastward zonal jets with a meridional scale of 1.5° is seen at the basin scale from 10°S to 10°N. These alternating jets, with mean speeds about 5 cm s−1, are clearly present in the western and central parts of the basin but weaken and disappear approaching the eastern coast. They are stronger in the Southern Hemisphere. Along the equator at both 1000 and 1500 m, a westward jet is seen. The jets closer to the equator are remarkably zonally coherent across the basin, but the jets farther poleward appear broken in several segments. In the western half of the basin, the 1000-m zonal jets appear to slant slightly poleward from east to west. At the western boundary in the south (east of Solomon Islands and Papua New Guinea), the alternating jets appear to connect in narrow boundary currents. Seasonal zonal velocity anomalies at 1000 and 1500 m are observed to propagate westward across the basin; they are consistent with annual vertically propagating Rossby waves superimposed on the mean zonal jets. Their meridional structure suggests that more than one meridional mode is present.

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