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Julie Jakoboski, Robert E. Todd, W. Brechner Owens, Kristopher B. Karnauskas, and Daniel L. Rudnick

Abstract

The Equatorial Undercurrent (EUC) encounters the Galápagos Archipelago on the equator as it flows eastward across the Pacific. The impact of the Galápagos Archipelago on the EUC in the eastern equatorial Pacific remains largely unknown. In this study, the path of the EUC as it reaches the Galápagos Archipelago is measured directly using high-resolution observations obtained by autonomous underwater gliders. Gliders were deployed along three lines that define a closed region with the Galápagos Archipelago as the eastern boundary and 93°W from 2°S to 2°N as the western boundary. Twelve transects were simultaneously occupied along the three lines during 52 days in April–May 2016. Analysis of individual glider transects and average sections along each line show that the EUC splits around the Galápagos Archipelago. Velocity normal to the transects is used to estimate net horizontal volume transport into the volume. Downward integration of the net horizontal transport profile provides an estimate of the time- and areal-averaged vertical velocity profile over the 52-day time period. Local maxima in vertical velocity occur at depths of 25 and 280 m with magnitudes of (1.7 ± 0.6) × 10−5 m s−1 and (8.0 ± 1.6) × 10−5 m s−1, respectively. Volume transport as a function of salinity indicates that water crossing 93°W south (north) of 0.4°S tends to flow around the south (north) side of the Galápagos Archipelago. Comparisons are made between previous observational and modeling studies with differences attributed to effects of the strong 2015/16 El Niño event, the annual cycle of local winds, and varying longitudes between studies of the equatorial Pacific.

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Franz Philip Tuchen, Peter Brandt, Martin Claus, and Rebecca Hummels

Abstract

Besides 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.

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David Nieves and Michael Spall

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We present a simplified theory using reduced-gravity equations for North Atlantic Deep Water (NADW) and its variation driven by high-latitude deep-water formation. The theory approximates layer thickness on the eastern boundary with domain-averaged layer thickness and, in tandem with a mass conservation argument, retains fundamental physics for cross-equatorial flows on interannual and longer forcing time scales. Layer thickness anomalies are driven by a time-dependent northern boundary condition that imposes a southward volume flux representative of a variable source of NADW and damped by diapycnal mixing throughout the basin. Moreover, an outflowing southern boundary condition imposes a southward volume flux that generally differs from the volume flux at the northern boundary, giving rise to temporal storage of NADW within the Atlantic basin. Closed form analytic solutions for the amplitude and phase are provided when the variable source of NADW is sinusoidal. We provide a nondimensional analysis that demonstrates that solution behavior is primarily controlled by two parameters that characterize the meridional extent of the southern basin and the width of the basin relative to the equatorial deformation radius. Similar scaling applied to the time-lagged equations of Johnson and Marshall provides a clear connection to their results. Numerical simulations of reduced-gravity equations agree with analytic predictions in linear, turbulent, and diabatic regimes. The theory introduces a simple analytic framework for studying idealized buoyancy- and wind-driven cross-equatorial flows on interannual and longer time scales.

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Gregory C. Johnson

Abstract

The southern tropical Indian Ocean contains a striking forced annual Rossby wave studied previously using satellite altimeter sea surface height data, surface wind fields, expendable bathythermograph ocean temperature data, and models. Here, the deep reach of this wave and its velocity are analyzed using density–depth profiles and 1000-dbar horizontal drift data from Argo. Significant annual cycles in isopycnal vertical displacements and zonal velocity persist to the deepest pressures to which Argo data can be mapped reliably in the region, 1600–1900 dbar. Phase propagation of the annual cycle of the directly measured zonal velocities at 1000 dbar suggests a zonal wavelength of about 6000 km—about the length of the deep basin in which the wave is found—and a westward phase speed of ~0.2 m s−1. Apparent upward phase propagation in isopycnal vertical displacements suggests energy propagation downward into the abyss. This pattern is clearer when accounting for both the potential and kinetic energy of the wave. The largest zonal current associated with this wave has a middepth maximum that decays rapidly up through the pycnocline and less rapidly with increasing depth, suggesting a first-vertical-mode structure. The anomalous zonal volume transport of this annually reversing current is ~27 × 106 m3 s−1 across 80°E in mid-November. The peak zonal velocity of 0.06 m s−1 implies a maximum zonal excursion of about 600 km associated with the wave over an annual cycle.

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Fan Jia and Lixin Wu

Abstract

The response of the equatorial Pacific SST under CO2 doubling is investigated using Community Atmosphere Model, version 3.1 (CAM3.1)–1.5-layer reduced-gravity ocean (RGO) coupled model. A robust El Niño–like warming pattern is found in the equatorial Pacific. The surface heat budget analyses suggest the El Niño–like pattern results from a weakening of the Walker circulation. In the western equatorial Pacific, all the heat flux components are important to warm the ocean, with the vast majority canceled by entraiment cooling related to increased stratification. In the central-eastern Pacific, the oceanic horizontal advections along with longwave radiation and latent heat flux act to warm the ocean, with entrainment, shortwave radiation, and horizontal diffusion acting as damping terms. An enhanced annual cycle of SST in the equatorial Pacific is also found, which is driven by the ocean dynamical adjustments to changing winds in the eastern ocean.

Although the ocean model used here is a simple reduced-gravity model, the El Niño–like response supports the results of some full ocean–atmosphere general circulation models (GCMs) performed for the World Climate Research Programme (WCRP) Coupled Model Intercomparison Project (CMIP) phase-5, indicating that the CAM3.1–RGO model can be taken as a useful and efficient tool to study equatorial Pacific response under changing climate.

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Motoki Nagura, Yukio Masumoto, and Takanori Horii

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This study examines heat advection due to mixed Rossby gravity waves in the equatorial Indian Ocean using moored buoy observations at (0°, 80.5°E) and (0°, 90°E) and an ocean general circulation model (OGCM) output. Variability associated with mixed Rossby gravity waves is defined as that at periods of 10–30 days, where both observations and the OGCM results show high energy in meridional velocity and meridional gradient of temperature. The 10–30-day variability in meridional velocity causes convergence of heat flux onto the equator, the net effect of which amounts to 2.5°C month−1 warming at the depth of the thermocline. Detailed analysis shows that the wave structure manifested in temperature and velocity is tilted in the xz plane, which causes the phase lag between meridional velocity and meridional temperature gradient to be a half cycle on the equator and results in sizable thermocline warming. An experiment with a linear continuously stratified model shows that the contributions of many baroclinic modes, and the right zonal wavelength of wind forcing, are essential in generating the correct wave structure. It is also shown that contributions of mixed Rossby gravity waves to cross-equatorial heat transport are negligible, as temperature variability associated with this wave mode has a node on the equator.

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Madeleine K. Youngs and Gregory C. Johnson

Abstract

Equatorial deep jets (EDJs) are equatorially trapped, stacked, zonal currents that reverse direction every few hundred meters in depth throughout much of the water column. This study evaluates their structure observationally in all three oceans using new high-vertical-resolution Argo float conductivity–temperature–depth (CTD) instrument profiles from 2010 to 2014 augmented with historical shipboard CTD data from 1972 to 2014 and lower-vertical-resolution Argo float profiles from 2007 to 2014. The vertical strain of density is calculated from the profiles and analyzed in a stretched vertical coordinate system determined from the mean vertical density structure. The power spectra of vertical strain in each basin are analyzed using wavelet decomposition. In the Indian and Pacific Oceans, there are two distinct peaks in the power spectra, one Kelvin wave–like and the other entirely consistent with the dispersion relation of a linear, first meridional mode, equatorial Rossby wave. In the Atlantic Ocean, the first meridional mode Rossby wave signature is very strong and dominates. In all three ocean basins, Rossby wave–like signatures are coherent across the basin width and appear to have wavelengths the scale of the basin width, with periods of about 5 yr in the Indian and Atlantic Oceans and about 12 yr in the Pacific Ocean. Their observed meridional scales are about 1.5 times the linear theoretical values. Their phase propagation is downward with time, implying upward energy propagation if linear wave dynamics hold.

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Wilco Hazeleger, Richard Seager, Mark A. Cane, and Naomi H. Naik

1. Introduction The excess of net incoming radiation at the top of the atmosphere in the Tropics and the deficit of net radiation in the extratropics requires poleward energy transport in the atmosphere and oceans to mediate the radiative imbalance. Recent estimates, based on satellite-derived top of the atmosphere radiation and atmospheric reanalysis data, show that the ocean dominates the energy transport in the Tropics ( Trenberth and Caron 2001 ). Here we will focus on the part of the ocean

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Alison J. McLaren and Richard G. Williams

to 20 W m −2 over the subtropical gyre ( Fig. 5b ). The buoyancy input for subduction, H * sub , is negative over much of the North Atlantic with positive values confined to the subtropical gyre and Tropics ( Fig. 5c ). The crucial result is that the buoyancy input driving gyre-scale subduction over the North Atlantic (as depicted in Fig. 1b ) is provided by the Ekman redistribution of buoyancy ( Fig. 5b ) rather than the atmospheric input ( Fig. 5a ). These buoyancy diagnostics are broadly

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Lewis M. Rothstein, Rong-Hua Zhang, Antonio J. Busalacchi, and Dake Chen

1. Introduction Although the ocean circulations in the subtropics and Tropics have been investigated separately by many (e.g., Pedlosky 1990 ; Philander 1990 ), it was not until recently that their interactions have received significant attention ( Pedlosky 1991b ; McCreary and Yu 1992 ; McCreary and Lu 1994 ; Liu 1994 ; Liu et al. 1994 ; Lu and McCreary 1995 ; Pedlosky 1997 ). Subtropical/tropical water pathways are fundamentally defined by the basin-scale structure of the thermocline

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