Search Results

You are looking at 1 - 10 of 11 items for

  • Author or Editor: Eric S. Johnson x
  • Refine by Access: All Content x
Clear All Modify Search
Douglas S. Luther and Eric S. Johnson

Abstract

Eddy energetics in the central equatorial Pacific Ocean is examined using Acoustic Doppler Current Profiler velocities and CTD densities collected during the Hawaii-to-Tahiti Shuttle Experiment, in 1979–80. Three distinct sources of eddy energy are identified with varying degrees of statistical reliability, and are interpreted as evidence for three separate instabilities of the mean flow field. An instability at and just north of the equator occurs primarily in boreal summer and fall. It arises from the cyclonic shear between the Equatorial Undercurrent and the South Equatorial Current (SEC) north of the equator. The instability is present only when and where both currents are well developed, and there is little involvement of the shear between the SEC and the North Equatorial Countercurrent (NECC). The instability is characterized by local maxima in zonal and meridional eddy velocity variance, strong U*V* Reynolds stress, and large mean flow to eddy kinetic energy conversion. Despite seasonal variability of the eddy kinetic energy production, no annual cycle energy is converted to eddy energy. A second instability occurs at the equatorial front at 3°N to 6°N, primarily during boreal winter. The instability is identified by large mean-to-eddy potential energy conversion. Finally, a third instability is evidenced by strong downgradient (northward) eddy heat flux and large mean flow to eddy potential energy conversion, in the thermocline of the NECC during boreal spring. Both features are confined below 60 m at 5°N–9°N. While the eddies gain potential energy from these last two instabilities, they are losing kinetic energy to the mean flow at a somewhat slower rate.

Nonlinear advection appears to be unimportant in the total eddy energy balance, but the meridional diffusion of eddy energy represented by the meridional divergence of eddy pressure work is large and significant The latter redistributes eddy energy into (not out of) the region of the barotropic instability just north of the equator.

Full access
Eric S. Johnson and Jeffrey A. Proehl

Abstract

Shipboard acoustic Doppler current profiler (ADCP)-derived zonal currents from 170° to 110°W are assembled into composite seasonal and ENSO cycles to produce detailed representations of large-scale ocean flow regimes that favor tropical instability waves (TIWs). The instability-favorable portion of these cycles, namely, the August– October period of the seasonal cycle and the pre-December period of the ENSO cold phase, both have intense westward flow in the South Equatorial Current, most particularly the branch north of the equator (SECN), and strengthened eastward flows in the North Equatorial Countercurrent (NECC) and the Equatorial Undercurrent (EUC). Taken together these flows enhance current shear in the two regions generally associated with TIW activity, namely, the cyclonic and anticyclonic shear regions located to the south and north of the SECN, respectively. Direct correlation of ADCP currents and CTD densities to an instability index derived from equatorial 13–30-day meridional velocities confirms the importance of the strengths of the SECN and NECC in determining the timing of TIW events. Very little correlation was found in the EUC, implying that its strength is not a determining factor in such timing. Reynolds stress and density flux calculations indicate that in a time-averaged sense TIWs derive energy from both the cyclonic and anticyclonic flanks of the SECN, and from both sides of the equatorial cold tongue. During low-instability periods these Reynolds stresses and fluxes substantially vanish, indicating that eddy energy production ceases. This is in marked contrast to Baturin and Niller's study, which indicated that eddy energy production was relatively continuous at 110°W. The current structures of individual months associated with TIW activity show substantial variability among themselves. Combined with previous findings of multiple modes of instabilities, this indicates that caution is required when attempting to model instabilities from averages of observed background flows such as those presented here.

Full access
Eric S. Johnson and Michael J. Mc Phaden

Abstract

Previous studies have shown that intraseasonal Kelvin waves are a prominent mode of variability in the eastern and central equatorial Pacific. These waves appear to be remotely forced by wind variations in the western Pacific and propagate eastward at approximately first baroclinic mode phase speeds. In this study, moored temperature and velocity time series data from 1984 to 1987 between 110° and 140°W are studied to isolate the Kelvin wave structures and to document their wave-mean flow interactions with the South Equatorial Current/Equatorial Undercurrent (RUC) system. Observed structures at periods of 59–125 days have rms amplitudes of about 10 cm s−1 for zonal velocity and about 1°C for temperature in the thermocline. These structures exhibit significant departures from linear no-mean-flow theory: zonal velocity is intensified below the EUC core, and temperature variability in the thermocline is weaker on the equator than 1.5° off the equator. Additionally, the wave temperature signal at the surface is nonzero and lags both zonal velocity and deeper temperatures by about 90°. These observed structures are compared with those from a linearized wave-mean flow interaction channel model, with the model mean flow specified to match the observed mean flow as closely as possible. The model qualitatively reproduces both the intensification of wave zonal velocity below the EUC and the weakening of temperature variability an the equator where the mean stratification is weaker. The model cannot reproduce the observed SST signal since it is zonally invariant in its basic state, and hence, perturbation SSTs are zero. However, an estimate of temperature advection due to wave zonal velocities advecting the observed sea surface temperature gradients accounts for the phase and the O (0.1°C) rms amplitude of observed SST variations at intraseasonal periods.

Full access
Eric S. Johnson and Bruce A. Warren

Abstract

A model for the structure of deep western boundary currents, based on linear momentum equations and lateral mixing of density, is applied to data from a recent section across the Ninetyeast Ridge current in the eastern basin of the Indian Ocean. It accounts moderately well for the breadth of the current, as inferred from the density distribution, and for the form of its velocity profile, as inferred (roughly) from the silica distribution.

Full access
Eric S. Johnson, Lloyd A. Regier, and Robert A. Knox

Abstract

Continuous velocity measurements from a shipboard Doppler acoustic long on the NORPAX shuttle experiment in the central equatorial Pacific are presented. The time mean of these velocities shows the classical zonal equatorial currents as well as their meridional circulation. The velocities are used with concurrent CTD data to examine the geostrophic balance of zonal currents in the upper 117 m. Estimates of the errors of the acoustic data are produced from a comparison between that data and concurrent profiling current-meter data, and are used to establish the reliability of the balances observed. Both the time mean and the time varying balances are investigated, as well as the departures from geostrophic balance. The mean zonal velocities between 4°S and 10°N are found to be in approximate geostrophic balance. Departures from geostrophy in the mean are observed near the surface at the equator. The meridional advection of meridional momentum appears to be only partly responsible for this departure. The time varying flow was partly geostrophic poleward of 1°, but not so equatorward. A large excess of geostrophic velocity variance (relative to observed velocity variance) exists near the equator, probably due to high frequency internal wave signals in the density data. North of 4°N an excess of observed velocity variance was found, due probably to near-internal waves. In both latitude bands the fluctuating departures from geostrophy are probably to near-inertial waves. In both latitude bands the fluctuating departures from geostrophy are probably balanced by meridional acceleration. More than sufficient acceleration exists near the equator to account for the observed imbalance, while in the North Equatorial Countercurrent the acceleration is barely sufficient, implying a close balance between acceleration and nongeostrophic pressure gradients.

Full access
Robert W. Helber, Robert H. Weisberg, Fabrice Bonjean, Eric S. Johnson, and Gary S. E. Lagerloef

Abstract

The relationships between tropical Atlantic Ocean surface currents and horizontal (mass) divergence, sea surface temperature (SST), and winds on monthly-to-annual time scales are described for the time period from 1993 through 2003. Surface horizontal mass divergence (upwelling) is calculated using surface currents estimated from satellite sea surface height, surface vector wind, and SST data with a quasi-linear, steady-state model. Geostrophic and Ekman dynamical contributions are considered. The satellite-derived surface currents match climatological drifter and ship-drift currents well, and divergence patterns are consistent with the annual north–south movement of the intertropical convergence zone (ITCZ) and equatorial cold tongue evolution. While the zonal velocity component is strongest, the meridional velocity component controls divergence along the equator and to the north beneath the ITCZ. Zonal velocity divergence is weaker but nonnegligible. Along the equator, a strong divergence (upwelling) season in the central/eastern equatorial Atlantic peaks in May while equatorial SST is cooling within the cold tongue. In addition, a secondary weaker and shorter equatorial divergence occurs in November also coincident with a slight SST cooling. The vertical transport at 30-m depth, averaged across the equatorial Atlantic Ocean between 2°S and 2°N for the record length, is 15(±6) × 106 m3 s−1. Results are consistent with what is known about equatorial upwelling and cold tongue evolution and establish a new method for observing the tropical upper ocean relative to geostrophic and Ekman dynamics at spatial and temporal coverage characteristic of satellite-based observations.

Full access
Eric S. Johnson, Fabrice Bonjean, Gary S. E. Lagerloef, John T. Gunn, and Gary T. Mitchum

Abstract

Comparisons of OSCAR satellite-derived sea surface currents with in situ data from moored current meters, drifters, and shipboard current profilers indicate that OSCAR presently provides accurate time means of zonal and meridional currents, and in the near-equatorial region reasonably accurate time variability (correlation = 0.5–0.8) of zonal currents at periods as short as 40 days and meridional wavelengths as short as 8°. At latitudes higher than 10° the zonal current correlation remains respectable, but OSCAR amplitudes diminish unrealistically. Variability of meridional currents is poorly reproduced, with severely diminished amplitudes and reduced correlations relative to those for zonal velocity on the equator. OSCAR’s RMS differences from drifter velocities are very similar to those experienced by the ECCO (Estimating the Circulation and Climate of the Ocean) data-assimilating models, but OSCAR generally provides a larger ocean-correlated signal, which enhances its ratio of estimated signal over noise. Several opportunities exist for modest improvements in OSCAR fidelity even with presently available datasets.

Full access
Sharon Stammerjohn, Ted A. Scambos, Susheel Adusumilli, Sandra Barreira, Germar H. Bernhard, Deniz Bozkurt, Seth M. Bushinsky, Kyle R. Clem, Steve Colwell, Lawrence Coy, Jos De Laat, Marcel D. du Plessis, Ryan L. Fogt, Annie Foppert, Helen Amanda Fricker, Alex S. Gardner, Sarah T. Gille, Tessa Gorte, Bryan Johnson, Eric Keenan, Daemon Kennett, Linda M. Keller, Natalya A. Kramarova, Kaisa Lakkala, Matthew A. Lazzara, Jan T. M. Lenaerts, Jan L. Lieser, Zhi Li, Hongxing Liu, Craig S. Long, Michael MacFerrin, Michelle L. Maclennan, Robert A. Massom, David Mikolajczyk, Lynn Montgomery, Thomas L. Mote, Eric R. Nash, Paul A. Newman, Irina Petropavlovskikh, Michael Pitts, Phillip Reid, Steven R. Rintoul, Michelle L. Santee, Elizabeth H. Shadwick, Alessandro Silvano, Scott Stierle, Susan Strahan, Adrienne J. Sutton, Sebastiaan Swart, Veronica Tamsitt, Bronte Tilbrook, Lei Wang, Nancy L. Williams, and Xiaojun Yuan
Full access
E. Povl Abrahamsen, Sandra Barreira, Cecilia M. Bitz, Amy Butler, Kyle R. Clem, Steve Colwell, Lawrence Coy, Jos de Laat, Marcel D. du Plessis, Ryan L. Fogt, Helen Amanda Fricker, John Fyfe, Alex S. Gardner, Sarah T. Gille, Tessa Gorte, L. Gregor, Will Hobbs, Bryan Johnson, Eric Keenan, Linda M. Keller, Natalya A. Kramarova, Matthew A. Lazzara, Jan T. M. Lenaerts, Jan L. Lieser, Hongxing Liu, Craig S. Long, Michelle Maclennan, Robert A. Massom, François Massonnet, Matthew R. Mazloff, David Mikolajczyk, A. Narayanan, Eric R. Nash, Paul A. Newman, Irina Petropavlovskikh, Michael Pitts, Bastien Y. Queste, Phillip Reid, F. Roquet, Michelle L. Santee, Susan Strahan, Sebastiann Swart, and Lei Wang
Free access
Molly Baringer, Mariana B. Bif, Tim Boyer, Seth M. Bushinsky, Brendan R. Carter, Ivona Cetinić, Don P. Chambers, Lijing Cheng, Sanai Chiba, Minhan Dai, Catia M. Domingues, Shenfu Dong, Andrea J. Fassbender, Richard A. Feely, Eleanor Frajka-Williams, Bryan A. Franz, John Gilson, Gustavo Goni, Benjamin D. Hamlington, Zeng-Zhen Hu, Boyin Huang, Masayoshi Ishii, Svetlana Jevrejeva, William E. Johns, Gregory C. Johnson, Kenneth S. Johnson, John Kennedy, Marion Kersalé, Rachel E. Killick, Peter Landschützer, Matthias Lankhorst, Tong Lee, Eric Leuliette, Feili Li, Eric Lindstrom, Ricardo Locarnini, Susan Lozier, John M. Lyman, John J. Marra, Christopher S. Meinen, Mark A. Merrifield, Gary T. Mitchum, Ben Moat, Didier Monselesan, R. Steven Nerem, Renellys C. Perez, Sarah G. Purkey, Darren Rayner, James Reagan, Nicholas Rome, Alejandra Sanchez-Franks, Claudia Schmid, Joel P. Scott, Uwe Send, David A. Siegel, David A. Smeed, Sabrina Speich, Paul W. Stackhouse Jr., William Sweet, Yuichiro Takeshita, Philip R. Thompson, Joaquin A. Triñanes, Martin Visbeck, Denis L. Volkov, Rik Wanninkhof, Robert A. Weller, Toby K. Westberry, Matthew J. Widlansky, Susan E. Wijffels, Anne C. Wilber, Lisan Yu, Weidong Yu, and Huai-Min Zhang
Free access