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Chen, R, Thompson A, Flierl G.  2016.  Time-dependent eddy-mean energy diagrams and their application to the ocean. Journal of Physical Oceanography. 46(9):2827-2850.  
Chen, R, Flierl G.  2015.  The contribution of striations to the eddy energy budget and mixing: diagnostic frameworks, and results in a quasi-geostrophic barotropic system with mean flow. Journal of Physical Oceanography. 45(8):2095-2113.   10.1175/JPO-D-14-0199.1   Abstract

Low-frequency oceanic motions have banded structures termed “striations.” Since these striations embedded in large-scale gyre flows can have large amplitudes, the authors investigated the effect of mean flow on their directions as well as their contribution to energetics and mixing using a β-plane, barotropic, quasigeostrophic ocean model. In spite of the model simplicity, striations are always found to exist regardless of the imposed barotropic mean flow. However, their properties are sensitive to the mean flow. Rhines jets move with the mean flow and are not necessarily striations. If the meridional component of the mean flow is large, Rhines jets become high-frequency motions; low-frequency striations still exist, but they are nonzonal, have small magnitudes, and contribute little to energetics and mixing. Otherwise, striations are zonal, dominated by Rhines jets, and contribute significantly to energetics and mixing. This study extends the theory of β-plane, barotropic turbulence, driven by white noise forcing at small scales, to include the effect of a constant mean flow. Theories developed in this study, based upon the Galilean invariance property, illustrate that the barotropic mean flow has no effect on total mixing rates, but does affect the energy cascades in the frequency domain. Diagnostic frameworks developed here can be useful to quantify the striations’ contribution to energetics and mixing in the ocean and more realistic models. A novel diagnostic formula is applied to estimating eddy diffusivities.

Chen, R, Gille S, McClean J, Flierl G, Griesel A.  2015.  A Multiwavenumber Theory for Eddy Diffusivities and Its Application to the Southeast Pacific (DIMES) Region.. Journal of Physical Oceanography. 45:1877–1896.   10.1175/JPO-D-14-0229.1   Abstract

A multiwavenumber theory is formulated to represent eddy diffusivities. It expands on earlier single-wavenumber theories and includes the wide range of wavenumbers encompassed in eddy motions. In the limiting case in which ocean eddies are only composed of a single wavenumber, the multiwavenumber theory is equivalent to the single-wavenumber theory and both show mixing suppression by the eddy propagation relative to the mean flow. The multiwavenumber theory was tested in a region of the Southern Ocean (70°–45°S, 110°–20°W) that covers the Drake Passage and includes the tracer/float release locations during the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). Cross-stream eddy diffusivities and mixing lengths were estimated in this region from the single-wavenumber theory, from the multiwavenumber theory, and from floats deployed in a global ° Parallel Ocean Program (POP) simulation. Compared to the single-wavenumber theory, the horizontal structures of cross-stream mixing lengths from the multiwavenumber theory agree better with the simulated float-based estimates at almost all depth levels. The multiwavenumber theory better represents the vertical structure of cross-stream mixing lengths both inside and outside the Antarctica Circumpolar Current (ACC). Both the single-wavenumber and multiwavenumber theories represent the horizontal structures of cross-stream diffusivities, which resemble the eddy kinetic energy patterns.

Chen, R, Flierl G, Wunsch C.  2015.  Quantifying and interpreting striations in a subtropical gyre: a spectral perspective. Journal of Physical Oceanography. 45(2):387–406.   10.1175/JPO-D-14-0038.1   AbstractWebsite

The amplitude, origin and direction of striations in the subtropical gyre are investigated using simulated and analytical multi-dimensional spectra. Striations, defined as banded structures in the low-frequency motions, account for a noticeable percentage of zonal velocity variability in the East North Pacific (ENP: 25°N – 42°N, 150°W – 130°W) and Central North Pacific (CNP: 10°N – 22°N, 132°E – 162°W) regions in an eddying global ocean model. Thus, they likely are non-negligible in mixing and transport processes. Striations in the ENP region are non-zonal and are embedded in the non-zonal gyre flow; whereas striations in the CNP region are more zonal, as are the mean gyre flows. An idealized 1.5-layer model shows the gyre flow partially determines their directions, which qualitatively resemble those in the global eddying model. In the linear limit, structures are quasi-stationary (frequency ω → 0) linear Rossby waves and the gyre flow influences the direction by influencing the nature of the zero Rossby wave frequency curve. In the nonlinear regime, striations are consistent with the non-dispersively propagating eddies, whose low-frequency component has banded structures. The gyre flow influences the striation direction by changing the eddy-propagation direction. Their origin in the nonlinear regime is consistent with the existence of a non-dispersive line in the frequency-wavenumber spectra. We do not exclude other striation mechanisms from literature, considering that the interpretations here are based on an idealized model and only from a spectral perspective.

Chen, R, Flierl G, Wunsch C.  2014.  A description of local and nonlocal eddy-mean flow interaction from an eddying state estimate. Journal of Physical Oceanography. 44:2336-2352.   10.1175/JPO-D-14-0009.1   AbstractWebsite

The assumption that local baroclinic instability dominates eddy–mean flow interactions is tested on a global scale using a dynamically consistent eddy-permitting state estimate. Interactions are divided into local and nonlocal. If all the energy released from the mean flow through eddy–mean flow interaction is used to support eddy growth in the same region, or if all the energy released from eddies through eddy–mean flow interaction is used to feed back to the mean flow in the same region, eddy–mean flow interaction is local; otherwise, it is nonlocal. Different regions have different characters: in the subtropical region studied in detail, interactions are dominantly local. In the Southern Ocean and Kuroshio and Gulf Stream Extension regions, they are mainly nonlocal. Geographical variability of dominant eddy–eddy and eddy–mean flow processes is a dominant factor in understanding ocean energetics.

Chen, R, McClean J, Gille S, Griesel A.  2014.  Isopycnal eddy diffusivities and critical layers in the Kuroshio Extension from an eddying ocean model. Journal of Physical Oceanography. 44:2191–2211.   10.1175/JPO-D-13-0258.1   AbstractWebsite

High spatial resolution isopycnal diffusivities are estimated in the Kuroshio Extension (KE) region (28°–40°N, 120°–190°E) from a global ° Parallel Ocean Program (POP) simulation. The numerical float tracks are binned using a clustering approach. The number of tracks in each bin is thus roughly the same leading to diffusivity estimates that converge better than those in bins defined by a regular geographic grid. Cross-stream diffusivities are elevated in the southern recirculation gyre region, near topographic obstacles and downstream in the KE jet, where the flow has weakened. Along-stream diffusivities, which are much larger than cross-stream diffusivities, correlate well with the magnitudes of eddy velocity. The KE jet suppresses cross-stream mixing only in some longitude ranges. This study estimates the critical layer depth both from linear local baroclinic instability analysis and from eddy phase speeds in the POP model using the Radon transform. The latter is a better predictor of large mixing length in the cross-stream direction. Critical layer theory is most applicable in the intense jet regions away from topography.

Chen, R.  2013.  Energy pathways and structures of oceanic eddies from the ECCO2 state estimate and simpli ed models. ( Wunsch C, Flierl G, Eds.).: Massachusetts Institute of Technology and Woods Hole Oceanographic Institution Joint Program Abstract

Studying oceanic eddies is important for understanding and predicting ocean circulation and climate variability. The central focus of this dissertation is the energy exchange between eddies and mean flow and banded structures in the low-frequency component of the eddy field. A combination of a realistic eddy-permitting ocean state estimate and simplified theoretical models is used to address the following specific questions. (1) What are the major spatial characteristics of eddy-mean flow interaction from an energy perspective? Is eddy-mean flow interaction a local process in most ocean regions? (2) The banded structures in the low-frequency eddy field are termed striations. How much oceanic variability is associated with striations? How does the time-mean circulation, for example a subtropical gyre or constant mean flow, influence the origin and characteristics of striations? How much do striations contribute to the energy budget and tracer mixing?

Chen, R, Liu Q, Hu H.  2009.  Analysis of formation mechanism about SST anomaly persistence during late winter in the North Pacific. Oceanologia Et Limnologia Sinica. 2009-04 Abstract

Based on the reanalysis data (monthly mean of temperature and salinity) and observation data (monthly mean of heat flux), it is detected that in the west area (38°—42°N, 158°E—172°W) and the east area (35°—42°N, 172°—145°W), the late winter Sea Surface Temperature Anomaly(SSTA) persists quite well. The formation mechanism for persistence phenomenon of SSTA is analyzed in both areas. It is found that the “reemergence mechanism” of the mixed layer is dominating formation mechanism of the SSTA persistence phenomenon in the west area; while the contributes of heat flux persistence anomalies is dominating for the persistence of late winter SST anomalies in the east area. In addition, in the west area, considering the decadal variability of the seasonal different of the mixed layer depth, the role of “reemergence mechanism” is more important after the 1976 climate shift due to the increase of the seasonal different of mixed layer depth by the intensification of the westerly in the North Pacific.

Jiang, H, Farrar T, Beardsley R, Chen R, Chen C.  2009.  Zonal surface wind jets across the Red Sea due to mountain gap forcing along both sides of the Red Sea. Geophysical Research Letters. 36(L19605)   10.1029/2009GL040008   AbstractWebsite

Mesoscale atmospheric modeling over the Red Sea, validated by in-situ meteorological buoy data, identifies two types of coastal mountain gap wind jets that frequently blow across the longitudinal axis of the Red Sea: (1) an eastward-blowing summer daily wind jet originating from the Tokar Gap on the Sudanese Red Sea coast, and (2) wintertime westward-blowing wind-jet bands along the northwestern Saudi Arabian coast, which occur every 10–20 days and can last for several days when occurring. Both wind jets can attain wind speeds over 15 m s−1 and contribute significantly to monthly mean surface wind stress, especially in the cross-axis components, which could be of importance to ocean eddy formation in the Red Sea. The wintertime wind jets can cause significant evaporation and ocean heat loss along the northeastern Red Sea coast and may potentially drive deep convection in that region. An initial characterization of these wind jets is presented.