NASA Project Report Year 2:
Dynamics of anisotropic mean and time-varying structures of
ocean circulation
Georgia Tech Report
Co-PI:
Contact Information:
Phone Number:
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Applicant/Institution:
Dr. Emanuele Di Lorenzo
Georgia Institute of Technology
Earth & Atmospheric Sciences
311 Ferst Drive
Atlanta, GA 30332-0340
404-788-8035
404-894-5638
edl@gatech.edu
Georgia Tech Research Corporation
Office of Sponsored Programs
505 Tenth Street, N.W.
Atlanta, GA 30332-0420
Summary of modeling activities and findings
The main goal of the ocean modeling research at Georgia Tech is to investigate the
emergence and dynamics of ocean striations in the North Pacific circulation. Using the Regional
Ocean Modeling System (ROMS) we have continued to explore the emergence of ocean striations in
the North Pacific mean circulation and their sensitivity to different boundary conditions (e.g. flat
bathymetry, weakly nonlinear regimes, etc.) (Figure 1). The ROMS control simulations were
configured over the Northeast Pacific region [180W -105W; 9N-53N] with 20km horizontal resolution
and 30 terrain following vertical layers. The development of the ocean striations is observed by
starting the ocean model from a state of rest with a uniform vertical density profile typical of the
North Pacific, and by forcing the ocean with a climatological seasonal cycle of the winds (2.5 degree
NCEP Reanalysis) with no buoyancy fluxes. This configuration insures that the only source of energy
at the scale of the striations is internal to the ocean and from interactions with bathymetry and
coastal geometry.
Figure 1: The ROMS ocean model (red box shows the control configuration) is used to explore the
emergence of ocean striations in the North Pacific and their sensitivity to different boundary conditions (e.g.
flat bathymetry, weakly nonlinear regimes, etc.) (blue box shows a list of the sensitivity experiments).
To isolate the signature of the ocean striations we initially focused on the 300 -meter zonal
velocity and being addressing some fundamental questions:
Q1: Do the striations emerge uniformly over the entire North Pacific or do they originate in the
eastern boundary and propagate westward?
By analyzing the spinup of the ROMS model we were able to show that ocean striations are
not directly forced by surface fluxes of momentum or buoyancy, but can develop from vorticity
sources associated with topography and/or with instabilities along the eastern boundaries. During
year 0 of the ROMS control run (NEPD 20km) we see the development of small scale zonal velocity
anomalies along the eastern boundary (red rectangle) and major topographic features (blue
rectangle) (see Figure 2 below).
Figure 2: The emergence of striations in the ROMS ocean model.
The zonal velocity anomalies (also evident in the vortices field) begin to intensity and propagate
westward from the eastern boundary (see snapshot for year 2) and develop into quasi -steady
striations (as evident in the 10-year mean from model year 60-70).
Q2: Is non-linearity (e.g. eddies) important in the development and maintenance of these features?
We also explored the role of nonlinearity in the development and maintenance of the
striations by decreasing the resolution of the ocean model to 40 km (to reduce the effect of
mesoscale eddies) and by decreasing the strength of the surface forcing by an order of magnitude.
We found that changing the resolution (Figure 3, arrow 1), the striations still develop. The striations
that have a topographic source in the ocean interior remain unchanged. However , the striations that
rely on the eastern boundary source change in location but maintain the same character. The
change in location is associated with changes in the small scale vorticity source along the eastern
boundary, which is a consequence of changing the model resolution. As we transition towards a
more linear regime by reducing the strength of the surface forcing (Figure 2 arrow 2), we found that
the striations are still a dominant feature in the mean and the topographic striations remain
unchanged. However, the striations with an eastern boundary source become more coherent and
appear to behave as beta-plumes. We also noticed that the striations become more zonal due to a
decrease in the mean circulation associated with the reduced surface forcing.
Figure 3: Results from ROMS sensitivity experiments.
Q3: What is the role of topography and eastern boundary coastal geometry in generating the
striations?
We next explored the relative importance of topography and of eastern boundary coastal
geometry in the development and maintenance of the striations. A new sensitivity experiment with
the control integration in the weakly nonlinear regime (reduce surface forcing) was conducted using
flat bathymetry in the ocean interior and a uniform continental slope along the eastern boundary
(Figure 3, arrow 3). The presence of the continental slope and the coastline along the eastern
boundary preserve the eastern boundary’s small-scale voritcity sources from which the striations
develop. We found that the mean circulation is still characterized by striations, however, the
toopograhic striations are no longer evident and the striations form the eastern boundary become
more coherent with no signatures of eddy-scale variability. This suggest that topography, even
though is not always a source of vorticity for the striations in the ocean interior, is still generating
local variability that affects the signature of striations originating from the eastern boundary. In
contrast the eastern boundary plays an important role in the development of the striations in that it
provides steady sources of energy and vorticity that spread westward in the ocean interior.
The role of the eastern boundary was further explored by removing the coastlines of the
eastern boundary and setting a wall of constant depth along 125W (see Figure 3, arrow 4). The
resulting mean circulation from this experiment is characterized by large-scale features of the gyrescale circulation with no clear signature of striations. Inspection of the time dependent evolution of
the 300-meter zonal field shows that this integration is still characterized by striations, however, the
long-term mean removes their signature because the sources of eastern boundary vorticity are no
longer steady but alternate in sign along the eastern boundary.
Future plans for modeling activities
In the next year we plan to use more advanced statistical analysis and vorticity budgets to
quantify these preliminary findings. We also plan to expand our mode l archive to include higher
resolution runs (10km) for critical sensitivity integrations. We also plan to essntialize some of these
findings using very idealized ROMS model setup that explore the fundamental elements of the
eastern boundary voriticity sources and their interior spreading (e.g. beta-plume linear and nonlinear
dynamics).
Outreach and modeling data archive
In order to improve visibility of this research we have developed a project website
(www.oceanjets.org). From this project website is possible to have direct access to the entire ROMS
model data archive (see figure 4) through the Georgia Tech OpenDAP data server. We also plan to
use this website to post the findings of the project as they become available in the refereed literature
and to exchange material/information/data within the project collaborating institutions. The website is
currently still under development and not all the components are available.
Figure 4: Project website at www.oceanjets.org