Notes on OFES meeting November 2010, JAMSTEC, Yokohama

Development and research with the FRAM and OCCAM ocean models - David Webb

• topography of the ACC is key to its dynamics.

• OCCAM data are available from a dataserver

• Deacon Cell as an helix:

  the zonal flow shallows and flows northward after passing the Drake Passage and then deepens going back south on its way sout of South Africa. This geometry shows up as a meridional-vertical circulation in the zonal average.

• The ACC is responding to changes due to wind stress within days. The dynamics are mostly barotropic.

Regional ocean modelling - Lie-Yauw Oey

• Open boundary conditions (OBC): - Sommerfeld radiation condition: we want the incoming waves to be zero == we want the wave to be able to leave the domain. - Orlanksi method #1: estimate the phase speed velocity at the boundary (see Orlanski, Journal of Computational Physics) - Orlanksi method #2: - nonlinear conditions -> condition not satisfied with more than one wave -> do not use Orlanski - Flather condition: successful - Flow Relaxation Scheme: relaxe the interior solution to the outer solution. This method is simple and effective and can be coupled with the Flather condition. - To estimate the error: use same grid for the interior model as for the outer model and the difference between the interior and outer solution inside the regional grid is due only to OBC.
• wind blowing over a warm eddy (see one or more of these papers: Chang and Oey, Ocean Discussion, 2010, Chang and oey, JPO, 2010, Fujisaki and Oey, JGR, 2010): destabilization on one side, stabilization on the other side. This remains me of the large vertical velocity on one side of two eddies in Fig. 5 of this note (see Thomas et al. 2007 for a similar discussion on wind blowing over ocean front and eddies).

Decadal predictions of the dynamic state of the Kuroshio Extension (KE) system - Bo Qiu

• Creation of an index for the KE which correlates well with an index from SSH.
• OFES hindcast simulation models well the KE variability
• Change in regime after 1976-77: after 1976-77, the KE variability is stronger and has a longer time scale
• Due to PDO that changes the wind forcing in the eastern North Pacific
• There is a feedback mechanism into the wind forcing
• A model that includes surface-forced Rossby wave dynamics predicts the KE variability well over the next 2-4 years
• A model that includes also the feedback mechanism lengthens the skill of the prediction by 2 to 3 more years

Internal variability of the Kuroshio Extension Current in OFES North Pacific regional model: a preliminary result - Masami Nonaka

• difference between OFES hindcast integration and climatological integration shows that some variability is created internally
• ensemble hindcast predictions using different initial conditions but same wind forcing
• I am not convinced that differences between the different simulations are necessarily indicative of internal variability

The barotropic response of the Kuroshio to wind forcing observations and OFES data - Magdalena Andres

• Unlike at the latitudes of the KE, the hindcast from SSH anomaly at the latitudes of the East China Sea (ECS) is doing a poor job in reproducing SSH observations
• Weak correlation between the SSH variability and PDO at latitudes of ECS
• SSH variability is correlated positively with the wind curl at 37N and negatively at 27N.
• Conceptual model involving barotropic Rossby waves following geostrophic contours that turn southwestward due to the topography of the western North Pacific – SSH contains both barotropic and baroclinic motions.

Simulated oceanic evolutions associated with the 2006 Indian Ocean Dipole event - Yukio Masumoto

• The OFES simulation succeeds to reproduce with remarkable details the evolution of the thermal structure of the ocean during an Indian Ocean Dipole event

Modeling intraseasonal variations in the Bay of Bengal - P.N. Vinayachandran

• The Bay of Bengal stays always warmer and has much stronger warming intraseasonal warming events than the Ariabian Sea.
• As in the previous talk, the model –here the POP model– succeeds to reproduce with remarkable details some of the observed features (upper-ocean velocity at the equator for instance). This is not the case, however, for salinity.

Deep oceanic zonal jets driven by fine-scale wind stress curls in a high-resolution coupled GCM - Bunmei Taguchi

• See Kessler and Gourdeau (2006, GRL) for fine-scale wind stress (QuickSCAT) generating the small-meridional-scale zonal jets

Layering and stratified turbulence surrounding an anticyclonic eddy: geoseismic observations and numerical simulations - Lien Hua

• Find hot spots of dissipation away from boundaries and surface from a cascade toward small scales
• Strong layering surrounding a subsurface eddy above and below (see Menesguen et al., GRL 2009; Papenberg et al. GRL 2010)
• Potential energy is found to follow a 5/3 power law in the horizontal and estimated dissipation is 100 times higher than an estimate from NATRE observations)
• Numerical simulations starting with a balanced subsurface eddy reproduce the layering process after a perturbation (noise) is added
• Transition scale: L > 5km, -3 spectrum (quasi-geostrophy turbulence?) and L < 5 km, -5/3 spectrum (Boussinesq billows of radial velocity), similar of wind observations (Takahashi et al. 2007)
• It takes about 100 days for the eddy to disappear which corresponds to reasonable eddy lifetime

Internal waves simulated by 1/10 deg. OGCMs - Jin-Song von Storch

• The aliasing error in the 3-day snapshot OFES data indicates the existence of near-inertial oscillations
• strong short waves traveling along strong currents –due to instability of strong currents
• long inertial oscillations having amplitude up to 0.8 cm/s propagating in regions away from strong currents

The combined pressure and potential energy flux in the global ocean - Hidenori Aiki

• For the Kuroshio and Gulf Stream, eddy stresses decelerate mean currents in the separation region and accelerate them downstream the extension

Impact of submesoscales on the large-scale oceanic circulation from high-resolution simulations - Patrice Klein

• See if Okubo-Weiss in the subtropical gyre does correspond to submesoscale features below the surface mixed layer
• See how the spectrum changes between the surface mixed layer and below
• See Roullet and Klein, PRL, 2010
• with stronger stratification, smaller variability in vertical velocity, much steeper velocity spectrum and smaller total kinetic energy
• the transfer from potential to kinetic energy is enhanced with submesocale processes