OS52B-01 10:20h
Spontaneous interannual to decadal scale variations in the Kuroshio and its recirculation in a high-resolution North Pacific Model
Interannual to decadal scale variations of the Kuroshio and its recirculation are investigated using an eddy-permitting North Pacific numerical model. The model is a z-coordinate free-surface OGCM (one version of MRI.COM; MRI Community Ocean Model). Horizontal resolution is $1/6^{\circ}$ in the latitudinal and $1/4^{\circ}$ in the longitudinal direction, with 44 vertical levels. The model domain is the Pacific north of $15^{\circ}$S. Integration is carried out for 150 years, including the initial restoring body forcing of temperature and salinity to the observed state for the first six years. Data from the last 50 years of integration are used for analysis. The model is forced by climatological wind stresses, and the temperature and salinity are restored to climatology at the surface, at the southern boundary, and in the Okhotsk and Bering Seas. Without interannual change in the forcing, it is found that there are variations of about three-year period in mass transports of the Kuroshio through the Tokara Strait and the Ryukyu Current, flowing northeastward east of the Ryukyu Islands. The Tokara transport lags that of the Ryukyu Current for about 9 months. The Tokara transport is also found to be in opposite phase with the extent of the recirculation gyre, which is defined here as an area encircled by some closed contour of the barotropic stream function. During the analysis period, the Kuroshio south of Japan takes the large meander path, the strait path, or unstable paths. The path variation is rather irregular in comparison with the mass transport described above. The stable large meander path that persists for longer than a couple of years appears four times within the fifty-year period. Thus, the periods between the transitions from the strait path ( or unstable paths ) to the large meander path are about twelve years on average, which is longer than the time scale of the mass transport variation, and the path variation looks uncorrelated to the mass transport.
OS52B-02 10:35h
Intraseasonal to Interannual Variability in the Tropical Indo-Pacific of Global POP.
Remote forcing from both the Pacific and Indian Ocean basins, as well as regional monsoon forcing, complicates property flux estimates within the Indonesian seas and causes variability over a range of time scales. Two global configurations of the Parallel Ocean Program (POP), one eddy-permitting (1/3$\deg$, 32-levels) and the other eddy-resolving (1/10$\deg$, 40-levels), both forced with synoptic atmospheric fluxes are used to characterize the variability on scales ranging from intraseasonal to interannual. Mesoscale variability is examined in the tropical Indo-Pacific of the eddy-resolving model (1994-2001), particularly intraseasonal Throughflow eddies to the west of Timor Strait. Extended Empirical Orthogonal Function (EEOF) analyses of the eddy-permitting model (1979-1997) for both biennial (1.9-3.0 years) and narrow-band interannual (3.2-6.3 years) periodicities in the tropical western Pacific reveal that the strongest variability can be identified as off-equatorial Rossby waves. These off-equatorial Rossby waves interact with the North Pacific western boundary to form coastally trapped waves that propagate through the Indonesian Seas (Ombai Strait) and along the northwest Australian coast of Australia, progressively pulling off into the Indian Ocean as free Rossby waves. Cross-spectra analyses confirm that these coastally trapped waves on interannual scales, but not biennial, partially account for the strong low-frequency signal in the southern tropical Indian Ocean off Australia. They also confirm the contribution of coastally-trapped waves excited by Pacific equatorial Rossby at the tip of Irian Jaya that propagate along the Arafura/Timor Strait waveguide as documented in observational studies.
http://www.oc.nps.navy.mil/navypop
OS52B-03 10:50h
The low frequency variability of the South Atlantic circulation
In this presentation, we analyze the structure of the low frequency variability in the South Atlantic circulation from nearly ten years of altimeter data. The annual variability is distinguished by an absolute maximum in the southwestern Atlantic that is characterized by a strengthening of the Brazil Current transport during the austral summer and a weakening towards the winter. These seasonal variations do not appear to be driven by the wind stress curl over the subtropical basin, but by the winds located in the region between 40?S and 50?S. The interannual variability of the basin-scale circulation is characterized by a dipole-type mode with peaks of opposing phases in the subtropics and the western subpolar region. This mode is modulated by a time series, characterized by a 4-5 year period. The combination of the spatial and temporal patterns indicates a strengthening of the oceanic circulation in the subtropical gyres from 1992 to 1996 and a weakening from 1996 to 2001. The subpolar expression of this dipole is characterized by a maximum over the region dominated by the Zapiola eddy (~47?S 45?W). The anticyclonic circulation over the Zapiola eddy weakened from 1992 to 1996 and strengthened from 1996 to 2001. The sea level pressure and the wind stress curl showed basin scale variations similar to those of the SSHA. The lowest mode of sea level pressure variability represents changes of the anticyclonic circulation over the subpolar basin. The structure of the wind stress curl show changes similar to those of the SSHA. Our analysis indicates the low frequency variability of the ocean circulation is related to a direct Ekman response in the subpolar basin and an indirect, geostrophic change of the circulation in the subtropics.
OS52B-04 11:05h
Stochastic Forcing of the North Atlantic Wind-Driven Ocean Circulation
At midlatitudes, the magnitude of stochastic wind stress forcing due to atmospheric weather is comparable to that associated with the seasonal cycle. Stochastic forcing is therefore likely to have a significant influence on the ocean circulation. In this work, we examine the influence of the stochastic component of the wind stress forcing on the large-scale, wind-driven circulation of the North Atlantic Ocean. To this end a quasi-geostrophic model of the North Atlantic was forced with estimates of the stochastic component of wind stress curl obtained from the NCAR Community Climate Model. Analysis reveals that much of the stochastically-induced variability in the ocean circulation occurs in the vicinity of the western boundary and some major bathymetric features. Using the ideas of generalized stability theory (GST), we find that the patterns of wind stress curl that are most effective for inducing variability in the model have their largest projection on the most nonnormal eigenmodes of the system. These eigenmodes are confined primarily to the western boundary region and are composed of long Rossby wave packets that are Doppler shifted by the Gulf Stream to have eastward group velocity. Linear interference of these eigenmodes yields transient growth of stochastically-induced perturbations, and it is this process that maintains the variance of the stochastically-induced circulations. By examining the model pseudospectra, we find that the nonnormal nature of the system enhances the transient growth of perturbation enstrophy and therefore elevates and also maintains the variance of the stochastically-induced circulations in the aforementioned regions.
OS52B-05 11:20h
Assessment of The New Fsu Winds Climatology
A new objective time series of in situ based monthly surface winds has been developed as a replacement for the subjective Tropical Pacific FSU winds. The new time series begins in Jan. 1978, and it is ongoing. The objective method distinguishes between observations from volunteer observing ships (VOS) and buoys, allowing different weights for these different types of observations. An objective method is used to determine these weights, and accounts for the differences in error characteristics and in spatial/temporal sampling. A comparison is made between the objective and subjective products, as well as scatterometer winds averaged monthly on the same grid. The scatterometer fields are a good proxy for truth. These three sets of fields have similar magnitudes, directions, and derivative fields. Both in situ wind product underestimate convergence about the Intertropical convergence zone; however, the objective FSU product is a much better match to the scatterometer observations. Furthermore, the objective winds are less variable in time than the subjective winds. The strengths and weakness of the objective and subjective winds are discussed.
OS52B-06 11:35h
Three-Dimensional Modeling of Air-Sea Interaction
Deep-water wave breaking is crucial in the transfer of heat, gases, and momentum between the ocean and the atmosphere. Observations of these events have provided qualitative support to this end, and yet accurate quantification of momentum transfer for strong winds and nonlinear waves has remained a challenge. In recent years, advances have been made in the development of numerous algorithms to capture and describe air-sea interaction. Most, however, are idealized and only capable of modeling fluid flow within the two-dimensional approximation. Thus, many important characteristics of the flow composition and breaking process are ignored, oversimplified, or remain unknown. We employ a three-dimensional, time-dependent, finite difference, volume of fluid model, including both the flow of air and water, entitled TRUCHAS, to address the issue of deep-water breaking waves. Our model utilizes the multidimensional piecewise linear interface calculation method to assess the volume fraction of each fluid material in every mesh cell. The model solves conservation equations for mass and momentum for multiple fluids within the domain and tracks the interfaces between them. A great many details of the flow development are available for analysis from the model output. These include wind and water velocities, pressure gradients in both the air and sea around a breaking wave, the development and evolution of wind-generated waves, and the corresponding transfer of momentum from the atmosphere to the ocean. Our results are correlated with laboratory experiments conducted at the University of Miami's Air-Sea Interaction Salt-water Tank that possesses both wind and wave generating capabilities. Preliminary model results show good qualitative agreement to laboratory data.
OS52B-07 11:50h
A fully implicit formulation of a layered wind-driven ocean model with outcropping
We have implemented an implicit solver for a layered formulation of the primitive equations. The advantage of the layered formulation is that there is no spurious diapycnal diffusivity due to the numerical representation of the advective terms. The numerical scheme is based on Hsu and Arakawa (1990) and allows isopycnic layers to outcrop at the surface. This makes it possible for us to explore solutions where the deeper layers are forced directly by the wind along their surface outcrop. The stationary solutions of the model are solved using an arc-length continuation approach based on Newton's method. Here we report some preliminary results about the bifurcations and stability of the ventilated thermocline as a function of the eddy-viscosity.
OS52B-08 12:05h
A Finite Difference Ocean Model With Adaptive Mesh Refinement Capability
This research integrates the concepts of Adaptive Mesh Refinement (AMR) into a well accepted finite difference ocean model known as the Modular Ocean Model (MOM) Traditional models require up to months of run time to simulate the globe at high resolution. This proves to be inefficient when high resolution is only desired in localized areas of interest. AMR offers a flexible alternative by allowing increased refinement on desired sections of the grid that can move with the area of interest. A high order of accuracy is obtained and run time is drastically reduced as a result. The software package SAMRAI (Structured Adaptive Mesh Refinement Application Infrastructure) is used to aid in memory management and data communication. Standard MOM features of our model that are uncommon in AMR include leapfrog time integration, staggered tracer and momentum control volumes, and time step splitting of barotropic and baroclinic modes. We demonstrate how these features are implemented in the model and present test case comparisons of AMR verses fine resolution.