North Atlantic circulation in three simulations of 1/12, 1/25, and 1/50

Transcription

1 North Atlantic circulation in three simulations of 1/12, 1/2, and 1/ Xiaobiao Xu and Eric Chassignet Center for ocean-atmospheric prediction studies Florida State University

2 Motivation Numerical models with different goals vary widely on horizontal resolutions, from 1 long-term climate simulations (no eddy), to 1/1 ocean simulations (resolve most of mesoscale eddies) and, more recently, to 1/6 simulations (resolve some submesoscale physics). Will (or when) the ocean model results converge? Specifically, how sensitive is large-scale circulation to the horizontal resolution in eddying regime?

3 Experiments 3 nearly identical experiments with 1/12, 1/2, 1/. All initialized from climatological T/S and zero velocity Latitude Same 32 layers in the vertical. Bathymetry in 1/2 and 1/ is linearly interpolated from 1/ km Same climatological forcing from monthly ECMWF 1 Reanalysis (ERA4) and high-frequent wind from 2 NOGAPS for 23 (no interannual variability) Longitude 1 3 Diffusion Parameters 1/12 1/2 1/ Laplacian deformation-dependent viscosity coefficient... Laplacian (background) viscosity for momentum 2 m2/s 1 m2/s 1 m2/s Biharmonic diffusion velocity for momentum 1 cm/s 1 cm/s 4 cm/s Biharmonic diffusion velocity for layer thickness 1 cm/s 1 cm/s 4 cm/s Laplacian diffusion velocity for temperature/salinity. cm/s. cm/s 1 cm/s

4 Kinetic Energy Kinetic Enery, cm 2 s /2 Laplacian diffusivity 1/8 Bihamonic diffusivity ½ Biharmonic diffusivity (for momentum and layer thickness) 2 1/12 1/2 1/ model year Time evolution of the 3-d domain averaged kinetic energy in three simulations

5 Surface eddy activity 1/12 1/ Ratio of relative vorticity to Coriolis parameter f

6 Eddy Kinetic Energy (EKE)

7 Impact of filtering to EKE

8 Mean SSH AVISO ( ) 1/12 (years 16-2) 1/ (years 16-2) 1/2 (years 16-2)

9 Mean flow Kinetic Energy (MKE)

10 Mean flow and EKE at W Model Observations Zonal Velocity, u / /12 4 N N N Latitude 3 N / shows a section of average zonal velocity across W calculated from hydrographic data with the geostrophic relation and by assuming zero at the sea floor. Because this figure only shows velocity relative to the bottom absolute velocity, the deep Gulf Stream and countercurrents which are nearly ic (Fig. 3a) do not appear. Despite this problem, there are some similarities 2 trophic velocity Eddy Kinetic Energy (EKE) 2 3 near-surface Gulf Stream appears to run almost exactly eastward (Wyrtki et 6; Emery, 1983; Robinson et al., 1979); the mean direction of the surface jet ifters is 93 (see Fig. la). The mean Gulf Stream is also nearly eastward-92 m, 86 at 2 m, and 92 at 4 m. Therefore, this problem is probably not nt here. Larger boxes, two degrees in latitude, resulted in more observations and a higher statistical accuracy, but the increased size tended to blur the deep ream and countercurrents. Calculations were repeated in one-degree boxes that en shifted northward half a degree. Although the velocity profiles varied at from the first set of calculations, there was good agreement in the main and in estimates of transport from Richardson (198) Depth, m LATITUDE 1/ 1 3" / Depth, m a;<::l '>, ~ ~ a. Contoured zonal velocity section (cm S-I) along W and through the Gulf Stream drifters, floats, and current meters. Eastward velocity is shaded. Dots indicate centers of used in calculating velocity except at 4 m, where they show current meter locations. ottom profile is from W; the average bottom profile between -6W is shifted ward from this by about one degree in latitude (see Fig. la). e l 1 2 Eddy Kinetic Energy (EKE) 2 [43, 1 Zonal velocity, u Journal of Marine Research 1/12 4 N 4 N 3 N Latitude 3 N

11 Summary I Increasing the horizontal resolution to 1/ leads to a significant improvement on the representation of the Gulf Stream penetration and the associated recirculation gyres, not seen in simulations at 1/12 or 1/2. This is consistent with earlier results obtained by Hurburt and Hogan (2) using a 6-layer hydrodynamic model NLOM in the subtropical North Atlantic configuration and by Levy et al. (21) using NEMO model in an idealized configuration. More details, including spectra analysis, in the revised manuscript to JPO (Chassignet and Xu)

12 How about even larger scale circulation patterns AMOC Subtropical gyre Subpolar gyre

13 AMOC streamfunction (z) 1/12 1/2 1/ Depth, km Latitude

14 Potential density AMOC streamfunction (σ2) 1/ / Latitude 1/

15 Basin-scale barotropic streamfunction OVIDE Latitude 4 3 RAPID Longitude

16 Subtropical North Atlantic (26 N) Zonal Integration Zonal Structure potential density, σ Northward limb Southward limb 1/12 1/2 1/ Transport, Sv depth, km 1 3 Northward limb Southward limb Transport, Sv Longitude

17 WBC east of Abaco at 26 N (Bryden et al., 2) 1 2 Depth, m 3 4 Observations 1/ Depth, m 3 Depth, m /12 1/ Distance (km) Distance (km)

18 Subpolar (OVIDE section) Section map 33.7 Overturning streamfunction potential density, σ /12 1/2 1/ Transport, Sv Observations from Daniaultet al. (216)-PO

19 Spatial distribution Full water column transport Two-layer transport 2 1 1/12 1/2 1/ 2 1 1/12 1/2 1/ Transport, Sv 1 2 Transport, Sv Sverdrup flow depth, km 3 bathymetry depth, km Longitude Longitude Observation from Daniault et al (216)-PO

20 SSH in the subpolar North Atlantic CNES-CLS-213 1/ HYCOM 1/12 HYCOM 1/2 HYCOM

21 Summary II The transport structure of the large-scale AMOC and the subtropical/subpolar gyres is consistent among the 1/12, 1/2, and 1/ simulations.