Studies of the Atlantic Ocean Climate with a Sigma Coordinate Coastal Ocean Model

High resolution coastal ocean models, with a free surface, a bottom-following vertical sigma coordinate and an imbedded turbulence closure scheme such as the Princeton Ocean Model, POM, (Blumberg and Mellor 1987) have been limited in the past to studies of estuaries, semi-enclosed seas and regional simulations of coastal areas. However, simulations of basin and global scale phenomena are now possible with these models. Processes such as Gulf Stream Separation (Ezer and Mellor 1992) and flow over sills where variations in bottom topography may be important are simulated well with sigma coordinate models; these processes are important for climate studies since they affect heat flux transport in middle latitudes and deep water formation in high latitude s. These models are also attractive for studies of the influence of climate changes in the open ocean on possible effects in coastal regions. In this note we report on some of the progress achieved in the use of POM for basin-scale climate problems.

Previous studies by Greatbatch et al. [1991] indicate significant changes in the North Atlantic thermohaline structure and circulation between the pentads 1955-1959 and 1970-1974, using hydrographic data analyzed by Levitus, COADS wind stress analyze d by DaSilva, and a simple diagnostic model by Mellor et al. [1982]. The results show for example that the Gulf Stream was considerably weaker (by about 30 Sv) during the 1970s compared to the 1950s. The above calculations are verified using the full th ree-dimensional model applied to the North Atlantic ocean, where the same data are used (Ezer et al. 1995). Diagnostic and short term prognostic calculations are used to infer the dynamically adjusted fields corresponding to the observed hydrographic and wind stress climatology of each pentad (the technique is discussed by Ezer and Mellor 1994). The results also indicate possible changes that have occurred in the poleward heat transport. To verify the interpentadal changes in thermohaline circulation, we compare model results with independent coastal sea level data. The comparison shows good agreement between model and observed sea level change (Figure 1), indicating a sea level rise along the U.S. east coast and a sea level drop at Bermuda. Note the large spatial variations in sea level change along the coast that relate to changes in the open circulation (e.g., the weakening of the subtropical gyre during the early 1970’s) and the shape of the coastline.

Figure 1. Sea level change in the western North Atlantic between 1955-1959 and 1970-1974, from model calculations ("+") and from observations ("o") at the following stations: 1.) Pt. Isabel, TX, 2.) St. Petersburg, Fl, 3.) Bermuda, 4.) K ey West, FL, 5.) Miami, FL, 6.) Mayport, FL, 7.) Pulaski, GA, 8.) Charleston, NC, 9.) Wilmington, NC, 10.) Hampton Rd., VA, 11.) Lewes, DE, 12.) New York, NY, 13.) Boston, MA, 14.) Portland, ME, 15.) Halifax, Nova Scotia. (From Ezer et al ., 1995)

Simulations of the entire Atlantic Ocean from 80S to 80N are now being analyzed. The variable horizontal resolution is about 1/4 deg. to 1 deg. (with higher resolution at the Antarctic Circumpolar Current, ACC, and in the Gulf Stream region), but wil l increase in the future. Forcing is monthly mean winds and monthly mean surface temperatures. One year, mean horizontal and (smoothed) vertical velocities after 10 years of simulations, at 100 m and near the bottom, are shown in Figures 2 and 3 respec tively. The horizontal velocity shows quite realistic Gulf Stream and ACC structure in the upper layers, and the Deep Western Boundary Current near the bottom. Small scale variations of bottom flow relate to variations in the bottom topography. Vertica l velocity in the upper ocean is mostly due to wind stress-driven Ekman pumping in middle latitudes and in the Greenland sea; a strong upwelling is seen along the South Atlantic Bight, where the Gulf Stream flows along the sloping bottom, and in the equatorial region. Vertical velocities near the bottom indicate strong upwelling and downwelling south and north of the Gulf Stream, and mostly north-south patches of downwelling along the western boundary and the mid-ocean ridge. Strong downslope flows near Greenland in the north and near the South America coast in the south may indicate the areas of deep water formation.

Figure 2. Annual mean velocity fields at 100 m after 10 years of integration. (a) Horizontal velocity vectors. (b)Vertical velocity field (smoothed); shaded areas represent downward direction, the contour interval is 0.5 x 10**-5 m/s.

Figure 3. Same as Figure 2, but for the lowest sigma level (i.e., the horizontal and the vertical velocities are the components of the along-bottom flow). The contour interval in (b) is 2 x 10**-5 m/s.

Tal Ezer and George L. Mellor
Program in Atmospheric and Oceanic Sciences
Princeton University
Princeton, New Jersey 08544

REFERENCES

Blumberg, A. F. and G. L. Mellor. (1987). A description of a three-dimensional coastal ocean circulation model. Three-Dimensional Coastal Ocean Models, ed. N. Heaps. Vol. 4, 208 pp. American Geophysical Union.

Ezer, T. and G. L. Mellor. (1992). A numerical study of the variability and the separation of the Gulf Stream induced by surface atmospheric forcing and lateral boundary flows, J. Phys. Oceanogr. 22: 660-682.

Ezer, T. and G. L. Mellor. (1994). Diagnositc prognostic calculations of the North Atlantic circulation and sea level using a sigma coordinate ocean model. J. Geophys. Res. 99: 14, 159- 14,171.

Ezer, T., G. L. Mellor and R. J. Greatbatch. (1995). On the interpentadal variability of the North Atlantic Ocean: Model simulated changes in transport, meriodional heat flux and coastal sea level between 1955-1959 and 1970-1974. J. Geophys. Res. In P ress.

Greatbatch, R. J., A. F. Fanning, A. D. Goulding, and S. Levitus. (1991). A diagnosis of interpentadal circulation changes in the North Atlantic, J. Geophys. Res. 96: 22,009-22,023.

Mellor, G. L., C. Mechoso, and E. Keto. (1982). A diagnostic calculation of the general circulation of the Atlantic Ocean, Deep Sea Res. 29: 1171-1192.

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