Pacific Fisheries Environmental Laboratory
National Marine Fisheries Service
Southwest Fisheries Science Center
Calculation Details and Differences
FNMOC 181 x 360 Monthly Mean Interpolated Sea Level Pressure and calculated Air/Ocean Indices
The air/ocean indices and upwelling indices are calculated in a similar way to PFEL's traditional
15-location coastal North American products (Bakun, 1973) with the following exceptions:
- These global products are derived from 1-degree interpolated pressure fields.
- Historically, PFEL has provided upwelling products derived from a 3-degree interpolation
of the pressure field. Mesh lengths less than 3-degrees were extrapolated to a 3-degree mesh length using Bessel's central difference formula.
The grid size of the pressure field provided by FNMOC has changed
over the years as follows:
- 1967 - 1980 (available for the Northern Hemisphere only) - 63 x 63 polar steriographic projection
- 1981 - 1996 (available globally) - 144 x 73 spherical projection
- Nov 1996 - present (available globally) - 360 x 181 spherical projection
- For the products on these pages, PFEL has created 1-degree interpolated 6-hourly pressure fields from the historical pressure
files. The interpolation is a simple linear interpolation from the native grid. Monthly averages of these fields are available from 1967 - present.
- The geostropic winds used to calculate Ekman transport are monthly averages of winds calculated
from 6-hourly pressure values.
- Historically, due to limited computational ability, the geostropic winds used to compute
upwelling index were derived from monthly averaged pressure fields. For consistency, the products
provided at the 15 standard North American coastal locations are still computed in this manner.
- The global products obtained here are computed from 6-hourly averages of geostropic winds, which are in turn computed from
the 1-degree 6-hourly pressure fields described above. Each of the air/ocean index products is thus an
average of 6-hourly values, rather than being computed from a monthly mean pressure field
- The parameterization of the drag coefficient (CD) used in the wind stress calculation is taken as a function of wind
speed (W) rather than a constant. :
- Historically, CD was taken as a constant value of 0.0013 to calculate indices from 6-hourly pressure fields and was increased to 0.0026 to calculate indices from a monthly pressure field.
- For the calculations on this page, a non-linear drag coefficient was used (based on Large and Pond (1981) modified for low wind speeds as in
Trenberth et al. (1990)):
- CD = 0.00218 for W < or = 1m/s
- = (0.62+1.56/W) x .001 for 1m/s < W < 3m/s
- = 0.00114 for 3m/s < or = W < 10m/s
- = (0.49 + 0.065W) x .001 for W > or = 10m/s
Non-Linear Drag Coefficient
References:
- Large, W., and S. Pond, 1981: Open Ocean momentum flux measurements in moderate to strong winds. J. Phys. Oceanogr., 11, 324-336.
- Trenberth, K. E., W. G. Large and J. G. Olson, 1990: The mean annual cycle in global ocean wind stress. J. Phys. Oceanogr., 20, 1742-1760.
The effects of the above differences in calculation methods can be seen in the following examples, which compare the
products on this page with PFEL's standard versions of the products at two locations. The magnitude of the effect depends on
location, pressure gradient, wind speed, and other factors such as the original mesh size and possibly proximity to the coast.
The factors affecting the calclation of upwelling index are an area of on-going research at PFEL.
Click on the plot to view a full size image:
