function sigma_t, t, s

; Uses the 1980 UNESCO International Equation of State (IES80) to
; derive the density of seawater at surface pressure, given the
; temperature and salinity.
;
; Inputs:
;
;    t         temperature (degrees Celsius)
;
;    s         salinity (psu)
;
; Output:
;
;    sigma_t   density(t, 0, s) - 1000  (kg/m^3)
;
; Written by:
;
; Steve Phipps  31 January 2002

; Derive coefficients as a function of temperature
a = 9.99842594d2 + 6.793952d-2*t - 9.095290d-3 * t^2 + $
      1.001685d-4 * t^3 - 1.120083d-6 * t^4 + 6.536332d-9 * t^5
b = 8.24493d-1 - 4.0899d-3 * t + 7.6438d-5 * t^2 - $
      8.2467d-7 * t^3 + 5.3875d-9 * t^4
c = -5.72466d-3 + 1.0227d-4 * t - 1.6546d-6 * t^2
d = 4.8314d-4

; Derive density at surface pressure (p = 0)
rho_zero = a + b * s + c * s^1.5 + d * s^2

; Derive sigma_t
sigma_t = rho_zero - 1000.0d

return, sigma_t

end