Observations and Modelling of Fronts and Frontogenesis

temperatures for January and February from 100 years of ship
observations and 27 years of hydrocasts (Robinson, 1976).
Figure 11.6 shows the probability density of the deviation of
15 m temperatures from a fit, linear in latitude in the
intervals 253O0 N and 30-35° N, to the January climatology.
As noted by Roden (1981), the surface layer temperatures are
colder on average than the climatological means. The
apparent bimodality is probably due to poor statistics. The
standard deviation is roughly 0.5 °C. Relative to the
climatological gradient, this would correspond to an
adiabatic meridional displacement of 80-100 km. From
drifters deployed near 30° N, 150° W, Niiler and Reynolds
(1984) compute a mean northward surface velocity of 1-2 cm
s1- during 27 January through 30 April 1980. From infrared
imagery, Van Woert (1982) estimates an e-folding time of 60
days for a large frontal meander near 30 N during January and
February 1980. The meridional displacement scale is
comparable to the product of these velocity and time scales.
Standard mixing-length arguments yield an eddy diffusivity
(as velocity times length or length squared divided by time)
of 1-2 x l0 m2 s1-. This is an order of magnitude smaller
than typical values used in numerical models (e.g., Lativ,
1987)
Horizontal 100 m mean temperature gradients were
calculated from differences of adjacent 15 m temperatures.
15