Seasonal stratification and the spring bloom - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 189

18

16

14

12

10

8

6

May July Sept Nov Jan Mar

Months 1975-1976

Figure 6.9 Annual cycle of surface ( þ ) and bottom ( D ) temperature observed at a strongly
stratified site on the Nymphe Bank, Celtic Sea (lat. 50 40' N, long. 7 30' W). This is an early
data set, collected by UK Admiralty vessels which, for secret naval reasons, visited the
Nymphe Bank several times per week for a whole year and recorded the temperature profile.
Release of the data for civilian use required weekly averaging to avoid giving away the position
of Her Majesty's ships. Adapted from Simpson and Bowers, 1984 , with permission from
the European Geosciences Union.

An interesting feature of such simulations in the stratified regions is that the
bottom temperature, which remains low for almost the whole summer, exhibits a
short sharp rise to a maximum at the overturn. This 'cusp' in the bottom temperature
cycle is confirmed by observations, an early example of which is shown in Fig. 6.9 .
It means that the temperature cycle of the benthic environment contrasts sharply
with that of the waters above the thermocline.
In spite of its simplicity, the TML model does a surprisingly good job at providing
a realistic account of the seasonal cycle in the different mixing regimes of the shelf
seas. In the stratified regions, it predicts surface and bottom temperatures and mixed
layer depths with reasonable accuracy (Elliott and Clarke, 1987 ). To see how a
sequence of runs of the TML model can be used to simulate the structure of a Tidal
Mixing Front, try running the TMFI module from the topic website. The two-layer
model also illustrates how seasonal heat storage in the water column is influenced by
vertical mixing. Compare the heat storage time series in Fig. 6.8a , b . In strongly
mixed regions ( Fig. 6.8b ) heat is stored uniformly throughout the water column,
which limits the rise in surface temperature. In a stratifying system ( Fig. 6.8a ), the
incoming heat is concentrated in the upper layer, leading to higher surface tempera-
tures and greater heat loss back to the atmosphere, which reduces the total amount of
heat absorbed by the water column. As a result, the mixed water column achieves