Physics of the EUC: Preliminaries
329
discuss theories which assume fundamentally that a purely conservative model
of the dynamics is the appropriate, order one, model for the EUC dynamics,
and we ignore vertical mixing of momentum as well as density at lowest order.
It is interesting to note that recent measurements (Johnson and Luther 1994)
do indicate that vertical mixing is probably not significant at the level of the
core of the undercurrent.
We therefore take as a starting point the conservative model as a null
hypothesis in which the ideal fluid theory of the thermocline, appropriately
modified and extended to the equator, is suggested as the fundamental explanation of the EUC. In this view the link between the regimes of the midlatitude thermocline and the equatorial current system is fundamental in
explaining equatorial motion.
It is important to emphasize that this linkage is suggested for the time
mean, general circulation. The isolation of the equatorial zone on short time
scales, over which the equatorial band acts as a wave guide for Kelvin and
Rossby waves is well known (see Philander 1990; Moore and Philander 1977,
for excellent reviews of equatorial wave dynamics). The trapping scale for lowfrequency Rossby waves is shown in these reviews to be of the order of
cK(kjw{J) 1 1 2 , where cK is the Kelvin wave speed, and k is the zonal wave
number of the wave whose frequency is w. Note that for low frequencies the
"trapping" becomes very feeble. On general circulation scales we are interested
in w R:j 0, and in spite of the equator's effectiveness as a wave guide for higher
frequency waves it thus is clear that for steady motions the linkage with higher
latitudes is to be naturally expected.
6.2 Physics of the EUC: Preliminaries
The solution for the layer thicknesses in midlatitude for the ventilated thermocline, i.e., ( 4.4.18) in the two moving layer model, gives a prediction for each
of the layer thicknesses which is well behaved as the equator is approached.
This, of course, does not mean that the solution is correct. In fact, at the
equator it is not.
For the case when the wind stress is purely zonal and independent of
longitude the Ekman pumping is given by:
wE = - R co~e ae ( r ;:;
0 )
(6.2.1)
where r is the zonal wind stress. It follows then that the function fYo is given by:
D 2 = 2 R(¢e- ¢) [sine 8 ' --'-]
(6.2.2)
0
Y2Po
ae cos e
so that, from (4.4.18):
329
discuss theories which assume fundamentally that a purely conservative model
of the dynamics is the appropriate, order one, model for the EUC dynamics,
and we ignore vertical mixing of momentum as well as density at lowest order.
It is interesting to note that recent measurements (Johnson and Luther 1994)
do indicate that vertical mixing is probably not significant at the level of the
core of the undercurrent.
We therefore take as a starting point the conservative model as a null
hypothesis in which the ideal fluid theory of the thermocline, appropriately
modified and extended to the equator, is suggested as the fundamental explanation of the EUC. In this view the link between the regimes of the midlatitude thermocline and the equatorial current system is fundamental in
explaining equatorial motion.
It is important to emphasize that this linkage is suggested for the time
mean, general circulation. The isolation of the equatorial zone on short time
scales, over which the equatorial band acts as a wave guide for Kelvin and
Rossby waves is well known (see Philander 1990; Moore and Philander 1977,
for excellent reviews of equatorial wave dynamics). The trapping scale for lowfrequency Rossby waves is shown in these reviews to be of the order of
cK(kjw{J) 1 1 2 , where cK is the Kelvin wave speed, and k is the zonal wave
number of the wave whose frequency is w. Note that for low frequencies the
"trapping" becomes very feeble. On general circulation scales we are interested
in w R:j 0, and in spite of the equator's effectiveness as a wave guide for higher
frequency waves it thus is clear that for steady motions the linkage with higher
latitudes is to be naturally expected.
6.2 Physics of the EUC: Preliminaries
The solution for the layer thicknesses in midlatitude for the ventilated thermocline, i.e., ( 4.4.18) in the two moving layer model, gives a prediction for each
of the layer thicknesses which is well behaved as the equator is approached.
This, of course, does not mean that the solution is correct. In fact, at the
equator it is not.
For the case when the wind stress is purely zonal and independent of
longitude the Ekman pumping is given by:
wE = - R co~e ae ( r ;:;
0 )
(6.2.1)
where r is the zonal wind stress. It follows then that the function fYo is given by:
D 2 = 2 R(¢e- ¢) [sine 8 ' --'-]
(6.2.2)
0
Y2Po
ae cos e
so that, from (4.4.18):