Dipole Mode events manifest air- sea interaction
at work in the equatorial Indian
Ocean.A
difference map between positive
and negative
phases of the Dipole Mode is
presented to
highlight the active role of
ocean dynamics
in producing the dipole structure
in sea
surface temperature anomalies
(Fig 1 a).
The response of the oceanic
thermocline
during Dipole Mode events is
inferred from
observational evidence and from
the simulations
of an Oceanic General Circulation
Model (OGCM)which
was forced with observed winds.In
fig 1 b,the
sea surface height (SSH)structure
obtained
from the TopeX/Poseidon satellites
is shown.Fig
1 c shows the difference map
of oceanic heat
content anomalies simulated by
the OGCM.Simulated
thermocline shifts at the equator
are depicted
in Fig 1 d.
The structure of the oceanic response(Fig
1 (b- d))is consistent with our current understanding
of equatorial ocean dynamics.The strong easterlies
seen in Fig 1 a,pulls up the thermocline
on it's east and pushes down the thermocline
to it's west (Fig 1 d).This dynamic adjustment
to the winds is achieved through Kelvin waves
and Rossby waves,thereby imparting the unique
spatial structure seen in Fig 1 b and Fig
1 c.The effect of coastal Kelvin waves and
the Rossby waves resulting from reflection
of the equatorial Kelvin wave off the eastern
boundary is seen to be modifying the structure
in the eastern side. The mechanism through
which the thermocline affects SST and the
subsequent one through which SST anomalies
affect the surface winds need to be investigated
in order to carry forward our understanding
of Dipole Mode events.The climatic implications
of this phenomenon also needs to be investigated.
 |
Fig 1:
October composite
difference maps between
the positive phase
(1994,1997) and negative
phase (1996) of the
Dipole Mode. |
| (a) |
SST (units=deg. Celsius) and surface winds
(units=metres/sec). |
| (b) |
TopeX/Poseidon SSH (units=cm) |
| (c) |
Heat Content simulated by the GFDL OGCM
(units=metres deg Celsius) |
| (d) |
Subsurface thermal structure at the equator
simulated by the OGCM. The shaded quantity
is the difference map of subsurface temperature
(units=deg. Celsius). Also are plotted the
25 degree isotherm (white curve) and 20 degree
isotherms (black curve) as simulated during
opposite phases of the Dipole Mode. |
|
|
