RESULT FRONTIER Newsletter No.10 Mar.2000 


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 The Ocean at work during the Indian Ocean Dipole Mode


Saji N Hameed (FRSGC)

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.


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