Structure and Dynamics of the Surface Branch of the Meridional Cell in the Indian Ocean

1
Equatorial phenomena
Is there an Equatorial Undercurrent in the Indian
Ocean, dynamically like those in the Pacific and
Atlantic Oceans?
2
Equatorial phenomena
Is there an Equatorial Undercurrent in the Indian
Ocean, dynamically like those in the Pacific and
Atlantic Oceans? No. IO undercurrents are
mostly vertically propagating waves.
3
Equatorial phenomena
Is there an Equatorial Undercurrent in the Indian
Ocean, dynamically like those in the Pacific and
Atlantic Oceans? No. IO undercurrents are
mostly vertically propagating waves. What are the
dynamics of Wyrtki Jets?
4
Equatorial phenomena
Is there an Equatorial Undercurrent in the Indian
Ocean, dynamically like those in the Pacific and
Atlantic Oceans? No. IO undercurrents are
mostly vertically propagating waves. What are the
dynamics of Wyrtki Jets? What is the cause of IO
intraseasonal variability? Is it unstable
currents or wind forcing?
5
Equatorial phenomena
Is there an Equatorial Undercurrent in the Indian
Ocean, dynamically like those in the Pacific and
Atlantic Oceans? No. IO undercurrents are
mostly vertically propagating waves. What are the
dynamics of Wyrtki Jets? What is the cause of IO
intraseasonal variability? Is it unstable
currents or wind forcing? What is the cause of
deeper equatorial currents? Are they vertically
propagating signals, basin resonance, or Deep
Equatorial Jets?
6
Equatorial phenomena
Is there an Equatorial Undercurrent in the Indian
Ocean, dynamically like those in the Pacific and
Atlantic Oceans? No. IO undercurrents are
mostly vertically propagating waves. What are the
dynamics of Wyrtki Jets? What is the cause of IO
intraseasonal variability? Is it unstable
currents or wind forcing? What is the cause of
deeper equatorial currents? Are they vertically
propagating signals, basin resonance, or Deep
Equatorial Jets? Both.
7
Dynamics of Wyrtki jets
8
Intraseasonal oscillations
9
Kindle and Thompson (1989) and Woodberry et al.
(1989) produced a 28-day signal in their
solutions to 1½-layer models forced by monthly
climatological winds, concluding that they were
generated by instabilities of the Somali Current
as it crossed the equator.
10
The instabilities were seasonally modulated, and
radiated Yanai waves into the interior ocean.
11
Current-meter observations from the central and
eastern IO, however, show a prominent biweekly
peak.
Sengupta et al. (2001 2004) forced an OGCM with
NCEP and QuikSCAT winds, and showed that the
oscillations are primarily wind-driven Yanai
waves. Miyama et al. (2004) considered their
dynamics in detail using an OGCM and a linear,
continuously stratified model, and showed that an
oceanic selection mechanism favors biweekly
variability.
12
Time series of zonal and meridional currents at
0N, 90E
Masumoto et al. (2004)
13
Power spectrum density of currents at 0N, 90E
10-20 days variability above 100 m depth, with
20-30 variability below
Broad time-scale with strong 3050 days
variability, shallower than 120 m depth