2007 Annual Report - jamstec japan agency for marine-earth ...

Ocean around Nov. 2005, more than 9 months before the
appearance of the IOD event Fig.1, and subsequent eastward
propagation of the upwelling Kelvin waves along the
equator, showing the importance of the ocean dynamics for the
IOD predictability.
In 2007, the positive IOD event occurred again in the tropical
Indian Ocean, generating the first consecutive positive IOD
years ever since early 1950's, when the recent comprehensive
observations started to take place. At the same time, La Nina
phenomenon appeared in the Pacific Ocean, which provided
quite rare condition with positive IOD in the Indian Ocean, 40
years after the previous event of the same situations in the tropical
Indo-Pacific sector in 1967.
Figure2 indicates horizontal distribution of the SSTA averaged
over three months during Sep. to Nov. 2007, predicted
from Apr. 2007 as the initial condition for the SINTEX-F
CGCM forecast experiment. The typical SSTA pattern of the
positive IOD event can be seen in the tropical Indian Ocean,
while the eastern equatorial Pacific demonstrate large negative
SSTA associated with strong La Nina event. Successful prediction
of the both climate events in the two basins suggests the
significant performance of SINTEX-F model in reproducing not
only the climate modes themselves but also the climatological
seasonal cycle as basic states for these climate modes.
Most of the results were published in major scientific journals,
and the press release on the 2007 IOD prediction caught
strong attention of public. We will continue the study on the
processes affecting the ENSO/IOD predictability and will try to
enhance our contribution to disaster prevention activities in the
regions surrounding the Indian Ocean, through improving the
prediction skills of the short-term climate variability over Indo-
Pacific sector.
b. Studies on oceanic variability based on the OFES simulation
Collaborating with the Earth Simulator Center, we have
conducted very high resolution ocean simulations using the
Ocean GCM for the Earth Simulator OFES. Those are
highly valuable to improve our understanding of various spatiotemporal
oceanic variations not only in the surface layer, which
interact with the atmosphere directly, but also those in the subsurface
ocean interiors. In our program, we investigate oceanic
variability from extensive viewpoints: behaviors of eddies, their
impacts on large-scale ocean circulation, global ocean energy
balance, and so on. Meanwhile, here a particular study on
decadal variability in the Pacific Ocean is reported.
For decadal variability in the tropical Pacific Ocean, many
hypotheses have been proposed. One of them suggests that
oceanic temperature anomalies subduct into the subsurface
layer in the subtropics, move equatorward, and surface there to
induce equatorial SST anomalies. For the North Pacific Ocean,
this has been denied. On the contrary, in the South Pacific, such
equatorward temperature anomaly propagation has been found
in observation, ocean GCMs, and a coupled GCM.
Interestingly, cool warm subsurface temperature anomalies
tend to be formed when SSTs have warm cool anomalies;
their formation mechanism is not intuitive and was not understood.
While importance of diapycnal mixing at the bottom of
the surface mixed layer was suggested recently, Nonaka and
Sasaki 2007, J. Climate gave a simple explanation for this
counter-intuitive temperature anomaly formation by paying
attention to large meridional migration of outcrop line of isopycnal
surfaces along which the temperature anomalies subduct.
Figure3 shows that subsurface temperature anomalies
propagate from the southeastern subtropical South Pacific to the
western boundary region panel b, and further extend to the
equatorial region panel a. On the one hand, in the formation
region of the anomalies, warm cool anomalies tend to
be formed panels b and c when the latitudes where the
isopycnal surface that the anomalies sudbuct intersects with the
sea surface migrates equatorward poleward, as warm
Fig.2 Predicted sea surface temperature anomaly SSTA during
Sep./Oct./Nov. 2007, starting from Apr. 1st, 2007. Typical SSTA patterns
of the positive IOD event and La Nina phenomenon can be seen in
the tropical Indian Ocean and Pacific Ocean, respectively.
cool water exists in lower latitudes. On the other hand, the
outcrop latitudes tend to migrate equatorward poleward