The use of physical decomposition to analyze interannual climate variability in
the southern Indian Ocean
Yi Cai
National Marine Environmental Forecasting Center, State Oceanic Administration,
Beijing, PR China. E-mail: caiy@nmefc.gov.cn
This study physically decomposes SODA (Simple Ocean Data Assimilation) SST (sea
surface temperature) data for the Indian Ocean from January 1945–December 2003,
with each component resulting from the decomposition being analyzed and discussed.
On a basin-wide scale, the zonally averaged heat flux in the Indian Ocean can be
divided into three latitudinal zones: tropical (6°N–15°S), subtropical-mid-latitude
(15°–40°S) and high-latitude (40°–75°S). The heat flux between the mid- to
high-latitude zones takes approximately half a year, and is largely positive from the
mid- to high-latitude zones (although negative heat fluxes in the same direction may
occur); a similar pattern of heat flux occurs between the subtropical to mid-latitude
regions within the second zone. Additionally, there exists a dipole-like SST structure
in the tropical Indian Ocean, whose relationship with the Pacific ENSO (El
Niño/Southern Oscillation) is closer than that between the ENSO and the Indian
Ocean dipole. The correlation coefficient between this dipole-like structure and the
ENSO is -0.87, and the pattern is entirely the result of the monsoons in the Indian
Ocean, with the correlation between the wind fields of a given month and the SST
structure of the subsequent month reaching a coefficient of 0.84. It can be seen that
the physical decomposition method is superior to the usual method of using the mean
monthly climate and decomposing the anomalies. An REOF (rotated empirical
orthogonal function) decomposition of the second component arising from a physical
decomposition of the surface wind stress data for the Indian Ocean reveal a
subtropical tripole structure.