International Journal of Marine Science2016, Vol.6, No.56, 1-9
5
Figure 3 Monthly mean anomalies of Ekman pumping/suction (W
EK
) averaged over the region same as in figure 2, covering (a)
1982-83 El Nino and (b) 1988-89 La Nina event
3.2 Positive and Negative IOD events
The Indian Ocean Dipole is a coupled ocean and atmosphere phenomenon in the equatorial Indian Ocean that
affects the climate of Australia and other countries that surround the Indian Ocean basin (Saji et al., 1999). The
strength of IOD event is commonly measured by an index called the Dipole Mode Index (DMI), which is the
difference between SST anomalies in the western (50°E to 70°E and 10°S to 10°N) and eastern (90°E to 110°E
and 10°S to 0°S) equatorial Indian Ocean. The strong positive IOD events during 1994 and 1997 and the strong
negative IOD event of 1992 are considered for computation of Ekman pumping.
Figures 5 shows monthly anomalies of (a) τ
alongshore
, (b) Ekman pumping, (c) MLD and (d) SST averaged over the
region (97°- 100°E, 2°S - 5°S) in the south east equatorial Indian Ocean, from January 1992 to December 1998
which includes the IOD years 1992, 1994 and 1997. During the negative IOD year 1992, τ
alongshore
anomalies in the
eastern box are negative (minimum ~ 2.5 *10
-2
Nm
-2
in September 1992) from figure 5(a) showing reduction in
along shore wind stress which would deepen the thermocline. Also negative Ekman pumping anomalies (~-9 *10
-5
m/s) are playing role in deepening the thermocline and effectively increasing the mixed layer depth (Fig. 5b &c).
Positive MLD anomalies of the order of 20 m are clearly seen in figure 5(c). Positive SST anomalies of the order
of (> 0.5
˚
C) exist due to downwelling in the eastern box (Fig. 5d).
In the positive IOD year 1994, positive τ
alongshore
anomalies of the order of 5*10
-2
Nm
-2
are seen in the month of
June associated with positive Ekman pumping anomalies of magnitude ~12*10
-5
m/s from figure 5(a &b). The
negative MLD anomalies (> 20 m) and negative SST anomalies (> 2
˚
C) are also evident from figures 5(c &d).
Similarly during another positive IOD event in 1997, positive τ
alongshore
anomalies (>4*10
-2
Nm
-2
), and positive
Ekman pumping anomalies (> 6*10
-5
m/s) and negative MLD anomalies (>20 m) and negative SST anomalies
(>2.5
˚
C), are readily seen from the figures 5(a, b, c &d). These results are consistent with the earlier study in
which positive wind curl anomaly was found in the eastern tropical Indian Ocean between 0
˚
-10
˚
S during the
positive phase of the IOD (Saji et al., 1999; Huang and Kinter III, 2002) causing cooler sea surface due to
upwelling.
