Figure 1b illustrates the time series of the dipole mode index (DMI) from January 1993 to December 2009, which is defined as SSTA differences between the western and eastern equatorial Indian Ocean (Saji et al. 1999). There were three major IOD events during this time, namely, in 1994, 1997 and 2006 when the DMI was about twice its standard deviation. In addition, we also observed three weak positive IOD events in 2003, 2007 and 2008. The time series also shows two significant negative IOD events in 1996 and 1998. These events are consistent with those identified by the Bureau of Meteorology of Australia (http://www.bom.gov.au/climate/iod/). In this study, we focus on the termination of positive IOD events, with emphasis on the strongest of these in 1997.
The contribution of horizontal heat advection to the termination of IOD event was quantified in terms of the mixed layer temperature balance defined as (Vialard et al. 2008)
$$ \frac{\partial T}{{\partial t}} = \frac{Q + q}{{\rho Ch}} - u\frac{\partial T}{{\partial x}} - v\frac{\partial T}{{\partial y}} + R, $$
(1)
where T is mixed layer temperature, which we will use as a proxy for SST (Horii et al. 2013). In Eq. (1), t indicates time, while x and y denote the zonal and meridional direction, respectively. ρ is the density of seawater (1026 kg m−3), C is the specific heat capacity of seawater (3986 J kg−1 K−1), h is the depth of mixed layer, and u and v are the zonal and meridional velocity averaged over the mixed layer, respectively. Q is the net surface heat flux and q is the downward shortwave radiation at the bottom of the mixed layer. The later is estimated as q = − 0.47 Qsw e(−0.04 h), where Qsw indicate the shortwave radiation on the ocean surface (Wang and McPhaden 1999). The last term in Eq. (1) is the residual (R), which consists of physical processes that cannot be explicitly calculated, namely, a combination of horizontal and vertical diffusion and vertical entrainment at the bottom of the mixed layer. The residual also contains computational errors associated with terms that are explicitly estimated. However, we assume that these computational errors are small and interpret the residual in terms of the neglected physical processes, especially associated with vertical turbulent entrainment and diffusion as is commonly done (e.g., McPhaden 1982; Vialard et al. 2008).
Subsurface temperature and salinity data from Argo observations from January 2005 to December 2009 were used to calculate the monthly mixed layer depth (h). Considering the effect of salinity on the stratification in the eastern equatorial Indian Ocean, we estimate the mixed layer depth as a depth at which the density is 0.2 kg m−3 larger than the surface density (Kumar et al. 2011). Since there is no observed temperature and salinity data for the period from January 1993 to December 2004, we used the mean climatology of the mixed layer based on the time series over the period of January 2005–December 2009.
The mixed layer temperature balance was calculated for a region in the eastern equatorial Indian Ocean between 8°S–2°N and 90°E–100°E (Fig. 1a). SSTA in this region is highly correlated with SSTA in the eastern pole of the IOD (r = 0.93 which is significantly nonzero with 95% confidence) for the period of January 1993–December 2009 (Fig. 1c) and so is representative of variability in the eastern pole of the dipole. It has the advantage though of being away from the irregular eastern boundary and in the equatorial waveguide, where wave processes can be readily diagnosed with the linear wave model.
Horizontal advection is calculated based on the formula proposed by Lee et al. (2004) in terms of heat fluxes across each interface of the designated domain relative to a reference temperature. Here, we used a spatially averaged SST over the region between 8°S–2°N and 90°E–100°E (Fig. 1a) as the reference temperature.
We start by examining the evolution of the Dipole Mode Index (DMI) associated with the IOD event in 1997. A positive DMI started to develop in mid-June corresponding to SST cooling in the eastern pole of the IOD (Fig. 2a, b). In September, a rapid increase of the DMI coincided with a rapid SST cooling event in the eastern Indian Ocean that began towards the end of August and continued until mid-November. There were two negative SST maxima in the eastern pole of the IOD occurring in early October and mid-November (Fig. 2a). We note that the onset of SST cooling in September corresponds well with zonal heat advection that shows an anomalous cooling from early September through early October, suggesting the critical role of zonal heat advection in the development of the IOD event (Fig. 2a, b). Furthermore, the inferred vertical entrainment and diffusion across the base of the mixed layer exhibits a cooling tendency at about the same time as SST begins to cool (Fig. 2c). The termination of the IOD is identified by a rapid decrease of the DMI from mid-November to early December before it completely returns to normal condition in mid-January 1998 (Fig. 2a). Interestingly, the timing of DMI decrease and the SST warming tendency in the eastern pole during October is associated with the weakening of vertical entrainment as well as zonal heat advection (Fig. 2c).
We next examine the relative role of different terms in the mixed layer temperature balance (Fig. 2c). We can see that during the development of the IOD event in September, the cooling tendency was associated with westward (negative) zonal heat advection linked to anomalous westward currents driven by easterly wind anomalies (Fig. 3a, b). In addition, vertical processes also tend to cool (Fig. 2c) consistent with earlier studies that noted anomalously strong upwelling-mediated entrainment and turbulent mixing generated cold SSTs in the eastern pole of the IOD during positive events (Murtugudde et al. 2000; Du et al. 2008). Later, during the termination of the event, net surface heat flux plays a role in SST warming. However, there is a phase lag between the net surface heat flux variations and the weakening of warming tendency. In particular, net surface heat flux leads the surface warming by about 1 month. Interestingly, there was abrupt SST warming and DMI decrease during October, which is associated with a weakening of both zonal heat advection and vertical entrainment/diffusion (Fig. 2c). From November toward the end of the event in December, the surface heat flux becomes effective at warming the surface layer after zonal heat advection and vertical processes diminish in intensity. Thus, the weakening of zonal heat advection and inferred vertical turbulent entrainment and diffusion provide conditions favorable for surface heat flux to warm SST during the termination of the IOD event. Note that meridional advection has little influence on the overall balance (Fig. 2c).
In order to examine the dynamics of the zonal current anomalies along the equator, we evaluate the results of our linear wave model (Fig. 4). The model simulates reasonably well the anomalous westward zonal currents during the peak phase of the IOD in October–November (Figs. 3a, 4d, e). It is shown that westward zonal currents in the eastern basin are mainly due to wind-forced Kelvin waves, while westward currents in the central and western basins are mainly due to Rossby waves (Fig. 4a, c). As previously noted by Nagura and McPhaden (2010a), zonal currents along the equator eventually become eastward in late November although the zonal winds continue to be westward (Fig. 4d). Our model clearly indicates that this reversal is mainly generated by eastern boundary-generated Rossby waves (Fig. 4b, d). The timing of this zonal current reversal during the termination of the IOD event in late November/early December coincides with the weakening of the zonal heat advection in mixed layer temperature analysis (Fig. 2c). The combination of Kelvin and Rossby waves to the total model zonal velocity along the equator in our target region of 8°S–2°N, 90°E–100°E (Fig. 3) highlights the central role of eastern-boundary-generated Rossby waves in the termination of the IOD event. Note that the contributions from the wind-forced Rossby and the reflected Kelvin waves in this region are negligible (not shown).
