5.3.4 Indian Ocean

The upper Indian Ocean has been warming everywhere except in a band centred at about 12°S (South Equatorial Current), as seen in Section 5.2 (Figure 5.3). In the tropical and eastern subtropical Indian Ocean (north of 10°S), warming in the upper 100 m (Qian et al., 2003) is consistent with the significant warming of the sea surface from 1900 to 1999 (see Section 3.2.2 and Figure 3.9). The surface warming trend during the period 1900 to 1970 was relatively weak, but increased significantly in the 1970 to 1999 period, with some regions exceeding 0.2°C per decade.

The global-scale circulation includes transport of warm, relatively fresh waters from the Pacific passing through the Indonesian Seas to the Indian Ocean and then onward into the South Atlantic. Much of this throughflow occurs in the tropics south of the equator, and is strongly affected by ENSO and the Indian Ocean Dipole (see Section 3.6.7.2). The latter causes pronounced thermocline variability (Qian et al., 2003) and includes propagation of upper-layer thickness anomalies by Rossby waves (Xie et al., 2002; Feng and Meyers, 2003; Yamagata et al., 2004) in the 3°S to 15°S latitude band that includes the westward-flowing throughflow water.

Long-term trends in transport and properties of the throughflow have not been reported. The mean transport into the Indonesian Seas measured at Makassar Strait from 1996 to 1998 was 9 to 10 Sv (Vranes et al., 2002), matching transports exiting the Indonesian Seas (e.g., Sprintall et al., 2004). Large variability in this transport is associated with varying tropical Pacific and Indian winds (Wijffels and Meyers, 2004), including a strong ENSO response (e.g., Meyers, 1996), and may be associated with changes in SST in the tropical Indian Ocean.

Models suggest that upper-ocean warming in the south Indian Ocean can be attributed to a reduction in the southeast trade winds and associated decrease in the southward transport of heat from the tropics to the subtropics (Lee, 2004). The export of heat from the northern Indian Ocean to the south across the equator is accomplished by a wind-driven, shallow cross-equatorial cell; data assimilation analysis has shown a significant decadal reduction in the mass exchange during 1950 to 1990 but little change in heat transport (Schoenefeldt and Schott, 2006).

Changes in Indian subtropical gyre circulation since the 1960s include a 20% slowdown from 1962 to 1987 (Bindoff and McDougall, 2000) and a 20% speedup from 1987 to 2002 (Bryden et al., 2003; McDonagh et al., 2005), with the speedup mainly between 1995 and 2002 (Palmer et al., 2004). The upper thermocline warmed during the slowdown, and then cooled during speedup. Simulations of this region and the analysis of climate change scenarios show that the slowdown and speedup were part of an oscillatory pattern in the upper part of this gyre over periods of decades (Murray et al., 2007; Stark et al., 2006). On the other hand, the lower thermocline (<10°C) freshened and warmed from 1936 to 2002 (Bryden et al., 2003), consistent with heat content increases discussed in Section 5.2 and earlier results.