Evolution Mechanisms For The Two Poles Of The Indian Ocean Dipole And The Relationship Between Them

The Indian Ocean Dipole (IOD) is an air-sea coupled phenomenon. On the interannual time scale, it is one of the dominant modes in the tropical Indian Ocean. The IOD is considered as an important mechanism for the changes in the tropical Indian Ocean and global climate due to its significant impact on the climate of both the surrounding and remote regions. Understandings of the mechanisms of IOD have important significance on studies of climate change.Interannual variations in the surface and subsurface tropical Indian Ocean are studied using HadISST and SODA datasets. The relative role of ocean dynamics and surface heat flux in the initiation and development of the IOD is investigated by analyzing results from a general circulation ocean model (HYCOM).A series of experiments are conducted to assess the relative importance of the eastern and western Indian Ocean forcing fields in driving the IOD.The anomalous ocean circulation during IOD is also studied. The main conclusions are as follows:1.The surface and subsurface variations of the tropical Indian Ocean during IOD events are significantly different. A prominent characteristic of the eastern pole is the SSTA rebound after a cooling process, which does not take place at the subsurface layer. In the western pole, the surface anomalies last longer than the subsurface anomalies.The subsurface anomalies are strongly correlated with ENSO, while the relationship between the surface anomalies and ENSO is much weaker. The subsurface anomalies of the two poles are negatively correlated while they are positively correlated at the surface layer.2.The mixed layer heat budget analysis indicates that the vertical advection and the surface heat flux are both important to the western warming. In IOD years without El Nino,the positive heat flux anomalies are not obvious, and the vertical advection persists shorter than in the years that IOD co-occurred with El Nino.So the western warming is not evident in these years.The development of eastern cooling is mainly caused by vertical advection, the cooling begins earlier in IOD years without El Nino. As the IOD decays, the eastern pole begins to warm up.It is primarily induced by anomalous surface heat flux. In IOD years without El Nino, the surface heat flux anomalies are weaker. Also the eastern pole begins to warm in September, when the vertical advection caused cooling is still strong, so the eastern rebound in IOD years without El Nino is much weaker than IOD years with El Nino.3.The EOF analysis is applied to ocean circulation anomalies in several vertical sections of the tropical Indian Ocean. The results indicate that the first modes of anomalous current in these sections are all determined by IOD events. The influence of El Nino is relatively weaker.4.Ttest runs are performed to assess the relative importance of eastern and western Indian Ocean forcing fields in driving the IOD. In these runs the interannual variability of all atmospheric forcing fields are separately suppressed in the western and eastern tropical Indian Ocean. The results indicate that the SST anomalies of the eastern and western pole are mainly determined by local forcing fields.The long persistence warming of the western pole can mainly be attributed to local forcing. Different form the SST, the thermocline depth variations in the eastern and western Indian Ocean has the characteristic of entirety. The western deepening and eastern shoaling always happen together. Both the eastern and western forcing test runs can trigger this pattern.