The Indian Ocean, which is tropical in nature, plays a significant role in global climate variability. In recent decades, extreme weather events, such as heatwaves, cyclones, droughts and floods, are becoming increasingly intense and frequent. This is mainly the case for the World’s coastal communities, which make up 30% of the global population and are highly vulnerable to extreme climatic conditions and sea level rises.
Ocean warming, in addition to several other climate change factors, has a direct impact on sea level rises. Therefore, a better understanding of climate variations over several decades in the Indian Ocean, defined as “decadal variability”, helps in improving the ability to predict the sea surface temperature and various climate parameters on a regional and global level.
Measuring and understanding decadal variability, and using it as a tool to predict climate changes, is the focus of my research at the University of Tasmania in Australia. Climate predictability will help society to prepare more effectively for any adverse effects resulting from future climate change.
Why is the tropical Indian Ocean an ideal candidate to study climatic impacts?
The Indian Ocean is currently receiving global attention and is an ideal case study due to the simultaneous occurrence of several complex processes that drive the global climate. My studies show that the tropical Indian Ocean has shown a strong basin-wide warming trend in recent decades, relative to other tropical oceans (Figure 1).
Several observational and modeling studies have speculated that the warming of the Indian Ocean has been caused by natural factors (such as volcanic eruptions and variations in solar radiation) and anthropogenic factors (such as increased greenhouse gases), as well as internal processes (such as the transfer of heat from the Pacific Ocean).
Apart from substantial ocean warming, considerable evidence shows that the decadal variability of the Indian Ocean’s sea surface temperature and surface wind, has had significant climatic impacts both regionally and globally.
The role of multidecadal climate oscillations in the Indian Ocean
Multidecadal climate oscillations are climate patterns associated with the interactions of atmospheric and oceanic conditions, measured over several decades.
Multidecadal climate oscillations have several climate impacts. Specifically, Interdecadal Pacific Oscillation (IPO) over the Pacific Ocean and Atlantic Multidecadal Oscillation (AMO) over the Atlantic Ocean, are known to influence the Indian Ocean and the Indian monsoon, through oceanic and atmospheric pathways, that regulate excess (deficit) rainfall over India.
The impacts of IPO and AMO on the Indian Ocean and the Indian monsoon have been reduced since the 1990s, primarily due to increased concentrations of anthropogenic aerosols and volcanic eruptions (Figure 2). However, my research, conducted in 2020 at the Indian Institute of Tropical Meteorology, shows the Indian Ocean still displays decadal/multidecadal variability.
The data from my research suggests that several underlying internal/local processes are the driving forces for the decadal and multidecadal variability observed in the temperatures of the Indian Ocean.
The link between the Eastern equatorial Indian Ocean and the Indian Monsoon
The Indian Monsoon is the world’s most powerful monsoon system. It mainly affects India and its surrounding water bodies, and turns large chunks of the country from a semi-desert into flourishing green lands.
Importantly, the Indian Monsoon is heavily impacted by changes in sea surface and subsurface temperatures.
The Indian Ocean Dipole is characterised by an opposing sea surface temperature anomaly between the western equatorial Indian Ocean and the southeastern equatorial Indian Ocean. It has two phases: positive and negative. Events usually start around May or June, peaking between September and November. The strong positive Indian Ocean Dipole events, i.e., warming over the western equatorial Indian Ocean and cooling over the southeastern Indian Ocean, correspond to excess rainfall over India.
Other than surface temperature, subsurface temperatures over the eastern equatorial Indian Ocean (one of the critical regions for Indian Ocean Dipole events) shows multidecadal and decadal signals observed since the mid-1990s. This is a unique feature compared to the multidecadal variations observed in other regions of the Indian Ocean. Subsurface variability is crucial, as it interacts with the surface and consequently affects the air-sea interaction processes. As compared to surface variability, subsurface variability and its consequences are relatively under-explored.
The eastern equatorial Indian Ocean region is significant for climate research due to its association with the Indian Ocean Dipole and, therefore, the Indian Monsoon.
The role of surface wind
Consistent with the eastern equatorial Indian Ocean subsurface temperature, surface winds have also shown significant multidecadal/decadal variability after the mid-1990s.
From 1962 to 1993, Interdecadal Pacific Oscillation (IPO) was strongly correlated with the equatorial Indian Ocean wind. Prior to the mid-1990s, decadal changes observed in the equatorial Indian Ocean winds and the eastern equatorial Indian Ocean subsurface temperature were closely associated with internal local factors, such as Indian Ocean warming, low-level jet, Mascarene high, and precipitation over the Indian landmass (Figure 3). This association highlights the growing role of local processes internal to the Indian Ocean region. In short, it implies a major decadal climate shift since the mid-1990s.
Overall, the Indian Ocean is becoming more active and independent rather than being forced by Pacific climate variability. A recent paper, published by myself and colleagues in the journal Global and Planetary Change, further highlighted that the oceanic heat in the upper 500m of ocean levels has several climate consequences, including heat transport across the Indian Ocean and subsequent regulation on a decadal time scale.
What does this research teach us about climate variability?
Climate variability in decadal timescales has been identified, and extensively studied in the Atlantic and Pacific Oceans. However, in the case of the Indian Ocean, it remains far less understood. This is critical, as studies have shown that the decadal variability in the Indian Ocean has affected the North Atlantic and Pacific climate regimes. In other words, the Indian Ocean has become more active and has started modulating the global climate. Moreover, the Indian Ocean and its overlying wind circulation are widely recognised as essential factors for regional and global climate.
The Indian ocean shows climate variability in temperature on both decadal to a multidecadal time scales, with the most significant variations seen at the subsurface level. It is not surprising therefore, that communities residing in coastal regions of the Indian Ocean are becoming increasingly anxious about climate change and its adverse impact on the environment. My research suggests that the rest of the world should take notice of these shifts too, as they are starting to impact climates and sea levels on a more regional and global scale.