Indian Summer Monsoon                                              

Connections and teleconnections 

During summers, the Indian subcontinent experiences high temperatures establishing low pressure conditions over the land. On the other hand, the Mascarene High located between 25 and 35 degrees south and 40 and 90 degrees east in the Indian Ocean has high pressure conditions. This difference in the pressure over the land and ocean gives rise to winds from the south-west that carry moisture from the underlying ocean. When the cloud-laden winds reach the land, it rains. With the Mascarene High being well connected with the other parts of the globe, the Indian Summer Monsoon also gets influenced by the changes happening elsewhere. So, along with nearby connections, the Indian Summer Monsoon also has distant relationships known as ‘teleconnections’.  

Schematic of the connections and teleconnections of the Indian Summer Monsoon on different timescales from the present studies

Recently, C. C. Bajish and team from NCPOR have studied one such teleconnection between the monsoon and the Southern Ocean. They worked with atmospheric and oceanic data from observations, satellites and simulations for the period 1979 to 2016. The researchers checked the relation between the monsoon rainfall and the Southern Ocean sea ice. They found that an increase in the monsoon rainfall co-occurred with a decrease in the sea ice extent in the Bellingshausen and the Amundsen Sea which is located below the Pacific Ocean adjacent to the Antarctica. Similarly, when the sea ice extent increased in this sector of the Southern Ocean, the rainfall saw a deficit. This inverse relation between the monsoon and the sea ice extent was stronger during March to May, say the researchers.   

But how does the sea ice around 13000 km away from India affect the monsoon? To find out, the team checked physical parameters in the nearby oceans. The researchers explained that the atmospheric circulation affecting temperatures over the Bellingshausen and Amundsen Sea influences the sea ice extent. The extent decreases when the air circulation increases the temperature. And when the circulation lowers the temperature, the sea ice extent increases. The same atmospheric circulation travels to the tropics through planetary Rossby waves that are generated due to earth’s rotation. These waves carry the signals generated in the Southern Ocean via the Pacific Ocean to the Indian Ocean, and thus impact the monsoon.  

Good thing is that the changes in the Southern Ocean sea ice are happening during March to May, well before the monsoon. Thus, it may give a beforehand information of what coherent responses we can expect in the coming monsoon.  

For details: Is the Indian monsoon rainfall linked to the Southern Ocean sea ice conditions?

If we want to know about past monsoons, information can be retrieved from paleorecords taken from sediment or ice cores. The deeper sections of the cores take us back to the time when the sediments were deposited and these are used by researchers to understand about the past climate.  

Vikash Kumar and team from NCPOR recently used a sediment core drilled from a southern mid-latitude location at 40 degree south and 48 degree east, near the Mascarene High. The core took them around 40 thousand years back. From the different sections of the core, the researchers separated Globigerinabulloides- a planktic foraminifera whose calcite shell records environmental changes. They reconstructed the sea surface temperature by calculating the Magnesium to Calcium ratio of the shells, which has a direct exponential relationship with the temperature.  

The team found that the southern mid-latitude data was coherent with the North Atlantic and Antarctic climate, thus establishing a teleconnection with the Indian Summer Monsoon. They explained that these regions are linked with the monsoon through both atmospheric and oceanic pathways. While the atmospheric pathways act at shorter timescales, the oceanic pathways operate via currents that link these geographically distant locations at a millennial timescale.

For details:  SST Changes in the Indian Sector of the Southern Ocean and their teleconnection with the Indian Monsoon during the Last Glacial Period

Let us dig a bit beyond. With more carbon dioxide concentrations, the atmosphere was warmer during 2.6 to 3.4 million years ago than the present. How was the monsoon then? 

To study this, a team led by Manish Tiwari investigated a sediment core from the eastern Arabian Sea that covered this period. To reconstruct the monsoon of that time, they checked the distribution of elements that indicate monsoonal weathering and erosion.  

With a prevailing intense monsoon, the team noticed a jump in the intensity at around 2.95 million years ago in the core data. Around the same time, there was a northward shift of the New Guinea island. This shrunk the Indonesian Throughflow, the oceanic connection between the western Pacific Ocean and the eastern Indian Ocean. The shrinking led to the inflow of colder north Pacific waters to the Mascarene High increasing the sea-level pressure over the region, say the researchers. This increased the pressure difference between the land and the ocean which resulted in the very intense Indian Summer Monsoon at 2.95 million years ago. And after this event, the monsoon declined due to global cooling.  

For details: South Asian Summer Monsoon precipitation variability during late Pliocene: Role of Indonesian Throughflow

Thus, the Indian Summer Monsoon has evolved and is evolving over time by various connections and teleconnections through the involvement of the Mascarene High. If we can gather all such links together, we may bring the monsoon forecast closer to reality.