The core of the southwest monsoon regime has been showing a tendency to shift north along India’s West coast, leaving the south increasingly dry.
This shift has been documented over the past 30 years. Going forward, the region south of Goa could gradually dry up while rains grow heavily over northwest India.
Fate of ghats
This is suggested in a paper co-authored by S Sandeep and R Ajayamohan of the Centre for Prototype Climate Modelling of New York University at Abu Dhabi.
Titled ‘Poleward shift in Indian summer monsoon low-level jet stream under global warming,’ the paper raises concerns about the fortunes of the ecologically fragile Western Ghats in the affected region.
The low-level jet comprised of super-fast winds forms the core of the southwest monsoon. The jet has been shifting to the north along the west coast of India.
It transports moisture from the surrounding oceans to the Indian landmass and is hence an important component of the southwest monsoon.
The decreased precipitation over the southern regions of the Ghats could lead to water stress in those regions.
In the northern regions, however, heavy precipitation events may be due to increased wind speeds and higher moisture content in the air.
The low-level jet has been strengthening north of the 15 deg North latitude (around Goa) and weakening south of it during the last three decades.
Part of the weakening of the jet could be compensated by the increased supply of moisture as oceans heat up in a warming environment.
This, the authors suspect, may prevent a stronger decrease in precipitation south of the Goan latitude.
At the same time, northwestern India has a stronger possibility of getting increased precipitation in a warming environment.
This is due to the dual impact of the northward shift of the jet and enhanced moisture content of the air, which is what creates rainfall.
Separately, the authors feared that the shift in the jet could cool the sea-surface temperatures over the North Arabian Sea.
Since the warmth of the sea decides the extent of evaporation and cloud formation, this can affect the mid-season variability (sudden shift to a lull or high-intensity precipitation) of the monsoon.
The general northward shift in the monsoon is also reflected in future projections in a warming scenario, with the magnitude depending on the degree of warming.
The monsoon represents the flow of winds from the sea (high pressure) towards the landmass that represents an area of lower pressure from sustained summer heating.
The land surface experiences a faster rate of heating compared to the oceans as the atmosphere warms under the influence of enhanced GHGs.
Enhanced land-sea contrast has resulted in a strengthening of the sea-level atmospheric pressure gradient over the Indian Ocean.
The faster rate of heating over land results in deepening of the low pressure over land and subsequently strengthens the cross-equatorial pressure gradient.
The stronger cross-equatorial pressure gradient pushes the monsoon circulation to the north. The land-sea contrast will amplify in a warming scenario.
The magnitude of the shift would be proportional to the degree of heating of land and sea.