Weather Bee: What is the southwest monsoon?

Hindustan Times

3 days ago

This question became all important on May 26, the day the India Meteorological Department (IMD) declared monsoon onset (the monsoon at this time of the year is called the southwest monsoon) over Mumbai. As HT's Mumbai edition reported on May 27, scientists not affiliated with the IMD debated if the monsoon had indeed arrived over the city. Without knowing what the southwest monsoon is, one cannot decide who among the two is right.
So who among the IMD and non-IMD scientists is right? They both are in their own way, although it'll perhaps be prudent for the IMD to address the concerns raised by scientists elsewhere. Here is why.
The short reason why IMD was not wrong in declaring the arrival of monsoon in Mumbai is that there are no strict criteria for declaring monsoon onset for places other than Kerala. The long reason can be understood by reading IMD's procedure for declaring monsoon onset, which is as follows:
'At present IMD uses a new-criteria adopted in 2016 for declaring the onset of monsoon over Kerala which was based on the daily rainfall of 14 stations over Kerala and neighbouring area along with wind field and Outgoing Longwave Radiation (OLR) over southeast Arabian Sea. The new criteria emphasize on the sharp increase in rainfall over Kerala along with setting up of large-scale monsoon flow and extension of westerlies up to 600 hPa. However, IMD declares monsoon onset/progress dates for other regions operationally in a subjective manner considering the sharp increase in rainfall and its characteristic sustenance for a few days and associated changes in the atmospheric circulation features'.
The FAQ then goes on to lay down the precise values of the different criteria used for Kerala, which HT explained here. However, no such precise values are laid down for the rest of the country. Therefore, it is not surprising that the IMD declared the arrival of monsoon in Mumbai after the city saw a 'sharp increase in rainfall and its characteristic sustenance for a few days'. To be sure, rain alone would not be enough, as even for regions beyond Kerala, the IMD does look for 'associated changes in the atmospheric circulation features'. One such change it would have looked for is shown in the accompanying map for winds at the height of 850 millibar (or hectopascal) pressure or 1-1.5 km, which are currently blowing from the west (called westerly) over the entire coast of Maharashtra.
Why are the scientists disputing the IMD's declaration then? As IMD's procedure copied above suggests, a rainy day or two does not make the monsoon. The sustenance of this rain over a prolonged period – the season is around four-months long although it does not rain every day of the season – depends on things changing in the atmosphere. One such change is the westerly winds shown above, which have origins south of the equator and start off blowing from a south-west direction, giving the season its 'southwest' prefix. The reason scientists disputed the IMD despite this wind being seen at 850 mb is that these winds were not high enough. As the map below shows, at a height of 700 hectopascal (around 2-3 km), the winds even on May 30 are blowing from the north rather than the west on the northern coast of Maharashtra, where Mumbai is.
How high then should the westerly winds be for one to know that the monsoon has arrived? Since the atmosphere is not just winds at two levels, relying on this metric alone can also mislead sometimes; but one can answer this question for settling the debate in this particular monsoon. This can be done by looking at the typical monsoon weather and comparing it with what happened in the current monsoon. To do so, HT looked at the average 700 mb wind on July 7 from 1971 to 2020, the period currently considered the Long Period Average (LPA) for tracking monsoon's performance. HT chose July 7 because monsoon onset over Mumbai – as declared by the IMD – varied from May 31 to June 28 from 1971 to 2024. Therefore, July 7 likely puts one in a period when monsoon had likely arrived over Mumbai even if one were using stricter criteria for the city. And as the map for July 7 from earlier years shows, 700 mb winds have been decidedly more westerly over northern Maharashtra coast during the monsoon in the past than they are on May 30.
Clearly, westerly winds should develop at least up to the height of 700 mb for the rains to be called monsoon rains at the Maharashtra coast. As the IMD itself suggests that the monsoon is not just rain, perhaps it can lay down more specific criteria for places beyond Kerala in its guidelines for declaring monsoon arrival. This will also help it avoid the kind of situation that has happened this year. While monsoon onset was declared over Mumbai on May 26, there was no rain in Mumbai or Mumbai suburban districts just four days later (on May 30). That is hardly a Mumbai monsoon.