2023 Southwest Monsoon Seasonal Outlook

We are rapidly approaching the Southwest Monsoon season across the northern Indian Ocean. While there are several large-scale, climatological controls affecting the onset and intensity of the Southwest (SW) monsoon, a couple of factors stand out this year. In terms of ship routing, more direct routes may be possible across the Arabian Sea this season (depending on vessel size and cargo). As always, we will monitor SW monsoon strength and keep our clients advised.

SW Monsoon Background

The SW monsoon is persistent southwesterly winds blowing near the equator in the Indian Ocean northeastward into the Arabian Sea and Bay of Bengal. These winds are generated by broad low pressure across the Indian subcontinent into Southeast Asia. Intense late spring and summer sunshine leads to differential heating between land and sea - temperatures over the subcontinent quickly increase relative to air temperature over the ocean. This creates low pressure and rising air over land, which then causes air from over the ocean to move northeastward toward land. Because of how large the subcontinent is, the low pressure and southwesterly winds persist with only occasional interruptions throughout the summer season. The SW monsoon generally starts in late May into early June, peaks in late June into August, then decreases into early autumn as air temperatures cool over the subcontinent.

Forecast Discussion

There are three prominent influences expected to affect the SW monsoon in 2023. Firstly, after three years, La Ni?a (cooler waters in the eastern tropical Pacific) has dissipated. The Climate Prediction Center now calls for El Ni?o (warmer waters in the eastern tropical Pacific) to develop over the next few months. In general, El Ni?o disrupts the monsoonal flow over the subcontinent, such that the southwesterly winds are not as persistent or strong.

Also, the Indian Ocean Dipole (IOD) is presently forecast to become positive over the next couple of months, meaning above normal sea surface temperatures (SSTs) will develop over the western Indian Ocean, with below normal SSTs west of Sumatra. Warmer waters in the western part of the basin lead to stronger surface convergence over the region. This means southwesterly winds will likely be weaker over the Arabian Sea.??

Finally, looking at the Madden-Julian Oscillation (MJO) gives a good indication of how soon the SW monsoon will kick into gear. The MJO is an atmospheric wave near the equator that travels west to east around the planet every 30-60 days. An MJO wave has profound effects on weather near the tropics. The rising part of the wave initiates rising motion and thunderstorms, while the sinking portion does the opposite. The latest MJO forecast suggests that sinking motion will be present over/near the subcontinent through early June. This will help to delay the onset of strong southwesterly winds over the Arabian Sea.