Early look at India's southwest monsoon

For my first global blog entry, I was going to go through some of our early thinking for the upcoming India southwest monsoon.

To begin, the southwest monsoon in India typically starts around June 1 in Kerala and lasts through September. There are several teleconnections that we have been watching to work up the forecast.

One of the first teleconnections is Eurasian snowcover. Below-normal snowcover from late winter into spring usually means a good monsoon. Looking at data from the Rutgers University Global Snow Lab, we can see February 2020 snow cover anomalies were at the third-lowest level only behind 2002 and 1995. The seasonal snow extent for the winter of 2019/2020 as a whole was at its lowest level since 2007; see graphics below.

Another area we are watching is the Quasi-biennial Oscillation (QBO) which is a measure of the winds in the stratosphere. A negative value means an easterly wind, while a positive value means a westerly wind. Papers have shown that an easterly (negative) QBO at 15 mb during the winter typically translates to a weakened monsoon. An easterly QBO at 50 mb during the monsoon can also mean a weak monsoon.

As we can see, the winds at Singapore were easterly at 15 mb over this past winter with easterlies attempting to descend to 50 mb. The descent has been slow, but there are signs that easterly winds are possible in the lower stratosphere during the summer. We will have to watch.

There are also some water-temperature-based connections to rainfall in India. The big one that helped produce a near- to above-normal monsoon in 2019 was the development of the strongest positive Indian Ocean Dipole (IOD) during the summer into autumn. In a +IOD, waters off Indonesia cool below normal, while the waters off eastern Africa are warmer than normal. This setup enhances the monsoon circulation and leads to wetter-than-normal conditions across much of India.

Unlike last year, most indications are the IOD will remain neutral or perhaps trend weakly negative in late summer into autumn. This will likely mean the IOD will be less of a factor during the 2020 Indian monsoon.

Another big water-temperature-based teleconnection is related to the sea surface temperature anomalies over the equatorial Pacific Ocean which is referred to as ENSO. A weak central Pacific El Nino developed over this past summer which means warmer than normal waters near the date line. El Nino conditions typically mean below-normal rainfall across India during the monsoon season, but the weak CP El Nino was washed out by the strong +IOD.

Overall ENSO will be largely neutral into early summer, but there are some signals that La Nina conditions can develop late in 2020. La Nina conditions usually mean good rainfall during the Indian summer monsoon. The wildcards are: 1. Does a La Nina develop and 2. Just how quickly does the phenomenon develop, so we will continue to watch.

One of the last areas we will touch on is sea-surface temperatures over the Atlantic Ocean. Warm waters in the Atlantic Ocean tend to favor a more active and wetter monsoon. The effect is amplified when this is in sync with ENSO. As we can see above, waters across the Atlantic are expected to remain warmer than normal which can mean a positive signal for India's summer monsoon especially if a La Nina develops as some models suggest.

Several seasonal forecast models from March show near- to above-normal rainfall across India during the 2020 monsoon season.

Our official monsoon forecast will be released in April, but early indications are that rainfall across India from June through September will average at least near normal. Our analogs suggest rainfall across India during the summer will average 99-101% of normal. The timing of the onset is a harder forecast, but we are leaning toward an onset around the normal start date of 1 June (+/- 5 days).

Continue to check AccuWeather.com for more blog entries and to catch the official monsoon forecast.

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