Indian Summer Monsoon 2018

 

A few days back, both Skymet weather services (a private weather forecasting firm) and the Indian Meteorological Department (IMD, a government organization) gave their predictions for the Indian summer monsoon 2018. Overall, they painted a positive picture, where both confirmed that monsoon 2018 is likely to remain normal at 100% (with error margin of +/-5%) of the long period average (LPA), where the nationwide LPA stands at 887 mm for the four-month period from June to September (for more details on this prediction see https://www.skymetweather.com/content/tag/monsoon-2018/). To elaborate, >105% of LPA indicates excess rainfall, <95% of LPA indicates below normal rainfall, and <90% of LPA indicates drought. Their prediction is based on the nationwide average and hence does not account for the state- and region-wise variabilities. Hence, although the prediction for 2018 is ‘normal rainfall’, this does not exclude the possibility of excessive rainfall in some regions and deficit rainfall in other regions. This is why IMD ventured into a state-wise monsoon forecast starting 2017 (press release: https://www.livemint.com/Politics/xUCzF905YWmJZm6MuXktNL/IMD-to-start-making-statewise-monsoon-forecast-from-next-ye.html), which is going to continue for 2018 (see the two week state-wise forecast on http://www.imd.gov.in/pages/press_release.php).

So how do these weather service providers come up with the predictions for monsoon a few months ahead of its arrival? The two most important phenomena that have been been linked to the tropical monsoon season are ElNino Southern Oscillations (ENSO) and Madden-Julian Oscillation (MJO). ENSO is specific to the equatorial Pacific zone, associated with change in the sea surface temperature (SST) patterns between the equatorial eastern and western Pacific. Normally, the equatorial eastern Pacific is cooler than the  central and the western Pacific. The reversal of these temperatures (warmer eastern Pacific and cooler western Pacific) leads to ENSO. The reversal and and hence the onset of ENSO starts in May of a given year (say for e.g. May 2017) and reaches a mature phase during February/March of the following year (say for e.g. Feb 2018). Hence, the weather services wait until Febraury or March to declare their monsoon forecast, since by then the state of ENSO would be known (i.e. whether ENSO is weak, neutral, or strong). In general, weak ENSO means good rains for India, neutral means normal rainfall, and strong ENSO means drought.

The SST charts are available on National Oceanic and Atmospheric Administration (NOAA) website . A sample plot for April 2018 is shown in Figure 1, which shows that the eastern Pacific is cooler than the western Pacific. The difference is not much, and hence it suggests neutral ENSO conditions. Imagine that weak ENSO would have blue color (cold waters) in region A and bright red in region B (hot waters). Some weather service agencies also look for the SST anomaly, defined as departure from a reference value or long-term average. A positive  anomaly  indicates that the observed  temperature  was warmer than the reference value, while a negative anomaly indicates that the observed temperature was cooler than the reference value. The anomaly plot for Pacific ocean is shown in Figure 2, where we see cooler water in east and warm water in the west. Again the magnitude of anomaly helps in figuring out the weak, neutral, and strong ENSO. In Figure 2, the negative anomaly is in the eastern Pacific and positive in the western Pacific, which is representative of neutral ENSO situation.

The other phenomenon that affects the monsoon, and is of interest to weather service providers, is Madden Julian Oscillation (MJO). This, unlike ENSO, is not a localized phenomenon but propagates eastwards in various phases (phase 1 to phase 8). Of these 8 phases, phases 3 & 4 and a little of phases 2 & 5 are associated with rainfall activity over India. The transition from Phase 3 to 4 is generally seen as start of monsoon over India. The Phase 2 is when an array of clouds are hanging near the equatorial region (as shown in Figure 3), and slowly will move north-east to transition to Phase 3. During this transition, pre-monsoon showers occur over some parts of India. Each phase lasts anywhere between 60-90 days. Hence, phases 3 & 4 are associated with southwest monsoon, and phases 4 & 5 are associated with the north-east monsoon.

These two phenomena help the meteorologists give a nationwise prediction about the overall tendency (i.e. normal, excess, drought) of the Indian monsoon. It precludes one from giving an exact region-wise rainfall prediction. This is because the winds, moisture (water) content, and the cloud patterns depend strongly on the orography, vegetation, and local urban canopy.

To sum up, the positive monsoon outlook for 2018, i.e. normal rainfall for India, given by Skymet and IMD may be good enough, but this does not rule out any short-term extreme events that may be local to a state or region. It should be emphasized that the normal rainfall prediction is for entire India, which means some state may get excess and some may be deficit (as has been happening for the past few years).

Some of my personal opinions (based on the analysis of various diagnostics listed on numerical weather prediction - IMD) for monsoon 2018:

(a) Based on the good pre-monsoon showers in the south (thanks to a rather active MJO phase 2 and its transition to phase 3), it seems like Kerala, North Karnataka, and coastal regions may have above normal rainfall .

(b) North east region may also end with up above normal rainfall.

(c) Western/central parts of India (Maharastra, Madhya Pradesh, Goa etc.) will have normal rainfall.

(d) The north Indian states (Delhi, UP, Harayana, etc.) and indo-gangetic plains may experience below normal rainfall and hence may end with some deficit.

(e)  As of now a clear picture of the north east monsoon (which brings rainfall to Tamil Nadu, coastal Andhra Pradesh, and some parts of Karnataka and Kerala) cannot be given. But due to neutral ENSO conditions, overall it is expected that north east monsoon will also be normal. A state-wise prediction (excess and deficit) can be given as and when the southwest monsoon starts its withdrawal (i.e. around September 2018).

Disclaimer: The content given here are all personal views and the author will not be held responsible or held liable for any oversight or misinterpretation.