Currently, both the El Niño Southern Oscillation(ENSO) and the IOD are in neutral state. Forecasts are indicating that ENSO will remain neutral or reach weak La Niña levels later this year, while there is growing confidence for a negative IOD to also form sometime between July and September according to the Australian Bureau of Meteorology(BoM). ENSO events co-occurring with IOD events are not uncommon at all though. In fact, studies show that the IOD is highly related to ENSO as most positive IOD events occurred during El Niño and most negative IOD events during La Niña. IOD events usually begin their transition in May-June, peak in fall (October-November) and dissipate by early Winter (December). ENSO events also usually peak late in the year but they can span/continue through multiple consecutive years.
Warmer sea surface temperatures(SSTs) in the eastern equatorial IO and cooler SSTs in the western side during a negative IOD event promote enhanced convection in the BOB and conversely, a suppressed state in the ARB. La Niña events also result to anomalously warm SSTs in the eastern equatorial IO as well as in the neighboring Maritime Continent. Under such conditions, the primary formation location of TCs in the NIO are also found to shift eastward, east of 87E. La Niña and negative IOD events each have their own influence on the steering patterns in the BOB, as studies show that La Niña events tend to steer TCs north or northwest towards the northern BOB (defined as north of 17N) while TCs are more into westward tracks during a negative IOD. Some sources each have slightly different lists of ENSO and IOD events due to different definitions or methodologies in determining them. For this purpose, ENSO years are taken from the CPC and ggweather website, while IOD events are from the BoM.
Past cyclone seasons when La Niña and/or negative IOD conditions were present indeed show that these events result to increased TC frequency in the BOB. Moderate to strong La Niña events even during a neutral IOD are highly capable of triggering strong/major cyclones (category 3+ on the Saffir-Simpson scale) that track into the northern BOB during October-November, as seen on the years 1970, 1974, 1988, 1995, 1998, 1999, 2007, and 2010. Except for 1998 and 2010, these years had neutral IODs. Weak La Niñas alone seem to have little or no influence in the intensity of post-monsoon cyclones in the BOB. However, when a negative IOD co-occurs with weak La Niña or even non-La Niña conditions (like 1989, 1992, 1996, and 2014), the chance of a major post-monsoon cyclone in the BOB may be equal to the chances during a moderate to strong La Niña event, but they appear to have a higher probability of moving west rather than towards the northern BOB. The case with pre-monsoon cyclones seem to be quite different, however, and may need further study. It can be seen on some seasons that even during La Niña and/or negative IOD years, major pre-monsoon cyclones still develop in the ARB, and likewise, they are able to develop in the BOB during El Niño and/or positive IOD years.
If the forecasts come into fruition, 2020 would be a weak La Niña year (or "cool neutral" at least) and/or negative IOD year that was preceded by a weak El Niño and positive IOD year. I picked years (beginning from 1964) which I think are analogs/matches for 2020.
1964 (WL preceded by ME ; -IOD preceded by +IOD)
1978 (NT preceded by WE ; NT IOD preceded by NT IOD)
1980 (NT preceded by WE ; NT IOD preceded by NT IOD)
2005 (WL preceded by WE ; NT IOD preceded by NT IOD)
2016 (WL preceded by VSE ; -IOD preceded by +IOD)
*(NT: Neutral | WL: Weak La Niña | WE: Weak El Niño | ME: Moderate El Niño | VSE: Very Strong El Niño)
All analog years listed above (except 2016) were years that began as having weak El Niño or "warm neutral" conditions, then transitioned to weak La Niña or "cool neutral" in the succeeding months but didn't reach moderate to strong La Niña thresholds. 2016's first few months were the continuation of a very strong El Niño event, but transitioned to weak La Niña during the year's 2nd half and is also a negative IOD year preceded by a positive IOD year. Among the five analogs, 1964 developed weak La Niña conditions the earliest, and like 2016, it is also a negative IOD year preceded by a positive IOD year. I actually find 1964 quite similar with 2016. The years 1978, 1980, and 2005 are also good analogs with pretty much similar beginnings with 2020 and may also reflect what this season would be like if the progress to La Niña threshold is slower. These three years had neutral IODs however, but for 2005, conditions during that time were pretty much negative IOD-like as seen on this graph. All analogs had activities that were focused in the BOB while ARB TC activity was almost non-existent. The pre-monsoon season was also weak/quiet for the whole NIO except 1964. Except for 1978, these years featured late-season tropical activities into mid/late-December, with 1964 and 2016 each having a typhoon/hurricane-strength system in the BOB during that month. Even so, these seasons were below-average to average in terms of accumulated cyclone energy(ACE) (probably with the exception of 1964 which had an intense late-season cyclone).
-----------------------
2020 North Indian Ocean(NIO) Cyclone Season Outlook/Forecast:
Currently, both the El Niño Southern Oscillation(ENSO) and the Indian Ocean Dipole(IOD) are in neutral state. Forecasts indicate that the ENSO will remain neutral or reach weak La Niña levels later this year, while there is growing confidence for a negative IOD to also form sometime between July and September according to the Australian Bureau of Meteorology(BoM). Such conditions favor increased tropical cyclone(TC) frequency in the Bay of Bengal(BOB) and conversely, suppressed activity in the Arabian Sea(ARB).
Based on the analog years for 2020 (1964, 1978, 1980, 2005, 2016) and as seen on past La Niña and/or negative IOD years, tropical activity is expected to be focused in the BOB this 2020 NIO Cyclone Season, with the primary TC formation location east of 87E (eastern-half of the BOB). The total storm count for the season is forecast to be average, while in terms of accumulated cyclone energy(ACE), the season is expected to end up being below-average to average. I also expect activity during the pre-monsoon season (late-April to early-June) to be weak/quiet, with the first cyclone-strength system (typhoon/hurricane) occurring during the post-monsoon season (October to November). However, tropical activities in the BOB could extend into mid/late-December, and I think it wouldn't be a surprise to see a late-season cyclone there. TC activity in the ARB is usually suppressed under La Niña and/or negative IOD conditions. I think that the best chance for a TC to form in the ARB this season is on late-May to June, which are climatologically the most favored times for TC development there.
The co-occurrence of a negative IOD with weak La Niña or even non-La Niña conditions usually increase the probability of a major cyclone (category 3+ on the Saffir-Simpson scale) developing in the BOB during the post-monsoon season. Taking this into consideration, zero to one major cyclone is forecast to develop in the NIO this 2020. Should one develop during that time, I also think that the probability of it impacting the coasts of the northern BOB (north of 17N ; Ganges Delta, Odisha, and northwestern Myanmar) is low since such occurrences are most-favored during moderate to strong La Niña events, however, this means that the risk increases for areas south of 17N (such as South India).
My numbers for the 2020 NIO Cyclone Season are...
Tropical Storms:
-BOB: 4
-ARB: 1
-NIO TOTAL: 5 [average: 5]
Cyclones (Typhoons/Hurricanes):
-BOB: 1-2
-ARB: 0
-NIO TOTAL: 1-2 [average: 2]
Major Cyclones (Category 3+):
-BOB: 0-1
-ARB: 0
-NIO TOTAL: 0-1 [average: 1]
Accumulated Cyclone Energy (ACE):
-NIO TOTAL: 15-20 [average: 21]
*Numbers in brackets[] are the climatological average (1980-2019) based on data by the Joint Typhoon Warning Center(JTWC). Figures obtained from Colorado State University(CSU).
-BOB: 4
-ARB: 1
-NIO TOTAL: 5 [average: 5]
Cyclones (Typhoons/Hurricanes):
-BOB: 1-2
-ARB: 0
-NIO TOTAL: 1-2 [average: 2]
Major Cyclones (Category 3+):
-BOB: 0-1
-ARB: 0
-NIO TOTAL: 0-1 [average: 1]
Accumulated Cyclone Energy (ACE):
-NIO TOTAL: 15-20 [average: 21]
*Numbers in brackets[] are the climatological average (1980-2019) based on data by the Joint Typhoon Warning Center(JTWC). Figures obtained from Colorado State University(CSU).
-----------------------
These are just my thoughts and by no means an expert/official forecast. If there is anything that I overlooked please let me know. I would also like to know what your thoughts are for the 2020 NIO Cyclone Season.
The effects of ENSO and IOD on NIO TC activity are discussed in the following papers:
Impacts of ENSO and IOD on tropical cyclone activity in the Bay of Bengal
North Indian Ocean tropical cyclone activities influenced by the Indian Ocean Dipole Mode
Environmental Influences on the Frequency and Intensity of North Indian Ocean Tropical Cyclones
Statistical forecasting of tropical cyclone landfall activities in the North Indian Ocean rim countries
).
GFS is now showing development as well, but it only has the one near India. The CMC model also has one TC but originating near the Arabian Peninsula.
