Dear Suresh Kumar Govindarajan,

Because the Indian Ocean does not has enough large surface of water as the Pacific Ocean, and because its surroundings has too much continental land near the Equator.

Regards,

Milivoj B. Gavrilov

Mode of variability is particular climate pattern having some periodicity and oscillations. This modes have interaction and teleconnection with both global and regional scale. You should explore about modes.

• El Nino southern oscillation (ENSO) is a strong mode in tropical Pacific (including El Nino and La Nina).
• Likewise Indian ocean basin-wide mode (IOB) and Indian ocean dipole (IOD) are strong modes in Indian ocean.

I will suggest you Suresh Kumar Govindarajan to read more about climate modes and Walker circulation.

The Indian ocean has another major weather influence, that the Pacific mostly lacks, that can cancel out the monsoonal moisture action--the "Pakistan-Arabia Dust Cloud" that can eat Category-5 cyclones, or form a wall that the cyclones bounce off and force the rainfall in a different direction, or frequently, catch the monsoon moisture and stall it and cause floods. It can also cause droughts, even though monsoon moisture wants to get through, the wall of the Dust Cloud stops that rainfall from proceeding. If you search "Pakistan-Arabia Dust Cloud" on Research Gate you will see some of the images I have posted with questions.

Teleconnection studies are often based on correlations between SST anomalies in specific tropical regions and variations in atmospheric circulation patterns. Tropical oceans play the most important role in terms of seasonal predictability, while the internal variability of atmosphere has little or no predictability at a long-term time scales.

In the Indopacific region, the relationship between SST in the Indian and Pacific basins show a clear seasonal dependency (Schott et al., 2009) connecting the SST anomalies associated with ENSO and SST variability in the western and eastern Indian ocean. When you look at SST, the Indian ocean dipole is mainly a (boreal) autumn phenomenon: only in this season an anti-correlation is present between the western and eastern side of the Indian ocean, with the two regions having opposite relationship with the central Pacific. Conversely, during the boreal winter, SST across the whole tropical Indian ocean tends to be the same sign, and be positively correlated with El Nino anomalies. Several studies on the teleconnections of the Indian ocean dipole (mainly during late summer and autumn) have made a clear distinction between signal coming from the western and eastern side of the tropical Indian ocean. Experiments with AGCM have shown that the Indopacific region is separated in two domains with a boundary at 120E, so that their Indian ocean domain ranges from the eastern Africa coast to the longitude of Borneo. However, during the boreal winter season the western and eastern part of the Indian ocean behave differently in terms of correlation of SST with either rainfall or thermocline depth. Correlation between SST and rainfall is weak or negative in the eastern Indian ocean, especially in the region west of Sumatra. Looking at the near-equatorial belt, regions of large positive correlation can be found in the western to central Indian ocean and the central to eastern Pacific. For these regions, it is acceptable to assume that SST variability forces rainfall (and diabatic heating) anomalies of the same sign. Instead, in the

eastern Indian ocean and the seas surrounding the maritime continents, especially south of the equator, SST-rainfall correlation is either weakly positive or negative.

Most theories of ENSO require large scale eastward (Kelvin) then reflected westward (Rossby) waves to propagate across the basin during a cycle the oscillation. This takes months for the Pacific due to its large size and this is thought to provide the long timescale of ENSO via the 'delayed oscillator' mechanism. The equatorial Pacific is bounded on the eastern side by a solid boundary (S America) which allows reflection of the initial Kelvin wave, although opinions differ on how important this part is. The lack of obstacles such as large equatorial islands or nearby coastlines in the central tropical Pacific compared to the Indian Ocean also help to allow the waves to propagate freely, supporting the El Nino - La Nina oscillation. These are a few reasons the Indian Ocean does not have an ENSO in it but note that the Indian Ocean dipole has some similarities to ENSO, but occurs on a shorter timescale.

You can watch the battle between the monsoonal moisture over India today with the Pakistan-Arabia Dust Cloud, by overlaying the cloud images from Infrared NHC Enhancement - Indian Ocean - Meteosat-7 from http://tropic.ssec.wisc.edu/real-time/imagemain.php?&basin=indian&prod=irn&sat=m5 with the dust images from NAAPS at https://www.nrlmry.navy.mil/shared-bin/display_image.cgi?URL=/aerosol_web/globaer/ops_01/india/current.gif

And if you have a cyclone in the Indian Ocean in the mix, you can overlay the detailed dust cloud images from CIRA from their "Storm Relative 16 km Geostationary Water Vapor Imagery" at http://rammb.cira.colostate.edu/products/tc_realtime/

You can see that the Dust Cloud is so powerful, the the lowest amount of atmospheric dust that can be currently detected, only 20-40 micrograms per cubic meter, can stall or form a wall against the monsoonal moisture trying to move westward.

Of course the Dust Cloud always wins, so what that means for the people of India over the next few days, is the Dust Cloud has caught and stalled the monsoon clouds, so expect massive flooding in areas. Fortunately or unfortunately, the Dust Cloud is man-made, by our farming and grazing the native plant cover from western India to Arabia creating the bare soil that is the home of the Dust Cloud.

If we just allowed for the local native plants to recover those areas, instead of plowing or grazing those lands to dust, then the monsoon moisture would continue and green up those lands and make the streams and rivers flow again, see http://www.ecoseeds.com/cool.html

About 10,000 years ago, North Africa, Arabia, the Middle East to western India and the Gobi desert had regular annual rainfall, at least 1.0 meter and in places as much as 2.0 meters a year, based on the stream beds that still exist in those areas.

Picture of those stream beds from Syria attached. For example, what is now the driest place on the planet, the Empty Quarter of Arabia, was a huge marsh with one of the world's largest lakes.

When that lake dried up so recently, you can still see the minerals from water at the bottom as a white-blue layer from satellite images, attached. That lake measured 200 by 400 miles and 100 meters deep.

Once humans invented agriculture and domesticated grazing animals that all changed, because they created the Dust Clouds that now stretch from Morocco to western India, and those man-made Dust Clouds cancel out the rainclouds that are trying to form in those areas.

What is now Pakistan, 3,500 years ago, the Indus Valley Civilization chopped down all of the trees to fire the bricks to build their 1,000 cities with 5 million people, and the rain stopped because those trees were forming the rainclouds for that civilization--then every one of those 1,000 cities were abandoned forever.

How we can know that these areas that were covered with vegetation not too many centuries ago, is that it still exists in the mountains above Salalah Oman, in a 10 mile by 40 mile strip of vegetation, that creates rainfall each summer, where you go outside of that very small zone of natural vegetation, then the rainfall ceases.

The edge of the Dust Cloud in the last few years, has continued to move eastward into India, and may eventually engulf half or more of India, because of the grazing and agriculture of marginal arid lands, that is spreading that Dust Cloud's edge into India Proper, image attached.

When you look close-up at the edge of the THAR Desert, you see how the grazing and farming in spreading the Dust Cloud, just like our farmers did in our US Mid-West 1930s Dust Bowl era.

And the Dust Cloud is also moving eastward into northern China for the same reason. with only a 100-mile wide mountain range between all of its farmlands and the deserts to the West, image attached.

See http://www.ecoseeds.com/cool.html and http://www.ecoseeds.com/cool2.html

Our knowledge of El Niño indicates that the process manifests the chaotic dynamics of the surface layer of the Pacific Ocean, driven by the interaction of the Poincaré waves and the Rossby waves in the basin with very specific geometric parameters. The corresponding parameters are very different in the case of the Indian Ocean, which leads to completely different chaotic dynamics having the form of the positive and negative Indian Ocean Dipole events.

These observations can be verified after analysis of a reduced-gravity shallow-water-ocean model, see for example Dijkstra and Burgers (2001): “Fluid Dynamics of El Niño variability”

https://dspace.library.uu.nl/bitstream/handle/1874/13613/dijkstra_02_fluiddynamics.pdf?sequence=1&isAllowed=y

Some authors debate the possible connection of the negative and positive Indian Ocean dipole (IOD) events (nIOD and pIOD, respectively) with the transition toward, respectively, El Niño and La Niña events one year later:

https://journals.ametsoc.org/doi/10.1175/JCLI-D-15-0481.1

Looking at the interesting answers describing the role of dust clouds, I wonder if we should include their impact in our attempts to predict the coupling between IOD and ENSO.