Why ENSO is local to the Pacific Ocean?

-
ElNino Southern Oscillation, abbreviated as ENSO, is an interesting and a well-established interseasonal phenomenon that has an immense effect on the tropical monsoons. Very briefly, it is associated with the weakening or full reversal of the atmospheric Walker circulation cell, which forms in the Pacific Ocean. The Walker cell's preferential direction is east-west (easterly), created by the motion of trade winds, supplying moisture to Australia & Indonesia. However, during an ElNino cycle, it either weakens (thereby slowing down the trade winds) or fully reverses, where the trade winds now blow west-east (westerlies). 


The Walker cell formation and its modification is driven by the ocean dynamics. During non-ElNino or neutral years, a cold pool exists in east Pacific - near South American coast due to upwelling (specifically Chile coast), and a warm pool is formed in the west Pacific - near Australia. This set up is quashed by a Kelvin wave, which one can  imagine as a hot water mass (with high grade fever) taking a dip in the west Pacific and moving eastward (at an average speed of around 150 km/day). As a consequence of this Kelvin wave, an upwelled cold Rossby wave is formed, which moves westwards at an average speed of around 70 km/day (Yes, Rossby travels slower than Kelvin). Now, this marks the beginning of an ElNino, also referred to as the initiation phase of ENSO.

The two waves move in opposite direction maintaining their characteristic behaviour (i.e. warm Kelvin wave moving east and cold Rossby wave moving west), thereby resulting in a developing stage of ENSO.

The Pacific is about 15,000 km long - which means it takes around 100 days for the Kelvin wave to reach the eastern Pacific (this is assuming there is no wind resistance). Assuming a resistance due to the easterly winds, which doesn't allow the wave to move fast, it will take, say, double the time to reach the eastern Pacific (specifically, the Chile coast), which is basically ~ 200 days. 

At the same time, a cold Rossby wave is moving west and it will reach the west coast (Australia) in approximately 200 days as well. For a Rossby wave, the wind will be a tail wind, since the winds are easterlies in the tropics, thereby assisting its motion.

Therefore, both these waves reach the opposite coasts at approximately the same time. As these waves reach the east and west coasts of Pacific respectively, they now change the Sea surface temperature (SST) pattern of the Pacific, where a cold pool now exists near Australia and warm pool in South America. This is now called the fully mature phase of ElNino, which is formed after about 7 months from its initiation.

In order for reversing the ENSO to neutral conditions, another set of Kelvin and Rossby waves should form to reverse the SST patterns and bring the Pacific back to neutral conditions (called receding phase of ElNino), which again will take ~7 months. Hence, a full ENSO cycle lasts for ~ 12-14 months.

What makes ENSO specific to the Pacific Ocean basin? 

The answer lies in the fact that the Pacific Ocean basin is the longest ocean basin that gives enough time and allows these two waves (Kelvin & Rossby) to nicely reverse the SST patterns. Any other basin, say Indian Ocean and Atlantic Ocean, are too small for these waves to reverse the pattern and the effect of the reversal to be felt on a phenomenon such as the Indian Monsoon. The short length-scale of a basin, like Indian Ocean, allows multiple reversals to happen and thereby an exact correlation on the tropical monsoon is not felt. The Pacific, on the other hand, is very long ocean and it behaves like a "delayed oscillator"; giving enough time for the effects of the Kelvin and Rossby waves characteristics to be felt on a synoptic scale features like monsoon.

As a comparison, Indian Ocean is only 6000 km long, so the Kelvin and Rossby waves will take a much lower time (~2.5x lower) to cover the basin, i.e. they will reverse the SST in 70-80 days time period - which is approximately 3 months. The monsoon time-scale being larger than this, therefore it probably doesn't see a direct correlation of this SST reversal pattern like the ENSO in the Pacific.

Owing to the smaller size of Indian Ocean basin, it has something called Indian Ocean Dipole Zonal Modes (IODZM), whose physics is different from ElNino and happens on a much faster time scale. A similar modal pattern is seen in the Atlantic Ocean, known as Atlantic Ocean Zonal Modes (AOZM).

You can follow my twitter handle @SriGMFL

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.


Comments

Post a Comment

Popular posts from this blog

Onset of Indian summer monsoon

-
Image
The onset of Indian summer monsoon (or southwest monsoon) is the most anticipated event for the Indian population. The first monsoon shower is the most awaited one as many seek respite from the scorching heat, little ones gleefully wanting to play in the cool showers, and the agricultural community get in full swing to prepare the land for farming. Over the past few years, owing to climate change, the onset has been delayed ( https://www.firstpost.com/economy/imd-forecasts-delay-in-onset-of-monsoon-may-hit-kerala-coast-around-5-june-1971211.html ) and the monsoon itself has been erratic, with brief periods of heavy rain (active phase) and periods of no rain (break phase). This is in contrast to a decade or two back, when the onset on monsoon would be well-timed and the nature of monsoon was also steady (with consistent rains through out the season). However, urbanisation, greenhouse pollution, deforestation, and many other anthropogenic activities have led to the late onset and incon...
Read more

Indian Summer Monsoon 2018

-
Image
  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 regio...
Read more

Cyclone ASANI

-
Image
Our team has been on the fence with regards to the SEVERE CYCLONIC STORM (SCS) ' ASANI' , since the ridge dynamics were highly tricky. The cone of uncertainty surrounding this cyclone is quite huge, with still some divergence in the weather models as to its exact track, landfall location, as well as intensity. This uncertainty is very uncommon given that the cyclone is just 48 hours within possible landfall. However, as an upcoming (read startup) private player in the field of weather and climate services, it is important to spell out what we feel is likely to happen based on the dynamics that we interpret. The Cyclone has enough fuel to maintain Cyclonic Storm status. The vertical wind shear is the only unfavourable factor, but it is not too high. The ridge action has been the main X-factor. The Indo-Pacific ridge movement has been very unstable for past few days and hence the models were divergent on the track. But now, it looks like the Indo-Pacific ridge has moved closer to...
Read more