Monsoon reaches Andaman & Nicobar Islands

FROM THE BLOG EDITION: http://www.downtoearth.org.in/content/monsoon-reaches-andaman-nicobar-islands

But with experts predicting 90 per cent chance of El Niño, uncertainty looms over monsoon's overall performance

Map showing the advance of south-west monsoon 2015. (Courtesy- India Meteorological Department)

India's much-awaited annual visitor, the south-west monsoon, has arrived at its first destination—Andaman and Nicobar Islands. On Saturday, May 16, the India Meteorological Department (IMD) announced that the monsoon advanced over south Andaman Sea, Nicobar Islands, some parts of south-east Bay of Bengal, north Andaman Sea and Andaman Islands (see image above). As per IMD, as of May 18 afternoon, the northern limit of monsoon continues to pass through 05.0°N/ 86.0°E, 10°N/90.0°E, Long Islands and 15.0°N/98.0°E. Usually, the monsoon reaches south Andaman Sea by May 15, and hence, as expected, it has arrived on time in Andaman and Nicobar Islands region.

For declaring the monsoon onset in Andaman Sea and Andaman and Nicobar Islands, IMD considers some basic conditions such as rainfall in these areas and direction of near surface winds. A nice flow of the near surface winds has been established between the southern Indian Ocean and northern Indian Ocean (particularly, Bay of Bengal). My previous blog explained how the monsoon winds originate in the southern hemisphere, cross the equator and then move towards India. Such a textbook flow is called cross-equatorial flow which is well established now and will be strengthening in the coming days. This will enable the monsoon to cover more parts of the Bay of Bengal region in the coming three to four days.

Weak onset predicted for Kerala

On May 14, IMD issued a press release  on monsoon's onset in Kerala. The Met officials are expecting the monsoon's arrival in Kerala on May 30 (onset window between May 26 and June 4). For declaring the monsoon's onset in Kerala, IMD has well-defined criteria. As per the present situation, the wind field parameter isn't in the acceptable range till May 30. But, the long range weather models indicate improvement in the situation from around June 1. Hence, if IMD decides to strictly follow the onset parameters, onset in Kerala would be declared by June 4. However, a weak onset phase is anticipated in Kerala and nearby areas.

El Niño intensity

Weak to moderate El Niño conditions are well established now in the equatorial Pacific Ocean as per Climate Prediction Centre (CPC) of USA's National Centers for Environmental Prediction (NCEP). As per Bureau of Meteorology of Australia, early stages of El Niño are appearing. Unlike last year, the Pacific Ocean and atmosphere above it are "in sync" now. Both Australian agency and CPC-NCEP agree on this. 
The sea surface of equatorial Pacific Ocean has been warmer than normal (positive sea surface temperature anomaly), mostly since last year. But the present temperature anomalies are greater than that of previous year.
  
On Thursday, May 14, CPC and International Research Institute for Climate and Society issued their monthly diagnostic discussion on El Niño Southern Oscillation (ENSO). According to this discussion, there is an approximately 90 per cent chance that El Niño will continue through the summer 2015 in Northern Hemisphere. Also, there is a greater than 80 per cent chance that El Niño will last through the entire 2015.

The image below shows various projections of the sea surface temperature (SST) anomalies in the Niño 3.4 region in the coming months. Niño 3.4 region lies between 120°W-170°W, 5°N-5°S and sea surface temperature anomalies for this region are mainly considered in ENSO forecasts. On the vertical axis, SST anomalies in degree celsius for this El Niño 3.4 region are represented and on the horizontal axis, groups of three months (for example, MJJ stands for May-June-July). Various curves indicate projections of various models (listed on the right side in the image), which are of two types—dynamical and statistical. Dynamical models are based on the usage of present weather conditions for forecasting. Statistical models give projections by applying statistics to past observations. Hence, dynamical models perform better than statistical models for ENSO forecasting.

As per the image, most of the models (both dynamical and statistical) are in agreement of warmer than normal SST in the Niño 3.4 region. Most of the curves tend to increase in the coming period, which means a possibility of strengthening of El Niño conditions. The average of dynamical models (shown as thick yellow coloured curve) shows that the SST anomalies in Niño 3.4 region would be between 1°C to 1.5°C in the coming period. That's why such a high percentage of possibility of El Niño conditions has been forecast for the coming months.

Plume of ENSO forecasts. (Courtesy- IRI/ CPC)

But there is catch, and it's due to the ongoing spring season. At this time of the year, seasonal transitions take place (like summer to monsoon for India). Likewise, a transition in the phases of ENSO also takes place in the Pacific Ocean. The phases like El Niño/ La Niña peak during Northern Hemisphere's winter season. During next year's spring season they are in transition. Suppose that we had an El Niño/ La Niña which peaked in the previous year's winter, then at present El Niño/ La Niña conditions would be weakening in the Pacific Ocean. Even an El Niño might be under the development in this period and hence there is a huge uncertainty in what the models expect about the status of ENSO in the coming months. Technically, this spring period in which models have a tough time in making forecasts is referred to as "spring predictability barrier". The discussion of CPC which has given a high percentage for El Niño conditions also mentions "there remains considerable uncertainty about how strong the El Niño event may become". 

Depth of warm water (shaded yellow/orange/red) in the equatorial pacific ocean as on 13th May 2015. (Courtesy- CPC)

India's summer monsoon season will be officially commencing from June 1, 2015.The Indian Ocean Dipole is anticipated to remain in the neutral stage as per Bureau of Meteorology, Australian, and this definitely wouldn't be a good news for the monsoon. Also, it is certain that El Niño or El Niño like conditions will be present at least till the half of the monsoon season.

The above image shows the location and depth of warm water in the equatorial Pacific Ocean. Yellow/orange/red shades indicate warmer water and the depth scale is given on the vertical axis. It is clear that a large pool of warm water has spread till a depth of about 250-300m in the central Pacific Ocean. It is even deeper in the eastern equatorial Pacific Ocean (east of 100°W). So, even if we assume that in the coming months no fresh heating of the surface water takes place through direct ways, this pool of warm sub-surface water will in itself be sufficient to keep the surface water of Pacific Ocean warm in the months to come. This would keep the atmospheric circulation affected and hence the monsoon also would remain affected.

In a nutshell, as per the present scenario, a weaker than normal monsoon is expected in at least first half of the monsoon season. Rainfall situation wouldn't be good in areas of Maharashtra like Marathwada.

Akshay Deoras is an independent weather forecaster

Countdown to monsoon arrival starts

4

BLOG EDITION : http://www.downtoearth.org.in/content/countdown-monsoon-s-arrival-starts

Will the southwest monsoon arrive on time over the Andaman and Nicobar Islands?

Normal dates for onset of southwest monsoon over India (Photo courtesy: India Meteorological Department)

As May begins, the countdown to the onset of the southwest monsoon in India has started. As per the standard timetable, it is normally expected to reach the Andaman and Nicobar Islands region about two weeks from now. Usually, by May 15, monsoon reaches the south Andaman Sea. By May 20, it covers almost the entire Andaman and Nicobar Islands. By May 25, it covers most of Sri Lanka, some parts of Myanmar and by June 1, it arrives in Kerala, most of Tamil Nadu and southeast Andhra Pradesh. 

However, it's not that easy for the monsoon to follow a strict timetable, just like we find it hard to do so in our daily lives. There are many factors which determine not only monsoon's performance, but also its timetable in a given year. The monsoon onset dates (shown in the image above) have been calculated by India Meteorological Department (IMD) based on rainfall recorded decades ago.

People often say that when they were children, monsoon used to arrive over the mainland of India on time, but they have been seeing variability in the onset of monsoon over the last few years. As a result of the natural variability in the monsoon system, these onset dates rarely hold true now (the southwest monsoon used to reach central India around June 10 earlier, but it doesn't anymore). Hence, there is a strong need to follow new monsoon onset dates or a new timetable. Weather factors are also responsible for the variations in the onset date. 

Monsoon arrived late over the Andaman and Nicobar Islands in 2005, 2011, 2012. But it also arrived before time in this region in 2007, 2008 and 2010. Let us assume the southwest monsoon is like a train going from the Andaman and Nicobar Islands towards Pakistan, stopping at several stations on the way. We have seen how a train reaches its final destination on time even if it has turned up late at the intervening stations by making up for lost time. The same thing happened in 2012 when the monsoon was "running" late near the Andaman and Nicobar Islands, but it reached Pakistan on July 11, before the normal date of July 15. 

But sometimes, the monsoon tends to stall at some places on its way across the Indian subcontinent. An on time or early arrival at the Andaman and Nicobar Islands doesn't guarantee a timely arrival in the rest of the country. 

It has been observed that low pressure systems (such as cyclones) in the Arabian Sea and the Bay of Bengal during May and June can interfere with the onset of the monsoon. The position, intensity and timing of such low pressure systems determine whether their presence would help the monsoon or not. Monsoon's bizarre progress in 2014 due to Cyclone Nanauk is a classic example. It helped the monsoon progress along the west coast of India, but couldn't help its progress in the interiors. 

There are so many possibilities every year that the accuracy of a long-term forecast of monsoon onset dates is always low. 

The monsoon journey in 2015 

In my previous blog, I had explained how some basic parameters play a vital role in the monsoon's performance. 

The first thing to look at will be the status of Mascarene High or the "powerhouse of monsoon". The above image of April 30, 2015 of the higher latitudes in the southern hemisphere shows "H 1028" written in a red circle. H 1028 means a high pressure region having mean sea level pressure of 1028 millibar (we consider 1013 millibar as standard mean sea level pressure) and theoretically, this is the position of Mascarene High. It must also be borne in mind that no two monsoon setups can be exactly similar and hence, some variation occurs. However, this year's setup is fairly impressive if one compares it with that of last year when the monsoon had reached Andaman and Nicobar on time. 

Last year, on the evening of May 1, a strong high pressure region was powering the winds in the southern hemisphere. This high pressure region can be seen here as a whitish patch off the southwest coast of Australia. Its highest mean sea level pressure was 1036 millibar. The star-trail like features from the west coast of Australia are monsoon winds which need to be fast enough to reach the equator and, then, come towards India. This high pressure region shifted westward and on May 5, 2014, it was located southeast of Madagascar as can be seen here. 

If we look at the situation on the evening of May 1, 2015, the star-trails (the winds) seem to be moving much slower as compared to last year. 

The good news as of now is that long range weather models show the possibility of an improvement in this setup in the coming days. They also show an increase in the rainfall activity in the Andaman and Nicobar Islands towards May 15 with better organisation of surface winds. But such long range weather models fluctuate a lot and, hence, blindly believing them will not be a good idea. 

As of now, the probability of an on-time arrival of the monsoon over the Andaman and Nicobar Islands is higher. In the coming days, we can expect to get a better picture of the expected status of El Niño and the Indian Ocean Dipole during the monsoon period and a close monitoring of conditions will help us have a better idea about this year's monsoon. 

Akshay Deoras is an independent weather forecaster

The Great Indian Monsoon System

BLOG EDITION : http://www.downtoearth.org.in/content/great-indian-monsoon-and-its-friends

A look at the association between the southwest monsoon and its companions

Henry Ford, the founder of Ford Motor Company, once said, "My best friend is the one who brings out the best in me."

This fact is observed not only in our daily lives, but also in the case of the Indian summer monsoon's (also called southwest monsoon) performance every year.
The southwest monsoon is so powerful that its performance from June to September decides the state of agricultural productivity in India, which in turn, decides the state of the economy. Thus, it is no less important than the finance minister himself.

The monsoon affects a region comprising nearly 25 countries and its influence stretches across a distance of 18,000 kilometres (from east to west) and 6,000 kilometres (from south to north).

Just like the performance of the monsoon is often compared to a game of cricket, one can also compare it to friends (here it means various atmospheric and oceanic conditions).

We gauge only the performance of the monsoon, but in reality, its friends are also working round-the-clock in the background to bring out its best performance. 
However, the twist in the story is that some of monsoon's friends are prone to changes. They can even become its enemies in a particular year and bat on behalf of the opponent team.

It is this mutability which causes variations in monsoon's performance and hence, no two monsoons are similar in nature.

Five special conditions
The Earth System Science Organisation (ESSO) and the India Meteorological Department (IMD) take into account five conditions while making forecasts about the monsoon in April every year.

These special conditions are as follows:

1) The Sea Surface Temperature (SST) gradient between the north Atlantic and the north Pacific (conditions during December of the previous year and January of present year)
2) SST of the equatorial south Indian Ocean (conditions during February and March of the present year)
3) Mean Sea Level pressure in east Asia (conditions during February and March of the present year)
4) The surface air temperature over northwest Europe (conditions during January of the present year)
5) The warm water volume of the equatorial Pacific Ocean (conditions during February and March of the present year)

Here are some friends of the Indian monsoon which are responsible for its existence and good performance over the years.

Mascarene High: monsoon's best buddy
The figure shows how Mascarene High works as the powerhouse of monsoon winds. Shades as per the scale represent the mean sea level pressure and the arrows indicate the direction of surface winds (Credit: Penn State Meteorology Department, ESRL, Philip Lutzak page)
The southwest monsoon derives its name from winds which blow from a south-westerly direction in the Indian subcontinent. We often read about "southwest winds" in various news reports or weather forecasts. But rarely do we think about the "source" of these winds. These come from a powerhouse located more than 4,000 kilometres from India. This powerhouse is known as the Mascarene High. This high-pressure region is located between 25°S-35°S and 40°E-90°E near theMascarene Islands in the southern Indian Ocean. 

Normally, this high-pressure region starts forming by mid-April and its strength is an important factor which determines the intensity of monsoon in India.

A stronger high pressure will produce stronger winds or monsoon current. If there is a delay in the formation of Mascarene High, there is also the possibility of a delay in the onset of monsoon in India. Mascarene High has been a subject of research for many years. Most research says that its strength is determined by the happenings in the Antarctic region.

But overall, this factor is not often held responsible for delays and poor performance of the monsoon in India. As a result of its punctuality, Mascarene High is considered its best buddy.

Coriolis Force: monsoon's trusted lieutenant
Winds from Mascarene High head in a north-westerly direction towards the east coast of Africa (Somalia). (See image 1)

Here, the topography of Somalia deflects the winds towards the east. Also, after crossing the equator, these winds experience the Coriolis Effect.

Coriolis Force is a pseudo force which exists only because of the Earth's rotational effect. Rotational motion observed in a tropical cyclone is also due to this force. Hence, these monsoon winds get deflected eastwards and now they blow from south-west to the north-east direction. They split into two branches—the Arabian Sea branch and the Bay of Bengal branch.

See the motion of these monsoon winds on June 21, 2014, here. 
In this graphic, the green-coloured moving star trail-like features are the winds at the surface level. Notice how they come from the southern hemisphere and then get deflected eastwards in the northern hemisphere. As the Earth's rotational velocity is constant, the Coriolis Force experienced at a given latitude by a given parcel of air having a definite velocity is also constant.

That is why the Coriolis Force is monsoon's trusted friend.

Indian summer: monsoon's guide 
The figure shows the mean sea level pressure during July 2014. The purple colour shows regions of low pressure whereas the yellow-shaded regions over the Indian Ocean show areas of higher pressure (Credit: earth.nullschool.net)
A mechanism is needed to attract the monsoon winds from the Arabian Sea or the Bay of Bengal. We keep reading about tropical cyclones or troughs or low pressure regions which are great attractors of winds in the Earth's weather system.

Winds flow from high pressure to low pressure areas. We all are familiar with India's summer season or pre-monsoon season as defined by the IMD. The Himalayan range plays a vital role in summer heating by restricting the intrusion of cold air from the north and allowing heating to occur. It is during this season that the land of India, particularly Rajasthan and surrounding areas (Gujarat and also Pakistan) heat up extensively. The seas surrounding the country also see a temperature rise. As a result of the differential heating rates and capacities of air over the sea and the land, we observe a gradient between air pressure over the sea and that over India (especially Rajasthan). The air pressure over India is lower than that over the southern part of the Arabian Sea and the Bay of Bengal. This acts as an attractor mechanism for the monsoon winds.

Thus, the summer season of India acts as the monsoon's guide.

El Nino Southern Oscillation: the undependable friend

El Nino Southern Oscillation (ENSO) refers to the oscillatory mode of the sea surface temperatures near the equatorial Pacific Ocean in which a warming (El Nino or cooling (La Nina) or neutrality is observed. Due to its mostly unpredictable nature, ENSO has been a big challenge for forecasters since a long time. Even trusted models have failed many a time.

Take the case of last year. Right from February-March, there were many instances during which the models indicated a moderate to high probability of El Nino formation, but it did not turn up till almost the end of 2014.

This was because the atmosphere and the ocean could not synchronise in most of the cases.

But things started getting organised better as 2015 kicked off and El Nino conditions started appearing in the western and central equatorial Pacific Ocean since February.

As a result of this, the Climate Prediction Center/NCEP/NWS and the International Research Institute for Climate and Society issued an El Nino advisory in March 2015.

Their April discussion suggests that "there is an approximately 70 per cent chance that El Nino will continue through Northern Hemisphere summer 2015 and a greater than 60 per cent chance it will last through autumn".

ENSO has been a driver of global weather (particularly in countries surrounding the Pacific Ocean) as it affects atmospheric circulation.

The El Nino concept became more popular in the Indian media after the drought of 2009. At the time, India received 21.8 per cent less than normal rainfall (Long Period Average of 89 cm). In fact, the major droughts of 1982 (-14.5 per cent rainfall), 1987 (-19.4 per cent rainfall), 2002 (-19.2 per cent rainfall), 2004 (-13.8 per cent rainfall) and 2009 (-21.8 per cent rainfall) were accompanied by a full-blown El Nino. However, there have been many El Nino years during which India received near normal or above-normal rainfall such as 1983 (13 per cent), 1994 (12.5 per cent), 1997 (2.2 per cent) and 2006 (-0.4 per cent).

Thus, ENSO is monsoon's undependable friend.

Indian Ocean Dipole: monsoon's latest friend
The image shows phases of the Indian Ocean Dipole and its impact on India's weather (Credit: WHOI/NCU (Obtained from Introduction to Tropical Meteorology, 2nd edition)

In 1999, N H Saji of Japan's University of Aizu and others discovered an ENSO-like phenomenon in the Indian Ocean which they named the Indian Ocean Dipole (IOD).

Like ENSO, IOD also has three phases—positive, negative and neutral. During the positive phase of IOD, sea surface temperatures are warmer in the western Indian Ocean (which gives a boost to monsoon winds) as compared to the eastern Indian Ocean—hence a dipole nature.

The reverse happens during IOD negative and no gradient is observed during the IOD neutral period.

It has been observed that during the period of positive IOD, the Indian summer monsoon rainfall is considerably good as compared to the negative IOD period. Despite 1994 and 2006 being El Nino years, India did not witness a drought as IOD was significantly positive. Thus, in some ways a strong positive phase of IOD tries to counter the negative effects of El Nino. But the relation between IOD and monsoon rainfall is still being debated and has not been fully comprehended.

Compared to the roles of Mascarene High, Coriolis Force, India's summer season and ENSO, IOD's role has been discovered recently.
It will be interesting to see how these parameters evolve in the ongoing pre-monsoon period. 
 

Akshay Deoras is an independent weather forecaster