Rainfall, snow expected in northern India over weekend

From the blog edition: http://www.downtoearth.org.in/blog/rainfall-snow-expected-in-northern-india-over-weekend-51565

A variety of weather events will be witnessed in India in the coming week 

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The satellite image taken on October 23 shows the position of the western disturbance (circled in red) and the low pressure system (denoted by L) in the Bay of Bengal Credit: IMD

A powerful upper air trough (popularly called western disturbance by the India Meteorological Department) is all set to hit northern India by this weekend.

As a result of it, the hilly areas of Jammu and Kashmir, Himachal Pradesh and Uttarakhand will receive snowfall from Saturday (October 24) till Wednesday (October 28). 

The maximum amount of rainfall (to be witnessed in the low-altitude areas of these three states) and snowfall will be witnessed from Saturday to Monday. During this period, landslides will be witnessed on the Jammu-Srinagar highway, which will likely to remain shut for a few days.

Temperatures in much of the Kashmir Valley (including Srinagar) are already above the freezing point. Hence, the expected rainfall may create water logging and flooding. Travellers should remain cautious.

Along with rainfall and snowfall, thundershowers are also expected in Punjab, Chandigarh, parts of Haryana, Delhi and the National Capital Region from Sunday to Tuesday.

Arrival of winter

Winter has gradually started arriving in the hilly areas of north India. From Saturday onwards, maximum temperatures will decrease by an average 2-3° Celsius in Jammu and Kashmir, Himachal Pradesh, Punjab, Haryana and Chandigarh due to rainfall.

Rainfall will bring about a temporary rise in the minimum temperatures till October 26, but after that, minimum temperatures will significantly fall in these areas bringing colder nights.

Already several places in northern India are experiencing chilly conditions. Leh in Jammu and Kashmir recorded a minimum temperature of -1.9°Celsius on Friday morning.

Other places such as Gulmarg (J&K) recorded a minimum temperature of -1.4°Celsius (3° Celsius below normal), Srinagar recorded 5.5° Celsius, Pahalgam and Keylong (Himachal Pradesh) recorded 0.6° Celsius and Manali stood at 4.6° Celsius.

Not to be outdone, the plains in north India have also started witnessing mild winter conditions. Places in Punjab and Haryana are witnessing minimum temperatures in the range of 14-17°Celsius. Even Safdarjung in Delhi recorded a minimum temperature of 17.3°C on Friday morning.

Southern story

However, south tells a different story. If one travels a bit southwards, a contrast in the weather will be observed.

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The image shows maximum temperatures in several places across India on October 19, 2015. Red patches indicate maximum temperatures above 35°Celsius. Credit: RMC Delhi/IMD

The maximum temperatures in the past 15 days have been consistently in the range of 35-40°Celsius in states such as Maharashtra, parts of Karnataka, Telangana and Andhra Pradesh.

In western India, a few places in the Kutch area of Gujarat recorded maximum temperatures above 40°Celsius.

Mumbai is also "boiling" as was expected (read http://www.downtoearth.org.in/blog/tropical-depression-forms-in-the-arabian-sea-51447).

On October 16, a 43-year-old record was broken in Mumbai. The day was the hottest in October in Santa Cruz area. On this day, the maximum temperature stood at 38.6°Celsius, which was 5.2°C above normal. Earlier, 37.9°Celsius was recorded on October 23, 1972.

Generally, Mumbai is not known for so much heat, but the maximum temperatures have remained above 35°Celsius for several days. Places in Vidarbha are witnessing maximum temperatures in the range of 35-40°Celsius.

Such high temperatures are affecting Maharashtra's agriculture. The state's water stock is at its lowest in the past five years due to poor monsoon this year. 

Weather conditions will continue to stay hot in western and central India, as no significant change in weather conditions is anticipated in the remaining days of October.

The temperature contrast between north and south India is a typical feature the country witnesses every year in October after the south-west monsoon withdraws.

This year, the south-west monsoon withdrew from entire India on October 19, as per the IMD. Dry, northerly winds in north India are keeping the skies clear. This, in turn, is causing radiative cooling (a process in which heat from the Earth's surface is radiated back to the space). Central and western India witness high temperatures in October due to the prevalence of dry winds. The lack of any rain-bearing system during this period is also responsible for this situation. But, this time the duration of such high temperatures is more than normal.

Strong low-pressure system possible in the Bay of Bengal

At present, there are two low-pressure systems in the north Indian Ocean. One is in the Arabian Sea, off the shore of Lakshadweep and another is in the Bay of Bengal, off the shore of Sri Lanka.

Weather models suggest intensification of this system in the Bay of Bengal early next week. If we believe in the present forecast, the system will reach near the Tamil Nadu coast by Wednesday. It may bring heavy rainfall in the coastal areas of Tamil Nadu and Andhra Pradesh. With this, the much awaited north-east monsoon is anticipated to commence in southern India.

The southern peninsula (particularly the eastern coastal areas of South India) gets a large percentage of its annual rains because of the North-East Monsoon. Tamil Nadu is known to receive about 48 per cent of its annual rainfall between October and December, which is known as the North-East Monsoon period. There are several criteria for declaring the onset of the North-East Monsoon. IMD usually declares this around October 20, mainly after an increase in rainfall activity in the coastal areas of Tamil Nadu and Andhra Pradesh.

Also, we cannot rule out the possibility of the low pressure intensifying into a tropical depression or even a tropical storm. Hence, it will be monitored closely and an update will be given, if at all, it poses any significant risk for the south-east coastal areas of India.

El Niño not solely responsible for poor monsoon

FROM BLOG EDITION: http://www.downtoearth.org.in/blog/el-ni-o-not-solely-responsible-for-poor-monsoon-51201

This season, parts of India witnessed one of the driest monsoon spells

India's crucial south-west monsoon season is coming to an end. With less than ten days to go in the official wind up of the season, India is all set to witness another year of below normal rain. All India seasonal rainfall departure will set close to -15 per cent (with slight variance possible). A departure of -15 per cent means rain has been 15 per cent below normal. As per India Meteorological Department (IMD), a departure of -15 per cent comes under a broad "normal" rain category which runs from +19 per cent to -19 per cent. However, things aren't that normal in most of the 16 (out of 36) meteorological subdivisions which till date have recorded "deficient" rain. With a departure of -46 per cent between June 1 and September 20, Eastern Uttar Pradesh tops the list. It is followed by Western Uttar Pradesh at -44 per cent, Punjab at -41 per cent and Haryana at -39 per cent.

This year's monsoon has been dicey and full of ups and downs. Speculations about El Niño affecting India's monsoon were there since early 2015. But the first blow to the chances of a good monsoon came on April 22, when the IMD issued the first part of Long Range Forecast (LRF) of monsoon. This suggested that India would get 93 per cent of normal rain of 89cm (with a variance of plus minus 5 per cent). On June 2, in the second part of LRF, IMD downgraded their previous forecast. The updated forecast suggested that India would get 88 per cent of normal rain in the monsoon period (variance of plus minus 4 per cent). But, to everyone's surprise, the monsoon's mood remained very good during the onset phase (i.e in June). In this month, India received 16 per cent above normal rain. Thirteen subdivisions recorded excess rain, 20 recorded normal rain and only 3 recorded deficient rain. Performance of rain was impressive in Jammu and Kashmir, Rajasthan, Western Madhya Pradesh and other parts of Central India. This was welcoming news for the citizens and it even brought back their hopes of a normal monsoon. Already debt-ridden but encouraged by good rain, farmers in drought-prone areas of Maharashtra like Marathwada took fresh loans and invested heavily in their fields in June hoping for a good harvest. The situation was similar in other parts of India too. No one even imagined what was in the offing.

 

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IMAGE 1- All India rainfall in the month of June '15. Courtesy IMD

Come the fourth week of June, things started changing. Central India witnessed a big dry spell of the monsoon which lasted for around one month. Such dry and wet spells are regular features during the season. Their intensity, frequency and time of occurrence play a very vital role in monsoon's performance, which in turn, influences the agricultural productivity. But, the one month long dry spell which Maharashtra, Gujarat and Madhya Pradesh witnessed was very severe. It severely damaged the kharif crops and even led to their complete destruction at many places in Marathwada and western Maharashtra (Madhya Maharashtra) where irrigated area is too low. It also kicked off drought conditions and agrarian crisis in these areas, leading to acute shortage of drinking water and fodder for thousands of cattle. For the first time in the history of Maharashtra, cattle shelters were set up in the month of August in Marathwada. At the end of July, India's seasonal rain which was 16 per cent above normal in June came down to 5 per cent below normal. The trend continued in August, due to which the amount of seasonal rain further came down to 11 per cent below normal. And on September 20, we are at 14 per cent below normal. This is how June's rain fooled the farmers in suicide-prone areas of Maharashtra. The farmers wouldn't have gone for early sowing had June's rain been scanty and in such a case, the loss certainly would have been lesser than what it is right now.

The overall poor performance of the monsoon confirms the influence of suppressing factors like El Niño. But why was June an exception? Where was El Niño in June? Was it absent?

 

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IMAGE 2- Schematic representation of wet phase of MJO over India. This is reflected by stormy and wet weather over India. Courtesy: Climate.gov/Fiona Martin

El Niño was very much present in the month of June! It was trying its best to suppress India's monsoon but it was the Madden-Julian Oscillation (MJO) which gave it a "fight". MJO was discovered by two atmospheric researchers—Roland Madden and Paul Julian in the early 1970s. Simply put, MJO is an atmospheric disturbance which travels around the world in the tropical regions over an average period of 30-60 days. This disturbance propagates eastwards with time and with a varying intensity. The most important property of MJO is that it influences the clouds/rain i.e the process of convection in the areas it travels through. It has two "faces". One face or phase boosts the process of clouds/rain (called the wet phase) while the other one suppresses it (called the dry phase). India's monsoon gets benefitted whenever there are wet phases over the nation. This is because such a phase helps in the rising motion of moist air, which is necessary for cloud formation. The more intense it is, the more rain that particular region gets. In a sense, it counters the negative effects of El Niño for a certain period of time. But when a dry phase enters, it suppresses the rainfall and under the prevailing El Niño conditions, the suppression factor is pretty strong. MJO has dipole nature as can be seen from the above image. Over India, there is a wet phase, but over the Pacific there is dry phase at the same time. This is because the warm moist air that rises over India diverges and cools at upper levels of the atmosphere. This cool and dry air then sinks over the Pacific (see the arrow) causing lack of clouds/rain there.

MJO became active in early June of this year and the Indian subcontinent was blessed by one such wet phase. It continued its presence till the middle of the month which helped the monsoon onset and also gave bountiful rain. But shortly after this, a strong dry phase entered the subcontinent and maintained its presence till mid-July. This dry phase was likely the strongest one seen over India since the last few years. This was the reason why central and other parts of India witnessed such a big dry spell between June-end and July-end. MJO activity has been weak since July due to which the El Niño has maintained its hold over India's monsoon.

El Niño to strengthen further in the coming months

 

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IMAGE 3: Representation of Sea Surface Temperature anomalies in the Pacific Ocean between 31st August and 6th September 2015. Red colour depicts above normal temperatures and blue colour depicts below normal temperatures. Courtesy: NOAA

 

El Niño is pretty strong now and the Oceanic Nino Index for June-July-August is 1.22°C in the Niño 3.4 region. The Niño 3.4 region (situated at 5°N-5°S, 120°-170°W) is very important as it is that part of the Pacific Ocean whose sea surface temperature anomaly (difference between the observed value and the average value) will reveal if there is an El Niño or not. This means that the average of sea surface temperature between June and August is 1.2°C above normal. This is the third highest value since the year 1950 (we have records from 1950), with the highest being 1.42°C in 1997 (the strongest known El Niño). The latest weekly departure in the Niño 3.4 region is 2.3°C, which suggests a strong departure. The big red patch across the equatorial Pacific Ocean in the above image clearly reflects the strength of El Niño.

El Niño isn't just about warmer sea surface temperatures. The atmospheric circulations too demonstrate changes during it. Weakening or reversal of trade winds is one of the important atmospheric indicators of an El Niño. Gradually, a coupling between the Pacific Ocean and the atmosphere above it is observed and at present we have that! It is because of this coupling that the ocean and the atmosphere are able to influence each other. As per the Bureau of Meteorology, Australia (which independently monitors the Pacific Ocean), these present oceanic and atmospheric indicators are at record levels. El Niño isn't going to end soon! As per the latest advisory from Climate Prediction Centre, there is approximately a 95 per cent chance of it continuing through this winter of the Northern Hemisphere. In fact, these models suggest that El Niño will strengthen around the Northern Hemisphere's winter and it would be a strong one. El Niño's influence will likely be witnessed on India's winter also as it tends to bring warmer than normal winter in India.

 

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IMAGE 4: El Niño's impact during the winter Courtesy: CPC/NCEP

 Monsoon's withdrawal to resume in north India after some rain

The withdrawal of the south-west monsoon commenced on September 4 from western Rajasthan. After that, the monsoon withdrew from some parts of Punjab and Haryana but the process has been stalled since then. IMD has three criteria for declaring the monsoon's withdrawal. Out of them, two state that rainfall must cease in a particular region for (at least) five days and there must be considerable reduction in the moisture content. However, widespread rainfall is expected in the whole of North India and Rajasthan till Wednesday, September 23 due to the combined effect of a low pressure area and a western disturbance. Jammu and Kashmir will get light to moderate rain in this period due to which there will be a possibility of landslides and some water logging in areas like Srinagar. Rainfall amount will be higher on Tuesday, September 22 in North India. Delhi NCR has been severely hit by dengue. As light rain will continue in Delhi till Wednesday and this will be followed by clearing of weather, there will be a possibility of further increase in dengue cases in the remaining days of September if things aren't handled properly.

The monsoon's withdrawal would stay suspended in this week. But, starting from next week (September 28 onwards), the process of monsoon withdrawal will resume in North India as the rain will cease and humidity will considerably fall.

 

Southwest monsoon reaches Kerala

FROM BLOG EDITION - http://www.downtoearth.org.in/content/southwest-monsoon-reaches-kerala

As of now, conditions are favourable for its progress

Progress of southwest monsoon over India (Courtesy: IMD)
 

After a delay of four days, the southwest monsoon finally arrived in Kerala on Friday, June 5. According to a press release by India Meteorological Department (IMD), the southwest monsoon has advanced over the entire south Arabian Sea, some parts of central Arabian Sea, Lakshadweep, Kerala, parts of coastal and south interior Karnataka, Tamil Nadu and some parts of Bay of Bengal.

As of Friday afternoon, the Northern Limit of Monsoon (NLM) or its northern boundary was passing through Mangaluru (Mangalore) and Mysuru (Mysore) in Karnataka and Salem and Cuddalore in Tamil Nadu. The onset of monsoon was delayed in Kerala and southern India by five days even last year. 

This year, the monsoon reached the Andaman and Nicobar Islands region on time, but lost its momentum and lingered around Sri Lanka for more than a week. Thus, it missed both its normal onset date and the date forecast by IMD (May 30). 

It is also running late in the North East and eastern India as the Bay of Bengal branch of the monsoon hasn't made any significant progress when compared to the Arabian Sea branch. 

Despite this delay, India is seeing a "good opening" of the summer monsoon season. From June 1 to 5, rainfall in India was 4 per cent more than normal, thanks to a combination of a western disturbance in the north, pre-monsoon showers in the south, monsoon showers in the Andaman and Nicobar Islands and multiple localised thunderstorms in other parts of India. 

The monsoon has another 118 days (till September 30) to perform in India. Factors such as the state of El Niño Southern Oscillation, Indian Ocean Dipole and frequency of monsoon depressions will influence its performance. 

Arabian Sea low pressure to boost monsoon's progress

Monsoon could regain its lost momentum because of a low pressure system in the Arabian Sea. For the onset of the monsoon in Kerala, it's important for a few atmospheric parameters such as rainfall and wind to achieve threshold values. This low pressure helped build clouds near Kerala which brought widespread rains to the region. Plus, its presence influenced the wind speed and direction and made them favourable. In this way, the low pressure helped pull the monsoon that was stuck over Sri Lanka to southern India. As of the afternoon of June 5, this low pressure system was situated off the west coast of Kerala-Karnataka and had lowest mean sea level pressure of 1004mb. 

Initially, multiple weather models showed that this system would intensify into a tropical cyclone and move parallel to the western coast of India. This could have led to a June 2014-like situation of monsoon's bizarre progress due to Cyclone Nanauk. But as per the present weather model runs, this system would marginally intensify and head almost northwards in a curved path. As of now, the probability of formation of a cyclone like Nanauk is very low. This system would dissipate in the northern Arabian Sea by June 11 and, hence, there is no threat to India. 

To some extent, monsoon is expected to progress faster along the west coast as compared to the interior areas of southern India till June 10. In this period, the western coast (including Mumbai, Pune) will witness rainfall with windy weather due to this system. The Arabian Sea will see rough conditions due to the low pressure. It is expected that the monsoon will arrive in more parts of Karnataka, remaining parts of Tamil Nadu and, possibly, southern parts of Andhra Pradesh by June 8. Mostly dry and hot weather will be witnessed in northern India during this period. 

IMD forecasts deficient monsoon

IMD downgraded the monsoon forecast, from 93 per cent to 88 per cent, dashing hopes of relief from drought in many parts of India. Thus, the forecast is now of deficient rainfall instead of below normal rainfall. (Note that the error range for these estimates is +/- 4 per cent.)

IMD also gave a region-wise forecast in this update. It said that northwest India will receive 85 per cent of LPA (LPA stands for Long Period Average or the average rainfall recorded from 1951 to 2000). Central India will receive 90 per cent of LPA, south peninsula will receive 92 per cent of LPA and northeast India will receive 90 per cent of LPA. All these figures are subject to an error of +/- 8 per cent. 

A decrease of 5 per cent in the rainfall forecast is significant. Monsoon rainfall in India exhibits a large spatial variation and, hence, this 5 per cent decrease can bring a significant decrease in rainfall at many places in India. For example, in 2014, IMD had forecast a rainfall of 94 per cent of LPA for central India. The observed rainfall was 90 per cent of LPA, but the monsoon rainfall in places like Marathwada in Maharashtra was 42 per cent below normal. 

Likewise, rainfall in north-west India was forecast at 85 per cent of LPA. The observed rainfall was 79 per cent of LPA, but a strong rainfall deficiency was seen in the region. Haryana-Chandigarh-Delhi and western Uttar Pradesh recorded 56 per cent below normal rainfall. This is why IMD's downgraded forecast 

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

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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