Monsoon Watch - 4

This article is reproduced from http://rajesh26.blogspot.in/


Forecasting Methods adopted for the Indian Monsoons:


Gowariker(1989) developed parametric and multiple power regression (MPR) models with 15 predictors for LRF of AISMR, which were later modified in 1991 to include 16 predictor parameters. The parametric model is qualitative and indicates the likelihood of the monsoon rainfall to be excess or deficient.


Thapliyal (1990) evaluated the relative performance of multiple regression, and Navone and Ceccatto (1994) have used 'feed-forward' neural network technique for the prediction of Indian monsoon rainfall with two predictors (500 hPa ridge location and Darwin SLP tendency from January to April). 


The results of a recent work by Krishna Kumar (1997) indicate that a single component accounts for about half of the total variance in the predictors


Most of the studies on LRF of Indian monsoon rainfall were based on empirical or statistical techniques till 2010. 
IITM, Pune has recently implemented the state-of-the-art coupled climate model, the Coupled Forecasting System (CFS) developed by the National Centers for Environmental Prediction (NCEP), USA. These statistical techniques range from simple correlation analysis to advanced procedures.


I still have faith, and believe in the "Gowariker Method" with 16 parameters. The results of this old but proven method has been successful. For Vagaries' forecast and analysis made in the "Arrival and quantum Monsoon Watch" series, I combine a few of them alongwith Dynamical models for a personal analysis and estimate. 


15 initial parameters from the months March, April and May are used to analysis and forecast the simulation of Indian monsoon rainfall during June to October. These initial conditions were carefully selected by experts to span the evolution of both the atmosphere and ocean.


Calculating the amount of rainfall, region wise, is of utmost importance, as SWM contributesto almost 80% of the annual total rainfall over India.
The performance of Vagaries' Monsoon Watch Quantum analysis is evaluated for the amount of summer monsoon rainfall over Indian monsoon region during June to SWM withdrawal.


The 500 hpa parameter is of importance in April. The mean latitudinal location of the 500 hPa ridge along 75°E in April over India, first identified by Banerjee (1978), is considered to be one of the most important predictors. The mid-tropospheric anticyclone over southern India migrates from 11.5°N in January to its northernmost position of 28.5°N during July. From October, the ridge starts shifting back southward. 
A more Northward location indicates better performance of the monsoon and vice versa. It also showed that the negative correlation of the March ridge was more dominant with the monsoon rainfall of the peninsular India, while the positive correlation of the April ridge was more dominant with the monsoon rainfall of Northern India.


ENSO factor in arriving at the rainfall amount is also of importance. The interface is the sea surface: that is where the transfers of water (evaporation/precipitation) and momentum occur. An accurate coupling of the fast atmosphere to the slow ocean is essential to simulate the ENSO, which in turn can simulate the interannual variability of Indian monsoon. Unfortunately, getting an accurate, or near to accurate forecast of the ENSO for the next 2 months is difficult, with no guarantee of accuracy or performance surety.


We see neutral conditions now. But, there is a possibility of a mild El-Nino event occuring as early as June/July. Now, this would result in "severe" break Monsoon condition in July. That is the last thing we want. But I would not endorse the event as yet. I would give it a 50-50 chance.


Limited pulses from the Eastern Pacific area, that is S.China sea, would mean lesser depressions and systems originating from the Bay. 
'In Situ" systems and systems from the Far east would contribute to the rainfall to the East Coast thru Central India and into Gujarat and even into Sindh (Pakistan).
El-Nino, even a mild one, would sort of restrict these pulses, and in fact encourage WDs into coming down South, more South than normal. 
But, for this, we would have to hope and pray for the Neutral ENSO conditions (presently on) to prevail and hang on till September. Again, a 50-50 chance for Neutral conditions.


Regarding the IOD parameter, recent forecasts from a few  coupled models  suggest  the possibility of development of a weak negative Indian Ocean Dipole event after July 2012.


1. Seasonal Low: Indicator: -ve


The heat is about to pick up in the Rajasthan/Sindh regions from the 3rd/4th of May, so good chance for the seasonal low to form and establish itself, at least by the 10th.Today the pressure at MSL is 1000 mb, with the core at 1000 mb. 
The sub Continent hot spots have just about touched the 44c mark (it is already Late. Refer MW-1) and should head for the 50c mark by around 16th May.

2. ITCZ: Indicator: Normal
As it is by the 10th, that we should have the ITCZ nearing the Equator. Only then would the Bay branch of the SWM could commence in the Southern Bay by the 15th of May.


3. Jet Streams: Indicator:-ve


These are streaming away in a Westerly direction above the 25N line. They are just about getting Easterly around 5N. Needs a rapid change now.


4. Cross Equatorial Winds: Indicator: Normal in Bay Branch. -ve in Arabian Sea Branch


Well established in the Bay region. SW winds, with sufficient speed hitting the S.Myanmar coasts, covering the Bay Islands with 20-30 knts, and could hearald the arrival of the SWM on time there.


Arabian Sea winds are now picking up, and rubbing the Somali coast at sufficient speed in getting the "Somali Current" established. The SST off the Somali Coast is 22c, While the required SST in First week June is 17/18c. (Required to form masses of Monsoon clouds in the warm central Arabian Sea).

Very Briefly, explaining the Factors considered for Seasonal Forecast:


If we take into account, that the ENSO will be Neutral, but "leaning" towards a weakish El-Nino phase by, say, end August, then:


a) There will be a drop in the systems originating from the Bay. If at all there will be weak depressions just about reaching Central India and fizzling out. 
Hence could result in rainfall deficit for the NW regions, including lower Sindh in Pakistan. Central India and adjoining areas will be well covered.


b) In such a scenario, even the 200 hps jet streams would bend slightly Southwards, and the "low" at that level will be dislodged. could bring in the odd WD even in July. Could bring some heavy rainfall to Upper Pakistan regions. Hence, rainfall could be normal/excess in rest Northern Pakistan and Kashmir. Though technically they will be Monsoon rains.


c) Due to systems reluctant to track southwards, we may see lesser rains in the Southern Peninsula regions. Even the support to the west coast (South of Mumbai)may be lesser than normal. Hence, rain shadow regions in TN and interior Karnataka may be slightly deficient. Lesser gradient along the West coast will show lower rainfall in Kerala, specially in June.


d) Break monsoon situations, and winds bringing in moisture from the Bay (diverted to the NW directions) will result in excessive rainfall this season in Nepal, more so the Eastern regions. NE states of India will get continuous feeding from the Bay. (Cherrapunji seems the likely candidate for the poll).


This is a very early estimates based absolutely on the situation as it shows and stands today.  

Diminishing natural resources

• Petroleum reserve has been depleted day by day in an alarming rate. Government must act swiftly to distribute petrol and diesel in RATION. Every vehicle is entitled to have their legitimate right for the access of petrol / diesel products through ration shops or designated petroleum bunk / outlets.

1. Petroleum / Diesel products should be restricted of all private / public / government vehicle.  Two wheelers may be entitled for 12 litres per month; four wheelers may be entitled for 40 litres per month(including Government Vehicle).
2. The excess usage should have to be borne in competitive rates of the petroleum products by the individual/ owner or whoever they may be.
3. There should be entry restrictions for private vehicle including two wheeler in main through fares / arterial roads during the peak hour and appropriate entry tax need to be collected using appropriate toll gates.Encouraging MRTS/ and other mass rapid transports (buses/EMU) in second tier / third tier cities and big villages.
4. 18-20 % of daily commuter traffic in metropolitan cities can be curbed if e-governance or vantage location offices where every government/ private employee can work comfortably sitting in front of a terminal located  close to his residential locality.

Reposted from http://veeravanalluran.blogspot.com

KAAL BAISHAKHI (or) NOR’WESTER

KAAL-BAISHAKHI OR NOR’WESTER

Phenomena of curse and blessings

Introduction

The dramatic appearance of nor’wester in the afternoon or evening of pre-monsoon months over Bengal, Bihar, Assam and adjoining areas has been a matter of great interest not only to the meteorologists but to all sections of people and has a bright existence in Bengali literature. Famous Bengali poet Mohit Lal Majumder described nor’wester so nicely in his poem “KalBaisakhi” that a meteorologist can hardly do. However, Nor’wester is subject of universal interest for a number of reasons. It not only gives relief after mid-day heat but pours well its crops. Its nature is unique and the causes are really interesting.

During the hot weather period i.e from March to May the eastern and North-eastern states of the subcontinent like West Bengal, Bihar, Assam, Orissa (parts) and Bangladesh experience dramatic appearance of a special type of violent thunderstorm know as Nor’wester.

In Bengal it is known as ‘KalBaisakhi’ or calamity of the month of Baisakh (April,15-May,15). Apart from its destructive effects like sudden rise in wind speed, lightning, thunder  and hail the rainfall associated with the storm although small in amount, is extremely helpful for the pre-Kharif crops like jute, Aus paddy, summer till and a large number of vegetables and fruits and the sudden drop in temperature gives relief after unbearable mid-day heat.

General Characteristics

The first sign of nor‘wester is a low bank of dark cloud in the north-west, the upper outline of which has the appearance of an arch. It approaches at first slowly and then more and more rapidly with a strong gust or squall. There is frequent thunder and lightning followed by down pour of rain and sometimes accompanied with hail, which is driven by the strong wind. On some     occasions the wind blows with almost hurricane force. Nor’easter in Bengal starts as early as February. The number gradually increases reaching maximum in April and May and decreases thereafter along with the onset of monsoon. On an average the hot weather season experiences about 15 such thunder squalls (Violent wind) of which March records 3, April and May 5 to 6 each. But the frequency is highly variable from one year to the next and one region to the other. Usually these thunder squalls occur in the afternoon and evening but they are also observed at other times of the day and night. A large number of these thunder squalls strike from north-west for which they are called nor’wester. In a ‘KalBaisakhi’ wind speed from 55 to 80 km per hour is common but in some cases the speed may exceed 140 km per hour. On the approach of the squall, in general, the temperature drops and pressure rises suddenly due to descending cold air from aloft. Fall of temperature from 20-40c is very common but drop of temperature more then 120c has also been recorded. The amounts of rainfall from such squalls are highly variable. The amount varies from place to place and between individual squalls. Over the Gangetic West Bengal the amount of rainfall from a thunder squall varies from less than a millimeter to more than 80 millimeter and increases from west-north-west to east-south direction. Over North Bengal the amount increases from west to east depending on the source of moisture supply. Hail from the towering umulonimbus cloud occurs during the early part of the season and decreases gradually along with the progress of the season due to gradual rise of the freezing level as a result of increasing surface heating. All such thunder storms occur in association with cumulonimbus (CB) clouds. The bases of these clouds are as low as 450 to 600 meters from the ground surface while their tops frequently go above 12 to 15 kilometers. Duration of ‘KalBaisaki’ over a station varies from a few minutes to about an hour.

Tornado

A pendant-shaped cloud resembling an elephants’ trunk some times descends downwards from the base of CB cloud and may ultimately touch the ground with a great suction-effect, tremendous pressure fall added to a violent whirl wind. These whirl winds are extremely destructive in nature and are called tornadoes, same as ‘twister’ in USA. The tornadoes have a small diameter of a few hundred meters only while the length of their track may extend to only from 25 to 35 kilometers or even less. In case of a tornado the wind speed associated with the storm is much more. It may often go as high as 300 km per hour or more. Instances are on record where heavy materials like a passenger bus had been lifted sodily from the road and thrown into an open field at a distance near Diamond Harbour on 21st March, 1969. Due to the suction effect the whole quantity of water from a pond along with the fish is known to have been lifted up and was dropped at a distance showering fish and snake along with the rainwater in Cooch Behar on 19th April of 1963. On April 15th, 1977 a tornado lashed two villages ‘Bhaitgarh’ and ‘Bagbheria near Kanthi in Purba Medinipur. At least 10 people were killed, 50 injured and a large number of cattle heads destroyed. In the evening of 12^th April, 1983, about 21 villages were lashed and 27 persons were killed in the Gaighata block of North 24- arganas district due to tornado hit. In a tornado which occurred in the afternoon of 9th April, 1993, five villages were completely destroyed, 57 persons were killed, about 300 persons were injured and thousands of people rendered homeless in Kandi and Khargram Police station of Murshidabad district; on 24th March 1998 by 15-00 hours a violent tornado lashed 26 mouzas of Dantan and Egra Police Stations of Paschim and Purba Medinipur districts of West Bengal.

What causes a nor’wester?

For the occurrence of a nor’wester, the lower level moisture flow from the Bay of Bengal is essential. In the summer months the atmosphere becomes unstable due to intense insolation. On such days if there is a suitable synoptic situation such as a surface low pressure area over Jharkhand and adjoining districts of Gangetic West Bengal, the cyclonic vorticity reaching up to 1.5 kilometers above sea level with dry air aloft and presence of a jet stream between the levels 300 mb and 200 mb in the vicinity of Gangetic West Bengal are extremely helpful for the occurrence of nor’westers..

Mechanism

Any mechanism suggested to explain the occurrence of nor’westers in Bengal during the summer months should satisfy the following important facts associated with them:

(i) Thunderstorms approach most frequently from west to north-west and hence they are called nor’westers. The number of thunderstorms approaching from other directions is small.

(ii) The majority of nor‘westers occur between 17-00 and 21-00 hrs, IST.

(iii) Nor‘westers usually occur later over south-east than over southern West Bengal.

(iv) Nor’westers is usually more severe in south-east than in southern West Bengal.

(v) In the majority of nor’westers air pressure rises and temperature falls.

(vi) Nor’westers starts simultaneously or at different times over different places. Time sequence for some nor’westers occurring on the same day at different places is also noticed.

Meteorological studies reveal that during the summer months, there is normally moist southerly air in the lower levels over Bengal and dry westerly or northwesterly air in the upper levels. The depth of moist current is greater in the southeast than in the south west Bengal. Between the moist and dry air mass there is an isothermal region or region of contrast. This distribution of air masses gives rise to latent instability conditions over Bengal in summer months. From the upper air observation it is seen that this instability is always manifested but may exist in a latent state and this only requires a further impulse for the energy to be released sometimes with great violence. This impulse is known ‘trigger action’.

This ‘trigger’ or the impulse is supplied by one or more of the followings which help to rise the moist underlying air up to form cloud, rain and hail:

(a) Insolation-which reaches its maximum in the afternoon and cause large convectional updraught of air from the lower to the higher levels of the atmosphere. As a result of this process, the nor’wester or thunder squalls of Bengal mostly occur.

(b) Out-flow of relatively cold air in different directions from a parent thunderstorm.

(c) East ward passage of the cold front of a western disturbance generally in an occluded form.

(d) Convergence of air due to katabatic flow from the hills to the north and east. As a result of this process the thundersqualls of North Bengal i.e. Jalpaiguri and Coochbehar mostly occur during night.

(e) Inflow of moist wind from the Bay of Bengal due to the formation of a cyclonic storm.

Classification

Thunderstorms over Bengal have been classified as follows:

(i) Those which occur with the passage of a cold front in connection with the western disturbances moving east wards. In this case the nor’westers occurs along a line and one finds a time sequence. Nor’westers due to this mechanism can occur at any time of the day depending upon the time of the passage of the cold front.

(ii) Those which occur in connection with cyclonic storms or depressions in the Bay of Bengal approaching the coast. These

Thunder storms can approach a place from any direction and can occur at any time of the day.

(iii) Those which occur in the morning (after mid-night): These occur in the North Bengal plains and adjoining Assam when there is a flow of cold air from the adjoining hills. Cold air from these thunderstorms can start chains of fresh thunderstorms.

(iv) Those which occur simultaneously over a large area and generally in the

afternoon, evening or early part of night. These are associated with either the approach of the western disturbances or with the accentuation of the east west pressure gradient over Bengal. About 70% to 80% of the nor’westers are of this type. The initial trigger for the release of energy of latent instability and starting the primary or parent thunderstorm is insolation. The released heated moist air goes higher up to form thick CB or thunder cloud. The descending air is generally from higher levels i.e from regions where there is westerly to north westerly wind; being dry, it gets humidified and cooled due to evaporation of rain through which it passes and reaches the surface as cold air. The cold air rushed down with great violence gives rise to squall. Once a thunderstorm starts due to insolation acting as trigger, the cold air from primary or parent thunderstorm can spread easily in the direction of the descending current, i.e generally from north westerly to south easterly direction, in the form of fingers spreading from the palm of the hand. Secondary thunderstorms or nor’westers can start at the tip of each finger as the cold air moves forward. Tertiary, quarternary etc. thunderstorms can also start due to the same mechanism as cold air spreading from the secondary or tertiary etc. thunderstorms can act as trigger. Thus a number of thunderstorms can take place simultaneously and there will be a time sequence in the occurrence of nor’westers. Nor’westers are generally more severe in south east than in south west Bengal as the moist current is deeper there and hence more energy of latent instability can be released to increase the violence of the storm.

(v) Besides the above four types, one should also consider the thunderstorms which can start late in the night as a result of radiative cooling at the top of clouds, giving rise to sufficient instability of strong descending current.

Nor’wester forecasting

Very specific forecasting of nor’wester well ahead is not a very easy task. Apart from the analysis of surface synoptic charts, upper air charts, other charts and diagrams x-band weather radars are capable of detecting these violent storms in their formative stage and subsequent maturity and regeneration. With judicious use of these informations received  from such weather radars, the meteorologists can issue warning against such violent  storms about three hours before the nor’wester hits. Using doplar radar, mobile van equipped with radar, data from automatic weather stations and satellite picture at one hour interval, attempts are being made to issue specific forecast about the time and area of occurrence of Kalbaisakhi considerably earlier.

Nor’wester and agriculture in Bengal

When success of the cultivation of Rabicrops in the State during winter depend on favourable temperature regime and abundant sunshine, cultivation of pre-kharif crops depend on adequate summer rain or nor’wester rain. Broadly speaking entire rabi cultivation is solely dependent on irrigation while pre-kharif crops largely depend on the amount and distribution of nor’wester rain in the state. During the three summer months, from March to May the state receives an average rainfall varying from 135 mm at kaliachak in the southern end to 805 mm at Buxa Duar in the north-east corner of the sub-Himalayan West Bengal and 95 mm at Para in the western part to 280 mm at Tehatta in the eastern border of the Gangetic West Bengal which constitute 7 to 17 per cent of the average annual rainfall. The main source of rainfall of the season is nor’wester. Cultivation of Boro paddy, jute, Aush paddy, summer til, ground nut, maize, summer pulses and a large number of summer vegetables and fruits depend directly upon the amount and distribution of nor’wester rain. Area production and productivity of all the summer crops has a direct and positive co-relation with the nor’wester rain. Prolonged absence of nor’wester rain is also reflected on the quality of fruits of the season like mango, lichi, jack fruit etc. The rain also helps to cover the dry and empty fields with a thick carpet of grass, the major source of food for the cattle population of rural Bengal. Mohit Lal Majumder rightly said in his poem ‘Kalbaisakhi’, it not only destroys the old but creates new and contributes a lot.

The author is agricultural meteorologist & rainfall registration authority of West Bengal.

Searched BY: Mahendra Rai, At Hitech Computers and Services, 2010 

Taken from https://sites.google.com/site/mahendraat2044/essay/essay1

Big pattern changes in winter coming? Arctic Oscillation Flips to Negative

Our December 15th post Winter may have had a mild start but likely to be progressively harsh appears to be turning out prophetic. Big pattern changes in the global weather are on the way and we in India during the last few days got just a small taste of things to come during the last few days.

A strong western disturbance over the western Himalayan region caused cloudiness over northwest and adjoining central India. If snowfall played truant in Himachal Pradesh on Christmas and New Year's Eve, then it made up for the lapse with a vengeance. Snow walloped Shimla, Kufri , Narkanda , Chail , Manali , Chamba and Dalhousie. 

In fact the entire tribal belt of Kinnaur, Lahaul and Spiti, Kullu and Chamba districts witnessed heavy snowfall. Shahpur and parts of Hamirpur among other places also experienced snowfall and sleet after a gap of more than three decades. And it's snowing in 'Queen of Hills' Shimla as well. This after 8 years! The minimum temperature recorded in the hill station fell to minus 0.8 degree C.

Dharamsala received snow after a gap of 35 long years. Located at a height of only 1,350 metre and the abode of Tibetan Buddhist spiritual leader the Dalai Lama, Dharamsala was totally covered in white, making the town more picturesque. In fact the snowfall was unusually heavy with McLeodganj, its tiny hamlet, known for its strong British heritage, receiving some of the heaviest snowfall on record.

Even Nurpur experienced snow. This after 45 years. But what really took weather forecasters by surprise was heavy snow lower down in Kangra town. This was after a gap of a whopping 67 years! The peculiarity of Kangra, Nupur and Dharmasala are that these are foothills where snowfall is not normally expected because of their low elevation. But Himachal Pradesh is not complaining. Good amount of snowfall has not only rejuvenated its glaciers, but also proved beneficial for its tourism industry, with scores of tourists making a beeline to the hill state.  Hotel accommodation and flights are reportedly fully booked.

It gets even better. Probably for the first time in the recorded history of Punjab, several villages of Pathankot and Hoshiarpur districts experienced snowfall on Saturday morning. The two districts normally experience maximum temperature of up to 45 degrees Celsius in summer.  The Met office however dismissed the event as a 'freak' happening due to change in wind directions. 

 

Last Friday, parts of New Delhi and Amritsar woke up to be greeted by a hailstorm.  It is not that hailstorms are a rarity in these parts of the country. It usually takes place during the transition period where winter gives way to spring. This is when temperature rises initially in the southern parts of India, giving rise to thunderstorms and squally weather, including hailstorms in Northern India. The problem is that we are not in this transition period and in fact only now entering into the peak winter season. 

So what could have triggered the hailstorms? My friend, Rajesh Kapadia who administers the blog Vagaries of the Weather explain that conditions for snow are a) surface temperatures are below freezing upto 2 deg C  b)  atmospheric temperatures are below freezing c)  air contains a minimum moisture level.  Atmospheric temperatures were below freezing but conditions of surface temperature and minimum moisture levels for snowfall were not meet to create snow and so the western disturbance impact on New Delhi and Amritsar probably took the shape of hailstorms. This of course remains a logical conjecture. 

 

Meanwhile, more than 3 feet snow received over the past two days closed down Kashmir's main highway, stranding hundreds of travellers. The snowfall on either side of the Jawahar tunnel, blocked the only road link, connecting Kashmir to rest of the country, and traffic has been suspended. Trucks carrying supplies to Kashmir haven't moved a bit in the past three days because the Jammu-Srinagar highway that connects the Valley with the rest of India is closed due to heavy snowfall and landslides. 

The heavy snowfall also knocked out power and telephone lines after the 220 KV-Kishanpur-Pampore and 400 KV Kishanpur-Wagoora transmission lines snapped on other side of Jawhar Tunnel due to snowfall.  Against the required 950 MW of electric power being supplied to the valley, the availability this time is pegged at 120-150 MW. Among the available power, 40-50 MW is being supplied to consumers while rest is being used for running essential services. The plight of residents made worse as J&K is already facing shortage of fuel- LPG and kerosene- for cooking and heating purposes.

Temperatures at Srinagar plummeted to minus 5.5 degree C, the coldest night in the city this winter. Due to the extreme cold, water taps in most areas of Srinagar were frozen in the morning.  Picturesque tourist spot Gulmarg recorded a low of minus 16.5 degree C. Qazigund’s low dipped to minus 8.4 degree C followed by Kokernag (minus 6.3 degree C), Pahalgam (minus six degree C) and Kupwara (minus 5.6 degree C).

 

The Dal Lake was frozen at many places too. Kulgam town in south Kashmir received 2 feet of snow while Srinagar,  around 5 inches of snowfall since last evening. High-altitude areas including the ski resort of Kupwara district which recorded night temperature of minus 0.3 degrees Celsius, had 14.4mm of fresh snow till early morning.

Pahalgam, received fresh snow of 8cms during the time while the world famous hill resort in south Kashmir, recorded a low of minus 1.8 degree C. Kargil district in Ladakh witnessed the first heavy snowfall of the season with Kargil town recording 2 inches of snow while Drass had about 3.5 inches on the ground this morning. Taisuru and Rangdum also recorded 2 inches snowfall. In Leh, temperatures plunged to minus 18.8 deg C.

India is experiencing a harsh winter characteristic of a La Niña winter. So how does rest of the world shape up this winter?

GLOBAL WINTER 

Let’s look at the latest satellite temperature imagery for today.

From these temperature maps it is evident that most of the extreme cold is confined to Canada, Russia and Northern China.  Winter by and large had been mild in most other areas within the Northern Hemisphere. So mild that huge swathes of Northern India are currently colder or at comparable temperature to much of Western Europe.  

The table illustrates that El Niño-Southern Oscillation (ENSO) by itself is not sufficient conditions to determine the character of winters within the Northern Hemisphere. More important is the Arctic Oscillation (AO) Index. The erratic AO can generate strong shifts in the climate patterns that could overwhelm or amplify La Niña's or El Niño’s typical impacts.

Technically, the Arctic Oscillation is a measure of atmospheric pressure variations at sea level north of 20N latitude. Whenever an Arctic high or low develops, it affects weather thousands of miles away.

 

Last year the so-called "Arctic fence" that keeps cold air penned up in the north broke down, allowing frigid air to spill south. So far that's not happening this winter which should explain the mild winter at the mid latitudes. However, the AO is a fickle—not seasonal—phenomenon and can switch erratically between the two modes.

As seen in the NOAA graph, the AO has been in its positive phase for the last 4 months, being particularly very strong all through December and first week January this year. The exception was a couple of days around the middle of October and before Christmas when it briefly flipped to its weakly negative mode. But each time it showed a strong streak of resilience - rebounding quickly to its strongly positive mode again. 

But yesterday, it flipped back to its weakly negative mode again as that Northern India and Pakistan received a snow packing wallop.  At the same time, in Europe, Austria found itself buried under 4 feet of snow, creating chaos and disruption of traffic and power supply. Snowfall trapped some 15,000 tourists at ski resorts on the Arlberg Mountain on Friday, and while some roads are open there, others remain closed. High winds are also whipping the nation, and closed an important rail line. Most snow has fallen in the western states of Tyrol and Vorarlberg.  52 people were rescued by an army helicopter there. An important rail line that connects Vienna and Innsbruck with western Austria and Switzerland was to be closed until Sunday. Two rail lines that connect western Austria with Germany were also closed.

These two events may look unconnected but they need not.  They occurred the same time as the AO flipped to it weakly negative mode.  But the moot question is whether a negative AO is just a blip or something longer in duration?

So what do weather ensemble models indicate? As seen in the above graph, some show the AO index rebounding and going positive while others (the majority) have it trending negative and ending below zero.

Two weathermen think on similar lines. On Twitter, former Accuweather and now with WeatherBell, legendary weatherman Joe Bastardi observed: “10mb [stratospheric] warming on European [model] over the pole by 360 hrs reaches 50C!!!! This is a huge event and will have [northern] hemisphere cold implications…By that I mean, something major is about to happen.” 

Henry Margusity Accuweather also anticipating a major change similarly observed: “This is such an amazing weather pattern evolving. I told all the [meteorologists] that we have a lot of winter ahead of us.”

It was left to Dave Tolleris of Wxrisk to make a more nuanced statement about a possible pattern change. He stated:  “All this being said this does NOT mean the winter of 2011-12 is about to turn nasty for everyone and or the Northeast is going to see big snows or noreasters. …It might turn that way but we don’t know that yet. But the old winter pattern of 2011-12 that has featured sustained warm/ mild dry pattern is going to end soon. And whatever the new pattern … won’t be the same as what the last 45-60 days have been.”

The Washington Post’s blog, Capital Weather blog reacting to these twitter posts:

“Much of the excitement by Bastardi and Margusity probably stems from a strong stratospheric warming event that is now showing up on the model guidance and how it might lead to more blocking and a negative Arctic Oscillation (AO). Remember, a negative AO means a cold and sometimes snowy weather pattern over the eastern U.S...

The pattern change ideas stem from research by Baldwin and Dunkerton published in a 2001 “Science” article which documents how there is coupling between the stratosphere and troposphere and that sudden stratospheric warming events can have an impact on the AO (by helping modulate the strength of the polar vortex), flipping it negative... 

That research notes that once the warming propagates down sufficiently to produce a weather pattern favouring a negative AO (lots of high latitude blocking), the pattern tends to last for about two months. That probably explains why some forecasters think that if the AO goes negative it might stay that way through much of the remainder or the winter....

The potent polar vortex event and positive AO that has been going on for over two months already appears to be on the wane with or without help from the stratosphere. This strong vortex event has lasted longer than most such events so the forecasts of it weakening may have some merit.”

What all this means is that a big pattern change in winter outlook  is due in the next 7-14 days and the odds favour a strong negative AO. If Europeans and Northern Americans have put away their woollies and not stocked up on their heating oil, then it is time they did so in a hurry as a harsh winter is speeding their way and when it does will probably not go away for the next 2-3 months. 

In India, if you thought that winter would have peaked by month end, you could be likely to be wrong.  Instead, the cold wave could turn even harsher. Much of the cold wave and snow in the country is linked to what called western disturbance (WD), which is a term used in India, Pakistan, Bangladesh and Nepal to describe an extra-tropical storm, originating in the Mediterranean that brings sudden winter rain and snow to the northwestern parts of the Indian subcontinent. 

This is a non-monsoonal precipitation pattern driven by the Westerlies. The moisture in these storms usually originates over the Mediterranean Sea and the Atlantic Ocean. Extra-tropical storms are a global, rather than a localized, phenomena with moisture usually carried in the upper atmosphere (unlike tropical storms where it is carried in the lower atmosphere). Once their passage is hindered by the Himalayas, they are unable to proceed further, and they release significant precipitation over the southern Himalayas. In winter it brings snow and sleet. 

As a strong negative AO brings cooling to the Mediterranean Sea and the Atlantic Ocean, this in turn creates stronger WDs that in turn create higher likelihood of snow, sleet and plunging temperatures.

Then again, the AO is so unpredictable and could prove everyone wrong. 

taken from http://devconsultancygroup.blogspot.com