Tuesday, October 1

Climates of Nepal with glimpse on Post Monsoon

Nepal is a small landlocked country located between world’s two most populous countries China to the north and India to the east, west and south with a total land area of 147,181 sq km. Nepal stretches from 26º 22’ to 30º 27’ North latitude and from 80º 04’ to 88 º 12’ East longitudes. The country looks roughly rectangular in shape with the length from east to west of about 885 km and width ranging from 130 to 260 km. The elevation of the country increases from about 60 m above mean sea level in the south to more than 8800 m above mean sea level in the north. The highest peak Mt. Everest, known as Sagarmatha to the Nepalese, rises to 8848 m and lies at the northern part of east Nepal . The country is divided into three broad ecological regions, i) the mighty Himalayas in the north, ii) Hills and Valleys in the middle, and iii) Terai, an extension of Indo-Gangetic plain, in the south. Mountains, Hills and the Terai regions are homes to 7 %, 46 % and 47 % of Nepal’s population respectively. Terai is a low-lying plain, highly vulnerable to floods during the monsoon. Northwards, a series of complex valleys breaks up the simple pattern of parallel east-west mountain ranges, and one of these, the valley of Kathmandu, contains the capital of Nepal. Further to the north rises the Himalayas, the world's greatest mountain range.

Climate of Nepal is based upon monsoon with great deal of variation in climate. Altitudinal variation within a country ranges climate from tropical to arctic. 

Seasons or Climate of Nepal can be categorized as;

Pre-monsoon or Hot weather Season :                     March-May
Summer(southwest) Monsoon or Rainy Season:       June-September
Post-monsoon:                                                        October and November
Winter (Northeast) Monsoon:                                  December-February

Traditionally, 6 types of Nepali Seasons

Spring -     Basanta           Chaitra-Baisakh (~April-May)    
Summer -  Grishma            Jestha-Asar (~June-July)    
Monsoon - Barsha            Shrawn-Bhadra (~Auguste- September)    
Autumn -  Sharad              Ashoj-Kartik (~October-November)    
Winter -    Hemanta          Mangsir –Poush (~December-January)    
Windy -     Sisir                 Magh-Falgun (~February-March)  

As described in Wikipedia, the free encyclopedia,( http://en.wikipedia.org/wiki/Geography_of_Nepal), Nepal has tremendous variation in climate. Its latitude is about the same as that of Florida of the USA so Terai land up to 500 meters (1,640 ft) has a fully tropical climate, with a subtropical zone extending up to 1,200 meters (3,937 ft) which is the lower limit of frost in winter. Warm temperate climates prevail from 1,200 to 2,400 meters (3,937 to 7,874 ft) where snow occasionally falls. Then there is a cold zone to 3,600 meters (11,811 ft) (treeline), a subarctic or alpine zone to 4,400 meters (14,436 ft) and fully arctic climate above that. Precipitation generally decreases from east to west with increasing distance from the Bay of Bengal, source of the summer monsoon. Eastern Nepal gets about 2,500 mm (98.4 in) annually; the Kathmandu area about 1,400 mm (55.1 in) and western Nepal about 1,000 mm (39.4 in). This pattern is modified by adabiatic effects as rising air masses cool and drop their moisture content on windward slopes, then warm up as they descend so relative humidity drops. Annual precipitation reaches 5,500 mm (216.5 in) on windward slopes in the Annapurna Himalaya beyond a relatively low stretch of the Mahabharat Range. In rainshadows beyond the high mountains, annual precipitation drops as low as 160 mm (6.3 in), creating a cold semi-desert.

Fig.1; Nepal Map
(source: Wikipedia, the free encyclopedia)

Furthermore the year is divided into a wet season from June to September—as summer warmth over Inner Asia creates a low pressure zone that draws in air from the Indian Ocean—and a dry season from October to June as cold temperatures in the vast interior creates a high pressure zone causing dry air to flow outward.The monsoon also complicates transportation with roads and trails washing out while unpaved roads and airstrips may become unusable and cloud cover reduces safety margins for aviation. Rains diminish in September and generally end by mid-October, ushering in generally cool, clear, and dry weather, as well as the most relaxed and jovial period in Nepal. By this time, the harvest is completed and people are in a festive mood. The two biggest and most important Hindu festivals—Dashain and Tihar (Dipawali)—arrive during this period, about one month apart. The postmonsoon season lasts until about December.

After the postmonsoon comes the winter monsoon, a strong northeasterly flow marked by occasional, short rainfalls in the lowlands and plains and snowfalls in the high-altitude areas.In this season the Himalaya function as a barrier to cold air masses from Inner Asia, so southern Nepal and northern India have warmer winters than would otherwise be the case. April and May are dry and hot, especially below 1,200 meters (3,937 ft) where afternoon temperatures may exceed 40 °C (104 °F).

Below is the first day temperature plot of Post Monsoon 2013;

Fig.2: 1 Oct 2013, Temperature Plot
(Data Source: mfd.gov.np)

And the rainfall statistics for the October in Kathmandu as stated in mfd.gov.np, normal this month is 51.2 mm and the highest 24hrs amount ending at 8:45 AM ever recorded in this month on 20/1987 is 124 mm.

The post monsoon can also be called an autumn. This period is considered as retreating of summer monsoon and is transitional months when the pressure and upper wind systems undergo gradual change. The intertropical convergence zone (ITCZ) now moves towards equator and the temperature starts falling down. Also, autumn sees the southward swing of the Equatorial Trough and the zone of maximum convection, which lies just to the north of the weakening easterly jet. The break-up of the summer circulation systems is associated with the withdrawal of the monsoon rains, which is less clearly defined than their onset. By October, the easterly trades of the Pacific affects the Bay of Bengal at the 500mb level and generate disturbances at their confluence with the equatorial westerlies. Occasional showers with thunderstorm and wind directions undergoes a change i.e. winds start blow from west. 

Fig.3: Mean sea level isobars -October
Ways of the Weather, P A Menon, National Book Trust, India)

Weakening of low pressure system over the Indian subcontinent as a whole is the most important climatic characteristics feature. But the low pressure center is found over the Bay of Bengal where water surface temperature reach a maximum in this season. This is the major season for Bay of Bengal cyclones and it is these disturbances, rather than the onshore north-easterly monsoon, that cause the October/November rainfall. A number of cyclone form over the southern part of the Bay. Most of these cyclones curve round to the north of central low and reach the east coast from the west.  Sometimes these cyclones becomes very violent with formation of eye in Indian Ocean. There is also correspondence between the oscillation of ITCZ and the areas of rain incidence. 
Fig.4: Normal Upper Winds- October
Ways of the Weather, P A Menon, National Book Trust, India)

During October, the westerly jet re-establishes itself south of the Tibetan Plateau, often within a few days, and cool season conditions restored over most of south and East Asia. Temperatures decreases all over and the nights become cool. By November sub-zero temperatures are reached over high terrain. Hence, this is really the cool season.

Rather than Nepal mostly south India gets rainfall in this season. Further, rain occurrence is confined to periods when lows, depressions, cyclones or easterly waves move over or across the peninsula. Hence this season is one of retreating south west monsoon

Reference for Further Reading:
  1. Ways of the Weather, P A Menon, National Book Trust, India
  2. Atmosphere, Weather and Climate, Seventh Edition, Roger G. Barry and Richard J. Chorley, Routledge, London and New York
  3. General Climatology, Fourth Edition, Howard J. Critchfield, Prentic-Hall, India
  4. Climatology, D.S. Lal, Sharda Pustak Bhawan, Allahabad, India

Friday, September 27

MAJOR HIGHLIGHTS: IPCC WGI AR5, Climate Change 2013: The Physical Science Basis, Summary for Policymakers (SPM)

Warming of the climate system is unequivocal, and since the 1950s, many of the observe changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased

Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850. In the Northern Hemisphere, 1983–2012 was likely the warmest 30-year period of the last 1400 years (medium confidence).

Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010 (high confidence). It is virtually certain that the upper ocean (0−700 m) warmed from 1971 to 2010 , and it likely warmed between the 1870s and 1971.

Over the last two decades, the Greenland and Antarctic ice sheets have been losing mass, glaciers have continued to shrink almost worldwide, and Arctic sea ice and Northern Hemisphere spring snow cover have continued to decrease in extent (high confidence)

Sea Level:
The rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia (high confidence). Over the period 1901–2010, global mean sea level rose by 0.19 [0.17 to 0.21] m.

Carbon and Other Biogeochemical Cycles:
The atmospheric concentrations of carbon dioxide (CO2), methane, and nitrous oxide have increased to levels unprecedented in at least the last 800,000 years. CO2 concentrations have increased by 40% since pre-industrial times, primarily from fossil fuel emissions and secondarily from net land use change emissions. The ocean has absorbed about 30% of the emitted anthropogenic carbon dioxide, causing ocean acidification.

Drivers of Climate Change:
Total radiative forcing is positive, and has led to an uptake of energy by the climate system. The largest contribution to total radiative forcing is caused by the increase in the atmospheric concentration of CO2 since 1750.

Understanding the Climate System and its Recent Changes:
Human influence on the climate system is clear. This is evident from the increasing greenhouse gas concentrations in the atmosphere, positive radiative forcing, observed warming, and understanding of the climate system.

Evaluation of Climate Models:
Climate models have improved since the AR4. Models reproduce observed continental-scale surface temperature patterns and trends over many decades, including the more rapid warming since the mid-20th century and the cooling immediately following large volcanic eruptions (very high confidence).

Quantification of Climate System Responses:
Observational and model studies of temperature change, climate feedbacks and changes in the Earth’s energy budget together provide confidence in the magnitude of global warming in response to past and future forcing.

Detection and Attribution of Climate Change
Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes . This evidence for human influence has grown since AR4. It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.

Future Global and Regional Climate Change:
Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system. Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions.

Atmosphere: Temperature
Global surface temperature change for the end of the 21st century is likely to exceed 1.5°C relative to 1850 to 1900 for all RCP scenarios except RCP2.6. It is likely to exceed 2°C for RCP6.0 and RCP8.5, and more likely than not to exceed 2°C for RCP4.5. Warming will continue beyond 2100 under all RCP scenarios except RCP2.6. Warming will continue to exhibit interannual-to-decadal variability and will not be regionally uniform.

Atmosphere: Water Cycle
Changes in the global water cycle in response to the warming over the 21st century will not be uniform. The contrast in precipitation between wet and dry regions and between wet and dry seasons will increase, although there may be regional exceptions.

The global ocean will continue to warm during the 21st century. Heat will penetrate from the surface to the deep ocean and affect ocean circulation.

It is very likely that the Arctic sea ice cover will continue to shrink and thin and that Northern Hemisphere spring snow cover will decrease during the 21st century as global mean surface temperature rises. Global glacier volume will further decrease.

Sea Level
Global mean sea level will continue to rise during the 21st century . Under all RCP scenarios the rate of sea level rise will very likely exceed that observed during 1971–2010 due to increased ocean warming and increased loss of mass from glaciers and ice sheets.

Carbon and Other Biogeochemical Cycles:
Climate change will affect carbon cycle processes in a way that will exacerbate the increase of CO2 in the atmosphere (high confidence). Further uptake of carbon by the ocean will increase ocean acidification.

Climate Stabilization, Climate Change Commitment and Irreversibility:
Cumulative emissions of CO2 largely determine global mean surface warming by the late 21st century and beyond . Most aspects of climate change will persist for many centuries even if emissions of CO2 are stopped. This represents a substantial multi-century climate change commitment created by past, present and future emissions of CO2.