SEASONAL VARIATIONS IN REGIONAL CIRCULATION SYSTEMS: THE MONSOONS
Outline
The
Word "monsoon" is derived from the Arabic word "mausam" which means season and in its broadest definition describes those climates that are seasonally arid.
The Word “Monsoon”
Ramage’s criteria for a monsoon to exist are as follows :
Element Of a Monsoon Circulation
1) Seasonal Heating
2) Moisture Processes
3) The Earth’s Rotation
Element Of a Monsoon : 1) Seasonal Heating
Seasonal contrasts in land surface temperatures produce atmospheric pressure changes.
~ 30C
~ 86F
Element Of a Monsoon Circulation� 1) Seasonal Heating
As a result, there are major seasonal wind reversals referred to as “the monsoons”
Elements Of a Monsoon Circulation� 2) Moisture Processes
Energy released in the form of latent heat of condensation raises summer land-ocean pressure differences to a point higher than they would be in the absence of moisture in the atmosphere.
Moisture 🡪 Monsoon Magnitude
Elements Of a Monsoon� 3) The Earth’s Rotation
Winds change direction as they cross the equator because of changes in the Coriolis sign
Element Of Monsoon Circulation
1) Seasonal Heating
2) Moisture Processes
3) The Earth’s Rotation
Moiture releases energy (latent heat) that intensifies the monsoon
Seasonal temperature and pressure changes produce seasonal wind reversals
Air moves in curved paths and winds change direction as they cross the equator
INDIAN MONSOON
Indian Monsoon
The Indian monsoon is made up of a number of components:
The Indian Monsoon (surface features)
Monsoon trough:
- Formed over northern India;
- Northern Hemisphere Summer as part of the global ITCZ;
- Associated with surface low pressure.
Mascarene high:
- Situated over the south-west Indian Ocean (30o S, 50o E);
- Generates a large outflow of air;
- The air moves north over the equator where it becomes a south-
westerly flow known as the low level cross-equatorial jet.
The Indian Monsoon
Fig. 2 from Krishnamurti and Bhalme (1976) Mean Sea Level Pressure for July
The Indian Monsoon and surface circulation
Monthly progression of the low-level cross-equatorial jet
Fig 5 from Krishnamurti and Bhalme (1976)
Low level cross-equatorial jet:
Low pressure
Low level Convergence
Upward vertical movement transports air molecules to upper levels.
More molecules imply in the increase of the weight of the column of the air above a given upper level surface
High pressure
upper levels
High pressure
Low pressure
upper levels
Upper level surface
Low level Divergence
Northern Hemisphere
Reasons for the easterly jet:
20
During summer, the Tibetan Plateau, which is located in subtropical regions, intensify the ascent increasing air pressure in high levels
High level High Pressure
Hot surface
Low pressure system:
Ascending air
High Pressure
High level Easterly Jet
Temperature Gradient
+
-
INDIAN MONSOON
JAN
APRIL
JULY
OCT
High level winds
Figs. 6 from Krishnamurti and Bhalme (1976) Monthly mean 200 hPa wind field
Example of wind
(85 knots)
50 + 3x10 + 5
Convective cloud processes
in the Indian Monsoon
Fig. 1 from Krishnamurti and Bhalme (1976)
Cloud cover:
Onset of Monsoon in India
The guidelines to be followed for declaring the onset of monsoon over Kerala and
its further advance over the country are enlisted below:
If after 10th May, 60% of the available 14 stations enlisted*, viz. Minicoy, Amini,
Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi, Thrissur,
Kozhikode, Thalassery, Kannur, Kudulu and Mangalore report rainfall of 2.5 mm or
more for two consecutive days, the onset over Kerala be declared on the 2nd day,
provided the following criteria are also in concurrence.
b) Wind field
Depth of westerlies should be maintained upto 600 hPa, in the box equator to Lat. 10ºN
and Long. 55ºE to 80ºE. The zonal wind speed over the area bounded by Lat. 5-10ºN,
Long. 70-80ºE should be of the order of 15 – 20 Kts. at 925 hPa. The source of data
can be RSMC wind analysis/satellite derived winds.
c) Outgoing Longwave Radiation (OLR)
INSAT derived OLR value should be below 200 wm-2 in the box confined by Lat.
5-10ºN and Long. 70-75ºE.
www.imd.gov.in
EAST ASIA MONSOON
1) East Asia Monsoon Trough or convergence zones
2) Mei-Yu Front
Components of the East Asia Monsoon
High level High
3) Tibetan High
4) Western Pacific High:
Low level
5) High level easterly jet
6) Low-level southerly
7) Low-level cross eq. jet
8) Midlatitude disturbances
EAST ASIA MONSOON
Ding (2004) has summarised the climatological dates of the onset of the Asian summer monsoon in different monsoon regions based on various sources, with dividing the whole onset process into four stages :
(1) Stage 1 (late in April or early in May) : the earliest onset in the continental Asia is often observed in the central Indochina Peninsula late in April and early in May, but in some cases, the onset may first begin in the southern part or the western part of the Indochina Peninsula.
(2) Stage 2 (from mid to late May) : this stage is characterized by the areal extending of the summer monsoon, advancing northward up to the Bay of Bengal and eastward down to the SCS.
(3) Stage 3 (from the first dekad to second dekad of June) : this stage is well known for the onset of the Indian summer monsoon and the arrival of the East Asian rainy season such as the Meiyu over the Yangtze River Basin and the Baiu season in Japan.
(4) Stage 4 (the first or second dekad of July) : the summer monsoon at this stage can advance up to North China, the Korean Peninsula (socalled Changma rainy season) and even Central Japan.
Regional Monsoon Systems�East Asia Monsoon
East Asian Monsoon also originates in air flows from:
The low-level cross equatorial jet:
Monsoon trough:
Australian Monsoon /
East Asia Winter Monsoon
Regional Monsoon Systems� 3) Australian Monsoon / East Asia Winter Monsoon
Climatological (1971–2000) SST (C) around Australia: (a) February and (b) August
a
b
Mean air temperature (°C) over Australia and surrounding regions at 925 mb
(a) February and (b) August (Courtesy: NCEP/NCAR Reanalysis Project)
a
b
a
b
Isobaric height (m) fields at 925 hPa over Australia and surrounding areas during (a) February and (b) August (Courtesy: NCEP Reanalysis)
Mean wind field and circulation over Australia and surrounding regions at 925 and 300 hPa (a) February and (b) August (Courtesy: NCEP/NCAR Reanalysis Project)
Onset of Australian Monsoon�
Onset of Australian Monsoon
Onset of Australian Monsoon
Onset of Monsoon
Australian Monsoon Variability
McBride (1987) quotes an example from two contrasting years of rainfall, namely, 1983 and 1974, when the January SOI was highly negative and positive respectively. In January 1983, an SOI of –29.8 corresponded to a distribution of widespread much below average rainfall over northern Australia, whereas in January 1974 with a value of +21.7 for SOI, rainfall was very much above average over a much wider area of tropical Australia.
a. Variability of Australian Rainfall with ENSO (Seasonal Variability)
SOI
Rainfall
Over Southern Australia
Australian Monsoon Variability
The MJO is typically at its strongest and most coherent during the southern hemisphere summer. During this season, the MJO displays greatest variability of low-level winds and convection across the eastern Indian Ocean and western Pacific between the equator and about 15°S (e.g., Fig.1). This region encompasses the Australian and related Indonesian summer monsoons. Thus great potential for MJO-induced variability of these monsoon systems exists.
b. Variability of Australian Rainfall with MJO and Tropical Disturbance (Intraseasonal Variability)
The MJO is typically at its strongest and most coherent during the southern hemisphere summer. During this season, the MJO displays greatest variability of low-level winds and convection across the eastern Indian Ocean and western Pacific between the equator and about 15°S (e.g., Fig.1). This region encompasses the Australian and related Indonesian summer monsoons. Thus great potential for MJO-induced variability of these monsoon systems exists.
As in Fig.1, except for composites of rainfall and wind over Australia. The precipitation contour interval is 1 mm/day, with additional contours drawn at +-0.5 mm/day. Only those rainfall anomalies that are statistically significant at the 90% level are shaded
Tracks of named tropical depressions and cyclones which formed during the summer season (December–April) of 1999–2000 between 100 and 180◦ E. Name of the disturbance is given at the location of its first detection. Thearrowshows the direction of its movement (Paterson and Bate, 2001)
Climate of Africa
Onset of West African Monsoon
‘‘Preonset’’ of the summer monsoon, defining the beginning of the rainy season over the Sudano–Sahelian zone based on the northward migration of the northern limit of the southwesterly winds of the monsoon (called the intertropical front (ITF))
The real ‘‘onset’’ of the summer monsoon characterized by an abrupt northward shift of the ITCZ from 5°to 10°N (Sultan and Janicot 2000; Le Barbe´et al. 2002), and leading to major changes in the atmospheric circulation over West Africa.
Intraseasonal variability of West African Monsoon
(Eric D. Maloney and Jeffrey Shaman, 2007)
Seasonal variability of West African Monsoon
ENSO:
El Niño 🡪 Subsidence over west Africa and thus below average convection.
La Niña🡪 ascent over west Africa and thus above average convection.
AMERICAN MONSOON
AMERICAN MONSOON
NORTH AND CENTRAL
AMERICAN MONSOON
SOUTH AMERICAN MONSOON
A. SOUTH AMERICA
AMERICAN MONSOON
1. Introduction
A. South America
MEAN MONTHLY RAINFALL
The reason for not mentioning monsoon in the case of South America earlier,might have been frequent failure of the observed surface wind direction over the continent to show seasonal , reversal which was used criterion for definition of monsoon according Ramage 1971.
Some recent studies appear to have removed the doubt
🖎Zhou and Lau study(1997) showed not only the seasonal reversal of the wind at 850 hpa but also a mean meredional circulation
TRADE WIND
2. Physical Features and Environment
2.1 Physical Dimension of the Continent
2.2 Topography
☞ High Andreas Mountain
☞ Venezuelan Highland
☞ Amazon basin
2.3 Oceanic Environment
Pasific Ocean
Atlantic Ocean
Caribbean Ocean
Pasific Ocean
2.3 Oceanic Environment and its influence on climate
3. Climatological Features
3.1 Air temperature
1
2
3
3. Climatological Features
3.2 Wind
3. Climatological Features
3.3 Co-existense of Monsoon and hadley Circulations
3. Climatological Features
3.4 Rainfall over South America
4. Quasi Stationary Waves and their associated weather
4.1 Definition
The wave which are forced by land and sea thermal contrast and/or orography along the northern and the southern boundaries of the heat low over the continent
4. Quasi Stationary Waves and their associated weather
4.2 Quasi Stationary Waves consist:
4.2 1 Monsoon Stastionary 🡪 Northern Boundary
✍ Low Level Jet(LLJ)
4. Quasi Stationary Waves and their associated weather
4.2 Quasi Stationary Waves consist:
4.2 1 Monsoon Stastionary 🡪 Northern Boundary
✍ Low Level Jet(LLJ)
4. Quasi Stationary Waves and their associated weather
4.2 Quasi Stationary Waves consist:
4.2 1 Monsoon Stastionary 🡪 Northern Boundary
✍ Low Level Jet(LLJ)
4. Quasi Stationary Waves and their associated weather
4.2 Quasi Stationary Waves consist:
4.2 1 Monsoon Stastionary 🡪 Northern Boundary
✍ Zonal Anomaly of Rainfall over NE Brazil
4. Quasi Stationary Waves and their associated weather
4.2 Quasi Stationary Waves consist:
4.2.2 Subtropical Stasionary Wave 🡪 Sourthern Boundary
✓ South Atlantic Convergence Zone(SACZ)
A through of the’heat low’ over SE Brazil at low levels extends southeastward into southwestern Atlantic Ocean. Further southeastward from Brazil coast over the ocean,this through of the”heat low” appears to form an extended through with a quasistationary through in the middle latitude westerlies of the Southwestern Atlantic Ocean
4. Quasi Stationary Waves and their associated weather
4.2 Quasi Stationary Waves consist:
4.2.2 Subtropical Stasionary Wave 🡪 Sourthern Boundary
✓ South Atlantic Convergence Zone(SACZ)
5. Tropical Disturbances over South America
A Recent study(Saha and Saha, 2004b) using observed winds,identified two types of tropical disturbances which may form in the circulation around the heat low over South America, these are:
During January of period 4 years,1999-2002, Saha and Saha detected as many as nine synoptic scale tropical disturbances over South America.
Of the disturbances observed, four were monsoon lows and depressions over the southern part of Brazil and five upper tropospheric cyclonic vortices over the oceanic area close to the coast of NE Brazil
5. Tropical Disturbances over South America
1
2
3
4
6. A Tropical Cyclone over the South Atlantic Ocean
The formation of a tropical cyclone over the South Atlantic Ocean in March 2004 that hit Brazil on 28March was an event that took the meteorological community all over the world by surprise, since in the past there had been no evidence of a tropical cyclone over this ocean and there had been a general belief that no tropical cyclone could ever form over this ocean, since it was too cold to breed and sustain a tropical cyclone. The belief was not entirely unfounded, for Gray (1968) had shown (see Fig. 2.8) that during a 20-year period no tropical cyclone had ever been reported from the South Atlantic Ocean.(Saha,Khusdiran 2010)
6. A Tropical Cyclone over the South Atlantic Ocean
6. A Tropical Cyclone over the South Atlantic Ocean
1. Subtropical Cyclone of March 1974
2. Angola Tropical Storm of 1991
6. A Tropical Cyclone over the South Atlantic Ocean
3. Tropical Cyclone of January 2004
4. Cyclone Catharina
6. A Tropical Cyclone over the South Atlantic Ocean
5. Tropical Cyclone of February 2006
6. Subtropical Storm of January 2009
6. A Tropical Cyclone over the South Atlantic Ocean
7. Tropical Storm Anita
8. Subtropical Storm of November 2010
6. A Tropical Cyclone over the South Atlantic Ocean
9. Tropical Storm Arani
6. A Tropical Cyclone over the South Atlantic Ocean
There have been 9 officially recorded tropical and subtropical cyclones in the South Atlantic Ocean since 1974.
B. MONSOON OVER CENTRAL AMERICA
1. Introduction
WEST
EAST
3. The Climate of Central America
3.1 Surface Temperature and Winds
3.1 Surface Temperature and Winds
3.2 Upper Air Temperature
3.2 Upper Air Temperature
3.2 Upper Air Height (gpm)
3.2 Upper Air Height (gpm)
3.4 Upper Air Wind Field and Circulation
3.4 Upper Air Wind Field and Circulation
4.Rainfall over Central America
4.1 Annual Rainfall over Mexico
4.2 Source of Moisture for Rainfall over the Arizona Sonoran Desert
5. Some Characeristic Features of Weather over central America
5.1 Weather Associated with W’ly Waves
5.2 Weather Associated with ‘Northers’
5.3 Land and Sea Breezes on the Pacific Coast of mexico
5.4 Temporales of the Caribbean Sea and the Gulf of mexico
5.5 Hurricanes and Tropical Storm
EXTRATROPICAL MONSOON OVER NORTH AMERICA
1. Introduction
2. Climatological background of North American Monsoon
2.1 Physical Features of the land
2.2 Semi Permanent High and Low Pressure System
2.2.1 The Subtropical High Pressure System of the Pacific Ocean
2.2.2 The Subtropical High Pressure System over the Atlantic Ocean
2.2.3 The Artic Ocean High Pressure
2.2.4 Semi Permanent ‘Aelutian Low’ and’Icelandic Low’
3. The Seasonal Movement of Heat Sourecs and Sinks
4. Seasonal Circulation-Monsoons
4.1 The Winter Monsoon(Dec-Feb)
4.2 The Spring Transtition Season(Marc-May)
4.3 The Summer Monsoon(June-August)
4.3.1 Structure of the Monsoon Boundary Layer
4.3.2 Moisture Budget and Precipitation
4.3.3 Seasonality in the Distribution of Orographic Precipitation
4.4 The Autumn Transtition Season(Sept-Nov)
5. Interaction of Monsoon with W’ly Wave Disturbance
6. Some Characteristic Features of East Coast Monsoon
6.1 Seasonal Variations and Reversals
6.2 Monsoonal Characteristic of the East Coast Region
7. Role of the Appalachian Mountain range-Leesida clogenesisi-Northeas Storms
8. Interaction of Monsoon with Storms and Hurricanes
5. Tropical Disturbance over South America
5.1 Types of Disturbances
5.2 Monsoon Lows and Depressions
5.3 Upper Tropospheric Cyclonic Vortices
6. A Tropical Cyclone over the South Atlantic Ocean
6.1 Introduction
6.2 Formation of the initiaal vortex
6.3 Structure, Movement and Development oh the vortex