monsoons: a brief introduction
what is a monsoon • Monsoon is a climatological feature covering roughly half the tropics (1/4 of the global surface) • Strictly, a system where the winds and precipitation reverses (summer rain, winter dry) • Sufficiently reproducible to host the most successful agricultural system (5000 years of success) • Host 65% of the world’s population • Small changes in year-to-year climate can be catastrophic
a description of the south asian monsoon
Halley (of comet fame) was the first to describe monsoon and attribute differential heating between land and ocean as the cause
mean august and january precipitation from MSU satellite (mm/day)
Monsoon regions are the largest region where precipitation exceeds evaporation. This excess water must come from somewhere …
…….which defines the scale of the monsoon. The monsoon is intrinsically inter-hemispheric with the winter hemisphere being the source of moisture
monsoon of the americas • There are three major • monsoon systems: • Asian-Australian • African • Americas • Each has its own • characteristics and • differences but each is • characterized by summer • rains and seasonal wind- • reversals Rainfall Surface pressure trough
Annual cycle of radiative temperature “Cold” areas denote Cloudiness and precipit- ation. “Warm” areas show clear regions. Note that there are large gradients both east-west and north- south indicating that the monsoon is driven by heating gradients in both directions.
Climatological precipitation over South-Asia/Indian Ocean • Three maxima (Bay of Bengal, east coast India, and south of equator) • Equatorial maximum indicates importance of intraseasonal oscillations
variability of the south asian monsoon
Time Scales of Monsoon Variability • Interannual variability: Variations on the annual cycle of the monsoon producing anomalously wet or dry years. Generally influenced by sea-surface temperature variations associated with ENSO or inherent variability in the Indian Ocean. • Monsoon Weather: Weather events such as monsoon lows and depressions, tropical cyclones (hurricanes) and etc. Produce short-lived local flooding (or drought), erosion, high winds and etc. • Intraseasonal Variability: “Envelopes”: or clusters of weather events leading to 20-40 day droughts or flood periods. Most difficult to forecast but perhaps the most important of all time scales for economic and agricultural sectors.
El Nino influences Indian Ocean Changes SST, sea surface slope and regions of maximum precipitation However, other factors are also important …..
Indian Ocean Zonal Mode: Inherent mode of Indian Ocean invoked by outside perturbations including ENSO Time series of E-W SST gradient frequency spectra
Sea-level variations associated with IOZM: Positive phase: warm SST WIO, low SSH EIO Negative phase: warm SST EIO, high SSH EIO
Intraseasonal variability of the monsoon: Active and break periods of the Summer South Asian Monsoon The summer monsoon is divided into distinct periods of wet and dry. When it is dry on the plains, it is wet in the foothills of the Himalaya, just as occurred this year. The result was Nepal, Assam and Bangladesh floods, the latter due to strong Brahmaputra discharge.
Intraseasonal Modes: Impact on Rainfall • Intraseasonal variability imposes a distinct form to precipitation. • Histograms of precipitation, shown for 9 years show distinct wet periods with lulls in between. • Lower diagram shows the distinct spatial character Precipitation histograms in central India
Forecasting monsoon variability on 20-30 day time scales: Taking advantage of a new understanding of monsoon dynamics Peter J. Webster Georgia Institute of Technology
Differences between active and break periods of the monsoon Note distinct bands of precipitation excess or deficit. During breaks, there is a 20-40 day drought across plains of India and excess rain over southern, northern and north-eastern regions
Latitude-time section of OLR along 90E: 1995 Active phases of the monsoon commence near the equator and propagate northward (and southward) across South Asia. Active phase northward propagation commence active phase
Physical Ingredients for a Monsoon • Land-Sea differences: land and water have different heating capacities & water can store more heat because it is a fluid and can mix heat down for future release • Planetary rotation: introduces swirl and much stronger winds • Moisture: water vapor, collected over oceans through evaporation, condenses over land and marginal seas releasing vast quantities of heat • Orography: acts as elevated heat source which intensifies flow & also ducts flow.
Impacts of Elevated Heat Sources Note the dominating effect of the Himalayas
Changes in Tropospheric Temperature in Monsoon Region Temperature changes much larger over Himalayas than elsewhere.
Impact of East African Highlands Arguably, without the Himalayas & the East African Highlands, South Asia would be a desert like North Africa
Impact of Rotation: increases in intensity and scale
Impact of anomalous monsoon seasons Strong and weak monsoons invoke different upwelling effects creating SST gradients. Then …….
Explanation of why there is an IOZM with approximately biennial period Atmosphere responds to SST anomaly associated with strong/weak monsoon Ocean responds to anomalous winds producing ocean dynamic modes Anomalous SST produces a monsoon of opposite anomaly which wipes out IOZM
predictability of the south asian monsoon