monsoons: a brief introduction

Not quite an ordinary day in monsoon land! A B C D

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

Basic Aims

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 Rainfall Surface pressure trough 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

asian-australian & african monsoon systems rain

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.

annual cycle of surface winds over Indian Ocean and South Asia

Annual cycle of convection (OLR) in the Indian Ocean-South Asia region

Annual cycle of SSTin the Indian Ocean-South Asia region

Three maxima (Bay of Bengal, east coast India, and south of equator) Equatorial maximum indicates importance of intraseasonal oscillations Climatological precipitation over South-Asia/Indian Ocean

Longitudinal & latitudinal sections through monsoon region

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 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 ….. 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 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 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 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 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 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. commence active phase northward propagation Active phase

some basic physics

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.

Impact of Moisture

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 Rotation: increases in intensity and scale

Strong and weak monsoons invoke different upwelling effects creating SST gradients. Then ……. Impact of anomalous monsoon seasons

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 Explanation of why there is an IOZM with approximately biennial period Explanation of why there is an IOZM with approximately biennial period

Monsoon is a fully coupled ocean-atmosphere system

predictability of the south asian monsoon