Tropical Easterly Jet
The Tropical Easterly Jet (jet stream) is the meteorological term referring to an upper level easterly wind that forms in late June and continues until early September. This strong flow of air that develops in the upper atmosphere during the Asian monsoon is centred on 15°N, 50-80°E and extends from South-East Asia to Africa.
A much weaker easterly jet exists in the northeast Pacific associated with the North American Monsoon.
Meteorology
The strongest development of the jet is at about {{convert|15|km|mi|0}} above the Earth's surface with wind speeds of up to {{convert|40|m/s|km/h mph knots}} over the Indian Ocean.{{cite book |title=Climate and Circulation of the Tropics |last=Hastenrath |first=Stefan |year=1985|publisher=Kluwer Academic Publishers |isbn=90-277-2026-6 |pages=464}} This jet subsides at the Somalia coast with the Mascarene High,{{cite journal|journal=Journal of the Atmospheric Sciences|last1=Krishnamurte|last2=Bhalme|first1=T.N.|first2=H.N.|title=Oscillations of a Monsoon System. Part 1. Observational Aspects|date=October 1976|pages=1937–1954|volume=33|issue=10 |doi=10.1175/1520-0469(1976)033<1937:OOAMSP>2.0.CO;2 |bibcode=1976JAtS...33.1937K }} and in the Sahara Desert. It has been suggested that the subsidence in the northwestern quadrant of the Tropical Easterly Jet is an important factor producing the exceptional hyperaridity of the Sahara{{cite encyclopedia|last1=Webster|first1=Peter J.|last2=Fasullo|first2=John|year=2003|title=Monsoon: Dynamical Theory|encyclopedia=Encyclopedia of Atmospheric Sciences|editor-last1=Holton|editor-first1=James R.|editor-last2=Curry|editor-first2=Judith A.|pages=1370–1385|location=London|publisher=Academic Press}} – drier than any other desert outside of coastal upwelling zones.{{cite book|title=Dryland Climatology|isbn=9781139500241|page=303|date=27 October 2011|publisher=Cambridge University Press|first=Sharon E.|last=Nicholson}}{{cite book|last=Besla|first=Helga|chapter=The Tropical Easterly Jet as a Cause for Intensified Aridity in the Sahara|title=Palaeoecology of Africa, volume 16|isbn=9780203744512|editor-link1=Johanna Alida Coetzee|editor-first1=Johanna Alida|editor-last1=Coetzee|editor-first2=Eduard Meine|editor-last2=van Zinderen-Bakker|pages=163–172}}
The easterly jet induces significant vertical wind shear during the monsoonal months, especially from July to September, which suppresses any tropical cyclone activity.Monsoonal depressions, characteristic of the monsoon season in India, generally do not intensify into cyclones
History of study
Although the upper-level easterly wind flow over India during the summer monsoon was established as early as the 1930s, the term "easterly jet" originated with Indian researcher P.R. Krishna Rao in 1952.{{cite journal|journal=Current Science|publisher=Current Science Association|location=Bangalore|volume=21|last=Krishna Rao|first=P.R.|title=Probable Regions of Jet Streams in the Upper Air over India|year=1952}} P. Koteswaram later in the 1950s demonstrated that the easterly flow is a jet stream in the strict sense of the term.{{cite conference|chapter=The Indian Summer Monsoon and the General Circulation Over the Tropics|last=Koteswaram|first=P.|title=Symposium on Monsoons of the World|location=Meteorological Office, New Delhi|date=19–21 February 1958}} During the 1960s and 1970s it became recognised that the Tropical Easterly Jet arises due to the intense solar heating of the Tibetan Plateau,{{cite journal |last1=Mason |first1=Ralph B. |last2=Anderson |first2=Calvin E. |title=The Development and Decay of the 100 mb. Summertime Anticyclone over Southern Asia |journal=Monthly Weather Review |date=January 1963 |volume=91 |issue=1 |pages=3–12|doi=10.1175/1520-0493(1963)091<0003:TDADOT>2.3.CO;2 |bibcode=1963MWRv...91....3M }} and that it was closely linked to the Asian monsoon in both day-to-day fluctuations and overall seasonal strength.
Relationship with Asian monsoon
The Tropical Easterly Jet's outflow from the Tibetan Plateau is deeply connected to the low-level monsoonal flow over India. Studies have demonstrated that when the high-level easterly jet is weak, monsoonal rainfall over India{{cite journal |last1=Pattanaik |first1=D. R. |last2=Satyan |first2=V. |title=Fluctuations of Tropical Easterly Jet during contrasting monsoons over India: A GCM study |journal=Meteorology and Atmospheric Physics |year=2000 |volume=75 |issue=1–2 |pages=51–60|doi=10.1007/s007030070015 |bibcode=2000MAP....75...51P }} and seasonal rainfall as far east as Micronesia{{cite journal |author=Minoru Tanaka |title=Internal Fluctuations of the Tropical Easterly Jet and the Summer Monsoon in the Asian Region |journal=Journal of the Meteorological Society of Japan |date=June 1982 |volume=60 |issue=3 |pages=865–875}} will be deficient.
It has been more recently suggested that variations in surface rainfall can actually drive variations in the strength of the jet,{{cite journal|volume=75|issue=1–2|date=January 2011|pages=21–30|journal=
Global and Planetary Change|title=Variations of Indian summer monsoon rainfall induce the weakening of easterly jet stream in the warming environment?|first1=C.V.|last1=Naidu|first2=K. Muni|last2=Krishna|first3=S.|last3=Ramalingeswara Rao|first4=O.S.R.U.|last4=Bhanu Kumar|first5=K.|last5=Durgalakshmi|first6=S.S.V.S.|last6=Ramakrishna|doi=10.1016/j.gloplacha.2010.10.001 |bibcode=2011GPC....75...21N }} contradicting earlier suggestions that the surface southwesterly monsoon flow is actually a return flow from the upper-level easterlies.
Recent weakening
Since 1950, the Tropical Easterly Jet has weakened by as much as 25 percent, so that the contour of {{convert|30|m/s|km/h mph knots}} has disappeared.{{cite journal|volume=31|year=2004|journal=Geophysical Research Letters|title=Decreasing trend in the strength of Tropical Easterly Jet during the Asian summer monsoon season and the number of tropical cyclonic systems over Bay of Bengal|first1=B. R.|last1=Srinivasa Rao|first2=D. V.|last2=Bhaskar Rao|first3=V.|last3=Brahmananda Rao|publisher=Department of Meteorology and Oceanography, Andhra University|location=Visakhapatnam}}{{cite journal |last1=Abish |first1=B. |last2=Joseph |first2=P.V. |last3=Johannessen |first3=Ola M. |title=Weakening Trend of the Tropical Easterly Jet Stream of the Boreal Summer Monsoon Season 1950–2009 |journal=Journal of Climate |date=1 December 2013 |volume=26 |issue=23 |pages=9409–9414|doi=10.1175/JCLI-D-13-00440.1 |bibcode=2013JCli...26.9408A |doi-access=free }} This trend has been attributed to a combination of low-level global warming with cooling near the tropopause over continental Asia. Other studies show that the greatest weakening has occurred in the highest-altitude parts of the jet, and in the westerly section over the Arabian Sea.{{cite journal|title=Effect of changing tropical easterly jet, low level jet and quasi-biennial oscillation phases on Indian summer monsoon|first1=P.|last1=Rai|first2=A. P.|last2=Dimri|journal=Atmospheric Science Letters|volume=18|pages=52–59|year=2017}} The declining strength of the Tropical Easterly Jet has been tightly linked with declining rainfall over Arunachal Pradesh, Jammu and Kashmir and certain parts of central India and the Malabar Coast.{{cite conference|title=A study on the decreasing trend in tropical easterly jet stream (TEJ) and its impact on Indian summer monsoon rainfall|first1=P. P.|last1=Sreekala|first2=S. V.|last2=Bhaskara Rao|first3=M. S.|last3=Arunachalam|first4=C.|last4=Harikiran|conference=Theoretical and Applied Climatology|date=November 2013}} Further weakenings of the jet are expected under enhanced greenhouse gases – by at least 11 percent at the end of the twenty-first century.{{cite journal|journal=Journal of Climate|volume=33|issue=19|date=October 2020|pages=8439–8455|title=Dramatic Weakening of the Tropical Easterly Jet Projected by CMIP6 Models|author1=Sihua Huang|author2=Bin Wang|author3=Zhiping Wen|doi=10.1175/JCLI-D-19-1002.1 |bibcode=2020JCli...33.8439H }}
The northeastern Pacific easterly jet, however, is expected to weaken by as much as 77 percent under likely global warming scenarios, which could reduce annual rainfall over Central America and southern Mexico by as much as {{convert|1,000|mm|in|-1|disp=or}}.
See also
Notes
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