Fujiwhara effect

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When cyclones are in proximity of one another, their centers will circle each other cyclonically (counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere){{Cite web |last=Landsea, Chris |author-link=Chris Landsea |date=2009-02-06 |title=Subject: D3) Why do tropical cyclones' winds rotate counter-clockwise (clockwise) in the Northern (Southern) Hemisphere? |url=http://www.aoml.noaa.gov/hrd/tcfaq/D3.html |access-date=2009-12-28 |publisher=Atlantic Oceanographic and Meteorological Laboratory}} about a point between the two systems due to their cyclonic wind circulations. The two vortices will be attracted to each other, and eventually spiral into the center point and merge. It has not been agreed upon whether this is due to the divergent portion of the wind or vorticity advection.{{Cite journal |last=DeMaria |first=Mark |last2=Chan |first2=Johnny C. L. |date=August 1984 |title=Comments on "A Numerical Study of the Interactions between Two Cyclones |journal=Monthly Weather Review |volume=112 |issue=8 |pages=1643–1645 |bibcode=1984MWRv..112.1643D |doi=10.1175/1520-0493(1984)112<1643:CONSOT>2.0.CO;2 |doi-access=free}} When the two vortices are of unequal size, the larger vortex will tend to dominate the interaction, and the smaller vortex will circle around it. The effect is named after Sakuhei Fujiwhara, the Japanese meteorologist who initially described it in a 1921 paper about the motion of vortices in water.{{Cite journal |last=Fujiwhara |first=Sakuhei |year=1921 |title=The natural tendency towards symmetry of motion and its application as a principle in meteorology |journal=Quarterly Journal of the Royal Meteorological Society |volume=47 |issue=200 |pages=287–293 |bibcode=1921QJRMS..47..287F |doi=10.1002/qj.49704720010}}{{Cite news |date=November 1, 2007 |title=Fujiwhara effect describes a stormy waltz |url=https://www.usatoday.com/weather/wfujiwha.htm |access-date=2008-02-21 |work=USA Today}}

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{{See also|Tropical cyclone}}

File:Odette-Seroja fujiwhara effect.gif (left) and Seroja (right) engaged in a Fujiwhara interaction whilst intensifying between 7–9 April 2021.]]

Tropical cyclones can form when smaller circulations within the Intertropical Convergence Zone merge.{{Cite journal |last=Kieu |first=Chanh Q. |last2=Zhang |first2=Da-Lin |date=June 2010 |title=Genesis of Tropical Storm Eugene (2005) from Merging Vortices Associated with ITCZ Breakdowns. Part III: Sensitivity to Various Genesis Parameters |journal=J. Atmos. Sci. |volume=67 |issue=6 |pages=1745–1758 |bibcode=2010JAtS...67.1745K |doi=10.1175/2010JAS3227.1 |s2cid=55906577}} The effect is often mentioned in relation to the motion of tropical cyclones, although the final merging of the two storms is uncommon. The effect becomes noticeable when they approach within {{convert|1400|km|mi}} of each other. Rotation rates within binary pairs accelerate when tropical cyclones close within {{convert|650|km|mi}} of each other. Merger of the two systems (or shearing out of one of the pair) becomes realized when they are within {{convert|300|km|mi}} of one another.{{Cite journal |last=Prieto |first=Ricardo |last2=McNoldy |first2=Brian D. |last3=Fulton |first3=Scott R. |last4=Schubert |first4=Wayne H. |date=November 2003 |title=A Classification of Binary Tropical Cyclone–Like Vortex Interactions |journal=Monthly Weather Review |volume=131 |issue=11 |page=2659 |bibcode=2003MWRv..131.2656P |doi=10.1175/1520-0493(2003)131<2656:acobtc>2.0.co;2 |s2cid=18784887 |doi-access=free}}

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File:Severe Tornado Outbreak in the Southern United States.ogv shows two extratropical cyclones involved in Fujiwhara interaction across the Midwest and Great Lakes.]]

{{See also|Extratropical cyclone}}

Binary interaction is seen between nearby extratropical cyclones when within {{convert|2000|km|mi}} of each other, with significant acceleration occurring when the low-pressure areas are within {{convert|1100|km|mi}} of one another. Interactions between their circulations at the 500 hPa level ({{convert|18,000|ft|disp=or|order=flip}} above sea level) behave more predictably than their surface circulations.{{Cite journal |last=Ziv |first=B. |last2=Alpert |first2=P. |date=1995-05-01 |title=Rotation of Binary Cyclones – A Data Analysis Study |journal=J. Atmos. Sci. |volume=52 |pages=1357–1363 |bibcode=1995JAtS...52.1357Z |doi=10.1175/1520-0469(1995)052<1357:ROBCDA>2.0.CO;2 |doi-access=free |number=9}} This most often results in a merging of the two low-pressure systems into a single extratropical cyclone, or can less commonly result in a change of direction of one or both of the cyclones.{{Cite journal |last=Ziv |first=B. |last2=Alpert |first2=P. |date=December 2003 |title=Rotation of mid-latitude binary cyclones: a potential vorticity approach |journal=Theor Appl Climatol |volume=76 |issue=3–4 |pages=189–202 |bibcode=2003ThApC..76..189Z |doi=10.1007/s00704-003-0011-x |s2cid=54982309}} The precise results of such interactions depend on factors such as the size of the two cyclones, their distance from each other, and the prevailing atmospheric conditions around them.

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{{Portal|Tropical cyclones}}

• Satellite tornado

{{Reflist|2}}

• [http://www.aoml.noaa.gov/hrd/prelim/Iris_prelim.html Edward N. Rappaport, NOAA Hurricane Research Division – "Hurricane Iris Preliminary Report"]

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