Lecture 11 Tropical Cyclones - Atmospheric and Oceanic Sciences

Lecture 11 Tropical Cyclones

Tropical Cyclone• A cyclone that originates over the tropical ocean– Distinguished its “warm core”, i.e. the temperature in the centerof the tropical cyclone is relatively warm. In contrast, a middlelatitude or frontal cyclone is a “cold core” cyclone.– A mature TC can become extremely intense. We call a matureTC a:• "hurricane" (the North Atlantic Ocean, the Northeast Pacific Oceaneast of the dateline, or the South Pacific Ocean east of 160E)• "typhoon" (the Northwest Pacific Ocean west of the dateline)• "severe tropical cyclone" (the Southwest Pacific Ocean west of160E or Southeast Indian Ocean east of 90E)• "severe cyclonic storm" (the North Indian Ocean)• "tropical cyclone" (the Southwest Indian Ocean)

AtlanticHurricanesTyphoonsEastPacificHurricanesNorthern HemisphereSummer/FallCyclonesSevere TropicalCycloneSouthern HemisphereSummer/FallSevere TropicalCyclone

2004 Atlantic Hurricane Season

What role do TCs play in theAtmospheric and OceanicCirculation?• They are a (desperate?) mechanism through which the atmospheretransfers heat from the ocean (extracted from excessively warm water) outto space. TCs tend to “boil off” excess energy stored in the oceans– Ocean circulations and evaporation from the surface naturally transport oceanheat into the atmosphere where air currents transport it further. The atmospherecontinually radiates energy to space cooling at 2 degrees/day on the average .– In the tropics these processes do not cool the ocean enough to prevent it fromreaching 27C which is conducive to the formation of TCs under certaincircumstances (low wind shear).– TCs dramatically enhance the heat transfer process and leave a cooled ocean intheir wake• Certain atmospheric flow patterns cool the ocean more than others, thusreducing the need for TCs– Sheared wind patterns produce stronger drying (therefore cooling) surface windsand also inhibit TC formation (this year, Atlantic) and vise versa (last year,Atlantic)– Weakening of warm transport by ocean currents, i.e. warm conveyor belt, can letexcess warmth build in the tropics favoring more frequent and stronger TCs (lastyear)– Deepening of warm water layer can increase fuel for TCs

Rappaportet al, NHC

Rappaportet al, NHC

Hurricane Tracks• Hurricanes are steered bythe mean lower troposphericflow• When the flow patternmoves, the storm can beforced to curve or even doloopsTypicalSubtropicalHighUnusual

Ritaeye

Ritahttp://cimss.ssec.wisc.edu/tropic/archive/2005/storms/rita/rita.htmleye

RitaSpiral bandseyeEye wallhttp://cimss.ssec.wisc.edu/tropic/archive/2005/storms/rita/rita.html

Eddy circulations in the eyehttp://cimss.ssec.wisc.edu/tropic/archive/2003/storms/isabel/isabel.html

Initiation of Tropical Cyclones• Atlantic (Hurricanes)– Easterly waves moving off of Africa– ITCZ distrubances– Frontal remnants from middle latitudes– Convective systems over US that move over the water• East Pacific (Hurricanes)– Easterly Waves from Africa moving across Isthmus– Tropical cyclones from Atlantic moving across Isthmus– Storms initiated in Venezuela mountains– Storms initiated by flow across Sierra Madre Mountains of CentralAmerica• West Pacific– ITCZ disturbances• initiated from westerly wind bursts along ITCZ– Easterly waves from Africa (rarely)

ITCZInitiation

• Troughs and ridges ineasterly flow are invertedcompared to troughs andridges in westerly flow• Based in lowertroposphere– opposite to shortwave inthe westerlies which isbased in upper troposphere– Therefore, cloudiness isassociated withconvergence upstream ofthe trough which forces airupEasterly Wave500 mb850 mbridgetroughWesterly WaveridgetroughtroughEasterly Waveridge

Frontal Genesis

South Atlantic’s onlyHurricane(March, 2004)(frontal initiation)

Danny1997

PredictedMaximumIntensity

Classification of Tropical cyclones• 00 kts ≤ wind < 20 kts (23MPH)Tropical disturbance• 20 kts ≤ wind < 34 kts (39 MPH)Tropical depression• 34 kts ≤ wind < 64 kts (74 MPH)Tropical storm• 64 kts ≤ windHurricane

Saffir/Simpson Scale

Saffir/Simpson Scale

WISHE(Wind InducedSurface HeatExchange)