EESA09H3 Lecture 4: Midlatitude Cyclones - Hurricanes
30 May 2018
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EESA09 – Lecture 4: Midlatitude Cyclones
El Nino – Southern Oscillation (ENSO)
- Example of atmosphere-ocean circulation
- Consists of 2 components:
o El Nino, mainly oceanic component of ENSO
o Southern Oscillation, mainly atmospheric component of ENSO is due to Walker
circulation
- Hurricanes & Climate Change
o Are El Nino changing w/changing climate?
YES, climate is warming oceans which is where hurricanes originate. Because of
this, hurricanes are made up of warmer air which sinks and creates lower
centers; stronger hurricanes
o Are the patterns of Atlantic Hurricane activity changing w/ climate change?
YES , as climate is warming (spec. in northern hemisphere)
o Are we prepared adequately for hurricanes? (infrastructure/emergency preparedness)
o Are the nature of hurricanes changing?
YES, it is based on all the forecasts the central temps are dropping
2005 record # of Atl.basin hurricanes – 27 named storms (alpha to zeta needed),
15 hurricanes, 4 category 5 (unprecedented)
Stronger activity since 1995
Humans & Climate Change
- Scientific community divided on explaining the hurricane changes:
o William Gray – AMO Atl. Multidecadal Oscillation
AMO Variation in the 3D circulation of the Atlantic Ocean; change every 30-
40 years. Have neg. (cold) + positive (warm) phases which affect sea surface
temperatures.
Warm temps in 40s and 50s = more freq.hurricanes NOT global warming
Maintains that strong thermohaline circulation in Atl. Led to higher SSTs in
tropics + vice versa; current THC = should be stronger than usual
BUT paper by Bryden et al.2005 indicate THC was 30% lower that year
so Gray is wrong.
o Kerry Emanuel – Global Warming is the reason for extraordinary changes for hurricanes,
hurricane chaser
- Hurricane Damage: Natural disaster or human folly?
o Insurance Payouts Increasing due to…
Nature of physical disaster – frequency of hurricanes appears unchanged but
intensity + duration are increasing
Where people live – Coastalization
Relative wealth - more properties near coastal line, die more from natural
disasters; Americans are increasingly wealthy + Coastalization
-
Hurricanes Primer
DEFINITIONS: ms = meters per sec.
Typhoon (‘taifung’ chinese = big wind) Term used in Western Pacific
Cyclone Term used in Australia + Indian Ocean
Tropical Storm GENERIC Storm (tropical region) w/sustained winds of 18ms to 33ms. “babies
of the hurricane”.
Hurricane (tropical cyclone)
o NA term (atlantic hurricanes, eastern pacific hurricanes), Taino language of Caribbean
“god of evil”.
o Tropical storm w/sustained winds ranging from 33 to 50 ms. Exceeds 50ms, will be
called a major hurricane.
o They are initiated/formed in the tropical (0-20 deg. N and S) latitude.
o Saffir-Simpson Scale scale to use to categorize hurricanes (1-5 rating scale)
Herbert Saffir (consulting engineer) + Robert Simpson (director of US National
Hurricane Centre)
Tropical Storm Classifications: Increase in wind speed, lower center pressure
Category 1 Hurricane – 119-153 km/h (33-42.5 m/s), >980 mb
Category 2 Hurricane – 154-177 km/h (42.5-49 m/s), 965-979 mb
Category 3 Hurricane – 178-209 km/h (49-58 m/s), 945-964 mb
Category 4 Hurricane – 210-249 km/h (58-69 m/s), 920-944 mb
o E.g. Hurricane Irma
Category 5 Hurricane – greater than 249 km/h (>69 m/s), <920 mb
DYNAMICS:
How Hurricanes are formed:
o ALWAYS formed over oceans 26.5 C warm threshold =necessary but not sufficient
oConvergence at surface, warm air expands creating divergence aloft; warm air is rising
creating low, centre pressure is dropping.
Warm core = ‘eye of the hurricane’ the center where air is sinking
o Wind generates waves, increasing spray, increasing evaporation + latent heat
o Tropical storms fuelled by sea surface temperatures + latent heat release (heat
released/absorbed due to change of phase can’t be seen, water turned into moisture
vapour)
Tropical Storm Development – Atlantic basin
o Begins as a tropical wave (orig.in E.Africa) in the Intertropical convergence zone (ITCZ)
o June thru November – latitudes 5-20 N
o Group of thunderstorms that become organized + self-sustaining
o What affects length + strength of a tropical cyclone?
o SSTs – warmer the surface, stronger + longer the storms
o Upper wind structure – strong upper lvl winds inhibit tropical cyclone longevity
QBO – quasi-biennial oscillation of winds in the stratosphere (Lec.3)
El Nino – affects winds in E.Pacific (EP) + Atlantic (Atl) hurricanes
(enhances EP, suppresses Atl) global phenomenon so affects more
than 1 basin.
Landfall – cuts off cyclone from both sources of energy (SSTs + water
vapour) –increases surface roughness. HURRICANE DIESS R.I.P
o Atlantic Hurricanes + ENSO
1997 – El Nino Year; few hurricanes are forming
1998 – Non-El Nino Year
