// lecture 19 Flashcards
pacific decadal oscillation
related to ENSO, but lower frequency and more concentrated in the middle latitudes.
impacts of the PDO:
- affects US winter weather in the same way ENSO does. for example, a negative PDO encourages drought in California.
- affects salmon abundance on the west coast because of its influence on the ocean.
- affects the global mean temp. when in its cold phase, the cold SST in the tropics can lower the global mean temp. because the ocean stores more heat and cools the atmosphere. by transition from cold to warm during 2015 El Nino caused global warming to accelerate after long period of very slow warming.
atlantic multi-decadal oscillation (AMO)
average SST over north atlantic, subtract linear trend. warm SST in tropical atlantic makes more and bigger hurricanes, all else being equal.
impacts of AMO:
- arctic seems to be warmed by warming of North Atlantic Ocean.
- hurricanes in Atlantic respond to local warming of subtropical atlantic ocean.
- PDO and AMO are low frequency (decade time scales) natural variability that can be confused with, but also interact with, global warming.
profile of tropical cyclone
- eye - clear area in the center.
- eyewall - clouds immediately surrounding the eye (highest winds and rain).
- spiral rain bands - outer raining areas.
- rising areas have rain and clouds; sinking areas are dry.
hurricanes occur over the
warmest waters, but not right at the equator because the coriolis force is required.
saffir-simpson hurricane scale
higher category = higher winds = more storm surge = lower pressure in the eye.
category 1 (ss hurricane scale)
74 mph, 980+ mbars; minimal
category 2
96 mph, 979 mbars; moderate
category 3
111 mph, 964 mbars; extensive
category 4
121 mph, 944 mbars; extreme
category 5
156+ mph, <920 mbars; catastrophic
many strong hurricanes are around
Taiwan, Philippines.
tropical cyclone Yasi
155 mph winds, 930 mbar min. pressure. among top ten strongest cyclones to hit australia soil is already saturated from floods. la nina (extremely warm ocean in w. pacific)
water vapor
fuels a hurricane. when water vapor condenses, heat is released.
heat released from a hurricane:
- 1.5 cm/day of average rainfall in a circle of radius 660 km.
- the heat released from this condensation is 52 quintrillion joules per day of energy.
- 200 times the world electricity generating capacity.
condensation is the ____ and evaporation provides the ____
energy source; fuel. the strong winds in hurricanes causes more evaporation from the ocean surface.
warmer air can hold more
moisture. also evaporation can occur more easily from a warmer ocean.
warm ocean temps.:
first requirement for hurricanes. hurricanes weaken when they pass over land due to lack of evaporation.
requirements for hurricanes
- SSTs must be above 26 C (79 F). this will shift to a warmer temp. threshold in a warmer climate though.
- must be a least 5 degrees off the equator. Coriolis force is required (why storms rotate opposite in N. and S. hemispheres).
- not much wind sheer (when the winds change with height). this rips hurricanes apart.
- can’t be too cold below the surface either.
when hurricanes pass by, they
churn up colder water from below.
damages in hurricanes are caused by:
winds, storm surge (high winds pushing towards the land), and flooding.
financial damages from hurricanes is
increasing, but this is primarily due to more people living on the coast.
1991 Bangladesh cyclone
144,000 fatalities. left 10 million homeless.
superstorm sandy
deadliest and most destructive storm of the 2012 season in the USA.
- only category 3, but was the largest Atlantic hurricane by diameter, about 1,800 km, 1,100 miles.
- hit caribbean islands and affected 24 states, especially NJ and NY, where the storm surge hit hard.
233 deaths, mostly in US (157) and Haiti (54).
- about $68 billion in damage.
- track featured unusual left turn in midlatitudes to hit US coast.
sandy and grace
in oct. of 1991, a similar juxtaposition of a troopical storm and a midlatitude low brought a very strong storm off shore that sank fishing boats - Andrea Gail Sword boat.
strong shear would be when the
wind speed increases or changes direction with height.
- midlatitude storms can deal with this shear and actually like it.
- tropical storms tend to be ripped apart, since the upper part needs to be connected to the lower part for an intense tropical storm.
increased shear over the
gulf of mexico would act to weaken hurricanes. would offset some of the increase in strength due to increased temps. also the atmosphere tends to stabilize when there’s more moisture.
in 2005, in the atlantic, there were:
- 3 of the 6 strongest storms ever (Wilma, Rita, and Katrina).
- 27 named storms (previous record: 21). doesn’t use letters Q, U, X, Y, and Z. uses alpha, beta, gamma, delta, epsilon, and zeta.
- but only 10% of the world’s hurricanes occur in the atlantic.
claim that category
4 and 5 have increased 80%.
power dissipation index
better measure of strength that combines frequency, intensity, and duration.
PDI in the atlantic is
highly correlated with atlantic ocean temps.
study suggests that hurricanes will
continue to strengthen with global warming.
observational record of hurricanes with full coverage is
relatively short because before 1970s we didn’t have satellite data.
recent models suggest more
really strong storms in warmer climate, but fewer storms overall.
indian ocean dipole
difference between the east and west indian ocean.
1994 event of indian ocean dipole
wind blows along coast of sumatra, upwelling of cold water, suppression of precipitation (rains more where SST is higher).
tornadoes are
smaller in scale and shorter in duration than hurricanes. hurricanes often spawn tornadoes when they make landfall.
tornadoes occur with
thunderstorms, hurricanes contain many thunderstorms.
tornadoes occur most often
over land in springtime, usually, hurricanes occur exclusively over tropical oceans.
observed changes in tornadoes are even more
uncertain than observed changes in hurricanes.
hard to predict future tornados.
still warming means more energy in the form of latent heat of water vapor, and release of latent heat is what drives tornadoes, so this causes worry.
