Annual- and zonal-mean climate of the
tropics (NCEP)
Relative humidity [%]
subtropical
high
subtropical
high
equatorial trough
Temperature [degC]
surface pressure [mb]
Angular momentum conservation and
zonal acceleration
When parcel moves poleward, r decreases
so v must increase in order to keep L constant
Walker cell
The real Walker cell
(Trenberth et al. 2000)
Monsoon circulation
Asian monsoon
American monsoons
N. America
S. America
African monsoon
Moist static energy h (105 J/kg)
DJF
JJA
Components of moist static energy averaged
over Hadley cell (10S–30N, DJF)
pressure (hPa)
total
potential
sensible
latent
energy (105 J/kg)
Vertically averaged energy transport
(arrows) and its divergence (shade, W/m2)
(Trenberth&Stepaniak 2003)
Total
Dry
Moist
Net surface energy flux (NCEP, W/m2)
Tropical cyclones
•
•
Definition: cyclonic system driven principally by energy transfer from ocean.
Categories by max surface wind:
–
–
–
•
•
•
•
up to 17 m/s: tropical depression
18–32 m/s: tropical storm
over 32 m/s: hurricane/typhoon
Scale: 100–1000 km diameter
Frequency: ~ 80 tropical cyclones per year globally (very stable number)
Occur mostly in summer/early autumn
Hurricane-like storms also occur in Mediterranean (“medicanes”) and in Arctic
(“polar lows”)
Tropical cyclone tracks, 1985–2005
Distribution of cyclone occurrence by
intensity
Tropical cyclone structure
Tropical cyclone structure
Potential intensity theory
W = rate of working by cyclone
D = energy dissipation rate
Vmax = max surface wind speed
p(Vmax) = surface pressure at location of Vmax
Potential intensity as function of Ts, T0
Potential intensity compared with actual
intensity
Winds here are normalized by the maximum potential value.
Note that this is a cumulative distribution, ie. the y-axis
gives the probability of achieving a windspeed equal or greater
than the corresponding x-axis value.
Cold wakes
Increasing destructiveness
Power Dissipation Index: