Annual- and zonal-mean climate of the tropics (NCEP) Relative humidity [%] Temperature [degC] surface pressure [mb] equatorial trough subtropical high subtropical high
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. AmericaS. America
African monsoon
Moist static energy h (10 5 J/kg) DJF JJA
Components of moist static energy averaged over Hadley cell (10S–30N, DJF) pressure (hPa) energy (10 5 J/kg) latent potential sensible total
Vertically averaged energy transport (arrows) and its divergence (shade, W/m2) (Trenberth&Stepaniak 2003) Total Dry Moist
Net surface energy flux (NCEP, W/m 2)
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
Potential intensity theory W = rate of working by cyclone D = energy dissipation rate V max = max surface wind speed p(V max ) = surface pressure at location of V max
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: