Tropical climatology and general circulation SO442 Lesson 2 Tropical climatology and general circulation

Large-scale high pressure systems feature: Subsidence (sinking motion) Surface air divergence Upper-troposphere air convergence Large-scale low pressure systems feature: Ascent (rising motion) Surface air convergence Upper-troposphere air divergence

Scales in the atmosphere Atmospheric phenomena tend to self organize depending on their time and space scales Larger weather and climate phenomena tend to last longer Why is that?? Duration of the phenomenon tends to depend on how it relates to its “Rossby radius” Things that are bigger than the Rossby radius tend to last longer Similarly, weather phenomena that are smaller than their Rossby radius tend to be short-lived What is the “Rossby radius”?

Rossby radius First, the Rossby Number You heard about Rossby Number in SO335/SO414 It is defined as the ratio between the material derivative and the Coriolis acceleration If the Rossby # is small (R << 1), then Coriolis is important and the material derivative isn’t If the Rossby # is large (R >> 1), then Coriolis is not important and the material derivative is Rossby radius is similarly defined: A ratio between buoyancy (eg, vertical instability) and Coriolis U = characteristic horizontal velocity (typically 10 m s-1) f = characteristic Coriolis (10-4 s-1 in mid-latitudes) L = characteristic horizontal length scale (1000 km in mid-latitudes)

Implications of the Rossby radius

Scale analysis of the equation of motion What are the forces that matter in the tropics? How do they compare to the mid-latitudes? Expand the left-hand side into 3 terms; replace omega with f Write each term as scaling parameters U, L, p, and ρ

Scale analysis of the equation of motion: mid-latitudes Typical values of each characteristic scaling parameter in the mid-latitudes: U = 10 m s-1 W = 10-2 m s-1 (1 cm s-1) L = 1000 km Δp = 10 hPa (1000 Pa) ρ = 1 kg m-3 f = (2)(7.29x10-5)sin(φ). At 40S and 40N, f ≈ 10-4 s-1

Scale analysis of the equation of motion: mid-latitudes Typical values of each characteristic scaling parameter in the mid-latitudes: U = 10 m s-1 W = 10-2 m s-1 (1 cm s-1) L = 1000 km Δp = 10 hPa (1000 Pa) ρ = 1 kg m-3 f = (2)(7.29x10-5)sin(φ). At 40S and 40N, f ≈ 10-4 s-1

Scale analysis of the equation of motion: tropics Typical values of each characteristic scaling parameter in the tropics: U = 10 m s-1 W = 10-2 m s-1 (1 cm s-1) L = 1000 km Δp = 1 hPa (100 Pa) ρ = 1 kg m-3 f = (2)(7.29x10-5)sin(φ). At 10S and 10N, f ≈ 10-5 s-1

Scale analysis of the equation of motion: deep tropics (5°S-5°N) Typical values of each characteristic scaling parameter in the deep tropics: U = 1 m s-1 W = 10-2 m s-1 (1 cm s-1) L = 1000 km Δp = 1 hPa (100 Pa) ρ = 1 kg m-3 f = (2)(7.29x10-5)sin(φ). At 5S and 5N, f ≈ 10-5 s-1

Consequences of a rotating planet with unequal heating Because the planet rotates, and because the heating is concentrated near the equator, we get a general circulation that largely resembles a 3-cell model: Hadley cell Rising motion near the equator, sinking motion near 30S and 30N Ferrell cell Rising motion near 60S and 60N, sinking motion near 30S and 30N Polar cell Rising motion near 60S and 60N, sinking motion near 90S and 90N Connecting the rising and sinking branches of the Hadley, Ferrell and Polar cells are surface winds: Easterly trade winds near the equator Westerly winds in the mid-latitudes Easterly winds near the poles

Mean structure of the global atmosphere that results from the 3 circulation cells

Mean structure of sea level pressure and surface winds, by season

Winds have tremendous consequences for the oceans