Ripples on Water and the Winds of a Planet ✎

Original: "Zonal-mean circulation response to reduced air-sea momentum roughness"

arXiv:1606.00566 · 2016-06-02 · arXiv non-exclusive · independent analysis · 1 min

If you make the ocean surface smoother, the mid-latitude westerlies shift toward the poles, and the rain belt near the equator moves.

The winds on a planet can be compared to a tuning knob: turn it just a little, and the air currents shift. Ocean ripples, it turns out, work as such a knob. In a computer model of an Earth without land, the ocean surface was made smoother for weak winds. After that, the mid-latitude westerlies moved closer to the poles, and the rain belt near the equator shifted.

Surface roughness is a measure of how much the sea "grabs" the wind. Smooth water slows down the air near the surface less.

Why did this happen? Smooth water gives off less heat to the air, both directly and through evaporation. The tropical atmosphere cools as a result, and this cooling rearranges the entire circulation. To test this, scientists changed roughness only in the tropics or only in the mid-latitudes, and only for heat or only for friction. It turned out that the key factor is precisely the heat from the tropics. A rudder hidden in small details steers the planetary winds.

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In the original CAM3 parameterization, the drag coefficient under weak winds (less than 5 m/s) can be reduced by a factor of 100, and this still falls within the scatter of observations from field campaigns over the ocean.

(\tau, E, H) = \rho_A |\Delta v| (C_D \Delta v, C_E \Delta q, c_p C_H \Delta \theta)

The fluxes are proportional to air density, the difference in wind speed, and the difference in temperature or humidity, multiplied by the corresponding exchange coefficients.

C_{DN} = \frac{\kappa^2}{\ln^2(10/Z_{0m})}

The drag coefficient depends logarithmically on the ratio of the measurement height (10 m) to the surface roughness.

\partial_\phi \langle v h \rangle = \langle Q_R \rangle + L_v E + H \equiv F

The divergence of the moist static energy flux is balanced by the sum of radiative heating and latent and sensible heat fluxes.

Concepts

Hadley cell La Niña intertropical convergence zone Walker circulation Bjerknes feedback ocean-atmosphere coupling Southern Oscillation zebiak cane model El Niño North Equatorial Countercurrent ocean current Oceanic Niño index wind stress relative ONI atmospheric pressure ENSO Atlantic multidecadal oscillation sea surface temperature anomaly coupled general circulation model surface tension

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Ripples on Water and the Winds of a Planet ✎