How Ripples on Water Steer the Planet's Winds ✎

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

arXiv:1606.00566 · 2016-06-02 · arXiv non-exclusive · independent analysis · 2 min

A hundredfold reduction in the tiny roughness of the ocean surface at low wind speeds shifts the jet streams toward the poles.

The planet's winds are not free-flowing currents but rather obedient jets that can be nudged from an unexpected direction. The ocean surface, covered with waves, acts like a tuning knob: change its roughness, and giant air rivers shift by hundreds of kilometers.

In a model of an Earth without land, where the ocean is heated differently at different latitudes, the surface roughness at low wind speeds was reduced by a factor of one hundred. This is still within the range of field measurements over the sea. After this change, the mid-latitude westerlies moved poleward by several degrees, and the tropical circulation cell that lifts air near the equator weakened by about a quarter. The rain belt, which usually sits near the equator, also shifted toward the pole.

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

The mechanism turned out not to be friction. When the surface became smoother, the ocean began to release less heat to the air, both directly and through evaporation. The tropical atmosphere cooled as a result, and that cooling rearranged the entire circulation from the equator to the poles. To confirm this, experiments were run with individual effects "switched off": roughness was changed only in the tropics or only in the mid-latitudes, and only for heat or only for friction. The response was triggered specifically by reduced heat fluxes in the tropics.

The circulation response resembles the difference between La Niña and El Niño: the jet streams shift toward the poles, and the tropical cell weakens.

The rudder hidden in small details turned out to be more important than previously thought. Even small changes in sea surface parameters, within the uncertainties of observations, can cause wind shifts comparable to natural climate variability. Previously, such uncertainties were mainly linked to clouds and convection, while the surface was considered secondary. Now it is clear: to understand where the planet's winds blow, we must pay close attention to the ripples beneath them.

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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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How Ripples on Water Steer the Planet's Winds ✎