Physical and biological mechanisms for planetary waves observed in satellite‐derived chlorophyll

We examine the evidence for global propagation of planetary wavelike features in sea‐surface chlorophyll. Over much of the midlatitude ocean, westward propagating signals are seen that travel at the same speed as that predicted for long planetary waves. We then test three mechanisms for production of this signal. These are: horizontal (passive) north‐south advection by the wave against a mean background gradient; vertical upwelling of nitrate, which is converted into chlorophyll; and vertical upwelling of chlorophyll itself. The tests involve comparisons of the amplitude and phase of the predicted signal with observations. The horizontal advective process predicts an amplitude for chlorophyll fluctuations that is in fair agreement with the data, though both overestimating and underestimating in places. The predictions for the phase difference between the chlorophyll and sea surface height signatures are in good agreement with the data. The upwelling biological mechanism could potentially give a large signal in the chlorophyll field, but the predicted amplitude patterns and the predicted phase difference (which is everywhere negative) are not in accord with the observations. Except in a few regions, the amplitude predicted by upwelling of chlorophyll is small compared with the horizontal advection mechanism. We conclude that over most of the ocean, the chlorophyll signal is well explained by horizontal advective processes, although we cannot rule out that there exist locations where additional biological mechanisms may be responsible for at least part of the signal.