B. Mete Uz

Pumping of nutrients to ocean surface waters by the action of propagating planetary waves.

Primary productivity in the oceans is limited by the lack of nutrients in surface waters. These nutrients are mostly supplied from nutrient-rich subsurface waters through upwelling and vertical mixing, but in the ocean gyres these mechanisms do not fully account for the observed productivity. Recently, the upward pumping of nutrients, through the action of eddies, has been shown to account for the remainder of the primary productivity; however, these were regional studies which focused on mesoscale (100-km-scale) eddies. Here we analyse remotely sensed chlorophyll and sea-surface-height data collected over two years and show that 1,000-km-scale planetary waves, which propagate in a westward direction in the oceans, are associated with about 5 to 20% of the observed variability in chlorophyll concentration (after low-frequency and large-scale variations are removed from the data). Enhanced primary production is the likely explanation for this observation, and if that is the case, propagating disturbances introduce nutrients to surface waters on a global scale—similar to the nutrient pumping that occurs within distinct eddies.

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.

Laboratory Studies Of Wind Stress Over Surface Waves

Simultaneous laboratory observations of wind speed, wind stress, and surfacewind-wave spectra are made under a variety of wind forcing patterns using cleanwater as well as water containing an artificial surfactant. Under typical experimentalconditions, more than half of the total stress is supported by the wave-induced stressrather than by the surface viscous stress. When the surfactant reduces the shortwind-wave spectra, the wind stress also decreases by as much as 20–30% at agiven wind speed. When the wind forcing is modulated in time, the wind stresstends to be higher under decreasing wind than under increasing wind at a givenwind speed, mainly because the response of short wind-wave spectra to varyingwind forcing is delayed in time. These examples clearly demonstrate that therelationship between the wind speed and the wind stress can be significantlymodified if the surface wave field is not in equilibrium with the wind forcing.Next, we examine whether the wind stress is estimated accurately if the wave-inducedstress by all surface wave components is explicitly evaluated by linear superpositionand is added to the surface viscous stress. It is assumed that the surface viscous stressis uniquely related to the wind speed, and that the wind input rate is determined by thelocal, reduced turbulent stress rather than the total stress. Our wind stress estimatesincluding the wave contributions agree well with observed wind stress values, evenif the surface wave field is away from its equilibrium with the wind in the presenceof surface films and/or under time-transient wind forcing. These observations stronglysuggest that the wind stress is accurately evaluated as a sum of the wave-induced stressand the surface viscous stress. At very high winds, our stress estimates tend to be lowerthan the observations. We suspect that this is because of the enhancement of wind stressover very steep (or breaking) short wind-waves.

Argo floats complement biological remote sensing

Within the past decade, satellite observations of ocean color have provided a global view of biology within the oceans surface layer and revealed how phytoplankton abundance varies in response to various physical forcings ranging from the large scales of seasonal cycles and the El Nino-Southern Oscillation, to planetary waves, eddies, and coastal filaments at smaller scales. The maturing Argo network of profiling floats now provides a way to put these remote sensing measurements into a vertical context. The true test of any scientific discipline is prognostic capability. In physical oceanography, the ocean dynamics are fairly well understood, and their prediction hinges on the measurement of key variables at the proper spatial and temporal scales so as to validate and initialize models. Remote sensing coverage of these key parameters, such as sea surface temperature and height anomalies, has been available for 30 and 15 years, respectively. As a result, recent advances in numerical modeling are bringing operational forecasting of ocean circulation within reach.