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Submesoscale Variability of the Upper Ocean: Patchy and Episodic Fluxes Into and Through Biologically Active Layers

Daniel Rudnick,* Mary Jane Perry, John J. Cullen, Bess Ward,§ Kenneth S. Johnson||

THE UPPER OCEAN

The upper ocean, defined roughly as the upper few hundred meters, will continue to be the subject of intense research through 2025. As a practical matter, almost all human interaction with the ocean is in the upper ocean, including transportation, fishing, and national defense. Communication with the atmosphere takes place through the upper ocean, so studies of air-sea interaction of all scales from squalls to climate require knowledge of upper ocean processes. Finally, most ocean life and its variability is concentrated in the upper ocean because of the availability of light for photosynthesis, making the upper ocean a focus for problems spanning the boundaries of physics, biology and chemistry. It is a fair bet that significant progress will be made by 2025 on these problems.

LAYERS, LAYERS EVERYWHERE AND HARDLY TIME TO THINK

The upper ocean is often described as composed of layers, each defined by some set of properties. These layers may be coincident, may overlap, or may not exist at all under some circumstances. Because of

*

Scripps Institution of Oceanography, University of California, San Diego

School of Marine Sciences, University of Maine

Dalhousie University

§

Princeton University

||

Monterey Bay Aquarium Research Institute



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Submesoscale Variability of the Upper Ocean: Patchy and Episodic Fluxes Into and Through Biologically Active Layers Daniel Rudnick,* Mary Jane Perry,† John J. Cullen,‡ Bess Ward,§ Kenneth S. Johnson| | THE UPPER OCEAN The upper ocean, defined roughly as the upper few hundred meters, will continue to be the subject of intense research through 2025. As a prac- tical matter, almost all human interaction with the ocean is in the upper ocean, including transportation, fishing, and national defense. Commu- nication with the atmosphere takes place through the upper ocean, so studies of air-sea interaction of all scales from squalls to climate require knowledge of upper ocean processes. Finally, most ocean life and its vari- ability is concentrated in the upper ocean because of the availability of light for photosynthesis, making the upper ocean a focus for problems spanning the boundaries of physics, biology and chemistry. It is a fair bet that significant progress will be made by 2025 on these problems. LAYERS, LAYERS EvERYWHERE AND HARDLY TIME TO THINK The upper ocean is often described as composed of layers, each defined by some set of properties. These layers may be coincident, may overlap, or may not exist at all under some circumstances. Because of * Scripps Institution of Oceanography, University of California, San Diego † School of Marine Sciences, University of Maine ‡ Dalhousie University § Princeton University |Monterey Bay Aquarium Research Institute | 10

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10 OCEANOGRAPHY IN 2025 their prevalence in the literature, and likely in future research, some definitions of layers are in order. The mixed layer is bounded by the surface, and is usually defined as being uniform in such properties as temperature, salinity, or velocity. Although observations have shown these properties are not always uniform over the same depth interval, the concept of a strongly turbulent slab mixed layer has spawned a number of useful models. Observations show that the actively turbulent layer does not always coincide with the mixed layer. The mixing layer is the region bounded by the surface in which turbulent dissipation is strong. Biologically influenced tracers, such as optical estimates of particle con- centration, can reveal when and where active mixing slows, confining phytoplankton near the surface. Recent research has focused on the tran- sition layer, the region between the mixed layer and the weakly turbulent interior ocean. The transition layer is both turbulent and stratified, so fluxes of momentum and other properties are strong. As the transition layer often overlaps with the nitracline and the subsurface chlorophyll maximum, processes in the transition layer are crucial to the biogeochem- istry of the upper ocean. The subsurface chlorophyll maximum layer is the region of high chlorophyll concentration that is generally found near the base of the euphotic zone below a nutrient-depleted mixed layer, with a thickness of five to 20 meters. Phytoplankton within this layer typi- cally have higher cellular chlorophyll concentrations that compensates for low light; consequently this layer may or may not also be a particle maximum layer. The subsurface chlorophyll maximum layer is a persis- tent feature of tropical and subtropical oceans and is a seasonal feature in many mid- to high-latitude oceanic and coastal regions, developing after nutrients in surface waters are depleted by the phytoplankton spring bloom. Planktonic thin layers are features with thickness on the scale of several centimeters to several meters, persisting from hours to days, and often with distinct species assemblages and very high concentrations of chlorophyll. Mechanisms for thin layer formation include straining by shear and active aggregation of the organisms by buoyancy regulation or swimming behavior. These layers are often associated with sharp gradients of nutrients and density, and thus they represent hot spots of physical, chemical and biological interaction. FLUxES: A WAY OUT (OR IN) The layers defined above have certainly been observed, and have proven to be conceptually valuable. What we really need to know, how- ever, are the mechanisms of their formation and maintenance, and con- sequently the fluxes of materials and properties into and through them. In turn, a thorough understanding of fluxes would allow an explanation

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109 DANIEL RUDNICK ET AL. for the formation of any layer, and would lead to prediction as fluxes are either directly resolved or parameterized in numerical models. The vertical turbulent fluxes in the transition layer are an important topic for research in the coming years. This region is challenging, as both inter- nal waves and turbulence are strong, its depth changes as it follows the undulations of the mixed-layer base, and any strict assumption of one-dimensionality is unlikely to be satisfactory. A long-standing prob- lem in biological oceanography is that turbulent fluxes, determined over the years from physical measurements, have never seemed sufficient to describe the observed production. For example, throughout the oligo- trophic ocean dissolved inorganic carbon is depleted in the mixed layer each summer by biological processes, yet there are almost no detectable nutrients to support the carbon consumption. Do episodic events that are difficult to sample by conventional, shipboard programs control this biological production, which represents a major component of the ocean carbon cycle? With the continued improvement in biogeochemical sen- sors, fluorescence measurements and molecular probes that assess short term physiological responses to nutrient pulses, the near future holds promise for the solution of this problem, as biological and chemical vari- ables are measured on the same scales as physical variables (centimeters to meters vertically, over deployments long enough to detect episodic events that may occur on time scales of seconds to minutes at intervals of hours to days). THE SUBMESOSCALE: WHERE THE ACTION IS The most important vertical advective fluxes occur on scales smaller than the energetic mesoscale, whose study has dominated past decades. The submesoscale, horizontal length scales of order kilometers and smaller, is populated with ageostrophic processes causing vertical flows. The ageostrophic circulation cells at fronts and on the edges of eddies are a particular focus. At issue is an ongoing debate about what causes eddies to be sites of enhanced productivity. Exactly geostrophic dynam- ics do not include vertical flows, so ageostrophy is required, but what is the relevant process? Wind forcing of the eddy and instability at the eddy’s edge are candidate processes. The seasonal restratification of the mixed layer is a submesoscale phenomenon, and as it occurs near the same time as the spring bloom, the coupling of biology and physics is a distinct possibility. The very fact that such properties as nitrate and chlorophyll have fundamentally different structures, sources and sinks than temperature and salinity make them potentially valuable tracers for understanding submesoscale processes, especially considering that new sensors can reveal physiological properties of microbes that reflect recent

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110 OCEANOGRAPHY IN 2025 nutrient perturbations. As model resolution improves, the processes that are unresolved will continue to shrink, so the submesoscale will be a focus for theoretical studies aiming to improved parameterizations. SUMMARY The fluxes causing the layered structure of the upper ocean are likely be quantified within the next 15 years. Observations using new biological and chemical sensors will pave the way, as biogeochemical and physical variables are resolved at the same length and time scales. Submesoscale dynamics will be a focus, as ageostrophic processes cause the relevant vertical fluxes.