PC-Large Scale

## Large-Scale Spatial Studies (Latitudinal Studies, Including ENSO Forcing)

           Goals and Objectives
           Relevance of Large-Scale Spatial Studies
           Questions and Hypotheses
           Methods

The coastal ocean off western North American can be divided into at least four regions characterized by differences in wind stress, intensity of coastal upwelling, coastal morphology, freshwater inflow, and the influence of long-term and transient advection, turbulence, and buoyancy. These regions also differ somewhat in terms of the planktonic, benthic and fish assemblages present, the timing of plankton production cycles, feeding and reproductive activity in populations, and the presence or absence of specific life stages of key fish species. Critical mesoscale physical processes may be influenced by future changes in the global atmosphere and ocean, which in turn may impact populations. Regional differences in physical-biological linkages provide a natural laboratory for comparing potential changes in marine populations that may accompany different climate change scenarios.

The large-scale component of a U.S. GLOBEC CCS program will consist of field programs located at multiple sites that characterize and contrast the ecosystem's principal biophysical regions, but may also include large-scale drifter studies, satellite studies, regional numerical models nested in coarser basin-scale models, and the analysis of historical environmental data sets that will connect regions and areas of mesoscale studies. These will tie together comparative studies in the different regions and long-term monitoring. Although it is unrealistic to consider in situ measurements sufficient to monitor the entire latitudinal extent of the CCS, other aspects of the large-scale system can be addressed by comparisons of detailed mesoscale studies in different regions across the large, latitudinal gradients in the system. These include: 1) regions with similar ecosystems but different environmental conditions; 2) the boundaries of distinct biological `provinces' or physical regimes; and, 3) regions connecting the main body of the CCS to sites with long paleosediment records (the Southern California Bight). Investigations of the relation between the timing of life history stages and biological rates with respect to biophysical events are emphasized, as are investigations of genetic variability.

Although an ENSO cannot be guaranteed to occur during the field survey period, the program and monitoring will span the period of a typical El Nino/La Nina cycle. Thus much of the interannual variability observed may be related to that cycle and the field programs should be designed to make use of that variability, and address the following questions. Are El Nino and La Nina the extremes of the spectrum of interannual variability, or are these events unique in their physics and ecosystem impact? Can ecological conditions during ENSOs serve as useful proxies for the ecosystem alterations expected as a result of longer term climatic variability?

### Goals and Objectives

Although many of the key physical processes occur in all regions of the CCS, their relative importance, seasonal timing, and spatial extent and impact vary. Substantial biological differences also occur between regions, presumably due to these physical differences. The overall goal of the large-scale spatial studies is to determine how regional differences in physical processes lead to the observed differences in the distribution, abundance and variability of marine organisms, and to relate these regional differences to the expected ecosystem response to climate change. Other objectives include:

- Assess and quantify the relative impact of physical processes in different latitudinal regions on the distribution, abundance, vital rates and life history of key populations of marine animals.

- Understand the mechanisms behind these physical/biological linkages.

- Use interannual variability at given locations to further understand the mechanisms behind these physical/biological linkages.

- Identify variables and sites for monitoring, to document future ecosystem changes.

- Use the understanding of the mechanisms by which populations respond to present differences in forcing to formulate specific parameterizations for biophysical models, which will then project the responses of these populations to different climate change scenarios.

- Use the spatial and temporal variability determined by the large-scale study to develop a conceptual model for EBC ecosystem response to various climate change scenarios.

### Relevance of Large-Scale Spatial Studies

Changes in large-scale atmospheric and oceanic forcing due to climate variations have a major impact on EBC mesoscale circulation which, in turn, affects the persistence of key marine populations. This may result in shifts in physical and biological boundaries, as well as changes in the major physical processes within a region (e.g., changes in the intensity and timing of upwelling, seasonality and intensity of mesoscale jets and eddies, degree of vertical mixing, etc.). Understanding the mechanisms by which populations respond to present differences in forcing and circulation will allow us to predict how those same populations will respond to changes in climatic forcing.

The large-scale study tells us something about the entire California Current EBC system, and can be generalized to other EBCs. Interannual variability at the edges of species ranges and transition zones, where the strongest gradients in physical and biological constituents occur, is the best analogy for what might occur if the climate actually shifts. To some degree, the boundaries between physiographic regions correspond with zoogeographic and population boundaries. It is not known to what extent the movement of physical and biological transition regions in response to climate change will coincide. One specific response of the large-scale CCS to climatic changes in local and distant forcing may consist of simple shifts in the locations of the boundaries between the regions, with expansion and contraction of present ranges of species. Species ranges are delimited by either hydrographic/circulation (water mass) restrictions, topographic restrictions, or a combination of the two. Only species whose range is delimited by the first will likely change with changing climate. Species whose current center of population or range limits are determined, at least in part by unique interactions of coastal topography with physical forcing, may not be able to either exploit other habitats or persist in their present habitat with changing environmental conditions, and so will experience a collapse. Latitudinal shifts in these boundaries on interannual to interdecadal time scales provide one model of how climate change affects EBC ecosystems.

Some species (e.g., hake) must cope with processes in more than one CCS region. Although some species appear fixed to specific regions, others migrate along the coast, using different regions as the primary habitat for different life stages; others exist as distinct populations in several locations. An understanding of the linkages between the environment and a population's life cycle must consider significant processes occurring over a broad latitudinal range. Understanding why and how these organisms adapt to conditions in different regimes will provide a better indication of the consequences of climate change to the ecosystem.

### Questions and Hypotheses

The primary questions of interest in latitudinal variability include:

- What is the spatial variability in physical forcing among the four regions?

- How do ecosystem structure and dynamics differ in the four regions and how might they respond to climate change?

- To what degree are biophysical boundaries geographically anchored (i.e., do boundaries shift proportionally to variations in forcing, or is there a "threshold" value of climate change necessary to move boundaries)?

In addition, the large-scale studies should address the following set of hypotheses:

- Variability in large-scale atmospheric and oceanic forcing in EBC systems plays a critical role in the recruitment and maintenance of key marine animal populations through its influence on advection, turbulence and productivity.

- The success of key populations in EBC systems is set by the interaction of their life-history strategies with the speed, direction and timing of physical transport and retention of planktonic stages of these species, their predators and their prey.

- Major regional differences in the amount of advection due to the mesoscale features exists, and is related to regional differences in the recruitment dynamics of benthic invertebrates.

- Strong latitudinal differences in the interaction between mesoscale oceanic features and the distribution, abundance, vital rates and life history of key populations can be used to understand and predict future ecosystem structure and dynamics under different climate change scenarios.

- The interannual variability that accompanies the El Nino/La Nina cycle, while not an exact analog of climate change, causes changes in local surface forcing and basin-scale currents, which perturb the biophysical interactions within the CCS and provide insight into the mechanisms that determine how the system responds to changes in forcing, such as will be caused by climate change.

### Methods

A multi-year field program is suggested for several locations along the North American west coast to compare the relationship between large-scale atmospheric and ocean forcing mechanisms, the principal mesoscale circulation features, and the distribution, abundance, population dynamics and genetic composition of plankton, benthic organisms and fishes at each site. The focus of the field program is two-fold: (1) regional comparison of the effects of mesoscale circulation on the dynamics and life histories of marine populations-to understand the dynamics of similar ecosystems located in different physical environments; and (2) variability of transition regions in the flow field and faunal provinces-to understand the processes that cause and maintain these transition regions and to project the impact of future climate change scenarios on them. Field sampling should combine Eulerian "mapping" studies of physical and biological parameters at selected sites (in comparison with other locations) with Lagrangian "tracking" of water and organisms through and between regions. The surveys will employ the methods described in the Mesoscale Spatial Studies section (addressed next).

Comparative surveys should be conducted in the various physical/biological regions as well as near the transition zones between these regions; the location of these surveys must consider the range and life history of the key species being studied as well as the principal physical processes. For example, comparative studies could examine mesoscale features off central-northern Oregon (Region I), central-northern California (Region II), and the Southern California Bight (Region III). Collaborations with Mexico (Region IV) and Canada (north of Region I) should be established to further examine spatial variability on the largest scales. Transition studies should focus on the boundaries of the biological provinces and the physical regions, while realizing that not all species observe the same boundaries (which may provide useful information in itself). The study also should be part of a large international program whose focus is the comparison of EBC ecosystems.