EBC-CCS - Working Group Reports

## WORKING GROUP REPORTS

### El Niño/Southern Oscillation (ENSO) Effects Within the California Current System

Cochairs: T. Hayward and R. Smith

Participants: D. Ainley, P. Bernal, M. Eakin, R. Haney, A. Hollowed, P. Hsueh, W. Pearcy, I. Perry, L. Sautter, F. Schwing, and T. Strub

El Niño represents an environmental extreme in the eastern boundary regions of the Pacific Ocean. Because of the extent of its effect on ecological structure and the economy, it has high visibility in the media and in politics. There have been 43 "strong to very strong" El Niño events in the five centuries since written accounts of climate and weather were first made in the Americas, with the most recent strong events being in 1982-83, 1972-73, 1957-58, and 1940-41 (Quinn et al. 1987). If one includes "moderate" El Niño events as well, the total rises to 117, although not all were manifested to midlatitudes. A strong El Niño every 10 to 20 years appears to be typical. Any research program in the California Current system extending for a decade or longer should therefore expect to encounter El Niño conditions, and - because El Niño represents an important mode of environmental variability in this region - a study of these conditions should be included in any GLOBEC plan for the California Current system.

The working group focused on three topics:

1. Expression of El Niño in large-scale and mesoscale physical and biological oceanography, and the availability of historical information

2. Potential variation under climate change

3. Contingency planning for additions and modifications to the overall field sampling program during El Niño years.

#### OCEANOGRAPHIC EXPRESSION OF EL NIÑO: WHAT IT IS AND WHAT WE CAN EXPECT IN THE CALIFORNIA CURRENT SYSTEM

It is presently thought that El Niño events represent one phase of an irregular oscillation involving both the atmosphere and the ocean. Since the atmospheric component is called the Southern Oscillation, the complete, coupled phenomenon is referred to as the El Niño/ Southern Oscillation (ENSO). Physical scientists' under-standing of ENSO events has improved (Kerr 1992), but probably not yet enough to predict El Niño with the confidence desired for committing major resources requiring long lead time, especially ships, to a major field effort. The present conceptual and numerical models do, however, provide excellent diagnostic and descriptive information.

The oceanic component of El Niño begins in the western Pacific and reaches the California Current region by two routes: oceanic and atmospheric. The oceanic signal propagates eastward along the equator from the western Pacific as an equatorially trapped baroclinic Kelvin wave, manifested as a deepening of the thermocline and a rise in SST and sea level. Upon reaching the eastern boundary of the Pacific, it propagates poleward in both hemispheres as a baroclinic coastal-trapped Kelvin wave at a speed of about 25 m/s. Thus, the rise in sea level and deepening of the thermocline along the midlatitude coast (30° to 50° latitude) occurs within months of the appearance of El Niño in the eastern equatorial Pacific. This initial effect is confined to within about 100 km of the coast. Increased poleward geostrophic flow is consistent with a deeper thermocline and higher sea level along the coast; early in the 1982-83 El Niño, the coastal currents off Oregon and Peru flowed strongly poleward (Huyer and Smith 1985).

As the coastal Kelvin wave propagates poleward along the eastern boundary, some El Niño signal also radiates westward from the eastern boundary as a Rossby wave with phase speed of only a few cm/s (Johnson and O'Brien 1990). Thus, at latitudes of the California Current, one might observe an anomalous deepening of the thermocline several hundred km offshore a year or two after warming of coastal waters has been observed. While the propagation of El Niño effects along the equator and coast as Kelvin waves has been clearly observed, the westward offshore propagation of El Niño in the form of Rossby waves in midlatitudes has only been shown in theory and suggested as an explanation for anomalous warm water in the offshore region a year or so after El Niño.

The large change in the sea-surface temperature of the equatorial Pacific during El Niño can also cause major changes in the position and strength of the atmospheric pressure patterns affecting the California Current region. During the El Niño winter of 1982-83, the atmospheric Aleutian Low strengthened and moved southward, causing severe storms along the California and Oregon coasts. The result was increased rainfall, increased vertical mixing, and increased onshore Ekman transport. During some El Niño events, however, the Aleutian Low strengthens but remains farther offshore, diverting storms to the north of their normal track (e.g., the California drought during El Niño 1976-77). The response of the coastal winds, which drive coastal upwelling, to El Niño is varied: although the coastal upwelling index was anomalously low off California in early 1983, the index was higher than normal off Peru. But despite the upwelling-favorable winds, nutrient inputs to the surface layer off Peru were low because the nutricline was anomalously deep and beyond the effects of coastal upwelling.

The biological effects of El Niño are less well documented than the physical effects. This is partly because descriptions of "normal" conditions are insufficient. But some generalizations can be made. The biological effects of El Niño stem in part from the deepening of the nutricline, and from the possible decrease in coastal upwelling. Although a deepening nutricline in the eastern Pacific should always be expected during El Niño, coastal upwelling does not necessarily weaken. As noted above, the net effect on nutrient input maybe equivalent, since the nutricline is deepened . Analysis of remote sensing data suggests that chlorophyll and, presumably, primary productivity decrease, although this has not been clearly established. A change in phytoplankton species composition has been documented in the Southern Hemisphere (Avaria and Munoz 1987). A large decrease in macrozooplankton biomass and in the abundance of some fish has been documented in both hemispheres (Chelton et al. 1982; Carrasco and Santander 1987). Large changes in patterns of distribution and abundance of some species of macrozooplankton and nekton have also been observed (e.g. ,Pearcy and Schoener 1987). Some of these changes in distribution are due to active migration; some are due to passive transport with the water; and others are likely due to in situ changes in population dynamics.

The California Current contains a rich pattern of low-frequency variability in biological and physical structure. The interannual variability in some physical and biological properties (e.g., in offshore steric height and in zooplankton abundance) is larger than the annual cycle. The question can be asked whether El Niño represents an extreme condition along a continuum of interannual environmental variability, or a qualitative change in environmental structure as well as an extreme in the range of environmental condition. There is insufficient information to answer this question, but the conceptual model that is chosen will affect the structure of models of biological response to El Niño.

The oceanographic community is probably aware of most data sources useful for describing the mid-latitude effects of El Niño (see, for example, the collection of papers on El Niño in J. Geophys. Res. 92(Cl3), 1987). The observations and monitoring necessary to answer many of the questions concerning effects of El Niño on the California Current region and its mesoscale features and variability have not been made. Coastal sea-level records, SST data, and biological monitoring over the decades indicate clearly that El Niño occurs along the coast, but repeated observations in the full California Current are much rarer. Some useful data are available from CalCOFI cruises, and from cruises by Oregon State University in the 1960s and 1970s, but the observations were on spatial scales too large to answer questions about the mesoscale.

#### POTENTIAL VARIATION UNDER CLIMATE CHANGE

The working hypothesis appropriate at this time is that in a globally warmed climate, El Niño events would occur with a frequency and intensity at least as great as in the present climate. Some recent preliminary studies (Zebiak and Cane l991) with a dynamic ENSO prediction model suggest that a warmer ocean, such as might result from global climate change, could increase the frequency and intensity of El Niño events.

The discussion was broadened to include the response of eastern boundary currents to global warming. It is difficult to predict any change in eastern boundary currents resulting from global warming on the basis of present general circulation models, which indicate a zonally uniform warming (1-2°C) of the oceans at 30° to 50°N. The oceanic components of the models have latitude/longitude resolution of several degrees, and therefore cannot adequately resolve eastern boundary currents. Thus the potential for increased warming in coastal regions due to a weakening in the eastern boundary current, which is caused by weaker atmospheric circulation caused in turn by reduced poleward temperature gradients, is not included in these models. Observations and physical reasoning indicate that increased warming of coastal land surface relative to the coastal ocean would increase coastal circulation and upwelling (Bakun 1990). Thus global warming might bring about two competing, and possibly offsetting, effects: a widespread warming but a local cooling in the coastal region of eastern boundary currents. If so, this would increase the zonal temperature gradient between the coastal and offshore regions and would enhance the likelihood of mesoscale variability in the coastal transition zone.

#### SPECIAL REQUIREMENTS FOR SAMPLING DURING AN EL NI„O EVENT

On the basis of present understanding it should be assumed that El Niño events will continue to occur with at least the same frequency and intensity as during recent centuries. Because El Niño represents an expected environmental extreme and because we need to know how eastern boundary current ecosystems respond to environmental extremes, it should ideally be an important GLOBEC goal to ensure that a full field program takes place in the California Current ecosystem during El Niño conditions. This is because the present ecosystem must contend with these conditions on an irregular basis, and much can be learned from comparing the extreme structure with more normal conditions. However, present prediction schemes (models) cannot confidently predict El Niño far enough in advance to schedule a full field program complete with major ships. Nevertheless, at the least, a limited study aimed toward understanding the effects of El Niño in the context of the major GLOBEC field programs should be developed. Such studies could be planned and launched several months in advance on the basis of early El Niño predictions.

Some level of monitoring will be necessary throughout the period of GLOBEC studies in the California Current region, independent of the phenomenon of El Niño. It is important to include in the monitoring some of the phenomena that may be affected by, or transmit the effects of, El Niño. Several studies will have been made in the California Current region before GLOBEC study begins. These should be useful for designing both the monitoring and the field program for studying El Niño.

Although we cannot make complete recommendations for an El Niño study within the context of a GLOBEC field program until the nature of that overall field program becomes more definite, it is clear that research on El Niño should be an important part of the overall design. We can at this time list elements of an El Niño study that are likely to contribute valuable information to understanding the California Current in the context of the GLOBEC program. Some of the key components would be:

- Moorings to monitor the poleward undercurrent over the continental margin. Poleward flow along the coast probably increases during El Niño and may be the major contribution to the advective changes observed. The strength of the undercurrent, and its interaction with topography, would affect mesoscale variability in the region, since the poleward flow affects the dynamical stability of the flow regime.

- Moorings offshore to monitor propagation of Rossby waves westward from the coastal region. In the models, Rossby waves are the mechanism that carry El Niño to the offshore ocean.

- Sediment traps on the moorings to document variability in the amount and composition of plankton and sedimenting particulate matter.

- Analysis of existing data, especially historical data on plankton distributions, to better understand "normal" conditions so that changes due to El Niño can be documented.

- Observations to assess the mesoscale structure and spatial distributions of the biota. The satellite altimetry data becoming available should help identify regions where mesoscale activity is particularly large in the near-surface current fields. Past and future satellite color scanner data should be similarly useful for some biological fields. Results from recent studies in the northern California Current region within a couple of hundred kilometers of the coast, and data from the larger domain accessible to satellite sensors, should make it possible to develop a monitoring and field program plan that would be sensitive to mesoscale variability and the effects of El Niño.

- Coupling of the physical and biological models. This could be done now by including biology in the present El Niño models. This would help clarify the putative effects of El Niño on the ecology and environment. At present, El Niño effects are best resolved in some of the physical and higher-trophic-level data; the linkage through the phytoplankton is less clear. The early initiation of coupled physical/biological modeling would also provide scientific publicity for GLOBEC.

#### SUMMARY

1. El Niño is an important environmental extreme in the ecosystem, and its conditions should be carefully sampled during an eastern boundary current GLOBEC program. This should not be solely a contingency plan to deal with an unexpected event, but rather an active plan to ensure that sufficient sampling will take place during El Niño conditions.

2. We should encourage the development of coupled physical/ biological models in eastern boundary current regions, especially with respect to mesoscale structure; El Niño conditions should be included in these models. This effort will be most valuable if significant results are available before field programs are planned.

3. Some monitoring will be necessary to place GLOBEC field programs within the context of low-frequency variability. El Niño is only one aspect of a rich pattern of low-frequency variability, and other processes of forcing and response deserve study.

4. The nature and extent of field work targeted specifically toward El Niño will depend upon the nature of the GLOBEC field program. El Niño studies should be an integral part of a field program, and should be included in planning from the earliest possible stage.