The Do-Loop of ENSO Warm Events



A simplistic word picture of the sequence is an energetic "Do Loop" that travels around the globe, much as an "inchworm" might circle a tomato. The (upper ocean) locus of the "tail" of the process provides the base direction for the "front end" which reaches up and outward (with many degrees of freedom) to determine the next "landfall". As the selection is made, and the "front end" reattaches, the "tail" is brought up to, but not over the landing spot, to begin again the process.

Think of the "tail end" as starting at a particularly energetic location within the Warm Pool. That location expells a large amount of its energy, upward, and eastward, and usually poleward, only to resettle where cooler context is located. Meanwhile, everywhere under the shadow of the "inchworm's" cloudy corpus, an extra amount of heat is absorbed, leading toward a new, more energetic state. Wherever a "tail end" has departed, the local heat (energy) content has been substantially diminished. The thermal signatures will reflect this.

Meanwhile, all downstream locations will have been affected by the "shadow", or the clouds formed, and will have gained energy, generally leading toward more "Deep Convection" expulsions.

Let's start again with the Pacific Warm Pool, as a base location, with the easterly surface winds pushing west from across the eastern Pacific, where they have scoured both sensible heat and moisture (latent heat), thereby causing continous upwelling (shoal thermoclines) from east to west. Thermocline depths in the eastern Pacific are relatively shoal, with the 15C isotherm often within the photic zone (90m or less). In the western Warm Pool region, the ocean receives both stored (rainfall) and sensible heat, and through simple gravitational forces, attains deeper thermoclines as these processes accrete. It is not uncommon for the 15C isotherm in the midst of the Warm Pool to be found at depths of several hundred meters (350-500m). Between the equator and the Inter-Tropical Convergence Zone (the ITCZ is the seasonal atmospheric equator), the surface temperature gradient is warmest in the west, and cooler in the east. This is due to simply the surface easterlies, and a gradient of cloud cover.

Fishery and physiological responses to ENSO warming - and suppression of production - is nearly immediate, either in the open ocean or along coastlines.

As a consequence of ENSO warming and resulting enhanced cloud cover, there are usually lower light levels, as well as higher surface temperatures, and other related dynamics associated with clouds and humidity. Pelagic food webs are often perturbed. Larger predators must move to more productive areas, or die. When there is decreasing predation from higher trophic level predators, emergent production tends to be spread throughout lower trophic components. These smaller species are often in competition with early life history stages of the migratory pelagic fishes, or even become their major predators, leading to a period of decreased survival for all but the most isolated of these fish larvae.

Also, stress-related spawning of adult pelagic fishes is induced by the warmer temperatures, and their growth patterns are foreshortened. Biomass production slows, as most of the energy that they obtain or have stored is mobilized for reproduction. The resulting eggs and larvae are of lower energy content, and therefore less likely to survive the general fray for food, and predator avoidance.

As a primary general consequence of local warming and suppression of upwelling, the more mobile species tend to become concentrated, increasing competition, and providing both natural predators and fishers with optimal forage conditions. For example, this year, the Peruvian fishers were able to catch over 700,000 tons of anchoveta in one week. All this from hundreds of unrefrigerated vessels that operate within only a few travel hours of their three major home ports. In the 1972 El Niño catches were similarly affected, and the combination of events described above caused havoc within the region's ecosystems, and fisheries.

That was not the entire story, as the fishes had begun a general decline in the late 1960s, as part of a decadal scale climate regime change. There was a general slowing upwelling along the coast of South America, that induced onshore intrusion of oceanic water, and associated species. The resulting shifts in production centers, and species, is well described. The recent (1985-1991) reversion to a stronger coastal upwelling shifted the catches from sardines that emerged in the warmer, lesser upwelling period, back to anchoveta.