La Niña: The B-Side of Climatology, or Another State of Confusion
Gary SharpCenter for Climate/Ocean Resources Study
Monterey Bay, CA. USA
gsharp@montereybay.com
We have come a long way since the early 1980s. There are many linkages being made using satellite technologies and in situ observations that focus on the Pacific-wide El Niño-Southern Oscillation phenomenon. More recently, there have been efforts that also link the westward propagation from the Warm Pool into the Indian Ocean, and others such as Warren White at SIO/IRI and Jim O'Brien at FSU/COAP who have tracked heat and wind energy into the ocean, and watched their progression poleward and across oceans. These "footprints" can persist for nearly a decade, beyond the usual time periods allocated to El Niño Warm Events. These processes have important biological consequences that need to be recognized, and accounted for, in resource conservation and exploitation scenarios.
My work on high seas tropical tuna fisheries showed direct relationships between availability of tunas to seine gear and the upper ocean thermal structure. Similarly, regions with high primary production, hence low oxygen availability at depth, also cause exclusions of many metabolically active fish species such as the tunas, jacks, and large sharks. The combined relations between temperature and oxygen availabilities define the ecosystems, and provide partitioning that is noticeable on global and regional scales. The latitudinal patterning of temperature, alone, provides one set of zoogeographic partitionings, with the strong gradients occurring in conjunction with surface wind patterns, and seasonal heating and cooling. Add to this the ENSO-scale patterns of energy transfers, and a natural sequence is assured of quasi-decadal perturbations of the seasonal climatology. These effects are observed in pelagic fisheries around the world.
Beyond the definitions of "Events", oceanographers have deferred to technical jargon used by climatologists and meteorologists in defining "normal," warm and cold anomalies, and typically use a thirty year mean of monthly mean observations to characterize climatology, or the expected climate on a monthly mean seasonal basis. Deviations from climatology are categorized against these thirty-year mean expectations. The problem lies with the fact that climatology is the least likely state to occur in dynamical systems with embedded histories or latent dissipations of energy, such that decadal patterns often have built-in trends, that represent past energy transfers. Typically, these tend to work their way toward the poles at the surface, and toward the equator at depth, in an effort to achieve mass balance.
Tracking warm water events is somewhat easier than their counterparts, as the warm water tends to expand, float, and have a measurable sea height rise. The counter position is complicated by the interactions of surface winds as they blow across the sea surface, as heat is removed, in the forms of both water vapor and sensible heat. The seasonal patterns of insolation as the Earth passes around the sun are only slowly changing, in comparison to these upper ocean dynamics, providing strong seasonality, or regularity of energy input and polar dissipation. The moderator of these are the atmospheric clouds, at various levels, and are particularly important to the understanding of energy transfers and trapping in the upper ocean. A fact that eludes many trained experts is that a clear sky ocean anywhere on the planet will lose more heat at night than is absorbed during the daylight hours. It takes cloudy night skies to trap heat into the upper ocean, even at the equator. What form these clouds take determines the rates of heat transfer and maintenance in the upper ocean, as also affected by the direction and histories of surface winds.
The surface winds that we tend to associate with normal in the eastern Tropical Pacific blow from east to west, and have already been well warmed, saturated and wrung out as they cross the Isthmus of Panama, or Central America. As they progress westward, they remove water vapor, physically, and heat, allowing upwelling or shallowing of the isotherms as that warm water is replaced from below. These are good times for surface tuna fisheries.
Following that water laden warm air westward, into the Warm Pool, we discover a boundary (or threshold) condition that is defined by the upper ocean and lower atmosphere instabilities, leading to deep convection in both media. As solar heat is added to the 27.5°C or warmer ocean, the volatility of the energy increases, making for great inhomogeneities, and therefore, even more intense convection. In the ocean, night time convection tends to concentrate the heated areas, and separate them by less energetic areas, leading to plume formations emanating from these hot spots, as measured by LIDAR technologies in recent research cruises.
This scenario is strongest at the upper end of the SOI Index, or at that period recently dubbed La Niña, or the Cold Phase of the ENSO process. This is certainly a misnomer for the western Pacific, as it is at its most energetic, with deep 350-550 meter thermoclines and 30+°C SSTs, while the south and western Indian Ocean and eastern Pacific tend to have shallower thermoclines and less heat content during these conditions. Meanwhile, we have not yet described the poleward dynamics, that appear to run on longer 18.6 to 22 year patterns associated with solar and lunar cycles, and also reflect the previous histories of energetic events along the equator.
Patterned ecological responses with serial autocorrelations of faunal shifts are the norm for ocean environments. The stronger La Niña events tend to be associated with definite declines in high latitude populations of marine mammals as a consequence of the debilities of decreased production from the previous El Niño warming, and subsequent population stresses and disease outbreaks. The La Niña does not always directly cause the debilities, it only exacerbates the already debilitated situations, such as susceptibility to the emergent viruses, etc., that are associated with the non-Niño conditions. This is not very different from wheat rust, or grasshopper and locust outbreaks. There are always opportunists waiting in the proverbial wings.
Regional ocean primary production appears to be generally enhanced following cool events, leading toward a bottom-up resurgence of coastal upwelling and high latitude marine ecosystem productivity. The enhanced deep convection, associated divergences and upwelling in the tropics also appears to lead to more primary production and stimulates survival of early life stages of tunas and other predator fishes, creating extra abundant predator fields. These all have direct consequences on global fisheries production. The important factors that need to be understood are those that create the patterns of rise and fall of lower trophic levels, then the subsequent rise and collapse of the higher level predators as they reflect the competition for lower trophic level production. Life in the ocean is a race against both blooming predators and dwindling nutrients for primary production. Less available light, or lessening of surface winds both have generally negative impacts. Cloud cover and upper ocean temperature structures and surficial gradients may actually provide some of the more useful signals, beyond high resolution precipitation maps over continental systems.
One might say that the first climate models that produce forecasts of useful upper ocean thermal structure dynamics will also provide the most useful climate forecasts, in the sense that improved climate prediction is about thorough book-keeping of the upper ocean energy content and its interactions with the atmosphere. Ocean users do not really need SSTs, they need vertical thermal structures, gradient strengths, and wind speed and direction. Similarly, terrestrial users need soil moisture, surface winds, and air temperature predictions. They will benefit simply because the better upper ocean structure predictors will likely also be the best drivers of atmospheric and downstream precipitation models, leading to better tracking of soil moisture, as an analog of upper ocean processes, e.g., the situations that create those deep convection cells over the tropical rainforests, or not, depending upon the previous few weeks-to-months hydrology.
The obvious next steps are to converge the soil and ocean dynamics into analogous classes, and create better, more useful localized products for the Global user community, rather then focusing on large regional agricultural models. Agriculture economics issues have primarily been resolved through risk spreading via geographic diversification. The ocean fisheries industry is a complex of many participants, each with distinct set of objectives. Unfortunately, ocean resource managers have yet to achieve the same levels of understanding as agriculturists, but better ocean products will help that happen. Since there is so much redundancy (i.e., too many fishing boats, too many markets) competition is rampant in most ocean fisheries. The need for both economic and ecological rationalization and broader scale focus is long overdue. Present levels of subsidization of that redundancy is uncalled for, and only when ocean fisheries can be managed through adequate information sharing processes will they become, again, economically self-sustaining ventures.