El-Nino is classified as global atmosphere phenomenon influenced by temperature fluctuations in surface waters. As may be expected, there is more surface water in the north and west, whereas water yields decrease and become more erratic to the south (Brasseur et al 1999). Superimposed on this pattern, however, are deviations from the average in the form of major El Niño flood events, at least ten of which have occurred during the past century along with episodes of drought. Such deviations rarely occur simultaneously across the region, while some unusually rainy years appear to be independent of El Niño—Southern Oscillation (ENSO) forcing.
Moreover, over the longer term before instrumentation, tree-ring data indicate several periods of extended drought. Water resources are further modified by human activity. Water transfers and the presence of hundreds of dams and thousands of debris basins and other control structures have altered average and peak discharges, sediment loads, and groundwater flows in many watersheds (Brasseur et al 1999) (Appendix, Picture 1).
More opportunity forecasts are likely to present themselves as scientists refine their understanding of other cyclical large-scale ocean-atmosphere anomalies. “El Niño is the result of complex interactions between the atmosphere and the ocean” (Glantz 2001, p. 8). These are the stuff of the new science of climate prediction: changes in planetary characteristics with consequences that move from one timescale to another.
On the scale of weeks, an atmospheric wave that circles the equator every 45 to 60 days provokes tropical thunderstorm activity that leads to winter storminess over the Pacific Northwest. Another important influence on North American climate is associated with El Nino -Southern Oscillation (ENSO) events over the tropical Pacific Ocean. A composite of ENSO events indicates they are commonly triggered from January to June when sea-surface temperatures rise in the equatorial Pacific from the South American coast to the International Date Line. From March to June, a pressure rise over northern AustraliaIndonesia gives a reversal of the normal Tahiti-Darwin sea level pressure gradient (the negative or low phase of the Southern Oscillation).
“Warm water in the western equatorial part of the Pacific Ocean is a major source of heat that warms the atmosphere above it. This warming causes air to rise generating convection, produces rain-bearing clouds” (Glantz 2001, p. 53). During the July to December peak of the warm El Nino phase, convective activity normally located over Indonesia shifts into the central equatorial Pacific where there is strong wind convergence (Brasseur et al 1999).
Concurrently, westerly wind anomalies develop over the westcentral subtropical North Pacific, continuing through the following April. ENSO events tend to favor frequent and stronger westerly flow across western North America and into Canada during the subsequent winter. Positive temperature anomalies affect the west coast from California to Alaska and all across southern Canada. Conversely, during the corresponding winters of the La Niña (cold) phase with strong Pacific easterly trade winds, weaker zonal circulation in the midlatitudes allows arctic outbreaks and negative anomalies over the Canadian Prairies. The strongest precipitation signal of ENSO is for wetter conditions during the winter following the El Nino peak, over the Gulf Coast of the United States and northern Mexico (César 2001) (Appendix, Picture 2).
ENSO warm events may be associated with positive or negative PNA or there may be no PNA signature. With negative (reversed) PNA, 500-mb height anomalies are positive (negative) over Hudson Bay (the northern North Pacific and Gulf of Mexico). Anomalous easterly flow occurs across southern Canada and much of the United States, with anomalous southerly flow over the west coast. Winter temperatures are above average from the Pacific Northwest to the upper Midwest and below average in the southeastern United States. Precipitation is above average in the Southeast, along the Atlantic seaboard, and in New England, and below average over the northern Great Plains and the Midwest (Seinfeld and Pandis 2006).
For ENSO warm events with no PNA signature. The 500-mb height anomalies resemble those of the reversed PNA except that the negative height anomaly is displaced to the east coast. This trough displacement causes northwesterly flow over the eastern United States, giving large negative departures of temperature in the southeast and of precipitation south of the Great Lakes (César 2001).
Winter climate is also affected by the reoccurrence of El Niño—Southern Oscillation events, causing quasi-periodic increases in precipitation associated with above-normal sea-surface temperatures over the eastern Pacific Ocean Little rain falls in late winter, spring, and early summer, when the Sonoran Desert is affected by the eastern edge of the Pacific high pressure cell (Seinfeld and Pandis 2006).
During high summer, monsoonal circulation, associated with the import of moist air from the Pacific Ocean across the Gulf of California and northwestern Mexico, results in summer thunderstorms, but precipitation is less than that of winter. In addition to normal winter moisture sources, the region also experiences water years related to El Niño—Southern Oscillation (ENSO) events during which precipitation may double average annual values (Harrison and Larkin 1998).
When a plume of warm water migrates eastward from the western Pacific Ocean near Indonesia to the west coast of South America on an erratic 4- to 7-year cycle, sea-surface temperatures in the eastern Pacific may rise 3–7°C above normal. Winter storms gain additional moisture from this influx of tropical warm water into cyclonic systems that are migrating south and east from the north Pacific (Harrison and Larkin 1998).
This combination of warm seasurface temperatures and traveling winter cyclones enhances the region’s precipitation, as seen during the 1982– 83 and 1997–98 water years. Although numerous El Niño events have been documented or inferred over the past century, the net spatial effect of such events varies considerably (César 2001). Further, even though cumulative rainfall associated with El Niño may be quite high, some of the largest floods of the past century, notably in 1968–69, have in fact been associated with relatively weak or even non—El Niño conditions (Glantz, 2001).
Many of the canyon rivers had highly variable flow regimes before the reservoir construction of the twentieth century. The larger tributaries of the Colorado River are dominated by late spring—early summer snowmelt floods, whereas moderate and small channels may have flash floods caused by convective storms, dissipating tropical cyclones, or North Pacific frontal storms. The frequency of flooding has fluctuated during the late Holocene.
Intervals of cool, moist climate and frequent El Nino events coincided with numerous large floods 4800–3600 years ago, around 1000 years ago, and during the last 500 years (César 2001). Related flooding was widespread, with Clear Lake in northern California reaching its highest level since 1909. Earlier, during and after the record rainfall of 1969, soil slips, debris flows, and deeper-seated landslides were important throughout southern California (César 2001).
The impacts of El-Nino on society include upwelling of cold water and fish populations. The main problem is that El-Nino affects fishing industry and coastal populations. It shifts locations of fish and their types in a particular region. “What has changed has been society’s ability to capture fish in quantities that go much beyond just satisfying the local needs of humans for food” (Glantz 2001, p. 92). Such events are extreme because of their potential to cause enormous damage in a short period of time. For example, February 1998 rainfall totals related to El Nino conditions broke all-time records at many locations. “Because it began relatively late in the calendar year, societies affected by it were caught off guard, even those societies where the impacts related to El Niño were known to it” (Glantz 2001, p. 85).
El-Nino affects coastal environments in many ways—by modifying the thermal and chemical properties of coastal waters, by forming density currents in and beyond river mouths, and by yielding terrigenous sediment to coastal systems. Such impacts reflect the hydroclimates and erodibility of contributing basins. Sediment inputs are important where debris-laden streams and glacier meltwaters descend quickly from the erodible Pacific mountain rim, less so where streams must run out across the Gulf and Atlantic coastal plains and sediment-starved rivers from the Canadian Shield reach the sea, and least important where terrestrial drainage seeps seaward through the karst terrains of Florida and Yucatan.
Tides along the open Atlantic coast are typically semidiurnal (two high and two low tides of similar magnitude daily), with the mean range of spring tides along the open coast increasing erratically (César 2001). Following Glantz “The worldwide hazards spawned by these phenomena include droughts, floods, frosts, fires, landslides, and infectious disease outbreaks” (p. 27).
In sum, El-Nino is one of the meteorological phenomenon which affects the society in general and coastal populations in particular. Flooding and mass movement commonly accompany major El Niño events, the erratic nature of the associated storms yields highly variable erosional and depositional responses.
References
Brasseur, G. P., Orlando, J.J. Tyndall, G.S. (1999). Atmospheric Chemistry and Global Change. Oxford University Press, USA; 1st edition.
César N. Caviedes (2001). El Niño in History: Storming Through the Ages University Press of Florida.
Glantz, M. H. (2001). Currents of Change: Impacts of El Niño and La Niña on Climate and Society. Cambridge University Press.
Harrison, D.E., and N.K. Larkin. (1998). El Nino Oscillation sea surface temperature and wind anomalies, 1946–1993. Reviews of Geophysics, 36: 353–399.
Seinfeld, J.H., Pandis, S.N. (2006). Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. John Wiley & Sons; 2nd Edition edition.