Using paleoclimate records to trace El Niño and La Niña events over 400 years

A major challenge in understanding climate is the relatively short periods over which reliable weather observations are available. In Australia, there are few temperature or rainfall records that extend back more than 100 years, and highly-reliable satellite temperature records only extend back to the late 1970s. This makes the study of long-term climatic variations quite difficult, especially in Australia which has a highly variable climate.

One way to overcome this limitation is to use paleoclimate data to re-construct records of climate over an extended period. For example, comparing tree-ring chronologies from regions along the north American pacific coast with growth patterns that can be observed in coral fossils found in the South Pacific ocean provides information about historic changes in ocean temperatures in both regions. This enables the reconstruction of long-term changes in the Pacific Decadal Oscillation (PDO) and the Inter-decadal Pacific Oscillation (IPO), which are climatic indexes which are linked to the El Niño Southern Oscillation (ENSO) phenomena.

A number of scientists have independently carried out such analyses, and a comparison of their results shows good consistency between them in relation to the historical periods they have identified during which changes occurred in the PDO and IPO. This, in turn, indicates periods during which either El Niño or La Niña events were likely to be more frequent.

The following figure uses this information to provide a reconstruction of climate indicators over the last four hundred years, and to identify periods during that time which were dominated by either La Niña or El Niño events. Note the La Niña-dominated period from the 1940s to the 1970s, which coincided with what now appears to be a historically wetter period in Australia, and the period from the late 1970s to the present time which has been dominated by El Niño events, and coincides with a drier period. As can be observed, there appears to be nothing abnormal about the duration of the current period of El Niño dominance. The figure also highlights the difficulties associated with projecting longer-term changes in the Australian climate, given that reliable weather observations only extend back over approximately two cycles of this phenomena.

A historical review of flood events in northern NSW shows the higher frequency of major floods that occurred during the La Niña dominant period from the 1940s to the 1970s. This observation supports theories about the role of El Niño and La Niña events in the Australian climate, and also explains why recent Australian climatic conditions have been hotter and drier that the period from 1940 to 1980.




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