Signals Of Interannual Variability Research Articles

Interdecadal Variation of ENSO Predictability in Multiple Models

Abstract In this study, El Niño–Southern Oscillation (ENSO) retrospective forecasts were performed for the 120 yr from 1881 to 2000 using three realistic models that assimilate the historic dataset of sea surface temperature (SST). By examining these retrospective forecasts and corresponding observations, as well as the oceanic analyses from which forecasts were initialized, several important issues related to ENSO predictability have been explored, including its interdecadal variability and the dominant factors that control the interdecadal variability. The prediction skill of the three models showed a very consistent interdecadal variation, with high skill in the late nineteenth century and in the middle–late twentieth century, and low skill during the period from 1900 to 1960. The interdecadal variation in ENSO predictability is in good agreement with that in the signal of interannual variability and in the degree of asymmetry of ENSO system. A good relationship was also identified between the degree of asymmetry and the signal of interannual variability, and the former is highly related to the latter. Generally, the high predictability is attained when ENSO signal strength and the degree of asymmetry are enhanced, and vice versa. The atmospheric noise generally degrades overall prediction skill, especially for the skill of mean square error, but is able to favor some individual prediction cases. The possible reasons why these factors control ENSO predictability were also discussed.

The Recent Decline of the Arctic Summer Sea-Ice Cover in the Context of Internal Climate Variability

By means of a 21-year simulation of a coupled regional pan-Arctic atmosphere-ocean-ice model for the 1980's and 1990's and comparison of the model results with SSM/I satellite-derived sea-ice concentrations, the patterns of maximum amplitude of interannual variability of the Arctic summer sea-ice cover are revealed. They are shown to concentrate beyond an area enclosed by an isopleth of barotropic planetary potential vorticity that marks the edge of the cyclonic rim current around the deep inner Arctic basin. It is argued that the propagation of the interannual variability signal farther into the inner Arctic basin is hindered by the dynamic isolation of upper Arctic Ocean and the high summer cloudiness usually appearing in the central Arctic. The thinning of the Arctic sea-ice cover in recent years is likely to be jointly responsible for its exceptionally strong decrease in summer 2007 when sea-ice decline was favored by anomalously high atmospheric pressure over the western Arctic Ocean, which can be regarded as a typical feature for years with low sea-ice extent. In addition, unusually low cloud cover appeared in summer 2007, which led to substantial warming of the upper ocean. It is hypothesized that the coincidence of several favorable factors for low sea-ice extent is responsible for this extreme event. Owing to the important role of internal climate variability in the recent decline of sea ice, a temporal return to previous conditions or stabilization at the current level can not be excluded just as further decline.

Earth Rotation as a Proxy for Interannual Variability in Atmospheric Circulation, 1860-Present

Modern atmospheric and geodetic data sets have demonstrated that changes in the axial component of the atmosphere's angular momentum and in the rotation rate of the solid earth are closely coupled on time scales of up to several years. The feasibility of using a historical record of the earth's rotation as a proxy for year-to-year changes in the zonal wind field over the globe is examined. The bulk of the earth rotation series acquired for this purpose is based on telescopic observations of the occultation of stars by the moon; semiannual values of changes in the length of day derived from these observations have acceptably small errors from about 1860 onwards. These values are filtered to remove decade-scale fluctuations, which are driven primarily by nonatmospheric processes, and the resulting proxy series is examined to see if it contains a signal associated with one of the major modes of interannual variability in the atmosphere, namely that due to the El Nino/Southern Oscillation (ENSO). According to tests of statistical significance, such a signal is present in the historical earth rotation series, in that the day is typically longer during the year following an ENSO oceanic warm event than otherwise. Therefore other signals of interannual variability in the proxy series are considered. In particular, it is inferred that noteworthy trends in atmospheric interannual variability have occurred over the last century; for example, the decade of the 1920s was marked by much larger year-to-year changes in the zonal circulation over the globe than that of the 1940s. Based on modern atmospheric data, it is suggested that most of these circulation changes have resulted from anomalies in the region between 30 deg N and 30 deg S.