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Thomas R. Knutson, Syukuro Manabe, and Daifang Gu

Abstract

An analysis is presented of simulated ENSO phenomena occurring in three 1000-yr experiments with a low-resolution (R15) global coupled ocean–atmosphere GCM. Although the model ENSO is much weaker than the observed one, the model ENSO’s life cycle is qualitatively similar to the “delayed oscillator” ENSO life cycle simulated using much higher resolution ocean models. Thus, the R15 coupled model appears to capture the essential physical mechanism of ENSO despite its coarse ocean model resolution. Several observational studies have shown that the amplitude of ENSO has varied substantially between different multidecadal periods during the past century. A wavelet analysis of a multicentury record of eastern tropical Pacific SST inferred from δ 18O measurements suggests that a similar multidecadal amplitude modulation of ENSO has occurred for at least the past three centuries. A similar multidecadal amplitude modulation occurs for the model ENSO (2–7-yr band), which suggests that much of the past amplitude modulation of the observed ENSO could be attributable to internal variability of the coupled ocean–atmosphere system. In two 1000-yr CO2 sensitivity experiments, the amplitude of the model ENSO decreases slightly relative to the control run in response to either a doubling or quadrupling of CO2. This decreased variability is due in part to CO2-induced changes in the model’s time-mean basic state, including a reduced time-mean zonal SST gradient. In contrast to the weaker overall amplitude, the multidecadal amplitude modulations become more pronounced with increased CO2. The frequency of ENSO in the model does not appear to be strongly influenced by increased CO2. Since the multidecadal fluctuations in the model ENSO’s amplitude are comparable in magnitude to the reduction in variability due to a quadrupling of CO2, the results suggest that the impact of increased CO2 on ENSO is unlikely to be clearly distinguishable from the climate system “noise” in the near future—unless ENSO is substantially more sensitive to increased CO2 than indicated in the present study.

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Daifang Gu and S. G. H. Philander

Abstract

Wavelet transforms, which can unfold signals in both time and frequency domains, are used to analyze the Comprehensive Ocean and Atmospheric Data Sets for the period 1870–1988. The focus is on secular changes in the interannual variability and the annual cycle of selected equatorial regions. The amplitude of El Niño/Southern Oscillation (ENSO) is found to be large from 1885 to 1915, to be small during the period 1915–1950, and to increase rapidly after about 1960. Surprisingly, the decadal variations in the amplitude of ENSO are not matched by similar decadal variations in the amplitude of the annual cycle.

On short timescales of 2–5 years, ENSO strongly influences the annual cycle in certain parts of the central and eastern tropical Pacific where the thermocline is shallow. The annual cycle is weak in warm El Niño years and is strong in cold La Niña years. This result suggests that the amplitude of the seasonal cycle is affected by interannual variations in the depth of the thermocline and in the intensity of the trade winds.

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O. M. Phillips, Daifang Gu, and Mark Donelan

Abstract

This paper is concerned with the expected configuration in space and time surrounding extremely high crests in a random wave field, or, equivalently, the mean configuration averaged over realizations of extreme events. A simple, approximate theory is presented that predicts that the mean configuration ζ¯(x + r, t + τ) surrounding a crest at (x, t) that is higher than γσ (where σ is the overall rms surface displacement and γ ≫ 1), when normalized by ζ¯(x,t) for ζ > γσ, is the space-time autocorrelation function ρ(r, t) = ¯ζ(x, t)ζ(x + r, t + τ)/ ζ¯2 for the entire wave field. This extends and simplifies an earlier result due to Boccotti and is consistent with a precise calculation of the one-dimensional case with r = 0, involving the time history of measurements at a single point. The results are compared with buoy data obtained during the Surface Wave Dynamics Experiment and the agreement is found to be remarkably good.

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Daifang Gu, S. G. H. Philander, and Michael J. McPhaden

Abstract

Data for the period from 1985 to 1993 from TAO moorings along 110°W (5°S–5°N) and 140°W (2°S–9°N) describe the vertical, meridional, and temporal structure of the seasonal cycle of several variables. The results have a number of interesting features. The amplitude of the seasonal cycle is relatively constant in the surface layers but varies considerably at the depth of the equatorial thermocline where it was small before 1989, large thereafter. Also, vertical seasonal movements of the thermocline have little effect on sea surface temperatures. These seasonal variations are consistent with a westward propagating coupled ocean–atmosphere mode in the surface layers. Conversely, the low-frequency modulation of the seasonal cycle in the thermocline is associated with changes in the seasonal cycle of the zonal wind in the central and western tropical Pacific and might be attributable to equatorial Kelvin waves forced resonantly by the surface winds.

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O. M. Phillips, Daifang Gu, and Edward J. Walsh

Abstract

In a previous paper (Phillips et al.) an approximate theory was developed that predicted that the expected configuration of extreme waves in a random sea (or the average configuration of an ensemble of extreme waves) is proportional to the space-time autocorrelation function of the surface displacement of the wave field as a whole. This result is tested by examination of scanning radar altimeter measurements made during SWADE in four different sea states, including a unimodal mature wave field, a short fetch, a wind-generated sea crossing swell, a very broad directional spectrum, and a fetch-limited wind sea with opposing swell. In each of these, the spatial autocorrelation function was found directly from the SRA data. The highest waves in each dataset were selected and their configurations averaged with respect to the crest. These averaged configurations were in each case found to be consistent with the autocorrelation function.

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