Archives par mot-clé : Cyclicity

climate-debate

Climate Oscillations 11: Oceanic Niño Index (ONI)

by A. May, 4 August, 2025 in WUWT

The Oceanic Niño Index or ONI is NOAA’s primarily indicator for monitoring the sea surface temperature (SST) anomaly in the critical Niño 3.4 region. It is a 3-month running mean of ERSST.v5 SST anomalies in the Niño 3.4 region, defined as 5°N-5°S and 120°W-170°W. Figure 1 shows the ONI as computed from the NOAA ERSST dataset. ERSST is a two-degree gridded dataset, so the region averaged for figure 1 is 6°N-6°S and 120°W-170°W.

Figure 1. A plot of the ONI from 1850 – 2023. The ONI 3-month smoothed anomaly must be above +0.5 for 5 months for an El Niño and below -0.5 for 5 months to be a La Niña. In between the ENSO state is neutral, as it is today.
Per convention a three-month moving average has been applied to the raw ONI data in figure 1. Sometimes you will see the ONI detrended, but the curve in figure 1 is not detrended and has an upward slope of one-half degree per century. The 3-month moving average has to exceed 0.5°C for five consecutive months to define an El Niño, so the chart is colored red above 0.5°C. The same is true for La Niña, but in reverse. The white area between -0.5 and +0.5 is ENSO neutral.

The current ENSO state, as of July 2025, is ENSO neutral, with an average ONI of about zero. NOAA prefers to use a base period for their ONI anomalies of 1991-2020, but we use 1961-1990 to be consistent with the other posts in this series and with HadCRUT5. There is a visual trend over the past 175 years, Niños are more common now and stronger than in previous years. Climate models have a very hard time duplicating ENSO over both short and long periods of time (IPCC, 2021, p. 115). The Niño 3.4 region is shown in figure 2 in red.

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climate-debate

High-frequency climate forcing causes prolonged cold periods in the Holocene.

by E. van Dijk et al., May 08 2024, in Nature (OPEN ACCESS)

Abstract

Understanding climate variability across interannual to centennial timescales is critical, as it encompasses the natural range of climate fluctuations that early human agricultural societies had to adapt to. Deviations from the long-term mean climate are often associated with both societal collapse and periods of prosperity and expansion. Here, we show that contrary to what global paleoproxy reconstructions suggest, the mid to late-Holocene was not a period of climate stability. We use mid- to late-Holocene Earth System Model simulations, forced by state-of-the-art reconstructions of external climate forcing to show that eleven long-lasting cold periods occurred in the Northern Hemisphere during the past 8000 years. These periods correlate with enhanced volcanic activity, where the clustering of volcanic eruptions induced a prolonged cooling effect through gradual ocean-sea ice feedback. These findings challenge the prevailing notion of the Holocene as a period characterized by climate stability, as portrayed in multi-proxy climate reconstructions. Instead, our simulations provide an improved representation of amplitude and timing of temperature variations on sub-centennial timescales.

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