The Northern Hemisphere Extratropical Atmospheric Circulation Response to ENSO: How Well Do We Know It and How Do We Evaluate Models Accordingly?

Abstract

Application of random sampling techniques to composite differences between 18 El Niño and 14 La Niña events observed since 1920 reveals considerable uncertainty in both the pattern and amplitude of the Northern Hemisphere extratropical winter sea level pressure (SLP) response to ENSO. While the SLP responses over the North Pacific and North America are robust to sampling variability, their magnitudes can vary by a factor of 2; other regions, such as the Arctic, North Atlantic, and Europe are less robust in their SLP patterns, amplitudes, and statistical significance. The uncertainties on the observed ENSO composite are shown to arise mainly from atmospheric internal variability as opposed to ENSO diversity. These observational findings pose considerable challenges for the evaluation of ENSO teleconnections in models. An approach is proposed that incorporates both pattern and amplitude uncertainty in the observational target, allowing for discrimination between true model biases in the forced ENSO response and apparent model biases that arise from limited sampling of non-ENSO-related internal variability. Large initial-condition coupled model ensembles with realistic tropical Pacific sea surface temperature anomaly evolution during 1920–2013 show similar levels of uncertainty in their ENSO teleconnections as found in observations. Because the set of ENSO events in each of the model composites is the same (and identical to that in observations), these uncertainties are entirely attributable to sampling fluctuations arising from internal variability, which is shown to originate from atmospheric processes. The initial-condition model ensembles thus inform the interpretation of the single observed ENSO composite and vice versa.

Response of O<sub>2</sub> and pH to ENSO in the California Current System in a high-resolution global climate model

“…The climate system is highly nonlinear and generates variability unrelated to ENSO events, contributing noisiness in the physical and biogeochemical ENSO-related signals in the CalCS. The noise is quite large, as has been demonstrated by the spread in ensembles of model simulations with the same ENSO conditions in the tropical Pacific but slightly different initial conditions (e.g., Sardeshmukh et al, 2000;Alexander et al, 2002;Deser et al, 2017). Thus, the evolution of anomalies in the CalCS is expected to differ between ENSO events in both nature and models, which may partly explain the difference between the study by Nam et al (2011), which analyzed a single La Niña event, and the present study.…”

“…This study additionally demonstrates that the diversity of ENSO events might contribute to the variability of the physics and biogeochemistry in the CalCS, and thus emphasizes the importance of analyzing long-enough time series to include a variety of different events. In addition to the variability between different types of ENSO events, other sources of internal climate variability unrelated to ENSO contribute to noisiness in the physical and biogeochemical signals in the CalCS (e.g., Deser et al, 2017).…”

“…Both models also simulate the intensification (weakening) of the Aleutian Low during El Niño (La Niña), though the changes are overestimated and biases in orientation relative to the observed record are apparent. It should be noted that both the orientation and position of the Aleutian Low in the observations and in ESM2.6 are likely affected by the limited number of events present in both time series and could lead to a bias in the results (Deser et al, 2017).…”

Projected sea surface temperatures over the 21st century: Changes in the mean, variability and extremes for large marine ecosystem regions of Northern Oceans

“…The ocean model has 60 vertical levels, where the layer thickness varies from 10 m near the surface to 250 m at depth. CESM1 was ranked as a top-performing model in CMIP5 by Knutti et al (2013) and its climatic variability is similar to that in nature (Deser et al, 2017).…”

The salience of nonlinearities in the boreal winter response to ENSO: North Pacific and North America

“…a response not proportionate with the underlying tropical SST forcing-is in fact present in the extratropical response is not clear. While the observational composites of clearly show evidence for nonlinearity between EN and La Niña (LN), the observational composites of DeWeaver and Nigam (2002) which sample a different period, and those of Deser et al (2017) and Deser et al (2018) which sample the period 1920 to 2013, exhibit weaker nonlinearity. This difference could be due to decadal variability, though it is not clear whether this decadal variability is forced (Gershunov and Barnett 1998;Zhou et al 2014) or reflects internal atmospheric variability.…”