El Nino Southern Oscillation stability under global warming.
Ferrett, Samantha Joanne
Date: 21 May 2015
University of Exeter
PhD in Mathematics
Typically, multi-model ensemble studies show mixed responses of El Nino Southern Oscillation (ENSO) under global warming, so it is currently unknown how, or even if, global warming will impact ENSO and its teleconnections. ENSO is governed by various ocean-atmosphere interactions in the equatorial Pacific, which provide either positive ...
Typically, multi-model ensemble studies show mixed responses of El Nino Southern Oscillation (ENSO) under global warming, so it is currently unknown how, or even if, global warming will impact ENSO and its teleconnections. ENSO is governed by various ocean-atmosphere interactions in the equatorial Pacific, which provide either positive amplifying or negative damping feedbacks and are not always accurate in models. This results in uncertainty in projected ENSO responses. In a flux adjusted HadCM3 perturbed physics ensemble, the Bjerknes' stability index (BJ index), a measure of ENSO stability, has been used to analyse the strength of ENSO feedbacks and their response under the SRES A1B warming scenario with respect to mean climate conditions. Despite mean sea surface temperature biases being minimised by flux adjustment, the important dominant feedbacks, namely the latent heat flux feedback, shortwave flux feedback, the thermocline feedback and the zonal advective feedback are found to be too weak in the ensemble. Common model biases cause weak ocean-atmosphere interactions such as a weak response of ocean currents to wind stress anomalies, a weak thermocline slope response to wind stress anomalies and weak thermodynamic dampings. These biases are linked to overly strong zonal surface ocean currents and convective response biases. Under global warming, a large increase in thermodynamic damping, caused by increasing shortwave damping, is found. This increase is linked to a strong convective response and overrides other feedback responses, resulting in a weakening BJ index in contrast to increasing ENSO amplitude. Positive feedback responses are also found but counteract each other, so have relatively little impact on total ENSO stability. Results here show that common model biases, such as the cold tongue bias, are linked to persistent ENSO feedback biases pointing to areas of improvement in future models. Results also suggest that caution must be exercised when using the BJ index to assess ENSO, as the BJ index is not always representative of ENSO amplitude. This may be caused by non-linearities in ENSO feedbacks which are not accounted for by the linear approximations used in the BJ index, or by ENSO feedbacks not being directly comparable in magnitude, as assumed by the BJ index calculation.
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