Analysis of the Slab-Ocean El Nino Atmospheric Feedbacks in Observed and Simulated ENSO Dynamics.

Dommenget, Dietmar, Haase, Sabine, Bayr, Tobias and Frauen, Claudia (2014) Analysis of the Slab-Ocean El Nino Atmospheric Feedbacks in Observed and Simulated ENSO Dynamics. Climate Dynamics, 42 (11-12). pp. 3187-3205. DOI 10.1007/s00382-014-2057-0.

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Abstract

In a recent study it was illustrated that the El Nino Southern Oscillation (ENSO) mode can exist in the absence of any ocean dynamics. This oscillating mode exists just due to the interaction between atmospheric heat fluxes and ocean heat capacity. The primary purpose of this study is to further explore these atmospheric Slab Ocean ENSO dynamics and therefore the role of positive atmospheric feedbacks in model simulations and observations. The positive solar radiation feedback to sea surface temperature (SST), due to reduced cloud cover for anomalous warm SSTs, is the main positive feedback in the Slab Ocean El Nino dynamics. The strength of this positive cloud feedback is strongly related to the strength of the equatorial cold tongue. The combination of positive latent and sensible heat fluxes to the west and negative ones to the east of positive anomalies leads to the westward propagation of the SST anomalies, which allows for oscillating behavior with a preferred period of 6-7 years. Several indications are found that parts of these dynamics are indeed observed and simulated in other atmospheric or coupled general circulation models (AGCMs or CGCMs). The CMIP3 AGCM-slab ensemble of 13 different AGCM simulations shows unstable ocean-atmosphere interactions along the equatorial Pacific related to stronger cold tongues. In observations and in the CMIP3 and CMIP5 CGCM model ensemble the strength and sign of the cloud feedback is a function of the strength of the cold tongue. In summary, this indicates that the Slab Ocean El Nino dynamics are indeed a characteristic of the equatorial Pacific climate that is only dominant or significantly contributing to the ENSO dynamics if the SST cold tongue is sufficiently strong. In the observations this is only the case during strong La Nina conditions. The presence of the Slab Ocean ENSO atmospheric feedbacks in observations and CGCM model simulations implies that the family of physical ENSO modes does have another member, which is entirely driven by atmospheric processes and does not need to have the same spatial pattern nor the same time scales as the main ENSO dynamics.

Document Type: Article
Additional Information: WOS:000336984000021
Keywords: El Nino Southern Oscillation; ENSO; Cloud feedback; Climate sensitivity; General circulation models; Tropical Pacific cold tongue; Climate variability
Research affiliation: OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-ME Maritime Meteorology
Refereed: Yes
Open Access Journal?: No
DOI etc.: 10.1007/s00382-014-2057-0
ISSN: 0930-7575
Projects: ARC
Date Deposited: 20 Jan 2014 11:14
Last Modified: 13 Sep 2016 12:15
URI: http://oceanrep.geomar.de/id/eprint/23147

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