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Using seasonal hindcasts to understand the origin of the equatorial cold tongue bias in CGCMs and its impact on ENSO

Using seasonal hindcasts to understand the origin of the equatorial cold tongue bias in CGCMs and its impact on ENSO
Using seasonal hindcasts to understand the origin of the equatorial cold tongue bias in CGCMs and its impact on ENSO
The cold equatorial SST bias in the tropical Pacific that is persistent in many coupled OAGCMs severely impacts the fidelity of the simulated climate and variability in this key region, such as the ENSO phenomenon. The classical bias analysis in these models usually concentrates on multi-decadal to centennial time series needed to obtain statistically robust features. Yet, this strategy cannot fully explain how the models errors were generated in the first place. Here, we use seasonal re-forecasts (hindcasts) to track back the origin of this cold bias. As such hindcasts are initialized close to observations, the transient drift leading to the cold bias can be analyzed to distinguish pre-existing errors from errors responding to initial ones. A time sequence of processes involved in the advent of the final mean state errors can then be proposed. We apply this strategy to the ENSEMBLES-FP6 project multi-model hindcasts of the last decades. Four of the five AOGCMs develop a persistent equatorial cold tongue bias within a few months. The associated systematic errors are first assessed separately for the warm and cold ENSO phases. We find that the models are able to reproduce either El Niño or La Niña close to observations, but not both. ENSO composites then show that the spurious equatorial cooling is maximum for El Niño years for the February and August start dates. For these events and at this time of the year, zonal wind errors in the equatorial Pacific are present from the beginning of the simulation and are hypothesized to be at the origin of the equatorial cold bias, generating too strong upwelling conditions. The systematic underestimation of the mixed layer depth in several models can also amplify the growth of the SST bias. The seminal role of these zonal wind errors is further demonstrated by carrying out ocean-only experiments forced by the AOCGCMs daily 10-meter wind. In a case study, we show that for several models, this forcing is sufficient to reproduce the main SST error patterns seen after 1 month in the AOCGCM hindcasts.
0930-7575
963-981
Vannière, Benoît
fd9185a0-60c2-4130-ad43-16d235d0b888
Guilyardi, Eric
7a596a3f-7c16-4d5b-8fa8-13848959694d
Madec, Gurvan
ffb28deb-4bbd-4a4c-914f-492f813e4864
Doblas-Reyes, Francisco J.
19217cac-40aa-40d6-8d55-75fd9c39cf6c
Woolnough, Steve
8498b5e9-7476-490b-a693-833460d9f2e6
Vannière, Benoît
fd9185a0-60c2-4130-ad43-16d235d0b888
Guilyardi, Eric
7a596a3f-7c16-4d5b-8fa8-13848959694d
Madec, Gurvan
ffb28deb-4bbd-4a4c-914f-492f813e4864
Doblas-Reyes, Francisco J.
19217cac-40aa-40d6-8d55-75fd9c39cf6c
Woolnough, Steve
8498b5e9-7476-490b-a693-833460d9f2e6

Vannière, Benoît, Guilyardi, Eric, Madec, Gurvan, Doblas-Reyes, Francisco J. and Woolnough, Steve (2013) Using seasonal hindcasts to understand the origin of the equatorial cold tongue bias in CGCMs and its impact on ENSO. Climate Dynamics, 40 (3-4), 963-981. (doi:10.1007/s00382-012-1429-6).

Record type: Article

Abstract

The cold equatorial SST bias in the tropical Pacific that is persistent in many coupled OAGCMs severely impacts the fidelity of the simulated climate and variability in this key region, such as the ENSO phenomenon. The classical bias analysis in these models usually concentrates on multi-decadal to centennial time series needed to obtain statistically robust features. Yet, this strategy cannot fully explain how the models errors were generated in the first place. Here, we use seasonal re-forecasts (hindcasts) to track back the origin of this cold bias. As such hindcasts are initialized close to observations, the transient drift leading to the cold bias can be analyzed to distinguish pre-existing errors from errors responding to initial ones. A time sequence of processes involved in the advent of the final mean state errors can then be proposed. We apply this strategy to the ENSEMBLES-FP6 project multi-model hindcasts of the last decades. Four of the five AOGCMs develop a persistent equatorial cold tongue bias within a few months. The associated systematic errors are first assessed separately for the warm and cold ENSO phases. We find that the models are able to reproduce either El Niño or La Niña close to observations, but not both. ENSO composites then show that the spurious equatorial cooling is maximum for El Niño years for the February and August start dates. For these events and at this time of the year, zonal wind errors in the equatorial Pacific are present from the beginning of the simulation and are hypothesized to be at the origin of the equatorial cold bias, generating too strong upwelling conditions. The systematic underestimation of the mixed layer depth in several models can also amplify the growth of the SST bias. The seminal role of these zonal wind errors is further demonstrated by carrying out ocean-only experiments forced by the AOCGCMs daily 10-meter wind. In a case study, we show that for several models, this forcing is sufficient to reproduce the main SST error patterns seen after 1 month in the AOCGCM hindcasts.

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More information

Published date: February 2013
Organisations: Marine Systems Modelling

Identifiers

Local EPrints ID: 362990
URI: http://eprints.soton.ac.uk/id/eprint/362990
ISSN: 0930-7575
PURE UUID: 9a9627d2-089d-4ac6-89d5-953ce9ceee60

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Date deposited: 10 Mar 2014 16:10
Last modified: 14 Mar 2024 16:17

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Contributors

Author: Benoît Vannière
Author: Eric Guilyardi
Author: Gurvan Madec
Author: Francisco J. Doblas-Reyes
Author: Steve Woolnough

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