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Nonlinear processes reinforce extreme Indian Ocean Dipole events.

Ng B, Cai W, Walsh K, Santoso A - Sci Rep (2015)

Bottom Line: Under greenhouse warming, these nonlinear processes do not change significantly in amplitude, but the frequency of occurrences surpassing a threshold increases.As such, the magnitude of SSTAs required to shift convection westward is relatively smaller, allowing these convection shifts to occur more frequently in the future.The associated changes in wind and ocean current anomalies support the zonal and vertical advection terms in a positive feedback process and consequently, moderate pIODs become more extreme-like.

View Article: PubMed Central - PubMed

Affiliation: 1] CSIRO Marine and Atmospheric Research, Aspendale, Victoria, Australia [2] School of Earth Sciences, University of Melbourne, Parkville, Victoria, Australia.

ABSTRACT
Under global warming, climate models show an almost three-fold increase in extreme positive Indian Ocean Dipole (pIOD) events by 2100. These extreme pIODs are characterised by a westward extension of cold sea surface temperature anomalies (SSTAs) which push the downstream atmospheric convergence further west. This induces severe drought and flooding in the surrounding countries, but the processes involved in this projected increase have not been fully examined. Here we conduct a detailed heat budget analysis of 19 models from phase 5 of the Coupled Model Intercomparison Project and show that nonlinear zonal and vertical heat advection are important for reinforcing extreme pIODs. Under greenhouse warming, these nonlinear processes do not change significantly in amplitude, but the frequency of occurrences surpassing a threshold increases. This is due to the projected weakening of the Walker circulation, which leads to the western tropical Indian Ocean warming faster than the east. As such, the magnitude of SSTAs required to shift convection westward is relatively smaller, allowing these convection shifts to occur more frequently in the future. The associated changes in wind and ocean current anomalies support the zonal and vertical advection terms in a positive feedback process and consequently, moderate pIODs become more extreme-like.

No MeSH data available.


Related in: MedlinePlus

Heat budget analysis of extreme and moderate pIOD events in CMIP5 models.(a) Multi-model ensemble averaged historical (1911–2005) SON heat budget components for extreme pIODs. (b) As in (a) but for moderate pIODs. Moderate pIODs are when the Dipole Mode Index is greater than 0.75 standard deviations but not an extreme event. (c), (d) As in (a), (b) respectively but for the RCP8.5 period (2006–2100). The error bars indicate 1 standard deviation of the multi-model ensemble. The heat budget components are averaged over the equatorial IO (60°E-100°E, 5°S-5°N). Table S2 provides a description of each heat budget term. All plots were generated in NCL.
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f1: Heat budget analysis of extreme and moderate pIOD events in CMIP5 models.(a) Multi-model ensemble averaged historical (1911–2005) SON heat budget components for extreme pIODs. (b) As in (a) but for moderate pIODs. Moderate pIODs are when the Dipole Mode Index is greater than 0.75 standard deviations but not an extreme event. (c), (d) As in (a), (b) respectively but for the RCP8.5 period (2006–2100). The error bars indicate 1 standard deviation of the multi-model ensemble. The heat budget components are averaged over the equatorial IO (60°E-100°E, 5°S-5°N). Table S2 provides a description of each heat budget term. All plots were generated in NCL.

Mentions: Heat budget analysis of 19 models (see Table S1) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) reveals that nonlinear zonal and vertical advection dominate during historical extreme pIODs (Fig. 1a; for definitions of the individual terms, see the Heat budget analysis section in the Methodology and Table S2). For non-extreme pIODs, the magnitude of these two nonlinear advection terms is substantially weaker, almost one-third that of extreme events. The other heat budget terms are also weaker due to the reduced forcing but the dominance of the nonlinear zonal and vertical advection shows that they still play an important role in generating and sustaining moderate pIODs (Fig. 1b). The strong influence of nonlinear zonal and vertical advection during extreme pIOD events is also seen in data from the European Centre for Medium-Range Weather Forecasts – Ocean Reanalysis System 3 (ECMWF ORA-S3; Fig. S1), suggesting that the CMIP5 models are able to capture the dynamics associated with extreme events. Under increasing greenhouse gases, nonlinear zonal and vertical advection remain important contributors to the cooling of the equatorial IO during extreme pIODs (Fig. 1c) but their amplitude does not change noticeably from the historical simulation. In contrast, moderate pIODs exhibit a distinct increase in amplitude for the two terms (Fig. 1d).


Nonlinear processes reinforce extreme Indian Ocean Dipole events.

Ng B, Cai W, Walsh K, Santoso A - Sci Rep (2015)

Heat budget analysis of extreme and moderate pIOD events in CMIP5 models.(a) Multi-model ensemble averaged historical (1911–2005) SON heat budget components for extreme pIODs. (b) As in (a) but for moderate pIODs. Moderate pIODs are when the Dipole Mode Index is greater than 0.75 standard deviations but not an extreme event. (c), (d) As in (a), (b) respectively but for the RCP8.5 period (2006–2100). The error bars indicate 1 standard deviation of the multi-model ensemble. The heat budget components are averaged over the equatorial IO (60°E-100°E, 5°S-5°N). Table S2 provides a description of each heat budget term. All plots were generated in NCL.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4481856&req=5

f1: Heat budget analysis of extreme and moderate pIOD events in CMIP5 models.(a) Multi-model ensemble averaged historical (1911–2005) SON heat budget components for extreme pIODs. (b) As in (a) but for moderate pIODs. Moderate pIODs are when the Dipole Mode Index is greater than 0.75 standard deviations but not an extreme event. (c), (d) As in (a), (b) respectively but for the RCP8.5 period (2006–2100). The error bars indicate 1 standard deviation of the multi-model ensemble. The heat budget components are averaged over the equatorial IO (60°E-100°E, 5°S-5°N). Table S2 provides a description of each heat budget term. All plots were generated in NCL.
Mentions: Heat budget analysis of 19 models (see Table S1) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) reveals that nonlinear zonal and vertical advection dominate during historical extreme pIODs (Fig. 1a; for definitions of the individual terms, see the Heat budget analysis section in the Methodology and Table S2). For non-extreme pIODs, the magnitude of these two nonlinear advection terms is substantially weaker, almost one-third that of extreme events. The other heat budget terms are also weaker due to the reduced forcing but the dominance of the nonlinear zonal and vertical advection shows that they still play an important role in generating and sustaining moderate pIODs (Fig. 1b). The strong influence of nonlinear zonal and vertical advection during extreme pIOD events is also seen in data from the European Centre for Medium-Range Weather Forecasts – Ocean Reanalysis System 3 (ECMWF ORA-S3; Fig. S1), suggesting that the CMIP5 models are able to capture the dynamics associated with extreme events. Under increasing greenhouse gases, nonlinear zonal and vertical advection remain important contributors to the cooling of the equatorial IO during extreme pIODs (Fig. 1c) but their amplitude does not change noticeably from the historical simulation. In contrast, moderate pIODs exhibit a distinct increase in amplitude for the two terms (Fig. 1d).

Bottom Line: Under greenhouse warming, these nonlinear processes do not change significantly in amplitude, but the frequency of occurrences surpassing a threshold increases.As such, the magnitude of SSTAs required to shift convection westward is relatively smaller, allowing these convection shifts to occur more frequently in the future.The associated changes in wind and ocean current anomalies support the zonal and vertical advection terms in a positive feedback process and consequently, moderate pIODs become more extreme-like.

View Article: PubMed Central - PubMed

Affiliation: 1] CSIRO Marine and Atmospheric Research, Aspendale, Victoria, Australia [2] School of Earth Sciences, University of Melbourne, Parkville, Victoria, Australia.

ABSTRACT
Under global warming, climate models show an almost three-fold increase in extreme positive Indian Ocean Dipole (pIOD) events by 2100. These extreme pIODs are characterised by a westward extension of cold sea surface temperature anomalies (SSTAs) which push the downstream atmospheric convergence further west. This induces severe drought and flooding in the surrounding countries, but the processes involved in this projected increase have not been fully examined. Here we conduct a detailed heat budget analysis of 19 models from phase 5 of the Coupled Model Intercomparison Project and show that nonlinear zonal and vertical heat advection are important for reinforcing extreme pIODs. Under greenhouse warming, these nonlinear processes do not change significantly in amplitude, but the frequency of occurrences surpassing a threshold increases. This is due to the projected weakening of the Walker circulation, which leads to the western tropical Indian Ocean warming faster than the east. As such, the magnitude of SSTAs required to shift convection westward is relatively smaller, allowing these convection shifts to occur more frequently in the future. The associated changes in wind and ocean current anomalies support the zonal and vertical advection terms in a positive feedback process and consequently, moderate pIODs become more extreme-like.

No MeSH data available.


Related in: MedlinePlus