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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

Nonlinear zonal advection during extreme and moderate pIOD events.(a) Map showing the multi-model ensemble mean historical SON nonlinear zonal advection term during extreme pIODs. (b) As in (a) but for moderate pIODs. (c), (d) As in (a), (b) respectively but for the RCP8.5 period. The dashed black box in (a) marks the equatorial IO region (60°E-100°E, 5°S-5°N) and the green contours in (c) and (d) denote where the difference between the historical and RCP8.5 periods is significant at the 95% confidence level. The significance is calculated using a two-tailed Student’s t-test. All maps were generated in NCL.
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f3: Nonlinear zonal advection during extreme and moderate pIOD events.(a) Map showing the multi-model ensemble mean historical SON nonlinear zonal advection term during extreme pIODs. (b) As in (a) but for moderate pIODs. (c), (d) As in (a), (b) respectively but for the RCP8.5 period. The dashed black box in (a) marks the equatorial IO region (60°E-100°E, 5°S-5°N) and the green contours in (c) and (d) denote where the difference between the historical and RCP8.5 periods is significant at the 95% confidence level. The significance is calculated using a two-tailed Student’s t-test. All maps were generated in NCL.

Mentions: Figure 3a shows the multi-model ensemble grid-point nonlinear zonal advection for historical extreme pIOD events. Compared with Fig. 3c, it is clear that under global warming, there is no significant change in the nonlinear zonal advection term over the equatorial IO (60°E-100°E, 5°S-5°N). However, the importance of this term for the development of extreme pIODs can be seen by comparing it with that of moderate pIODs (Figs. 3b,d). For extreme pIOD events, nonlinear zonal advection induces cooling in the western and central equatorial IO (between 50°E-90°E). This allows the cold SSTA in the east to extend further westward (Figs. S2a,c), generating anomalously dry conditions along the equator and pushing the centre of convection further west, in contrast to the conditions during moderate events (Figs. 3b, S2b). As this occurs, a positive feedback process forms where the westward shift in convection drives a stronger anomalous negative zonal temperature gradient and anomalous easterly zonal winds8. These easterly zonal wind anomalies reduce the mean wind along the equator, generating upwelling Kelvin waves that strengthen the cooling in the eastern IO while the thermocline deepens in the west8. Consequently, the nonlinear zonal advection process is reinforced and this is conducive for extreme pIOD growth.


Nonlinear processes reinforce extreme Indian Ocean Dipole events.

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

Nonlinear zonal advection during extreme and moderate pIOD events.(a) Map showing the multi-model ensemble mean historical SON nonlinear zonal advection term during extreme pIODs. (b) As in (a) but for moderate pIODs. (c), (d) As in (a), (b) respectively but for the RCP8.5 period. The dashed black box in (a) marks the equatorial IO region (60°E-100°E, 5°S-5°N) and the green contours in (c) and (d) denote where the difference between the historical and RCP8.5 periods is significant at the 95% confidence level. The significance is calculated using a two-tailed Student’s t-test. All maps were generated in NCL.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: Nonlinear zonal advection during extreme and moderate pIOD events.(a) Map showing the multi-model ensemble mean historical SON nonlinear zonal advection term during extreme pIODs. (b) As in (a) but for moderate pIODs. (c), (d) As in (a), (b) respectively but for the RCP8.5 period. The dashed black box in (a) marks the equatorial IO region (60°E-100°E, 5°S-5°N) and the green contours in (c) and (d) denote where the difference between the historical and RCP8.5 periods is significant at the 95% confidence level. The significance is calculated using a two-tailed Student’s t-test. All maps were generated in NCL.
Mentions: Figure 3a shows the multi-model ensemble grid-point nonlinear zonal advection for historical extreme pIOD events. Compared with Fig. 3c, it is clear that under global warming, there is no significant change in the nonlinear zonal advection term over the equatorial IO (60°E-100°E, 5°S-5°N). However, the importance of this term for the development of extreme pIODs can be seen by comparing it with that of moderate pIODs (Figs. 3b,d). For extreme pIOD events, nonlinear zonal advection induces cooling in the western and central equatorial IO (between 50°E-90°E). This allows the cold SSTA in the east to extend further westward (Figs. S2a,c), generating anomalously dry conditions along the equator and pushing the centre of convection further west, in contrast to the conditions during moderate events (Figs. 3b, S2b). As this occurs, a positive feedback process forms where the westward shift in convection drives a stronger anomalous negative zonal temperature gradient and anomalous easterly zonal winds8. These easterly zonal wind anomalies reduce the mean wind along the equator, generating upwelling Kelvin waves that strengthen the cooling in the eastern IO while the thermocline deepens in the west8. Consequently, the nonlinear zonal advection process is reinforced and this is conducive for extreme pIOD growth.

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