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The role of the SST-thermocline relationship in Indian Ocean Dipole skewness and its response to global warming.

Ng B, Cai W, Walsh K - Sci Rep (2014)

Bottom Line: This asymmetric thermocline feedback drives IOD skewness despite positive IODs receiving greater damping from the SCR feedback.In response to global warming, although the thermocline feedback strengthens, its asymmetry between positive and negative IODs weakens.This behaviour change explains the reduction in IOD skewness that many models display under global warming.

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
A positive Indian Ocean Dipole (IOD) tends to have stronger cold sea surface temperature anomalies (SSTAs) over the eastern Indian Ocean with greater impacts than warm SSTAs that occur during its negative phase. Two feedbacks have been suggested as the cause of positive IOD skewness, a positive Bjerknes feedback and a negative SST-cloud-radiation (SCR) feedback, but their relative importance is debated. Using inter-model statistics, we show that the most important process for IOD skewness is an asymmetry in the thermocline feedback, whereby SSTAs respond to thermocline depth anomalies more strongly during the positive phase than negative phase. This asymmetric thermocline feedback drives IOD skewness despite positive IODs receiving greater damping from the SCR feedback. In response to global warming, although the thermocline feedback strengthens, its asymmetry between positive and negative IODs weakens. This behaviour change explains the reduction in IOD skewness that many models display under global warming.

No MeSH data available.


Related in: MedlinePlus

Mean state change over the tropical Indian Ocean.(a) Map showing the multi-model ensemble mean SON change (RCP8.5 minus historical) in SST (colours) and precipitation (contours, negative values are dashed). The green contours mark where the change in precipitation is significant at the 95% confidence level. The change in SST is significant everywhere at the 95% confidence level. (b) As in (a) but for thermocline depth (ZT, colours) and wind stress (vectors). Black vectors are where the zonal or meridional wind stress change is significant at the 95% confidence level. The green contours show where the ZT change is significant at the 95% confidence level. The significance for (a) and (b) is calculated using Student's two tailed t-test. (c) Scatter plot showing the relationship between the change in IODE SST skewness (RCP8.5 minus historical) and the ratio of mean thermocline depth for the RCP8.5 and historical simulations. All maps and plots were generated in NCL.
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f4: Mean state change over the tropical Indian Ocean.(a) Map showing the multi-model ensemble mean SON change (RCP8.5 minus historical) in SST (colours) and precipitation (contours, negative values are dashed). The green contours mark where the change in precipitation is significant at the 95% confidence level. The change in SST is significant everywhere at the 95% confidence level. (b) As in (a) but for thermocline depth (ZT, colours) and wind stress (vectors). Black vectors are where the zonal or meridional wind stress change is significant at the 95% confidence level. The green contours show where the ZT change is significant at the 95% confidence level. The significance for (a) and (b) is calculated using Student's two tailed t-test. (c) Scatter plot showing the relationship between the change in IODE SST skewness (RCP8.5 minus historical) and the ratio of mean thermocline depth for the RCP8.5 and historical simulations. All maps and plots were generated in NCL.

Mentions: Do the models exhibit this response, where the Walker circulation weakens due to increasing greenhouse gases? Most (18) of the 19 models display this behaviour with the eastern IO warming slower than the west, generating a pIOD-like pattern (Fig. 4a). Only the GFDL-ESM2G model shows an opposite change in SST, with slightly stronger warming over the eastern IO than the west (figure not shown). For the remaining models, this SST pattern leads to weaker westerly winds in RCP8.5 and thus, the difference between RCP8.5 and historical winds is easterly along the central equatorial IO. This leads to a shoaling of the mean IODE thermocline in the models. Figure 4b shows the MME mean change in thermocline depth and winds over the tropical IO. Strong shoaling over the IODE region is clear whilst in the western IO, the change due to global warming is much weaker and more varied. The change in precipitation can also be seen in Fig. 4a (contours) and a decrease (increase) in mean precipitation occurs over the eastern (western) IO. This is further evidence that the Walker circulation is weakening in the models, as the decrease in rainfall implies less convection occurring over the IODE region, which is where the rising branch is located. The importance of mean state change is highlighted in Fig. 4c, which shows the relationship between the change in IODE SST skewness and the RCP8.5/historical ratio of mean thermocline depth. The strong correlation implies that differences in skewness change are systematically related to changes in the mean depth of the thermocline. Smaller ratio values, which indicate greater shoaling, are associated with greater positive changes in IODE SST skewness (i.e., larger reductions in skewness).


The role of the SST-thermocline relationship in Indian Ocean Dipole skewness and its response to global warming.

Ng B, Cai W, Walsh K - Sci Rep (2014)

Mean state change over the tropical Indian Ocean.(a) Map showing the multi-model ensemble mean SON change (RCP8.5 minus historical) in SST (colours) and precipitation (contours, negative values are dashed). The green contours mark where the change in precipitation is significant at the 95% confidence level. The change in SST is significant everywhere at the 95% confidence level. (b) As in (a) but for thermocline depth (ZT, colours) and wind stress (vectors). Black vectors are where the zonal or meridional wind stress change is significant at the 95% confidence level. The green contours show where the ZT change is significant at the 95% confidence level. The significance for (a) and (b) is calculated using Student's two tailed t-test. (c) Scatter plot showing the relationship between the change in IODE SST skewness (RCP8.5 minus historical) and the ratio of mean thermocline depth for the RCP8.5 and historical simulations. All maps and plots were generated in NCL.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4129411&req=5

f4: Mean state change over the tropical Indian Ocean.(a) Map showing the multi-model ensemble mean SON change (RCP8.5 minus historical) in SST (colours) and precipitation (contours, negative values are dashed). The green contours mark where the change in precipitation is significant at the 95% confidence level. The change in SST is significant everywhere at the 95% confidence level. (b) As in (a) but for thermocline depth (ZT, colours) and wind stress (vectors). Black vectors are where the zonal or meridional wind stress change is significant at the 95% confidence level. The green contours show where the ZT change is significant at the 95% confidence level. The significance for (a) and (b) is calculated using Student's two tailed t-test. (c) Scatter plot showing the relationship between the change in IODE SST skewness (RCP8.5 minus historical) and the ratio of mean thermocline depth for the RCP8.5 and historical simulations. All maps and plots were generated in NCL.
Mentions: Do the models exhibit this response, where the Walker circulation weakens due to increasing greenhouse gases? Most (18) of the 19 models display this behaviour with the eastern IO warming slower than the west, generating a pIOD-like pattern (Fig. 4a). Only the GFDL-ESM2G model shows an opposite change in SST, with slightly stronger warming over the eastern IO than the west (figure not shown). For the remaining models, this SST pattern leads to weaker westerly winds in RCP8.5 and thus, the difference between RCP8.5 and historical winds is easterly along the central equatorial IO. This leads to a shoaling of the mean IODE thermocline in the models. Figure 4b shows the MME mean change in thermocline depth and winds over the tropical IO. Strong shoaling over the IODE region is clear whilst in the western IO, the change due to global warming is much weaker and more varied. The change in precipitation can also be seen in Fig. 4a (contours) and a decrease (increase) in mean precipitation occurs over the eastern (western) IO. This is further evidence that the Walker circulation is weakening in the models, as the decrease in rainfall implies less convection occurring over the IODE region, which is where the rising branch is located. The importance of mean state change is highlighted in Fig. 4c, which shows the relationship between the change in IODE SST skewness and the RCP8.5/historical ratio of mean thermocline depth. The strong correlation implies that differences in skewness change are systematically related to changes in the mean depth of the thermocline. Smaller ratio values, which indicate greater shoaling, are associated with greater positive changes in IODE SST skewness (i.e., larger reductions in skewness).

Bottom Line: This asymmetric thermocline feedback drives IOD skewness despite positive IODs receiving greater damping from the SCR feedback.In response to global warming, although the thermocline feedback strengthens, its asymmetry between positive and negative IODs weakens.This behaviour change explains the reduction in IOD skewness that many models display under global warming.

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
A positive Indian Ocean Dipole (IOD) tends to have stronger cold sea surface temperature anomalies (SSTAs) over the eastern Indian Ocean with greater impacts than warm SSTAs that occur during its negative phase. Two feedbacks have been suggested as the cause of positive IOD skewness, a positive Bjerknes feedback and a negative SST-cloud-radiation (SCR) feedback, but their relative importance is debated. Using inter-model statistics, we show that the most important process for IOD skewness is an asymmetry in the thermocline feedback, whereby SSTAs respond to thermocline depth anomalies more strongly during the positive phase than negative phase. This asymmetric thermocline feedback drives IOD skewness despite positive IODs receiving greater damping from the SCR feedback. In response to global warming, although the thermocline feedback strengthens, its asymmetry between positive and negative IODs weakens. This behaviour change explains the reduction in IOD skewness that many models display under global warming.

No MeSH data available.


Related in: MedlinePlus