BAMOS Vol 31 No.2 June 2018 | Page 11

BAMOS Jun 2018
11
Global Cooling
El Niño
La Niña
recent eruptions such as Pinatubo in 1991. Future scenarios with adjustments made to include the effect of volcanic forcing show an extremely high chance of a hiatus in the RCP4.5 scenario at the end of the Century in the event of a volcanic eruption( Figure 1b). In contrast, there is virtually no chance of a hiatus decade in the RCP8.5 scenario at the end of the Century( Figure 1b). As such, if we follow a high emissions trajectory, the occurrence of a hiatus period becomes very unlikely. Importantly, we subsequently found that there was no significant shift in the projected end of century multi-model mean warming when considering a subset of models that suitably captured the hiatus, suggesting that decadal scale hiatus periods have negligible influence on long term warming projections( England et al., 2015).
-ve IOD
La Niña
+ ve IOD
El Niño
2. Effects of volcanism on tropical variability
La Niña El Niño
Volcanic eruptions have been shown to influence SAT and other climate variables. SAT cools rapidly in the first 1-3 years after the eruption, returning to the post-volcanic value approximately 6-7 years after the eruption. The low number of large volcanic eruptions( n = 5) in the observational record precludes the separation of the volcanic forced response from internal variability and other forcings. Here, I evaluated 122 historical ensemble members( from 31 CMIP5 models) to quantify the probability for a particular tropical response occurring after large tropical volcanic eruptions.
Figure 2. Hovmöller plots averaged between 50N to 50S of A) SST( o C), B) SSH( m) and C) zonal wind stress anomalies( N / m 2)( relative to 5 years before the eruption) over the Indian and Pacific Oceans. The black line at time zero is the eruption peak, the dashed black line around t =-1year is the average eruption start time.
In this study I used the novel approach of assessing tropical Pacific variability using sea surface height( SSH) as well as sea surface temperature( SST)( Maher et al., 2015). Here, I was able to separate the volcanic cooling response from the dynamic forcing response to the eruption. I proposed that a volcanic eruption causes global cooling that increases the likelihood of an El Niñolike response in the Pacific Ocean and a co-occurring positive Indian Ocean Dipole( IOD) event in the Indian Ocean( Figure 2a, b) associated with a weakening of the trade winds( Figure 2c). There is also an increased likelihood of a La Niña pattern occurring in the third austral summer post-eruption( i. e. 14 months later than the peak of the El Niño), which is enhanced in the SST field due to volcanic cooling( Figure 2b) and associated with a strengthening of the trade winds( Figure 2c). This signal, coherent with a weak negative IOD may enhance the persistence of post-volcanic cooling seen in CMIP5. This result has implications for hindcast skill, which can be significantly reduced if the internal variability of an ENSO event does not match the multi-model mean response( Meehl et al., 2015).