12
BAMOS
Jun 2018
ITF
EUC
STC
Indian Ocean
STC
20 o N
Pacific Ocean
20 o S
Figure 3. Schematic of temperature and circulation trends over the period 1992–2011. Arrows and
annotations show the changes in the circulation, which are found to drive the temperature changes.
Note on acronyms: Indonesian through-flow (ITF), Equatorial Undercurrent (EUC) and Subtropical cells (STC).
3. Role of Pacific trade winds in driving ocean
temperatures
In the final section of this PhD I applied linear trends in all
atmospheric fields over the period 1992-2011 in the Pacific
in a globally forced ocean model to isolate the role of Pacific
atmospheric forcing, especially the acceleration of the trade
winds in driving the recent hiatus and the ocean processes
involved (Maher et al., Submitted). I showed how warming
trends in the subsurface western Pacific, a cooling surface
eastern Pacific and warming in the subsurface Indian Ocean
can be associated with an acceleration of the Pacific shallow
overturning cells (PSOC) and Indonesian through-flow (ITF)
(Figure 3). This result compares well with observed temperature
trends over the period 2003-2011 (post-Argo observing era).
Sensitivity experiments showed that trends in the atmospheric
Pacific winds in isolation can explain most of the subsurface
Indian Ocean response. I showed that the oceanic response
does not just involve a redistribution of heat in the Pacific, but
also a net increase in global heat content, which occurs in the
Indian and Pacific basins.
A symmetric reversal of the atmospheric trends revealed that the
heat content changes were not fully reversible, with the Indo-
Pacific having a net increase over the course of the combined
experiments. While the PSOC response was largely symmetric,
there was a clear asymmetry in the heat flux associated with the
ITF and in the surface heat fluxes. Additionally heat redistributed
to the subsurface Pacific Ocean was partially transported to the
Indian Ocean and mixed below the thermocline, two processes
which are not reversible. Here I suggested that a reversal in the
IPO phase will not result in a complete reversal in the ocean
heat content, even if this reversal were perfectly symmetric.
4. Conclusions
My thesis has investigated the role of both Pacific variability
and volcanism in driving changes in surface climate. In the first
section I have shown the role of both large tropical volcanic
eruptions and a transition to a negative phase of the IPO in
driving decades of cooling, otherwise known as hiatus decades.
The second section uses the novel method of assessing
Pacific Variability using SSH. I outline the role of large tropical
eruptions in increasing the probability of a El Niño-like response
and a co-occurring positive IOD event followed by an increased
likelihood of a La Niña, which may enhance the persistence of
post-volcanic cooling. Finally, I have shown that a strengthening
of the trade winds corresponding to observed strengthening
associated with a negative IPO over the period 1992-2011 drives
heat into the subsurface Pacific and Indian Oceans associated
with a strengthening of the PSOC and ITF. This increase in heat is
not reversible, even if the wind returns to its initial state.
References
Donat, M. G., A. L. Lowry, L. V. Alexander, P. A. O’Gorman, and
N. Maher (2016), More extreme precipitation in the world’s
dry and wet regions, Nature Climate Change, p. doi:10.1038/
nclimate2941.
England, M. H., J. B. Kajtar, and N. Maher (2015), Robust warming
projections despite the recent hiatus, Nature Climate Change, 5,
394–396.
Maher, N., A. S. Gupta, and M. H. England (2014), Drivers
of decadal hiatus periods in the 20th and 21st centuries,
Geophysical Research Letters, 41, 5978–5986.
Maher, N., S. McGregor, M. H. England, and A. Sen Gupta (2015),
Effects of volcanism on tropical variability, Geophysical Research
Letters, 42 (14), 6024–6033.
Maher, N., E. M.H., A. Sen Gupta, and P. Spence (Submitted), Role
of Pacific trade winds in driving ocean temperatures during the
recent hiatus and projections for a wind trend reversal, Climate
Dynamics.
Meehl, G. A., H. Teng, N. Maher, and M. H. England (2015), Effects
of the Mount Pinatubo eruption on decadal climate prediction
skill of Pacific sea surface temperatures, Geophysical Research
Letters, 42 (24), 10,840–10,846.