Article
Natural drivers of interannual to decadal
variations in surface climate
Nicola Maher 1,2
1
Climate Change Research Centre, University of New South Wales, NSW, Australia.
2
ARC Centre of Excellence for Climate System Science, University of New South Wales, NSW, Australia.
email:
My thesis aims to understand drivers of fluctuations in surface
climate that could result in hiatus decades and periods of
accelerated warming, specifically focusing on the Indo-Pacific
region. I demonstrate the role of the Inter-decadal Pacific
Oscillation (IPO) and volcanic eruptions in driving hiatus
periods. I further identify a multi-model mean response of the
tropical Pacific Ocean to large volcanic eruptions, which may
act to further cool the surface ocean. Finally I demonstrate an
increase in Indo-Pacific heat content associated with the recent
hiatus, which is not reversible even when the IPO returns to its
positive phase.
1.
Drivers of decadal hiatus periods in the
20th and 21st Centuries
While the long term trend in surface air temperature (SAT) is
one of warming, there was no significant warming trend in SAT
over the period 2001-2014. Decades where the surface of the
Earth does not warm significantly are known as hiatus decades.
1
(a) The role of anthropogenic forcing
The early 2000s hiatus is not unique in the historical SAT record
or within climate model simulations.
I used SAT output from a single ensemble member of 31 climate
models taking part in Coupled Model Intercomparison Project 5
(CMIP5) (Maher et al., 2014). Here, I considered both the historical
(1860-2006) plus two future scenarios RCP4.5 (moderate
warming) and RCP8.5 (strong warming), which extend to 2100.
Hiatus periods are identified in three categories, (i) those due
to volcanic eruptions, (ii) those associated with negative phases
of the Interdecadal Pacific Oscillation (IPO), where accelerated
warming is associated with the IPO positive phase, and (iii)
those affected by anthropogenic aerosols in the mid 20th
Century. Figure 1a shows that the likelihood of hiatus periods
is sensitive to the rate of change of anthropogenic forcing,
when decades influenced by large tropical volcanic eruptions
are excluded. While I showed that large volcanic eruptions have
dramatically increased the likelihood of hiatus periods in the
historical record pre-1980 (77-93% likelihood), the background
warming has acted to partially offset the cooling effect of
(b) Probability of a hiatus occuring in the future
historical (non−volcanic)
historical (volcanic)
future projections
0.9
10
BAMOS
Jun 2018
0.8
RCP4.5
RCP8.5
RCP4.5 + volc (−3.3 W/m −2 )
RCP4.5 + volc (−1.5 W/m −2 )
RCP8.5 + volc (−3.3 W/m −2 )
RCP8.5 + volc (−1.5 W/m −2 )
0.7
0.6
probability = 50%
pro
0.5
0.4
0.3
0.2
0.1
0
0
0.2
0.4
0.6
2
Gradient in anthropogenic forcing (W/m /Year)
2010
2030
2050
Year
2070
2090
Figure 1. a) Probability of a hiatus period versus the gradient in anthropogenic forcing in CMIP5 models, b) probability of a hiatus in
two future scenarios, with and without hypothetical volcanic eruptions added in years 2032 and 2087.