Palaeoclimate Update

Placing the 14-year ‘Big Dry’ in a long-term context

Recent decadal-scale drying – is it unusual in a longer-term context?– Only have 100 years of instrumental data– Difficult to assess decadal-scale variability using short time

series

?

SEA rainfall reconstruction pilot study (1783–1988)

• Pilot analysis using 12 well-dated, annually resolved records from Australasia with published climate sensitivity (largely ENSO) to develop an initial rainfall reconstruction for SE Australia

• Locations sensitive to large-scale climate modes: El Niño–Southern Oscillation, Indian Ocean variability, Southern Annular Mode

Corals

Trees

Ice cores

Southeast Australian rainfall signal in proxies

Locations show responses to SEA rainfall variability at annual and/or decadal scales

Interannual correlation Interdecadal correlation

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Using remote climate proxies

• Virtual no proxies are available from SEA (except Tasmanian tree rings)• Using remote teleconnections to estimate regional climate variability• Can a common climate signal can be extracted that is representative of

variations in southeast Australian rainfall?

Location verification using GHCN network

Can we reconstruct southeast Australian rainfall using an observational network?

Used the Global Historical Climate Network (GHCN) of observational stations

Closest possible locations to proxy data

30 years of data only

Responses from different variables– temperature, precipitation, pressure and sea-

surface temperatures

GHCN network and southeast Australian rainfall

Principal Component Analysis extracts the common signal in the GHCN network of ‘pseudo-proxies’

ENSO signal?

Strongest signal in southeast Australia – ENSO and other influences?

May – April year has strongest signal with southeast Australian rainfall

Reconstructing southeast Australian rainfall from remote observations

SEA rainfall variance explained by the remote observational network (dashed) and the palaeoclimate network (solid)

47.2%

85.4%

Annual

Decadal

GHCN versus Palaeo-proxies

Apply the same technique (PCA) to the palaeo-proxy network

Similar common signal is extracted to the GHCN network

GHCN observations

Palaeo records

Contributions from proxies to common signal

First three principal components (41% of palaeo network’s covariance) were significantly correlated with at least 20% of southeast Australian region

Projecting the signal onto SEA rainfall

Multiple linear regression to project the common signals onto southeast Australian rainfall

Independent ‘calibration’ and ‘verification’ periods to determine regression coefficients (αn)

α1x1 + α2x2 + α3x3

x1

x2

x3

rainfall =

Sensitivity of ‘calibration’ and ‘verification’

Model skill sensitive to choice of calibration and verification period

Statistic Definition

ar2 Variance explained adjusted for degrees of freedom

r Correlation

RE Reduction of Error

CE Coefficient of Efficiency

ST Sign Test

RMSE Root Mean Square Error

Skill on annual time scales

Estimate spread of model skill – better estimate of ‘truth’

Bootstrap resampling to generate (10 000 iterations):

– select randomised decades for calibration– remaining years used for model verification

Skill of the reconstruction ensemble

rcalibration ar2calibration rverification RE CE ST RMSE

0.85 ± 0.15

0.70 ± 0.25

0.85 ± 0.19

0.73 ± 0.32

0.66 ± 0.52

80% ± 20%

0.56σ ± 0.19σ

Statistic Definition

ar2 Correlation adjusted for degrees of freedom

r Correlation

RE Reduction of Error

CE Coefficient of Efficiency

ST Sign Test

RMSE Root Mean Square Error

Median of the reconstruction ensemble captures 72% of decadal variations in instrumental rainfall

?

How does the ‘Big Dry’ (1998–present) compare to rainfall estimations since 1783?

According to our reconstruction there is a 97% chance that the 1998–2008 rainfall deficit is the lowest since European settlement

2.9%

Unusual happenings: 1820s and 1830s

Independent records from Lake George report maxima in lake levels during 1820s– ‘A magnificent sheet of

water’ (H. C. Russell, 1821)

– ‘20 miles long and 8 miles wide’ (H. C. Russell, 1824)

Inter-decadal Pacific Oscillation (IPO)

• Decadal-scale variability in the Pacific Ocean primary modulator of east Australian rainfall

• Inter-decadal Pacific Oscillation: ‘Decadal-scale ENSO-like oscillations’ (Zhang et al., 1997)

• Positive phase: increased El Niño frequency and lower rainfall

• Negative phase: increased La Niña frequency and higher rainfall

Correlations (1900–1989) between the IPO and;• Instrumental southeast Australian rainfall = -0.77• Reconstructed southeast Australian rainfall = -0.64

Decadal variations in SEA rainfall: 1783–1989

Observed area-averaged rainfall from nine high-quality stations (1873–2006)

Observed area-averaged rainfall from spatially complete grids (1900–2009)Median of rainfall reconstruction ensemble

Low-frequency variations in reconstructed ENSO

Can use low-frequency variations in reconstructed ENSO and look at links with southeast Australian rainfall

McGregor et al. (2010) looked for a common signal in a number of ENSO reconstructions – proxy for decadal-scale variations in the Pacific

Interdecadal Pacific variability and southeast Australian rainfall: 1793–1970

• Breakdown is associated with pronounced wet period 1818–1833 (peaking at ~100mm above C20th average) is a real climate signal not a data issue (tested individual proxies, combinations and independent ENSO reconstructions)

• Pre 1840: early 1800s cooling (1oC colder than present), and period of high tropical volcanism e.g. 1816 Tambora eruption ‘year without a summer’…opportunity for pre-industrial detection and attribution studies

Summary

• Remote climate data represents common process, responsible for southeast Australian rainfall variations

• A network of 12 remote proxies can capture around 72% of the variations in decadal rainfall variations during 20th Century

• Bootstrapping provides better uncertainty estimates and more robust measure of skill of reconstruction

• Evidence of large decadal variations in southeast Australian rainfall

• Variations associated with ENSO-like interdecadal variability in Pacific Ocean

• Breakdown of teleconnection pre-1840s? Abnormally large rainfall variations compared to the rest of record

• 97% chance that the 1998–2008 rainfall deficit is the worst since European settlement

PAGES regional 2K network

- Global effort to consolidate regional palaeo data for IPCC AR5

- Australasia ‘Aus2K’ component: important area of southern mid-latitudes: ENSO, Southern Annual Mode (SAM), Indian Ocean Dipole (IOD)

- In June 2010 the 1st Aus2k workshop held at UniMelb, 70 attendees

Source: http://www.pages-igbp.org/index.html

Palaeoclimatology of the last 2,000 years

- The last 2K contains a period of marked temperature fluctuations: Medieval Warm Period, Little Ice Age and ~1870s transition into the industrial era: period contains solar, volcanic and anthropogenic forcing that models can assess climate sensitivity

- Period where we have the highest confidence in the palaeoclimate record e.g. more records, calendar dates

- Complementary aspects of climate system (e.g; marine vs. terrestrial, tropics vs. mid-latitudes) are needed to understand global and regional climate variability: major gaps in the Southern Hemisphere, particularly in mid-latitude regions across Australasia

- Comparing multiple records reduces uncertainty by focusing on large-scale forcing represented by as co-variability in multi-variate studies

IPCC (2007)

Availability of SH annually-resolved palaeoclimate records: an update

- New records identified/released for Aus2k synthesis effort: map currently only shows annually-resolved records, no Australian and NZ sedimentary records included yet

- Currently 178 records from Southern Hemisphere, 48 from Australasia: 14 extend back to the year 1000- First step is to develop a PAGES metadata ‘inventory’ of records, database of quantitative data is needed for climate analysis

- Dept Climate Change Eng Eff (DCCEE) funds to develop Aus2K reconstructions for IPCC AR5

Neukom and Gergis, in prep

DCCEE funding deliverables

1. Online database of annual–decadal Australasian palaeoclimate records covering last 2,000 years for climate and environmental change analyses

2. Peer-reviewed journal publications submitted in line with IPCC mid-2012 deadline:

(i) Proxy review: assessment of database, including quality of proxies

(ii) Australasian mean temperature reconstruction

(iii) Northern and Southern Australian rainfall reconstruction

3. Australasian temperature and rainfall reconstructions lodged in international databanks: NOAA World Data Centre for Palaeoclimatology

4. Policy briefing information sheets with key findings

5. Material for Aus2K book chapter for PAGES regional 2K synthesis (2013)

Review of Southern Hemisphere palaeo records

Neukom and Gergis, in prep for The Holocene

- Review paper assessing all annually-resolved Southern Hemisphere palaeoclimate records with Raphael Neukom (Swiss NSF post-doc, UniMelb/ LDEO Univeristy of Columbia)

- Correlation maps for Temp, Rainfall, MSLP, SSTs for Australia, Australasia and SH

- Testing seasonal and lag sensitivities

- WA tree rings vs. rainfall (summer and winter half year signals)

Reassessing proxy climate sensitivity

- WA tree rings published based on rainfall sensitivity but clearly responding to winter temp

- Assess all SH proxies for potential to develop Australian reconstructions

- Determine rainfall, temperature and ‘both’ proxy subsets for climate reconstruction

- Plan to develop Australasian temperature spatial field reconstruction next (extract regional index for SEA) in collaboration with Raphael Neukom (South America 2K group)

Neukom and Gergis, in prep for The Holocene