Climate Change During Intervals Of The Past Millennium In The Southwestern Tropical Pacific

Loading...
Thumbnail Image

Files

Publication or External Link

Date

2018

Citation

Abstract

Limited observations from the tropical Pacific over the past millennium make it

difficult to assess whether different time periods had significant variations

in El Nino-Southern Oscillation (ENSO) amplitude and frequency. Composited

simulation results from climate models participating in the Paleoclimate

Modeling Intercomparison Project suggest no difference in statistical variance

and ENSO event frequency for the Medieval Climate Anomaly (MCA), Little Ice Age

(LIA), and the modern Industrial Era. ENSO may not be sensitive to external

radiative forcings. Unforced variability arising from the coupled

ocean-atmosphere system could explain the observed past millennium results.

New coral geochemical measurements were collected from Aitutaki, southern Cook Islands

in the southwestern tropical Pacific and composited with existing coral

geochemical observations from Rarotonga to increase the temporal coverage of

climate data over the past millennium. Forward modelling of coral oxygen isotopes as a

function of sea surface temperature and the oxygen isotopic composition of seawater suggests this location

is sensitive to interannual variations in the position of the South Pacific

Convergence Zone (SPCZ) driven by ENSO activity. Analysis of observed

interannual \delo\ indicates interannual variations are driven primarily by sea

surface salinity but also sea surface temperature forcings. More negative

(positive) coral oxygen isotope results indicate warmer/wetter (cooler/dryer)

conditions that occur at Aitutaki when La Nina (El Nino) events

redistribute the South Pacific Convergence Zone away (towards) the equator.

Spatial correlation of the coral \delo\ signal with regional and tropical

climate variables support the interpretation that Aitutaki coral \delo\ varies

according to changes in the SPCZ and ENSO activity. Results from modern

Aitutaki coral oxygen isotopes may be used to interpret coral data collected from

earlier periods of time.

Paired coral oxygen isotopes and Sr/Ca measurements were made on diagenetically-screened

samples radiometrically dated to the Medieval Climate Anomaly. These results,

used to calculate interannual oxygen isotopic composition of seawater anomalies, show higher statistical

variance in the fossil record relative to the modern Aitutaki/Rarotonga

composite record. Singular spectrum analysis shows the first ten reconstructed

components explain 79-86% of the variability in the timeseries. Composited

interannual frequency (2-10 year period) components show variable oxygen isotopic composition of seawater

throughout the MCA suggesting an active ENSO period. Large variations of 0.6

permil in calculated oxygen isotopic composition of seawater suggest potential decadal shifts in oxygen isotopes

from warmer/wetter to cooler/dryer conditions. Long term trends in calculated

oxygen isotopic composition of seawater during the earlier MCA from more negative to more positive values

suggest a transition from warmer/wetter to cooler/dryer conditions. Together,

the results suggest a highly variable MCA period relative to the modern period.

This new data may be used in conjunction with other observations for data/model

comparisons to better understand hydroclimate variability over the past

millennium.

Notes

Rights