Intan Suci Nurhati, Ph.D.     
 

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Coral Records of 20th Century Tropical Pacific Climate
 

This research constitutes my Ph.D. dissertation at Georgia Tech with Dr. Kim Cobb, on investigating how anthropogenic climate change may impact tropical Pacific climate. Tropical Pacific climate variability such as the interannual El Nino-Southern Oscillaiton (ENSO) strongly regulates global temperature and rainfall patterns. Global Climate Model (GCMs) have provide differing results on projecting the character of tropical Pacific changes under greenhouse warming (see Vecchi et al. 2008). Studies on quantifying tropical Pacific changes using instrumental datasets has been challenged by the sparse availability of long, continuous climate records from the tropical Pacific (see Deser et al. 2010). Therefore here our approach is to reconstruct climate proxy records derived from coral skeletal geochemistry. A better understanding of tropical Pacific climate variability and its secular trend under greenhouse forcing is crucial in formulating adaptation strategies for climate change.


Kim Cobb & Jordan Watson retrieving coral core from Fanning Island, central tropical Pacific.


Scientific Motivation: A Trend Towards Warmer and/or More Precipitation over the 20th Century?
 

Coral oxygen isotopic (d18O) records from the central tropical Pacific exhibit late 20th century trends towards depleted coral d18O, suggesting that warming and/or freshening have occurred in the region (Evans et al. 1999; Urban et al. 2000; Cobb et al. 2001). The fact that this negative coral d18O trend is unprecedented in the last millennium (Fig.1) strongly suggests that anthropogenic climate forcing has caused appreciable warming and/or freshening in this region. 

In order to better understand the mechanism underlying this trend, I quantify the role of sea-surface temperature (SST) and salinity changes via paired measurements of coral d18O (a dual proxy for SST and salinity) and Sr/Ca ratio (as a SST proxy). I derive the values of d18O of seawater (d18Osw) as a salinity proxy in the central tropical Pacific by subtracting the SST contribution on coral d18O values.


FIG. 1. Modern and fossil coral d18O records from Palmyra Island (6N, 162W) spanning the last millennium. A late-twentieth-century trend toward lower coral d18O values is indicative of recent warming and/or freshening in the central tropical Pacific (modified after Cobb et al. 2003).

 


Spatial Study of Late 20th century (1972-1998) Tropical Pacific Climate Change


In Nurhati et al. (2009), I
reconstructed the histories of central tropical Pacific SST and salinity using corals from Palmyra, Fanning and Christmas Islands (2N-6N, 157W-162W) over the late 20th century (1972-1998). Collectively, the three islands are sensitive to ENSO and decadal-scale tropical Pacific climate variability. The islands span SST and salinity gradients due their relative distances to equatorial ocean currents and the Inter-Tropical Convergence Zone (ITCZ).

The late 20th century portion of this work shows warming and freshening trends in the central tropical Pacific, in line with climate model projections of weakened tropical Pacific zonal SST gradient (FIg. 2). Stronger warming trends towards the equator are consistent with a weakening of the equatorial upwelling inferred from instrumental data [McPhaden and Zhang, 1999]. Stronger freshening trends towards Palmyra suggests a strengthening and/or equatorward shift of the ITCZ that may support the enhancement of hydrological patterns (wet area is getting wetter, and dry area is getting drier) under global warming as projected by a majority of IPCC climate models [Held and Soden, 2000]. See Nurhati et al. (2009) for more details on scientific questions and findings.
 

FIG. 2. Central tropical Pacific coral climate proxy records. (a) A millennium-long coral d18O reconstruction from Palmyra Island (Cobb et al. 2003) showing the unprecedented late 20th century trend towards lower coral d18O values (warmer, wetter conditions; note inverted y-axis). (b) Coral d18O records from Palmyra (Cobb et al. 2001) (red), Fanning (green), and Christmas (blue) islands (note inverted y-axis). (c) Sr/Ca-derived SST reconstructions from Palmyra, Fanning, and Christmas. (d) d18Osw (salinity proxy) reconstructions from Palmyra, Fanning, and Christmas. (e) The Southern Oscillation Index (SOI).

 

       

Tropical Pacific SST and Salinity over the 20th century (1886-1998)
 

In Nurhati et al. (2011), I presented 20th century SST and salinity proxy record from Palmyra Island. A visual comparison of the three coral records (Fig. 3) reveals that the d18Osw trend is responsible for the large trend in coral d18O, with warming playing a secondary role.

The study highlights that central tropical Pacific SST and salinity linkages to modes of interannual climate variability (e.g. canonical eastern Pacific ENSO and central Pacific ENSO) and decadal-scale variability of North Pacific phenomenon (e.g. the Pacific Decadal Oscillation and the recently uncovered North Pacific Gyre Oscillaion (Di Lorenzo et al. 2008). See Nurhati et al. (2011) for more details on scientific questions and findings.


FIG. 3. Palmyra coral monthly resolved d18O, Sr/Ca-derived SST, and d18Osw records from 1886 to 1998. (a) Palmyra coral d18O record (Cobb et al. 2001), (b) Sr/Ca-derived SST (black) plotted with ERSST (gray; Smith et al. 2008), and (c) d18Osw-based salinity record.

 

  

Results from this work:

[1] Nurhati, I. S., Cobb, K. M., and E. Di Lorenzo. (2011). Decadal-scale SST and salinity variations in the central tropical Pacific: Signatures of natural and anthropogenic climate change. Journal of Climate, 24, doi: 10.1175/2011JCLI3852.1.

[2] Sayani, H. R., Cobb, K. M., Cohen, A. L., Elliot, W. C., Nurhati, I. S., Rose, K. A. and Zaunbrecher, L. K. (2011). Effects of diagenesis on paleoclimate reconstructions from modern and young fossil corals. Geochim. Cosmochim. Acta, doi: 10.1016/j.gca.2011.08.026.

[3] Nurhati, I. S., Cobb, K. M., Charles, C. D. and Dunbar, R. B. (2011). Correction to "Late 20th century warming and and freshening in the central tropical Pacific." Geophysical Research Letters. 

[4] LaVigne, M., I.S. Nurhati, K.M. Cobb, H.V. McGregors, D. Sinclair, and R.M. Sherrell (in review). Systematic ENSO-driven nutrient variability recorded by central Pacific corals.

[5] Nurhati, I. S. (2010). Coral records of central tropical Pacific sea-surface temperature and salinity variability over the 20th century. Ph.D. Dissertation. Georgia Institute of Technology.

[6] Nurhati, I. S., Cobb, K. M., Charles, C. D. and Dunbar, R. B. (2009). Late 20th century warming and and freshening in the central tropical Pacific. Geophysical Research Letters, 36, L21606, doi:10.1029/2009GL040270.