Interannual to decadal climate variability in the Pacific and Indian oceans revealed by isotopic studies of corals

Full text: Instrumental records of past variability and climatically important regions of the tropical Indian and Pacific Oceans are relatively scarce and short. Multi-century reconstructions of past climate change in these areas are now being made available through the isotopic and chemical analysis of coral skeletons collected from both the surface ocean and deep sea. Here we present a brief synthesis of multi-century trends in ocean climate as revealed by coral archives. In general, these records are consistent with century-scale trends towards a warmer tropical ocean with greater near-equatorial rainfall and greater ENSO variability in more recent decades. Very limited deep sea coral data reveal larger than expected variability in thermocline conditions during the past several centuries. Sufficient data is now available to allow comparisons of Pacific ENSO variability with records of the Monsoon from the Pacific Basin over timescales extending back at least 150 years. The last half of this talk is used to describe a new synthesis derived from East Africa coral records. We present a monthly resolution stable isotopic record acquired from several large living coral heads (Porites) from the Malindi and Watamu Marine Reserves, Kenya (3 deg. S, 40 deg. E). The annual chronology is precise and is based on exceptionally clear high and low density growth band couplets. The record extends from 1696 to 1996 A.D., making it the longest coral climate record from the Indian Ocean and one of the longest available worldwide. We have analyzed the uppermost portion of 8 separate coral cores. This upper section, used for calibration purposes, provides estimates of signal fidelity and noise in the climate recording system internal to the colony. Coral δ18O at this site primarily records SST; regression of monthly coral δ18O vs. SST yields a slope of -0.26 per mille δ18O per deg. C, and δ18O explains well over 50% of the variance in SST. Additional isotopic variability may result from changes in seawater d18O due to local runoff or regional evaporation/precipitation balance, but these changes are likely to be small because local rainfall δ18O is not strongly depleted relative to seawater and salinity gradients are small. The coral record indicates a clear warming trend of about 1.5 deg. C that accelerates in the latest 20th century, superimposed on strong decadal variability that persists throughout the record. In fact, d18O values in the 1990s exceed the 300 year envelope (they are lower) and correspond with apparently unprecedented coral bleaching in coastal East Africa. The decadal component of the Malindi coral record reflects a regional climate signal spanning much of the western equatorial Indian Ocean. In general, East African SST and rainfall are better correlated with Pacific ENSO indicators than with the Indian Monsoon at all periods (inter-annual through multi-decadal) but the correlation weakens after 1975. One dramatic new result we report here is a strong indication of a major cool and dry period from 1750-1820 A.D. This is the single largest multi-decadal anomaly of the past 300 years and correlates perfectly in time with the historically and anecdotally defined Lapanarat Drought. Our results indicate a strong link between multi-decadal tropical cold SST anomalies and far-reaching continental droughts in East Africa. Interannual-decadal SST variations are strongly coherent with ENSO indices and other ENSO-sensitive coral records on decadal and interannual time scales. The decadal component of the Malindi coral record reflects a regional climate signal spanning much of the western equatorial Indian Ocean. Previous work has argued that this component likely reflects a monsoonal influence. However, decadal variance in both Malindi and Seychelles coral records is more strongly coherent with ENSO indices than with the India or East Africa rain indices. The coherency of both coral records with Pacific indicators suggests instead that Indian Ocean variability reflects decadal ENSO -like variability originating in the Pacific. These records do not correlate significantly with the Pacific Decadal Oscillation implying a dominant role for the tropical Pacific (as opposed to extra-tropical regions) as a source of regional decadal variability in the western Indian Ocean. This work confirms that the tropical Pacific can act as an agent forcing decadal climate variability over a very large spatial scale on Earth. We conclude by examining the time-varying relationship between ENSO and the monsoons using available coral records from the Pacific and Indian basins. (author)