U32A-01 INVITED
How does Atlantic climate change affect the eastern Pacific: Coupled modeling and paleoclimate implications
Paleoclimate observations suggest coherent variability in various parts of the globe in step with the Dansgaard-
Oeschger cycles, which are often associated with Heinrich events of massive land ice sheet discharges into the
North Atlantic. Recent water-hosing experiments with coupled GCMs show that a shutdown of the Atlantic
meridional overturning circulation induces large ocean-atmospheric anomalies not only in the Atlantic but in the
Pacific as well. In particular, the intensified cross-Central American winds have been identified as a key conduit
for the tropical Pacific response.
Central America features narrow mountain ranges with three major gaps: Tehuantepec, Papagayo, and Panama.
The IPRC regional ocean-atmosphere model (iROAM), consisting of a regional atmospheric model (RAM) and a
Pacific Ocean basin model, is used to study the conduit mechanisms of narrow Central American Isthmus. The
RAM extends from the central Pacific to the western Atlantic, from 35S to 35N. The 0.5 deg resolution is a
significant improvement over global coupled GCMs in representing Central American mountains. In response to
the North Atlantic cooling, the SST annual cycle weakens on the equator, triggered by an intensified Panama wind
jet. The tropical eastern Pacific response is highly seasonal, with a major cooling during the February-April
season but much weaker SST anomalies in other seasons. The ocean-atmospheric processes leading to this
strong seasonality, and the cause of consistency and diversity among different GCM water-hosing experiments
will be discussed in detail in the presentation.
http:iprc.soest.hawaii.edu/~xie
U32A-02
Interannual to Decadal Predictability of Tropical and North Pacific Sea Surface Temperatures
A multivariate empirical model is used to show that predictability of the dominant patterns of tropical and North Pacific oceanic variability, El Nino-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO), is limited to a few years despite the presence of spectral peaks on decadal timescales. The model used is a linear inverse model (LIM) derived from the observed simultaneous and one-year lag correlation statistics of July-June averaged SST from the Hadley Centre Global Sea Ice and Sea Surface Temperature (HadISST) dataset for the years 1900-2002. The model accurately reproduces the power spectra of the data, including interannual and interdecadal spectral peaks that are significant relative to univariate red noise. Eigenanalysis of the linear dynamical operator yields propagating eigenmodes that correspond to these peaks but have very short decay times and thus limited predictability. Longer-term predictability does exist, however, due to two stationary eigenmodes that are more weakly damped. These eigenmodes do not strongly correspond to the canonical ENSO and PDO patterns. Instead, one is similar to the 1900-2002 trend and might represent anthropogenic effects, while the second represents multidecadal fluctuations of a pattern that potentially represents natural decadal variability. These eigenmode patterns are robust and can also be obtained by simpler EOF analysis of 11-year running mean SST data. Predictability of these two stationary eigenmodes is significantly enhanced by tropical-North Pacific coupling. Critically, neither stationary eigenmode is well captured in either the control run or 20th century simulation of any coupled GCM in the CMIP project of the IPCC Fourth Assessment Report (AR4), perhaps because in all the GCMs tropical SST decadal variability is too weak and North Pacific SSTs are too independent of the Tropics. A key implication of this analysis is that the PDO may represent not a single physical mode but rather the sum of several phenomena, each of which represents a different red noise process with its own autocorrelation timescale and spatial pattern. The sum of these red noises can give rise to apparent PDO "regime shifts" and seeming characteristics of a long memory process. Such shifts are not predictable beyond the timescale of the most rapidly decorrelating noise, less than two years, although the expected duration of regimes may be determined from the relative amplitudes of different eigenmodes.
U32A-03
The Incredible Shrinking Iguana: Impact of Galapagos
Galapagos Islands are well known for Darwin's epiphany about evolution after his 'discovery' of the islands. The incredible transformation that the key species on this island undergo on ENSO time-scales is recorded extensively. The impact of Galapagos on SSTs and ocean primary production has been conjectured but some recent work demonstrates that their influence extdends thousands of kilometers via stronger TIWs and modulations of the termination of EUC and related air-sea interactions. Tropical Pacific is not only the theater of action for ENSO dynamics but the ecosystem is the most studied iron limited region due to its HNLC characteristics. This study addresses the physical and biogeochemical impacts of the Galapagos islands with a OGCM-biogeochemical model coupled to a simple wind-anomaliy model. The impact of Galapagos is to warm the cold-tongue by upto 1C with a major impact on the termination of the EUC that results in large-scale circulation changes that extend to the South American coast as far south as 20S. If the islands are not included, then the equatorial Kelvin waves extend their impact on thermocline variability all the way to the eastern boundary and lead to a quasi-biennial ENSO whereas the impact of Galapagos islands shifts the frequency to a quasi- quadrennial time-scale. The impacts on the ecosystem are commensurate with the seasonal and interannual variability changes with the addition of dissolved iron supply related to EUC changes. Bio-physical feedbacks in the deep tropical Pacific also tend to warm SSTs and the impact of Galapagos is thus to flatten the east-west SST gradient resulting in less-frequent and weaker ENSO events. The relevance of these results to longer time-scale variability and any potential analogs to global warming and sea level change are discussed in detail.