U31A-01 INVITED
Global Warming and the Weakening of the Tropical Circulation
We explore the impact of CO2-induced reduction in the intensity of large-scale atmospheric circulation, resulting from global energetic constraints, on the tropical Pacific atmospheric and oceanic structure and circulation. We examine the projected 21st Century response of the tropical circulation using a set of 22 climate model experiments performed for the IPCC-AR4. In all models there is a robust decrease in the strength of the atmospheric overturning circulation decreases as the climate warms; the circulation weakens in a manner consistent with simple thermodynamic arguments. The weakening occurs preferentially in the zonally-asymmetric (i.e., Walker) rather than zonal-mean (i.e., Hadley) component of the tropical circulation, and results in a weakening the near-equatorial easterlies in the Pacific Ocean. These wind changes induce substantial changes to the thermal structure and circulation of the tropical oceans. Although many aspects of the model changes in both the atmospheric and ocean circulation resemble "El Niño-like" conditions, the mechanisms are shown to be distinct from those of El Niño and are reproduced in both mixed-layer and full ocean dynamics coupled climate models. Even though the mechanisms and structure differ from El Niño, aspects of climate teleconnections (in particular a strengthening of tropical Atlantic vertical wind shear) resemble those associated with El Niño. Changes seen in the consensus of models presented here are also consistent with recently detected changes in the tropical circulation since the 19th Century.
U31A-02
Fragile Reefs of the Eastern Pacific: Does low Cementation Provide a Model for Reefs in a High CO2 World?
Around the world, reefs will experience high pCO2, low pH, low carbonate concentrations, and low aragonite saturation state as atmospheric CO2 rises. Ocean carbon chemistry measurements show that eastern Pacific waters already exist at high pCO2 and low carbonate concentrations due to natural upwelling in the region. Because of the upwelling, this region may serve as a model for coral reef development under enhanced atmospheric CO2 and oceanic pCO2; that is, low coral growth, low secondary cementation, and high physical, chemical, and biological erosion. Reefs in the eastern Pacific Ocean are characterized by low biological diversity and relatively small size. Both past coring and recent analysis reveal that, while many reefs in the eastern Pacific are several thousand years old, they are fragile and lack significant cementation, even in the innermost, oldest structures. They are also extremely porous with high water throughflow. Without secondary cementation, branching coral frameworks are held together only by organically produced calcium carbonate (e.g. coralline algae), sponges, and other reef infauna, and contain a high proportion of loose sediments. The result is reef frameworks that are more susceptible to destruction from mechanical or biological erosion. The poorly cemented nature of eastern Pacific reefs is thus hypothesized to have been a factor in the severe bioerosion that occurred on these reefs after past bleaching events (1982-3, 1997-8). We will present data that indicate low rates of cementation and high rates of erosion on eastern Pacific coral reefs and will compare current carbonate chemistry in the eastern Pacific to model predictions of what reefs around the globe may experience in coming decades.
U31A-03 INVITED
Climate Impacts on the Fisheries of the North Pacific
The fisheries of the North Pacific and Bering Sea (NP/BS) have been fully exploited since the 1960s, with some of the more valuable fisheries dating back a century and longer. Many of the fisheries exhibit trends in productivity over a range of time scales. Debate on the causes of the productivity trends dates back to the 1950s (the "Thompson-Burkenroad" debate) and continues to this day. It is now widely recognized that environmental forcing, including the Pacific Decadal Oscillation and the El Nino-Southern Oscillation, contributes substantially to the observed variability in fisheries production. In this review, I will illustrate the historical trends in NP/BS fisheries, highlighting spatial and temporal commonalities/differences across species. Pacific halibut and Pacific salmon, which have been exploited for the longest period and for which the longest time series of data exist, will be discussed in greater detail. I conclude with a summary of proposed mechanisms linking climate variability and control of NP/BS fisheries production.
U31A-04 INVITED
What can we learn from corals about low-frequency (decadal-secular) climate variability?
Corals offer excellent reconstructions of interannual climate anomalies in the tropical oceans, but many studies have raised questions about their ability to faithfully track decadal variability and long-term trends. Yet resolving such variability in the tropical oceans is crucial for questions of climate dynamics and identification of anthropogenic impacts. Understanding the mechanisms of this low-frequency variability is critical because it provides a basis for prediction, and because such variability may interact with higher-frequency or secular patterns to alter predictability and produce unexpected climate extremes. Long-term climate variations and trends in the tropics have the potential to impact climate globally through altered atmospheric circulation, systems such as ENSO and the monsoons, and teleconnections to extratropical climates. The 20th-century record of instrumental observations is too short to address these issues with confidence. Paleoclimate records from corals can contribute to this effort, if subtle signals of low-frequency change can be disentangled from sources of noise. Here we present several approaches to this issue. First, a synthesis of 23 coral geochemical records suggests that a common pattern of decadal variation can be extracted from these records. This pattern closely resembles a 20th century mode identified from instrumental data, but our analysis indicates that this mode was significantly stronger in the 19th century (explaining nearly half the deseasonalized variance). This mode resembles ENSO in spatial pattern, implicating equatorial dynamics. Second, we explore longer-term modes in individual records and identify an interval in the 17th century during which several global and Pacific-sensitive records appear to follow a bidecadal beat. Finally, we analyze the warming/freshening trends apparent in nearly all coral climate records and assess their correlation to local SST and salinity changes. The amplitude of trends at most sites is near or beyond what can be explained with existing instrumental records, but the lack of local salinity records on reefs, coupled with potential changes in the isotopic content of precipitation, means that these trends cannot yet be dismissed as biological artifacts.
U31A-05
Atmospheric Amplification of Pacific Decadal Variability
The patterns of Pacific Decadal Variability (PDV) are now well-documented in sea surface temperature and sea level pressure. However, the mechanisms that give rise to these patterns are not well-understood. Much of the focus of previous studies has been on oceanic mechanisms, or ocean coupling to the lower atmosphere. Here we present evidence that atmospheric feedbacks involving clouds, water vapor and radiation also contribute to the pattern of PDV. First, a multi-variate index for PDV is constructed from reanalysis datasets. Then, this timeseries is used to extract the spatial pattern of PDV in several satellite-based observational datasets as well as surface-based cloud data. These multiple, independent datasets all show physically consistent signals of atmospheric feedbacks operating in the subtropical Pacific which would amplify the SST and SLP signals. We hypothesize that these atmospheric feedbacks are initiated as a remote response to changes in convection in the western tropical Pacific.
U31A-06
North Pacific Variability From a Data-Assimilating Model
A regional version of a data-assimilating model is used to study variability of circulation and property transports in the North Pacific from 1992-2004. The Estimating the Climate and Circulation of the Ocean (ECCO) model is a one-degree general circulation model which uses the adjoint method of data assimilation to determine a dynamically consistent time-varying estimate of the ocean state constrained by satellite and hydrographic data. In this study, different weighting schemes for the assimilation process are tested to determine the model's sensitivity and to improve the estimate in the region of interest. The effects of increasing vertical resolution from 23 to 50 levels are also studied. The resulting model output is compared with a long-term high resolution expendable bathythermograph (XBT) dataset. Time-varying budgets of heat and freshwater are calculated from the model and from the data for the closed region north of the XBT transect in the North Pacific. The heat budget shows large variability in advective transport leading to changes in heat storage in the region, while surface heat fluxes remain relatively constant. In freshwater, changes in advective transport are of similar magnitude to changes in surface fluxes. The largest signal in both heat and freshwater storage occurs during the 1997-98 El Nino event, when a northward meander of the North Equatorial Current causes an influx of heat and freshwater into the region of study. Further connections between these changes in advective transport, surface fluxes, and long-term modes of climate variability are explored.