Mechanisms of tropical Pacific climate variability and change are studied in numerical experiments of future anthropogenic global warming (AGW), the Last Glacial Maximum (LGM) 21,000 years ago, and decadal variability (PDV). According to these mechanisms the equatorial Pacific does not become either El Niño- or La Niña-like as the tropics warm up in response to increasing greenhouse gases. Because humidity increases much faster than precipitation as the tropical atmosphere warms up, theory and models anticipate a slowing-down of the Walker circulation in order to keep a balanced flow of water vapor into areas of convection. On long time scales characteristic of climate change, ocean dynamics oppose these changes in the Walker circulation. First, equatorial adjustment theory indicates that changes in the Walker circulation are not amplified via the Bjerknes feedback, as during El Niño or La Niña events. Second, during AGW, the ocean becomes more thermally stratified resulting in enhanced cooling of the equatorial cold tongue opposing the warming there. These ideas can be applied to interpret proxies of the LGM for which El Niño and La Niña analogies have been made. However, the LGM tropics are not an opposite analogue to future AGW because the Walker circulation is also sensitive to changes in the geography of the Maritime continent associated with lower sea level. Models simulate a stronger Walker circulation when the tropics cool in order to compensate for the decrease in moisture due to a cooler/drier atmosphere. However, this response is opposed by a weakening of the ascending branch of the Walker circulation because convection is suppressed over vast areas of the Maritime Continent exposed due to lower sea level. In general, the patterns of warming or cooling for AGW and LGM are not El Niño- or La Niña-like because of the opposing mechanisms presented here. As such, adherence to an ENSO analogy for interpreting tropical Pacific climate change can lead to serious misconceptions. However, proxies of the thermocline tilt can provide unequivocal evidence of changes in the Walker circulation because the pressure gradient associated with the thermocline tilt has to be in balance with the trade winds at all timescales.

Large-scale climate modes such as the El Niño/Southern Oscillation (ENSO), the Asian monsoon, and the Arctic Oscillation are responsible for much of the Earth’s climate variability. Despite the importance of these modes, we have limited understanding of how they vary on long (multidecadal to millennial) timescales due to the short length of instrumental climate records. Fortunately, climate information stored in natural archives can provide us with information on how these modes varied in the more distant past. Lake sediments are an ideal climate archive since they are continuous, have high temporal resolution, and contain many potential climate proxies. In the present study, I use lake sediment records to assess past climate and environmental changes associated with the El Niño/Southern Oscillation, the Asian monsoon, and the Arctic Oscillation. Exploring modern precipitation variability across the Asian monsoon region, I found that precipitation within this broad area is not coherent, which holds implications for paleorecords that are hypothesized to represent monsoon variability, including many lake sediment records on the Tibetan Plateau. Monsoon precipitation in the Arabian Sea is distinct from precipitation in India and China, and increased precipitation in the Arabian Sea coincides with decreased precipitation in the western North Pacific. Furthermore, only precipitation in southwestern Tibet responds to the Southwest monsoon, whereas precipitation in southeastern Tibet responds to the western North Pacific monsoon. In southwestern Tibet, I have reconstructed dust variability over the last millennium using the lake sediment record from Kiang Co. The sediment record shows a trend toward increasing dust over the 20th century, and our hypothesized dust proxy is positively correlated with the June-November Arctic Oscillation Index. A trend toward more positive Arctic Oscillation Index values as well as higher temperatures over the 20th century likely drove increased dustiness in southwestern Tibet, due the influence of temperature on glaciofluvial sediment availability in the Himalayas. Sediment trap, sediment core data, and modern measurements of local climate and lake water variables at Genovesa Crater Lake, Galápagos, indicate the lake and its sediments respond to local climate variability, with carbonate-rich sediments forming during prolonged dry periods (La Niña events), and organic-rich sediment forming during the warm season and El Niño events. The ratios of silica to calcium and strontium to calcium also reflect cool season SST. Thus, this lake sediment record has potential to provide a record of both seasonal and ENSO variability spanning the Holocene.

This study has been performed as a pilot study for a project regarding the meridional migration of the Intertropical Convergence Zone (ITCZ) and its relationship with lightning activity in the Eastern Tropical Pacific (ETP). Objectives of this study were to analyze and improve lightning data to be used for such a study and to decide on a method and proper time scale of data analysis and ITCZ index development for this study. Exploratory data analysis has been practiced with World Wide Lightning Location Network (WWLLN) data and ITCZ index data. Results suggest that the most beneficial time-scale to be used for the above study is 15 days and that ITCZ estimations can be obtained via the use of precipitation index and cloud top temperature. Lightning data originated from atmospheric systems not associated with the ITCZ has been analyzed. This report proposes that Uppsala University should become part of the World Wide Lightning Location Network, enabling further work regarding this and similar projects.

A systematic procedure is designed to evaluate the role of stochastic forcing (SF) in El Nino ? Southern Oscillation (ENSO) exhibited by coupled ocean-atmosphere general circulation models (CGCMs). The procedure is applied on a 163-year run of a CGCM which has realistic representation of ENSO and SF. The realism of ENSO in the CGCM is determined by statistical comparison with a 29-year global reanalysis product. SF is extracted from both the CGCM and reanalysis as residual atmospheric variability uncoupled to the ocean. Further, the Madden-Julian Oscillation (MJO) and non-MJO components are isolated from SF. The CGCM stochastic components are compared to those from the reanalysis to validate their representation. A coupled ocean-atmosphere model of intermediate complexity is first forced with stochastic components from the reanalysis. The resulting ENSO is examined for realism to evaluate the strengths and weaknesses of the intermediate coupled model, which is then forced with the stochastic components from the CGCM. Results are diagnosed to investigate the role of SF. It is found that the SF can play an important role in ENSO in the CGCM, especially in its warm events. The role is similar to reanalysis SF in generating ENSO period and spring predictability barrier. However, unlike in case of the reanalysis, the seasonal dependence of ENSO variance in the CGCM does not seem to be originating from its SF. The contribution to statistics appears to be higher from the MJO component of SF compared to the non-MJO component. The intermediate model simulations also suggest that both in CGCM and nature, the SF operates on a weakly stable coupled system to produce ENSO variability.

In the Eastern Tropical Pacific (ETP), the operation mode of purse-seine fishing can be divided into dolphin sets and non-dolphin sets. Dolphin sets can catch more yellowfin tuna, but cause a lot of dolphin deaths. On the contrary, non-dolphin sets can catch more skipjack tuna, but discard a lot of tuna. Therefore, the choice of the operation mode to the ETP purse-seine fishery is very influential. If the fishermen use more dolphin sets, they are able to catch high quantity and more valuable yellowfin tuna. Conversely, if the fishermen use more non-dolphin sets, they will not hurt dolphins. But the tuna fish caught by this operation mode is smaller in size and the discarding rate is extremely high which will affect the overall profits of the fishermen. The research will investigate how the fishermen make the choice of the operation mode in the dilemma between dolphin sets which cause dolphins-bycatch and non-dolphin sets which result in tuna-discarding, In addition, if managers can ban dolphin sets or non-dolphin sets to maximize the profit of the fishermen and maintain the ecological balance between tuna and dolphins.

Accurate forecasts of future regional temperature and rainfall patterns in many regions largely depend on characterizing anthropogenic trends in tropical Pacific climate. However, strong interannual to decadal-scale tropical Pacific climate variability, combined with sparse spatial and temporal coverage of instrumental climate datasets in this region, have obscured potential anthropogenic climate signals in the tropical Pacific. In this dissertation, I present sea-surface temperature (SST) and salinity proxy records that span over the 20th century using living corals from several islands in the central tropical Pacific. I reconstruct the SST proxy records via coral Sr/Ca, that are combined with coral oxygen isotopic (d18O) records to quantify changes in seawater d18O (hereafter d18Osw) as a proxy for salinity. Chapter 2 investigates the spatial and temporal character of SST and d18Osw-based salinity trends in the central tropical Pacific from 1972-1998, as revealed by corals from Palmyra (6ºN, 162ºW), Fanning (4ºN, 159ºW) and Christmas (2ºN, 157ºW) Islands. The late 20th century SST proxy records exhibit warming trends that are larger towards the equator, in line with a weakening of equatorial Pacific upwelling over this period. Freshening trends revealed by the salinity proxy records are larger at those sites most affected by the Inter-Tropical Convergence Zone (ITCZ), suggesting a strengthening and/or an equatorward shift of the ITCZ. Taken together, the late 20th century SST and salinity proxy records document warming and freshening trends that are consistent with a trend towards a weakened tropical Pacific zonal SST gradient under continued anthropogenic forcing. Chapter 3 characterizes the signatures of natural and anthropogenic variability in central tropical Pacific SST and d18Osw-based salinity over the course of 20th century using century-long coral proxy records from Palmyra. On interannual timescales, the SST proxy record from Palmyra tracks El Niño-Southern Oscillation (ENSO) variability. The salinity proxy record tracks eastern Pacific-centered ENSO events but is poorly correlated to central Pacific-centered ENSO events - the result of profound differences in precipitation and ocean advection that occur during the two types of ENSO. On decadal timescales, the coral SST proxy record is significantly correlated to the North Pacific Gyre Oscillation (NPGO), suggesting that strong dynamical links exist between the central tropical Pacific and the North Pacific. The salinity proxy record is significantly correlated to the Pacific Decadal Oscillation (PDO), but poorly correlated to the NPGO, suggesting that, as was the case with ENSO, these two modes of Pacific decadal climate variability have unique impacts on equatorial precipitation and ocean advection. However, the most striking feature of the salinity proxy record is a prominent late 20th century freshening trend that is likely related to anthropogenic climate change. Taken together, the coral data provide key constraints on tropical Pacific climate trends, and when used in combination with model simulations of 21st century climate, can be used to improve projections of regional climate in areas affected by tropical Pacific climate variability.

Ocean circulation changes in the tropical Pacific strongly influence global climate, as demonstrated during El Niño-Southern Oscillation (ENSO) extremes. Understanding the causes of past variability in tropical Pacific circulation and their relationship to climate change will help to predict how future climate may evolve under anthropogenic radiative forcing. I measure fossil coral radiocarbon (Δ¹⁴C) from Palmyra (6°N, 162°W) and Christmas (2°N, 157°W) Islands in the central tropical Pacific to reconstruct high-resolution records of tropical Pacific ocean circulation variability over the last millennium. Variations in coral Δ¹⁴C from Palmyra and Christmas reflect a combination of the atmospheric concentration of ¹⁴C at the time of growth, Δ¹⁴C-depleted waters associated with equatorial upwelling, and Δ¹⁴C -enriched waters advected from the western tropical Pacific. Existing oxygen isotopic (δ ¹⁸O) records of the Palmyra and Christmas fossil corals reveal a rich history of interannual to centennial variability in sea-surface temperature (SST) and salinity over the last millennium [Cobb et al., 2003b]. My approach targets specific time intervals associated with strong interannual to centennial-scale coral δ ¹⁸O anomalies for high-resolution Δ¹⁴C analysis. Seasonally-resolved Δ¹⁴C measurements are used to compare interannual Δ¹⁴C variability across the 10th, 13th, 15th, 17th, and 20th centuries. Annually-resolved Δ¹⁴C measurements are used to compare decadal to centennial-scale Δ¹⁴C variations from the 10th, 12th - 15th and 17th centuries. SEM photos are used to assess the fidelity of the coral Δ¹⁴C records with respect to post-depositional alteration of the coral skeleton. I find evidence for minor dissolution and addition of secondary aragonite, but my results indicate that coral Δ¹⁴C is only compromised after moderate to severe diagenesis. Despite strong ENSO signals in modern and fossil coral δ ¹⁸O, our data show no statistically significant interannual variability in coral ¹⁴C. There is a centennial-scale increase in coral radiocarbon from the Medieval Climate Anomaly (MCA, ~900-1200AD) to the Little Ice Age (LIA, ~1500-1800). I use a box model of central tropical Pacific Δ¹⁴C contributions to show that this centennial-scale trend over the last millennium is largely explained by centennial-scale changes in atmospheric ¹⁴C. However, large 12th century depletions in Palmyra coral ∆¹⁴C data cannot be explained by atmospheric ¹⁴C variability and likely reflect a roughly two-fold increase in upwelling and/or a significant change in the ¹⁴C of higher-latitude source waters reaching the equatorial Pacific during this time. Conversely, significantly enriched Christmas coral ∆¹⁴C values during the 16th century are consistent with a two-fold reduction in upwelling strength and/or the advection of high-¹⁴C waters to the equatorial thermocline from higher latitudes.

Finely laminated sediments within Bainbridge Crater Lake, Galapagos, provide a record of El Nino-Southern Oscillation (ENSO) events over the Holocene. Despite the importance of this sediment record, hypotheses for how climate variability is preserved in the lake sediments have not been tested. Here we present results of long-term monitoring of the local climate and limnology and a revised interpretation of the sediment record. Brown-green, organic-rich, siliciclastic laminae reflect warm, wet conditions typical of El Nino events, whereas carbonate and gypsum precipitate during cool, dry La Nina events and persistent dry periods, respectively. Applying this new interpretation, we find that ENSO events of both phases were generally less frequent during the mid-Holocene (similar to 6100-4000 calendar years B.P.) relative to the last similar to 1500 calendar years. Abundant carbonate laminations between 3500 and 3000 calendar years B.P. imply that conditions in the Galapagos region were cool and dry during this period when the tropical Pacific E-W sea surface temperature (SST) gradient likely strengthened. The frequency of El Nino and La Nina events then intensified dramatically around 1750-2000 calendar years B.P., consistent with a weaker SST gradient and an increased frequency of ENSO events in other regional records. This strong interannual variability persisted until similar to 700 calendar years B.P., when ENSO-related variability at the lake decreased as the SST gradient strengthened. Persistent, dry conditions then dominated between 300 and 50 calendar years B.P. (A.D. 1650-1900, +/- similar to 100 years), whereas wetter conditions and frequent El Nino events dominated in the most recent century. Plain Language Summary Sediments accumulating at the bottom of Bainbridge Crater Lake have provided a record of Galapagos climate and the frequency of El Nino events over the past similar to 6000 years. Motivated by the importance of this lake for our understanding of climate in the tropical Pacific Ocean, we have been monitoring the link between climate, lake conditions, and the physical and chemical properties of the lake sediments since 2009. Based on this long-term monitoring, we find that the Bainbridge sediment record preserves both El Nino and La Nina events. This makes Bainbridge a particularly valuable archive of past climate, as most sediment-based records typically preserve only one or the other key phase of tropical Pacific climate.

Understanding gene flow, diversity, and dispersal patterns is important for predicting effects of natural events and anthropogenic activities on dolphin populations. With the very recent exceptions of false killer whales (Pseudorca crassidens), spinner dolphins (Stenella longirostris), and common bottlenose dolphins (Tursiops truncatus), Hawaiian odontocete species are managed as single stocks within the U.S. Hawaiian Exclusive Economic Zone. These exceptions are a result of recent studies that have indicated that some species have populations that show fidelity to individual islands or groups of islands, resulting in genetic differentiation, often with management implications. The first part of my study (following the introductory chapter) focused on population structure of pantropical spotted dolphins (Stenella attenuata) near the Hawaiian Islands. Because of the level of human interaction, pantropical spotted dolphin populations need to be defined accurately to be managed in a way that will avoid local population losses, especially given that the commercial and recreational troll fisheries near the islands "fish on dolphins" to catch tuna. I analyzed genetic samples for mtDNA and microsatellite loci from four island regions: Hawai'i, the 4-islands area, O'ahu, and Kaua'i/Ni'ihau. My results support genetic differentiation among the regions of Hawai'i, the 4-islands area, and O'ahu and suggest that pantropical spotted dolphins near Kaua'i/Ni'ihau are likely transient and in very low numbers. There was no strong evidence to support sex-biased dispersal or group fidelity. Possibly, differentiation is mediated by behavior adapted to differing habitat types. From a management perspective, spinner and bottlenose dolphin populations near the Hawaiian Islands have been split into separate stocks for management based on levels of genetic differentiation similar to those found for pantropical spotted dolphins. These precedents suggest that comparable action should be taken to split pantropical spotted dolphin stocks near the Hawaiian Islands. Most population studies rely heavily upon fixation indicies like FST to determine whether populations are genetically differentiated. When FST values are low but significantly different from zero, it can be difficult to interpret the biological significance of these values. As part of my study, I suggest that one way to evaluate whether small FST values indicate significant differentiation is to compare FST values with other populations considered to be separate based on factors such as extreme distance or morphological differences. I examined pantropical spotted dolphins from the coastal and offshore Eastern Tropical Pacificm (ETP), Hawaiian Islands, and China/Taiwan to examine the utility of comparing FST values across separate populations. Among Hawaiian Island regions, FST values are significantly different from zero but small. The comparison of these FST values with more distant populations in the ETP and China/Taiwan indicated that differences among Hawaiian Island regions were similar in magnitude to those found between the offshore and coastal ETP sub-species, but smaller than between the Hawaiian Island regions and the other regions examined. This suggests a level of reproductive isolation among the Hawaiian Islands regions that is comparable to that of offshore and coastal ETP populations, and supports the value of fixation index comparisons in evaluating differentiation among putative populations. My results suggest that assigning specific numerical baseline FST values may not always be biologically meaningful but that determining whether related populations with geographic or other separation show a preponderance of similar, lower, or higher fixation index values can help evaluate whether genetic differences among sympatric or parapatric groups warrants designating them as separate populations for management. Lastly, I explore whether the fast evolving mtDNA control region may be more suited to phylogenetic comparisons among the Stenella than slower evolving gene regions and whether the small number of haplotypes generally used in phylogenetic analyses is adequate for defining relationships among dolphins. Usually, slow evolving regions, such as gene regions, are used in phylogenetic analyses because species and genera have been isolated long enough for variation to have accumulated in such regions but not so long that many reversals (i.e. a mutational change in sequence that later changes back to the original sequence) have occured. The mtDNA control region is typically used for population genetic comparisons rather than phylogenetic comparisons because it is considered to be a fast evolving region. Historically, dolphin phylogeny has been examined using gene regions, which have resulted in ambiguous and unexpected relationships. However, the lack of variation in the mtDNA control region for pantropical spotted dolphin populations and the fact that recent studies have found that the mtDNA control region in cetaceans evolves at about one quarter the rate of other mammals, raises the question as to whether this region would be better suited to phylogenetic studies for the Stenella (and potentially other dolphin species). In comparing 346 haplotypes from five species of Stenella world-wide, I found that the mtDNA control region is probably not a good region to use for phylogenetic analyses, and that even faster evolving regions might perform better. The differences in the mtDNA control region were not sufficient to distinguish clear relationships among the Stenella. I also found that when subsets of haplotypes chosen at random were compared, the results differed among comparisons, suggesting that there is value in using more than the usual one or two haplotypes when making phylogentic comparisons. Given the recent increases in sequence availability (e.g. GenBank) and computing power, researchers should strongly consider using many haplotypes from a variety of populations in their phylogenetic comparisons.

We improved the assimilated daily inversion method by conducting model simulation, satellite retrieval, and inverse modeling sequentially on a daily basis. The improved procedure was applied to GOME-2 and OMI NO₂ measurements over China in 2011, respectively. The new daily retrieval-inversion method significantly reduced the systematic bias in inverse modeling of NOₓ emission between using GOME-2 and OMI measurements, and detected more clear seasonal and weekly variations. OMI instrument observed NO₂ columns over China from 2005 to 2010 were analyzed in order to estimate the top-down anthropogenic NOₓ emission trends. The estimated average emission trend is slower than the trend reported for previous years. We find large regional, seasonal, and urban-rural variations in emission trends. These results appear to suggest that a number of factors have significantly reduced or even reversed the increasing trend of NOₓ emissions in more economically developed megacities and southern coastal regions, but their effects are not as significant in other major cities or less economically developed regions. A 1-D chemical transport model was applied to analyze OH and HO₂ radical observations during the Pacific Atmospheric Sulfur Experiment (PASE) near Christmas Island (Kiritimati, 1.52°N 157.24°W) from Aug. 2 through Sep. 10, 2007. In two of fourteen research flights, significantly higher HO₂/OH ratios in the buffer layer than the other flights were found. Model simulations indicated that fast-reacting oxygenated volatile organic compounds, which can react rapidly with OH and provide additional primary radical sources through photolysis, were necessary to explain the observations. During or right before these two flights, the WRF model simulated two strongest shallow convective events during this experiment, suggesting a transport pathway of ocean organics into the buffer layer. Ocean upwelling driven by atmospheric pressure depression during convection may expedite the release of ocean organics.