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Karnauskas, K. B., and S. Curtis, 2016: The American Midsummer Drought: Past, Future, and Research Challenges. US CLIVAR Variations, 14(1), 15–21.

A downscaling approach is applied to future projection simulations from four CMIP5 global climate models to investigate the response of the tropical cyclone (TC) climatology over the North Pacific basin to global warming. Under the influence of the anthropogenic rise in greenhouse gases, TC-track density, power dissipation, and TC genesis exhibit robust increasing trends over the North Pacific, especially over the central subtropical Pacific region. The increase in North Pacific TCs is primarily manifested as increases in the intense and relatively weak TCs. Examination of storm duration also reveals that TCs over the North Pacific have longer lifetimes under global warming.
Through a genesis potential index, the mechanistic contributions of various physical climate factors to the simulated change in TC genesis are explored. More frequent TC genesis under global warming is mostly attributable to the smaller vertical wind shear and greater potential intensity (primarily due to higher sea surface temperature). In contrast, the effect of the saturation deficit of the free troposphere tends to suppress TC genesis, and the change in large-scale vorticity plays a negligible role.

Zhang, L., K. B.  Karnauskas, J. P. Donnelly, and K. Emanuel, 2017: Response of the North Pacific Tropical Cyclone Climatology to Global Warming: Application of Dynamical Downscaling to CMIP5 Models. J. Climate, 30(4), 1233–1243, doi: 10.1175/JCLI-D-16-0496.1.

The five most severe and persistent droughts in the American West (AW) during the Common Era occurred during a 450 year period known as the Medieval Climate Anomaly (MCA—850–1299 C.E.). Herein we use timeseries modeling to estimate the probability of such a period of hydroclimate change occurring. Clustering of severe and persistent drought during an MCA-length period occurs in approximately 10% of surrogate timeseries that were constructed to have the same characteristics as a tree-ring derived estimate of AW hydroclimate variability between 850 and 2005 C.E. Periods of hydroclimate change like the MCA are thus expected to occur in the AW, although not frequently, with a recurrence interval of approximately 11 000 years. Importantly, a shift in mean hydroclimate conditions during the MCA is found to be necessary for drought to reach the severity and persistence of the actual MCA megadroughts. This result has consequences for our understanding of the atmosphere-ocean dynamics underlying the MCA and a persistently warm Atlantic Multidecadal Oscillation is suggested to have played an important role in causing megadrought clustering during this period.

Coats, S., J. E. Smerdon, Karnauskas, K. B., and R. Seager, 2016: The improbable but unexceptional occurrence of megadrought clustering in the American West during the Medieval Climate Anomaly. Environ. Res. Lett., 11, 074025, doi: 10.1088/1748-9326/11/7/074025.

Seasonal hurricane activity is a function of the amount of initial disturbances (e.g., easterly waves) and the background environment in which they develop into tropical storms (i.e., the main development region). Focusing on the former, a set of indices based solely upon the meridional structure of satellite-derived outgoing longwave radiation (OLR) over the African continent are shown to be capable of predicting Atlantic seasonal hurricane activity with very high rates of success. Predictions of named storms based on the July OLR field and trained only on the time period prior to the year being predicted yield a success rate of 87%, compared to the success rate of NOAA's August outlooks of 53% over the same period and with the same average uncertainty range (±2). The resulting OLR indices are statistically robust, highly detectable, physically linked to the predictand, and may account for longer-term observed trends.

Karnauskas, K. B., and L. Li, 2016: Predicting Atlantic seasonal hurricane activity using outgoing longwave radiation over Africa. Geophys. Res. Lett., 43(13), 7152–7159, doi: 10.1002/2016GL069792.

Karnauskas, K. B., J. P. Donnelly, and K. J. Anchukaitis, 2016: Future Freshwater Stress for Island Populations. Nature Climate Change, doi: 10.1038/nclimate2987.

Karnauskas, K. B., A. L. Cohen, and J. M. Gove, 2016: Mitigation of Coral Reef Warming Across the Central Pacific by the Equatorial Undercurrent: A Past and Future Divide. Nature Scientific Reports, 6, 21213, doi: 10.1038/srep21213.

Changes in larval import, export, and self-seeding will affect the resilience of coral reef ecosystems. Climate change will alter the ocean currents that transport larvae and also increase sea surface temperatures (SST), hastening development and shortening larval durations. Here, we use transport simulations to estimate future larval connectivity due to: 1) physical transport of larvae from altered circulation alone, and 2) the combined effects of altered currents plus physiological response to warming. Virtual larvae from islands throughout Micronesia were moved according to present-day and future-ocean circulation models. The Hybrid Coordinate Ocean Model (HYCOM) spanning 2004-2012 represented present-day currents. For future currents, we altered HYCOM using analysis from the National Center for Atmospheric Research Community Earth System Model, version 1-Biogeochemistry, Representative Concentration Pathway 8.5 experiment. Based on the NCAR model, regional SST is estimated to rise 2.74° C which corresponds to a ~17% decline in larval duration for some taxa. This reduction was the basis for a separate set of simulations. Results predict an increase in self-seeding in 100 years such that 62-76% of islands experienced increased self-seeding, there was an average domain-wide increase of ~1-3 percentage points in self-seeding, and increases of up to 25 percentage points for several individual islands. When changed currents alone were considered, approximately half (i.e. random) of all island pairs experienced decreased connectivity but when reduced PLD was added as an effect, ~65% of connections were weakened. Orientation of archipelagos relative to currents determined the directional bias in connectivity changes. There was no universal relationship between climate change and connectivity applicable to all taxa and settings. Islands that presently export large numbers of larvae but that also maintain or enhance this role into the future should be the focus of conservation measures that promote long term resilience of larval supply.

Kendall, M. S., M. Poti, and K. B. Karnauskas, 2016: Climate change and larval–transport in the ocean: Fractional effects from physical and physiological factors. Glob. Chang. Biol., 22, 1532–1547, doi: 10.1111/gcb.13159.

The Galápagos is a flourishing yet fragile ecosystem whose health is particularly sensitive to regional and global climate variations. The distribution of several species, including the Galápagos Penguin, is intimately tied to upwelling of cold, nutrient–rich water along the western shores of the archipelago. Here we show, using reliable, high–resolution sea surface temperature observations, that the Galápagos cold pool has been intensifying and expanding northward since 1982. The linear cooling trend of 0.8°C per 33 years is likely the result of long–term changes in equatorial ocean circulation previously identified. Moreover, the northward expansion of the cold pool is dynamically consistent with a slackening of the cross–equatorial component of the regional trade winds—leading to an equatorward shift of the mean position of the Equatorial Undercurrent. The implied change in strength and distribution of upwelling has important implications for ongoing and future conservation measures in the Galápagos.

Karnauskas, K. B., S. Jenouvrier, C. W. Brown, and R. Murtugudde, 2015: Strong sea surface cooling in the eastern equatorial Pacific and implications for Galápagos Penguin conservation. Geophys. Res. Lett., 42(15), 6432–6437, doi: 10.1002/2015GL064456.

The airline industry closely monitors the midlatitude jet stream for short-term planning of flight paths and arrival times. In addition to passenger safety and on-time metrics, this is due to the acute sensitivity of airline profits to fuel cost. US carriers spent US$47 billion on jet fuel in 2011, compared with a total industry operating revenue of US$192 billion. Beyond the timescale of synoptic weather, the El Niño/Southern Oscillation (ENSO), Arctic Oscillation (AO) and other modes of variability modulate the strength and position of the Aleutian low and Pacific high on interannual timescales, which influence the tendency of the exit region of the midlatitude Pacific jet stream to extend, retract and meander poleward and equatorward1, 2, 3. The impact of global aviation on climate change has been studied for decades owing to the radiative forcing of emitted greenhouse gases, contrails and other effects4, 5. The impact of climate variability on air travel, however, has only recently come into focus, primarily in terms of turbulence6, 7. Shifting attention to flight durations, here we show that 88% of the interannual variance in domestic flight times between Hawaii and the continental US is explained by a linear combination of ENSO and the AO. Further, we extend our analysis to CMIP5 model projections to explore potential feedbacks between anthropogenic climate change and air travel.

Karnauskas, K. B., J. P. Donnelly, H. C. Barkley, and J. E. Martin, 2015: Coupling between Air Travel and Climate. Nature Climate Change, 5, 1068–1073, doi: 10.1038/nclimate2715.

Internal waves (IWs) generated in the Luzon Strait propagate into the Northern South China Sea (NSCS), enhancing biological productivity and affecting coral reefs by modulating nutrient concentrations and temperature. Here, we use a state-of-the-art ocean data assimilation system to reconstruct water column stratification in the Luzon Strait as a proxy for IW activity in the NSCS, and diagnose mechanisms for its variability. Interannual variability of stratification is driven by intrusions of the Kuroshio Current into the Luzon Strait and freshwater fluxes associated with the El Niño Southern Oscillation. Warming in the upper 100 meters of the ocean caused a trend of increasing IW activity since 1900, consistent with global climate model experiments that show stratification in the Luzon Strait increases in response to radiative forcing. IW activity is expected to increase in the NSCS through the 21st century, with implications for mitigating climate change impacts on coastal ecosystems.

DeCarlo, T. M., K. B. Karnauskas, K. A. Davis, and G. T. F. Wong, 2015: Climate modulates internal wave activity in the Northern South China Sea. Geophys. Res. Lett., 42(3), 831–838, doi: 10.1002/2014GL062522.

Low oxygen levels in tropical oceans shape marine ecosystems and biogeochemistry, and climate change is expected to expand these regions. Now a study indicates that regional dynamics control tropical oxygen trends, bucking projected global reductions in ocean oxygen.

Doney, S. C. and K. B. Karnauskas, 2014: Oxygen and climate dynamics. Nature Climate Change, 4(10), 862–863, doi: 10.1038/nclimate2386.

Geophysical and sedimentary records from five lakes in Massachusetts reveal regionally coherent hydrologic variability during the Holocene. All of the lakes have risen since ~9.0 ka, but multicentury droughts after 5.6 ka repeatedly lowered their water levels. Quantified water level histories from the three best-studied lakes share >70% of their reconstructed variance. Four prominent low-water phases at 4.9–4.6, 4.2–3.9, 2.9–2.1, and 1.3–1.2 ka were synchronous across coastal lakes, even after accounting for age uncertainties. The droughts also affected sites up to ~200 km inland, but water level changes at 5.6–4.9 ka appear out of phase between inland and coastal lakes. During the enhanced multicentury variability after ~5.6 ka, droughts coincided with cooling in Greenland and may indicate circulation changes across the North Atlantic region. Overall, the records demonstrate that current water levels are exceptionally high and confirm the sensitivity of water resources in the northeast U.S. to climate change.

Newby, P. E., B. N. Shuman, J. P. Donnelly, K. B. Karnauskas, and J. Marsicek, 2014: Centennial–to–Millennial Hydrologic Trends and Variability along the North Atlantic Coast, U.S.A., during the Holocene. Geophys. Res. Lett., 41(12), 4300–4307, doi: 10.1002/2014GL060183.

Upwelling across the tropical Pacific Ocean is projected to weaken in accordance with a reduction of the atmospheric overturning circulation1, enhancing the increase in sea surface temperature relative to other regions in response to greenhouse-gas forcing. In the central Pacific, home to one of the largest marine protected areas and fishery regions in the global tropics, sea surface temperatures are projected to increase by 2.8 °C by the end of this century2, 3, 4. Of critical concern is that marine protected areas may not provide refuge from the anticipated rate of large-scale warming, which could exceed the evolutionary capacity of coral and their symbionts to adapt5. Combining high-resolution satellite measurements6, 7, an ensemble of global climate models4 and an eddy-resolving regional ocean circulation model8, we show that warming and productivity decline around select Pacific islands will be mitigated by enhanced upwelling associated with a strengthening of the equatorial undercurrent. Enhanced topographic upwelling will act as a negative feedback, locally mitigating the surface warming. At the Gilbert Islands, the rate of warming will be reduced by 0.7±0.3 °C or 25 ± 9% per century, or an overall cooling effect comparable to the local anomaly for a typical El Niño, by the end of this century. As the equatorial undercurrent is dynamically constrained to the Equator, only a handful of coral reefs stand to benefit from this equatorial island effect. Nevertheless, those that do face a lower rate of warming, conferring a significant advantage over neighbouring reef systems. If realized, these predictions help to identify potential refuges for coral reef communities from anticipated climate changes of the twenty-first century.

Karnauskas, K. B., and A. L. Cohen, 2012: Equatorial refuge amid tropical warming. Nature Climate Change, 2(7), 530–534, doi: 10.1038/nclimate1499.

Sea surface temperature (SST) across much of the tropics has increased by 0.4° to 1°C since the mid-1970s. A parallel increase in the frequency and extent of coral bleaching and mortality has fueled concern that climate change poses a major threat to the survival of coral reef ecosystems worldwide. Here we show that steadily rising SSTs, not ocean acidification, are already driving dramatic changes in the growth of an important reef-building coral in the central Red Sea. Three-dimensional computed tomography analyses of the massive coral Diploastrea heliopora reveal that skeletal growth of apparently healthy colonies has declined by 30% since 1998. The same corals responded to a short-lived warm event in 1941/1942, but recovered within 3 years as the ocean cooled. Combining our data with climate model simulations by the Intergovernmental Panel on Climate Change, we predict that should the current warming trend continue, this coral could cease growing altogether by 2070.

Cantin, N. E., A. L. Cohen, K. B. Karnauskas, A. M. Tarrant, and D. C. McCorkle, 2010: Ocean warming slows coral growth in the central Red Sea. Science, 329(5989), 322–325, doi: 10.1126/science.1190182.

The interannual variability of outgoing longwave radiation (OLR) over Africa from the Advanced Very High Resolution Radiometer (AVHRR) and zonal wind speed in the African easterly jet (AEJ) is analyzed and discussed in the context of Atlantic tropical cyclone activity. It is found that hurricane and tropical storm totals in the Atlantic basin are closely related to the African meridional OLR contrast (AMOC). It is suggested that the AMOC provides a simple yet novel way to simultaneously characterize the meridional temperature gradient and ITCZ activity, both of which play integral roles in generating African easterly waves. Complimentary to observed relationships between Sahel rainfall and Atlantic tropical cyclone activity, the potential for the AMOC to augment existing techniques used in preparing Atlantic hurricane season outlooks is also discussed.

Karnauskas, K. B., 2006: The African meridional OLR contrast as a diagnostic for Atlantic tropical cyclone activity and implications for predictability. Geophys. Res. Lett., 33(6), L06809, doi: 10.1029/2005GL024865.