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Cavaleri, M. A., Coble, A. P., Ryan, M. G., Bauerle, W. L., Loescher, H. W., & Oberbauer, S. F. (2017). Tropical rainforest carbon sink declines during El Niño as a result of reduced photosynthesis and increased respiration rates. New Phytol, 216(1), 136–149.
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Ducker, J. A., Holmes, C. D., Keenan, T. F., Fares, S., Goldstein, A. H., Mammarella, I., et al. (2018). Synthetic ozone deposition and stomatal uptake at flux tower sites. Biogeosciences, 15(17), 5395–5413.
Abstract: We develop and evaluate a method to estimate O-3 deposition and stomatal O-3 uptake across networks of eddy covariance flux tower sites where O-3 concentrations and O-3 fluxes have not been measured. The method combines standard micrometeorological flux measurements, which constrain O-3 deposition velocity and stomatal conductance, with a gridded dataset of observed surface O-3 concentrations. Measurement errors are propagated through all calculations to quantify O-3 flux uncertainties. We evaluate the method at three sites with O(3 )flux measurements: Harvard Forest, Blodgett Forest, and Hyytiala Forest. The method reproduces 83 % or more of the variability in daily stomatal uptake at these sites with modest mean bias (21 % or less). At least 95 % of daily average values agree with measurements within a factor of 2 and, according to the error analysis, the residual differences from measured O-3 fluxes are consistent with the uncertainty in the underlying measurements.
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Erb, M. P., Broccoli, A. J., Graham, N. T., Clement, A. C., Wittenberg, A. T., & Vecchi, G. A. (2015). Response of the Equatorial Pacific Seasonal Cycle to Orbital Forcing. J. Climate, 28(23), 9258–9276.
Abstract: The response of the equatorial Pacific Ocean's seasonal cycle to orbital forcing is explored using idealized simulations with a coupled atmosphere-ocean GCM in which eccentricity, obliquity, and the longitude of perihelion are altered while other boundary conditions are maintained at preindustrial levels. The importance of ocean dynamics in the climate response is investigated using additional simulations with a slab ocean version of the model. Precession is found to substantially influence the equatorial Pacific seasonal cycle through both thermodynamic and dynamic mechanisms, while changes in obliquity have only a small effect. In the precession experiments, western equatorial Pacific SSTs respond in a direct thermodynamic manner to changes in insolation, while the eastern equatorial Pacific is first affected by the propagation of thermocline temperature anomalies from the west. These thermocline signals result from zonal wind anomalies associated with changes in the strength of subtropical anticyclones and shifts in the regions of convection in the western equatorial Pacific. The redistribution of heat from these thermocline signals, aided by the direct thermodynamic effect of insolation anomalies, results in large changes to the strength and timing of the eastern equatorial Pacific seasonal cycle. A comparison of 10 CMIP5 mid-Holocene experiments, in which the primary forcing is due to precession, shows that this response is relatively robust across models. Because equatorial Pacific SST anomalies have local climate impacts as well as nonlocal impacts through teleconnections, these results may be important to understanding paleoclimate variations both inside and outside of the tropical Pacific.
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Hodgkins, S. B., Richardson, C. J., Dommain, R., Wang, H., Glaser, P. H., Verbeke, B., et al. (2018). Tropical peatland carbon storage linked to global latitudinal trends in peat recalcitrance. Nat Commun, 9(3640).
Abstract: Peatlands represent large terrestrial carbon banks. Given that most peat accumulates in boreal regions, where low temperatures and water saturation preserve organic matter, the existence of peat in (sub)tropical regions remains enigmatic. Here we examined peat and plant chemistry across a latitudinal transect from the Arctic to the tropics. Near-surface low-latitude peat has lower carbohydrate and greater aromatic content than near-surface high-latitude peat, creating a reduced oxidation state and resulting recalcitrance. This recalcitrance allows peat to persist in the (sub)tropics despite warm temperatures. Because we observed similar declines in carbohydrate content with depth in high-latitude peat, our data explain recent field-scale deep peat warming experiments in which catotelm (deeper) peat remained stable despite temperature increases up to 9 degrees C. We suggest that high-latitude deep peat reservoirs may be stabilized in the face of climate change by their ultimately lower carbohydrate and higher aromatic composition, similar to tropical peats.
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Ishtiaq, K. S., & Abdul-Aziz, O. I. (2015). Relative Linkages of Canopy-Level CO2 Fluxes with the Climatic and Environmental Variables for US Deciduous Forests. Environmental Management, 55(4), 943–960.
Abstract: We used a simple, systematic data-analytics approach to determine the relative linkages of different climate and environmental variables with the canopy-level, half-hourly CO2 fluxes of US deciduous forests. Multivariate pattern recognition techniques of principal component and factor analyses were utilized to classify and group climatic, environmental, and ecological variables based on their similarity as drivers, examining their interrelation patterns at different sites. Explanatory partial least squares regression models were developed to estimate the relative linkages of CO2 fluxes with the climatic and environmental variables. Three biophysical process components adequately described the system-data variances. The ‘radiation-energy’ component had the strongest linkage with CO2 fluxes, whereas the ‘aerodynamic’ and ‘temperature-hydrology’ components were low to moderately linked with the carbon fluxes. On average, the ‘radiation-energy’ component showed 5 and 8 times stronger carbon flux linkages than that of the ‘temperature-hydrology’ and ‘aerodynamic’ components, respectively. The similarity of observed patterns among different study sites (representing gradients in climate, canopy heights and soil-formations) indicates that the findings are potentially transferable to other deciduous forests. The similarities also highlight the scope of developing parsimonious data-driven models to predict the potential sequestration of ecosystem carbon under a changing climate and environment. The presented data-analytics provides an objective, empirical foundation to obtain crucial mechanistic insights; complementing process-based model building with a warranted complexity. Model efficiency and accuracy (R 2 = 0.55–0.81; ratio of root-mean-square error to the observed standard deviations, RSR = 0.44–0.67) reiterate the usefulness of multivariate analytics models for gap-filling of instantaneous flux data.
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Katsaros, K. B., A. Bentamy, M. Bourassa, N. Ebuchi, J. Gower, W. T. Liu, and S. Vignudelli. (2011). Climate Data Issues from an Oceanographic Remote Sensing Perspective. In Remote Sensing of the Changing Oceans (pp. 7–32). Berlin, Germany: Springer-Verlag.
Abstract: In this chapter we review several climatologically important variables
with a history of observation from spaceborne platforms. These include sea surface
temperature and wind vectors, altimetric estimates of sea surface height, energy and
water vapor fluxes at the sea surface, precipitation over the ocean, and ocean color.
We then discuss possible improvements in sampling for climate and climate change
definition. Issues of consistency of different data sources, archiving and distribution
of these types of data are discussed. The practical prospect of immediate international
coordination through the concept of virtual constellations is discussed and
applauded.
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Malone, S. L., Barr, J., Fuentes, J. D., Oberbauer, S. F., Staudhammer, C. L., Gaiser, E. E., et al. (2016). Sensitivity to Low-Temperature Events: Implications for CO2 Dynamics in Subtropical Coastal Ecosystems. Wetlands, 36(5), 957–967.
Abstract: We analyzed the ecosystem effects of low-temperature events (< 5 A degrees C) over 4 years (2009-2012) in subtropical short and long hydroperiod freshwater marsh and mangrove forests within Everglades National Park. To evaluate changes in ecosystem productivity, we measured temporal patterns of CO2 and the normalized difference vegetation index over the study period. Both water levels and distance from the coast influenced the ecosystem response to low-temperature events. Photosynthetic capacity, or the maximum CO2 uptake rate, and sensitivity to low-temperature events were much higher in mangrove forest than in freshwater marsh ecosystems. During low-temperature events photosynthetic capacity was enhanced in freshwater marsh while it declined in mangrove forests, and respiration rates declined across Everglades ecosystems. While the long hydroperiod freshwater marsh gained 0.26 g CO2 m(-2) during low-temperature events, the mangrove forest had the greatest C lost (7.11 g CO2 m(-2) low-temperature event(-1)) followed by the short hydroperiod freshwater marsh (0.37 g CO2 m(-2) low-temperature event(-1)). Results suggest that shifts in the frequency and intensity of weather anomalies with climate change can alter C assimilation rates in Everglades ecosystems through effects on the photosynthetic capacity of existing species, which might lead to changes in species composition and ecosystem productivity in the future.
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Potter, H., Drennan, W. M., & Graber, H. C. (2017). Upper ocean cooling and air-sea fluxes under typhoons: A case study. J. Geophys. Res. Oceans, 122(9), 7237–7252.
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Wanninkhof, R., & Trinanes, J. (2017). The impact of changing wind speeds on gas transfer and its effect on global air-sea CO2 fluxes. Global Biogeochem. Cycles, 31(6), 961–974.
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Weihs, R. R., & Bourassa, M. A. (2014). Modeled diurnally varying sea surface temperatures and their influence on surface heat fluxes. J. Geophys. Res. Oceans, 119(7), 4101–4123.
Abstract: A diurnal warming model is used to create a new data set of global, diurnally varying sea surface temperatures (dSSTs) and surface turbulent heat fluxes over a 5 year period. The magnitude of diurnal warming is primarily a function of low wind speed and net heat flux. Differences between each of the surface turbulent fluxes with and without a diurnally varying SST are examined on hourly, daily, and seasonal time scales. Over a 2 month period, maximum averaged diurnal warming is as large as 0.3°C, and latent heat flux is underestimated by as much as 8 W/m2 in the Indian Ocean. They also exceed roughly 0.7°C and 10 W/m2, respectively, up to 25% of the total daytime in the Atlantic. A best-case approach validation shows the model overestimates peak warming and underestimates the duration of the cycle, though the average error is quite small. The model is tested under a variety of wind speed, solar radiation, and precipitation conditions to examine the impact of potential biases or error in the input data. To test the impact of a positive bias in the wind speeds, diurnal warming magnitudes are recomputed with an adjusted wind under near-neutral conditions. Compared to the original data, diurnal warming can increase by as much as 1.5°C on an hourly scale but generally is <0.06°C. Although precipitation effects on dSSTs are small compared to winds and radiation, the model configuration wrongly causes diurnal warming to increase by precipitation, contrary to the underlying model physics.
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Xu, X., Rhines, P. B., & Chassignet, E. P. (2016). Temperature-Salinity Structure of the North Atlantic Circulation and Associated Heat and Freshwater Transports. J. Climate, 29(21), 7723–7742.
Abstract: This study investigates the circulation structure and relative contribution of circulation components to the time-mean meridional heat and freshwater transports in the North Atlantic, using numerical results of a high-resolution ocean model that are shown to be in excellent agreement with the observations. The North Atlantic circulation can be separated into the large-scale Atlantic meridional overturning circulation (AMOC) that is diapycnal and the subtropical and subpolar gyres that largely flow along isopycnal surfaces but also include prominent gyre-scale diapycnal overturning in the Subtropical Mode Water and Labrador Sea Water. Integrals of the meridional volume transport as a function of potential temperature θ and salinity S yield streamfunctions with respect to θ and to S, and heat functions. These argue for a significant contribution to the heat transport by the southward circulation of North Atlantic Deep Water. At 26.5°N, the isopycnic component of the subtropical gyre is colder and fresher in the northward-flowing western boundary currents than the southward return flows, and it carries heat southward and freshwater northward, opposite of that of the diapycnal component. When combined, the subtropical gyre contributes virtually zero to the heat transport and the AMOC is responsible for all the heat transport across this latitude. The subtropical gyre however significantly contributes to the freshwater transport, reducing the 0.5-Sv (1 Sv ≡ 106 m3 s–1) southward AMOC freshwater transport by 0.13 Sv. In the subpolar North Atlantic near 58°N, the diapycnal component of the circulation, or the transformation of warm saline upper Atlantic water into colder fresher deep waters, is responsible for essentially all of the heat and freshwater transports.
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