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1. Interannual variation in rainfall modulates temperature sensitivity of carbon allocation and flux in a tropical montane wet forest

   https://doi.org/10.1111/gcb.15664  

   Lyu, Maokui ; Giardina, Christian P. ; Litton, Creighton M. ( May 2021 , Global Change Biology)

   Tropical forests exert a disproportionately large influence on terrestrial carbon (C) balance but projecting the effects of climate change on C cycling in tropical forests remains uncertain. Reducing this uncertainty requires improved quantification of the independent and interactive effects of variable and changing temperature and precipitation regimes on C inputs to, cycling within and loss from tropical forests. Here, we quantified aboveground litterfall and soil‐surface CO₂efflux (“soil respiration”;F_S) in nine plots organized across a highly constrained 5.2°C mean annual temperature (MAT) gradient in tropical montane wet forest. We used five consecutive years of these measurements, during which annual rainfall (AR) steadily increased, in order to: (a) estimate total belowground C flux (TBCF); (b) examine how interannual variation in AR alters the apparent temperature dependency (Q₁₀) of above‐ and belowground C fluxes; and (c) quantify stand‐level C allocation responses to MAT and AR. Averaged across all years,F_S, litterfall, and TBCF increased positively and linearly with MAT, which accounted for 49, 47, and 46% of flux rate variation, respectively. Rising AR lowered TBCF andF_S, but increased litterfall, with patterns representing interacting responses to declining light. The Q₁₀ofF_S, litterfall, and TBCF all decreased with increasing AR, with peak sensitivity to MAT in the driest year and lowest sensitivity in the wettest. These findings support the conclusion that for this tropical montane wet forest, variations in light, water, and nutrient availability interact to strongly influence productivity (litterfall+TBCF), the sensitivity of above‐ and belowground C fluxes to rising MAT (Q₁₀ofF_S, litterfall, and TBCF), and C allocation patterns (TBCF:[litterfall+TBCF]).

    

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2. Three Regimes of Temperature Distribution Change Over Dry Land, Moist Land, and Oceanic Surfaces

   https://doi.org/10.1029/2020GL090997  

   Duan, Suqin Q. ; Findell, Kirsten L. ; Wright, Jonathon S. ( December 2020 , Geophysical Research Letters)

   Climate model simulations project different regimes of summertime temperature distribution changes under a quadrupling of CO₂for dry land, moist land, and oceanic surfaces. The entire temperature distribution shifts over dry land surfaces, while moist land surfaces feature an elongated upper tail of the distribution, with extremes increasing more than the corresponding means by ∼20% of the global mean warming. Oceanic surfaces show weaker warming relative to land surfaces, with no significant elongation of the upper tail. Dry land surfaces show little change in turbulent sensible (SH) or latent (LH) fluxes, with new balance reached with compensating adjustments among downwelling and upwelling radiative fluxes. By contrast, moist land surfaces show enhanced partitioning of turbulent flux toward SH, while oceanic surfaces show enhanced partitioning toward LH. Amplified warming of extreme temperatures over moist land surfaces is attributed to suppressed evapotranspiration and larger Bowen ratios.

    

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3. ENSO‐Influenced Drought Drives Methane Flux Dynamics in a Tropical Wet Forest Soil

   https://doi.org/10.1029/2018JG004832  

   Aronson, E. L. ; Dierick, D. ; Botthoff, J. K. ; Oberbauer, S. ; Zelikova, T. J. ; Harmon, T. C. ; Rundel, P. ; Johnson, R. F. ; Swanson, A. C. ; Pinto‐Tomás, A. A. ; et al ( July 2019 , Journal of Geophysical Research: Biogeosciences)

   Global atmospheric methane growth rates have wildly fluctuated over the past three decades, which may be driven by the proportion of tropical land surface saturated by water. The El Niño/Southern Oscillation Event (ENSO) cycle drives large‐scale climatic trends globally, with El Niño events typically bringing drier weather than La Niña. In a lowland tropical wet forest in Costa Rica, we measured methane flux bimonthly from March 2016 to June 2017 and using an automated chamber system. We observed a strong drying trend for several weeks during the El Niño in 2016, reducing soil moisture below normal levels. In contrast, soil conditions had high water content prior to the drought and during the moderate La Niña that followed. Soil moisture varied across the period studied and significantly impacted methane flux. Methane consumption was greater during the driest part of the El Niño period, while during La Niña and other time periods, soils had lower methane consumption. The mean methane flux observed was −0.022 mg CH₄‐C m⁻²hr⁻¹, and methane was consumed at all timepoints, with lower consumption in saturated soils. Our data show that month studied, and the correlation between soil type and month significantly drove methane flux trends. Our data indicate that ENSO cycles may impact biogenic methane fluxes, mediated by soil moisture conditions. Climate projections for Central America show dryer conditions and increased El Niño frequency, further exacerbating predicted drought. These trends may lead to negative climate feedbacks, with drier conditions increasing soil methane consumption from the atmosphere.

    

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4. A Hybrid Bulk Algorithm to Predict Turbulent Fluxes over Dry and Wet Bare Soils

   https://doi.org/10.1175/JAMC-D-20-0232.1  

   Grachev, A. ( January 2022 , Journal of Applied Meteorology and Climatology)

   Abstract Measurements made in the Columbia River Basin (Oregon) in an area of irregular terrain during the second Wind Forecast Improvement Project (WFIP 2) field campaign are used to develop an optimized hybrid bulk algorithm to predict the surface turbulent fluxes from readily measured or modelled quantities over dry and wet bare or lightly vegetated soil surfaces. The hybrid (synthetic) algorithm combines (i) an aerodynamic method for turbulent flow which is based on the transfer coefficients (drag coefficient and Stanton number), roughness lengths, and Monin-Obukhov similarity and (ii) a modified Priestley-Taylor (P-T) algorithm with physically based ecophysiological constraints which is essentially based on the surface energy budget (SEB) equation. Soil heat flux in the latter case was estimated from measurements of soil temperature and soil moisture. In the framework of the hybrid algorithm, bulk estimates of the momentum flux and the sensible heat flux are derived from a traditional aerodynamic approach, whereas the latent heat flux (or moisture flux) is evaluated from a modified P-T model. Direct measurements of the surface fluxes (turbulent and radiative) and other ancillary atmospheric/soil parameters made during WFIP 2 for different soil conditions (dry and wet) are used to optimize and tune the hybrid bulk algorithm. The bulk flux estimates are validated against the measured eddy-covariance fluxes. We also discuss the SEB closure over dry and wet surfaces at various timescales based on the modelled and measured fluxes. Although this bulk flux algorithm is optimized for the data collected during the WFIP 2, a hybrid approach can be used for similar flux-tower sites and field campaigns. 

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5. Regimes of Soil Moisture–Wet-Bulb Temperature Coupling with Relevance to Moist Heat Stress

   https://doi.org/10.1175/JCLI-D-23-0132.1  

   Kong, Qinqin ; Huber, Matthew ( November 2023 , Journal of Climate)

   Human heat stress depends jointly on atmospheric temperature and humidity. Wetter soils reduce temperature but also raise humidity, making the collective impact on heat stress unclear. To better understand these interactions, we use ERA5 to examine the coupling between daily average soil moisture and wet-bulb temperature (T_w) and its seasonal and diurnal cycle at global scale. We identify a global soil moisture–T_wcoupling pattern with both widespread negative and positive correlations in contrast to the well-established cooling effect of wet soil on dry-bulb temperature. Regions showing positive correlations closely resemble previously identified land–atmosphere coupling hotspots where soil moisture effectively controls surface energy partition. Soil moisture–T_wcoupling varies seasonally closely tied to monsoon development, and the positive coupling is slightly stronger and more widespread during nighttime. Local-scale analysis demonstrates a nonlinear structure of soil moisture–T_wcoupling with stronger coupling under relatively dry soils. Hot days with highT_wvalues show wetter-than-normal soil, anomalous high latent and low sensible heat flux from a cooler surface, and a shallower boundary layer. This supports the hypothesis that wetter soil increasesT_wby concentrating surface moist enthalpy flux within a shallower boundary layer and reducing free-troposphere-air entrainment. We identify areas of particular interest for future studies on the physical mechanisms of soil moisture–heat stress coupling. Our findings suggest that increasing soil moisture might amplify heat stress over large portions of the world including several densely populated areas. These results also raise questions about the effectiveness of evaporative cooling strategies in ameliorating urban heat stress.

   The purpose of this study is to provide a global picture of the relationship between soil moisture anomalies and a heat stress metric that includes the joint effects of temperature and humidity. This is important because a better understanding of this relationship will help improve the prediction of extreme heat stress events and inform strategies for ameliorating heat stress. We find a widespread positive correlation between soil moisture and heat stress, in contrast to studies relying on temperature alone. This raises the possibility that, over much of the world, and in the most populous regions, strategies like irrigation or “greening” that can reduce temperature might be ineffective or even harmful in reducing heat stress with humidity incorporated.

    

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