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Abstract. Recent earth system models predict a 10 %–20 % decrease in particulate organic carbon export from the surface ocean by the end of the21st century due to global climate change. This decline is mainly caused by increased stratification of the upper ocean, resulting in reducedshallow subsurface nutrient concentrations and a slower supply of nutrients to the surface euphotic zone in low latitudes. These predictions,however, do not typically account for associated changes in remineralization depths driven by sinking-particle size. Here we combinesatellite-derived export and particle size maps with a simple 3-D global biogeochemical model that resolves dynamic particle size distributions toinvestigate how shifts in particle size may buffer or amplify predicted changes in surface nutrient supply and therefore export production. We showthat higher export rates are empirically correlated with larger sinking particles and presumably larger phytoplankton, particularly in tropical andsubtropical regions. Incorporating these empirical relationships into our global model shows that as circulation slows, a decrease in export isassociated with a shift towards smaller particles, which sink more slowly and are thus remineralized shallower. This shift towards shallowerremineralization in turn leads to greater recycling of nutrients in the upper water column and thus faster nutrient recirculation into the euphoticzone. The end result is a boost in productivity and export that counteracts the initial circulation-driven decreases. This negative feedbackmechanism (termed the particle-size–remineralization feedback) slows export decline over the next century by ∼ 14 % globally (from −0.29to −0.25 GtC yr−1) and by ∼ 20 % in the tropical and subtropical oceans, where export decreases are currently predicted tobe greatest. Our findings suggest that to more accurately predict changes in biological pump strength under a warming climate, earth system modelsshould include dynamic particle-size-dependent remineralization depths.more » « less
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null (Ed.)Abstract. Accurate estimates of past global mean surface temperature (GMST) help tocontextualise future climate change and are required to estimate thesensitivity of the climate system to CO2 forcing through Earth's history.Previous GMST estimates for the latest Paleocene and early Eocene(∼57 to 48 million years ago) span a wide range(∼9 to 23 ∘C higher than pre-industrial) andprevent an accurate assessment of climate sensitivity during this extremegreenhouse climate interval. Using the most recent data compilations, weemploy a multi-method experimental framework to calculate GMST during thethree DeepMIP target intervals: (1) the latest Paleocene (∼57 Ma), (2) the Paleocene–Eocene Thermal Maximum (PETM; 56 Ma), and (3) the earlyEocene Climatic Optimum (EECO; 53.3 to 49.1 Ma). Using six differentmethodologies, we find that the average GMST estimate (66 % confidence)during the latest Paleocene, PETM, and EECO was 26.3 ∘C (22.3 to28.3 ∘C), 31.6 ∘C (27.2 to 34.5 ∘C), and27.0 ∘C (23.2 to 29.7 ∘C), respectively. GMST estimatesfrom the EECO are ∼10 to 16 ∘C warmer thanpre-industrial, higher than the estimate given by the Intergovernmental Panel on Climate Change (IPCC) 5thAssessment Report (9 to 14 ∘C higher than pre-industrial).Leveraging the large “signal” associated with these extreme warm climates,we combine estimates of GMST and CO2 from the latest Paleocene, PETM,and EECO to calculate gross estimates of the average climate sensitivitybetween the early Paleogene and today. We demonstrate that “bulk”equilibrium climate sensitivity (ECS; 66 % confidence) during the latestPaleocene, PETM, and EECO is 4.5 ∘C (2.4 to 6.8 ∘C),3.6 ∘C (2.3 to 4.7 ∘C), and 3.1 ∘C (1.8 to4.4 ∘C) per doubling of CO2. These values are generallysimilar to those assessed by the IPCC (1.5 to 4.5 ∘C per doublingCO2) but appear incompatible with low ECS values (<1.5 perdoubling CO2).more » « less
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Abstract Characteristics of tropical cyclones (TCs) in global climate models (GCMs) are known to be influenced by details of the model configurations, including horizontal resolution and parameterization schemes. Understanding model-to-model differences in TC characteristics is a prerequisite for reducing uncertainty in future TC activity projections by GCMs. This study performs a process-level examination of TC structures in eight GCM simulations that span a range of horizontal resolutions from 1° to 0.25°. A recently developed set of process-oriented diagnostics is used to examine the azimuthally averaged wind and thermodynamic structures of the GCM-simulated TCs. Results indicate that the inner-core wind structures of simulated TCs are more strongly constrained by the horizontal resolutions of the models than are the thermodynamic structures of those TCs. As expected, the structures of TC circulations become more realistic with smaller horizontal grid spacing, such that the radii of maximum wind (RMW) become smaller, and the maximum vertical velocities occur off the center. However, the RMWs are still too large, especially at higher intensities, and there are rising motions occurring at the storm centers, inconsistently with observations. The distributions of precipitation, moisture, and radiative and surface turbulent heat fluxes around TCs are diverse, even across models with similar horizontal resolutions. At the same horizontal resolution, models that produce greater rainfall in the inner-core regions tend to simulate stronger TCs. When TCs are weak, the radial gradient of net column radiative flux convergence is comparable to that of surface turbulent heat fluxes, emphasizing the importance of cloud–radiative feedbacks during the early developmental phases of TCs.
