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P. Michael Link, Tim Brücher, Martin Claussen, Jasmin S. A. Link, and Jürgen Scheffran
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Ping Chang, Link Ji, Bin Wang, and Tim Li

Abstract

The nonlinear interactions between the seasonal cycle and El Niño-Southern Oscillation (ENSO) in the coupled ocean-atmosphere system are examined using a newly developed intermediate coupled ocean-atmosphere model. The model permits coupling between total sea surface temperature (SST) and total surface winds and thus is able to produce its own seasonal cycle. This coupling approach allows for the examination of full dynamic interactions between the seasonal cycle and interannual oscillations. Numerical simulations with realistic surface heat fluxes indicate that this model is capable of capturing the essential variability of the coupled ocean-atmosphere system on seasonal-to-interannual timescale in the tropical Pacific.

Model sensitivity experiments were carried out by independently varying the external forcing strength and coupling strength. These experiments reveal a very different behavior of the coupled system with and without the seasonal cycle. In the presence of the seasonal cycle, the coupled model, in response to changes in the model parameters, undergoes several transitions between periodic (frequency-locking) and chaotic states. Chaotic response is found as the forcing amplitude approaches the observed value. In contrast, in the absence of the seasonal cycle, varying model coupling strength produces neither frequency-locking nor chaos. The coupled system simply undergoes a Hopf bifurcation from a nonoscillatory state to a periodic state as the coupling strength increases. This result suggests that nonlinear interactions between the forced seasonal mode and the intrinsic ENSO mode of oscillation are crucial for the irregular behavior of the model ENSO cycle. The experiments also show that a biennial oscillation can be excited by seasonal forcing even when air-sea coupling is so weak that a self-sustaining oscillation does not exist in the coupled system. This implies that the biennial oscillation observed as a fundamental element of ENSO variability in the low-latitude eastern Indian and western Pacific sector could be a subharmonic resonant to the seasonal forcing rather than a self-sustaining oscillation of the coupled system. Analysis of SST time series further demonstrates that major ENSO “episodes” in the coupled model exhibit a preferred phasing with the seasonal cycle. This phase-locking with the seasonal cycle occurs not only when the model ENSO cycle is periodic but also when it is chaotic. However, phase locking in the model appears to be tighter than that in nature. This study uncovers dual roles of the seasonal cycle in ENSO variabilities: it introduces a degree of regularity into the ENSO cycle by producing annual phase-locking and it generates chaos in the coupled system through inherent nonlinear interactions.

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John Pomeroy, Chad Ellis, Aled Rowlands, Richard Essery, Janet Hardy, Tim Link, Danny Marks, and Jean Emmanuel Sicart

Abstract

The spatial variation of melt energy can influence snow cover depletion rates and in turn be influenced by the spatial variability of shortwave irradiance to snow. The spatial variability of shortwave irradiance during melt under uniform and discontinuous evergreen canopies at a U.S. Rocky Mountains site was measured, analyzed, and then compared to observations from mountain and boreal forests in Canada. All observations used arrays of pyranometers randomly spaced under evergreen canopies of varying structure and latitude. The spatial variability of irradiance for both overcast and clear conditions declined dramatically, as the sample averaging interval increased from minutes to 1 day. At daily averaging intervals, there was little influence of cloudiness on the variability of subcanopy irradiance; instead, it was dominated by stand structure. The spatial variability of irradiance on daily intervals was higher for the discontinuous canopies, but it did not scale reliably with canopy sky view. The spatial variation in irradiance resulted in a coefficient of variation of melt energy of 0.23 for the set of U.S. and Canadian stands. This variability in melt energy smoothed the snow-covered area depletion curve in a distributed melt simulation, thereby lengthening the duration of melt by 20%. This is consistent with observed natural snow cover depletion curves and shows that variations in melt energy and snow accumulation can influence snow-covered area depletion under forest canopies.

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Richard Essery, Peter Bunting, Aled Rowlands, Nick Rutter, Janet Hardy, Rae Melloh, Tim Link, Danny Marks, and John Pomeroy

Abstract

Solar radiation beneath a forest canopy can have large spatial variations, but this is frequently neglected in radiative transfer models for large-scale applications. To explicitly model spatial variations in subcanopy radiation, maps of canopy structure are required. Aerial photography and airborne laser scanning are used to map tree locations, heights, and crown diameters for a lodgepole pine forest in Colorado as inputs to a spatially explicit radiative transfer model. Statistics of subcanopy radiation simulated by the model are compared with measurements from radiometer arrays, and scaling of spatial statistics with temporal averaging and array size is discussed. Efficient parameterizations for spatial averages and standard deviations of subcanopy radiation are developed using parameters that can be obtained from the model or hemispherical photography.

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