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Qi Quan, Zhongya Cai, Guangzhen Jin, and Zhiqiang Liu

Abstract

Topographic Rossby waves (TRWs) in the abyssal South China Sea (SCS) are investigated using observations and high-resolution numerical simulations. These energetic waves can account for over 40% of the kinetic energy (KE) variability in the deep western boundary current and seamount region in the central SCS. This proportion can even reach 70% over slopes in the northern and southern SCS. The TRW-induced currents exhibit columnar (i.e., in phase) structure in which the speed increases downward. Wave properties such as the period (5–60 days), wavelength (100–500 km), and vertical trapping scale (102–103 m) vary significantly depending on environmental parameters of the SCS. The TRW energy propagates along steep topography with phase propagation offshore. TRWs with high frequencies exhibit a stronger climbing effect than low-frequency ones and hence can move further upslope. For TRWs with a certain frequency, the wavelength and trapping scale are dominated by the topographic beta, whereas the group velocity is more sensitive to the internal Rossby deformation radius. Background circulation with horizontal shear can change the wavelength and direction of TRWs if the flow velocity is comparable to the group velocity, particularly in the central, southern, and eastern SCS. A case study suggests two possible energy sources for TRWs: mesoscale perturbation in the upper layer and large-scale background circulation in the deep layer. The former provides KE by pressure work, whereas the latter transfers the available potential energy (APE) through baroclinic instability.

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Chaim I Garfinkel, Ofer Shamir, Itzhak Fouxon, and Nathan Paldor

Abstract

Variability in the tropical atmosphere is concentrated at wavenumber–frequency combinations where linear theory indicates wave modes can freely propagate, but with substantial power in between. This study demonstrates that such a power spectrum can arise from small-scale convection triggering large-scale waves via wave–wave interactions in a moderately turbulent fluid. Two key pieces of evidence are provided for this interpretation of tropical dynamics using a nonlinear rotating shallow-water model: a parameter sweep experiment in which the amplitude of an external forcing is gradually ramped up, and also an external forcing in which only symmetric or only antisymmetric modes are forced. These experiments do not support a commonly accepted mechanism involving the forcing projecting directly onto the wave modes with a strong response, yet still simulate a power spectrum resembling that observed, though the linear projection mechanism could still complement the mechanism proposed here in observations. Interpreting the observed tropical power spectrum using turbulence offers a simple explanation as to why power should be concentrated at the theoretical wave modes, and also provides a solid footing for the common assumption that the background spectrum is red, even as it clarifies why there is no expectation for a turbulent cascade with a specific, theoretically derived slope such as −5/3. However, it does explain why the cascade should be toward lower wavenumbers, that is an inverse energy cascade, similar to the midlatitudes even as compressible wave modes are important for tropical dynamics.

Open access
Tao Zhu and Jing Yang

Abstract

Two types of mid-high-latitude low-frequency intraseasonal oscillations (LF-ISOs), featuring eastward and westward propagation, have been identified over the Eurasian continent in the past 37 summers (1982–2018). The eastward and westward propagating modes commonly have a dominant periodicity of 30–50 days near the Ural Mountains (UM) but have different origins and evolutions. The eastward propagating LF-ISO initiates over eastern North America, migrates northeastward across northeastern North America–western North Atlantic, central North Atlantic, western Europe, and the UM, then propagates southeastward to northwestern and eastern China, which is the Atlantic-Eurasian continental mode. In contrast, the westward propagating mode is quasi-circumpolar, initiating over the East Siberian Sea and moving southwestward across the UM and northern Europe and eventually reaching Greenland and the Canadian Arctic Archipelago. These two mid-high-latitude LF-ISOs are accompanied by significant tropical intraseasonal variations with evident tropical–extratropical interactions. Meanwhile, these two LF-ISOs have different decadal preferences before and after 2000, which are ascribed to the decadal change of both intraseasonal efficient kinetic energy obtained from the mean flow over their genesis region and their background flow associated with the North Atlantic Oscillation/Arctic Oscillation decadal change. This study deepens the understanding of subseasonal variations for mid-high latitudes and subseasonal prediction sources for low-latitude regions.

Open access
Timothy D. Mitchell and Joanne Camp

Abstract

The Conway–Maxwell–Poisson distribution improves the precision with which seasonal counts of tropical cyclones may be modeled. Conventionally the Poisson is used, which assumes that the formation and transit of tropical cyclones is the result of a Poisson process, such that their frequency distribution has equal mean and variance (“equi-dispersion”). However, earlier studies of observed records have sometimes found overdispersion, where the variance exceeds the mean, indicating that tropical cyclones are clustered in particular years. The evidence presented here demonstrates that at least some of this overdispersion arises from observational inhomogeneities. Once this is removed, and particularly near the coasts, there is evidence for equi-dispersion or underdispersion. To more accurately model numbers of tropical cyclones, we investigate the use of the Conway–Maxwell–Poisson as an alternative to the Poisson that represents any dispersion characteristic. An example is given for East China where using it improves the skill of a prototype seasonal forecast of tropical cyclone landfall.

Open access
Julia Jeworrek, Gregory West, and Roland Stull

Abstract

Physics parameterizations in the Weather Research and Forecasting (WRF) Model are systematically varied to investigate precipitation forecast performance over the complex terrain of southwest British Columbia (BC). Comparing a full year of modeling data from over 100 WRF configurations to station observations reveals sensitivities of precipitation intensity, season, location, grid resolution, and accumulation window. The choice of cumulus and microphysics parameterizations is most important. The WSM5 microphysics scheme yields competitive verification scores when compared to more sophisticated and computationally expensive parameterizations. Although the scale-aware Grell–Freitas cumulus parameterization performs better for summertime convective precipitation, the conventional Kain–Fritsch parameterization better simulates wintertime frontal precipitation, which contributes to the majority of the annual precipitation in southwest BC. Finer grid spacings have lower relative biases and a more realistic spread in precipitation intensity distribution, yet higher relative standard deviations of their errors—they produce finer spatial differences and local extrema. Finer resolutions produce the best fraction of correct-to-incorrect forecasts across all precipitation intensities, whereas the coarser 27-km domain yields the highest hit rates and equitable threat scores. Verification metrics improve greatly with longer accumulation windows—hourly precipitation values are prone to double-penalty issues, while longer accumulation windows compensate for timing errors but lose information about short-term precipitation intensities. This study provides insights regarding WRF precipitation performance in complex terrain across a wide variety of configurations, using metrics important to a range of end users.

Open access
Yann Y. Planton, Jérôme Vialard, Eric Guilyardi, Mathieu Lengaigne, and Michael J. McPhaden

Abstract

Unusually high western Pacific oceanic heat content often leads to El Niño about 1 year later, while unusually low heat content leads to La Niña. Here, we investigate if El Niño Southern Oscillation (ENSO) predictability also depends on the initial state recharge, and discuss the underlying mechanisms. To that end, we use the CNRM-CM5 model, which has a reasonable representation of the main observed ENSO characteristics, asymmetries and feedbacks. Observations and a 1007-years long CNRM-CM5 simulation indicate that discharged states evolve more systematically into La Niña events than recharged states into neutral states or El Niño events. We ran 70-members ensemble experiments in a perfect-model setting, initialized in boreal fall from either recharged or discharged western Pacific heat content, sampling the full range of corresponding ENSO phases. Predictability measures based both on spread and signal-to-noise ratio confirm that discharged states yield a more predictable ENSO outcome one year later than recharged states. As expected from recharge oscillator theory, recharged states evolve into positive central Pacific sea surface temperature anomalies in boreal spring, inducing stronger and more variable Westerly Wind Event activity and a fast growth of the ensemble spread during summer and fall. This also enhances the positive wind stress feedback in fall, but the effect is offset by changes in thermocline and heat flux feedbacks. The state-dependent component of westerly wind events is thus the most likely cause for the predictability asymmetry in CNRM-CM5, although changes in the low-frequency wind stress feedback may also contribute.

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Ivana ALEKSOVSKA, Laure RAYNAUD, Robert FAIVRE, François BRUN, and Marc RAYNAL

Abstract

Agriculture is a highly weather-dependent activity, climatic conditions impact both directly crop growth and indirectly diseases and pests developments causing yield losses. Weather forecasts are now a major component of various decision-support systems that assist farmers to optimize the positioning of crop protection treatments. However, properly accounting for weather uncertainty in these systems still remains a challenge. In this paper, three global and regional ensemble prediction systems (EPSs), covering different spatio-temporal scales, are coupled to a temperature-driven developmental model for grape vine moth in order to provide probabilistic forecasts of treatment dates. It is first shown that a parametric post-processing of the EPSs significantly improves the prediction of treatment dates. Anticipating the need for phytosanitary treatments also requires seamless weather forecasts from the next hour to sub-seasonal time scales. An approach is presented to design seamless ensemble forecasts from the combination of the three EPSs used. The proposed method is able to leverage the increased performance of high-resolution EPS at short ranges, while ensuring a smooth transition toward larger-scale EPSs for longer ranges. The added value of this seamless integration on agronomic predictions is, however, difficult to assess with the current experimental setup. Additional simulations over a larger number of locations and years may be required.

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David C. Eisenhauer

Abstract

This paper presents a case study of how boundary objects were deployed to support a collaborative knowledge production process that resulted in the creation of climate change knowledge usable to municipal governments in the New Jersey shore region. In doing so, a case is made that boundary objects are useful throughout the collaborative process in overcoming ambiguity and disagreement. This points to boundary objects possessing a wider array of capabilities than frequently theorized in the climate policy literature. Effectively designing and using boundary objects, however, requires carefully considering how they interface and interact with one another.

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Ruud Sperna Weiland, Karin van der Wiel, Frank Selten, and Dim Coumou

Abstract

Persistent hot-dry or cold-wet summer weather can have significant impacts on agriculture, health and the environment. For North-Western Europe, these weather regimes are typically linked to, respectively, blocked or zonal jetstream states. The fundamental dynamics underlying these circulation states are still poorly understood. Edward Lorenz postulated that summer circulation may be either fully or almost intransitive, implying that part of the phase space (capturing circulation variability) cannot be reached within one specific summer. If true, this would have major implications for the predictability of summer weather and our understanding of the drivers of interannual variability of summer weather. Here, we test the two Lorenz hypotheses (i.e. fully or almost intransitive) for European summer circulation, capitalising on a newly-available, very large ensemble (2000 years) of present-day climate data in the fully-coupled global climate model EC-Earth. Using Self-Organising Maps, we quantify the phase space of summer circulation and the trajectories through phase space in unprecedented detail. We show that, based on Markov assumptions, the summer circulation is strongly dependent on its initial state in early summer with the atmospheric memory ranging from 28 days up to ~45 days. The memory is particularly long if the initial state is either a blocked or a zonal flow state. Furthermore, we identify two groups of summers which are characterised by distinctly different trajectories through phase space, and which prefer either a blocked or zonal circulation state, respectively. These results suggest that intransitivity is indeed a fundamental property of the atmosphere and an important driver of interannual variability.

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Ryuichiro Inoue and Satoshi Osafune

Abstract

A part of near-inertial wind energies dissipates locally below the surface mixed layer. Here, their role in the climate system is studied by adopting near-inertial near-field wind-mixing parameterization to a coarse-forward ocean general circulation model. After confirming a problem of the parameterization in the equatorial region, we investigate effects of near-field wind mixing due to storm track activities in the North Pacific. We found that, in the center of the Pacific Decadal Oscillation (PDO) around 170°W in the mid latitude, near-field wind mixing transfers the PDO signal into deeper layers. Since the results suggest that near-field wind mixing is important in the climate system, we also compared the parameterization with velocity observations by a float in the North Pacific. The float observed abrupt and local propagation of near-inertial internal waves and shear instabilities in the main thermocline along the Kuroshio Extension for 460 km. Vertical diffusivities inferred from the parameterization do not reproduce the enhanced diffusivities in the deeper layer inferred from the float. Wave-ray tracing indicates that wave trapping near the Kuroshio front is responsible for the elevated diffusivities. Therefore, enhanced mixing due to trapping should be included in the parameterization.

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