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Shihe Ren, Xueming Zhu, Marie Drevillon, Hui Wang, Yunfei Zhang, Ziqing Zu, and Ang Li


A frontal detection algorithm is developed with the capability of detecting significant frontal segments of sea surface temperature (SST) in the high-resolution South China Sea Operational Forecasting System (SCSOFS). To effectively obtain frontal information, a gradient-based Canny edge detection algorithm is improved with postprocessing designed for high-resolution numerical models, aiming at extracting primary ocean fronts while ensuring the balance of frontal continuity and positioning accuracy. Metrics of frontal probability and strength are used to measure the robustness of the results in terms of mean state and seasonal variability of frontal activities in the South China Sea (SCS). Most fronts are found in the nearshore and form a strip shape extending from the Taiwan Strait to the coast of Vietnam. The SCSOFS is found to reproduce strong seasonal signals dominating the variability of the frontal strength and occurrence probability in the SCS. We implement the algorithm on the daily averaged SST derived from two other SST analyses for intercomparison in the SCS.

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Christophe Cassou, Clara Deser, Laurent Terray, James W. Hurrell, and Marie Drévillon


The origin of the so-called summer North Atlantic “Horseshoe” (HS) sea surface temperature (SST) mode of variability, which is statistically linked to the next winter's North Atlantic Oscillation (NAO), is investigated from data and experiments with the CCM3 atmospheric general circulation model (AGCM). Lagged observational analyses reveal a linkage between HS and anomalous rainfall in the vicinity of the Atlantic intertropical convergence zone. Prescribing the observed anomalous convection in the model generates forced atmospheric Rossby waves that propagate into the North Atlantic sector. The accompanying perturbations in the surface turbulent and radiative fluxes are consistent with forcing the SST anomalies associated with HS. It is suggested that HS can therefore be interpreted as the remote footprint of tropical atmospheric changes.

The ARPEGE AGCM is then used to test if the persistence of HS SST anomalies from summer to late fall can feed back to the atmosphere and have an impact on the next winter's North Atlantic variability. Observed HS SST patterns are imposed in the model from August to November. They generate a weak but coherent early winter response projecting onto the NAO and therefore reproduce the observed HS–NAO relationship obtained from lagged statistics. Changes in the simulated upper-level jet are associated with the anomalous HS meridional SST gradient and interact with synoptic eddy activity from October onward. The strength and position of the transients as a function of seasons are hypothesized to be of central importance to explain the nature, timing, and sign of the model response.

In summary, the present study emphasizes the importance of summer oceanic and atmospheric conditions in both the Tropics and extratropics, and their persistence into early winter for explaining part of the NAO's low-frequency variability.

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