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We theoretically investigate the electromagnetic enhancement by a T-shaped nanogroove in a metallic substrate. Compared with a single bare groove without the top trench, a T-shaped groove can achieve a much higher enhancement factor of electric field by more than two times. To explain this improved field enhancement, we build up an intuitive model that explicitly incorporates the mode resonance in the central groove and the excitation and collection of surface waves by the top trench. The model can accurately reproduce the fully vectorial data and thus allows an in-depth analysis of the physics of the field enhancement. Analysis of the model shows that the resonance in the groove can be further enhanced by the excited surface waves that are collected by the top trench and further coupled into the groove. Surface plasmon polariton is shown to be not the unique surface wave that enhances the groove resonance and taking into account another so-called quasi-cylindrical wave will give the full contribution. Our results are helpful for intuitively designing field-enhancement devices that employ surface waves to further enhance the field concentration or resonance.