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ERA5 data is produced and archived not on a Cartesian grid (a two-dimensional rectangular grid), but as spectral coefficients or on a reduced Gaussian grid - think of it as a globe with , which has quasi-uniform spacing over the globe. Reduced Gaussian grids have a series of evenly spaced data points along each parallel (latitude), and parallels spaced at quasi-regular intervals. So near Near the poles you have there are only a few points along a parallel, but close to the equator you have there are many more data points along a parallel.
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