The standard deviation reveals also a strong zonal variability, with minimum variability at the low latitudes and maximums at high latitudes, stronger at the Southern Hemisphere. The bottom map of Fig. 4 shows that the seasonal cycle of SLP has strong variations. The North Atlantic, including the whole European region, the Equatorial Pacific and the latitudes between 40S and 70S are regions with almost no seasonal signal in SLP, where the seasonal signal explains less than 30% of the total SLP variance. On the contrary, in the tropical regions the seasonal signal explains more than 70% of the total variance. In the Southern Ocean, below latitude 50S and over the North Atlantic, high-frequency signals (periods shorter than 6 months) contribute to a considerable fraction (30–50%) of the overall variance in sea-level pressure. In the Equatorial Pacific, large-scale variability associated with ENSO (El Nin˜o Southern Oscillation) contributes up to 40% of the total variance (Barbosa et al., 2009). For each station and epoch, four ZHD values, at sea level, were calculated: from in situ surface pressure, from ECMWF-025 SLP, from ERA SLP and from VMF1 grids. For each measurement of surface pressure Ps, the corresponding value of SLP, P0, is computed from Eq. (7) or Eq. (8) and then used in Eq. (6) to derive the corresponding ZHD value at sea level.