I am trying to calculate upwelling index by following method adapted from Gomez-Gesteira, M., C. Moreira, I. Alvarez, and M. deCastro (2006), Ekman transport along the Galician coast (northwest Spain) calculated from forecasted winds, J. Geophys. Res., 111, C10005, doi:10.1029/2005JC003331.
Ekman transport can be calculated in terms of the wind speed at the 10 m level, W, the seawater density, ρw = 1025 kg m−3, a dimensionless drag coefficient, Cd = 1.4 × 10−3, and the air density, ρa , by means of
where f is the Coriolis parameter defined as twice the vertical component of the Earth's angular velocity, Ω, about the local vertical given byf = 2Ω sin (θ) at latitude θ. Finally, x subscript corresponds to the zonal component and the y subscript to the meridional one. In addition, the Upwelling Index (UI) can be defined as the Ekman transport component in the direction perpendicular to the shoreline by means of UI= Q⊥ = −sin(θ)Qx + cos(θ)Qy, where θ = π/2 + ϕ and ϕ is the angle of the unitary vector perpendicular to the shoreline pointing landward.
I fail to understand the sentence in RED, can somebody explain to me by using "figure"?