A current velocity index of ocean variability

Abstract

We use daily time series of the longshore component of residual (detided) current velocity υ(z, t) measured at several depths at a single location on the continental slope to construct a current velocity index, Iυ ≡ α|υ′s∂υ/∂z|∂υ/∂z, of coastal ocean variability. Here z is depth, t is time, and α = −sgn(f)[(f2 + δ2)συ]−1 is a normalization factor constructed from the local Coriolis parameter f, a friction parameter δ, and the standard deviation συ of the near‐surface current velocity, , where υ′s is the fluctuation about the time series mean ; sgn(f) = +1 for the northern hemisphere and −1 for the southern hemisphere. The time series of vertical shear, ∂υ/∂z ≈ Δυ/Δz, is derived from the velocity difference Δυ between simultaneous current meter records separated by a vertical distance Δz within the upper half of the water column. The index characterizes low‐frequency baroclinic variability over the continental margin and is linked, through use of the thermal wind relation, to instability processes that effect the transfer of potential energy stored in the mean cross‐slope density field to the kinetic energy of mesoscale meanders and eddies. For eastern boundary regions the index ranges from large positive values during summer upwelling conditions to large negative values during winter downwelling conditions. The “transition” seasons, spring and fall, are characterized by Iυ ≈ 0. We demonstrate use of the index by applying it to the continental margin of southwest Vancouver Island using current velocity data collected from October 1989 to March 1995 at depths of 35, 100, 175, and 400 m at a long‐term mooring site. Results enable us to specify the start and end times of the oceanic seasons and to quantify the summer upwelling and winter downwelling intensities for the region. Gaps in the current meter time series prompted us to examine records of coastal sea level, sea surface temperature, sea surface salinity, and “Bakun” upwelling index (the wind‐driven cross‐shore component of surface Ekman transport) as possible surrogates for the velocity index. None of these time series were acceptable. Although the Bakun index is a logical substitute for the velocity index, differences between the two indices are significant and suggest that current velocity data are needed for accurate delineation of seasonal oceanic variability along continental margins.