Journal Highlights

Sloping Topography and Oceanic Surface Modes

Editor’s Highlight— 

An accurate understanding of the influence of ocean bottom topography helps to diagnose the velocities of subsurface currents. 

Currents in the ocean are continually changing, influenced by factors such as “eddies,” circular movements of water, and “planetary waves,” large-scales waves within an ocean basin. The vertical structure of eddies and linear waves is traditionally understood by dividing the flow into “modes,” an orthogonal basis set for the flow. There are an infinite number of these modes, but the barotropic (depth-averaged) and first baroclinic (top-to bottom velocity difference) modes are usually considered to dominate ocean variability. However, other orthogonal basis sets exist, including “surface modes” which are formulated to have negligible amplitude at the ocean floor. Recent analyses of observations suggest that ocean variance is dominated by surface modes. This letter by LaCasce [2017] demonstrates that the presence of sloping bottom topography or bottom friction is sufficient to constrain the vertical structure of the flow to a “surface mode” structure. This result provides a physical justification for the observations of de La Lama et al. [2016], and implies that surface modes may be used to project surface satellite observations into the interior and to predict the direction and propagation speed of oceanic planetary waves.

-- Andrew M. Hogg, Editor, Geophysical Research Letters,


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Featured Special Collection

A Census of Atmospheric Variability from Seconds to Decades 

The atmosphere varies naturally on all length scales from millimeters to thousands of kilometers, and on all time scales from seconds to decades and longer.  This special collection of Geophysical Research Letters synthesizes and summarizes that variability through a phenomenological census.  The collection brings together some of the most influential and definitive papers to have been published in this journal in recent years.  The topics covered include turbulence on time scales of seconds and minutes, gravity waves on time scales of hours, weather systems on time scales of days, atmospheric blocking on time scales of weeks, the Madden–Julian Oscillation on time scales of months, the Quasi-Biennial Oscillation and El Niño–Southern Oscillation on time scales of years, and the North Atlantic, Arctic, Antarctic, Pacific Decadal, and Atlantic Multi-decadal Oscillations on time scales of decades.  The collection is accompanied by a Commentary article, which provides an authoritative, concise, and accessible point of reference for the most important modes of atmospheric variability.

A Census of Atmospheric Variability from Seconds to Decades