OCEAN 320 Lecture 2: Unit 2D Part 2
Unit 2D: Part 2
Origin, Dynamics, & Evolution of Ocean Garbage Patches from Observed Surface Drifters
• Economic growth has contributes to heavy input of toxic debris into oceans
o Floating debris is carried by winds and currents; currents may converge and
subduct, bringing garbage with it, creating the great ocean garbage patches
o Formation of the patches is governed by well-established dynamics of Ekman
pumping in subtropical gyres, where wind-driven convergence of surface flow
leads to accumulation of surface water in center of gyres; debris is less dense than
seawater, floats and accumulates
o Ekman theory: does not predict the timescales of patch formation from debris
entering at coasts, or how eddy mixing/other processes can counteract the
accumulation and provide a flux out of the patch locations
▪ Needs to know how fast debris reaches patches & how fast it leaks to
monitor threat
o Maximenko Theory: idealized initial state (surface debris uniformly spread over
global ocean), then employed observed surface drifter data to show marine debris
forward in time
▪ Main subtropical garbage patches emerge accurately, but does not account
for high concentration of waste at coasts, where most garbage is output
▪ Assumed the rate of tracer to be constant, ignored the seasonal cycle of
surface ocean circulation
o This article’s approach: study transport of tracer away from coasts into ocean
open while incorporating seasonal cycle & marine debris source (as relative to
human population around coast)
▪ This method reveals sixth garbage patch, previously unknown
▪ Examine how garbage patches are connected, how inter-ocean exchange
mixes debris from different regions, how leaky the patches are & how they
evolve over centuries
• Methods
o Objective: asses evolution of debris in ocean
o Global Drifter Program (Niiler 2001, Lumpkin 2003, Lumpkin et al 2012): buoys
get advected with near-surface flow used to study where and over what timescales
marine debris accumulates; have battery life of 5 years & data yields geo-location
of buoys every 6 hours
▪ Drogue attached at 15m, but many buoys lose them; 48-52% of data
comes from buoys who have lost their drogues;
▪ non-drogued buoys usually end up in same regions are drogued buoys, but
dynamics are very different
• non-drogued buoys: more sensitive to direct wind forcing and
winds drift
• drogued buoys: track the ocean flow at 15m depth, therefore more
representative of upper ocean flow and Ekman transport
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