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Lecture 2

OCEAN 320 Lecture 2: Unit 2D Part 2Premium

4 pages53 viewsSummer 2018

Department
Oceanography
Course Code
OCEAN 320
Professor
K.Robinson
Lecture
2

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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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