December 2021

Frontiers in Ocean Observing: Documenting Ecosystems, Understanding Environmental Changes, Forecasting Hazards

DEEP ARGO – OBSERVING THE FULL

OCEAN VOLUME

Sustained measurements of ocean properties and circu-

lation are needed over the full water column to provide

fundamental insights into the spatial and temporal extent

of deep ocean warming, sea level rise resulting from the

expanded volume of deep ocean warming, and environ-

mental changes that affect the growth and reproduction

of deep-sea species. Deep ocean (>2,000 m) observing is

sparse in space and time compared to the upper 2,000 m.

Less than 10% of historical non-Argo T/S profiles extend

to depths greater than 2,000 m, with current high-quality

deep ocean measurements limited primarily to GO-SHIP

transects repeated on decadal timescales, ocean stations

located in special regions, and moored arrays set mainly

near the coasts of continents.

To address the void in deep ocean observing, new Deep

Argo float models are designed with high pressure toler-

ance in order to extend autonomous ocean observing to

the abyss. New Deep Argo CTD sensors have improved

temperature, salinity, and pressure accuracies and stability

to resolve deep ocean signals. Use of a bottom-detection

algorithm and bottom-detecting wires enables collection

of temperature, salinity, oxygen, and pressure to as close

as 1–3 m above the seafloor. The implementation of an

ice-avoiding algorithm on all Deep Argo floats deployed at

high latitudes enables deep ocean profiling under sea ice.

The Deep Argo fleet presently consists of pilot arrays

implemented in deep regions where GO-SHIP data show

strong ocean warming (Figure 7). Active float models

include those capable of sampling from the surface to

6,000 m depth, and others that can profile to 4,000 m

(Figure 8). Observations from the pilot arrays show float life-

times reaching 5.5 years and sensor accuracies approach-

ing GO-SHIP quality standards. Deep Argo’s ability is well

demonstrated to measure variability of deep ocean warm-

ing and large-scale deep ocean circulation, both regionally

and globally, at intraseasonal to decadal timescales. The

international Deep Argo community is committed to imple-

menting a global Deep Argo array of 1,250 floats in the next

five to eight years and to sustain Deep Argo observations in

the future (Zilberman et al., 2019).

With full implementation of the Deep Argo array, the

temporal and spatial resolution of deep ocean observa-

tions will improve by orders of magnitude, enabling new

insight into how the deep ocean responds to, distributes,

or influences signals of Earth’s changing climate. Deep

Argo’s homogeneous coverage of the full ocean volume

in all seasons will be particularly useful to constrain and

increase signal-to-error ratios in global ocean reanalyses

and to prevent unrealistic drift in coupled climate-ocean

models. Deep Argo will therefore increase our ability to

predict climate variability and change and to anticipate

and reduce the impact of more frequent extreme weather

events, warmer ocean temperatures, and sea level rise.

These all have damaging implications for various sectors

of the blue economy that nations increasingly depend

upon. Low-lying coastal communities and small island

developing states are especially vulnerable.

FIGURE 8. (a) Deployment of a 4,000 m capable

Deep Arvor float in the North Atlantic Ocean. Photo

courtesy of IFREMER/GEOVIDE (b) Deployment of

a 6,000 m capable Deep SOLO float in the North

Pacific Ocean. Photo credit: Richard Walsh

FIGURE 7. Location of the 191 Deep Argo floats active in October 2021,

including 4,000 m capable Deep Arvor and Deep NINJA, and 6,000 m

capable Deep SOLO and Deep APEX floats. The background colors indi-

cate ocean bottom depth: <2,000 m (white), 2,000–3,000 m (light gray),

3,000–4,000 m (light blue), 4,000–5,000 m (blue), and >5,000 m (dark

gray). Data courtesy of OceanOPS

SIO Deep SOLO (64)

MRV DSeep SOLO (38)

Deep Arvor (60)

Deep NINJA (1)

Deep APEX (28)

60°N

30°N

30°S

60°S

60°E

120°E

180°

120°W

60°W

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