Air-Sea Carbon Flux During Hurricanes Morgan O’Neill and Virginia Sawyer University of New Hampshire mep29@unh.edu, vrf2@unh.edu

Motivation

What is the influence of the surface wind

speed on air-sea CO2 flux?

What effect might tropical cyclone

activity have on the global carbon cycle?

We modeled the air-sea CO2 flux for Hurricane Katrina and for Hurricane Maria, a category

three storm that also formed in 2005. We discuss how increased tropical cyclone intensity

might affect the global climate.

High wind speeds over short time scales can cause carbon dioxide dissolved in the

oceanic mixed layer to outgas into the atmosphere.

Bates et al. (1998) estimates the ocean-to-atmosphere CO2 flux of tropical cyclones at

0.02 to 0.20 Pg C/yr over the Northern Hemisphere.

For comparison, the average air-sea CO2 flux for a single ocean basin is 0.36 to 0.72

Pg C/yr.

General circulation models (GCMs) used for climate prediction do not model any effects

of tropical cyclones because they occur over limited geographical areas and for short

periods of time compared to the resolution constraints of a gridded global model. The

exclusion of tropical cyclone activity from climate models is of particular concern

because of the prediction made in Emanuel (1987) and other literature that the intensity

of these storms may increase due to global climate change. If so, the windspeed-

dependent CO2 flux becomes a positive feedback mechanism in the climate system.

Figure 1. Potential minimum central pressure of tropical cyclones forming under a,

present-day September sea surface temperatures and b, September sea surface

temperatures in a doubled atmospheric CO2 scenario. From Emanuel (1987), 910 mb

and 880 mb contours colored for visibility.

a b Model Description

Assumptions

Figure 2. Model diagram in Stella for whole-storm instantaneous CO2 flux.

Initial pCO2 values and subsurface ocean conditions are prescribed from typical values,

as data specific to Hurricanes Katrina and Maria were not available.

Ocean mixed layer assumed to be well-mixed, with uniform temperature and pCO2

Aftereffects of storm—such as long-lasting sea surface temperature change and possible

nutrient mixing—not considered in this model.

The model begins with the gas flux equation

F = ks(pCO2w – pCO2a)

k ~ gas transfer velocity (windspeed and viscosity dependent)

k = 0.31u2(Sc/660)-½ where

Sc = 1911.1 - 118.11*SST + 3.4527*SST2 - 0.04132*SST3 and

SST is the sea surface temperature ( C here, K below)

s ~ solubility of CO2 in water (temperature dependent)

s =(1/29.4)exp[-2400(1/SST - 1/298)]

pCO2 ~ partial pressure of CO2 present in water and air,

respectively (μatm)

To extend the gas flux per m2 to the entire hurricane, it is

multiplied over the area of the storm. The National

Hurricane Center reports quadrant radii for three different

windspeeds, as shown in Figure 3. The model uses five

NHC observations representing significant stages of storm

development.

Figure 3. Hurricane Katrina, 2100Z 8/21/2005

(NHC Forecast Advisory archive)

370 km

18 m/s

26 m/s 33 m/s

75 m/s

NE

SE SW

NW

PCO

2w

PCO

2a

→ Sum the fluxes from all 3 wind radii, yield the total flux at any given point during the storm

→ Sum the areas from all 3 wind radii and all 4 quadrants, as they vary through the lifetime of the storm

→ Tropospheric pCO2 will vary according to the gas flux equation

→ The gas flux equation governs how much CO2 moves between the ocean and the atmosphere

→ The upper-level oceanic pCO2 will depend on both the gas flux, and the upwelling of colder, CO2-rich water

→ Wind speed increases the depth of the top ocean layer, increasing pCO2 and decreasing sea surface temperature

Results

Discussion

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Hours

Tera

gram

s [1

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ram

s]

Hours

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gram

s [1

0*1

2 g

ram

s]

Hours

Tera

gram

s [1

0*1

2 g

ram

s]

For every hurricane that becomes a

Saffir-Simpson category 5 instead of a

category 3, 140 teragrams of additional

CO2 are released to the atmosphere.

Figure 5. Sea Surface Temperatures [K] Before & After Hurricane Maria

0

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Tera

gram

s [1

0*1

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s]

Hours

Tera

gram

s [1

0*1

2 g

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s]

Figure 4.

Hurricane Maria Cumulative CO2 Flux

The above graphs (Fig. 4 & 6) illustrate the notable difference in CO2 efflux between

Hurricane Maria, a typical tropical cyclone, and Hurricane Katrina, a massive and intense

category 5 storm. According to our model, Katrina expelled more than 4 times more CO2 to

the atmosphere as did Maria.

We provide temperature graphs to show that our model produced reasonable results.

Katrina experienced a warmer initial ocean temperature than Maria because the Gulf of

Mexico at that time was unusually warm.

Figure 6.

Hurricane Katrina Cumulative CO2 Flux

Figure 7. Sea Surface Temperatures [K] Before & After Hurricane Katrina

Our model suggests that higher wind speeds cause higher rates of CO2 flux to the

atmosphere. We demonstrate that a category 5 storm outputs higher rates of CO2 from

the ocean that a category 3 storm. This has implications for global climate change: as the

ability for the ocean to act as a CO2 sink slows down and the CO2 content of the

atmosphere increases, the amount of CO2 released to the atmosphere by tropical

cyclones will likely increase.

Further work would involve adding phytoplankton blooms to the trails of tropical cyclones.

These blooms likely act as a CO2 sink, mitigating the cyclone’s net efflux.

303.25 303.25 303.25

302.12

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301.26

300

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302.29 302.29 302.29

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34 knot winds 50 knot winds 64 knot winds