**Potential role of ocean color in modulating tropical cyclogenesis**

Anand Gnanadesikan, Whit G. Anderson, Kerry Emanuel, Gabriel A. Vecchi,
S.M. Griffies and Robert W. Hallberg

In prep. for Nature Geosciences

The distribution of absorbing and scattering materials in the upper
ocean (often referred to as "ocean color") governs the vertical
distribution of shortwave radiation within this domain.^1,2^ This means
that changes in ocean color affects the patterns of sea surface
temperature. Increasing shortwave penetration by removing ocean color
will tend to warm the waters below the oceanic mixed layer.^3-5^ The
waters thus warmed will then tend to be brought back to the surface at
some location thus potentially altering the atmospheric
circulation.^6-9^ Ocean color can thus affect not only sea surface
temperatures, but large-scale atmospheric stability, shear and humidity,
all factors known to play a role in altering hurricane formation. Here
we show that the total impact of ocean color on hurricane formation is
extremely large- so much so that relatively small details in how to
parameterize shortwave penetration may nonetheless have significant
implications for the frequency with which cyclones would be expected to
form in coupled climate models.

Tropical cyclones are organized convective systems requiring for their
formation environmental conditions that promote both convective
instability and organization. The first is provided by a vertical
atmospheric profile a strong contrast in temperatures between the sea
surface and top of the troposphere. Emanuel and Nolan^10^ defined the
maximum potential intensity (MPI) as the maximum windspeed that could be
associated with releasing this energy- an index that will play a role
later in this paper. Additionally, there must be sufficient moisture in
the lower atmosphere that rising plumes do not begin to evaporate and
slow down. Organization is enhanced by the presence of horizontal shear
and limited by the presence of vertical shear. All of these properties
can be influenced by the pattern of tropical sea surface temperature,
which is well known to play a primary role in controlling the tropical
circulation.

Recent studies have examined the impact of changing ocean color in
comprehensive coupled climate models.^6-9^ The general finding of these
models is that increasing penetration tends to produce warming along the
equator and cooling in the subtropical gyres, though the relative size
of these two perturbations depends on the location and nature of the
changes to shortwave absorption. We begin by examine three pairs of
coupled model simulations in which shortwave absorption due to suspended
and dissolved absorbers and scatterers was changed relative to a control
simulation. In the first pair, the control case (*Green*) uses observed
chlorophyll concentrations to parameterize the penetration of a single
band of light (21% of the total incoming radiation- essentially that
contained in the blue-green bands) below a minimum mixed layer depth of
10m. This run is compared to case where the chlorophyll-dependent part
of the absorption is removed only in the already clear Pacific gyres
where chlorophyll concentrations are less than 0.2 mg/m^3^ (run
*BLPGyre*). In the second pair of simulations, the chlorophyll remains
constant, but the minimum mixed layer depth is only 2m and instead of
resolving only blue-green light (*Oneband)* we calculate the penetration
of blue-green, yellow-red, and near infrared penetrating bands
(*Threeband*). Finally, using a different ocean model, we compare a
formulation of shortwave absorption which parameterizes the existence of
a deep chlorophyll maximum (*Morel*) to one which does not (*Manizza*).
Details about the formulation of the models and their representation of
solar absorption are provided in the methods section.

As all three pairs of simulations produce an increase in the depth at
which radiation is absorbed, the result is to cool the large parts of
the subtropical gyres (Figure 1a-c) and to warm the subsurface. These
warm subsurface waters are brought back to the surface in upwelling
zones particularly the equatorial cold tongue. The near-equatorial winds
respond as would be expected, flowing away from the colder regions
towards the warmer ones. In the *BLPGyre-Green* simulations, this
results in an increase in the Hadley and decrease in the Walker
circulation, with increases in off-equatorial zonal shear and large
decreases (-20%) in mid-tropospheric relative humidity (Figure 1d). In
the *Threeband-Oneband* pair of simulations, the warming of the eastern
Pacific, with an associated increase in the Walker circulation seems to
dominate. As the SST changes are much weaker, so are the off-equatorial
changes in shear and relative humidity (Figure 1b,e). In the
*Manizza-Morel* pair of simulations (Figure 1c,f) the increase in
convergence is centered over the eastern Pacific, but the changes to
shear and relative vorticity are more zonally uniform.

Because the resolution of the coupled climate models is too coarse to
permit formation of hurricanes, we cannot directly examine the impact of
these changes on hurricane counts and paths. Instead, we employ two
separate approaches to look at the potential size of changes. The first
involves the Genesis Potential Index (GPI), an empirical measure
proposed by Emanuel and Nolan^8^ which has been shown to have some skill
in predicting both the spatial patterns of cyclongenesis and the
year-to-year variability in cyclogenesis.^9^ The GPI depends on the
maximum potential intensity (MPI), the shear between 200 and 850 mb, the
relative humidity at 700mb and the vorticity at 850mb. It represents the
number of storms that would be expected to traverse a 2.5x2.5 degree
square over the course of a decade. As seen in Figure 2a the GPI changes
in the BLPGyre-Green pair of simulations result in a shift of storms
towards the equator and an overall drop in the intensity. It is possible
to break down the role of different changes in the GPI over different
regions by changing all but one variable. When this is done for the
BLPGyre-Green pair of simulations for the Northwest Pacific (Figure 2d)
we see that the 61% decrease in GPI is affected by all the variables,
but that shear (-33%) and relative humidity (-27%) play a more
significant role than temperature structure as filtered through MPI
(-16%), and vorticity (-10%). Although this is the result of an
extremely unrealistic change- it dwarfs the O(5%) changes associated
with a 1C global warming. It seems worthwhile then to examine the
smaller changes associated with the changes in representation of
shortwave absorption.

The overall pattern of change in GPI in the simulations where the
representation of shortwave absorption is all that changes (Figures
2b-c) is similar to that seen in *BLPGyre-Green* pair of simulations,
with decreases at the edge of the subtropical gyres and some increases
within the equatorial region. Although the changes are in general
smaller, they can still be significant on a regional scale. For example
in the NE Pacific (15N-25N,150W-110W) the *Threeband* model shows a 26%
decrease in GPI relative to the *OneBand* model (Figure 2e). As in the
BLPGyre-Green case, shear and relative humidity play the most important
role (causing decreases of 10% on their own) while MPI (-7%) and
vorticity (-3.6%) play a much smaller role. In the western Atlantic
(15-25N, 100W-60W) the *Manizza* model which assumes vertically uniform
shows an 18% decrease in GPI relative to the *Morel* model which
parameterizes a subsurface chlorophyll maximum. Here too the shear is
the dominant player (-10%) with relative humidity (-5%), MPI (-4%) and
vorticity (-1%) all less important.

EMANUEL ET AL. DOWNSCALING RESULTS

These results suggest a number of directions for future inquiry. First,
the details of how solar heating is depositing in the water column turn
out to project strongly onto the susceptibility of the tropical
circulation to cyclogenesis. Much more attention needs to be paid to the
details of solar absorption and mixing in the upper 150m of the water
column, particularly in the region below the mixed layer. This involves
both understanding how different substances absorb solar radiation and
continuing to measure such substances. Second, it has been suggested
that global warming will act to decrease vertical exchange and
chlorophyll concentrations-implying a change in shortwave absorption in
the same general direction as those studied here. Our study suggests
that such changes could act to reduce tropical cyclogenesis. Third, iron
deposition is known to affect chlorophyll concentrations, including in
the subsurface chlorophyll maximum. Changes in iron deposition- either
as a result of climate change^19^ or of pollution^20^ could have an
indirect impact on hurricane paths.

**Methods**

Global coupled climate model: The atmospheric model used in the coupled
climate model simulations is the atmospheric component of the GFDL
global coupled climate model, with a resolution of 2x2.5 degrees, 24
vertical levels and up-to-date physical representations of radiation,
gravity wave drag, convection, and surface boundary layers. Three
different ocean models are used in this study. The first model, used in
the *Green* and *BLPGyre* simulations, uses the Hallberg Isopycnal Model
code, an isopycnal layer-coordinate ocean model with realistic equation
of state, Richardson-number dependent entrainment, and a four-layer
representation of the mixed layer. Previous work published by our group
has used this code base. The second pair of models, used in the
*ThreeBand* and *OneBand* simulations, are based on an updated version
of HIM known as the Generalized Ocean Layer Dynamics (GOLD) code.
Simulations done with GOLD are very similar to those done with HIM but
with slightly smaller mean sea surface temperature and sea surface
salinity errors. The final pair of models, used in the *Manizza* vs.
*Morel* simulations utilize the level-coordinate Modular Ocean Model,
v.4.1 (MOM4.1), a state-of-the-art level coordinate model that is mass
rather than volume conserving. All models have a horizontal resolution
of 1˚ in the east-west direction with a north-south resolution of 1˚in
mid-latitudes increasing to \~1/3˚ near the equator.

The optical schemes used in these three models are all based on the data
of Morel^1^, who produced an absorption profile that implicitly
parameterized the impact of a deep chlorophyll maximum when surface
chlorophyll concentrations are low. Such a formulation is problematic
for models that explicitly predict chlorophyll concentrations, so
Manizza et al.^5^ reanalyzed Morel's data to produce a three-band
formulation of shortwave absorption, breaking the incoming shortwave
radiation at the surface into 58% near_IR, 21% red-yellow, and 21%
blue-green and fitting three chlorophyll-dependent attenuation scales to
each of these bands. This is the scheme used in the ThreeBand model, in
which the minimum mixed layer depth is set to 2m. In both the HIM and
MOM runs, the surface layer was 10m deep, so that it made little sense
to resolve the near-IR and red-yellow bands with e-folding depths less
than 5m. Thus our Blue, BLPGyre, OneBand and Manizza simulations, the
Manizza scheme was further simplified to only include the blue-green
portion, with the remaining 79% of incoming shortwave radiation being
effectively added at the surface of the mixed layer. Chlorophyll
concentrations were taken from the SeaWiFS satellite.

MPI and GPI:

Dynamical downscaling:

Acknowledgments: WGA was supported by the Earth

Statement of work: WGA and AG set up and performed the experiments
looking at impacts of ocean color. GAV developed the analysis codes to
do the MPI and GPI calculations, and helped set up the downscaling
analysis with the simplified dynamical model. KE performed the
downscaling analysis using the simplified dynamical model. RWH and SMG
implemented the parameterizations of shortwave absorption within their
ocean codes.

References

1\. Morel, A., Optical modeling of the upper ocean in relation to its
biogenous matter content (case-I waters), J. Geophys. Res., 93,
10,749--10,768, (1988).

2\. Lewis, M. R., Carr, M.E., Feldman, G.C., Esaias, W. & McClain, C.,
Influence of penetrating solar radiation on the heat budget of the
equatorial Pacific Ocean, *Nature*, 347, 543-- 545, (1990).

3\. Nakamoto, S., *et al*, Response of the equatorial Pacific to
chlorophyll pigment in a mixed layer isopycnal ocean general circulation
model, *Geophys. Res. Lett.,* 28, 2021-- 2024, (2001).

4\. Manizza, M., Le Quere,C., Watson, A.J.& Buitenhuis, E.T.
,Bio-optical feedbacks among phytoplankton, upper ocean physics and
sea-ice in a global model, *Geophys. Res. Lett.*, 32, L05603,
doi:10.1029/2004GL020778, (2005).

5\. Sweeney, C. *et al* Impacts of shortwave penetration depth on
large-scale ocean-circulation and heat transport, *J. Phys. Oceanogr.*,
35(6), 1103-1119, (2005).

6\. Shell, K. M., Nakamoto, S. & Somerville, R.C., Atmospheric response
to solar radiation absorbed by phytoplankton, *J. Geophys. Res*.,
108(D15), 4445, doi:10.1029/2003JD003440, (2003).

7\. Wetzel, P. et al., Effects of ocean biology on the penetrative
radiation in a coupled climate model, *J. Climate*, 19, 3973-3987,
(2006).

8\. Lengaigne, M. et al., Influence of the oceanic biology on the
tropical Pacific climate in a coupled general circulation model, *Clim.
Dyn.,* 28, 503-516, (2007).

9\. Anderson, W.G., Gnanadesikan, A., Hallberg, R.W., Dunne, J.P. &
Samuels, B.L., Impact of ocean color on the maintenance of the Pacific
Cold Tongue, *Geophys. Res. Lett*., 34, L11609,
doi:10.1029/2007GL030100, (2007).

10\. Emanuel and Nolan

11\. Vecchi and Soden

![](media/image1.wmf){width="5.9875in" height="4.104166666666667in"}

**Figure 1:** Atmospheric circulation changes associated with changes in
oceanic shortwave absorption. (A)-(C) SST change (C, colors) and wind
stress change (Pa, arrows). Arrow scale in (B) and (C) 1/5 that of (A).
(A) BLPGyre-Green. (B) Three Band-One Band. (C) Manizza-Morel. (D)-(F).
Change in Shear (200mb-850mb in m/s, colors) and relative humidity at
700mb (contours). Note differences in color scale. Contour interval is
2% for (D) and 0.5% for (E),(F).

![](media/image2.wmf){width="5.990972222222222in"
height="4.222916666666666in"}

**Figure 2:** Changes in genesis potential index. (A) BLPGyre-Green,
Jun.-Nov. (B) ThreeBand-OneBand, Jun.-Nov. (C) Manizza-Morel, Jun.-Nov.
(D)-(F) GPI changes for specific regions within each model pair. In each
plot, the green line shows the control case, the solid blue line the
perturbation case, and the blues with symbols or dashes the GPI computed
by replacing one component only. (D) BLPGyre-Green over just the NW
Pacific (10-30N, 100E-150E). Shear, relative humidity, temperature and
vorticity all contribute to the 61% decrease in GPI, with shear playing
the biggest role. (E) ThreeBand-OneBand over the Northeast Pacific
(15-25N, 150W-110W) (F) Manizza-Morel for just the western Atlantic
(15-25N, 100W-60W)