I. The latitude of the maximum SST shifts poleward and the maximum value

increases as the cross-equatorial heat flux increases in the slab ocean runs, while the SST maximum remains the same (by design) in the fixed-SST simulations. The meridional SST gradient is greater in the slab ocean runs.

II. The ITCZ moves poleward in both sets of simulations and is located equatorward of the maximum SST.

s

High Resolution Atmospheric Model (HiRAM)

• Aquaplanet general circulation modeling experiments have been useful tools for

understanding the response of large scale circulations to climate change.

• We perform high-resolution simulations in two different aquaplanet configurations (with fixed-SST and slab ocean boundary conditions) to study changes in the frequency and intensity of tropical cyclones (TCs).

• We explore the sensitivity of TC frequency to the ITCZ position and strength of convergence because the ITCZ provides favorable conditions for TC genesis (e.g., a region of low-shear and high tropospheric relative humidity).

• As the off-equatorial thermal forcing increases, the frequency of TC genesis increases and there is a shift in the distribution toward more intense storms.

Influence of the zonal mean circulation on tropical cyclone frequency Andrew Ballinger1, Timothy Merlis1,2, Isaac Held1,2 and Ming Zhao2

1Program in Atmospheric and Oceanic Sciences, Princeton University, Princeton, New Jersey

2Geophysical Fluid Dynamics Laboratory / NOAA, Princeton, New Jersey

Introduction

Fixed Sea Surface Temperature experiments

Slab ocean experiments

Results: ITCZ location and strength

Results: ITCZ and Genesis latitude

• Control SST profiles (zonally symmetric):

• Tropical SST perturbations:

• In a response to increased off-equatorial thermal forcing in a series of aquaplanet

simulations, we find an increase in TC genesis frequency and a shift in the intensity distribution toward more intense TCs.

• A tropical warming (cooling) perturbation generates more (fewer) TCs, along with a relative decrease (increase) in the intensity distribution, while a globally-uniform warming (cooling) yields fewer (more) TCs.

• The changes in TC genesis frequency are partially related to changes in the mean vertical velocity weighted over the genesis region.

• Where a poleward shift of the genesis region occurs, the associated increases in mean absolute vorticity may contribute to the higher TC genesis frequency.

• The idealized aquaplanet configuration provides a useful framework for exploring the relationship between TC genesis frequency and the large scale environment, and the testing of different TC genesis indices.

• These results may have implications for TC frequency and intensity in a changing climate.

Results: Frequency Distribution of TCs

1

0

-1

Lat

itud

e

Longitude

Vort

icit

y (

10

-5 s

-1)

850

hP

a

SL

P(h

Pa) / v

10m

(m

/s)

• We use 6-hourly fields of 850hPa relative vorticity, sea level pressure, surface

wind-speed (10m), and upper-tropospheric temperature to detect and track TCs, requiring a surface wind-speed of >15.2 ms-1 to exist for at least 3 days (total).

• Prescribed warming and cooling in the extratropics:

• Implies a cross-equatorial ocean heat flux:

N S Eq .

F0

WARM

COOL

N S N S

• GFDL’s atmospheric GCM (adapted from AM2.1), with cubed-sphere topology

and ~50km horizontal resolution.

• A series of previous papers have demonstrated HiRAM’s ability to simulate tropical cyclone statistics consistent with observations when forced with monthly-averaged SSTs, including the geographical distribution, seasonal and interannual variability across different ocean basins.

(Zhao et al. 2009, 2010, 2012; Zhao and Held 2010, 2012; Held and Zhao 2011)

• Using HiRAM in two different aquaplanet configurations we perform a series of 10 yr simulations seeking to test the sensitivity of genesis frequency to changes in the meridional SST profile (fixed-SST boundary conditions), and to changes in the cross-equatorial ocean heat flux (20m slab ocean boundary condition).

Follows Kang et al. 2008

Perturbed Controls

I. As the off-equatorial thermal forcing increases and the peak-SST and ITCZ

moves poleward, the number of TCs (genesis < 30°N) increases. II. The latitude of peak-genesis is not solely related to the latitude of the peak-SST

or ITCZ location, but is influenced by other factors (not shown).

Fixed-SST control and slab ocean experiments

Fixed-SST perturbation experiments

I. Increase in frequency of TCs as the maximum temperature shifts poleward in

the fixed-SST runs, and a slight poleward expansion of the genesis region. II. Increase in TCs with increased cross-equatorial heat flux in the slab ocean runs

(with evidence of saturation), and a marked poleward shift of genesis. III. Decrease (increase) in TC frequency for a globally-uniform heating (cooling). IV. Increase (decrease) in TC frequency for a tropical heating (cooling).

Results: Intensity Distribution of TCs

Detecting and tracking Tropical Cyclones

Latitude Latitude I. Increased intensity of TCs as the maximum SST shifts poleward in the fixed-

SST runs, as well as with an increasing heat flux in the slab ocean runs. II. The shapes of the intensity distributions are notably different between the

various experiments, with flatter meridional SST gradients yielding a shift toward longer-lived and more intense storms.

Latitude Latitude

III. The location of the ITCZ is relatively insensitive to a globally-uniform SST perturbation, but moves toward (away from) the maximum SST as the SST gradient is increase (decreased).

Conclusions

Email: aballing@princeton.edu

Geographical distribution

Interannual variability

Seasonal cycle