Ken Takahashi, Ph. D.

Thermotidal and land-heating forcing of the diurnal cycle of oceanic surface winds in the eastern tropical Pacific*

Reunión LMI DISCOH, 29 de marzo, 2012IMARPE, Callao

* Geophysical Research Letters, 39, L04805, doi:10.1029/2011GL050692, 2012

Diurnal variation in Quikscat winds (6 am minus 6 pm, local time)

Gille et al., 2003

“Upsidence wave”: Diurnal cycle in vertical velocity(wave forced by heating of the Andes, Garreaud & Muñoz, 2004)

Rahn & Garreaud, 2010

Diurnal cycle in surface winds in

the tropical Pacific

Data: TAO/TRITONBased in diurnal harmonic

Ueyama & Deser, 2008

01 Local time

04 Local time

07 Local time

10 Local time

13 Local time

Linear theory of the sea breeze(Rotunno, 1983)

|Latitude|> 30° : Coastal trapping< 30° : Wave propagation=30° : Singular

MM5 v3 simulation (Δx=120 km)(Gayno-Seaman, Grell, CCM2)BC: NNRP Oct. 2008 mean conditions, fixed SST

Surface wind diurnal variation(6 am minus 6 pm, local time, m/s)

MM5 v3

Takahashi, 2012

Simulated diurnal cycle in surface

wind and sea level pressure (SLP) (in local time)

MM5 v3 Δx=120 km(GS, Grell, CCM2)

Takahashi, 2012

Migratory diurnal thermal

tide

Observational estimate

Lieberman & Leovy, 1995

Model with tropospheric solar absorption

Tidal component in SLP (6 UTC)

Trenberth et al., 2009

Global energy budget (Wm-2)

SOLAR RADIATIONTERRESTRIAL RADIATION

Hartmann 1994High absorptionAtm

osph

eric

abs

orpti

on (%

) Visible Near IR Infrared (IR)

Atmospheric absorption

Absorción atmosféricaGa

ses i

nver

nade

ro

Radiación solar Radiación terrestre (infrarrojo)

Hartmann, 1994

H2O

CO2

O2, O3

CH4

N2O

El vapor de agua es el responsable principal de la absorción de radiación solar en la atmósfera

ExperimentsControl

Diurnal land heating suppressed*

Absorption of solar radiation (near-IR) by atmospheric water vapor suppressed**

Modeled diurnal amplitude of• Land surface temperature• SLP

*Land-slab layer heat capacity and relaxation time-scale multiplied by 100.** Corresponding absorption coefficients set to zero in radiation code.

b) + c)

Takahashi, 2012

Modeled diurnal variations in SLP and surface wind

Control Thermal tide Extended sea breeze

x

x

x

Solar “speed” = 462 m/s

~60

m/s

Takahashi, 2012

Zonal mean thermal tideColors:Temperature (°C)Contours: Pressure (hPa)Vectors: (v,w) (m/s, cm/s)

MM5 v3 Δx=120 km(GS, Grell, CCM2)

Takahashi, 2012

Low pressure

Radiative air heating

West

East

Sun

Equatorward surface wind

High pressureNot to scale

Migratory atmospheric thermal tide

Diurnal cycle off Ica (central-southern coast of Peru)7 pm LT 11 pm 3 am

7 am 11 am 3 pmMaximumWind (m/s) at

40 m above surface

Takahashi et al, in preparation

Diurnal anomalies7 pm LT 11 pm 3 am

7 am 11 am 3 pm

Wind (m/s) at 40 m above surface

Takahashi et al, in preparation

Conclusions

• The diurnal cycle of surface winds has a substantial large-scale contribution (not only “sea breeze”)

• Atmospheric shortwave absorption by water vapor is an important forcing of this diurnal cycle (thermal tides)

• Diurnal land-heating produces an important contribution to the diurnal cycle within 2000 km from the coast.