Importance of the Low-level Diabatic Heating on the Formation of theSummertime Subtropical Highs: Sensitivity to Diabatic Heating DataSetTakafumi Miyasaka1, Hisashi Nakamura1,2
1

Departiment of Earth and Planetary Science, The University of Tokyo, Japan
2
Frontier Research Center for Global Change, JAMSTEC, Japan
Correspondence: miyasaka@eps.s.u-tokyo.ac.jp

INTRODUCTION
Summertime subtropical highs exist over the eastern portions of the subtropical oceans. The equatorward
along-shore winds along the eastern flanks of the highs act to maintain cool sea-surface temperatures (SSTs)
underneath by enhancing surface evaporation, mixing and coastal upwelling. The presence of the cool SSTs and a
mid-tropospheric subsidence favors the local development of marine stratus in the planetary boundary layer.
The formation mechanisms of the summertime subtropical highs have been investigated through numericalexperiments. Rodwell and Hoskins (2001), in which a primitive-equation planetary-wave model was forced withdiabatic heating at all vertical levels in some regions, emphasized an effect of monsoonal convective heating on theformation of the summertime subtropical highs. Shaffrey et al. (2001) supported the conclusion of Rodwell andHoskins (2001) through AGCM experiments forced with modified boundary conditions such as topography, albedo
and SST. However, it may be problematic that marine stratus was poorly reproduced in their model. Importance ofmarine stratus on the formation of the summertime subtropical highs is suggested by Miyasaka and Nakamura (2005)
through numerical experiments forced with diabatic heating of the National Centers for Environmental Prediction(NCEP)-the Department of Energy (DOE) reanalysis (Kanamitsu et al. 2002). They concluded that the summertime
subtropical highs are formed mainly by low-level land-sea heating-cooling couplets, which are associated withsensible heat flux over dry continents and radiative cooling due to marine stratus. However, diabatic heating ofreanalysis may have uncertainty quantitatively. Therefore, to confirm the conclusion of Miyasaka and Nakamura(2005), similar numerical experiments are performed with diabatic heating of the JRA-25 reanalysis (Onogi et al.
2007).
DATA AND A MODEL
The model used in this study is same as in Miyasaka and Nakamura (2005); a global primitive-equation
planetary-wave model (T42L30) with simplified physical processes such as Newtonian cooling, Rayleigh friction andhorizontal hyper-diffusion. In this model, zonal-mean state in the model is fixed to zonal mean of NCEP-DOE
reanalysis. Diabatic heating is taken from the NCEP-DOE or JRA-25 reanalyses. Diabatic heating and zonal-meanstate in experiments for the Northern and Southern Hemispheres are climatology (1979-1998) in July and January,respectively.
SLP RESPONSES TO DIABATIC HEATING
A control run forced with diabatic heating of the JRA-25 reanalysis assigned at all vertical model levels globallyand global topography indicates that the summertime subtropical highs in the Northern Hemispheres can be
reproduced realistically as well as counterpart forced with diabatic heating of the NCEP-DOE reanalyses (Figure 1).
When diabatic heating is assigned only at lower tropospheric layers (0.667 < σ < 1) in the land-sea heating-cooling

(a) Observation (Zonally asymmetric component of NCEP-DOE reanalysis)(b) Model response to diabatic heating of NCEP-DOE reanalysis(c) Model response to diabatic heating of JRA-25 reanalysis

Figure 1: (a) Zonally asymmetric component of climatological sea-level pressure (SLP) in July. (b) SLP
response of the control run for July forced with global topography and global diabatic heating at all
vertical levels. Diabatic heating is taken from the NCEP-DOE reanalyses. (c) As in (b) but for diabatic
heating of the JRA-25 reanalyses.
(a) Zonally asymmetric diabatic heating of NCEP-DOE (0.667<σ<1)(b) Zonally asymmetric diabatic heating of JRA-25 (0.667<σ<1)(c) Zonally asymmetric diabatic heating of NCEP-DOE (20N-50N)(d) Zonally asymmetric diabatic heating of JRA-25 (20N-50N)

Figure 2: Zonally asymmetric July diabatic heating of (a and b) the NCEP-DOE and (b and d) JRA-25
reanalyses. (a and b) Diabatic heating is averaged (a and b) in the lower troposphere (0.667<σ<1) and (cand d) in subtropics (20°~50°N). Thick rectangles signify forcing domains for model simulations for thelocal low-level diabatic heating experiments.
(a) SLP response (North Pacific, NCEP-DOE)(b) SLP response (North Pacific, JRA-25)(c) SLP response (North Atlantic, NCEP-DOE)(d) SLP response (North Atlantic, JRA-25)

Figure 3: SLP response to diabatic heating of (a and c) the NCEP-DOE and (b and d) JRA-25 reanalysesin (a and b) the North Pacific and (c and d) Atlantic. Thick rectangles signify forcing domains for model
simulations for the local low-level diabatic heating experiments.

couplet regions (indicated by rectangles in Figures 2 and 3), subtropical highs over the North Pacific and Atlantic canbe reproduce with ~67% and ~60% strengths of the control run, respectively (Figures 3). Diabatic heating of theJRA-25 reanalyses shows that the land-sea heating-cooling couplet is comprised of sensible heating over dry continentand radiative cooling due to marine stratus (Figure 2). These results are similar to the counterparts in Miyasaka andNakamura (2005), therefore their conclusion is confirmed. Also in the Southern Hemisphere, it is confirmed that thesummertime subtropical highs can be reproduced mainly by the local low-level land-sea heating-cooling couplet ofboth reanalysis diabatic heating (Table 1).
MARITIME COOLING IN TWO REANALYSES
It is noteworthy that the responses to diabatic heating of the JRA-25 reanalyses are weaker than counterpart ofthe NCEP-DOE reanalyses (Table 1), although low-level cloud of the JRA-25 reanalyses is grater than of theNCEP-DOE reanalyses (Figures 4a and 4d). This relation seems to be conflicted because it is thought that themore marine stratus develops the stronger radiative cooling is. Radiative heating components of both reanalyses in
the North Pacific are shown in Figures 4b and 4e. Consistently with grater cloud cover in the JRA-25 reanalyses
than in the NCEP-DOE reanalyses, radiative cooling in the JRA-25 reanalysis is stronger than in the NCEP-DOE
reanalyses (Figures 4b and 4e). However, vertically diffusive heating component of diabatic heating in the
JRA-25 reanalyses shows strong heating in marine stratus unlike in the NCEP-DOE reanalysis (Figures 4c andTable 1. SLP responses to the local low-level diabatic heating and their percentages to response in thecontrol runs
North PacificNorth Atlantic
South PacificSouth Atlantic
South Indian Ocean

NCEP-DOE diabatic heating
8 hPa (~80%)
8 hPa (~57%)
4 hPa (~80%)
4 hPa (~80%)
3 hPa (~100%)

JRA-25 diabatic heating
6 hPa (~67%)
6 hPa (~60%)
3 hPa (~75%)
3 hPa (~75%)
2 hPa (~50%)(a) Cloud cover (NCEP-DOE)(b) Radiative heating (NCEP-DOE)(c) Vertically diffusive heating (NCEP-DOE)(d) Cloud cover (JRA-25)(e) Radiative heating (JRA-25)(f) Vertically diffusive heating (JRA-25)

Figure 4: Longitude-height cross sections of zonally asymmetric component averaged for 20~50N in theNortheastern Pacific. (a and d) Cloud cover. (b and e) radiative and (c and f) vertically diffusive heating
component of diabatic heating. (a, b and c) The NCEP-DOE and (d, e and f) JRA-25 reanalyses.

4f). As a result, total maritime cooling in the JRA-25 reanalyses is weaker than in the NCEP-DOE reanalyses, andSLP responses to diabatic heating of the JRA-25 are weaker than counterpart of the NCEP-DOE reanalyses.
SUMMARY
It is confirmed that the summertime subtropical highs are mainly formed by the local land-sea heating-cooling
couplets through numerical experiments forced with diabatic heating of the JRA-25 reanalysis as well as
Miyasaka and Nakamura (2005), in which similar experiments performed with diabatic heating of the NCEP-DOE
reanalyses. However, diabatic heating of the JRA-25 reanalyses shows stronger radiative cooling associated withgrater marine stratus and stronger vertically diffusive heating in marine stratus unlike in the NCEP-DOEreanalysis. These differences suggest that diabatic heating of reanalyses have negligible uncertainty quantitatively
and a quantitative discussion on the formation mechanisms of the summertime subtropical highs is difficult.

ACKNOWLEDGMENTS
This work is supported by a Grant-in-Aid for Scientific Research (18204044) of the Japan Society for thePromotion of Science and by the Global Environment Research Fund (S-5) of the Ministry of the Environment,
Japan. One of datasets used for this study is provided from the cooperative research project of the JRA-25 long-term
reanalysis by Japan Meteorological Agency (JMA) and Central Research Institute of Electric Power Industry(CRIEPI). The other datasets is provided from the cooperative research project of the NCEP–DOE Atmospheric
Model Intercomparison Project (AMIP-II) reanalysis (R-2) by NCEP and DOE. Figures were drawn by GrADS.

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