Impact of resolving the diurnal cycle in an ocean–atmosphere GCM. Part 1: a diurnally forced OGCM |
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| Authors: | D J Bernie E Guilyardi G Madec J M Slingo S J Woolnough |
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| Institution: | (1) National Centre for Atmospheric Science - Climate, Department of Meteorology, University of Reading, Reading, UK;(2) Laboratoire d‘Océanographie et du Climat, Expérimentation et Approches Numériques, IPSL, Paris, France;(3) Met Office Hadley Centre, Fitzroy road, Exeter, EX1 3PB, UK |
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| Abstract: | The diurnal cycle is a fundamental time scale in the climate system, at which the upper ocean and atmosphere are routinely
observed to vary. Current climate models, however, are not configured to resolve the diurnal cycle in the upper ocean or the
interaction of the ocean and atmosphere on these time scales. This study examines the diurnal cycle of the tropical upper
ocean and its climate impacts. In the present paper, the first of two, a high vertical resolution ocean general circulation
model (OGCM), with modified physics, is developed which is able to resolve the diurnal cycle of sea surface temperature (SST)
and current variability in the upper ocean. It is then validated against a satellite derived parameterization of diurnal SST
variability and in-situ current observations. The model is then used to assess rectification of the intraseasonal SST response
to the Madden–Julian oscillation (MJO) by the diurnal cycle of SST. Across the equatorial Indo-Pacific it is found that the
diurnal cycle increases the intraseasonal SST response to the MJO by around 20%. In the Pacific, the diurnal cycle also modifies
the exchange of momentum between equatorially divergent Ekman currents and the meridionally convergent geostrophic currents
beneath, resulting in a 10% increase in the strength of the Ekman cells and equatorial upwelling. How the thermodynamic and
dynamical impacts of the diurnal cycle effect the mean state, and variability, of the climate system cannot be fully investigated
in the constrained design of ocean-only experiments presented here. The second part of this study, published separately, addresses
the climate impacts of the diurnal cycle in the coupled system by coupling the OGCM developed here to an atmosphere general
circulation model. |
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