Coupled processes for equatorial Pacific interannual variability (original) (raw)
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Climate Dynamics, 1999
The mechanisms responsible for the mean state and the seasonal and interannual variations of the coupled tropical Pacific-global atmosphere system are investigated by analyzing a thirty year simulation, where the LMD global atmospheric model and the LODYC tropical Pacific model are coupled using the delocalized physics method. No flux correction is needed over the tropical region. The coupled model reaches its regime state roughly after one year of integration in spite of the fact that the ocean is initialized from rest. Departures from the mean state are characterized by oscillations with dominant periodicites at annual, biennial and quadriennial time scales. In our model, equatorial sea surface temperature and wind stress fluctuations evolved in phase. In the Central Pacific during boreal autumn, the sea surface temperature is cold, the wind stress is strong, and the Inter Tropical Convergence Zone (ITCZ) is shifted northwards. The northward shift of the ITCZ enhances atmospheric and oceanic subsidence between the equator and the latitude of organized convention. In turn, the stronger oceanic subsidence reinforces equatorward convergence of water masses at the thermocline depth which, being not balanced by equatorial upwelling, deepens the equatorial thermocline. An equivalent view is that the deepening of the thermocline proceeds from the weakening of the meridional draining of near-surface equatorial waters. The inverse picture prevails during spring, when the equatorial sea surface temperatures are warm. Thus temperature anomalies tend to appear at the thermocline level, in phase opposition to the surface conditions. These subsurface temperature fluctuations propagate from the Central Pacific eastwards along the thermocline; when reaching the surface in the Eastern Pacific, they trigger the reversal of sea surface temperature anomalies. The whole oscillation is synchronized by the apparent meridional motion of the sun, through the seasonal oscillation of the ITCZ. This possible mechanism is partly supported by the observed seasonal reversal of vorticity between the equator and the ITCZ, and by observational evidence of eastward propagating subsurface temperature anomalies at the thermocline level.
Climate Dynamics, 2011
The study compares the simulated poleward migration characteristics of boreal summer intraseasonal oscillations (BSISO) in a suite of coupled ocean-atmospheric model sensitivity integrations. The sensitivity experiments are designed in such a manner to allow full coupling in specific ocean basins but forced by temporally varying monthly climatological sea surface temperature (SST) adopted from the fully coupled model control runs (ES10). While the local air-sea interaction is suppressed in the tropical Indian Ocean and allowed in the other oceans in the ESdI run, it is suppressed in the tropical Pacific and allowed in the other oceans in the ESdP run. Our diagnostics show that the basic mean state in precipitation and easterly vertical shear as well as the BSISO properties remain unchanged due to either inclusion or exclusion of local air-sea interaction. In the presence of realistic easterly vertical shear, the continuous emanation of Rossby waves from the equatorial convection is trapped over the monsoon region that enables the poleward propagation of BSISO anomalies in all the model sensitivity experiments. To explore the internal processes that maintain the tropospheric moisture anomalies ahead of BSISO precipitation anomalies, moisture and moist static energy budgets are performed. In all model experiments, advection of anomalous moisture by climatological winds anchors the moisture anomalies that in turn promote the northward migration of BSISO precipitation. While the results indicate the need for realistic simulation of all aspects of the basic state, our model results need to be taken with caution because in the ECHAM family of coupled models the internal variance at intraseasonal timescales is indeed very high, and therefore local air-sea interactions may not play a pivotal role.
Climate Dynamics, 1999
The thirty year simulation of the coupled global atmosphere-tropical Pacific Ocean general circulation model of the Laboratoire de Métérologie Dynamique and the Laboratoire d’Océanographie Dynamique et de Climatologie presented in Part I is further investigated in order to understand the mechanisms of interannual variability. The model does simulate interannual events with ENSO characteristics; the dominant periodicity is quasi-biennial, though strong events are separated by four year intervals. The mechanism that is responsible for seasonal oscillations, identified in Part I, is also active in interannual variability with the difference that now the Western Pacific is dynamically involved. A warm interannual phase is associated with an equatorward shift of the ITCZ in the Western and Central Pacific. The coupling between the ITCZ and the ocean circulation is then responsible for the cooling of the equatorial subsurface by the draining mechanism. Cold subsurface temperature anomalies then propagate eastward along the mean equatorial thermocline. Upon reaching the Eastern Pacific where the mean thermocline is shallow, cold subsurface anomalies affect surface temperatures and reverse the phase of the oscillation. The preferred season for efficient eastward propagation of thermocline depth temperature anomalies is boreal autumn, when draining of equatorial waters towards higher latitudes is weaker than in spring by a factor of six. In that way, the annual cycle acts as a dam that synchronizes lower frequency oscillations.
The tropical intraseasonal oscillation in a coupled ocean‐atmosphere general circulation model
Geophysical Research Letters, 1999
In this study we report the simulation of the intraseasonal oscillation of the tropical atmosphere (TIO) in the coupled model GIOTTO. The model produces a zonal wave number one eastward propagating disturbance with a mean period of 42 days, which is in good agreement with the observed TIO periodicity. The simulated TIO, however, seems to be influenced by errors in the model mean state. In particular, the errors in the sea surface temperature of the tropical Pacific appear to have a significant impact on the oscillation, affecting its spatial distribution and producing an excess of TIO activity over the eastern Pacific. Despite of these shortcomings, the model seems to be able to reproduce some of the air-sea interaction mechanisms which characterize the observed TIO.
Interactions between ENSO, Transient Circulation, and Tropical Convectionover the Pacific
Journal of Climate, 1999
The interannual variability of transient waves and convection over the central and eastern Pacific is examined using 30 northern winters of NCEP-NCAR reanalyses (1968/69-1997/98) and satellite outgoing longwave radiation data starting in 1974. There is a clear signal associated with the El Niño-Southern Oscillation, such that differences in the seasonal-mean basic state lead to statistically significant changes in the behavior of the transients and convection (with periods less than 30 days), which then feed back onto the basic state.
Journal of Climate, 2004
This study compares the tropical intraseasonal oscillation (TISO) variability in the Geophysical Fluid Dynamics Laboratory (GFDL) coupled general circulation model (CGCM) and the stand-alone atmospheric general circulation model (AGCM). For the AGCM simulation, the sea surface temperatures (SSTs) were specified using those from the CGCM simulation. This was done so that any differences in the TISO that emerged from the two simulations could be attributed to the coupling process and not to a difference in the mean background state. The comparison focused on analysis of the rainfall, 200-mb velocity potential, and 850-mb zonal wind data from the two simulations, for both summer and winter periods, and included comparisons to analogous diagnostics using NCEP-NCAR reanalysis and Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) rainfall data.
Journal of the Meteorological Society of Japan. Ser. II
Low-frequency oscillations appearing in three GCM seasonal cycle integrations are compared with the analyses of the European Centre for Medium-Range Weather Forecasts (ECMWF). All three models have the same resolution: 4 degrees latitude by 5 degrees longitude, with 9 levels. The GLAS GCM simulates a realistic eastward propagation of the 30-60 day oscillation in the tropical upperlevel divergent flow. The eastward travelling planetary scale structure becomes more stationary over the Indonesian region and accelerates over the central Pacific, as observed. In the GLA GCM, the oscillation propagates into the higher latitudes of both hemispheres as the waves leave the convective region. The presence of the eastward propagating oscillation is not obvious in the UCLA GCM. The wavenumber-frequency spectra of the 200 mb velocity potential reveal that all the GCMs have a significantly weaker signal for eastward propagation in the 30-60 day range than the analyses. The spectrum for the GLAS GCM is dominated by 20-60 day periods, while the GLA GCM has a spectral peak around 20-30 days. There is a weak eastward propagating peak near 15 days in the UCLA GCM. The dominant phase speeds and the different vertical structures of the heating profiles in the GCMs are in general agreement with current theory involving the positive feedback between latent heating and moist static stability. The composited patterns of the observations indicate that in the tropics a Kelvin wave-type structure is dominant near the center of the oscillation. The simulated winds are fairly realistic, although the meridional component is too strong, especially in the GLA GCM. The vertical structures of the zonal wind component and moisture suggest that a mobile wave-CISK (Lau and Peng,1987) is an important mechanism in maintaining the intraseasonal oscillation in these GCMs. The vertical distribution of the moisture field further suggests that evaporation-wind feedback (Neelin, et al., 1987) may play a role in maintaining the eastward propagating tropical waves. The differences in the structure of the oscillation in the GLAS GCM and GLA GCM appear to be a consequence of the different numerical schemes used. The GCMs have preferred zones for diabatic heating, with a turn-on heating occurring when the rising branch of the intraseasonal oscillation passes over these convective regions. All three GCMs fail to capture the detailed evolution in the different stages of the development and decay of the oscillation. The results suggest that an improvement in the boundary layer moisture processes may be crucial for a better simulation of the oscillation.
Annual Cycle and ENSO in a Coupled Ocean Atmosphere General Circulation Model
Monthly Weather Review, 1997
Results from multiyear integrations of a coupled ocean-atmosphere general circulation model are described. The atmospheric component is a rhomboidal 15, 18-level version of the Center for Ocean-Land-Atmosphere Studies atmospheric general circulation model. The oceanic component is the Geophysical Fluid Dynamics Laboratory ocean model with a horizontal domain extending from 70ЊS to 65ЊN. The ocean model uses 1.5Њ horizontal resolution, with meridional resolution increasing to 0.5Њ near the equator, and 20 vertical levels, most in the upper 300 m. No flux adjustments are employed.
2006
Intraseasonal oscillations (ISOs) are important large-amplitude and large-scale elements of the tropical Indo-Pacific climate with time scales in the 20-60-day period range, during which time they modulate higher-frequency tropical weather. Despite their importance, the ISO is poorly simulated and predicted by numerical models. A joint diagnostic and modeling study of the ISO is conducted, concentrating on the period between the suppressed and active (referred to as the "transition") period that is hypothesized to be the defining stage for the development of the intraseasonal mode and the component that is most poorly simulated.