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In this study the role of atmospheric boundary layer schemes in climate models is investigated. Including a boundary layer scheme in an Earth system model of intermediate complexity (EMIC) produces only minor differences in the estimated global distribution of sensible and latent heat fluxes over land (upto about 15% of the net radiation at the surface). However, neglecting of boundary layer processes, such as the development of a well-mixed layer over land or the impact of stability on the exchange coefficient in the surface layer, leads to erroneous surface temperatures, especially in convective conditions with low wind speeds. As these conditions occur frequently, introducing a boundary layer scheme in an EMIC gives reductions in June-July-August averaged surface temperature of 1–2 °C in wet areas, to 5–7 °C in desert areas. Even a relatively simple boundary layer scheme provides reasonable estimates of the surface fluxes and surface temperatures. Detailed schemes that solve explicitly the turbulent fluxes within the boundary layer are only required when vertical profiles of potential temperature are needed. 相似文献
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The Bjerknes feedback in the tropical Atlantic in CMIP5 models 总被引：1，自引：1，他引：0
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The anthropogenic changes during boreal winter in the thermal and zonal flow structure over Eastern Atlantic and Western Europe (EAWE) have been investigated using an ensemble of CMIP3 and CMIP5 models. The ensemble mean change in the zonal wind at 500 hPa over this region is characterized by an eastward extension of the belt of zonal winds. Using the thermal wind relation these wind changes are found to be consistent with the changes in the tropospheric temperature profile. An enhanced warming is simulated in the subtropical upper troposphere and a relative surface cooling in the mid-latitudes. The subtropical upper tropospheric warming is related to the downward branch of the mean meridional circulation, whereas the mid-latitude lower tropospheric relative cooling is linked to the ocean processes that govern changes in its surface temperatures. Inter-model differences in the simulated change of the zonal wind over the EAWE by the CMIP3 and CMIP5 models relate well with differences in the upper tropospheric subtropical warming and the mid-latitude lower tropospheric relative cooling. The simulated change of the zonal wind over the EAWE region by the CMIP3 and CMIP5 models correlates well with changes in the meridional SST gradient. We conclude that uncertainties in the projected changes of the zonal flow over Europe are at least partly due to uncertainties in the response of the North Atlantic Ocean to increased levels of greenhouse gases. 相似文献
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Rapid transitions and ultra-low frequency behaviour in a 40 kyr integration with a coupled climate model of intermediate complexity 总被引：1，自引：0，他引：1
A 40 kyr integration with the coupled atmosphere/ocean/sea-ice model of intermediate complexity ECBilt for present boundary
conditions has been performed. The climate of ECBilt displays quasi-periodical behaviour with a period of approximately 13 kyr.
The quasi-periodical behaviour is characterized by large changes in the overturning cell in the Southern Ocean. The southern
cell fluctuates between two quasi-stationary states, with accompaning changes in the atmospheric circulation in the Southern
Hemisphere. The transition between these states is rapid and resembles the polar halocline catastrophes and flushes as observed
in ocean general circulation models under mixed boundary conditions. The sea-ice influence on both the surface heat and fresh
water flux appears to be crucial for the existence and the prolongation of the quasi-stationary states. The atmospheric circulation
of those two quasi-stationary states displays large regional differences over Antarctica, resulting in even opposite surface
air temperature trends for certain locations during the transition from one state to another.
Received: 7 October 1999 / Accepted: 28 August 2000 相似文献
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The sensitivity of tropical Atlantic climate to upper ocean mixing is investigated using an ocean-only model and a coupled
ocean–atmosphere model. The upper ocean thermal structure and associated atmospheric circulation prove to be strongly related
to the strength of upper ocean mixing. Using the heat balance in the mixed layer it is shown that an excessively cold equatorial
cold tongue can be attributed to entrainment flux at the base of the oceanic mixed layer, that is too large. Enhanced entrainment
efficiency acts to deepen the mixed layer and causes strong reduction in the upper ocean divergence in the central equatorial
Atlantic. As a result, the simulated sea surface temperature, thermocline structure, and upwelling velocities are close to
the observed estimates. In the coupled model, the seasonal migration of the Intertropical Convergence Zone (ITCZ) reduces
when the entrainment efficiency in the oceanic mixed layer is enhanced. The precipitation rates decrease in the equatorial
region and increase along 10°N, resulting in a more realistic Atlantic Marine ITCZ. The reduced meridional surface temperature
gradient in the eastern tropical Atlantic prohibits the development of convective precipitation in the southeastern part of
the tropical Atlantic. Also, the simulation of tropical Atlantic variability as expressed in the meridional gradient mode
and the eastern cold tongue mode improves when the entrainment efficiency is enhanced. 相似文献
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A robust change in the tropical circulation induced by anthropogenic warming in CMIP3 models is a weakening of the Walker circulation. This weakening affects the upper tropospheric divergence thereby modifying the propagation of Rossby waves from the tropics into the extra-tropics. It can be modeled by the barotropic vorticity equation forced with a Rossby wave source that is computed from the upper tropospheric divergence. Using the BVE as a diagnostic tool it is demonstrated for the CMIP3 models that the weakening of the Walker circulation has a significant impact on the extra-tropical planetary wave structure and to a large extent explains the projected changes in the mid tropospheric meridional wind in the CMIP3 models. The dominant response is a wave number five pattern similar to the circumglobal waveguide pattern. This analysis implies that a correct simulation of the Walker circulation and its response to anthropogenic changes are crucial for a correct simulation of the anthropogenic change in the extra-tropical planetary wave structure. Structure and intensity of the Walker circulation of the CMIP3 models show significant deviations from the Walker circulation as diagnosed from the ERA-interim and NCEP/NCAR reanalysis. Improving the simulation of the Walker circulation is a prerequisite to narrow the uncertainty in the projected anthropogenic change in the extra-tropical planetary wave structure. 相似文献
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Hylke de Vries Tim Woollings James Anstey Reindert J. Haarsma Wilco Hazeleger 《Climate Dynamics》2013,41(9-10):2643-2654
The future changes of atmospheric blocking over the Euro-Atlantic sector, diagnosed from an ensemble of 17 global-climate simulations obtained with the ECHAM5/MPI-OM model, are shown to be largely explainable from the change of the 500 hPa mean zonal circulation and its variance. The reduction of the blocking frequency over the Atlantic and the increased frequency of easterly upper-level flow poleward of 60°N are well explained by the changes of mean zonal circulation. In winter and autumn an additional downstream shift of the frequency maximum is simulated. This is also seen in a subset of the CMIP5 models with RCP8.5. To explain this downstream shift requires the inclusion of the changing variance. It is suggested that the increased downstream variance is caused by the stronger, more eastward extending future jet, which promotes Rossby wave breaking and blocking to occur further downstream. The same relation between jet-strength and central-blocking longitude is found in the variability of the current climate. 相似文献
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Reindert J. Haarsma Edmo J. D. Campos Sybren Drijfhout Wilco Hazeleger Camiel Severijns 《Climate Dynamics》2011,36(5-6):989-1003
In this paper we use a coupled ocean?Catmosphere model to investigate the impact of the interruption of Agulhas leakage of Indian ocean water on the tropical Atlantic, a region where strong coupled ocean?Catmosphere interactions occur. The effect of a shut down of leakage of Indian ocean water is isolated from the effect of a collapse of the MOC. In our experiments, the ocean model is forced with boundary conditions in the southeastern corner of the domain that correspond to no interocean exchange of Indian ocean water into the Atlantic. The southern boundary condition is taken from the Levitus data and ensures an MOC in the Atlantic. Within this configuration, instead of warm and salty Indian ocean water temperature (cold) and salinity (fresh) anomalies of southern ocean origin propagate into the South Atlantic and eventually reach the equatorial region, mainly in the thermocline. This set up mimics the closure of the ??warm water path?? in favor of the ??cold water path??. As part of the atmospheric response, there is a northward shift of the intertropical convergence zone (ITCZ). The changes in trade winds lead to reduced Ekman pumping in the equatorial region. This leads to a freshening and warming of the surface waters along the equator. Especially in the Cold Tongue region, the cold and fresh subsurface anomalies do not reach the surface due to the reduced upwelling. The anomaly signals are transported by the equatorial undercurrent and spread away from the equator within the thermocline. Part of the anomaly eventually reaches the Tropical North Atlantic, where it affects the Guinea Dome. Surprisingly, the main effect at the surface is small on the equator and relatively large at the Guinea Dome. In the atmosphere, the northward shift of the ITCZ is associated with a band of negative precipitation anomalies and higher salinities over the Tropical South Atlantic. An important implication of these results is that the modified water characteristics due to a shut down of the Agulhas leakage remain largely unaffected when crossing the equatorial Atlantic and therefore can affect the deepwater formation in the North Atlantic. This supports the hypothesis that the Agulhas leakage is an important source region for climate change and decadal variability of the Atlantic. 相似文献