共查询到20条相似文献,搜索用时 31 毫秒
1.
The present study investigates the sensitivity of the COupled Hydrodynamical–Ecological model for REgioNal and Shelf seas (COHERENS) to predict sea surface temperature (SST) patterns in the Sea of Iroise (western end of French Brittany) in relation to the spatial and temporal resolutions of open boundary conditions (OBCs). Two sources of daily operational OBCs of temperature are considered, derived from (1) the Mercator Global Ocean and (2) the Iberian Biscay Irish analysis and forecasting systems delivering predictions at spatial resolutions of 1/12° and 1/36°, respectively. Coastal model performance is evaluated by comparing SST predictions with recently available field data collected (1) along the route of a vessel travelling between the coast and the isle of Ushant and (2) at two offshore stations. The comparison is extended to SST spatial distribution derived from remote-sensing observations. The influence of OBC spatial resolution is exhibited in the north-eastern area of the Sea of Iroise in relation to the intrusion of cold surface waters. OBC temporal resolution is found to have a lower impact advocating for the implementation of climatological temperature forcings to predict major SST patterns in the Sea of Iroise. 相似文献
2.
The influence of a summer storm event in 2007 on the North Sea and its effects on the ocean stratification are investigated using a regional coupled ocean (Regional Ocean Modeling System, ROMS)-atmosphere (Weather Research & Forecasting model, WRF) modeling system. An analysis of potential energy anomaly (PEA, Φ) and its temporal development reveals that the loss of stratification due to the storm event is dominated by vertical mixing in almost the entire North Sea. For specific regions, however, a considerable contribution of depth-mean straining is observed. Vertical mixing is highly correlated with wind induced surface stresses. However, peak mixing values are observed in combination with incoming flood currents. Depending on the phase between winds and tides, the loss of stratification differs strongly over the North Sea. To study the effects of interactive ocean-atmosphere exchange, a fully coupled simulation is compared with two uncoupled ones for the same vertical mixing parameters to identify the impact of spatial resolution as well as of SST feedback. While the resulting new mixed layer depth after the storm event in the uncoupled simulation with lower spatial and temporal resolution of the surface forcing data can still be located in the euphotic zone, the coupled simulation is capable to mix the entire water column and the vertical mixing in the uncoupled simulation with higher resolution of the surface forcing data is strongly amplified. These differences might have notable implications for ecosystem modeling since it could determine the development of new phytoplankton blooms after the storm and for sediment modeling in terms of sediment mobilization. An investigation of restratification after the extreme event illustrates the persistent effect of this summer storm. 相似文献
3.
Analysis and modeling of the seasonal South China Sea temperature cycle using remote sensing 总被引:1,自引:0,他引:1
Daniel J. Twigt Erik D. De Goede Ernst J. O. Schrama Herman Gerritsen 《Ocean Dynamics》2007,57(4-5):467-484
The present paper describes the analysis and modeling of the South China Sea (SCS) temperature cycle on a seasonal scale.
It investigates the possibility to model this cycle in a consistent way while not taking into account tidal forcing and associated
tidal mixing and exchange. This is motivated by the possibility to significantly increase the model’s computational efficiency
when neglecting tides. The goal is to develop a flexible and efficient tool for seasonal scenario analysis and to generate
transport boundary forcing for local models. Given the significant spatial extent of the SCS basin and the focus on seasonal
time scales, synoptic remote sensing is an ideal tool in this analysis. Remote sensing is used to assess the seasonal temperature
cycle to identify the relevant driving forces and is a valuable source of input data for modeling. Model simulations are performed
using a three-dimensional baroclinic-reduced depth model, driven by monthly mean sea surface anomaly boundary forcing, monthly
mean lateral temperature, and salinity forcing obtained from the World Ocean Atlas 2001 climatology, six hourly meteorological
forcing from the European Center for Medium range Weather Forecasting ERA-40 dataset, and remotely sensed sea surface temperature
(SST) data. A sensitivity analysis of model forcing and coefficients is performed. The model results are quantitatively assessed
against climatological temperature profiles using a goodness-of-fit norm. In the deep regions, the model results are in good
agreement with this validation data. In the shallow regions, discrepancies are found. To improve the agreement there, we apply
a SST nudging method at the free water surface. This considerably improves the model’s vertical temperature representation
in the shallow regions. Based on the model validation against climatological in situ and SST data, we conclude that the seasonal
temperature cycle for the deep SCS basin can be represented to a good degree. For shallow regions, the absence of tidal mixing
and exchange has a clear impact on the model’s temperature representation. This effect on the large-scale temperature cycle
can be compensated to a good degree by SST nudging for diagnostic applications. 相似文献
4.
B. Meyssignac C. G. Piecuch C. J. Merchant M.-F. Racault H. Palanisamy C. MacIntosh S. Sathyendranath R. Brewin 《Surveys in Geophysics》2017,38(1):187-215
We analyse the regional variability in observed sea surface height (SSH), sea surface temperature (SST) and ocean colour (OC) from the ESA Climate Change Initiative datasets over the period 1993–2011. The analysis focuses on the signature of the ocean large-scale climate fluctuations driven by the atmospheric forcing and do not address the mesoscale variability. We use the ECCO version 4 ocean reanalysis to unravel the role of ocean transport and surface buoyancy fluxes in the observed SSH, SST and OC variability. We show that the SSH regional variability is dominated by the steric effect (except at high latitude) and is mainly shaped by ocean heat transport divergences with some contributions from the surface heat fluxes forcing that can be significant regionally (confirming earlier results). This is in contrast with the SST regional variability, which is the result of the compensation of surface heat fluxes by ocean heat transport in the mixed layer and arises from small departures around this background balance. Bringing together the results of SSH and SST analyses, we show that SSH and SST bear some common variability. This is because both SSH and SST variability show significant contributions from the surface heat fluxes forcing. It is evidenced by the high correlation between SST and buoyancy-forced SSH almost everywhere in the ocean except at high latitude. OC, which is determined by phytoplankton biomass, is governed by the availability of light and nutrients that essentially depend on climate fluctuations. For this reason, OC shows significant correlation with SST and SSH. We show that the correlation with SST displays the same pattern as the correlation with SSH with a negative correlation in the tropics and subtropics and a positive correlation at high latitude. We discuss the reasons for this pattern. 相似文献
5.
Globally coupled climate models are generally capable of reproducing the observed trends in the globally averaged atmospheric temperature. However, the global models do not perform as well on regional scales. Here, we present results from four 100-year, high-resolution ocean model experiments (resolution less than 1 km) for the western Baltic Sea. The forcing is taken from a regional atmospheric model and a regional ocean model, imbedded into two global greenhouse gas emission scenarios, A1B and B1, for the period of 2000 to 2100 with each two realisations. Two control runs from 1960 to 2000 are used for validation. For both scenarios, the results show a warming with an increase of 0.5–2.5 K at the sea surface and 0.7–2.8 K below 40 m. The simulations further indicate a decrease in salinity by 1.5–2 practical salinity units. The increase in water temperature leads to a prolongation of heat waves based on present-day thresholds. This amounts to a doubling or even tripling of the heat wave duration. The simulations show a decrease in inflow events (barotropic/baroclinic), which will affect the deepwater generation and ventilation of the central Baltic Sea. The high spatial resolution allows us to diagnose the inflow events and the mechanism that will cause future changes. The reduction in barotropic inflow events correlates well with the increase in westerly winds. The changes in the baroclinic inflows can be consistently explained by the reduction of calm wind periods and thus a weakening of the necessary stratification in the western Baltic Sea and the Danish Straits. 相似文献
6.
Nisha Kurian Matthieu Lengaigne Gopalakrishna Venkata Vissa Jerome Vialard Stephane Pous Anne-Charlotte Peter Fabien Durand Shweta Naik 《Ocean Dynamics》2013,63(4):329-346
Active and break phases of the Indian summer monsoon are associated with sea surface temperature (SST) fluctuations at 30–90 days timescale in the Arabian Sea and Bay of Bengal. Mechanisms responsible for basin-scale intraseasonal SST variations have previously been discussed, but the maxima of SST variability are actually located in three specific offshore regions: the South-Eastern Arabian Sea (SEAS), the Southern Tip of India (STI) and the North-Western Bay of Bengal (NWBoB). In the present study, we use an eddy-permitting 0.25° regional ocean model to investigate mechanisms of this offshore intraseasonal SST variability. Modelled climatological mixed layer and upper thermocline depth are in very good agreement with estimates from three repeated expendable bathythermograph transects perpendicular to the Indian Coast. The model intraseasonal forcing and SST variability agree well with observed estimates, although modelled intraseasonal offshore SST amplitude is undere-stimated by 20–30 %. Our analysis reveals that surface heat flux variations drive a large part of the intraseasonal SST variations along the Indian coastline while oceanic processes have contrasted contributions depending of the region considered. In the SEAS, this contribution is very small because intraseasonal wind variations are essentially cross-shore, and thus not associated with significant upwelling intraseasonal fluctuations. In the STI, vertical advection associated with Ekman pumping contributes to ~30 % of the SST fluctuations. In the NWBoB, vertical mixing diminishes the SST variations driven by the atmospheric heat flux perturbations by 40 %. Simple slab ocean model integrations show that the amplitude of these intraseasonal SST signals is not very sensitive to the heat flux dataset used, but more sensitive to mixed layer depth. 相似文献
7.
The near-sea surface meteorological conditions associated with the Mediterranean heavy precipitation events constitute, on
a short time scale, a strong forcing on the ocean mixed layer. This study addresses the question of the optimal time frequency
of the atmospheric forcing to drive an ocean model in order to make it able to capture the fine scale ocean mixed layer response
to severe meteorological conditions. The coupling time frequency should allow the ocean model to reproduce the formation of
internal low-salty boundary layers due to sudden input of intense precipitation, as well as the cooling and deepening of the
ocean mixed layer through large latent heat fluxes and stress under the intense low-level jet associated with these events.
In this study, the one-dimensional ocean model is driven by 2.4-km atmospheric simulated fields on a case of Mediterranean
heavy precipitation, varying the time resolution of the atmospheric forcing. The results show that using a finer temporal
resolution than 1 h for the atmospheric forcing is not necessary, but a coarser temporal resolution (3 or 6 h) modifies the
event course and intensity perceived by the ocean. Consequently, when using a too coarse temporal resolution forcing, typically
6 h, the ocean model fails to reproduce the ocean mixed layer fine scale response under the heavy rainfall pulses and the
strong wind gusts. 相似文献
8.
A high-resolution, regional coupled atmosphere–ocean model is used to investigate strong air–sea interactions during a rapidly developing extratropical cyclone (ETC) off the east coast of the USA. In this two-way coupled system, surface momentum and heat fluxes derived from the Weather Research and Forecasting model and sea surface temperature (SST) from the Regional Ocean Modeling System are exchanged via the Model Coupling Toolkit. Comparisons are made between the modeled and observed wind velocity, sea level pressure, 10 m air temperature, and sea surface temperature time series, as well as a comparison between the model and one glider transect. Vertical profiles of modeled air temperature and winds in the marine atmospheric boundary layer and temperature variations in the upper ocean during a 3-day storm period are examined at various cross-shelf transects along the eastern seaboard. It is found that the air–sea interactions near the Gulf Stream are important for generating and sustaining the ETC. In particular, locally enhanced winds over a warm sea (relative to the land temperature) induce large surface heat fluxes which cool the upper ocean by up to 2 °C, mainly during the cold air outbreak period after the storm passage. Detailed heat budget analyses show the ocean-to-atmosphere heat flux dominates the upper ocean heat content variations. Results clearly show that dynamic air–sea interactions affecting momentum and buoyancy flux exchanges in ETCs need to be resolved accurately in a coupled atmosphere–ocean modeling framework. 相似文献
9.
The Mediterranean Sea is a region of intense air–sea interactions, with in particular strong evaporation over sea which drives
the thermohaline circulation. The Mediterranean region is also prone to strong precipitation events characterized by low spatial
extent, short duration, and high temporal variability. The impacts of intense offshore precipitation over sea, in the Gulf
of Lions which is a spot for winter deep convection, are investigated using four sensitivity simulations performed at mesoscale
resolution with the eddy-resolving regional ocean model NEMO-MED12. We use various atmospheric fields to force NEMO-MED12,
downscaled from reanalyses with the non-hydrostatic mesoscale Weather Research and Forecasting model but differing in space
resolutions (20 and 6.7 km) or in time frequencies (daily and three-hourly). This numerical study evidences that immediate,
intense, and rapid freshening occurs under strong precipitation events. The strong salinity anomaly induced extends horizontally
(≃50 km) as vertically (down to 50 m) and persists several days after strong precipitation events. The change in the space
resolution of the atmospheric forcing modifies the precipitating patterns and intensity, as well as the shape and the dynamics
of the low-salinity layer formed are changed. With higher forcing frequency, shorter and heavier precipitation falls in the
ocean in the center of the Gulf of Lions, and due to a stronger vertical shear and mixing, the low-salinity anomaly propagates
deeper. 相似文献
10.
Berina Mina Kilicarslan Ismail Yucel Heves Pilatin Eren Duzenli Mustafa Tugrul Yilmaz 《水文研究》2021,35(9):e14338
This study investigates the impact of the spatio-temporal accuracy of four different sea surface temperature (SST) datasets on the accuracy of the Weather Research and Forecasting (WRF)-Hydro system to simulate hydrological response during two catastrophic flood events over the Eastern Black Sea (EBS) and the Mediterranean (MED) regions of Turkey. Three time-variant and high spatial resolution external SST products (GHRSST, Medspiration and NCEP-SST) and one coarse-resolution and time-invariant SST product (ERA5- and GFS-SST for EBS and MED regions, respectively) already embedded in the initial and the boundary conditions datasets of WRF model are used in deriving near-surface atmospheric variables through WRF. After the proper event-based calibration is performed to the WRF-Hydro system using hourly and daily streamflow data in both regions, uncoupled model simulations for independent SST events are conducted to assess the impact of SST-triggered precipitation on simulated extreme runoff. Some localized and temporal differences in the occurrence of the flood events with respect to observations depending on the SST representation are noticeable. SST products represented with higher cross-correlations (GHRSST and Medspiration) revealed significant improvement in flood hydrographs for both regions. The GHRSST dataset shows a substantial improvement in NSE (~70%), RMSE reduction up to 20%, and an increase in correlation from 0.3 to 0.8 with respect to the invariable SST (ERA5) in simulated runoffs over the EBS region. The use of both GHRSST and Medspiration SST data characterized with high spatio-temporal correlation resulted in runoff simulations exactly matching the observed runoff peak of 300 m3/s by reducing the overestimation seen in invariable SST (GFS) in the MED region. Improved precipitation simulation skills of the WRF model with the detailed SST representation show that the hydrographs of GHRSST and Medspiration simulations show better performance compared to the simulated hydrographs by observed precipitation. 相似文献
11.
P. C. Chu 《Pure and Applied Geophysics》1989,130(1):31-45
An important part of the influence of the oceans on the atmosphere is through direct radiation, sensible heat flux and release of latent heat of evaporation, whereby all of these processes are directly related to the surface temperature of the oceans. A main effect of the atmosphere on the oceans is through momentum exchange at the air-ocean interface, and this process is directly related to the surface wind stress. The sea surface temperature (SST) and the surface wind stress are the two important components in the air-ocean system. If SST is given, a thermally forced boundary layer atmospheric circulation can be simulated. On the other hand, if the surface wind stress is given, the wind-driven ocean waves and ocean currents can be computed.The relationship between SST and surface wind is a coupling of the atmosphere and the oceans. It changes a one-way effect (ocean mechanically driven by atmosphere, or atmosphere thermally forced by oceans) into two-way air-sea interactions. Through this coupling the SST distribution, being an output from an ocean model, leads to the thermally forced surface winds, which feeds back into the ocean model as an additional forcing.Based on Kuo's planetary boundary layer model a linear algebraic equation is established to link the SST gradient with the thermally forced surface wind. The surface wind blows across the isotherms from cold to warm region with some deflection angle to the right (left) in the Northern (Southern) Hemisphere. Results from this study show that the atmospheric stratification reduces both the speed and the deflection angle of the thermally forced wind, however, the Coriolis' effect increases the wind speed in stable atmosphere (Ri>10–4) and increases the deflection angle. 相似文献
12.
13.
The 1D version of the Model for Applications at Regional Scale is used to parameterize the effects of sea surface waves in
2D in a horizontally homogeneous offshore zone of the Iroise sea. Here we present the first simulation of the Iroise sea including
sea surface waves forcing, and more generally, the first study of a boundary layer including the Hasselmann force with a tidal
wave. We use a single equation turbulence closure based on a non-local diagnosis for energetic and dissipation length scales.
The turbulent energy flux at the surface due to whitecaps and the Hasselmann force induced by Stokes drift are assessed using
the whole sea surface waves spectrum given by the Wave Watch Third generation model. The ability of the parameterization to
reproduce surface currents over a period of 1 year (2007) is tested with high frequency radar using spectral and time-frequency
analysis. One problem with 1D modelling, corresponding to overestimation of current oscillating at inertial frequency is illustrated
by comparing 1D and 3D simulations. We found an overall improvement by including the Hasselmann force mainly within the bandwidth
of less than one cycle per day to one cycle per day for surface currents. Turbulence is induced by whitecaps decaying rapidly
below the ocean surface but the mixed layer below 40 m is deeper due to waves breaking on the sea surface. 相似文献
14.
Global coupled climate models are generally capable of reproducing the observed trends in the globally averaged atmospheric
temperature. However, the global models do not perform as well on regional scales. Here, we present results from a 20-year,
high-resolution ocean model experiment for the Atlantic and Arctic Oceans. The atmospheric forcing is taken from the final
20 years of a twentieth-century control run with a coupled atmosphere–ocean general circulation model. The ocean model results
from the regional ocean model are validated using observations of hydrography from repeat cruises in the Barents Sea. Validation
is performed for average quantities and for probability distributions in space and time. The validation results reveal that,
though the regional model is forced by a coupled global model that has a noticeable sea ice bias in the Barents Sea, the hydrography
and its variability are reproduced with an encouraging quality. We attribute this improvement to the realistic transport of
warm, salty waters into the Barents Sea in the regional model. These lateral fluxes in the ocean are severely underestimated
by the global model. The added value with the regional model that we have documented here lends hope to advance the quality
of oceanic climate change impact studies. 相似文献
15.
Steven L. Morey Mark A. Bourassa Dmitry S. Dukhovskoy James J. O’Brien 《Ocean Dynamics》2006,56(5-6):594-606
A coupled ocean and boundary layer flux numerical modeling system is used to study the upper ocean response to surface heat
and momentum fluxes associated with a major hurricane, namely, Hurricane Dennis (July 2005) in the Gulf of Mexico. A suite
of experiments is run using this modeling system, constructed by coupling a Navy Coastal Ocean Model simulation of the Gulf
of Mexico to an atmospheric flux model. The modeling system is forced by wind fields produced from satellite scatterometer
and atmospheric model wind data, and by numerical weather prediction air temperature data. The experiments are initialized
from a data assimilative hindcast model run and then forced by surface fluxes with no assimilation for the time during which
Hurricane Dennis impacted the region. Four experiments are run to aid in the analysis: one is forced by heat and momentum
fluxes, one by only momentum fluxes, one by only heat fluxes, and one with no surface forcing. An equation describing the
change in the upper ocean hurricane heat potential due to the storm is developed. Analysis of the model results show that
surface heat fluxes are primarily responsible for widespread reduction (0.5°–1.5°C) of sea surface temperature over the inner
West Florida Shelf 100–300 km away from the storm center. Momentum fluxes are responsible for stronger surface cooling (2°C)
near the center of the storm. The upper ocean heat loss near the storm center of more than 200 MJ/m2 is primarily due to the vertical flux of thermal energy between the surface layer and deep ocean. Heat loss to the atmosphere
during the storm’s passage is approximately 100–150 MJ/m2. The upper ocean cooling is enhanced where the preexisting mixed layer is shallow, e.g., within a cyclonic circulation feature,
although the heat flux to the atmosphere in these locations is markedly reduced. 相似文献
16.
Influence of high-resolution wind forcing on hydrodynamic modeling of the Gulf of Lions 总被引:2,自引:2,他引:0
Amandine Schaeffer Pierre Garreau Anne Molcard Philippe Fraunié Yann Seity 《Ocean Dynamics》2011,61(11):1823-1844
The impact of the choice of high-resolution atmospheric forcing on ocean summertime circulation in the Gulf of Lions (GoL;
Mediterranean Sea) is evaluated using three different datasets: AROME (2.5 km, 1 h), ALADIN (9.5 km, 3 h), and MM5 (9 km,
3 h). A short-term ocean simulation covering a 3-month summer period was performed on a 400-m configuration of the GoL. The
main regional features of both wind and oceanic dynamics were well-reproduced by all three atmospheric models. Yet, at smaller
scales and for specific hydrodynamic processes, some differences became apparent. Inertial oscillations and mesoscale variability
were accentuated when high-resolution forcing was used. Sensitivity tests suggest a predominant role for spatial rather than
temporal resolution of wind. The determinant influence of wind stress curl was evidenced, both in the representation of a
mesoscale eddy structure and in the generation of a specific upwelling cell in the north-western part of the gulf. 相似文献
17.
A global eddy-admitting ocean/sea-ice simulation driven over 1958–2004 by daily atmospheric forcing is used to evaluate spatial
patterns of sea level change between 1993 and 2001. In the present study, no data assimilation is performed. The model is
based on the Nucleus for European Models of the Ocean code at the 1/4° resolution, and the simulation was performed without
data assimilation by the DRAKKAR project. We show that this simulation correctly reproduces the observed regional sea level
trend patterns computed using satellite altimetry data over 1993–2001. Generally, we find that regional sea level change is
best simulated in the tropical band and northern oceans, whereas the Southern Ocean is poorly simulated. We examine the respective
contributions of steric and bottom pressure changes to the total regional sea level changes. For the steric component, we
analyze separately the contributions of temperature and salinity changes as well as upper and lower ocean contributions. Generally,
the model results show that most regional sea level changes arise from temperature changes in the upper 750 m of the ocean.
However, contributions of salinity changes and deep steric changes can be locally important. We also propose a map of ocean
bottom pressure changes. Finally, we assess the robustness of such a model by comparing this simulation with a second simulation
performed by MERCATOR-Ocean based on the same core model, but differing by its short length of integration (1992–2001) and
its surface forcing data set. The long simulation presents better performance over 1993–2001 than the short simulation, especially
in the Southern Ocean where a long adjustment time seems to be needed.
In memory of my little brother Jean-Eudes, whose thirst for science filled out the rich discussions we had about my investigations
and his job as user-service provider for MERCATOR-Ocean. 相似文献
18.
Debora Bellafiore Edoardo Bucchignani Silvio Gualdi Sandro Carniel Vladimir Djurdjević Georg Umgiesser 《Ocean Dynamics》2012,62(4):555-568
Modeling studies of future changes in coastal hydrodynamics, in terms of storm surges and wave climate, need appropriate wind
and atmospheric forcings, a necessary requirement for the realistic reproduction of the statistics and the resolution of small
scale features. This work compares meteorological results from different climate models in the Mediterranean area, with a
focus on the Adriatic Sea, in order to assess their capability to reproduce coastal meteorological features and their possibility
to be used as forcings for hydrodynamic simulations. Five meteorological datasets are considered. They are obtained from two
regional climate models, implemented with different spatial resolutions and setups and are downscaled from two different global
climate models. Wind and atmospheric pressure fields are compared with measurements at four stations along the Italian Adriatic
coast. The analysis is carried out both on simulations of the control period 1960–1990 and on the A1B Intergovernmental Panel
for Climate Change scenario projections (2070–2100), highlighting the ability of each model in reproducing the statistical
coastal meteorological behavior and possible changes. The importance of simulated global- and regional-scale meteorological
processes, in terms of correct spatial resolution of the phenomena, is also discussed. Within the Adriatic Sea, the meteorological
climate is influenced by the local orography that controls the strengthening of north-eastern katabatic winds like Bora. Results
show indeed that the increase in spatial resolution provides a more realistic wind forcing for the hydrodynamic simulations.
Moreover, the chosen setup and the global climate models that drive the regional downscalings appear to play an important
role in reproducing correct atmospheric pressure fields. The comparison between scenario and control simulations shows a small
increase in the mean atmospheric pressure values, while a decrease in mean wind speed and in extreme wind events is observed,
particularly for the datasets with higher spatial resolution. Finally, results suggest that an ensemble of downscaled climate
models is likely to provide the most suitable climatic forcings (wind and atmospheric pressure fields) for coastal hydrodynamic
modeling. 相似文献
19.
The aim of this work is to compare the relative impact of land and sea surface anomalies on Sahel rainfall and to describe the associated anomalies in the atmospheric general circulation. This sensitivity study was done with the Météo-France climate model: ARPEGE. The sensitivity to land surface conditions consists of changes in the management of water and heat exchanges by vegetation cover and bare soil. The sensitivity to ocean surfaces consists in forcing the lower boundary of the model with worldwide composite sea surface temperature (SST) anomalies obtained from the difference between 4 dry Sahel years and 4 wet Sahel years observed since 1970. For each case, the spatiotemporal variability of the simulated rainfall anomaly and changes in the modelled tropical easterly jet (TEJ) and African easterly jet (AEJ) are discussed. The global changes in land surface evaporation have caused a rainfall deficit over the Sahel and over the Guinea Coast. No significant changes in the simulated TEJ and an enhancement of the AEJ are found; at the surface, the energy budget and the hydrological cycle are substantially modified. On the other hand, SST anomalies induce a negative rainfall anomaly over the Sahel and a positive rainfall anomaly to the south of this area. The rainfall deficit due to those anomalies is consistent with previous diagnostic and sensitivity studies. The TEJ is weaker and the AEJ is stronger than in the reference. The composite impact of SST and land surfaces anomalies is also analyzed: the simulated rainfall anomaly is similar to the observed mean African drought patterns. This work suggests that large-scale variations of surface conditions may have a substantial influence on Sahel rainfall and shows the importance of land surface parameterization in climate change modelling. In addition, it points out the interest in accurately considering the land and sea surfaces conditions in sensitivity studies on Sahel rainfall. 相似文献
20.
This study examines the circulation and associated monthly-to-seasonal variability in the Caribbean Sea using a regional ocean
circulation model. The model domain covers the region between 99.0 and 54.0°W and between 8.0 and 30.3°N, with a horizontal
resolution of 1/6°. The ocean circulation model is driven by 6-hourly atmospheric reanalysis data from the National Center
for Environmental Prediction and boundary forcing extracted from 5-day global ocean reanalysis data produced by Smith et al.
(Mercator Newsletter 36:39–49, 2010), and integrated for 7 years. A comparison of model results with observations demonstrates that the regional ocean circulation
model has skill in simulating circulation and associated variability in the study region. Analysis of the model results, as
well as a companion model run that uses steady annual mean forcing, illustrates the role of Caribbean eddies for driving monthly-to-seasonal
circulation variability in the model. It is found that vertically integrated transport between Nicaragua and Jamaica is influenced
by the interaction between the density perturbations associated with Caribbean eddies and the Nicaraguan Ridge. The impact
of Caribbean eddies squeezing through the Yucatan Channel is also discussed. 相似文献