Regional Ocean-atmosphere Model Research Articles

Origins of mesoscale mixed-layer depth variability in the Southern Ocean

Abstract. Mixed-layer depth (MLD) exhibits significant variability, which is important for atmosphere–ocean exchanges of heat and atmospheric gases. The origins of the mesoscale MLD variability in the Southern Ocean are studied here in an idealised regional ocean–atmosphere model (ROAM). The main conclusion from the analysis of the upper-ocean buoyancy budget is that, while the atmospheric forcing and oceanic vertical mixing, on average, induce the mesoscale variability of MLD, the three-dimensional oceanic advection of buoyancy counteracts and partially balances these atmosphere-induced vertical processes. The relative importance of advection changes with both season and average MLD. From January to May, when the mixed layer is shallow, the atmospheric forcing and oceanic mixing are the most important processes, with the advection playing a secondary role. From June to December, when the mixed layer is deep, both atmospheric forcing and oceanic advection are equally important in driving the MLD variability. Importantly, buoyancy advection by mesoscale ocean current anomalies can lead to both local shoaling and deepening of the mixed layer. The role of the atmospheric forcing is then directly addressed by two sensitivity experiments in which the mesoscale variability is removed from the atmosphere–ocean heat and momentum fluxes. The findings confirm that mesoscale atmospheric forcing predominantly controls MLD variability in summer and that intrinsic oceanic variability and surface forcing are equally important in winter. As a result, MLD variance increases when mesoscale anomalies in atmospheric fluxes are removed in winter, and oceanic advection becomes a dominant player in the buoyancy budget. This study highlights the importance of oceanic advection and intrinsic ocean dynamics in driving mesoscale MLD variability and underscores the importance of MLD in modulating the effects of advection on upper-ocean dynamics.

Thermal air–sea coupling in hindcast simulations for the North Sea and Baltic Sea on the NW European shelf

This article compares interactively coupled atmosphere–ocean hindcast simulations with stand-alone runs of the atmosphere and ocean models using the recently developed regional ocean–atmosphere model NEMO-Nordic for the North Sea and Baltic Sea. In the interactively coupled run, the ocean and the atmosphere components were allowed to exchange mass, momentum and heat every 3 h. Our results show that interactive coupling significantly improves simulated winter sea surface temperatures (SSTs) in the Baltic Sea. The ocean and atmosphere stand-alone runs, respectively, resulted in too low sea surface and air temperatures over the Baltic Sea. These two runs suffer from too cold prescribed ERA40 SSTs, which lower air temperatures and weaken winds in the atmosphere only run. In the ocean-only run, the weaker winds additionally lower the vertical mixing thereby lowering the upward transport of warmer subpycnocline waters. By contrast, in the interactively coupled run, the ocean–atmosphere heat exchange evolved freely and demonstrated good skills in reproducing observed surface temperatures. Despite the strong impact on oceanic and atmospheric variables in the coupling area, no far reaching influence on atmospheric variables over land can be identified. In perturbation experiments, the different dynamics of the two coupling techniques is investigated in more detail by implementing strong positive winter temperature anomalies in the ocean model. Here, interactive coupling results in a substantially higher preservation of heat anomalies because the atmosphere also warmed which damped the ocean to atmosphere heat transfer. In the passively coupled set-up, this atmospheric feedback is missing, which resulted in an unrealistically high oceanic heat loss. The main added value of interactive air–sea coupling is twofold: (1) the elimination of any boundary condition at the air–sea interface and (2) the more realistic dynamical response to perturbations in the ocean–atmosphere heat balance, which will be essential in climate warming scenarios.

Causes and consequences of mid–21st-century rapid ice loss events simulated by the Rossby centre regional atmosphere-ocean model

Recent observations and modelling studies suggest that the Arctic climate is undergoing important transition. One manifestation of this change is seen in the rapid sea-ice cover decrease as experienced in 2007 and 2012. Although most numerical climate models cannot adequately reproduce the recent changes, some models produce similar Rapid Ice Loss Events (RILEs) during the mid–21st-century. This study presents an analysis of four specific RILEs clustered around 2040 in three transient climate projections performed with the coupled Rossby Centre regional Atmosphere-Ocean model (RCAO). The analysis shows that long-term thinning causes increased vulnerability of the Arctic Ocean sea-ice cover. In the Atlantic sector, pre-conditioning (thinning of sea ice) combined with anomalous atmospheric and oceanic heat transport causes large ice loss, while in the Pacific sector of the Arctic Ocean sea-ice albedo feedback appears important, particularly along the retreating sea-ice margin. Although maximum sea-ice loss occurs in the autumn, response in surface air temperature occurs in early winter, caused by strong increase in ocean-atmosphere surface energy fluxes, mainly the turbulent fluxes. Synchronicity of the events around 2040 in the projections is caused by a strong large-scale atmospheric circulation anomaly at the Atlantic lateral boundary of the regional model. The limited impact on land is caused by vertical propagation of the surface heat anomaly rather than horizontal, caused by the absence of low-level temperature inversion over the ocean.

Modelling the Caspian Sea and its catchment area using a coupled regional atmosphere-ocean model (RegCM4-ROMS): model design and preliminary results

Abstract. We describe the development of a coupled regional atmosphere-ocean model (RegCM4-ROMS) and its implementation over the Caspian Sea basin. The coupled model is run for the period 1999–2008 (after a spin up of 4 yr) and it is compared to corresponding stand alone model simulations and a simulation in which a distributed 1d lake model is run for the Caspian Sea. All model versions show a good performance in reproducing the climatology of the Caspian Sea basin, with relatively minor differences across them. The coupled ROMS produces realistic, although somewhat overestimated, Caspian Sea Surface Temperature (SST), with a considerable improvement compared to the use of the simpler coupled lake model. Simulated near surface salinity and sea currents are also realistic, although the upwelling over the eastern coastal regions is underestimated. The sea ice extent over the shallow northern shelf of the Caspian Sea and its seasonal evolution are well reproduced, however, a significant negative bias in sea-ice fraction exists due to the relatively poor representation of the bathymetry. ROMS also calculates the Caspian Sea Level (CSL), showing that for the present experiment excessive evaporation over the lake area leads to a drift in estimated CSL. Despite this problem, which requires further analysis due to many uncertainties in the estimation of CSL, overall the coupled RegCM4-ROMS system shows encouraging results in reproducing both the climatology of the region and the basic characteristics of the Caspian Sea.

A high-resolution ocean-atmosphere coupled downscaling of the present climate over California

A fully coupled regional ocean-atmosphere model system that consists of the regional spectral model and the regional ocean modeling system for atmosphere and ocean components, respectively, is applied to downscale the present climate (1985–1994) over California from a global simulation of the Community Climate System Model 3.0 (CCSM3). The horizontal resolution of the regional coupled modeling system is 10 km, while that of the CCSM3 is at a spectral truncation of T85 (approximately 1.4°). The effects of the coupling along the California coast in the boreal summer and winter are highlighted. Evaluation of the sea surface temperature (SST) and 2-m air temperature climatology shows that alleviation of the warm bias along the California coast in the global model output is clear in the regional coupled model run. The 10-m wind is also improved by reducing the northwesterly winds along the coast. The higher resolution coupling effect on the temperature and specific humidity is the largest near the surface, while the significant impact on the wind magnitude appears at a height of approximately 850-hPa heights. The frequency of the Catalina Eddy and its duration are increased by more than 60 % in the coupled downscaling, which is attributed to enhanced offshore sea-breeze. Our study indicates that coupling is vital to regional climate downscaling of mesoscale phenomena over coastal areas.

Influence of air–sea interaction on the simulation of East Asian summer monsoon: A case study

A coupled, regional ocean–atmosphere model was developed to study the role of air–sea interaction in East Asian summer monsoon. The coupled model used a coupler that was based on the model coupling toolkit (MCT) to couple the regional climate model (RegCM3) as atmospheric component and the Princeton Ocean Model (POM) as oceanic component. The simulation was carried out for the East Asian summer case in 1998, and the results were compared with observations and with simulation results of uncoupled RegCM3 forced by sea surface temperature (SST) derived from the coupled regional model. It was found that the coupled model was able to simulate the temporal and spatial distributions of SST in the East Asian summer monsoon region. Compared with the uncoupled model results, the coupled model showed the essence of air–sea interaction in the East Asian summer more realistically, leading to higher simulation skills on the simulation of monsoon circulation and rainfall. Local and remote air–sea interaction had great influences on the simulation of East Asian summer monsoon. Due to the local negative feedback of atmosphere on ocean, the rainfall over the adjacent ocean surrounding East Asia simulated by the coupled model was less than that in the uncoupled atmospheric model. However, the rainfall over the East Asian continent in the coupled run was less than that in the forced run due to remote air–sea interaction through a positive feedback process. The reduction of rainfall over the oceanic region due to air–sea coupling led to an increase in local specific humidity, as less moisture was used for local precipitation. The increased moisture due to less rainfall over the oceanic region was transport to East Asia by the summer monsoon flow, which led to more East Asian monsoon rainfall and enhanced condensation heating. Increased positive vorticity associated with enhanced condensation heating strengthened the monsoon flow through vorticity balance, which further strengthened the surface evaporation and local moisture in the adjacent ocean surrounding East Asia, leading to an enhanced moisture transport.

Sensitivity of natural vegetation to climate change in the Euro-Mediterranean area

This study explores the sensitivity to future climate change of natural vegetation patterns in the Euro-Mediterranean area under the IPCC-A1B emission scenario, using a dynamic vegetation model forced by fully coupled high resolution regional ocean-atmosphere model simula- tions. Our results indicate that a future warmer climate could not only affect the regional terrestrial carbon cycle, but also significantly impact vegetation dynamics. Specifically, by 2021-2050, temper- ate deciduous vegetation is projected to replace boreal and grass vegetation in parts of northeastern Europe, while in North Africa, simulations show a progressive desertification. The changes in domi- nant vegetation are mainly related to the increased drought stress on vegetation and the enhanced fire frequency. As for the carbon cycle, large increases in net primary production (NPP) are found in northern Europe resulting from higher temperature and precipitation, as well as higher atmospheric CO2 levels. In contrast, smaller NPP increases are found in the Mediterranean region, where reduced precipitation and increased temperature leads to an increase in drought years and, hence, water- stress for vegetation.

Intraseasonal variability in the far-east pacific: investigation of the role of air–sea coupling in a regional coupled model

Intraseasonal variability in the eastern Pacific warm pool in summer is studied, using a regional ocean–atmosphere model, a linear baroclinic model (LBM), and satellite observations. The atmospheric component of the model is forced by lateral boundary conditions from reanalysis data. The aim is to quantify the importance to atmospheric deep convection of local air–sea coupling. In particular, the effect of sea surface temperature (SST) anomalies on surface heat fluxes is examined. Intraseasonal (20–90 day) east Pacific warm-pool zonal wind and outgoing longwave radiation (OLR) variability in the regional coupled model are correlated at 0.8 and 0.6 with observations, respectively, significant at the 99% confidence level. The strength of the intraseasonal variability in the coupled model, as measured by the variance of outgoing longwave radiation, is close in magnitude to that observed, but with a maximum located about 10° further west. East Pacific warm pool intraseasonal convection and winds agree in phase with those from observations, suggesting that remote forcing at the boundaries associated with the Madden–Julian oscillation determines the phase of intraseasonal convection in the east Pacific warm pool. When the ocean model component is replaced by weekly reanalysis SST in an atmosphere-only experiment, there is a slight improvement in the location of the highest OLR variance. Further sensitivity experiments with the regional atmosphere-only model in which intraseasonal SST variability is removed indicate that convective variability has only a weak dependence on the SST variability, but a stronger dependence on the climatological mean SST distribution. A scaling analysis confirms that wind speed anomalies give a much larger contribution to the intraseasonal evaporation signal than SST anomalies, in both model and observations. A LBM is used to show that local feedbacks would serve to amplify intraseasonal convection and the large-scale circulation. Further, Hovmöller diagrams reveal that whereas a significant dynamic intraseasonal signal enters the model domain from the west, the strong deep convection mostly arises within the domain. Taken together, the regional and linear model results suggest that in this region remote forcing and local convection–circulation feedbacks are both important to the intraseasonal variability, but ocean–atmosphere coupling has only a small effect. Possible mechanisms of remote forcing are discussed.

Influences of Atlantic Climate Change on the Tropical Pacific via the Central American Isthmus*

Abstract Recent global coupled model experiments suggest that the atmospheric bridge across Central America is a key conduit for Atlantic climate change to affect the tropical Pacific. A high-resolution regional ocean–atmosphere model (ROAM) of the eastern tropical Pacific is used to investigate key processes of this conduit by examining the response to a sea surface temperature (SST) cooling over the North Atlantic. The Atlantic cooling increases sea level pressure, driving northeasterly wind anomalies across the Isthmus of Panama year-round. While the atmospheric response is most pronounced during boreal summer/fall when the tropical North Atlantic is warm and conducive to deep convection, the Pacific SST response is strongest in winter/spring when the climatological northeast trade winds prevail across the isthmus. During winter, the northeasterly cross-isthmus winds intensify in response to the Atlantic cooling, reducing the SST in the Gulf of Panama by cold and dry advection from the Atlantic and by enhancing surface turbulent heat flux and mixing. This Gulf of Panama cooling reaches the equator and is amplified by the Bjerknes feedback during boreal spring. The equatorial anomalies of SST and zonal winds dissipate quickly in early summer as the seasonal development of the cold tongue increases the stratification of the atmospheric boundary layer and shields the surface from the Atlantic influence that propagates into the Pacific as tropospheric Rossby waves. The climatological winds over the far eastern Pacific warm pool turn southwesterly in summer/fall, superimposed on which the anomalous northesterlies induce a weak SST warming there. The ROAM results are compared with global model water-hosing runs to shed light on intermodel consistency and differences in response to the shutdown of the Atlantic meridional overturning circulation. Implications for interpreting paleoclimate changes such as Heinrich events are discussed. The results presented here also aid in understanding phenomena in the present climate such as the Central American midsummer drought and Atlantic multidecadal oscillation.

A Regional Ocean–Atmosphere Model for Eastern Pacific Climate: Toward Reducing Tropical Biases*

AbstractThe tropical Pacific Ocean is a climatically important region, home to El Niño and the Southern Oscillation. The simulation of its climate remains a challenge for global coupled ocean–atmosphere models, which suffer large biases especially in reproducing the observed meridional asymmetry across the equator in sea surface temperature (SST) and rainfall. A basin ocean general circulation model is coupled with a full-physics regional atmospheric model to study eastern Pacific climate processes. The regional ocean–atmosphere model (ROAM) reproduces salient features of eastern Pacific climate, including a northward-displaced intertropical convergence zone (ITCZ) collocated with a zonal band of high SST, a low-cloud deck in the southeastern tropical Pacific, the equatorial cold tongue, and its annual cycle. The simulated low-cloud deck experiences significant seasonal variations in vertical structure and cloudiness; cloud becomes decoupled and separated from the surface mixed layer by a stable layer in March when the ocean warms up, leading to a reduction in cloudiness. The interaction of low cloud and SST is an important internal feedback for the climatic asymmetry between the Northern and Southern Hemispheres. In an experiment where the cloud radiative effect is turned off, this climatic asymmetry weakens substantially, with the ITCZ migrating back and forth across the equator following the sun. In another experiment where tropical North Atlantic SST is lowered by 2°C—say, in response to a slow-down of the Atlantic thermohaline circulation as during the Younger Dryas—the equatorial Pacific SST decreases by up to 3°C in January–April but changes much less in other seasons, resulting in a weakened equatorial annual cycle. The relatively high resolution (0.5°) of the ROAM enables it to capture mesoscale features, such as tropical instability waves, Central American gap winds, and a thermocline dome off Costa Rica. The implications for tropical biases and paleoclimate research are discussed.

Hurricane-induced destratification and restratification in a partially-mixed estuary

Hurricane Isabel made landfall at the Outer Banks of North Carolina and moved past Chesapeake Bay on 18 –19 September 2003. The baroclinic response of this partially-mixed estuary to the passage of Isabel is investigated using the output from a regional atmosphere-ocean model. The hurricane-forced winds caused gradual deepening of the surface mixed layer, followed by rapid destratification in the water-column. The mixed-layer deepening appears to be driven by velocity shear and can be interpreted by a gradient Richardson number. Although strong winds caused complete mixing locally, a large longitudinal salinity gradient of about 10 4 psu m 1 persisted between the estuary’s head and mouth. After passage of the storm, the horizontal baroclinic pressure gradient drove restratification and a two-layer circulation in the estuary. The averaged buoyancy frequency increased linearly with time during an initial stage, and reached about 0.03 s 1 one day after the destratification. The model results are in good agreement with the theoretical prediction based on gravitational adjustment. Subsequently, turbulent diffusion works against the longitudinal advection to produce quasi-steady salinity distribution.

Carbon cycling and climate change: Predictions for a High Arctic marine ecosystem (Young Sound, NE Greenland)

This chapter reviews current predictions of future changes in the High Arctic marine ecosystem Young Sound, NE Greenland. A high-resolution regional atmosphere-ocean model predicts an increase in atmospheric temperature of 6–8°C and in precipitation of 20–30% by the end of this century (2071–2100), leading to increased freshwater runoff, thinning of sea ice, and an increase in open-water period from 2.5 months to 4.7–5.3 months. Evaluation of the consequences of enhanced freshwater runoff to the fjord revealed that the mixed layer thickness of the water column will change only marginally, whereas the transport of saltwater from the Greenland Sea to Young Sound below the halocline is predicted to increase considerably due to stimulated estuarine circulation.
 The thinning of sea ice and the increase in the open-water period is expected to enhance primary productivity in the area due to a c. 50% increase in light availability. The phytoplankton bloom will continue to occur in a sub-surface layer, but as the exchange between the fjord and the Greenland Sea increases, production will benefit from increased import of nutrients. We estimate that primary productivity in the area will have tripled by the end of the century compared with present-day levels. The longer ice-free period will induce a shift in the pelagic food web structure, from a copepod-dominated grazer community to a situation with growing influence of protozooplankton.
 The increased pelagic production will enhance sedimentation and thus intensify bacterial mineralization at the sea floor along with carbon burial. This will reduce oxygen availability in the sediment and the relative importance of anaerobic degradation will increase. The rise in sedimentation will also improve food availability for the benthic animals and thus stimulate growth and production until a certain threshold, where sulphide released from anaerobic sulphate reduction may become inhibitory. Finally, an increase in the ice-free period will prolong the period in which birds and marine mammals – e.g. walruses – have access to the food-rich coastal area, and thus improve their foraging conditions. All in all, conditions in Young Sound in 2071–2100 are predicted to resemble present-day conditions c. 450 km further south, e.g. Scoresby Sound.

Simulated Distributions of Baltic Sea-ice in Warming Climate and Consequences for the Winter Habitat of the Baltic Ringed Seal

Sea-ice in the Baltic Sea in present and future climates is investigated. The Rossby Centre Regional Atmosphere-Ocean model was used to perform a set of 30-year-long time slice experiments. For each of the two driving global models HadAM3H and ECHAM4/OPYC3, one control run (1961-1990) and two scenario runs (2071-2100) based upon the SRES A2 and B2 emission scenarios were conducted. The future sea-ice volume in the Baltic Sea is reduced by 83% on average. The Bothnian Sea, large areas of the Gulf of Finland and Gulf of Riga, and the outer parts of the southwestern archipelago of Finland will become ice-free in the mean. The presented scenarios are used to study the impact of climate change on the Baltic ringed seal (Phoca hispida botnica). Climate change seems to be a major threat to all southern populations. The only fairly good winter sea-ice habitat is found to be confined to the Bay of Bothnia.

Physical Conditions, Carbon Transport, and Climate Change Impacts in a Northeast Greenland Fjord

This paper presents data on physical conditions and carbon transport in a typical northeast Greenland fjord along with predictions of expected changes in the area due to climate change. The fjord has an average depth of 100 m; the maximum depth is 360 m, and a sill at a depth of 45 m is found at its entrance. Sea ice covers the fjord from early October to late July. The freshwater input to the fjord, occurring from June to September, is 1063 × 106 m3 from the catchment area (3109 km2) and 440 × 106 m3 from melting of sea ice. During the ice-free period this buoyancy input and mixing by wind and tides results in an estuarine circulation in which lighter low-salinity water is moved seaward above denser water from the Greenland Sea. The tidal amplitude is 0.8 to 1.5 m, and the transport of tides from the outer parts of the fjord to the inner parts is delayed less than 15 min due to low friction in the fjord system. During the ice-free period, a net carbon input of 15–50 t C d−1 occurs in the outer region of the fjord due to transport from land and the adjacent Greenland Sea. A regional atmosphere-ocean model predicts a temperature increase of 6–8°C at the end of this century (2071–2100) that will lead to increase in freshwater runoff, thinning of the sea ice, and an increase in ice-free conditions from 2.5 mo to 4.7–5.3 mo in Young Sound. The increased freshwater input will greatly enhance the estuarine circulation and nutrient input to the fjord and is expected to increase biological productivity.