Coupled Ocean Atmosphere Model Research Articles

Regional downscaled future change of clear‐air turbulence over East Asia under RCP8.5 scenario within the CORDEX‐EA‐II project

AbstractDriven by four global coupled atmosphere–ocean models (GCMs) from the Coupled Model Intercomparison Project Phase 5 (CMIP5) ensemble (CNRM‐CM5, GFDL‐ESM2M, EC‐EARTH, and MPI‐ESM‐LR), future changes (2040–2059) of clear‐air turbulence (CAT) in winter over East Asia under Representative Concentration Pathway 8.5 (RCP8.5) scenario are projected with the regional climate model (RCM) RegCM4 and 18 CAT diagnostics based on the second phase of Coordinated Regional Downscaling Experiment East Asia (CORDEX‐EA‐II) framework. Evaluation of RegCM4 with ERA‐Interim shows that RegCM4 and diagnostics can well reproduce the spatial distribution of CAT potential. The probability of CAT potential is expected to increase over most East Asia between 35°N and 50°N, Southeast China, western Pacific, and North India in the future. The CAT potential that significantly affects aviation safety shows a remarkable increase over North and Northeast China, part of East China, and Tibet‐Plateau. In the whole domain of East Asia, the multi‐diagnostics ensemble‐average frequency increase is 6.9% for light CAT (with an intra‐ensemble range of 4.1–18%), 9.1% (5.0–24%) for light‐to‐moderate CAT, 12% (5.9–26%) for moderate CAT, 13% (7.2–29%) for moderate‐to‐severe CAT, and 15% (8.5–33%) for severe CAT. Stronger CAT is more likely to happen, which would be a constant threat, although greater scatter exists among these diagnostics, which implies the uncertainties are also significant in the projection.

The Role of Air–Sea Interactions in Atmospheric Rivers: Case Studies Using the SKRIPS Regional Coupled Model

AbstractAtmospheric rivers (ARs) play a key role in California's water supply and are responsible for most of the extreme precipitation and major flooding along the west coast of North America. Given the high societal impact, it is critical to improve our understanding and prediction of ARs. This study uses a regional coupled ocean–atmosphere modeling system to make hindcasts of ARs up to 14 days. Two groups of coupled runs are highlighted in the comparison: (1) ARs occurring during times with strong sea surface temperature (SST) cooling and (2) ARs occurring during times with weak SST cooling. During the events with strong SST cooling, the coupled model simulates strong upward air–sea heat fluxes associated with ARs; on the other hand, when the SST cooling is weak, the coupled model simulates downward air–sea heat fluxes in the AR region. Validation data shows that the coupled model skillfully reproduces the evolving SST, as well as the surface turbulent heat transfers between the ocean and atmosphere. The roles of air–sea interactions in AR events are investigated by comparing coupled model hindcasts to hindcasts made using persistent SST. To evaluate the influence of the ocean on ARs we analyze two representative variables of AR intensity, the vertically integrated water vapor (IWV) and integrated vapor transport (IVT). During strong SST cooling AR events the simulated IWV is improved by about 12% in the coupled run at lead times greater than one week. For IVT, which is about twice more variable, the improvement in the coupled run is about 5%.

The Zealandia Switch: Ice age climate shifts viewed from Southern Hemisphere moraines

Two fundamental questions about the ice-age climate system await satisfactory resolution. First, if summer solar radiation intensity truly controls the orbital signature of the last glacial cycle, then why were major climatic shifts, including the last termination, globally synchronous? Second, what caused the millennial-scale climate oscillations superimposed on this cycle? We address these questions from a Southern Hemisphere perspective focused on mid-latitude mountain ice fields. We put particular emphasis on the last glacial termination, which involved both orbital-scale and millennial-scale climate elements and has generally well-resolved chronological control.Sustained retreat of mountain glaciers, documented by detailed mapping and chronology of glacial landforms in the Southern Alps and southern Andes, marked the termination of the last ice age, beginning ∼18 kyrs ago and involved a change from glacial to near-interglacial atmospheric temperature within a millennium or two. A rapid poleward shift of the Subtropical Front, delineating the northern margin of the Southern Ocean, ∼18 kyrs ago implies a concurrent poleward shift of the austral westerlies and leads us to hypothesize a southern origin for the dominant phase of the last glacial termination. Together with interhemispheric paleoclimate records and with results of coupled ocean-atmosphere climate modeling, these findings suggest a big, fast, and global end to the last ice age in which a southern-sourced warming episode linked the hemispheres. We posit that a shift in the Southern Ocean circulation and austral westerly wind system, tied to southern orbital forcing, caused this global warming episode by affecting the tropical heat engine and hence global climate.Central to this hypothesis, dubbed the ‘Zealandia Switch’, is the location of the Australia and Zealandia continents relative to Southern Hemisphere oceanic and atmospheric circulation. Coupled ocean-atmosphere climate modeling shows that the locus of the austral westerlies, whether in a more equatorward position representing a glacial-mode climate or in a poleward-shifted position marking interglacial-mode climate, has profound effects on oceanic and associated atmospheric linkages between the tropical Pacific and the Southern Ocean. Shifts in the austral westerlies have global climatic consequences, especially through resulting changes in the greenhouse gas content of the atmosphere and altered heat flux from the tropical Pacific into the Northern and Southern Hemispheres. We suggest that the last glacial termination was a global warming episode that led to extreme seasonality in northern latitudes by stimulating a flush of meltwater and icebergs into the North Atlantic from adjoining ice sheets. This fresh-water influx resulted in widespread North Atlantic sea ice that caused very cold boreal winters, thus amplifying the annual southward shift of the Intertropical Convergence Zone and the monsoonal rain belts. We further suggest that muted manifestations of the Zealandia Switch mechanism were responsible for smaller, recurring millennial-scale climate oscillations within the last glacial cycle.

Data for Double ITCZ in Coupled Ocean-Atmosphere Models in CMIP3, CMIP5 and CMIP6

data for Double ITCZ in Coupled Ocean-Atmosphere Models in CMIP3, CMIP5 and CMIP6

Two Tropical Routes for the Remote Influence of the Northern Tropical Atlantic on the Indo–Western Pacific Summer Climate

AbstractThis study investigates the influence of sea surface temperature (SST) in the northern tropical Atlantic (NTA) on the Indo–western Pacific summer climate by analyzing record-high NTA SSTs in summer 2010. In that time, a decaying El Niño and developing La Niña were accompanied by widespread anomalous climate conditions in the Indo–western Pacific. These conditions are typical of summers that follow El Niño events and are often explained as being due to the influence of Indian Ocean warming induced by El Niños. Meanwhile, the record-high NTA SSTs that resulted from the influence of El Niño, the negative phase of the North Atlantic Oscillation and the interdecadal-and-longer NTA SST variability are one of the possible causes of anomalous conditions in the Indo–western Pacific. The results of sensitivity experiments using a coupled atmosphere–ocean model clearly indicate that the high NTA SSTs had a considerable influence on the summer weather in the Indo–western Pacific via two tropical routes: an eastbound route that involved a reinforcement of the atmospheric equatorial Kelvin wave and a westbound route that involved altering the Walker circulation over the Atlantic–Pacific region. The altered Walker circulation facilitated the transition to La Niña, amplifying the impact on the western North Pacific monsoon. Further evaluation reveals that both the interannual and interdecadal-and-longer variability of the NTA SST contributed to the anomalous Indo–western Pacific summer. The results highlight the interannual to multidecadal predictability of the Indo–western Pacific summer climate that originates in the NTA.

Estimating the Thermodynamic and Dynamic Contributions to Hydroclimatic Change over Peninsular Florida

AbstractIn this study we examine the thermodynamically and dynamically forced hydroclimatic changes in the four representative seasons over Peninsular Florida (PF) from an unprecedented pair of high-resolution regional coupled ocean–atmosphere model simulations conducted at 10-km grid spacing for both the atmospheric and the oceanic components forced by one of the global climate models that participated in CMIP5. The model simulation verifies reasonably well with the observations and captures the distinct seasonal cycle of the region. The projected change in the freshwater flux in the mid-twenty-first century (2041–60) relative to the late twentieth century (1986–2005) shows a precipitation deficit in the summer over PF, which is statistically significant. This projected change in freshwater flux over PF is enabled by the strengthening of the anticyclonic North Atlantic subtropical high circulation with corresponding changes in divergence of moisture, advection of moisture from changes in the winds and in the change in humidity gradient, and from the change in moisture flux convergence by the transient eddies. These changes suggest that a future warm climate could witness a drier summer over PF at the expense of a wetter West Florida Shelf. The analysis conducted in this study reveals that the changes in atmospheric circulation have a significant impact on the hydroclimate, far more than that implied by the Clausius–Clapeyron equation from changes in temperature.

Quantification of ocean-atmosphere energy exchange during super cyclone Amphan in the Bay of Bengal using Indian Ocean moored observatories

Based on the in-situ subsurface thermal and salinity measurements from the Ocean Moored buoy network for Northern Indian Ocean (OMNI) during the Super Cyclone Amphan in the Bay of Bengal during 14–18 May 2020, we have identified that the ocean-atmosphere interaction is limited to 80 m, the depth of the pycnocline. Using the OAEE-TS algorithm and dynamic changes in the sea subsurface temperature, salinity and precipitation as inputs, we have calculated the water vapor generation rates for a range of wind speeds. Based on the salinity changes, the water vapor generation rates for cyclone period-averaged wind speeds of 9, 16 and 25 m/s were 0.86, 4.33 and 11.24 kg/m2/h, respectively. For the same wind speeds, based on the Ocean Heat Content (OHC) changes, ∼76, 86 and 95 % of the OHC changes were in the form of latent heat. The identified evaporation rate parameters along with the previous study results are presented as model which could be used as inputs for ocean-atmosphere coupled models and tropical cyclone intensification studies in the Bay of Bengal.

A simplified atmospheric boundary layer model for an improved representation of air–sea interactions in eddying oceanic models: implementation and first evaluation in NEMO (4.0)

Abstract. A simplified model of the atmospheric boundary layer (ABL) of intermediate complexity between a bulk parameterization and a three-dimensional atmospheric model is developed and integrated to the Nucleus for European Modelling of the Ocean (NEMO) general circulation model. An objective in the derivation of such a simplified model, called ABL1d, is to reach an apt representation in ocean-only numerical simulations of some of the key processes associated with air–sea interactions at the characteristic scales of the oceanic mesoscale. In this paper we describe the formulation of the ABL1d model and the strategy to constrain this model with large-scale atmospheric data available from reanalysis or real-time forecasts. A particular emphasis is on the appropriate choice and calibration of a turbulent closure scheme for the atmospheric boundary layer. This is a key ingredient to properly represent the air–sea interaction processes of interest. We also provide a detailed description of the NEMO-ABL1d coupling infrastructure and its computational efficiency. The resulting simplified model is then tested for several boundary-layer regimes relevant to either ocean–atmosphere or sea-ice–atmosphere coupling. The coupled system is also tested with a realistic 0.25∘ resolution global configuration. The numerical results are evaluated using standard metrics from the literature to quantify the wind–sea-surface-temperature (a.k.a. thermal feedback effect), wind–current (a.k.a. current feedback effect), and ABL–sea-ice couplings. With respect to these metrics, our results show very good agreement with observations and fully coupled ocean–atmosphere models for a computational overhead of about 9 % in terms of elapsed time compared to standard uncoupled simulations. This moderate overhead, largely due to I/O operations, leaves room for further improvement to relax the assumption of horizontal homogeneity behind ABL1d and thus to further improve the realism of the coupling while keeping the flexibility of ocean-only modeling.

The inter-annual variations of the significant wave height in the Western North Pacific and South China Sea region

The spatio-temporal variations of the significant wave height (SWH) in the Western North Pacific and South China Sea (WNP-SCS) region, as well as their driving mechanisms, are investigated based on the long-term (1981–2014) simulation by a coupled ocean–atmosphere model and a WAVEWATCH III model. The Empirical Orthogonal Function modes of SWH anomalies show different patterns in the cold and warm seasons. In winter, the first mode (explaining 40.63% of the variance) shows a monopole pattern with large loadings lying in SCS and the WNP to the east of the Philippine Islands, which is primarily associated with the El Nino-Southern Oscillation (ENSO) on inter-annual time scales. The second mode (explaining 19.62% of the variance) shows a dipole pattern with negative loadings in the northeastern SCS and positive loadings to the east of Japan, which is prominently connected with the intensity variation and longitudinal shift of Aleutian Low on decadal time scales. In summer, the first leading mode (explaining 73.47% of the variance) presents large loadings located mainly in the WNP region between 10° N and 30° N and secondarily in the central SCS, which is associated with the ENSO-affected tropical cyclone activities and South China Sea summer monsoon, respectively.

Future Changes of a Slow-Moving Intense Typhoon with Global Warming: A Case Study Using a Regional 1-km-mesh Atmosphere–Ocean Coupled Model

Future Changes of a Slow-Moving Intense Typhoon with Global Warming: A Case Study Using a Regional 1-km-mesh Atmosphere–Ocean Coupled Model

Projection of Future Enhancement of Heavy Rainfalls Associated with Typhoon Hagibis (2019) Using a Regional 1-km-mesh Atmosphere-Ocean Coupled Model

Projection of Future Enhancement of Heavy Rainfalls Associated with Typhoon Hagibis (2019) Using a Regional 1-km-mesh Atmosphere-Ocean Coupled Model

Sensitivity of the Southern Hemisphere circumpolar jet response to Antarctic ozone depletion: prescribed versus interactive chemistry

Abstract. Southern Hemisphere lower-stratospheric ozone depletion has been shown to lead to a poleward shift of the tropospheric jet stream during austral summer, influencing surface atmosphere and ocean conditions, such as surface temperatures and sea ice extent. The characteristics of stratospheric and tropospheric responses to ozone depletion, however, differ among climate models depending on the representation of ozone in the models. The most appropriate way to represent ozone in a model is to calculate it interactively. However, due to computational costs, in particular for long-term coupled ocean–atmosphere model integrations, the more common way is to prescribe ozone from observations or calculated model fields. Here, we investigate the difference between an interactive and a specified chemistry version of the same atmospheric model in a fully coupled setup using a nine-member chemistry–climate model ensemble. In the specified chemistry version of the model the ozone fields are prescribed using the output from the interactive chemistry model version. We use daily resolved ozone fields in the specified chemistry simulations to achieve a very good comparability between the ozone forcing with and without interactive chemistry. We find that although the shortwave heating rate trend in response to ozone depletion is the same in the different chemistry settings, the interactive chemistry ensemble shows a stronger trend in polar cap stratospheric temperatures (by about 0.7 K decade−1) and circumpolar stratospheric zonal mean zonal winds (by about 1.6 m s−1 decade−1 as compared to the specified chemistry ensemble. This difference between interactive and specified chemistry in the stratospheric response to ozone depletion also affects the tropospheric response. However, an impact on the poleward shift of the tropospheric jet stream is not detected. We attribute part of the differences found in the experiments to the missing representation of feedbacks between chemistry and dynamics in the specified chemistry ensemble, which affect the dynamical heating rates, and part of it to the lack of spatial asymmetries in the prescribed ozone fields. This effect is investigated using a sensitivity ensemble that was forced by a three-dimensional instead of a two-dimensional ozone field. This study emphasizes the value of interactive chemistry for the representation of the Southern Hemisphere stratospheric-jet response to ozone depletion and infers that for periods with strong ozone variability (trends) the details of the ozone forcing could also have an influence on the representation of southern-hemispheric climate variability.

Dynamical Structures of Cross-Domain Forecast Error Covariance of a Simulated Tropical Cyclone in a Convection-Permitting Coupled Atmosphere–Ocean Model

AbstractUnderstanding the dynamics of the flow-dependent forecast error covariance across the air–sea interface is beneficial toward revealing the potential influences of strongly coupled data assimilation on tropical cyclone (TC) initialization in coupled models, and the fundamental dynamics associated with TC air–sea interactions. A 200-member ensemble of convection-permitting forecasts from a coupled atmosphere–ocean regional model is used to investigate the forecast error covariance across the oceanic and atmospheric domains during the rapid intensification of Hurricane Florence (2018). Forecast uncertainties in both atmospheric and oceanic domains, from an Eulerian perspective, increase with forecast lead time, mainly from TC displacement errors. In a storm-relative framework, the ensemble forecast uncertainties in both domains are predominantly caused by differences in the simulated storm intensity and structure. The largest ensemble spread in the atmospheric pressure, temperature, and wind fields can be found within the TC inner-core region. Alternatively, the largest ensemble spread in the upper-ocean currents and temperature fields are located along the cold wake behind the storm. Cross-domain ensemble correlations between simulated atmospheric (oceanic) observations and oceanic (atmospheric) state variables in the storm-relative coordinates are highly anisotropic, variable dependent, and ultimately driven by the dynamics of TC air–sea interactions. Meaningful and dynamically consistent cross-domain ensemble correlations suggest that it is possible to use atmospheric and oceanic observations to simultaneously update state variables associated with the coupled ocean–atmosphere prediction of TCs using strongly coupled data assimilation. Sensitivity experiments demonstrate that at least 60–80 ensemble members are required to represent physically consistent cross-domain correlations and minimize sampling errors.

Skill Assessment of Seasonal-to-Interannual Prediction of Sea Level Anomaly in the North Pacific Based on the SINTEX-F Climate Model

Extreme sea level rise seriously impacts habitation and is indicative of changes in primary production in the North Pacific. Because of its rising trend associated with global warming, skillful seasonal-to-interannual predictions have become increasingly valuable to guide the introduction of suitable adaptation measures that help us reduce the risks of socio-economic losses. Here, we have used a dynamical coupled ocean–atmosphere model called “SINTEX-F” to revisit the potential predictability of sea level anomalies at a lead of up to about two years. Skillful prediction is found mainly in the tropical Pacific as shown in previous work. Here, we found a new skillful prediction region in the North Pacific (30o-40oN, 180oW-150oW) at about two years lead time. We also analyzed the co-variability among ensemble members and found the long-lasting ENSO/ENSO-Modoki in the tropical Pacific seems to contribute to the predictability source. The result may be useful to develop systematic and synergistic attempts to predict marine ecosystem responses to regional and global climate variations.

Impact of Annual Cycle on ENSO Variability and Predictability

AbstractLow-order linear inverse models (LIMs) have been shown to be competitive with comprehensive coupled atmosphere–ocean models at reproducing many aspects of tropical oceanic variability and predictability. This paper presents an extended cyclostationary linear inverse model (CS-LIM) that includes the annual cycles of the background state and stochastic forcing of tropical sea surface temperature (SST) and sea surface height (SSH) anomalies. Compared to a traditional stationary LIM that ignores such annual cycles, the CS-LIM is better at representing the seasonal modulation of ENSO-related SST anomalies and their phase locking to the annual cycle. Its deterministic as well as probabilistic hindcast skill is comparable to the skill of the North American Multimodel Ensemble (NMME) of comprehensive global coupled models. The explicit inclusion of annual-cycle effects in the CS-LIM improves the forecast skill of both SST and SSH anomalies through SST–SSH coupling. The impact on the SSH skill is particularly marked at longer forecast lead times over the western Pacific and in the vicinity of the Pacific North Equatorial Countercurrent (NECC), consistent with westward propagating oceanic Rossby waves that reflect off the western boundaries as eastward propagating Kelvin waves and influence El Niño development in the region. The higher CS-LIM skill is thus associated with the improved representation of both ENSO phase-locking and Pacific NECC variations. These improvements result from explicitly accounting for not only the annual cycle of the background state, but also that of the stochastic forcing.

Does regional air–sea coupling improve the simulation of the summer monsoon over the western North Pacific in the WRF4 model?

A new regional coupled ocean–atmosphere model, WRF4-LICOM, was used to investigate the impacts of regional air–sea coupling on the simulation of the western North Pacific summer monsoon (WNPSM), wi...

Improving the ocean and atmosphere in a coupled ocean–atmosphere model by assimilating satellite sea‐surface temperature and subsurface profile data

AbstractAn ensemble‐based data assimilation framework for a coupled ocean–atmosphere model is applied to investigate the influence of assimilating different types of ocean observations on the ocean and atmosphere simulation. The data assimilation is performed with the parallel data assimilation framework (PDAF) for the climate model AWI‐CM. Observations of the ocean, namely satellite sea‐surface temperature (SST) and temperature and salinity profiles, are assimilated into the ocean component. The atmospheric state is only influenced by the model dynamics. Different assimilation scenarios were carried out with different combinations of observations to investigate to what extent the assimilation into the coupled model leads to a better estimation of the state of the ocean as well as the atmosphere. The influence of the data assimilation is assessed by comparing the ocean prediction with dependent and independent ocean observations. For the atmosphere, the assimilation result is compared with the ERA‐Interim atmospheric reanalysis data. The ocean temperature and salinity are improved by all the assimilation scenarios in the coupled system. The assimilation leads to a response of the atmosphere throughout the troposphere and impacts the global atmospheric circulation. Globally the temperature and wind speed are improved in the atmosphere on average.

El Ni\xf1o/Southern Oscillation response to low-latitude volcanic eruptions depends on ocean pre-conditions and eruption timing

Proxy-based reconstructions of the past suggest that the Pacific ocean has often shown El Niño-like warming after low-latitude volcanic eruptions, while climate model simulations have suggested diverse responses. Here we present simulations from a coupled ocean–atmosphere model that illuminate the roles of ocean preconditioning, eruption magnitude and timing, and air–sea feedbacks in the El Niño/Southern Oscillation (ENSO) response to these eruptions. A deterministic component of the response, which dominates for boreal summer eruptions, leads to cooler tropical Pacific sea surface temperatures in the eruption year and El Niño-like warming the following year. A stochastic component is also important, especially for boreal winter eruptions. The simulated ENSO response depends nonlinearly on the eruption magnitude and the tropical Pacific conditions before the eruption. We conclude that adequate sampling is critical to accurately assess the ENSO responses in both models and observations.

Stability Analysis of Interface Conditions for Ocean–Atmosphere Coupling

In this paper we analyze the stability of different coupling strategies for multidomain PDEs that arise in general circulation models used in climate simulations. We focus on fully coupled ocean–atmosphere models that are needed to represent and understand the complicated interactions of these two systems, becoming increasingly important in climate change assessment in recent years. Numerical stability issues typically arise because of different time-stepping strategies applied to the coupled PDE system. In particular, the contributing factors include using large time steps, lack of accurate interface flux, and single-iteration coupling. We investigate the stability of the coupled ocean–atmosphere models for various interface conditions such as the Dirichlet–Neumann condition and the bulk interface condition, which is unique to climate modeling. By analyzing a simplified model, we demonstrate how the parameterization of the bulk condition and other numerical and physical parameters affect the coupling stability and establish stability conditions for different coupling strategies.

A review of progress in coupled ocean-atmosphere model developments for ENSO studies in China

El Nino-Southern Oscillation (ENSO) is the strongest interannual signal that is produced by basinscale processes in the tropical Pacific, with significant effects on weather and climate worldwide. In the past, extensive and intensive international efforts have been devoted to coupled model developments for ENSO studies. A hierarchy of coupled ocean-atmosphere models has been formulated; in terms of their complexity, they can be categorized into intermediate coupled models (ICMs), hybrid coupled models (HCMs), and fully coupled general circulation models (CGCMs). ENSO modeling has made signifiscant progress over the past decades, reaching a stage where coupled models can now be used to successfully predict ENSO events 6 months to one year in advance. Meanwhile, ENSO exhibits great diversity and complexity as observed in nature, which still cannot be adequately captured by current state-of-the-art coupled models, presenting a challenge to ENSO modeling. We primarily reviewed the long-term efforts in ENSO modeling continually and steadily made at different institutions in China; some selected representative examples are presented here to review the current status of ENSO model developments and applications, which have been actively pursued with noticeable progress being made recently. As ENSO simulations are very sensitive to model formulations and process representations etc., dedicated efforts have been devoted to ENSO model developments and improvements. Now, different ocean-atmosphere coupled models have been available in China, which exhibit good model performances and have already had a variety of applications to climate modeling, including the Coupled Model Intercomparison Project Phase 6 (CMIP6). Nevertheless, large biases and uncertainties still exist in ENSO simulations and predictions, and there are clear rooms for their improvements, which are still an active area of researches and applications. Here, model performances of ENSO simulations are assessed in terms of advantages and disadvantages with these differently formulated coupled models, pinpointing to the areas where they need to be further improved for ENSO studies. These analyses provide valuable guidance for future improvements in ENSO simulations and predictions.