Search

Impacts of temperature assimilation on Venusian thermal tides are investigated by the observing system simulation experiments assuming Akatsuki Longwave Infrared Camera (LIR) observations. Synthetic temperature data are prepared by a Venusian general circulation model (VGCM) to test if LIR temperature data resolves a discrepancy in the structure of thermal tides between observations and the VGCM. They are assimilated at 70 km altitude with several combinations of frequency and horizontal region. The result shows that the three-dimensional structure of thermal tides is significantly improved not only in temperature but also in horizontal wind, even if observations are available only at a limited frequency of 6-hourly or on the dayside. The zonal–mean zonal wind and temperature fields are also modified at 60–80 km altitudes globally through the vertical momentum transport of thermal tides. It would be promising to assimilate Akatsuki LIR observations to produce realistic objective analysis of the Venus atmosphere.

An ensemble-based forecast sensitivity to observations (EFSO) diagnosis has been implemented in an atmospheric general circulation model–ensemble Kalman filter data assimilation system to estimate the impacts of specific observations from the quasi-operational global observing system on weekly short-range forecasts. It was examined whether EFSO reasonably approximates the impacts of a subset of observations from specific geographical locations for 6-hour forecasts, and how long the 6-hour observation impacts can be retained during the 7-day forecast period. The reference for these forecasts was obtained from 12 data denial experiments in each of which a subset of three radiosonde observations launched from a geographical location was excluded. The 12 locations were selected from three latitudinal bands comprising (i) four Arctic regions, (ii) four midlatitude regions in the Northern Hemisphere, and (iii) four tropical regions during the Northern Hemisphere winter of 2015/16. The estimated winter-averaged EFSO-derived observation impacts well corresponded to the 6-hour observation impacts obtained by the data denials and EFSO could reasonably estimate the observation impacts by the data denials on short-range (6-hour to 2-day) forecasts. Furthermore, during the medium-range (4-day to 7-day) forecasts, it was found that the Arctic observations tend to seed the broadest impacts and their short-range observation impacts could be projected to beneficial impacts in Arctic and midlatitude North American areas. The midlatitude area located just downstream of dynamical propagation from the Arctic toward the midlatitudes. Results obtained by repeated Arctic data-denial experiments were found to be generally common to those from the non-repeated experiments., ひとつひとつの観測データが気象予測に与える影響を簡易に評価する手法を確認 --北極の観測データは7日先の北米気象予測の改善に貢献することも明らかに--. 京都大学プレスリリース. 2021-04-30.

A quasi-global eddying ocean hindcast simulation using a new version of our model called OFES2 (Ocean General Circulation Model for the Earth Simulator version 2) was conducted to overcome several issues with unrealistic properties in its previous version OFES. This paper describes the model and the simulated oceanic fields in OFES2 compared with OFES and also observed data. A sea-ice model and a tidal mixing scheme were implemented in OFES2, which was forced by a newly created surface atmospheric dataset called JRA55-do and simulated the oceanic fields from 1958 to 2016. We found several improvements in OFES2 over OFES: smaller biases in the global sea surface temperature and sea surface salinity and the water properties in the Indonesian and Arabian Seas. The time series of the Niño3.4 and Indian Ocean Dipole (IOD) indexes are somewhat better in OFES2 than in OFES. Unlike the previous version, OFES2 reproduces more realistic anomalous low sea surface temperatures during a positive IOD event. One possible cause for these improvements in El Niño and IOD events is the replacement of the atmospheric dataset. On the other hand, several issues remained unrealistic, such as the pathways of the Kuroshio and Gulf Stream and the unrealistic spreading of salty Mediterranean overflow. Given the worldwide use of the previous version and the improvements presented here on it, the output from OFES2 will be useful in studying various oceanic phenomena with broad spatiotemporal scales.

Ensemble-based atmospheric data assimilation (DA) systems are sometimes afflicted with an underestimation of the ensemble spread near the surface caused by the use of identical boundary conditions for all ensemble members and the lack of atmosphere–ocean interaction. To overcome these problems, a new DA system has been developed by replacing an atmospheric GCM with a coupled atmosphere–ocean GCM, in which atmospheric observational data are assimilated every 6 h to update the atmospheric variables, whereas the oceanic variables are subject to no direct DA. Although SST suffers from the common biases among many coupled GCMs, two months of a retrospective analysis–forecast cycle reveals that the ensemble spreads of air temperature and specific humidity in the surface boundary layer are slightly increased and the forecast skill in the midtroposphere is rather improved by using the coupled DA system in comparison with the atmospheric DA system. In addition, surface atmospheric variables over the tropical Pacific have the basinwide horizontal correlation in ensemble space in the coupled DA system but not in the atmospheric DA system. This suggests the potential benefit of using a coupled GCM rather than an atmospheric GCM even for atmospheric reanalysis with an ensemble-based DA system.

Local air–sea interactions over the high sea surface temperature (SST) band along the Hawaiian Lee Countercurrent (HLCC) are examined with a focus on dynamical feedback of SST-induced wind stress to the ocean using the atmosphere–ocean coupled general circulation model (CGCM). A pair of ensemble CGCM simulations are compared to extract the air–sea interactions associated with HLCC: the control simulations and other simulations, the latter purposely eliminating influences of the high SST band on the sea surface flux computations in the CGCM. The comparison reveals that oceanic response to surface wind convergence and positive wind stress curl induced by the high SST band increases (decreases) the HLCC speed in the southern (northern) flank of the HLCC. The HLCC speed changes are driven by the Ekman suction associated with positive wind stress curl over the warm HLCC via the thermal wind balance. The HLCC speed increase is more significant than its decrease. This dynamical feedback is likely to be important to sustain the extension of the HLCC far to the west. The heat budget analysis confirms that advection of warm water from the west associated with this significant current speed increase plays a role in the southward shift of the HLCC axis. The dynamical feedback with the HLCC speed increase can potentially amplify the seasonal and interannual variations of HLCC.

An integration of a high-resolution coupled general circulation model whose ocean component is eddy permitting and thus able to reproduce a sharp gradient in sea surface temperature (SST) is analyzed to investigate air–sea heat exchanges characteristic of the midlatitude oceanic frontal zone. The focus of this paper is placed on a prominent SST front in the south Indian Ocean, which is collocated with the core of the Southern Hemisphere storm track. Time-mean distribution of sensible heat flux is characterized by a distinct cross-frontal contrast. It is upward and downward on the warmer and cooler flanks, respectively, of the SST front, acting to maintain the sharp gradient of surface air temperature (SAT) that is important for preconditioning the environment for the recurrent development of storms and thereby anchoring the storm track. Induced by cross-frontal advection of cold (warm) air associated with migratory atmospheric disturbances, the surface flux is highly variable with intermittent enhancement of the upward (downward) flux predominantly on the warmer (cooler) flank of the front. Indeed, several intermittent events of cold (warm) air advection, whose total duration accounts for only 21% (19%) of the entire analysis period, contribute to as much as 60% (44%) of the total amount of sensible heat flux during the analysis period on the warmer (cooler) flank. This antisymmetric behavior yields the sharp cross-frontal gradient in the time-mean flux. Since the flux intensity is strongly influenced by local magnitude of the SST–SAT difference that tends to increase with the SST gradient, the concentration of the flux variance to the frontal zone and cross-frontal contrasts in the mean and skewness of the flux all become stronger during the spinup of the SST front. Synoptically, the enhanced sensible heat flux near the SST front can restore SAT toward the underlying SST effectively with a time scale of a day, to maintain a frontal SAT gradient against the relaxing effect of atmospheric disturbances. The restoration effect of the differential surface heating at the SST front, augmented by the surface latent heating concentrated on the warm side of the front, represents a key process through which the atmospheric baroclinicity and ultimately the storm track are linked to the underlying ocean.

A variational data assimilation system is presented for the initialization of an ocean surface circulation forecast system. The authors’ variational data assimilation system is designed to satisfy both statistical and dynamical constraints. As is usual, the statistical part of the assimilation scheme corresponds to the optimal interpolation scheme while the dynamical part works as a weak constraint for the model equations except for the time differential terms. Thus this variational data assimilation scheme can be regarded as an extended optimal interpolation capable of obtaining the analysis field that satisfies the model dynamics. Comparison with the results of assimilating altimetric data into a 1.5-layer primitive equation model by the classical optimal interpolation clearly shows the advantage of this assimilation method. For example, the analysis field is significantly improved, in particular, in the western boundary current regions and their extensions, where some assumptions inherent in t...

[1] Oceanic alternating zonal jets at depth have been detected ubiquitously in observations and ocean general circulation models (GCMs). Such oceanic jets are generally considered as being generated by purely oceanic processes. Here we explore a possible air-sea interaction induced by surface signatures of the deep zonal jets using an eddy-permitting coupled atmosphere-ocean GCM (CGCM). The 23-year CGCM integration reproduces bands of latitudinally-narrow alternating jets in the Southeast Pacific. They extend from the sea surface to well below the main thermocline and are embedded in the large-scale westward-flowing South Equatorial Current, the latter mostly confined above the thermocline. These jets generate fine-scale sea surface temperature (SST) anomalies through the advection of zonal temperature gradients. The atmospheric boundary layer appears to respond thermally to this fine-scale SST field, which induces fine-scale wind stress anomaly through atmospheric pressure adjustment, as indicated by a good spatial correlation between the SST Laplacian field and the fine-scale wind stress curl. A Sverdrup calculation on the wind stress field of the CGCM predicts fine-scale zonal currents driven by the meridional gradient of the fine-scale wind stress curl. The positions of these Sverdrup currents are generally coincident with those of the original zonal jets and the Sverdrup prediction explains roughly half of the amplitudes of the jets. While the original cause of the deep zonal jets simulated in our CGCM is unidentified, this analysis suggests that there is likely a positive air-sea feedback: the jets generate fine-scale wind stress curl that reinforces themselves through the Sverdrup dynamics.

Using numerical simulations forced by a uniform realistic wind time series, the authors show that the presence of a mesoscale eddy field at midlatitudes accelerates the vertical propagation of the wind-forced near-inertial waves (NIW) and produces the emergence of a maximum of vertical velocity into the deep ocean (around 2500 m) characterized by a mean amplitude of 25 m day−1, a dominant 2f frequency, and scales as small as O(30 km). These results differ from previous studies that reported a smaller depth and larger scales. The authors show that the larger depth observed in the present study (2500 m instead of 1700 m) is due to the wind forcing duration that allows the first five baroclinic modes to disperse and to impact the deep NIW maximum (instead of the first two modes as reported before). The smaller scales (30 km instead of 90 km) are explained by a resonance mechanism (described in previous studies) that affects the high NIW baroclinic modes, but only when small-scale relative vorticity structures (related to the mesoscale eddy field) have an amplitude that is large enough. These results, which point out the importance of the wind forcing duration and the resolution, indicate that the emergence of a deep NIW maximum with a 2f frequency reported before is a robust feature that is enhanced with more realistic conditions. Such 2f frequency in the deep interior raises the question of the mechanisms, still unresolved, that may ultimately transfer this superinertial energy into mixing at these depths.

[1] The relative importance of remote and local influences in the development of Benguela Ninos is examined using observations and a coupled general circulation model capable of simulating interannual variability in the tropical Atlantic. While previous studies have emphasized the role of equatorially excited Kelvin waves, the present study finds that meridional wind anomalies along the southwest African coast contribute substantially. Both observations and model output indicate that sea-surface temperatures along the southwest African coast respond rapidly to changes in meridional wind stress. These wind anomalies form part of a basin-scale weakening of the subtropical anticyclone that extends to the equator. As the weakening begins three months before the peak of the event it might have some predictive potential. Results also indicate that the close correlation between Benguela and Atlantic Ninos in observations might result from the large spatially coherent wind stress anomalies associated with the weakened anticyclone.

[1] In order to investigate the deep ocean inertia-gravity waves, a high-resolution global coupled atmosphere–ocean simulation is carried out with a coupling interval of 20 minutes. Large (∼10−3 m s−1) root-mean-square variability of vertical velocity is found in middepths (2000–4000 m), which is not reported in previous studies using realistic ocean simulations. Horizontal distribution of the large variability roughly corresponds to the wintertime atmospheric storm tracks and is stretched equatorward due to β-dispersion in open ocean with some “shadow regions” behind the obstacles. Frequency spectrum of vertical velocity has strong peaks at around f and 2f (f is the local inertial period) in midlatitudes, and has additional peak at around (3/2)f or 3f at some points. These results suggest necessity of re-evaluation of wind-induced near-inertial energy with high-frequency atmospheric forcing.

The Gulf Stream is a warm Atlantic current that transports heat northward, keeping Western Europe significantly warmer than North America in winter. It is known to influence short-term weather phenomena such as surface winds and cyclone formation, but its effects on longer-term climate and at higher levels in the atmosphere are poorly understood. Now a combination of weather analyses, satellite data and an atmospheric general circulation model reveals that the Gulf Stream's influence is felt well above the near-surface portion of the atmosphere. The current anchors a tall wall of atmospheric upward motion that penetrates into the upper troposphere and supports deep raining clouds. This provides a pathway by which the Gulf Stream can affect local climate, and possibly climate in remote regions via an effect on planetary wave propagation. The cover graphic represents surface current speeds in blue-white colours (white is the fastest) and upward wind velocities in yellow-red colours (red for stronger winds), along with land-surface topography in eastern North America. The Gulf Stream's influence on the atmosphere is examined using a combination of operational weather analyses and satellite observations. The results indicate that the Gulf Stream anchors a rain band in which upward motion of air penetrates deep into the upper troposphere, well above the near-surface portion of the atmosphere. These mechanisms provide a pathway by which the Gulf Stream can affect local climate, and possibly also climate in remote regions. The Gulf Stream transports large amounts of heat from the tropics to middle and high latitudes, and thereby affects weather phenomena such as cyclogenesis1,2 and low cloud formation3. But its climatic influence, on monthly and longer timescales, remains poorly understood. In particular, it is unclear how the warm current affects the free atmosphere above the marine atmospheric boundary layer. Here we consider the Gulf Stream’s influence on the troposphere, using a combination of operational weather analyses, satellite observations and an atmospheric general circulation model4. Our results reveal that the Gulf Stream affects the entire troposphere. In the marine boundary layer, atmospheric pressure adjustments to sharp sea surface temperature gradients lead to surface wind convergence, which anchors a narrow band of precipitation along the Gulf Stream. In this rain band, upward motion and cloud formation extend into the upper troposphere, as corroborated by the frequent occurrence of very low cloud-top temperatures. These mechanisms provide a pathway by which the Gulf Stream can affect the atmosphere locally, and possibly also in remote regions by forcing planetary waves5,6. The identification of this pathway may have implications for our understanding of the processes involved in climate change, because the Gulf Stream is the upper limb of the Atlantic meridional overturning circulation, which has varied in strength in the past7 and is predicted to weaken in response to human-induced global warming in the future8.

[1] A 4-dimensional variational data assimilation system has been used to better define the mean seasonal state of the North Pacific. The synthesis of available observational records and a sophisticated general circulation model produces a dynamically consistent time-varying dataset which exhibits realistic features of the global ocean circulation and requires no artificial sources or sinks for the temperature and salinity fields. The dataset enables us to clarify the water mass formation and movement processes. A sensitivity experiment using our system reveals that the origin of the North Pacific Intermediate Water can be traced back to the Okhotsk and Bering Seas in the subarctic region and to the subtropical Kuroshio region further south, consistent with recent observational findings. This result illustrates that the ocean state derived from our data assimilation has greater information and forecast potential than that obtained from earlier methods.

[1] Capability for short-range forecasting of the Kuroshio path variabilities south of Japan is studied by assimilating the absolute sea surface dynamic height data, which are derived from in situ hydrography and TOPEX/Poseidon (T/P) altimetry, into a high-resolution 1-1/2 layer primitive equation model every 15 days for 5 years from 1993 to 1997. The initialization scheme used is the variational method, which contains the nonlinear dynamics of the model as a weak constraint and is suitable for initialization in the western boundary current regions where initialization by simple optimal interpolation is inappropriate. The time series of the analysis field well represents the Kuroshio path variations during the study period. For example, the root-mean-square (RMS) differences from the observations falls within 0.3° for the location in latitude of the current axis off Enshu-nada and 11.4 cm for the sea level at Hachijo-jima Island while their observed RMS variabilities are 0.5° and 30.7 cm, respectively. By using the resulting fields as the initial conditions, 116 cases of 90-day forecast experiments for the Kuroshio path variations are then performed to assess the statistical properties of our assimilation system. Though significant bias is found in the forecast field southeast of Kyushu, the accuracy of our forecasted field south of Japan is much higher than that of persistence experiments, and the typical evolution of the Kuroshio into a large-amplitude path off Enshu-nada is successfully reproduced after the initialization. In particular, the eastward progression speed and the horizontal scale of the meander are quite similar to those observed. These results and the comparison with the RMS variability of the analysis field enable us to infer that roughly 60-day forecasts of the Kuroshio path variabilities south of Japan are possible. This indicates that our assimilation system is suitable for the operational short-range forecasting of the upper Kuroshio.