Coupled General Circulation Model Experiments Research Articles

Critical Role of Tropical North Atlantic SSTA in Boreal Summer in Affecting Subsequent ENSO Evolution

AbstractThe season‐dependent impacts of the tropical North Atlantic (TNA) sea surface temperature anomaly (SSTA) on subsequent El Niño‐Southern Oscillation (ENSO) evolution were investigated through observational and modeling studies. The results indicate that, although the maximum amplitude of the TNA SSTA occurs during boreal spring, the TNA SSTA in boreal summer generates a stronger rainfall response in situ, which can further induce a significantly stronger zonal wind anomaly over the equatorial western Pacific via Kelvin and Rossby wave processes. The cause of a stronger precipitation response in boreal summer is attributed to the northward migration of the climatological Atlantic warm pool and the Inter‐Tropical Convergence Zone. Idealized Coupled General Circulation Model experiments further demonstrate that a persisting TNA SSTA forcing up to boreal summer is critical in conveying the TNA impact to subsequent ENSO evolutions in the Pacific.

Impact of air\u2013sea coupling on the probability of occurrence of heat waves in Japan

In extreme event attribution, which aims to answer whether and to what extent a particular extreme weather event can be attributed to global warming, the probability of an event is generally estimated through large ensemble simulations, using an atmospheric general circulation model (AGCM). In islands, such as Japan, it has been considered that surface air temperature (SAT) can be significantly affected by the surrounding sea surface temperature (SST), which mostly is affected by atmospheric circulation at mid- and high-latitudes. Therefore, the absence of SST responses to atmospheric variability in AGCMs impacts the estimation of the occurrence of extreme events, such as heat waves in Japan. In this study, we examined the impact of air–sea coupling on the probability of occurrence of severe heat waves that occurred in Japan in the summer of 2010 by analyzing the probability differences obtained from AGCM and coupled general circulation model (CGCM) large-ensemble experiments. The observed ocean temperature, salinity, and sea ice were assimilated in the 100-member CGCM experiments, as they were assigned as boundary conditions in the 100-member AGCM experiments. The SAT around Japan in the northern summer is largely related to the Bonin high, whose interannual variability is largely affected by the Silk Road and Pacific-Japan (PJ) pattern teleconnections in the real world. The SAT anomaly over Japan was related to the pressure variability due to the Silk Road and PJ patterns in the CGCM experiment. By contrast, the SAT over Japan simulated by AGCM was less sensitive to such pressure variability, and the SAT ensemble spread became narrower in AGCM. The results suggest that the probability of occurrence of the 2010 heat wave in Japan would tend to be underestimated by the AGCM ensemble compared to the CGCM ensemble, provided that the ensemble averages of the SAT anomalies were equal between CGCM and AGCM experiments. This study raised the issue of the absence of SST response to atmospheric variability in AGCMs, which can critically impact the estimation of extreme event probability, particularly in mid-latitude islands, such as Japan.

Role of Weddell Sea ice in South Atlantic atmospheric variability

CR Climate Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials CR 74:171-184 (2017) - DOI: https://doi.org/10.3354/cr01495 Role of Weddell Sea ice in South Atlantic atmospheric variability Yushi Morioka1,*, Francois Engelbrecht2,3, Swadhin K. Behera1 1Application Laboratory, JAMSTEC, Yokohama, Japan 2Natural Resources & the Environment, CSIR, Pretoria, South Africa 3School of Geography, Archaeology and Environmental Sciences, University of the Witwatersrand, South Africa *Corresponding author: morioka@jamstec.go.jp ABSTRACT: We investigated the role of Weddell Sea ice in atmospheric variability over the South Atlantic by conducting observational data analysis and coupled general circulation model (CGCM) experiments. Weddell Sea ice exhibits a pronounced interannual variability in early austral summer (November-January). Low sea-ice concentration (SIC) anomalies in the Weddell Sea are strongly associated with anticyclonic atmospheric circulation anomalies in the South Atlantic. Composite analysis reveals that the low SIC anomalies in the Weddell Sea may be attributed to increased solar radiation and northwesterly wind anomalies. The low SIC anomalies, in turn, contribute to warmer skin temperature in the band of 60-70°S and enhance the near-surface atmospheric stability north of this band, implying favorable conditions for sustaining the anticyclonic circulation anomalies in the South Atlantic. This intriguing association between the SIC and atmospheric circulation anomalies is also simulated in CGCM experiments, e.g. when the interannual sea surface temperature variability in the tropics and mid-latitudes is suppressed. These results suggest that Weddell Sea ice, which may undergo interannual variation via air-sea-ice interactions in the high latitudes, influences atmospheric variability over the South Atlantic. KEY WORDS: Air–sea–ice interaction · Interannual variability · South Atlantic · Atmospheric teleconnection · Coupled general circulation model Full text in pdf format Supplementary material PreviousCite this article as: Morioka Y, Engelbrecht F, Behera SK (2017) Role of Weddell Sea ice in South Atlantic atmospheric variability. Clim Res 74:171-184. https://doi.org/10.3354/cr01495 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in CR Vol. 74, No. 2. Online publication date: December 27, 2017 Print ISSN: 0936-577X; Online ISSN: 1616-1572 Copyright © 2017 Inter-Research.

Strongly Coupled Data Assimilation Using Leading Averaged Coupled Covariance (LACC). Part II: CGCM Experiments*

Abstract This paper uses a fully coupled general circulation model (CGCM) to study the leading averaged coupled covariance (LACC) method in a strongly coupled data assimilation (SCDA) system. The previous study in a simple coupled climate model has shown that, by calculating the coupled covariance using the leading averaged atmospheric states, the LACC method enhances the signal-to-noise ratio and improves the analysis quality of the slow model component compared to both the traditional weakly coupled data assimilation without cross-component adjustments (WCDA) and the regular SCDA using the simultaneous coupled covariance (SimCC). Here in Part II, the LACC method is tested with a CGCM in a perfect-model framework. By adding the observational adjustments from the low-level atmosphere temperature to the sea surface temperature (SST), the SCDA using LACC significantly reduces the SST error compared to WCDA over the globe; it also improves from the SCDA using SimCC, which performs better than the WCDA only in the deep tropics. The improvement in SST analysis is a result of the enhanced signal-to-noise ratio in the LACC method, especially in the extratropical regions. The improved SST analysis also benefits the subsurface ocean temperature and low-level atmosphere temperature analyses through dynamic and statistical processes.

The influence of ENSO on the equatorial Atlantic precipitation through the Walker circulation in a CGCM

The link between El Nino/Southern Oscillation (ENSO) and the equatorial Atlantic precipitation during boreal spring (March–April–May) is explored using a coupled general circulation model (CGCM). Interannual variability of the equatorial Atlantic sea surface temperature (SST) in the CGCM is excluded by nudging the modeled SST toward the climatological monthly mean of observed SST in the equatorial Atlantic, but full air–sea coupling is allowed elsewhere. It is found that the equatorial Atlantic precipitation is reduced (increased) during El Nino (La Nina) in the case where the interannual variability of the equatorial Atlantic SST is disabled. The precipitation anomalies in the equatorial Atlantic during ENSO are not strongly associated with the meridional migration of the Atlantic inter-tropical convergence zone. We find the reduced precipitation in the equatorial Atlantic during El Nino is associated with an enhanced Atlantic Walker circulation characterized by strengthened low-level easterlies and anomalous dry, downward winds over the equatorial Atlantic, while the Pacific Walker circulation is weakened. The upper-level anomalous westerlies over the equatorial Atlantic are consistent with a Matsuno–Gill-type response to heating in the eastern equatorial Pacific. Our results of the CGCM experiments suggest that changes to the Walker circulation induced by ENSO contribute significantly to changes in precipitation over the equatorial Atlantic.

Mid‐Holocene Asian summer climate and its responses to cold ocean surface simulated in the PMIP2 OAGCMs experiments

AbstractIn this study, the outputs from four Ocean‐Atmosphere Coupled General Circulation Model experiments within the Paleoclimate Modelling Intercomparison Project phase 2 and four sets of Atmospheric General Circulation Model experiments were used to analyze the Asian summer climate during the mid‐Holocene (6 ka, about 6000 years ago). Additionally, the role of the orbital forcings and the effects of a cold ocean surface for the 6 ka were investigated by comparing the Atmospheric General Circulation Model simulations forced by different combinations of forcing parameters. The results indicated that in the 6 ka summer, the orbital forcings were the prime drivers of the increased temperature and precipitation and the strengthened summer monsoon over the Asian continent. On the other hand, these different orbital forcings also resulted in a colder Indian Ocean‐northwestern Pacific during this period. Our results suggested that this cold ocean surface could reduce the warming amplitude and precipitation enhancement over the Asian monsoon area in the 6 ka summer. The changes caused by the different ocean surface conditions were comparable with simulated 6 ka climate changes. The cold ocean surface also suppressed the Asian summer monsoon circulations. Therefore, the influences from anomalous ocean surface conditions played an important role on regulating the Asian summer climate during the 6 ka. In addition, it was found that the summer climate in the South Asian monsoon area was more sensitive to the changes in the orbital forcings and ocean surface conditions than that in the East Asian monsoon area.

Role of the Indo-Pacific Interbasin Coupling in Predicting Asymmetric ENSO Transition and Duration

Warm and cold phases of El Niño–Southern Oscillation (ENSO) exhibit a significant asymmetry in their transition/duration such that El Niño tends to shift rapidly to La Niña after the mature phase, whereas La Niña tends to persist for up to 2 yr. The possible role of sea surface temperature (SST) anomalies in the Indian Ocean (IO) in this ENSO asymmetry is investigated using a coupled general circulation model (CGCM). Decoupled-IO experiments are conducted to assess asymmetric IO feedbacks to the ongoing ENSO evolution in the Pacific. Identical-twin forecast experiments show that a coupling of the IO extends the skillful prediction of the ENSO warm phase by about one year, which was about 8 months in the absence of the IO coupling, in which a significant drop of the prediction skill around the boreal spring (known as the spring prediction barrier) is found. The effect of IO coupling on the predictability of the Pacific SST is significantly weaker in the decay phase of La Niña. Warm IO SST anomalies associated with El Niño enhance surface easterlies over the equatorial western Pacific and hence facilitate the El Niño decay. However, this mechanism cannot be applied to cold IO SST anomalies during La Niña. The result of these CGCM experiments estimates that approximately one-half of the ENSO asymmetry arises from the phase-dependent nature of the Indo-Pacific interbasin coupling.

Impacts of Atlantic sea surface temperature anomalies on Indo-East Asian summer monsoon-ENSO relationship

In this study, the effect of the tropical North Atlantic (TNA) sea surface temperature (SST) variation in inducing the circulation anomaly in the Indo-East Asian monsoon (IEAM) region is investigated through the observational analysis and numerical modeling. The observational analysis shows that the TNA summer SST is positively correlated with the preceding winter Nino3 SST and is simultaneously correlated with the circulation in the IEAM region. The simultaneous circulation pattern resembles that of the ENSO-decaying summer. The positive correlation between the TNA SST and the Nino3 region SST is primarily ascribed to the surface latent heat flux and short wave radiation anomalies induced by the ENSO teleconnection. Coupled general circulation model experiments show that, while including the air-sea coupling in the Atlantic, the model can reproduce the main features of the IEAM circulation, such as an anomalous anticyclone over the western North Pacific (WNP) and southerly anomalies over southeast China. While the climatological Atlantic SST is prescribed, the circulation over the WNP displays a significantly different pattern, with an eastward migration of the WNP anticyclone and the associated northerly anomalies over southeast China. It is argued that anticyclonic shear and Ekman divergence associated with the atmospheric Kelvin wave response to the TNA warm SSTA forcing is the primary mechanism for the generation of the anomalous anticyclone in WNP. The results presented in this study provide a teleconnection pattern between TNA and short-term climate variability in IEAM region.

Seasonally Different Response of the Indian Ocean to the Remote Forcing of El Nino: Linking the Dynamics and Thermodynamics

Physical processes involved in basin-wide warming (BW) in the Indian Ocean (IO) and the IO “Dipole” mode (IOD) are investigated, using solutions to an air-sea coupled general circulation model (CGCM). In this study, we pay attention to the seasonal difference of the IO SST in response to the ENSO. A suite of time-slice CGCM experiments, by imposing the El Nino-related SST anomalies in the Pacific for the boreal summer and fall, are conducted to study the El Nino impact on the IO SST anomalies. The impacts of El Nino largely depend on the monsoon seasonality, and significantly contribute to the IOD and IO BW. Additional experiments demonstrate the salient seasonal difference in the SST sensitivity to the ocean dynamics.

An asymmetric mode of tropical Indian Ocean rainfall variability in boreal spring

An analysis of observational estimates has revealed an asymmetric mode of boreal spring rainfall and wind variability over the tropical Indian Ocean. The asymmetric mode is characterized by opposite rainfall and surface wind anomalies north and south of the equator. This mode is associated with a cross‐equatorial gradient in sea surface temperature anomalies. The evolution of this mode is related to air‐sea interactions in the tropical Indian Ocean. In particular, surface heat fluxes play an important role in the initiation, development, and decay of this mode. The occurrence of this mode in observations is closely linked to both El Niño‐Southern Oscillation (ENSO) and the Indian Ocean Dipole mode. Coupled general circulation model experiments indicate that this mode can occur in the absence of ENSO. The variability associated with this mode enhances ENSO's teleconnection to the Indian Ocean and affects the seasonal transition in the tropical Indian Ocean.

The Role of the Indonesian Throughflow in the Indo–Pacific Climate Variability in the GFDL Coupled Climate Model

AbstractThe impacts of the Indonesian Throughflow (ITF) on the tropical Indo–Pacific climate, particularly on the character of interannual variability, are explored using a coupled general circulation model (CGCM). A pair of CGCM experiments—a control experiment with an open ITF and a perturbation experiment in which the ITF is artificially closed—is integrated for 200 model years, with the 1990 values of trace gases. The closure of the ITF results in changes to the mean oceanic and atmospheric conditions throughout the tropical Indo–Pacific domain as follows: surface temperatures in the eastern tropical Pacific (Indian) Ocean warm (cool), the near-equatorial Pacific (Indian) thermocline flattens (shoals), Indo–Pacific warm-pool precipitation shifts eastward, and there are relaxed trade winds over the tropical Pacific and anomalous surface easterlies over the equatorial Indian Ocean. The character of the oceanic changes is similar to that described by ocean-only model experiments, though the amplitude of many features in the tropical Indo–Pacific is amplified in the CGCM experiments.In addition to the mean-state changes, the character of tropical Indo–Pacific interannual variability is substantially modified. Interannual variability in the equatorial Pacific and the eastern tropical Indian Ocean is substantially intensified by the closure of the ITF. In addition to becoming more energetic, El Niño–Southern Oscillation (ENSO) exhibits a shorter time scale of variability and becomes more skewed toward its warm phase (stronger and more frequent warm events). The structure of warm ENSO events changes; the anomalies of sea surface temperature (SST), precipitation, and surface westerly winds are shifted to the east and the meridional extent of surface westerly anomalies is larger.In the eastern tropical Indian Ocean, the interannual SST variability off the coast of Java–Sumatra is noticeably amplified by the occurrence of much stronger cooling events. Closing the ITF shoals the eastern tropical Indian Ocean thermocline, which results in stronger cooling events through enhanced atmosphere–thermocline coupled feedbacks. Changes to the interannual variability caused by the ITF closure rectify into mean-state changes in tropical Indo–Pacific conditions. The modified Indo–Pacific interannual variability projects onto the mean-state differences between the ITF open and closed scenarios, rectifying into mean-state differences. These results suggest that CGCMs need to reasonably simulate the ITF in order to successfully represent not just the mean climate, but its variations as well.

An Impact of SST Anomalies in the Indian Ocean in Acceleration of the El Nino to La Nina Transition

One possible impact of sea surface temperature (SST) anomalies in the Indian Ocean (IO) on the on- going El Nino is investigated using an air-sea coupled general circulation model (CGCM). A suite of CGCM experiments by imposing the IO basin-wide warming (BW) and IO dipole/zonal mode (IODZM) are conducted to assess the feedback effects of the El Nino-related SST anomalies on the Pacific. While the IODZM during boreal fall does not have a significant impact on the Pacific sector, the BW during bo- real winter enhances the surface easterlies over the equatorial Western Pacific (WP) during the mature- decay phase of El Nino. The strengthened easterlies act to enhance the SST cooling over the WP, thereby the zonal gradient of SST between the IO and WP is greater than the climatology. These enhanced WP easterlies induce an advanced transition to the La Nina phase, which is caused by the upwelling ocean Kelvin waves. These results imply that the BW in the IO, to some extent, can be hastening the El Nino to La Nina transition.

Origin of decadal El Niño–Southern Oscillation–like variability in a coupled general circulation model

The origin of decadal El Niño–Southern Oscillation–like variability (hereinafter referred to as DEV) is analyzed in coupled general circulation model (CGCM) simulations. The CGCM experiments test the null hypothesis for the existence of DEV, that is, that DEV is linear, damped, and stochastically forced by atmospheric internal dynamics. In order to test this hypothesis we employ the interactive ensemble coupling strategy. It is found that the spatially averaged variance of sea surface temperature anomaly (SSTA) and heat content anomaly (HCA) associated with the DEV is linearly related to the amplitude of atmospheric internal dynamics. This result suggests that the DEV is primarily noise forced but that damped coupled feedbacks and/or internal ocean dynamics enhance this variability. The SSTA variability associated with the DEV appears to be of a basin‐type standing mode with a triangular shape. However, the decadal evolution of the HCA associated with the DEV shows a systematic westward propagation from the central North Pacific and the eastern South Pacific. Both a singular vector analysis and noise variance analysis based on the coupled simulation results support the argument that atmospheric noise in the subtropics leads to the excitation of the DEV.