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[en] Using an ensemble of nine El Niño/Southern Oscillation (ENSO) reconstructed proxies and volcano eruption proxies for the past 1500 years, this study shows that a significant La Niña state emerges in the second year (year (2) hereafter) after large tropical volcanic eruptions. The reasons for the development of La Niña are investigated using the Community Earth System Model (CESM). In the volcanic eruption experiment (Vol), a robust La Niña signal occurs in year (2), resembling the proxy records. The eastward positioning of the western North Pacific anomalous anticyclone (WNPAC) in Vol plays a critical role in the advanced decay of year (2) warming and the strong intensification of cooling in the equatorial eastern Pacific. The enhanced easterlies located on the southern edge of the WNPAC can stimulate consecutive oceanic upwelling Kelvin waves, shallowing the thermocline in the eastern Pacific, thereby resulting in a greater cooling rate by the enhanced thermocline feedback and cold zonal advection. Over the equatorial eastern Pacific, the reduced shortwave radiation contributes to the advanced decay of warming, while the upward latent heat flux augments the strong intensification of the cooling. Essentially, the eastward positioning of the WNPAC is a result of the volcanic forcing. The volcanic effect cools the maritime continent more than its adjacent oceans, thus pushing convective anomalies eastward during year (1). This induces vertical thermal advection and upward surface latent heat flux, thereby suppressing the development of warm Sea Surface Temperature over the central-western Pacific and causing the eastward positioning of the WNPAC in Vol.
[en] The 1925 El Niño (EN) event was the third strongest in the twentieth century according to its impacts in the far-eastern Pacific (FEP) associated with severe rainfall and flooding in coastal northern Peru and Ecuador in February–April 1925. In this study we gathered and synthesised a large diversity of in situ observations to provide a new assessment of this event from a modern perspective. In contrast to the extreme 1982–1983 and 1997–1998 events, this very strong “coastal El Niño” in early 1925 was characterised by warm conditions in the FEP, but cool conditions elsewhere in the central Pacific. Hydrographic and tide-gauge data indicate that downwelling equatorial Kelvin waves had little role in its initiation. Instead, ship data indicate an abrupt onset of strong northerly winds across the equator and the strengthening/weakening of the intertropical convergence zones (ITCZ) south/north of the equator. Observations indicate lack of external atmospheric forcing by the Panama gap jet and the south Pacific anticyclone and suggest that the coupled ocean–atmosphere feedback dynamics associated with the ITCZs, northerly winds, and the north–south SST asymmetry in the FEP lead to the enhancement of the seasonal cycle that produced this EN event. We propose that the cold conditions in the western-central equatorial Pacific, through its teleconnection effects on the FEP, helped destabilize the ITCZ and enhanced the meridional ocean–atmosphere feedback, as well as helping produce the very strong coastal rainfall. This is indicated by the nonlinear relation between the Piura river record at 5°S and the SST difference between the FEP and the western-central equatorial Pacific, a stability proxy. In summary, there are two types of EN events with very strong impacts in the FEP, both apparently associated with nonlinear convective feedbacks but with very different dynamics: the very strong warm ENSO events like 1982–1983 and 1997–1998, and the very strong “coastal” EN events like 1925.
[en] The El Niño/Southern Oscillation (ENSO) exhibits considerable differences between the evolution of individual El Niño and La Niña events (‘ENSO diversity’), with significant implications for impacts studies. However, the degree to which external forcing may affect ENSO diversity is not well understood, due to both internal variability and potentially compensatory contributions from multiple forcings. The Community Earth System Model Last Millennium Ensemble (CESM LME) provides an ideal testbed for studying the sensitivity of twentieth century ENSO to forced climate changes, as it contains many realizations of the 850–2005 period with differing combinations of forcings. Metrics of ENSO amplitude and diversity are compared across LME simulations, and although forced changes to ENSO amplitude are generally small, forced changes to diversity are often detectable. Anthropogenic changes to greenhouse gas and ozone/aerosol emissions modify the persistence of Eastern and Central Pacific El Niño events, through shifts in the upwelling and zonal advective feedbacks; these influences generally cancel one another over the twentieth century. Other forcings can also be quite important: land use changes amplify Eastern Pacific El Niño events via modulating zonal advective heating, and orbital forcing tends to preferentially terminate twentieth century Central Pacific El Niño events due to enhanced eastern Pacific cooling during boreal winter and spring. Our results indicate that multiple anthropogenic and natural forcings can have substantial impacts on ENSO diversity, and suggest that correctly representing the net ENSO diversity response to climate change will depend on the precise balance between all these influences.
[en] Interannual precipitation and temperature variations during 1979–2014 are investigated by examining the effects of two distinct flavors of the El Niño-Southern Oscillation (ENSO), i.e., the tropical eastern Pacific (EP) and central Pacific (CP) ENSO events. Satellite- and ground-based observations with global coverage are applied including the monthly precipitation data from the Global Precipitation Climatology Project (GPCP) and surface temperature anomalies from the NASA-GISS surface temperature anomaly analysis. Related variations in other water-cycle components including atmospheric moisture transport are also examined by using the outputs from the NASA-Modern Era Retrospective-analysis for Research and Applications (MERRA). While the second leading mode from an EOF analysis of sea surface temperature (SST) anomalies between 30°N and 30°S is dominated by interdecadal-scale variability that is not a focus of this study, the first and third leading modes represent well the EP and CP events, respectively. The corresponding principal components (PC1 and PC3) are then applied as indices to estimate the influences of the two ENSO flavors on various physical components through linear regression. Because of their distinct SST configurations in the tropical Pacific, the two ENSO flavors manifest different spatial features of precipitation anomalies as shown in past studies. Differences can also be readily seen in satellite-retrieved tropospheric layered temperatures and oceanic columnar water vapor content. General agreements between observations and MERRA outputs can be obtained as judged by consistent respective anomalies corresponding to the two ENSO flavors, suggesting that MERRA could provide an accurate account of variations on the interannual time scale. Interannual variations in MERRA vertically integrated moisture transport (VIMT) are further examined to explore possible relations between precipitation and tropospheric moisture transport corresponding to the two flavors during two contrasting seasons: December–March (DJFM) and June–September (JJAS). Anomalies of zonal moisture transport in the deep tropics following the variations in the Pacific Walker Circulation are distinctly different for two ENSO flavors and also manifest evident seasonal variations for each flavor. Differences in the zonal mean VIMT (both zonal and meridional components) are also evident between the two flavors, consistent with the differences in zonal mean precipitation anomalies from both GPCP and MERRA. Furthermore, the ENSO flavors are associated with distinct precipitation anomaly patterns over various land areas, which can be further traced to the differences in their associated VIMT anomalies, particularly during DJFM when the warm ENSO events usually reach their mature phase.
[en] Precipitation and temperature in freeze–thaw agricultural area have different patterns under global warming. In this study, a statistical relationship between large-scale changes in climate variables and local weather data was built by applying an automated statistical downscaling (ASD) model in the Sanjiang Plain in China. We evaluated the prediction ability of the ASD model in terms of spatial–temporal changes in freeze–thaw agricultural area, and the temperature and precipitation changes in the twenty-first century under the representative concentration pathway 4.5 (RCP4.5) scenario were estimated. The results revealed that the explained variances in temperature were higher than 0.93 during the calibration and verification periods, which demonstrated good simulation capacity. The R2 of precipitation was acceptable due to the randomness and complexity of daily precipitation. Based on the Geophysical Fluid Dynamics Laboratory Earth System Model with the Generalized Ocean Layer Dynamics component (GFDL-CM3), the regional climate simulation provided good predictions. By 2100, the average, maximum and minimum temperatures in this area could increase by 2.0–2.5 °C, 2.5 °C and 2.5–4.0 °C, respectively. In terms of the spatial distribution, temperatures could increase faster in the northern region and slower in the central and southern regions. The warming trends in summer and winter were more significant than those in spring and autumn. There was no significant change in annual precipitation in the twenty-first century (increased approximately 10 mm by 2100). Precipitation decreased obviously in July and August (approximately 0.4 mm/day), and other months showed an increasing trend (approximately 0.5–0.9 mm/day). There will be large spatial variation of precipitation in the future changes. The results could serve as a reference for assessing non-point source pollution and agricultural management.
[en] The reforecast of 11 models in the sub-seasonal to seasonal (S2S) prediction project has been analyzed to investigate the effects of the Madden–Julian Oscillation (MJO) on the prediction skill of winter 2-m air temperature (T2M) over China. Most of the S2S models have useful prediction skills (correlation skill ≥ 0.5) before pentads 3 and 4. ECMWF model can possess a good prediction skill for almost four pentads and perform the best among the 11 models. ECCC and ECMWF models have more reliable ensemble prediction and better ensemble strategies than the other models. All the models tend to have lower T2M prediction skill over the Tibetan Plateau than that over the other regions of China. Moreover, initial state and model resolution have important influences on S2S prediction skill. In most of the models at pentads 3 and 4, T2M prediction skill of forecast with MJO at initial time is significantly higher than that without over parts of China. However, the spatial distributions of the prediction skill differences due to MJO are not consistent among the 11 models. This indicates that there is an uncertainty of the effects of MJO on T2M prediction over China at pentads 3 and 4. Planetary-scale teleconnection pattern excited by MJO over the Northern Hemisphere is the possible reason for the effect of MJO on T2M prediction skill. Because most of the models can maintain this teleconnection pattern for 3–4 forecast pentads, MJO can affect the atmospheric circulation over China during this period, and improve the T2M prediction skill in the models. This finding suggests that the prediction of winter T2M over China initialized with MJO can be more skillful at pentads 3 and 4 than that without MJO in the initial conditions.
[en] Accurate and precise forecasting of the Indian monsoon is important for the socio-economic security of India, with improvements in agriculture and associated sectors from prediction of the monsoon onset. In this study we establish the skill of the UK Met Office coupled initialized global seasonal forecasting system, GloSea5-GC2, in forecasting Indian monsoon onset. We build on previous work that has demonstrated the good skill of GloSea5 at forecasting interannual variations of the seasonal mean Indian monsoon using measures of large-scale circulation and local precipitation. We analyze the summer hindcasts from a set of three springtime start-dates in late April/early May for the 20-year hindcast period (1992–2011). The hindcast set features at least fifteen ensemble members for each year and is analyzed using five different objective monsoon indices. These indices are designed to examine large and local-scale measures of the monsoon circulation, hydrological changes, tropospheric temperature gradient, or rainfall for single value (area-averaged) or grid-point measures of the Indian monsoon onset. There is significant correlation between onset dates in the model and those found in reanalysis. Indices based on large-scale dynamic and thermodynamic indices are better at estimating monsoon onset in the model rather than local-scale dynamical and hydrological indices. This can be attributed to the model’s better representation of large-scale dynamics compared to local-scale features. GloSea5 may not be able to predict the exact date of monsoon onset over India, but this study shows that the model has a good ability at predicting category-wise monsoon onset, using early, normal or late tercile categories. Using a grid-point local rainfall onset index, we note that the forecast skill is highest over parts of central India, the Gangetic plains, and parts of coastal India—all zones of extensive agriculture in India. El Niño Southern Oscillation (ENSO) forcing in the model improves the forecast skill of monsoon onset when using a large-scale circulation index, with late monsoon onset coinciding with El Niño conditions and early monsoon onset more common in La Niña years. The results of this study suggest that GloSea5’s ensemble-mean forecast may be used for reliable Indian monsoon onset prediction a month in advance despite systematic model errors.
[en] The strength of the simultaneous linear relationship between El Niño/Southern Oscillation (ENSO) and Indian summer monsoon (ISM) precipitation show strong variations on a decadal timescale. While some studies attribute this to shift in the state of the climate and consequent teleconnection pattern, some other argue this as natural variability between two random time series. In this study, we show that the relationship between West African Summer Monsoon (WASM) precipitation with ENSO also experiences decadal timescale oscillation. While the ENSO–ISM relationship weakened during the past seven decades, ENSO–WASM relationship strengthened to above the 95% significance level. We explain this multi-decadal see-saw of strong–weak impact of ENSO on ISM and WASM through a common mechanism. ENSO impacts ISM and WASM rainfall by modulating the upper tropospheric temperature of subtropical Africa and South Asia. While the impact of ENSO on this temperature anomaly was strong and concentrated over the northwest of Indian region before the 1980, the anomalies are spatially discontinuous and weak after 1980. Moreover, a westward shift of the center of this anomaly after 1980 help strengthen the ENSO–WASM relationship. We also show a dramatic change in the relationship between Atlantic Niño and ENSO before and after the 1980s. While before 1980 ENSO did not have much impact on Atlantic Nino index-3 (ATL3), after 1980 El Niño (La Niña) is coincidental with negative (positive) ATL3 index. Since a negative (positive) ATL3 reduce (enhance) WASM by increased south-westerly moisture flux, the ENSO–WASM relationship strengthens after 1980. Our study suggests that the decadal variations of ENSO–ISM and ENSO–WASM relationship is physically linked and possibly could not be due to pure noise in the time series.
[en] The connections between European rainfall variability in winter (from January to March) and Northern Hemisphere circulation patterns, as represented by 500-hPa geopotential height, are investigated in the period 1900–2014. Initially, three statistically significant pairs of linearly-related circulation and precipitation patterns, explaining 45% of the variance in the latter field, are identified. The first two essentially represent the North Atlantic Oscillation (NAO) and the East Atlantic (EA) patterns, while the third corresponds to a structure described in recent literature as a blend of the Pacific North America (PNA) pattern and the so-called Asia Bering North America (ABNA) pattern. The residual precipitation field is then examined for patterns that can introduce non-linear modulations into the circulation-precipitation links. It is so found that the NAO impact on European rainfall is modulated by North American and Pacific factors controlling cyclogenesis over Newfoundland. The positive-phase EA rainfall anomalies over Central Europe and the British Isles seem to be markedly affected by the NAO phase. Finally, a possible signature from the Circumglobal Teleconnection Pattern disrupting the NAO influence on East Mediterranean precipitation anomalies is detected.
[en] The Atlantic meridional overturning circulation (AMOC) plays a fundamental role in the climate system, and long-term climate simulations are used to understand the AMOC variability and to assess its impact. This study examines the basic characteristics of the AMOC variability in 44 CMIP5 (Phase 5 of the Coupled Model Inter-comparison Project) simulations, using the 18 atmospherically-forced CORE-II (Phase 2 of the Coordinated Ocean-ice Reference Experiment) simulations as a reference. The analysis shows that on interannual and decadal timescales, the AMOC variability in the CMIP5 exhibits a similar magnitude and meridional coherence as in the CORE-II simulations, indicating that the modeled atmospheric variability responsible for AMOC variability in the CMIP5 is in reasonable agreement with the CORE-II forcing. On multidecadal timescales, however, the AMOC variability is weaker by a factor of more than 2 and meridionally less coherent in the CMIP5 than in the CORE-II simulations. The CMIP5 simulations also exhibit a weaker long-term atmospheric variability in the North Atlantic Oscillation (NAO). However, one cannot fully attribute the weaker AMOC variability to the weaker variability in NAO because, unlike the CORE-II simulations, the CMIP5 simulations do not exhibit a robust NAO-AMOC linkage. While the variability of the wintertime heat flux and mixed layer depth in the western subpolar North Atlantic is strongly linked to the AMOC variability, the NAO variability is not.