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[en] We examine the connection between interannual anomalies of sea surface temperature (SST) in the central and far-eastern equatorial Pacific associated with basin-scale and coastal El Niños. Variations of the SST anomalies in these two regions are largely coherent, meaning coastal El Niños mostly occur together with the commonly studied basin-scale El Niños. Of particular interest for this study though is the understanding of the coastal El Niños that are not accompanied by basin-scale El Niños or that follow basin-scale El Niños. Such coastal El Niños can have catastrophic societal consequences in western South America. We identify seven coastal El Niños during 1979–2017, namely 1983, 1987, 1998, 2008, 2014, 2015, and 2017. These coastal El Niños are driven by different mechanisms. The coastal El Niños in 1983, 1987 and 1998 occurred after basin-scale El Niños. A unique feature of such extreme basin-scale El Niños like in 1982–1983, 1986–1987, and 1997–1998 is an equatorially centered intertropical convergence zone during its decaying phase. As a result, positive SST anomalies persist, and sometimes even strengthen, in the eastern Pacific in the subsequent boreal spring/early-summer, leading to coastal El Niños. The coastal El Niños in 2014 and 2015 on the other hand resulted from westerly wind bursts in the western Pacific that forced downwelling Kelvin waves and a thermocline depression in the far eastern Pacific. The formation of coastal El Niños in 2008 and 2017 were associated with westerly surface wind anomalies in the eastern equatorial Pacific and largely driven by ocean surface heat flux anomalies. These two coastal El Niños occur during the warm phase of the seasonal cycle, so that warm SSTs are amplified and/or the warm season is extended along the west coast of South America. Thus, there is a wide variety of the coastal El Niños in terms of evolution, mechanism, and timing.
[en] Recent research on the El Niño–Southern Oscillation (ENSO) phenomenon increasingly reveals the highly complex and diverse nature of ENSO variability. A method of quantifying ENSO spatial pattern uniqueness and diversity is presented, which enables (1) formally distinguishing between unique and “canonical” El Niño events, (2) testing whether historical model simulations aptly capture ENSO diversity by comparing with instrumental observations, (3) projecting future ENSO diversity using future model simulations, (4) understanding the dynamics that give rise to ENSO diversity, and (5) analyzing the associated diversity of ENSO-related atmospheric teleconnection patterns. Here we develop a framework for measuring El Niño spatial SST pattern uniqueness and diversity for a given set of El Niño events using two indices, the El Niño Pattern Uniqueness (EPU) index and El Niño Pattern Diversity (EPD) index, respectively. By applying this framework to instrumental records, we independently confirm a recent regime shift in El Niño pattern diversity with an increase in unique El Niño event sea surface temperature patterns. However, the same regime shift is not observed in historical CMIP5 model simulations; moreover, a comparison between historical and future CMIP5 model scenarios shows no robust change in future ENSO diversity. Finally, we support recent work that asserts a link between the background cooling of the eastern tropical Pacific and changes in ENSO diversity. This robust link between an eastern Pacific cooling mode and ENSO diversity is observed not only in instrumental reconstructions and reanalysis, but also in historical and future CMIP5 model simulations.
[en] The decadal modulation of East China winter precipitation by the El Niño-Southern Oscillation (ENSO) is examined using both observational data and coupled global climate model simulations. The co-variability between 68-year (1948–2015) observed East China precipitation and tropical Pacific sea surface temperature (SST) is quantified by the singular value decomposition (SVD) method. The first SVD mode relates Southeast China winter pluvial (drought) to the tropical Pacific El Niño (La Niña) SST. A comparison between two 480-year model simulations with and without ENSO suggests that ENSO can modulate both the intensity and frequency of East China winter precipitation. In the presence of ENSO, maximum precipitation anomalies over Southeast China can be increased by 50% and largely on the interannual timescale (3–6 years). It is also demonstrated that there is an asymmetry in the precipitation and circulation responses to warm and cold phases of ENSO. The responses are sensitive to the intensity of SST anomalies during El Niño, but less sensitive to SSTs during La Niña. This sensitivity, together with the decadal variations of ENSO, helps understand the observed decadal changes in the strength of the association between wintertime tropical Pacific SST and East China precipitation. The association is relatively weak during 1948–1977 when La Niña occurred more frequently, but strong during 1978–1999 when El Niño occurred more frequently. In the last 16 years (2000–2015) the association is weakest, likely due to the weakened variability of tropical Pacific SST since 2000.
[en] Recurrent convection regimes are identified during the extended West African Monsoon (WAM) season (May–Nov) using a clustering of 1980–2013 NOAA daily Outgoing Longwave Radiation (OLR), and are well reproduced in 1996–2015 ECMWF week-1 reforecasts despite systematic biases. One regime of broad drying across the Sahel in the early (May–Jun) and late (Oct) WAM is of particular interest regarding the prediction of onset date. This regime is associated with an anticyclonic cell along the Atlantic coast of West Africa leading to a weakened monsoon flow and subsiding anomalies across the Sahel. Teleconnections of this regime with the Indian monsoon sector are identified through modulations of the Walker circulation alongside relationships to MJO phase 3 more than 10 days in advance, when convection is enhanced over the Indian Ocean. Other regimes are associated with westward propagating anomalous convective cells along two distinct wave trains at and during the core (Jul–Sep) and late (Oct–Nov) WAM, respectively, and translate into wet anomalies transiting across the Sahel. A regime of broad Sahel wetting in the core WAM, more frequent since the 1990s, is related to global SST warming, agreeing with the observed recovery of Sahel rainfall. ECMWF skill in forecasting regime sequences decreases from week-1 to -4 leads, except in the case of the above-mentioned regime associated with early season dry spells, translating into the potential for skillful WAM onset date predictions. Our analysis suggests that sources of predictability include relationships to the MJO and the Indian monsoon sector, which need to be further examined to benefit subseasonal forecasting efforts in West Africa, and ultimately agricultural planning and food security across the Sahel.
[en] This study undertakes a multi-model comparison with the aim to describe and quantify systematic changes of the global energy and water budgets when the horizontal resolution of atmospheric models is increased and to identify common factors of these changes among models. To do so, we analyse an ensemble of twelve atmosphere-only and six coupled GCMs, with different model formulations and with resolutions spanning those of state-of-the-art coupled GCMs, i.e. from resolutions coarser than 100 km to resolutions finer than 25 km. The main changes in the global energy budget with resolution are a systematic increase in outgoing longwave radiation and decrease in outgoing shortwave radiation due to changes in cloud properties, and a systematic increase in surface latent heat flux; when resolution is increased from 100 to 25 km, the magnitude of the change of those fluxes can be as large as 5 W m−2. Moreover, all but one atmosphere-only model simulate a decrease of the poleward energy transport at higher resolution, mainly explained by a reduction of the equator-to-pole tropospheric temperature gradient. Regarding hydrological processes, our results are the following: (1) there is an increase of global precipitation with increasing resolution in all models (up to 40 × 103 km3 year−1) but the partitioning between land and ocean varies among models; (2) the fraction of total precipitation that falls on land is on average 10% larger at higher resolution in grid point models, but it is smaller at higher resolution in spectral models; (3) grid points models simulate an increase of the fraction of land precipitation due to moisture convergence twice as large as in spectral models; (4) grid point models, which have a better resolved orography, show an increase of orographic precipitation of up to 13 × 103 km3 year−1 which explains most of the change in land precipitation; (5) at the regional scale, precipitation pattern and amplitude are improved with increased resolution due to a better simulated seasonal mean circulation. We discuss our results against several observational estimates of the Earth's energy budget and hydrological cycle and show that they support recent high estimates of global precipitation.
[en] El Niño, due to its global impact on weather patterns, ecosystems, agriculture and public health, has become as commonly known to the public as the recent global warming. But why we have El Niño is not yet as well answered as it may have been assumed. Linear theories have been successful in explaining the transition from the warm phase to the cold phase of the eastern tropical Pacific that results from the rise and fall of El Niño, but failed to explain the asymmetry between the two phases. A nonlinear theory for El Niño has suggested that there exist two equilibrium states for the tropical Pacific—one is zonally symmetric (or nearly so) with the warm-pool extending all the way to the eastern Pacific, and the other is strongly zonally asymmetric with the warm-pool confined to the western half of the tropical Pacific. Under this hypothesis, ENSO results from the fact that under the current radiative heating, both states are unstable, resulting in the apparent “wandering” behavior in between these two states as seen in the observations. To test this hypothesis, the authors have obtained the best approximations for the two equilibrium states empirically using updated ocean assimilation data, and quantified the stability of these two empirically obtained equilibrium states using two stability analysis methods. The results suggest that the two states are unstable, offering support for the nonlinear view of why we have El Niño.
[en] Noting a strong imperative to understand precipitation extremes, and that considerable uncertainty affects observational data sets, this paper compares the representation of extremes in a number of widely used daily gridded products, derived from rain gauge data, satellite retrieval and reanalysis for the conterminous United States. Analysis is based upon the concept of “tail dependence” arising in multivariate extreme value theory, and we infer the level of temporal dependence in the joint tail of the precipitation probability distribution for pairwise comparisons of products. In this way, we consider the range of products more like an ensemble and examine the relationships between members, and do not attempt to define, or compare products to, some ground truth. Linear correlation between products is also computed. Considerable discrepancy between groups of products, both annually and seasonally, is linked to source data and complex terrain. In particular, products based on rain gauge data showed remarkable similarity, but differed considerably, showing almost total loss of extremal dependence during DJF in mountainous regions, when compared with satellite products. Additionally, simulated re-forecasts revealed reasonable temporal agreement with large scale generated extremes. The diversity and extent of discrepancies identified across all products raises important questions about their use, and we urge caution, particularly for products derived from satellite data.
[en] The Gulf of Maine is undergoing rapid environmental and ecological changes, yet our spatial and temporal understanding of the climatic and hydrographic variability in this region, including extreme events, is limited and biased to recent decades. In this study, we utilize a highly replicated, multi-century master shell growth chronology derived from the annual increments formed in the shells of the long-lived bivalve Arctica islandica collected in 38 m from the central coastal region in the Gulf of Maine. Our results indicate that shell growth is highly synchronous and inversely related to local seawater temperatures. Using composite analyses of extreme shell growth events from CE 1900 to 2013, we extend our understanding of the factors driving oceanic variability and shell growth in the Northwestern Atlantic back to CE 1761. We suggest that extreme shell growth events are primarily controlled by Gulf of Maine sea surface temperature (SST) and stratification conditions, which in turn appear to be largely influenced by SST patterns in the Pacific Ocean through their influence on mid-latitude atmospheric circulation patterns and the location of the eddy-driven jet. The large-scale jet dynamics during these extreme years manifest as precipitation and moisture transport anomalies and regional SST conditions in the Gulf of Maine that either enhance or inhibit shell growth. Pacific climate variability is thus an important, yet understudied, influence on Gulf of Maine ocean conditions.
[en] In this paper we investigate the evolution of moderate El Niño events during their developing phase with the objective to understand why some of them did not evolve as extreme events despite favourable conditions for the non-linear amplification of the Bjerknes feedback (i.e. warm SST in Austral winter in the eastern equatorial Pacific). Among the moderate events, two classes are considered consisting in the Eastern Pacific (EP) El Niño events and Central Pacific (CP) events. We first show that the observed SST variability across moderate El Niño events (i.e. inter-event variability) is largest in the far eastern Pacific (east of ~ 130°W) in the Austral winter prior to their peak, which is associated to either significant warm anomaly (moderate EP El Niño) or an anomaly between weak warm and cold (moderate CP El Niño) as reveals by the EOF analysis of the SST anomaly evolution during the development phase of El Niño across the El Niño years. Singular value decomposition (SVD) analysis of SST and wind stress anomalies across the El Niño years further indicates that the inter-event SST variability is associated with an air–sea mode explaining 31% of the covariance between SST and wind stress. The associated SST pattern consists in SST anomalies developing along the coast of Ecuador in Austral fall and expanding westward as far as 130°W in Austral winter. The associated wind stress pattern features westerlies (easterlies) west of ~ 130°W along the equator peaking around June–August for EP (CP) El Niño events. This air–sea mode is interpreted as resulting from a developing seasonal Bjerknes feedback for EP El Niño events since it is shown to be associated to a Kelvin wave response at its peak phase. However equatorial easterlies east of 130°W emerge in September that counters the growing SST anomalies associated to the air–sea mode. These have been particularly active during both the 1972 and the 2015 El Niño events. It is shown that the easterlies are connected to an off-equatorial southerly wind off the coast of Peru and Ecuador. The southerly wind is a response to the coastal SST anomalies off Peru developing from Austral fall. Implications of our results for the understanding of the seasonal ENSO dynamics and diversity are discussed in the light of the analysis of two global climate models simulating realistically ENSO diversity (GFDL_CM2.1 and CESM).
[en] Based on previous study by Xu and Chan (J Clim 14:418–433, 2001), two types of El Niño distinguished by the onset time, a Spring (SP) type and a Summer (SU) type, have been investigated from 1871 through 2011. As can be classified by the spatial patterns of sea surface temperature anomaly into the Warm Pool (WP) and Cold Tongue (CT) El Niño, the temporal features of the CT are dominated by the SP events whereas the SU events mostly display the spatial pattern of WP or Mixed events. The approximate 140-year data analysis shows that the frequency of SP events tends to increase in the most recent 30 years (1980–2009) while the SU events show very strong activity in the beginning of the twentieth century (1900–1929), which are closely associated with the decadal changes in oceanic and atmospheric background conditions. The air-sea processes indicate that the pattern of sea surface temperature (SST) gradient between tropical and extratropical Pacific Ocean on decadal time scales is related to the sea level pressure distribution, which tends to produce wind anomalies. The wind anomalies in turn affect the SST anomalies on inter-annual time scales over the equatorial areas and finally result in the early onset of El Niño in SP time or late onset of El Nino in SU time. A spring onset El Niño favors a Kelvin wave that propagates across the basin and a summer onset favors a Kelvin wave that does not traverse the basin or the related effects are not strong enough. The early or late onset of El Niño significantly impacts the precipitation distribution correlated with the monsoon systems including the Asian–Australian monsoon and North–South American monsoon. The El Niño–monsoon relationship is modulated by decadal changes in atmospheric and oceanic background conditions. The precipitation in the monsoonal area circling the Pacific Ocean exhibits characteristic quasi-biennial variations that are closely associated with the onset time of El Niño events, especially with the early onset of El Niño. For the Spring (SP) type, drought is observed over the central China, Australia, southwestern North America and northern South America in boreal summer, but the opposite pattern appears in the subsequent summer of the following year.