Contrasting global teleconnection features of the eastern Pacific and central Pacific El Niño events

Being triggered by different physical processes, the eastern Pacific (EP) and central Pacific (CP) El Niño events have several different teleconnection features around the globe. Using the ERA-Interim re-analysis monthly data during the period 1980–2016, the El Niño-Southern Oscillation (ENSO) teleconnections on the global scale and their statistical significance are investigated, with an emphasis on the contrasting features of the EP and CP El Niño events. With some exceptions, the EP El Niño and La Niña have generally similar teleconnection patterns with the reversed sign, while in some parts of the globe different and occasionally contrasting teleconnections of the EP and CP El Niño events are identified. Compared to the CP El Niño, more regions of the world are influenced by the statistically significant positive surface pressure anomalies during the EP El Niño, particularly over the Indian Ocean, tropical Atlantic and Northern Africa. It is found that the mid-tropospheric geopotential height anomalies across the globe are significantly different during the EP and CP El Niño events. Associated with different surface pressure and mid-tropospheric geopotential height anomalies, precipitation anomalies in many regions of the world are found different during the EP and CP El Niño events, particularly over the tropical Pacific, central to eastern equatorial Atlantic and the eastern Sahara. While central and eastern equatorial Atlantic experience statistically significant negative (positive) rainfall anomalies during the EP El Niño (La Niña), the CP El Niño does not have a strong influence on the amount of annual rainfall over the equatorial Atlantic. For the first time, statistically significant anomalously dry conditions are found over some parts of the Middle East and Southwest Asia during La Niña, and over the eastern Sahara during the EP El Niño.     

The asymmetric effects of El Niño and La Niña on the East Asian winter monsoon and their simulation by CMIP5 atmospheric models

El Niño–Southern Oscillation (ENSO) events significantly affect the year-by-year variations of the East Asian winter monsoon (EAWM). However, the effect of La Niña events on the EAWM is not a mirror image of that of El Niño events. Although the EAWM becomes generally weaker during El Niño events and stronger during La Niña winters, the enhanced precipitation over the southeastern China and warmer surface air temperature along the East Asian coastline during El Niño years are more significant. These asymmetric effects are caused by the asymmetric longitudinal positions of the western North Pacific (WNP) anticyclone during El Niño events and the WNP cyclone during La Niña events; specifically, the center of the WNP cyclone during La Niña events is westward-shifted relative to its El Niño counterpart. This central-position shift results from the longitudinal shift of remote El Niño and La Niña anomalous heating, and asymmetry in the amplitude of local sea surface temperature anomalies over the WNP. However, such asymmetric effects of ENSO on the EAWM are barely reproduced by the atmospheric models of Phase 5 of the Coupled Model Intercomparison Project (CMIP5), although the spatial patterns of anomalous circulations are reasonably reproduced. The major limitation of the CMIP5 models is an overestimation of the anomalous WNP anticyclone/cyclone, which leads to stronger EAWM rainfall responses. The overestimated latent heat flux anomalies near the South China Sea and the northern WNP might be a key factor behind the overestimated anomalous circulations.     

Extra-tropical atmospheric response to ENSO in the CMIP5 models

The seasonal mean extra-tropical atmospheric response to El Niño/Southern Oscillation (ENSO) is assessed in the historical and pre-industrial control CMIP5 simulations. This analysis considers two types of El Niño events, characterized by positive sea surface temperature (SST) anomalies in either the central equatorial Pacific (CP) or eastern equatorial Pacific (EP), as well as EP and CP La Niña events, characterized by negative SST anomalies in the same two regions. Seasonal mean geopotential height anomalies in key regions typify the magnitude and structure of the disruption of the Walker circulation cell in the tropical Pacific, upper tropospheric ENSO teleconnections and the polar stratospheric response. In the CMIP5 ensembles, the magnitude of the Walker cell disruption is correlated with the strength of the mid-latitude responses in the upper troposphere i.e., the North Pacific and South Pacific lows strengthen during El Niño events. The simulated responses to El Niño and La Niña have opposite sign. The seasonal mean extra-tropical, upper tropospheric responses to EP and CP events are indistinguishable. The ENSO responses in the MERRA reanalysis lie within the model scatter of the historical simulations. Similar responses are simulated in the pre-industrial and historical CMIP5 simulations. Overall, there is a weak correlation between the strength of the tropical response to ENSO and the strength of the polar stratospheric response. ENSO-related polar stratospheric variability is best simulated in the “high-top” subset of models with a well-resolved stratosphere.     

Changes in El Niño and La Niña teleconnections over North Pacific–America in the global warming simulations

The change in the teleconnections of both El Niño and La Niña over the North Pacific and American regions due to a future greenhouse warming has been analyzed herein by means of diagnostics of the Intergovernmental Panel on Climate Change-Fourth Assessment Report (IPCC-AR4) coupled general circulation models (CGCMs). Among the IPCC-AR4 CGCM simulations, the composites of the eight-member multimodel ensemble are analyzed. In most CGCMs, the tropical Pacific warming due to the increase of CO₂ concentration in the atmosphere promotes the main convection centers in the equatorial Pacific associated with both El Niño and La Niña to the east. The eastward shift of the convection center causes a systematic eastward shift of not only El Niño but also La Niña teleconnection patterns over the North Pacific and America, which is demonstrated in the composite maps of 500 hPa circulation, surface temperature, and the precipitation against El Niño and La Niña, as observed in a comparison between the pre-industrial and CO₂ doubling experiments. Thus, a systematic eastward migration of convection centers in the tropical Pacific associated with both El Niño and La Niña due to a future global warming commonly causes the eastward shift of the atmospheric teleconnection patterns over the Northern Hemisphere.     

Multi-model changes in El Niño teleconnections over North America in a future warmer climate

Previous studies with single models have suggested that El Niño teleconnections over North America could be different in a future warmer climate due to factors involving changes of El Niño event amplitude and/or changes in the midlatitude base state circulation. Here we analyze a six-member multi-model ensemble, three models with increasing future El Niño amplitude, and three models with decreasing future El Niño amplitude, to determine characteristics and possible changes to El Niño teleconnections during northern winter over the North Pacific and North America in a future warmer climate. Compared to observed El Niño events, all the models qualitatively produce general features of the observed teleconnection pattern over the North Pacific and North America, with an anomalously deepened Aleutian Low, a ridge over western North America, and anomalous low pressure over the southeastern United States. However, associated with systematic errors in the location of sea surface temperature and convective heating anomalies in the central and western equatorial Pacific (the models’ anomaly patterns are shifted to the west), the anomalous low pressure center in the North Pacific is weaker and shifted somewhat south compared to the observations. For future El Niño events, two different stabilization experiments are analyzed, one with CO₂ held constant at year 2100 concentrations in the SRES A1B scenario (roughly doubled present-day CO₂), and another with CO₂ concentrations held constant at 4XCO₂. Consistent with the earlier single model results, the future El Niño teleconnections are changed in the models, with a weakened as well as an eastward- and northward-shifted anomalous low in the North Pacific. This is associated with weakened anomalous warming over northern North America, strengthened cooling over southern North America, and precipitation increases in the Pacific Northwest in future events compared to present-day El Niño event teleconnections. These changes are consistent with the altered base state upper tropospheric circulation with a wave-5 pattern noted in previous studies that is shown here to be consistent across all the models whether there are projected future increases or decreases in El Niño amplitude. The future teleconnection changes are most consistent with this anomalous wave-5 pattern in the models with future increases of El Niño amplitude, but less so for the models with future decreases of El Niño amplitude.     

Summer monsoon circulation and precipitation over the tropical Indian Ocean during ENSO in the NCEP climate forecast system

This study investigates the El Niño Southern Oscillation (ENSO) teleconnections to tropical Indian Ocean (TIO) and their relationship with the Indian summer monsoon in the coupled general circulation model climate forecast system (CFS). The model shows good skill in simulating the impact of El Niño over the Indian Oceanic rim during its decay phase (the summer following peak phase of El Niño). Summer surface circulation patterns during the developing phase of El Niño are more influenced by local Sea Surface Temperature (SST) anomalies in the model unlike in observations. Eastern TIO cooling similar to that of Indian Ocean Dipole (IOD) is a dominant model feature in summer. This anomalous SST pattern therefore is attributed to the tendency of the model to simulate more frequent IOD events. On the other hand, in the model baroclinic response to the diabatic heating anomalies induced by the El Niño related warm SSTs is weak, resulting in reduced zonal extension of the Rossby wave response. This is mostly due to weak eastern Pacific summer time SST anomalies in the model during the developing phase of El Niño as compared to observations. Both eastern TIO cooling and weak SST warming in El Niño region combined together undermine the ENSO teleconnections to the TIO and south Asia regions. The model is able to capture the spatial patterns of SST, circulation and precipitation well during the decay phase of El Niño over the Indo-western Pacific including the typical spring asymmetric mode and summer basin-wide warming in TIO. The model simulated El Niño decay one or two seasons later, resulting long persistent warm SST and circulation anomalies mainly over the southwest TIO. In response to the late decay of El Niño, Ekman pumping shows two maxima over the southern TIO. In conjunction with this unrealistic Ekman pumping, westward propagating Rossby waves display two peaks, which play key role in the long-persistence of the TIO warming in the model (for more than a season after summer). This study strongly supports the need of simulating the correct onset and decay phases of El Niño/La Niña for capturing the realistic ENSO teleconnections. These results have strong implications for the forecasting of Indian summer monsoon as this model is currently being adopted as an operational model in India.     

Predictability of the western North Pacific summer climate associated with different ENSO phases by ENSEMBLES multi-model seasonal forecasts

Predictability of the western North Pacific (WNP) summer climate associated with different El Niño–Southern Oscillation (ENSO) phases is investigated in this study based on the 1-month lead retrospective forecasts of five state-of-the-art coupled models from ENSEMBLES. During the period from 1960 to 2005, the models well capture the WNP summer climate anomalies during most of years in different ENSO phases except the La Niña decaying summers. In the El Niño developing, El Niño decaying and La Niña developing summers, the prediction skills are high for the WNP summer monsoon index (WNPMI), with the prediction correlation larger than 0.7. The high prediction skills of the lower-tropospheric circulation during these phases are found mainly over the tropical western Pacific Ocean, South China Sea and subtropical WNP. These good predictions correspond well to their close teleconnection with ENSO and the high prediction skills of tropical SSTs. By contrast, for the La Niña decaying summers, the prediction skills are considerably low with the prediction correlation for the WNPMI near to zero and low prediction skills around the Philippines and subtropical WNP. These poor predictions relate to the weak summer anomalies of the WNPMI during the La Niña decaying years and no significant connections between the WNP lower-tropospheric circulation anomalies and the SSTs over the tropical central and eastern Pacific Ocean in observations. However, the models tend to predict an apparent anomalous cyclone over the WNP during the La Niña decaying years, indicating a linearity of the circulation response over WNP in the models prediction in comparison with that during the El Niño decaying years which differs from observations. In addition, the models show considerable capability in describing the WNP summer anomalies during the ENSO neutral summers. These anomalies are related to the positive feedback between the WNP lower-tropospheric circulation and the local SSTs. The models can capture this positive feedback but with some uncertainties from different ensemble members during the ENSO neutral summers.     

The seasonally-varying influence of ENSO on rainfall and tropical cyclone activity in the Philippines

An observational study covering the period 1950–2002 examines a seasonal reversal in the ENSO rainfall signal in the north-central Philippines. In boreal Summer of El Niño (La Niña) events, above (below) average rainfall typically occurs in this area. Rainfall anomalies of opposite sign develop across the country in the subsequent fall. This study investigates the seasonal evolution of the anomalous atmospheric circulation over the western North Pacific (WNP) during both El Niño and La Niña and places these features in the context of the large-scale evolution of ENSO events, including an analysis of changes in tropical cyclone activity affecting the Philippines. The results show that during boreal summer of El Niño (La Niña) events, a relatively narrow, zonally elongated band of enhanced (reduced) low-level westerlies develops across the WNP which serves to increase (decrease) the summer monsoon flow and moisture flux over the north-central Philippines and is associated with an increase (decrease) in the strength of the WNP monsoon trough via the anomalous relative vorticity. Tropical cyclone activity is shown to be enhanced (reduced) in the study region during boreal summer of El Niño (La Niña) events, which is related to the increase (decrease) of mid-level atmospheric moisture, as diagnosed using a genesis potential index. The subsequent evolution shows development of an anomalous anticyclone (cyclone) over the WNP in El Niño (La Niña) and the well-known tendency for below (above) average rainfall in the fall. Prolonged ENSO events also exhibit seasonal rainfall sign reversals in the Philippines with a similar evolution in atmospheric circulation.     

ENSO teleconnections in projections of future climate in ECHAM5/MPI-OM

The teleconnections of the El Niño/Southern Oscillation (ENSO) in future climate projections are investigated using results of the coupled climate model ECHAM5/MPI-OM. For this, the IPCC SRES scenario A1B and a quadrupled CO₂ simulation are considered. It is found that changes of the mean state in the tropical Pacific are likely to condition ENSO teleconnections in the Pacific North America (PNA) region and in the North Atlantic European (NAE) region. With increasing greenhouse gas emissions the changes of the mean states in the tropical and sub-tropical Pacific are El Niño-like in this particular model. Sea surface temperatures in the tropical Pacific are increased predominantly in its eastern part and redistribute the precipitation further eastward. The dynamical response of the atmosphere is such that the equatorial east–west (Walker) circulation and the eastern Pacific inverse Hadley circulation are decreased. Over the subtropical East Pacific and North Atlantic the 200 hPa westerly wind is substantially increased. Composite maps of different climate parameters for positive and negative ENSO events are used to reveal changes of the ENSO teleconnections. Mean sea level pressure and upper tropospheric zonal winds indicate an eastward shift of the well-known teleconnection patterns in the PNA region and an increasing North Atlantic oscillation (NAO) like response over the NAE region. Surface temperature and precipitation underline this effect, particularly over the North Pacific and the central North Atlantic. Moreover, in the NAE region the 200 hPa westerly wind is increasingly related to the stationary wave activity. Here the stationary waves appear NAO-like.     

Volcanoes and ENSO in millennium simulations: global impacts and regional reconstructions in East Asia

The impacts and cooperative effects of volcanic eruptions and ENSO (El Niño/Southern Oscillation) are analyzed in a millennium simulation for 800–2005 AD using the earth system model (ESM) ECHAM5/MPIOM/JSBACH subject to anthropogenic and natural forcings. The simulation comprises two ensembles, a first with weak (E1, five members) and a second with strong (E2, three members) variability total solar irradiance. In the analysis, the 21 most intense eruptions are selected in each ensemble member. Volcanoes with neutral ENSO states during two preceding winters cause a global cooling in the year after eruptions up to ?2.5°C. The nonsignificant positive values in the tropical Pacific Ocean indicate an El Niño-like warming. In the winter after an eruption, warming is mainly found in the Arctic Ocean and the Bering Sea in E2 warming extends to Siberia and central Asia. The recovery times for the volcano-induced cooling (average for 31 eruptions) vary globally between 1 and 12 years. There is no significant increase of El Niño events after volcanic eruptions in both ensembles. The simulated temperature and the drought indices are compared with corresponding reconstructions in East Asia. Volcanoes cause a dramatic cooling in west China (?2°C) and a drought in East China during the year after the eruption. The reconstructions show similar cooling patterns with smaller magnitudes and confirm the dryness in East China. Without volcanoes, El Niño events reduce summer precipitation in the North, while South China becomes wetter; La Niña events cause opposite effects. El Niño events in the winters after eruptions compensate the cooling caused by volcanoes in most regions of China (consistent with reconstructions), while La Niña events intensify the cooling (up to ?2.5°C). The simulated and reconstructed drought indices show tripole patterns which are altered by El Niño events. The simulated impact of the Tambora eruption in 1815, which caused the “year without summer” of 1816 in Europe and North America and led to coldness and famines in the Chinese province Yunnan, depends crucially on the ENSO state of the coupled model. A comparison with reconstructed El Niño events shows a moderate cool climate with wet (in the south) and extreme dry anomalies (in the north) persisting for several years.     

Quasi-3-yr Cycle of Rainy Season Precipitation in Tibet Related to Different Types of ENSO during 1981–2015

The rainy season precipitation in Tibet (RSPT) is a direct cause for local floods/droughts. It also indirectly affects the thermal conditions of the Tibetan Plateau, which can result in anomalous patterns of atmospheric circulation over East Asia. The interannual variability of the RSPT is often linked with the El Niño–Southern Oscillation (ENSO), but the relevant mechanisms are far from being understood, particularly for different types of ENSO events. We investigated the interannual variation of the RSPT in association with different types of ENSO. A quasi-3-yr period of the RSPT (less–more–more precipitation) was significant at the 95% confidence level. A joint multi-taper method with singular value decomposition analysis of the coupled field between the RSPT and the sea surface temperature (SST) revealed that the developing eastern Pacific type El Niño was accompanied by a decrease in the RSPT. The shift from the central Pacific type El Niño to the eastern Pacific La Niña was accompanied by an increase in the RSPT. Weakening of the central Pacific La Niña was accompanied by an increase in the RSPT. Analysis of the mechanism of this coupling, using the same analysis method but other climatic factors, indicated that the gradually strengthening eastern Pacific El Niño can inhibit the Walker circulation, weakening the South Asian summer monsoon, and resulting in transport of less water vapor from the Bay of Bengal to Tibet. The change from the central Pacific El Niño to the eastern Pacific La Niña led to continued strengthening of the Walker circulation with westward movement of the ascending area. This enhanced the South Asian summer monsoon over the Arabian Sea and transported more water vapor to Tibet. The decreasing central Pacific La Niña accompanied by persistent cooling of SSTs in the equatorial Pacific led to a strong eastern North Pacific summer monsoon, causing an anomaly in the easterly transport of water vapor from the Sea of Japan to Tibet and increased RSPT.     

ENSO evolution asymmetry: EP versus CP El Niño

Through an oceanic mixed-layer heat budget analysis, the dominant processes contributing to the largest decay rate (− 0.37 °C/mon) in EP El Nino, the moderate delay rate (− 0.22 °C/mon) in CP El Nino and the smallest decay rate (0.13 °C/mon) in La Nina, are identified. The result shows that both dynamic (wind induced equatorial ocean waves and thermocline changes) and thermodynamic (net surface solar radiation and latent heat flux changes) processes contribute to a fast decay and thus phase transition in EP El Niño composite, whereas the thermodynamic process has less effect on the decay rate for both CP El Niño and La Niña due to the westward shift of sea surface temperature anomaly (SSTA) centers. Thus, the difference in surface wind stress forcing is critical in contributing to evolution asymmetry between CP El Niño and La Niña, while the difference in both the wind stress and heat flux anomalies contribute to evolution asymmetry between EP El Niño and La Niña. It is interesting to note that El Nino induced anomalous anticyclone over the western North Pacific is stronger and shifts more toward the east during EP El Niño than during CP El Niño, while compared to CP El Niño, the center of an anomalous cyclone during La Niña shifts further to the west. As a consequence, both EP and CP El Niño decay fast and transform into a La Niña episode in the subsequent year, whereas La Niña has a much slower decay rate and re-develops in the second year.

     

Analysis of the non-linearity in the pattern and time evolution of El Niño southern oscillation

In this study the observed non-linearity in the spatial pattern and time evolution of El Niño Southern Oscillation (ENSO) events is analyzed. It is shown that ENSO skewness is not only a characteristic of the amplitude of events (El Niños being stronger than La Niñas) but also of the spatial pattern and time evolution. It is demonstrated that these non-linearities can be related to the non-linear response of the zonal winds to sea surface temperature (SST) anomalies. It is shown in observations as well as in coupled model simulations that significant differences in the spatial pattern between positive (El Niño) versus negative (La Niña) and strong versus weak events exist, which is mostly describing the difference between central and east Pacific events. Central Pacific events tend to be weak El Niño or strong La Niña events. In turn east Pacific events tend to be strong El Niño or weak La Niña events. A rotation of the two leading empirical orthogonal function modes illustrates that for both El Niño and La Niña extreme events are more likely than expected from a normal distribution. The Bjerknes feedbacks and time evolution of strong ENSO events in observations as well as in coupled model simulations also show strong asymmetries, with strong El Niños being forced more strongly by zonal wind than by thermocline depth anomalies and are followed by La Niña events. In turn strong La Niña events are preceded by El Niño events and are more strongly forced by thermocline depth anomalies than by wind anomalies. Further, the zonal wind response to sea surface temperature anomalies during strong El Niño events is stronger and shifted to the east relative to strong La Niña events, supporting the eastward shifted El Niño pattern and the asymmetric time evolution. Based on the simplified hybrid coupled RECHOZ model of ENSO it can be shown that the non-linear zonal wind response to SST anomalies causes the asymmetric forcings of ENSO events. This also implies that strong El Niños are mostly wind driven and less predictable and strong La Niñas are mostly thermocline depth driven and better predictable, which is demonstrated by a set of 100 perfect model forecast ensembles.     

Impacts of the central and eastern Pacific types of ENSOon sea surface temperature in the South Pacific

The present study examines the relationship between two types of El Niño–Southern Oscillation (ENSO), the central Pacific (CP) ENSO and the eastern Pacific (EP) ENSO, and the sea surface temperature (SST) variability over the South Pacific (SP) (20° S–60° S, 145° E–70° W) using NOAA OI SST for the period 1982–2006. The SP SST variability associated with the two types of ENSO varies with season. These two types of ENSO can excite different atmospheric patterns associated with the Pacific–South American mode, through which they influence the SP SST variability. Both the surface turbulent air–sea heat fluxes and the heat advection by Ekman currents (i.e., Ekman heat fluxes) have an important impact on the SST variability. An analysis of the surface mixed layer heat budget indicates that the heat fluxes (the sum of turbulent heat fluxes and Ekman heat fluxes) can effectively explain much of the SST variability related to the two types of ENSO.     

Cross-season relation of the South China Sea precipitation variability between winter and summer

The present study reveals cross-season connections of rainfall variability in the South China Sea (SCS) region between winter and summer. Rainfall anomalies over northern South China Sea in boreal summer tend to be preceded by the same sign rainfall anomalies over southern South China Sea in boreal winter (denoted as in-phase relation) and succeeded by opposite sign rainfall anomalies over southern South China Sea in the following winter (denoted as out-of-phase relation). Analysis shows that the in-phase relation from winter to summer occurs more often in El Niño/La Niña decaying years and the out-of-phase relation from summer to winter appears more frequently in El Niño/La Niña developing years. In the summer during the El Niño/La Niña decaying years, cold/warm and warm/cold sea surface temperature (SST) anomalies develop in tropical central North Pacific and the North Indian Ocean, respectively, forming an east–west contrast pattern. The in-phase relation is associated with the influence of anomalous heating/cooling over the equatorial central Pacific during the mature phase of El Niño/La Niña events that suppresses/enhances precipitation over southern South China Sea and the impact of the above east–west SST anomaly pattern that reduces/increases precipitation over northern South China Sea during the following summer. The impact of the east–west contrast SST anomaly pattern is confirmed by numerical experiments with specified SST anomalies. In the El Niño/La Niña developing years, regional air-sea interactions induce cold/warm SST anomalies in the equatorial western North Pacific. The out-of-phase relation is associated with a Rossby wave type response to anomalous heating/cooling over the equatorial central Pacific during summer and the combined effect of warm/cold SST anomalies in the equatorial central Pacific and cold/warm SST anomalies in the western North Pacific during the mature phase of El Niño/La Niña events.     

The spatio-temporal characteristics of total rainfall during September in South Korea according to the variation of ENSO

A significant negative correlation between the total rainfall averaged over South Korea and the Niño-3.4 index was found for the month of September. To find out the reason for this negative correlation, composite analyses were carried out for the highest and lowest 8 years of the Niño-3.4 index. During the strong El Niño year, an anomalous anticyclone occurs in the continental East Asia, while an anomalous cyclone emerges in the subtropical western Pacific. The resultant eastward pressure gradient force induces anomalous northerlies in most regions of East Asia, which produces anomalous cold and dry conditions throughout the troposphere between 120° and 140°E, reducing the Korean rainfall. It is also found that during El Niño year, tropical cyclones (TCs) tend to recurve far east offshore of Japan because the weakening of the western North Pacific subtropical high (WNPSH). During La Niña years, on the other hand, the strengthening and westward extension of the WNPSH render more TCs influencing the Korean peninsula. Therefore, the TC track changes associated with El Niño-Southern Oscillation is another contributor to change of the Korean rainfall.     

La Niña diversity and Northwest Indian Ocean Rim teleconnections

The differences in tropical Pacific sea surface temperature (SST) expressions of El Niño-Southern Oscillation (ENSO) events of the same phase have been linked with different global atmospheric circulation patterns. This study examines the dynamical forcing of precipitation during October–December (OND) and March–May (MAM) over East Africa and during December–March (DJFM) over Central-Southwest Asia for 1950–2010 associated with four tropical Pacific SST patterns characteristic of La Niña events, the cold phase of ENSO. The self-organizing map method along with a statistical distinguishability test was used to isolate La Niña events, and seasonal precipitation forcing was investigated in terms of the tropical overturning circulation and thermodynamic and moisture budgets. Recent La Niña events with strong opposing SST anomalies between the central and western Pacific Ocean (phases 3 and 4), force the strongest global circulation modifications and drought over the Northwest Indian Ocean Rim. Over East Africa during MAM and OND, subsidence is forced by an enhanced tropical overturning circulation and precipitation reductions are exacerbated by increases in moisture flux divergence. Over Central-Southwest Asia during DJFM, the thermodynamic forcing of subsidence is primarily responsible for precipitation reductions, with moisture flux divergence acting as a secondary mechanism to reduce precipitation. Eastern Pacific La Niña events in the absence of west Pacific SST anomalies (phases 1 and 2), are associated with weaker global teleconnections, particularly over the Indian Ocean Rim. The weak regional teleconnections result in statistically insignificant precipitation modifications over East Africa and Central-Southwest Asia.     

Interdecadal changes in the relationship between Southern China winter-spring precipitation and ENSO

Winter-spring precipitation in southern China tends to be higher (lower) than normal in El Niño (La Niña) years during 1953–1973. The relationship between the southern China winter-spring precipitation and El Niño-Southern Oscillation (ENSO) is weakened during 1974–1994. During 1953–1973, above-normal southern China rainfall corresponds to warmer sea surface temperature (SST) in the equatorial central Pacific. There are two anomalous vertical circulations with ascent over the equatorial central Pacific and ascent over southern China and a common branch of descent over the western North Pacific that is accompanied by an anomalous lower-level anticyclone. During 1974–1994, above-normal southern China rainfall corresponds to warmer SST in eastern South Indian Ocean and cooler SST in western South Indian Ocean. Two anomalous vertical circulations act to link southern China rainfall and eastern South Indian Ocean SST anomalies, with ascent over eastern South Indian Ocean and southern China and a common branch of descent over the western North Pacific. Present analysis shows that South Indian Ocean SST anomalies can contribute to southern China winter-spring precipitation variability independently. The observed change in the relationship between southern China winter-spring rainfall and ENSO is likely related to the increased SST variability in eastern South Indian Ocean and the modulation of the Pacific decadal oscillation.     

Role of the Indian Ocean sea surface temperature in shaping the natural variability in the flow of Nile River

A significant fraction of the inter-annual variability in the Nile River flow is shaped by El Niño Southern Oscillation (ENSO). Here, we investigate a similar role for the Indian Ocean (IO) sea surface temperature (SST) in shaping the inter-annual variability of the Nile River flow. Using observations of global SST distribution and river flow in addition to atmospheric general circulation model sensitivity experiments, we show that North and Middle IO SSTs play a significant intermediate role in the teleconnection between ENSO and the Nile flow. Applying partial coherency analyses, we demonstrate that the connection between North and Middle IO SSTs and Nile flow is strongly coupled to ENSO. During El Niño events, SST in the North and Middle IO increases in response to the warming in the Tropical Eastern Pacific Ocean and forces a Gill-type circulation with enhanced westerly low-level flow over East Africa and the Western IO. This anomalous low-level flow enhances the low-level flux of air and moisture away from the Upper Blue Nile (UBN) basin resulting in reduction of rainfall and river flow. SSTs in the South IO also play a significant role in shaping the variability of the Nile flow that is independent from ENSO. A warming over the South IO, generates a cyclonic flow in the boundary layer, which reduces the cross-equatorial meridional transport of air and moisture towards the UBN basin, favoring a reduction in rainfall and river flow. This independence between the roles of ENSO and South IO SSTs allows for development of new combined indices of SSTs to explain the inter-annual variability of the Nile flow. The proposed teleconnections have important implications regarding mechanisms that shape the regional impacts of climate change over the Nile basin.     

Combined effect of El Niño-Southern Oscillation and Pacific Decadal Oscillation on the East Asian winter monsoon

Using long-term observational data and numerical model experiments, the combined effect of the El Niño-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) on the variability of the East Asian winter monsoon is examined. In the observations, it is found that when the ENSO and PDO are in-phase combinations (i.e., El Niño/positive PDO phase and La Niña/negative PDO phase), a negative relationship between ENSO and East Asian winter monsoon is significantly intensified. In other words, when El Niño (La Niña) occurs with positive (negative) PDO phase, anomalous warm (cold) temperatures are dominant over the East Asian winter continent. On the other hand, there are no significant temperature anomalies when the ENSO and PDO are out-of-phase combinations (i.e., El Niño/negative PDO phase and La Niña/positive PDO phase). Further analyses indicate that the anticyclone over the western North Pacific including the East Asian marginal seas plays an essential role in modulating the intensity of the East Asian winter monsoon under the changes of ENSO–PDO phase relationship. Long-lasting high pressure and warm sea surface temperature anomalies during the late fall/winter and following spring over the western North Pacific, which appear as the El Niño occurs with positive PDO phase, can lead to a weakened East Asian winter monsoon by transporting warm and wet conditions into the East Asian continent through the southerly wind anomalies along the western flank of the anomalous high pressure, and vice versa as the La Niña occurs with negative PDO phase. In contrast, the anomalous high pressure over the western North Pacific does not show a prominent change under the out-of-phase combinations of ENSO and PDO. Numerical model experiments confirm the observational results, accompanying dominant warm temperature anomalies over East Asia via strong anticyclonic circulation anomalies near the Philippine Sea as the El Niño occurs with positive PDO phase, whereas such warming is weakened as the El Niño occurs with negative PDO phase. This result supports the argument that the changes in the East Asian winter monsoon intensity with ENSO are largely affected by the strength of the anticyclone over the western North Pacific, which significantly changes according to the ENSO–PDO phase relationship.