Negative Atlantic Multidecadal Oscillation Phase Research Articles

Asymmetrical Modulation of the Relationship Between the Western Pacific Pattern and El Niño–Southern Oscillation by the Atlantic Multidecadal Oscillation in the Boreal Winter

AbstractTropical atmospheric convection generated by the El Niño–Southern Oscillation (ENSO) plays a crucial role in affecting the western Pacific pattern (WP) in the boreal winter by triggering an atmospheric teleconnection. Here we show from analysis of observations and model simulations that the Atlantic Multidecadal Oscillation (AMO) asymmetrically modulates the relationship between ENSO and WP. We find a significant modulatory effect of AMO on the relationship between wintertime El Niño and WP. A robust El Niño−WP relation can be attributed to the negative AMO phase (−AMO), yet a weak relationship during the positive AMO phase (+AMO). In contrast, the relationship between La Niña and WP is independent of AMO modulation. Furthermore, during the −AMO period, stronger El Niño amplitudes lead to stronger atmospheric convection anomalies over the tropical western North Pacific, which excites stronger atmospheric teleconnection and thus has a more significant effect on WP than during the +AMO period.

Modulation of Tropical Cyclone Tracks and Rainfall by the North Atlantic Oscillation

AbstractUnderstanding and quantifying the large‐scale environmental control of spatio‐temporal tropical cyclone (TC) variability beyond a few weeks remains a challenge with significant implications for societal impacts. This study focuses on the relationship between the North Atlantic Oscillation (NAO) and year‐to‐year changes in TC activity and rainfall using observational data and reanalysis products. We use Poisson regression models to show that low‐frequency NAO (LF‐NAO) variability is associated with a distinct pattern of TC activity, which extends across the western North Atlantic, the Caribbean Basin, and the Gulf of Mexico. The negative LF‐NAO phase is characterized by enhanced TC activity: an interquartile range decrease in the NAO corresponds to a 30%–40% increase in TC track density. While the NAO is known to affect the weather regimes of the mid‐latitudes, we show that its low‐frequency component has a strong correlation to the large‐scale environment across the Main Development Region of TCs. The negative LF‐NAO phase is associated with two favorable environmental conditions for TCs: significantly higher sea‐surface temperature and weaker deep‐tropospheric wind shear. The LF‐NAO relationship to TC activity is strongest during El Niño Southern Oscillation positive or neutral conditions and during the negative Atlantic Multidecadal Oscillation phase, and it can also be detected in the basin‐scale variations of TC rainfall. By developing annual TC rainfall composites from satellite data and reanalysis products, we show that TC rainfall is strongly enhanced in the Caribbean and in the Gulf of Mexico during the negative LF‐NAO phase.

WINTER ATMOSPHERIC BLOCKINGS IN THE NORTHERN HEMISPHERE UNDER CLIMATE CHANGES IN RECENT DECADES (1980–2018)

The analysis of winter atmospheric blockings in the Northern Hemisphere (NH) under climate changes in 1980–2018 was carried out. The results of the analysis indicate a significant increase in the total duration of winter atmospheric blockings τ in the NH in recent decades. The noted trend was revealed against the background of a rapid increase in the NH surface air temperature T in winter seasons. The parameter of sensitivity of the atmospheric blockings duration to changes in the NH surface air temperature in winter seasons for the period 1980–2018 was estimated. The statistically significant estimate obtained means an increase in τ by more than one and a half times with an increase in the mean winter surface air temperature of the NH by 1K. A more significant relationship with T was obtained for values of τ above the average value of τc for the period 1980–2018, while at τ τc, the relationship was found to be statistically insignificant. The noted features are associated with differences in different phases of the key modes of climatic variability. In particular, the mean value of the surface air temperature of the NH in winter in the positive phase of the Atlantic Multidecadal Oscillation (AMO) within the analyzed period of 1980–2018 significantly exceeded the corresponding mean value of T in the negative phase of AMO. At the same time, it was found that the mean value of the winter blockings duration in the NH in the positive AMO phase is one and a half times longer than in the negative AMO phase. For winter seasons, the estimates of the relationship between τ and the intensity of the Arctic anticyclonic center of action depending on the AMO phase were obtained.

Multidecadal Changes in Zonal Displacement of Tropical Pacific MJO Variability Modulated by North Atlantic SST

Abstract The Madden–Julian oscillation (MJO) exhibits the most vigorous activity over the Indo-Pacific warm pool during the boreal winter. In this study, we find the interbasin connection between the tropical Pacific MJO activity and North Atlantic sea surface temperature at the interdecadal time scale. Accompanied with the negative phase of the Atlantic multidecadal oscillation (AMO), the enhanced MJO variability (i.e., larger amplitude of 20–90-day convection) tends to extend eastward into the central tropical Pacific (20°S–0°, 170°E–150°W), while the vigorous MJO variability is confined to the Indo-Pacific warm pool during the positive phase of AMO. The eastward displacement of MJO convection into the central tropical Pacific during the negative AMO phase is supported by the increased moist static energy and low-level moisture tendencies there when the low-frequency background (>90 days) westerly transports the moisture perturbation [] and the MJO-associated easterly perturbation transports the background moisture [] toward the central tropical Pacific. This suggests that the interdecadal changes in zonal displacement of MJO activity over the tropical Pacific are closely linked with the impacts of AMO on the background flow and moisture condition over the central tropical Pacific. The results of coupled model sensitivity experiments and preindustrial simulations of phase 6 of the Coupled Model Intercomparison Project (CMIP6) support the observational findings.

Atlantic Multidecadal Oscillation Modulates ENSO Atmospheric Anomaly Amplitude in the Tropical Pacific

Abstract Previous studies have demonstrated that the Atlantic multidecadal oscillation (AMO) could affect El Niño–Southern Oscillation (ENSO) through thermocline adjustment, with a stronger ENSO sea surface temperature (SST) amplitude during a negative AMO phase than during a positive phase. In this study, we find that the ENSO atmospheric anomaly amplitudes in the tropical Pacific during different AMO phases are not necessarily consistent with these ENSO SST changes. For El Niño episodes, the low-level wind and precipitation anomalies over the tropical Pacific in the boreal winter are more pronounced during the negative AMO phase than during the positive phase, corresponding well to the stronger SST anomalies. However, La Niña events during the negative AMO phase are accompanied by weaker atmospheric anomalies in the tropical Pacific, although their SST anomalies are stronger than those during the positive phase. We suggest that this mismatch between La Niña SST and atmospheric anomalies can be largely attributed to AMO decadal modulation. A positive AMO favors intensified trade winds and weakened precipitation in the central tropical Pacific by modifying Walker circulation. Therefore, when La Niña coincides with a positive AMO, the low-level easterly and negative precipitation anomalies are superimposed, which gives rise to stronger atmospheric perturbations. In contrast, under a negative AMO background, the atmospheric anomalies induced by La Niña anomalous SST are partly counteracted by the AMO remote decadal modulation, thereby resulting in weaker anomaly amplitudes. Here, we highlight that AMO decadal forcing needs to be considered when investigating ENSO atmospheric variabilities and related regional climate impacts.

A Seesaw Variability in Tropical Cyclone Genesis between the Western North Pacific and the North Atlantic Shaped by Atlantic Multidecadal Variability

AbstractVariabilities in tropical cyclone (TC) activity are commonly interpreted in individual TC basins. We identify an antiphase decadal variation in TC genesis between the western North Pacific (WNP) and North Atlantic (NA). An inactive (active) WNP TC genesis concurs with an enhanced (suppressed) NA TC genesis. We propose that the transbasin TC connection results from a subtropical east–west “relay” teleconnection triggered by Atlantic multidecadal oscillation (AMO), involving a chain atmosphere–ocean interaction in the North Pacific. During a negative AMO phase, the tropical NA cooling suppresses local convective heating that further stimulates a descending low-level anticyclonic circulation in the tropical NA and eastern North Pacific as a Rossby wave response, inhibiting the NA TC genesis. Meanwhile, the anomalous southwesterly to the western flank of the anomalous anticyclonic circulation tends to weaken the surface evaporation and warm the SST over the subtropical eastern North Pacific (southwest–northeast-oriented zone from the tropical central Pacific to the subtropical west coast of North America). The SST warming further sustains a cyclonic circulation anomaly over the WNP by local atmosphere–ocean interaction and the Bjerknes feedback, promoting the WNP TC genesis. This transbasin linkage helps us interpret the moderate amplitude of variations in TC genesis frequency in the Northern Hemisphere.

Atlantic Multidecadal Oscillation Modulates the Relation of ENSO With the Precipitation in the Central-Western Indian Ocean

It is well known that the El Niño-Southern Oscillation (ENSO) could affect the precipitation anomalies in the central-western Indian Ocean (CWIP) through modifying the Walker circulation, with an El Niño generally accompanied by an enhanced CWIP. In this study, we find that this positive association is modulated by the Atlantic Multidecadal Oscillation (AMO). When ENSO and AMO are out-of-phase combinations (i.e., AMO-/El Niño and AMO+/La Niña), the CWIP is significantly stronger than that when they are in-phase cooperated. It is suggested that the AMO’s modulating effect mainly comprises two pathways that influence ENSO’s linkage with the CWIP. On one hand, AMO could modulate the SST variability in the central-eastern tropical Pacific with a stronger ENSO SST amplitude during its negative phase, thus influencing the ENSO-CWIP relationship. On the other hand, AMO is associated with a multidecadal atmospheric variation in the Walker circulation. The weakened circulation during the negative AMO phase favors an anomalous ascending flow over the central-western Indian Ocean, thereby favoring an enhanced CWIP there. Therefore, El Niño is accompanied by a more pronounced CWIP during the negative AMO phase compared to that during a positive AMO phase. For La Niña episodes, however, these two pathways have opposite modulation effects. Although AMO+/La Niña is weaker than AMO-/La Niña, the accompanied CWIP is relatively stronger as an multidecadal dry background induced by the Atlantic warming reinforces the negative CWIP anomaly generated by La Niña. We here highlight that the AMO decadal forcing needs to be considered when investigating the Indian Ocean atmospheric variabilities during ENSO events.

Interpreting the nonstationary relationship between El Niño–Southern Oscillation and the winter precipitation over southeast China

AbstractThe amount of winter precipitation over southeast China (SCWP) is significantly affected by the El Niño–Southern Oscillation (ENSO), with generally an enhanced SCWP response to El Niño events. In this paper, we find that this relationship shows remarkable nonstationarities on the multidecadal timescale. Running ENSO–SCWP correlation maintains statistically significant before the mid‐1960s and after the mid‐1990s but not during the interim period, which exhibits an approximate synchronized temporal evolution with the Atlantic Multidecadal Oscillation (AMO). A statistical method for interpreting nonstationary relationships is then applied to quantitatively verify and measure this possible AMO modulation. It is demonstrated that the nonstationary ENSO–SCWP correlation can be expressed to a large extent by the influence of the AMO–ENSO nonlinear effect (AMO*Niño3.4) on the SCWP. This influence intensifies the ENSO–SCWP relationship during the positive AMO phase but weakens it during the negative AMO phase. Therefore, the nonstationarity in the ENSO–SCWP relationship comes predominantly from an AMO modulation via this key nonlinear effect. We further suggest that a positive AMO favours a westward shift of ENSO sea surface temperature (SST) anomaly pattern in the tropical Pacific. Correspondingly, the anomalous western North Pacific (WNP) anticyclone also migrates westward, which gives rise to pronounced moisture transports and vertical motions over the adjacent southeast China, thereby resulting in a significant ENSO–SCWP relationship. In contrast, during the negative AMO phase, the eastward‐shifted ENSO SST and WNP anticyclonic anomalies produce significant atmospheric anomalies primarily to the east of the Chinese mainland, thus having less effect on the SCWP.

Impact of Atlantic multidecadal oscillation on interannual relationship between ENSO and East Asian early summer monsoon

AbstractImpact of the Atlantic multidecadal oscillation (AMO) on the interannual relationship between the El Niño‐Southern Oscillation (ENSO) and East Asian early summer monsoon (EAESM) is investigated using observations and community atmosphere model (CAM) outputs. We find that the interannual ENSO–EAESM relationship displays a prominent decadal variation during 1951–2018, which is consistent with the AMO phase change, that is, a high ENSO–EAESM correlation appears during the negative AMO phase, while no significant correlation is found during the positive AMO phase. Further analyses reveal that the AMO‐related sea surface temperature anomalies (SSTAs) in the western North Pacific, rather than in the North Atlantic, modulate the interannual relationship between the El Niño and EAESM. These SSTAs can prolong the existence of the anomalous western North Pacific anticyclone (WNPAC) from the ENSO mature winter to the subsequent early summer, thus affecting the ENSO–EAESM relationship for the El Niño events. In contrast, the AMO modulates the interannual relationship between the La Niña and EAESM through the North Atlantic and North Pacific. The North Atlantic warming‐induced anomalous anticyclone weakens the background cyclone over the western North Pacific during the positive AMO phase, and the mid‐latitude air–sea interaction in the North Pacific also influence the anomalous cyclone. The observed AMO's impact on the interannual relationship between the ENSO and EAESM is confirmed by atmosphere general circulation model simulations using the CAM.

Interdecadal modulation of ENSO amplitude by the Atlantic multi-decadal oscillation (AMO)

The impact of the Atlantic Multi-decadal Oscillation (AMO) on the ENSO amplitude was investigated through observational analyses. During the past 90 years the interdecadal variability of ENSO intensity is significantly correlated with the AMO. ENSO variability was strengthened (weakened) during a negative (positive) AMO phase. An ocean mixed layer heat budget analysis reveals that the thermocline feedback is the main process regulating AMO negative phase dependent ENSO growth characteristic. A further examination indicates that a strengthened atmospheric response to unit SST anomaly, an enhanced thermocline response to unit wind stress forcing and a strengthened subsurface temperature response to unit thermocline variation all contribute to the enhanced thermocline feedback during the negative phase of AMO. Such changes are attributed to the increase of background moisture, the weakening of mean subtropical cell (STC) and increase of upper ocean vertical temperature gradient respectively.

Environmental influences on Norwegian spring-spawning herring (Clupea harengusL.) larvae reveal recent constraints in recruitment success

AbstractThe lack of any abundant recruiting year class of Norwegian spring-spawning (NSS) herring between 2005 and 2015 contributed to an approximate reduction of 40% in the spawning-stock biomass since 2009, i.e. from 7 to 4 million tonnes. Warming of the North Atlantic is suggested to contribute to this reduction in recruitment. In the past, a warm phase induced by a positive Atlantic Multidecadal Oscillation (AMO) in the North Atlantic was positively correlated to the NSS herring stock size. Recent unprecedented ocean warming in the Norwegian Sea ecosystem, besides elevated temperatures due to a positive AMO, seems to be outside optimal environmental conditions for early life history stages of NSS herring. We analysed 28 years of survey data using generalized additive models to reconstruct environmental conditions for drifting yolksac and preflexion stage larvae. Our results indicate that strong recruitment years were more likely when the larvae occurred simultaneously with a negative AMO during positive temperature anomalies. The transition from yolksac stages towards preflexion stages occurred while there was a slow increase in water temperature during the larval drift. Weak recruitment years generally occurred when larvae experienced elevated temperatures during the life stage transition under a positive AMO. These results augment evidence that the historical positive relationship between AMO and stock dynamics is reversed between 1988 and 2015. Albeit not implying any specific mechanistic biological interactions, we can assume that the unprecedented warming has modified the ecosystem drivers that negatively affect drifting larvae. Since 2016, the North Atlantic is shifting into a negative AMO phase, possibly resulting in the 10-year recruitment suppression of NSS herring ending soon.

Modulation of the Relationship between ENSO and Its Combination Mode by the Atlantic Multidecadal Oscillation

AbstractRecent studies demonstrated the existence of a conspicuous atmospheric combination mode (C-mode) originating from nonlinear interactions between El Niño–Southern Oscillation (ENSO) and the Pacific warm pool annual cycle (AC). Here we find that the C-mode exhibits prominent decadal amplitude variations during the ENSO decaying boreal spring season. It is revealed that the Atlantic multidecadal oscillation (AMO) can largely explain this waxing and waning in amplitude. A robust positive correlation between ENSO and the C-mode is detected during a negative AMO phase but not during a positive phase. Similar results can also be found in the relationship of ENSO with 1) the western North Pacific (WNP) anticyclone and 2) spring precipitation over southern China, both of which are closely associated with the C-mode. We suggest that ENSO property changes due to an AMO modulation play a crucial role in determining these decadal shifts. During a positive AMO phase, ENSO events are distinctly weaker than those in an AMO negative phase. In addition, El Niño events concurrent with a positive AMO phase tend to exhibit a westward-shifted sea surface temperature (SST) anomaly pattern. These SST characteristics during the positive AMO phase are both not conducive to the development of the meridionally asymmetric C-mode atmospheric circulation pattern and thus reduce the ENSO/C-mode correlation on decadal time scales. These observations can be realistically reproduced by a coupled general circulation model (CGCM) experiment in which North Atlantic SSTs are nudged to reproduce a 50-yr sinusoidally varying AMO evolution. Our conclusion carries important implications for understanding seasonally modulated ENSO dynamics and multiscale climate impacts over East Asia.

Interdecadal Modulation of AMO on the Winter North Pacific Oscillation-Following Winter ENSO Relationship

It is known that the wintertime North Pacific Oscillation (NPO) is an important extratropical forcing for the occurrence of an El Nino-Southern Oscillation (ENSO) event in the subsequent winter via the “seasonal footprinting mechanism” (SFM). This study reveals that the Atlantic Multidecadal Oscillation (AMO) can notably modulate the relationship between the winter NPO and the following winter ENSO. During the negative AMO phase, the winter NPO has significant impacts on the following winter ENSO via the SFM. In contrast, the influence of the winter NPO on ENSO is not robust at all during the positive AMO phase. Winter NPO-generated westerly wind anomalies over the equatorial western Pacific during the following spring are much stronger during negative than positive AMO phases. It is suggested that the AMO impacts the winter NPO-induced equatorial westerly winds over the western Pacific via modulating the precipitation climatology over the tropical central Pacific and via modulating the connection of the winter NPO with spring sea surface temperature in the tropical North Atlantic.

Does summer precipitation in China exhibit significant periodicities?

Identification of the periodic variability of precipitation is of paramount importance for rainfall modeling and forecasting, but it is also a challenge. In this article, we (1) identify the periodic variability of summer precipitation in China at interannual and decadal scales; and (2) explore its physical connection to the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO). The summer precipitation data in 1961–2013 from 520 meteorological stations across China and the empirical mode decomposition-based method are used for this study. Results show that at the interannual and decadal timescales, summer precipitation exhibits significant periodicities, which can be attributed to the variability of PDO and AMO, with high correlations, only in four small regions: northwest corner of China, most parts of the Songliao River Basin (SRB), the Huang-Huai-Hai Plain, and the south boundary of the Yangtze River Basin (YRB). However, their effects with spatial variability in strength and direction are distinguished as four types, due to the spatial variability in the changes in water vapor flux between warm and cold PDO (AMO) phases. Precipitation is mainly connected to: (1) AMO in the northwest corner of China, with the same phase; (2) PDO in most parts of SRB and lower reach of YRB, keeping the same phase; (3) PDO in the mid-reach of the Yellow River basin, eastward to the northern Huai River basin and the Southeast River basin, with the inverse phase; and (4) both PDO and AMO in the mid-reach of YRB, where a positive PDO and a negative AMO phase cause precipitation surplus. For the “four-horizontal-three-vertical” national water transfer framework in China, the decadal variability of summer precipitation between the water exporting and importing regions keeps an inverse phase, due to the PDO and AMO effects. Although allocation of water resources over a large region through huge water transfer projects in China (and elsewhere) often yield favorable results for water security, the periodic variability of precipitation, and especially the anthropogenic effects, needs to be carefully considered and evaluated, to maximize the benefits.

The Decadal-Multidecadal Changes of Extreme Air Temperature in the Black Sea Region

The decadal-multidecadal changes of extreme air temperature characteristics during winter and summer in the Black Sea region are studied in the paper. The 5th and 95th percentiles of intra-seasonal daily temperature anomalies and number of days with air temperature below the 5th and above the 95th percentile were used to describe moderate extremes. The average daily air temperature data from 13 hydrometeorological stations in 1950–2014 and from several stations in 1900–2005 were analyzed.Pacific Decadal oscillation (PDO) and the Atlantic Multidecadal oscillation (AMO) were considered as the main global processes in the ocean-atmosphere system responsible for regional climate change on decadal-multidecadal scale. The differences of extreme air temperature characteristics between positive and negative AMO and PDO phases are analyzed in the paper. It is found that the extreme winter air temperature anomalies are increased by about 2 °C during the positive PDO phase and negative AMO phase. The winter frequency of extreme low temperature is decreased by two times during the positive PDO phase and negative AMO phase. The summer changes of extreme temperature characteristics, associated with the PDO and AMO phases are insignificant.

Disentangling the influencing factors driving the cooling trend in boreal summer over Indo-Gangetic river basin, India: role of Atlantic multidecadal oscillation (AMO)

Using the Community Earth System Model (CESM)-Large Ensemble (LE) surface air temperature (SAT) data, we investigate the multidecadal changes in SAT variability over Central Indian landmass, particularly the Indo-Gangetic (IG) river basin. This region comes under the active influence of the Indian summer monsoon, and during the summer monsoon months (JJA), we observe an amplified cooling (< − 3 °C) trend (1961–2000) in SAT. This SAT trend is considered as a superposition of external forcings and natural climatic variability. The forced response is computed by averaging the trend in 35 ensemble members, which displays a moderate cooling trend due to aerosol-, ozone-, and volcano-only forcings. But the internal variability introduces a wide range of uncertainties in SAT, with majority of the members display a strong cooling trend in the Central Indian region. During the entire period, natural climatic variability dominates over the forced response, which strongly overrides the greenhouse gas (GHG) warming. Here, we separate out the influence of global climate variability on regional climate variability and identify the specific internal variability which is responsible for the multidecadal cooling trend in the analyzed region. Furthermore, we investigate the specific physical mechanism driving the cooling trend and analyze the role of Atlantic multidecadal oscillation (AMO) in its negative phase. The covariability is − 0.74, i.e., AMO accounts for ~ 55% of total variance in the multidecadal variability. In the negative phase of AMO, strong signals of Rossby waves emanating from North Atlantic Ocean propagate across the Eurasian continent, and in the latter half of the twentieth century, the effect of this cold sea surface temperature (SST) anomaly is felt in the Central Indian landmass (particularly over IG river basin) through teleconnection. This study will increase the predictability of multidecadal variability in SAT during summer monsoon season over the Central Indian region with AMO as a strong driving component.

Teleconnection between mean sea level pressure in the North Atlantic for September, the AMO phase and mean temperature in Central Europe for December (1896–2015)

According to the present study, an atmospheric teleconnection is likely to exist between mean sea level pressure (MSLP) in the North Atlantic at early autumn (September) and temperature in Central Europe at early winter (December) based on the last 120 years. Atlantic Multidecadal Oscillation (AMO) might play an important role in this teleconnection, determining its direction. During a positive AMO phase, a North Atlantic MSLP above average in September is frequently followed by a colder‐than‐average December in Central Europe (in 80% of cases) whereas a warmer‐than‐average December is often observed in Central Europe after an under‐average MSLP in the North Atlantic in September. According to the present investigation, this is caused by a strong tendency for blocking patterns over Northern Europe (mostly over the British Isles and Scandinavia) in the last month of the year, in the first case, and a weak tendency for that, in the second case. During a negative AMO phase, this connection is less certain. In the strong negative AMO phase between 1903 and 1925, the connection became reversed compared with that observed during the positive AMO phase. However, it was significantly strong. Within the much weaker negative AMO phase between 1963 and 1994, the connection was weaker, but rather similar to that observed during positive AMO phases. Since the revealed atmospheric teleconnection is characterized by a three‐month delay, it possesses a prognostic value and can be used to predict the probability of temperature anomaly.

A Nonstationary ENSO–NAO Relationship Due to AMO Modulation

Abstract Many previous studies have demonstrated a high uncertainty in the relationship between El Niño–Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). In the present work, decadal modulation by the Atlantic multidecadal oscillation (AMO) is investigated as a possible cause of the nonstationary ENSO–NAO relationship based on observed and reanalysis data. It is found that the negative ENSO–NAO correlation in late winter is significant only when ENSO and the AMO are in phase (AMO+/El Niño and AMO−/La Niña). However, no significant ENSO-driven atmospheric anomalies can be observed over the North Atlantic when ENSO and the AMO are out of phase (AMO−/El Niño and AMO+/La Niña). Further analysis indicates that the sea surface temperature anomaly (SSTA) in the tropical North Atlantic (TNA) plays an essential role in this modulating effect. Because of broadly analogous TNA SSTA responses to both ENSO and the AMO during late winter, a warm SSTA in the TNA is evident when El Niño occurs during a positive AMO phase, resulting in a significantly weakened NAO, and vice versa when La Niña occurs during a negative AMO phase. In contrast, neither the TNA SSTA nor the NAO shows a prominent change under out-of-phase combinations of ENSO and AMO. The AMO modulation and the associated effect of the TNA SSTA are shown to be well reproduced by historical simulations of the HadCM3 coupled model and further verified by forced experiments using an atmospheric circulation model. These offer hope that similar models will be able to make predictions for the NAO when appropriately initialized.

Interannual Relationship between ENSO and Atlantic Storm Track in Spring Modulated by the Atlantic Multidecadal Oscillation

It has been well documented that storm track activity are closely related to the weather and short-term climate variability in the extratropics, which is affected by sea surface temperature anomalies over the tropical eastern Pacific Ocean. Interannual relationship between the El Niño-Southern Oscillation (ENSO) and the Atlantic storm track (AST) in spring modulated by the Atlantic multidecadal oscillation (AMO) was investigated using reanalysis data and model simulations in this study. The meridional displacement of the AST is significantly correlated with ENSO during negative AMO phase, while no significant relationship is found during positive AMO phase. This may be due to the difference of 500-hPa geopotential height anomalies induced by ENSO in different AMO phases. For an El Niño event during the negative AMO phase, an anomalous 500-hPa wave train propagates eastward across the North American continent, with positive height anomalies at the high latitudes, extending from South Canada to Newfoundland. Thus, easterly wind anomalies appear over central North America, upstream of the negative AST anomaly. Accordingly, the local eddy growth rate (EGR) and baroclinic energy conversion (BC) are obviously reduced, which weaken (strengthen) the southern (northern) part of the climatological AST. As a result, the AST is shifted northward significantly. During the positive AMO phase, the ENSO-related anomalous wave train at 500 hPa only propagates northeastward and is largely suppressed over Northwest Canada, with positive height anomalies confined to the northwest of North America. Therefore, no significant changes of the westerly jet, EGR and BC are found in the upstream region of the AST, and the meridional location of the AST generally remains unchanged. Most previous studies investigate AST variabilities in winter, and few focus on AST in spring. This work may be helpful in understanding more about the interannual and interdecadal variations of springtime AST and in further studying the weather and short-term climate changes caused by AST.

Coupled impacts of ENSO AMO and PDO on temperature and precipitation in the Alabama–Coosa–Tallapoosa and Apalachicola–Chattahoochee–Flint river basins

ABSTRACTLarge‐scale patterns of ocean surface temperature can influence weather across the globe and understanding their interaction with the local climate can improve seasonal forecasting of local temperature and precipitation. Here we focus on the combined interactions of the El Niño‐Southern Oscillation (ENSO), the Atlantic Multidecadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO) in the Alabama–Coosa–Tallapoosa (ACT) and Apalachicola–Chattahoochee–Flint (ACF) river basins of the southeastern United States. Nonparametric ranks‐sum tests of individual and coupled impacts of these teleconnections on the annual study area climate (1895–2009) found significant impacts. A positive AMO phase was associated with decreased precipitation and increased mean temperature while the negative AMO phase was associated with increased precipitation and decreased temperature. While an El Niño event generally increases regional precipitation, El Niño during a positive AMO or PDO phase resulted in precipitation below the long‐term average in our study area. Because of many instances of El Niño being shared between AMO and PDO phase, the effects of the PDO and AMO on El Niño could not be distinguished. La Niña was associated with negative precipitation and increased temperature. The effects of La Niña on the temperature and precipitation anomaly were significantly increased during positive AMO and PDO phases. The coupled impacts of the aforementioned teleconnections demonstrate the necessity of including the effects of the AMO and the PDO when using ENSO‐based forecasts. The significant shifts on the effects of teleconnections on area climate from AMO negative phase to AMO positive phase cast doubt on seasonal prediction for the study area based on the recent history (i.e. the use of the period 1950–2000 to predict seasonal climate since 2000).