Phase Of Atlantic Multidecadal Oscillation Research Articles

Intensified Impact of Spring Tibetan Plateau Snow Cover on Summer Compound Heat Waves in Western Europe after 1998

Abstract This study revealed a significant interdecadal change in the impact of spring western Tibetan Plateau (TP) snow cover (TPSC) on subsequent summer compound heat waves (CHWs) in western Europe (WE) after 1998. This interdecadal change is attributed to a change in a western Europe high–western TP low (WEH–TPL) atmospheric circulation pattern. This pattern arises due to both the inherent variability of TPSC and the phase transition of the Atlantic multidecadal oscillation (AMO) after 1998. The increased magnitude and persistence of western TPSC from spring to summer after 1998 enhanced the snow–atmosphere coupling effect, intensifying ascent and decent motion over the TP and WE, respectively, and strengthening the WEH–TPL pattern. In addition, the post-1998 positive AMO phase favors continuous and stable downstream Rossby wave propagation, enhancing the WEH–TPL pattern and the TPSC–CHWs relationship. Further analyses reveal that the interdecadal changes in the TPSC and the AMO around 1998 contribute to the presence of “double jets” over the North Atlantic–central Eurasian sectors. The TPSC–related anomalous atmospheric circulation and AMO phase shift contribute to the southern and northern branches of the intensified westerly jet, respectively. These conditions create a favorable environment for the formation and persistence of summer CHWs in WE. Numerical modeling experiments with a linear baroclinic model confirm these findings. Our findings suggest that in the context of a changing climate, TPSC plays a pivotal role in the genesis of summer CHWs in WE and may serve as a valuable predictor for CHWs. Significance Statement This study discovered that starting from 1998 there was a significant change in how spring snow cover on the western Tibetan Plateau affects summer compound heat waves in western Europe. After 1998, the snow cover on the western Tibetan Plateau increased in size and lasted longer from spring to summer, intensifying the interaction between the snow and the atmosphere. This led to more rising and sinking air over the Tibetan Plateau and western Europe, respectively. Also, after 1998 a positive phase of the Atlantic multidecadal oscillation (AMO) was favorable for the circulation connection between western Europe and the Tibetan Plateau. Further analysis showed that these changes in snow cover and the AMO after 1998 caused “double jets” in the North Atlantic and central Eurasia that created better conditions for summer heat waves in western Europe. Numerical models are used to confirm these findings. Our research indicates that, in a changing climate, the snow cover on the western Tibetan Plateau plays a crucial role in the development of summer heat waves in western Europe and can be a useful predictor for these heat waves.

Influence of the Atlantic Multidecadal Oscillation on the Rapid Intensification Magnitude of Tropical Cyclones over the Western North Pacific

Abstract Rapid intensification (RI) of tropical cyclones (TCs), which refers to an explosive increase of TC intensity exceeding a certain threshold [e.g., 30 kt (1 kt ≈ 0.51 m s−1) in 24 h] within a short time period, poses a great challenge to both forecasting and disaster prevention efforts. Recent studies have documented a significant increase in the magnitude of TC RI (RIM; measured as the mean intensification rate of all TC RI records in a TC season) over the western North Pacific (WNP) since 1979, and have attributed it to the impacts of global warming. In this study, results from statistical analyses show that the TC RIM over the WNP during 1951–2021 exhibits significant interdecadal variability, which is found to be closely related to the Atlantic multidecadal oscillation (AMO). Further analyses indicate that the response of the local thermodynamic conditions to the AMO plays a dominant role in shaping this relationship. The positive AMO phase fosters a high TC RIM over the WNP by producing significant warm sea surface temperature (SST) anomalies, which in turn enhances TC heat potential and the midtropospheric relative humidity in the main region of TC RI occurrence. Results from both data analyses and numerical model experiments demonstrate that the AMO modulates thermodynamic conditions over the WNP, such as SST and ocean heat content, by affecting local heat fluxes and the Ekman heat transport in the WNP via the modulation of Walker circulation from the Atlantic to the Pacific.

The Role of the Indian Ocean in Controlling the Formation of Multiyear El Niños through Subtropical ENSO Dynamics

Abstract This study explores the key differences between single-year (SY) and multiyear (MY) El Niño properties and examines their relative importance in causing the diverse evolution of El Niño. Using a CESM1 simulation, observation/reanalysis data, and pacemaker coupled model experiments, the study suggests that the Indian Ocean plays a crucial role in distinguishing between the two types of El Niño evolution through subtropical ENSO dynamics. These dynamics can produce MY El Niño events if the climatological northeasterly trade winds are weakened or even reversed over the subtropical Pacific when El Niño peaks. However, El Niño and the positive Indian Ocean dipole (IOD) it typically induces both strengthen the climatological northeasterly trades, preventing the subtropical Pacific dynamics from producing MY events. MY events can occur if the El Niño fails to induce a positive IOD, which is more likely when the El Niño is weak or of the central Pacific type. Additionally, this study finds that such a weak correlation between El Niño and the IOD occurs during decades when the Atlantic multidecadal oscillation (AMO) is in its positive phase. Statistical analyses and pacemaker coupled model experiments confirm that the positive AMO phase increases the likelihood of these conditions, resulting in a higher frequency of MY El Niño events.

Modulation of the link between the Hadley circulation and the meridional structure of tropical SSTs by the Atlantic multidecadal oscillation

This paper investigates the effects of the Atlantic multidecadal oscillation (AMO) on the relationship between the Hadley circulation (HC) and the different meridional structures of the tropical sea surface temperature (SST). The response of the HC to SSTs shows inconsistent variations between the warm and cold phases of the AMO. The response of the HC to SSTs during the cold phase of the AMO is similar to that seen in the long-term and seasonal cycles. However, during the warm AMO phase, the magnitude of this response is significantly reduced. This significant difference in the response is caused mainly by the weakened response amplitude of the equatorially asymmetric HC and SST. The potential mechanisms associated with this suppression are also investigated, and they relate primarily to differences in the SST meridional anomalies within the Indo-Pacific warm pool (IPWP). During the AMO warm phase, the profile of SST anomalies in the IPWP exhibits characteristics similar to an equatorially symmetric distribution, which weakens the equatorially asymmetric component of the tropical SST and then contributes to the suppression of the response ratio. However, the amplitudes of the insignificant negative SST anomalies in the northern and southern IPWP are similar during the cold AMO phase. This distribution of SST anomalies has a minimal impact on the variation of SST gradients. Therefore, the response ratio during the cold AMO phase is comparable to that observed in the long-term and seasonal cycles. Other atmospheric reanalysis data are used to further confirm these results.

Extreme waves in the Caribbean Sea: spatial regionalization and long-term analysis

The extreme wave height distribution in the Caribbean Sea is studied using a new method based on the maximum basin-wide aggregate of significant wave height, Hs, values per month. Besides, by means of the Self-Organizing Maps (SOM) technique, we identify coherent geographical regions with similar extreme wave height variability in the Caribbean Sea. Our findings revealed three primary regions: the eastern side with comparatively lower values, the central region with intermediate values, and the western side with the highest extreme wave heights. The study also examines the wind forcing conditions driving the spatial and temporal variability of the extreme waves, highlighting the influence of the low-pressure belt dynamics as well as the role played by the Caribbean Low-Level Jet (CLLJ) index, and the impact of cold fronts and hurricanes on extreme wave heights. Additionally, we explore the relationship between the extreme wave height distribution and climatic indices, such as the Atlantic Multidecadal Oscillation (AMO), the Atlantic Meridional Mode (AMM), the North Atlantic Oscillation (NAO) and the Oceanic Niño (ONI). The results reveal that the spatial distribution of extreme wave heights in the Caribbean Sea is mostly ruled by the influence of the CLLJ, with correlations close to 80%. In addition, significant correlations were observed between the extreme wave heights and the ENSO in the central Caribbean, as well as positive correlations between the extreme wave heights and NAO in the eastern part of the basin, and significant values of correlation with the negative phases of AMO and AMM in the whole basin. We show that, unlike conventional (or broadly used) methods deployed to identify extreme wave height, such as percentile 99th, Hs99, our methodology allows a further assessment of the wind and climate forcing conditions associated with the extreme wave events. Although, we acknowledge that the method here presented has limitations to capture extreme wave height outliers, it has the advantage of being used concomitant with the wind forcing to develop multivariate wave climate analysis at basin scale, and could be extended to a more local scale when studying coastal processes.

РОЛЬ ИЗМЕНЕНИЙ АТМОСФЕРНОЙ ЦИРКУЛЯЦИИ В УВЕЛИЧЕНИИ ПОВТОРЯЕМОСТИ ЛЕТНИХ ЗАСУХ НА ЕВРОПЕЙСКОЙ ЧАСТИ РОССИИ

According to the Standardized Precipitation Evapotranspiration Index (SPEI), most extensive summer droughts in European Russia south of 55°N in 1950-2021 were observed during the extremely negative phases of the Eastern Atlantic/Western Russia (EAWR) and West Pacific (WP) atmospheric circulation patterns characterized by abnormal high atmospheric pressure and an increased frequency of the number of days with atmospheric blocking over European Russia. It is shown that the frequency of droughts in the study area in the years of the negative phases of both circulation indices and their extremes in the summer months as compared to other years was higher by five droughts per decade in the Volga and Central Chernozem regions and by three droughts per decade in the northwestern Caspian region. A statistically significant correlation was found between the EAWR in summer and the multidecadal variability of sea surface temperature in the North Atlantic. It was revealed that the increase in the drought frequency in the study area in recent decades has been caused by the restructuring of atmospheric circulation in the Euro-Atlantic sector accompanying the transition of the Atlantic Multidecadal Oscillation (AMO) to a positive phase. An increase in the stability of the EAWR and WP atmospheric circulation patterns was observed, as well as a related significant increase in the frequency of extensive droughts in the study area in 2010-2021. They were accompanied by the weakening of zonal atmospheric circulation in the Northern Hemisphere, the combination of the positive AMO phase and the effects of anthropogenic warming on the atmospheric circulation in the

Combined Impacts of the Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation on Summer Precipitation in Eastern China During the Medieval Climate Anomaly and Little Ice Age

AbstractWe investigate the joint effects of the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) on eastern China summer precipitation (ECSP) during two typical periods in the last millennium [i.e., the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA)] using simulations from five selected climate models. The multimodel mean indicates that the ECSP anomalies under the four combined AMO and PDO phases differ between the MCA and the LIA. During the MCA, the meridional wave train induced by the PDO‐related convective heating over the northwestern Pacific is dominant and the zonal wave train related to the AMO is secondary. Independent of the AMO phase, deficient precipitation appears over South China in both PDO+ phases and vice versa. Meanwhile, most areas north of the Yangtze River valley feature positive anomalies in the PDO+/AMO+ phase but deficits in the PDO+/AMO− phase. In comparison, the Huanghe‐Huaihe River valley and southern Northeast China receive excessive and deficient precipitation in the PDO−/AMO+ and PDO−/AMO− phases, respectively. During the LIA, the zonal wave train induced by the AMO locates more southeastward than that during the MCA and has an overwhelming impact on the ECSP. Specifically, regardless of the PDO phase, excessive ECSP exists over Northeast China and South China in both AMO+ phases and vice versa. Moreover, precipitation between the two regions is excessive when the PDO is in phase with the AMO but deficient when they are out of phase.

Increased Summer European Heatwaves in Recent Decades: Contributions From Greenhouse Gases‐Induced Warming and Atlantic Multidecadal Oscillation‐Like Variations

AbstractSummer heatwaves over Europe, which can cause many deaths and severe damage, have become increasingly frequent over central and eastern Europe and western Russia in recent decades. In this paper, we estimate the contributions of the warming due to increased greenhouse gases (GHG) and nonlinear variations correlated with the Atlantic Multidecadal Oscillation (AMO) to the observed heatwave trend over Europe during 1980–2021, when the GHG‐induced warming over Europe exhibits a linear trend. It is found that GHG‐induced warming contributes to ∼57% of the European heatwave trend over 1980–2021, while the cold‐to‐warm phase shift of the AMO‐like variations accounts for ∼43% of the trend via the intensification of midlatitude North Atlantic jet. The recent trend of heatwaves over western and northern Europe is mainly due to GHG‐induced warming, while that over central and eastern Europe and western Russia is primarily related to the combined effect of the AMO‐like variations and GHG‐induced warming. To some extent, GHG‐induced warming is an amplifier of the increasing trend of recent AMO‐related European heatwaves. Moreover, European blocking (Ural blocking, UB) is shown to contribute to 55% (42%) of the AMO‐related heatwave trend via the influence of midlatitude North Atlantic jet. In the presence of a strong North Atlantic jet during the recent warm AMO phase, UB events concurrent with positive‐phase North Atlantic Oscillation can cause intense, persistent and widespread heatwaves over Europe such as that observed in the summer of 2022.

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.

Atmospheric patterns and winter precipitation over north‐western South Asia

AbstractMid‐latitude storms act as crucial atmospheric bridges between global teleconnection patterns and regional precipitation distribution during boreal winter over South Asia. In this work, we study the spatio‐temporal variability of seasonal precipitation over South Asia over the last four decades, which reveals distinct regional precipitation patterns across seasons. For the winter region of South Asia (WRSA), the second Empirical orthogonal function (EOF2) of 500 hPa geopotential heights shows a significant positive correlation with precipitation. Furthermore, using regression analysis, we estimate that the first six EOFs explain most of the interannual variability in winter precipitation during the positive phase of AMO. The results show that six standard climate indices influence winter precipitation over WRSA. We observed a decadal variability in the relationship between ENSO and winter precipitation over WRSA that is modulated by Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO). The positive phase of AMO strengthens the positive relationship between ENSO and winter precipitation over WRSA. The model results also show that two other oscialltions, DMI and AAO, significantly explain the variability of winter precipitation over WRSA. The atmospheric patterns extracted in this study significantly improve winter precipitation forecasting skill of linear regression models compared to those based on standard climate indices, especially during positive AMO winters. The results from this study can be used to improve seasonal predictability of winter precipitation over WRSA.

What Causes the Arabian Gulf Significant Summer Sea Surface Temperature Warming Trend?

The present study investigated the significant sea surface temperature (SST) warming trend during the summer season over the Arabian Gulf (AG) and its links with the large-scale atmospheric driver, namely, the Atlantic multidecadal oscillation (AMO), from 1900 to 2021. The link between the AMO and the AGs oceanic circulations has received little scientific attention. It has been found that there is a significant spatial positive trend, with a maximum of up to 0.6 °C per decade over the far northern end, while the time series trend shows a significant shift after 1995, with an average value of about 0.36 °C per decade. The spatial trend in the AG is eight times and four times higher than the global value from 1980 to 2005 using HadISST and OISST, respectively. The AMOs significant role in the AGs SST significant warming trend has been confirmed by the spatial and temporal correlation coefficient, which is above 0.50 and 0.48, respectively, with statistical significance at the 99% level. The underlying mechanisms that explained the AMO-related AGs SST decadal variability can be explained as follows: when the AMO is in a positive phase, the surface northwesterly wind weakens, leading to (1) less advection of the relatively cold air masses from Southern Europe to the AG and surrounding areas, (2) less evaporation, and thus less surface cooling (3); thus, this enhances the water masses stratification and decreases the upwelling process, and vice versa occurs for the negative phase. For the air temperature, the positive AMO phase coincides with the occurrence of warm air masses covering all of the Arabian Peninsula, North Africa, and Southern Europe. These processes prove that the AMO is a possible candidate for the AGs SST decadal variability, hence enabling a better evaluation of future climate scenarios for this important region. Our results provide initial insights into the AMO-driven spatiotemporal variability in the SST over the AG and prove that the relation is nonstationary over time. Further analyses are required to explore whether the impacts of the AMO are extended to other oceanic variables such as evaporation rate, heat transport, etc.

Manifestation of the Early 20th Century Warming in the East-European Plain: Atmospheric Circulation Anomalies and Its Connection to the North Atlantic SST and Sea Ice Variability

A study of the climatic characteristics and annual runoff of the Volga and Severnaya Dvina rivers demonstrates that, on the East European Plain (EEP), Early Twentieth Century Warming (ETCW) manifested in a multiyear drought between 1934 and 1940; this drought has no analogues in this region in terms of intensity and duration according to Palmer’s classification, and caused extreme hydrological events. The circulation conditions during this event were characterized by an extensive anticyclone over Eastern Europe, combined with a cyclonic anomaly in the circumpolar region. An analysis of the spatial features of sea surface temperature (SST) anomalies indicate that the surface air temperature (SAT) anomalies in July on the EEP during ETCW were related not only to the North Atlantic (NA) warming and positive AMO phase, but also to a certain spatial pattern of SST anomalies characteristic of the 1920–1950 period. The difference between the SST anomalies of the opposite sign in the different NA zones, used as the indicator of the obtained spatial pattern, shows the quite close relations between the July SAT anomalies on the EEP and the atmospheric circulation patterns responsible for them. The positive phase of the Atlantic Multidecadal Oscillation (AMO) and the expansion of the subtropical high-pressure belt to the north and to the east can be considered as global-scale drivers of this phenomenon. The AMO also impacts the sea ice cover in the Barents–Kara Sea region, which, in turn, could have led to specific atmospheric circulation patterns and contributed to droughts on the EEP in the 1930s.

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.

Influence of the Atlantic Multidecadal Oscillation on South American Atmosphere Dynamics and Precipitation

The Atlantic Multidecadal Oscillation (AMO) is coherently linked to climate variations over many parts of the globe. Despite recent achievements, the mechanism by which the AMO influences regional precipitation over South America is not well understood. In this study, we isolate the atmospheric response to the AMO using a water isotope-enabled version of the Community Earth System Model version 1.2 (iCESM1.2) and determine its influence on (sub)tropical South American regional precipitation and atmospheric circulation. The results suggest an interhemispheric seesaw in Hadley circulation strength and that the section of the Atlantic Hadley cell is marked by a stronger upward air component south of the equator during the cold AMO phase. We also find that the precipitation anomalies over (sub)tropical South America during AMO phases are mainly related to changes in the Atlantic Intertropical Convergence Zone (ITCZ) core strength, where in the cold (warm) AMO phase the core region strengthens (weakens) from February to July, while from July to November the core region weakens (strengthens). Our results stress the importance of acknowledging the dynamics of season- and regional-dependent ITCZ responses, as they are sufficient to produce observed AMO-related signals even in the absence of marked changes in the ITCZ position.

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.

Effects of decadal climate variability on spatiotemporal distribution of Indo-Pacific yellowfin tuna population

Spatial variations in tuna population and abundance are strongly linked to large-scale climate fluctuations, such as the Pacific decadal oscillation (PDO) and Atlantic multidecadal oscillation (AMO). However, the mechanisms underlying the association of climate indices with yellowfin tuna (YFT) abundance and habitat preference remain unclear. We analysed long-term longline fishery data for YFT and oceanic climate variability index data for 1971–2018. The standardized catch per unit effort (CPUE) of Indo-Pacific Ocean YFT was higher during negative AMO and positive PDO phases. In tropical Pacific Ocean, the trend of YFT habitat preference exhibited seesaw patterns because of the distinct environmental factors influenced by the PDO phase. The PDO changed the environmental parameters throughout the tropical Indian Ocean such that the habitat preference of YFT remained consistent throughout. However, the variations in habitat suitability did not correspond to the distribution or standardized CPUE of YFT throughout the Pacific Ocean during AMO events. Moreover, the changes in habitat suitability had a positive periodicity of 8–16 years with AMO in the Indian Ocean, but revealed opposite trends with the distribution or standardized CPUE of YFT. Our results provide sufficient information to distinguish the variations between PDO phase changing and YFT standardized CPUE/ habitat preference. Furthermore, the AMO phase shift period 60–100 years longer than that of the PDO (20–30 years), and models employing time series of fishery and environmental data must be extended the time period of our study to make the AMO match the fishery data more complete.

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.

Potential fire risks in South America under anthropogenic forcing hidden by the Atlantic Multidecadal Oscillation

Fires in South America have profound effects on climate change and air quality. Although anthropogenic forcing has exacerbated drought and fire risks, the fire emissions and aerosol pollution in the southern Amazon and the Pantanal region showed a consistent long-term decrease during the dry season (August–October) between 2003 and 2019. Here, we find that the decreasing trend in fire emissions, mainly located in the non-deforested region, was associated with climatic conditions unfavorable for intensifying and spreading fires, including increased humidity and slower surface wind speed. These climatic trends can be attributed to weakening of the positive phase of the Atlantic Multidecadal Oscillation, which has strengthened the northeast trade winds within the region (3°S–13°N) and the northwest winds east of the Andes that transport more moisture into the southern Amazon and the Pantanal region. Our findings show the mitigating effects of weakening of the positive Atlantic Multidecadal Oscillation phase on human-induced intensification of fire risks in South America and warn of potentially increased risks of fires and aerosol pollution under intensified anthropogenic forcing in the future.

Interbasin Interactions between the Pacific and Atlantic Oceans Depending on the Phase of Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation

Abstract The authors investigated the interbasin interactions between the Pacific and Atlantic Oceans depending on the phase relationship of Pacific decadal oscillation (PDO)/Atlantic multidecadal oscillation (AMO) based on observations and idealized model experiments. When the AMO and the PDO are in phase (i.e., +PDO/+AMO or −PDO/−AMO), the Pacific Ocean regulates the SST anomalies in the equatorial Atlantic Ocean with altering of the Walker circulation. During this period, there is a negative SST–precipitation relationship in the equatorial Atlantic Ocean where the atmosphere forces the ocean. In contrast, when they are out of phase (i.e., either +PDO/−AMO or −PDO/+AMO), the Atlantic Ocean influences the equatorial Pacific Ocean by modifying the Walker circulation, resulting in a westward shift of a center of convective forcing in the equatorial Pacific Ocean compared to that during an in-phase relationship of PDO/AMO. During this period, a positive SST–precipitation relationship is dominant in the equatorial Atlantic Ocean where the ocean forces the atmosphere. To verify this result, we conducted pacemaker experiments using the Nanjing University of Information Science and Technology Earth System Model version 3 (NESM3). Model results supported our findings obtained from the observations. We infer that the characteristics of the Pacific–Atlantic interbasin interactions depend on whether the PDO and AMO phases are either in phase or out of phase.

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.