Cold Pacific Decadal Oscillation Research Articles

Modulation of Pacific decadal oscillation on the relationship between El Niño–Southern Oscillation and rainy season onset over the Indo‐China Peninsula

AbstractMonsoon precipitation variability over the Indo‐China Peninsula (ICP) has become more complicated affected by global warming. In this study, the modulation of Pacific decadal oscillation (PDO) on the relationship between El Niño–Southern Oscillation (ENSO) and the rainy season onset over the ICP were investigated. The results showed that the ICP rainy season onset were predominantly correlated with winter sea surface temperature anomalies (SSTAs) in the East Pacific Ocean, with late and early onsets following El Niño and La Niña events, respectively. During the warm and cold PDO phase, the correlations tended to be substantially strengthened and weakened, respectively. Further analysis indicates that PDO significantly influenced the effects of ENSO on the ICP rainy season onset by modulating SSTAs and low‐level wind fields. During the El Niño events, abnormal easterlies over the Bay of Bengal (BoB) and southern ICP suppressed water vapour transporting to the ICP, which may be related to the zonal SST anomaly gradient between the Indian Ocean and the Northwest Pacific Ocean. When the El Niño occurred during the warm PDO phase, the rainy season onsets were later. The anomalous easterlies became stronger corresponds to the increasing zonal sea surface temperature anomaly (SSTA) gradient between the Indian Ocean and the Northwest Pacific Ocean. There was no significant anomaly on the rainy season onset during the cold PDO phase. During the La Niña events, the abnormal westerlies in BOB accelerated water vapour transport, and the rainy season onset were earlier during the warm and cold PDO phase. The modulating effects of PDO on La Niña were less than those on El Niño. These results suggest that the predictability of rainy season onset over the ICP can be improved through PDO and thus help agricultural planning and water resources management.

On the association of Pacific Decadal Oscillation with the interdecadal variability in Asian-Pacific Oscillation

The Asian-Pacific Oscillation (APO), featuring a seesaw pattern in the upper-tropospheric temperatures between Asia and the North Pacific, plays profound roles in the Northern Hemispheric climate anomalies. So, its variability and associated physical mechanisms are of great interest. Although progress has been achieved for the APO variability at the interannual time scale, the understanding of its interdecadal variability remains relatively poor. Based on the twentieth-century reanalysis and the simulations of the preindustrial control, Pacific pacemaker, and large ensemble experiments, this study reports a salient contribution of the Pacific Decadal Oscillation (PDO) to the interdecadal variability of the APO during summer, featured as a warm (cold) PDO phase accompanying a negative (positive) APO polarity. During the warm PDO phase, the sea surface temperature (SST) warming over the eastern Pacific is conducive to local ascending anomalies, which teleconnect with descending anomalies over the Tibetan Plateau through zonal vertical circulations. The anomalous ascent (descent) over the eastern Pacific (Tibetan Plateau) tends to strengthen (weaken) the atmospheric heating, beneficial for a warming (cooling) of the troposphere in situ. In addition, the PDO-related meridional SST gradient anomalies favor a southward shift of the subtropical jet stream, weakening the South Asian high and the North Pacific trough in the upper troposphere, which corresponds to the atmospheric situation of a negative APO phase. Quantitative analysis from the large ensemble simulations indicates that the PDO may contribute to approximately 48% of recent APO interdecadal variation from the negative phase to the positive phase in the late 1990s.

Characteristics of Marine Heatwaves in the Indonesian Waters during the PDO, ENSO, and IOD Phases and Their Relationships to Net Surface Heat Flux

We conducted an investigation into the characteristics of marine heatwaves (MHWs) in Indonesian waters, aiming to understand the underlying mechanisms responsible for their formation, particularly the ones generated by net surface heat flux. To accomplish this, we utilized remote sensing data from the National Oceanic and Atmospheric Administration and the European Centre for Medium-Range Weather Forecasts. The dataset covered a 40-year period (1982–2021) encompassing both warm (1982–2007) and cold (2008–2021) phases of the Pacific Decadal Oscillation (PDO). Statistical analysis methods were employed to process the data. Our study reveals significant findings regarding MHWs in Indonesian waters. We observed the highest average frequency and maximum intensity of MHWs, occurring approximately 2–3 times a year and exceeding 1.5 °C, respectively, during the warm phase of PDO (with El Niño events occurring more frequently than La Niña). Conversely, the longest durations of MHWs in Indonesian waters were recorded during the cold phase of PDO (with La Niña events occurring more frequently than El Niño), spanning approximately 7–15 days. We identified local forcing in the form of net surface heat flux as the primary driver of MHW frequency and maximum intensity. During the warm phase of PDO, the net surface heat flux was notably higher, primarily due to increased shortwave radiation (heat gain) and reduced latent heat flux (heat loss) through the evaporation process. These factors collectively contributed to maintaining warmer ocean temperatures. Moreover, our study provides valuable insights into the interannual variability of MHWs through the application of composite calculations. We discovered a strong correlation between the occurrence of El Niño and positive Indian Ocean Dipole (IOD) events during the warm PDO phase and the highest frequency and maximum intensity of MHWs, with approximately 2.52 events and 1.54 °C, respectively. In contrast, we found that MHWs with the longest durations were closely linked to La Niña and negative IOD events during the cold PDO phase, lasting approximately 10.90 days. These findings highlight the complex interplay between climate phenomena and MHW characteristics, further deepening our understanding of their dynamics.

Multiscale Interactions of Climate Variability and Rainfall in the Sogamoso River Basin: Implications for the 1998–2000 and 2010–2012 Multiyear La Niña Events

In this research, we explored rainfall variability in the Sogamoso River Basin (SRB), its relationship with multiple scales of variability associated with El Niño–Southern Oscillation (ENSO), and the implications for rainfall prolongation during multiyear La Niña events. First, we examined time-frequency rainfall variations in the SRB based on the standardized precipitation index (SPI) from 1982 to 2019, using wavelet transform and principal component analysis (PCA). In addition, we applied wavelet analysis to investigate the links at different time scales between ENSO and the main mode of rainfall variability in the SRB. Finally, we explored the role that each scale of variability played in the prolongation and intensity of rainfall in the SRB during the 1998–2000 and 2010–2012 multiyear La Niña events. The results of the wavelet analyses revealed significant ENSO relationships affecting SRB rainfall at three different scales: quasi-biennial (2–3-years) between 1994 and 2002, as well as from 2008 to 2015; interannual (5–7 years) from 1995 to 2011; and quasi-decadal (9–12 years) from 1994 to 2012. This indicates that multiyear events are a consequence of the interaction of several scales of variability rather than a unique scale. During the 1998–2000 event, El Niño conditions were observed during the first half of 1998; subsequently, a cooling of the central and eastern tropical Pacific (western tropical Pacific) on the quasi-biennial (interannual) scale was observed during 1999; in 2000, only La Niña conditions were observed on the interannual scale. Therefore, during this event, the quasi-biennial (interannual) scale promoted wet conditions in the Caribbean, the Andes, and the Colombian Pacific from June–August (JJA) 1998 to JJA 1999 (during 1999–2000). During the 2010–2012 La Niña event, the interbasin sea surface temperature gradient between the tropical Pacific and tropical North Atlantic contributed to strengthening (weakening) of the Choco jet (Caribbean low-level jet) on the quasi-biennial scale during 2010, and the interannual scale prolonged its intensification (weakening) during 2011–2012, acting to extend the rainy periods over most of the Colombian territory. Variations on quasi-decadal scales were modulated by the Pacific decadal oscillation (PDO), resulting in a further intensification of the 2010–2012 La Niña event, which developed under conditions of the cold PDO (CPDO) phase, whereas the 1998–2000 La Niña occurred during the transition from warm (WPDO, 1977–1998) to cold (CPDO, 2001–2015) conditions. These results indicate that the interaction of quasi-biennial to quasi-decadal scales of variability could play a differential role in the configuration and prolongation of rainfall events in the SRB.

Role of solar activity and Pacific decadal oscillation in the hydroclimatic patterns of eastern China over the past millennium

The hydroclimatic patterns in eastern China (EC) over the past millennium, both influenced by solar activity and Pacific decadal oscillation (PDO), are presented based on the climatic reconstructions and simulations. For strong solar activity, a consistent pattern exists where drought appears in the majority of EC initialized from the cold PDO phase, which is related to less moisture caused by the negative East Asia/Pacific-like (EAP-like) pattern and the enhanced western Pacific subtropical high (WPSH). When the initial condition with a neutral or warm PDO phase is set up, there is a tripole-like pattern from north to south, i.e., drought–flood–drought for the neutral PDO phase and flood–drought–flood for the warm PDO phase, which is related to the positive EAP-like pattern with various moisture transportation paths. For weak solar activity, there are dipole-like patterns where drought occurs in the northern part of the Yangtze River and flood appears in the southern part initialized from the cold or warm PDO phase, which is possibly related to the influence of the weakened WPSH. When the initial condition is the neutral PDO phase, there is a tripole-like pattern from north to south (flood–drought–flood) in EC affected by the weakened WPSH and the intensive India–Burma trough. Notably, the vertical velocity anomalies of airflow, representing the vertical moisture transportations, correspond well with the hydroclimatic patterns in EC whenever the various initialization strategies of PDO phases are employed. • Hydroclimatic patterns in eastern China (EC) are related to solar activity and PDO. • A consistent pattern in EC for strong solar period (SS) and cold PDO initial phase. • The tripole-like patterns for SS and the neutral or warm PDO initial phase. • A tripole-like pattern for weak solar period (WS) and neutral PDO initial phase. • The dipole-like patterns for WS and the cold or warm PDO initial phase.

Synchronous spatio-temporal changes in potential habitats of Trachurus murphyi and Dosidicus gigas off Chile in relation to regime shift of Pacific decadal oscillation

Understanding concurrent responses of habitat pattern of pelagic fish species to climate variability is favorable for sustainable exploitation and fisheries management. In this study, the key environmental factors affecting Dosidicus gigas (sea surface salinity (SSS), sea surface height anomaly (SSHA) and water temperature at 400 m (Temp_400m), and Trachurus murphyi (sea surface temperature (SST), mixed layer depth (MLD) and Temp_400m), were used in combination with the Pacific Decadal Oscillation (PDO) index to examine synchronous habitat variations off Chile based on habitat suitability index model (HSI). All environmental factors were significantly related to the PDO. A significantly negative relationship was found between the HSI of D.gigas and the PDO index, while a significantly positive correlation was observed in the HSI of T.murphyi. In the warm PDO regime, MLD was shallower, SST increased, and SSHA decreased from the northeast to the southwest off Chile. SSS and Temp_400m in northern waters off Chile were higher than those in southern waters. The suitable habitats of D.gigas contracted and shifted southwestward. While the area and distribution of T.murphyi changed little, but its habitat quality enhanced. In the cold PDO regime, SST and SSHA decreased, and MLD deepened. Variations in SSS and Temp_400m were consistent with those in the warm PDO regime. The suitable habitats of D.gigas enlarged and moved northeastward. Whereas the suitable habitats of T.murphyi slightly reduced with small change occurred with its spatial location. Our findings suggested that the PDO played important roles in the long-term concurrent habitat variations of Chilean T.murphyi and D.gigas.

Pacific Decadal Oscillation modulates the Arctic sea-ice loss influence on the midlatitude atmospheric circulation in winter

Abstract. The modulation of the winter impacts of Arctic sea-ice loss by the Pacific Decadal Oscillation (PDO) is investigated in the IPSL-CM6A-LR ocean–atmosphere general circulation model. Ensembles of simulations are performed with constrained sea-ice concentration following the Polar Amplification Model Intercomparison Project (PAMIP) and initial conditions sampling warm and cold phases of the PDO. Using a general linear model, we estimate the simulated winter impact of sea-ice loss, PDO and their combined effects. On the one hand, a negative North Atlantic Oscillation (NAO)-like pattern appears in response to sea-ice loss together with a modest deepening of the Aleutian Low. On the other hand, a warm PDO phase induces a large positive Pacific–North America pattern, as well as a small negative Arctic Oscillation pattern. Both sea-ice loss and warm PDO responses are associated with a weakening of the poleward flank of the eddy-driven jet, an intensification of the subtropical jet and a weakening of the stratospheric polar vortex. These effects are partly additive; the warm PDO phase therefore enhances the response to sea-ice loss, while the cold PDO phase reduces it. However, the effects of PDO and sea-ice loss are also partly non-additive, with the interaction between both signals being slightly destructive. This results in small damping of the PDO teleconnections under sea-ice loss conditions, especially in the stratosphere. The sea-ice loss responses are compared to those obtained with the same model in atmosphere-only simulations, where sea-ice loss does not significantly alter the stratospheric polar vortex.

Interdecadal Modulation of ENSO‐Related Anomalous Equatorial Intermediate Currents in the Western Pacific by the PDO

AbstractAs the most prominent decadal variability in the Pacific Ocean, the Pacific Decadal Oscillation (PDO) can influence the El Niño‐Southern Oscillation‐related atmospheric and upper oceanic motions. Based on results from ocean reanalysis products and linear continuously stratified model experiments, we reveal that the PDO can further extend its interdecadal modulation effect to the equatorial intermediate currents below the thermocline. For the maximum El Niño‐related intermediate eastward transport anomaly in the western Pacific, its magnitude during the warm PDO is nearly twice as large as that during the cold PDO, and its occurrence time in the warm PDO leads that in the cold PDO by about 2 months. Such modulation is achieved through westward and downward propagation of the reflected Kelvin‐to‐Rossby waves, triggered by enhanced and eastward‐displaced anomalous westerly wind. The decisive wind forcing region for the modulation is located over the equatorial central Pacific of 5°S–5°N and 180°–140°W.

Potential caveats in land surface model evaluations using the US drought monitor: roles of base periods and drought indicators

The US drought monitor (USDM) has been widely used as an observational reference for evaluating land surface model (LSM) simulation of drought. This study investigates potential caveats in such evaluation when the USDM and LSMs use different base periods and drought indices to identify drought. The retrospective national water model (NWM) v2.0 simulation (1993–2018) was used to exemplify the evaluation, supplemented by North American land data assimilation system phase 2 (NLDAS-2). Over their common period (2000–2018), in distinct contrast with the USDM which shows high drought occurrence (>50%) in the western half of the continental US (CONUS) and the southeastern US with low occurrence (<30%) elsewhere, the NWM and NLDAS-2 based on soil moisture percentiles (SMPs) consistently show higher drought occurrence (30%–40%) in the central and southeastern US than the rest of the CONUS. Much of the differences between the LSMs and USDM, particularly the strong LSM underestimation of drought occurrence in the western and southeastern US, are not attributed to the LSM deficiencies, but rather the lack of long-term drought in the LSM simulations due to their relatively short lengths. Specifically, the USDM integrates drought indices with century-long periods of record, which enables it to capture both short-term (<6 months) drought and long-term (⩾6 months) drought, whereas the relatively short retrospective simulations of the LSMs allows them to adequately capture short-term drought but not long-term drought. In addition, the USDM integrates many drought indices whereas the NWM results are solely based on the SMP, further adding to the inconsistency. The high occurrence of long-term drought in the western and southeastern US in the USDM is further found to be driven collectively by the post-2000 long-term warm sea surface temperature (SST) trend, cold Pacific decadal oscillation and warm Atlantic multi-decadal oscillation, all of which are typical leading patterns of global SST variability that can induce drought conditions in the western, central, and southeastern US. Our findings highlight the effects of the above caveats and suggest that LSM evaluation should stay qualitative when the caveats are considerable.

Variability of PDO identified by a last 300-year stalagmite δ18O record in Southwest China

Stalagmite oxygen isotope (δ18O) record has been widely used in characterizing Asian Monsoon variations. In southwest (SW) China, although the stalagmite δ18O was usually suggested as a precipitation proxy, it remains a subject of debate. Here we present two replicated, high-precise-dated stalagmite δ18O records for the interval 1710–2003 AD from Yunnan province, SW China. The stalagmite δ18O shows no correlation with the local monsoon precipitation, but a significantly positive correlation with the Pacific Decadal Oscillation (PDO) index. It demonstrates that the stalagmite δ18O in SW China is most likely influenced by the atmospheric circulation associated with the PDO through the upstream depletion mechanism. During the warm (cold) PDO period, the deficit rainfall over India would make the δ18O of water vapor transporting to SW China less (more) negative due to the weaker Rayleigh distillation process, which would lead to the less (more) negative stalagmite δ18O. Consequently, the stalagmite δ18O in SW China is suitable to reconstruct the PDO variations. It is important to note that the other factors, such as El Niño–Southern Oscillation (ENSO) or sea surface temperature (SST) also possibly affect the stalagmite δ18O in SW China, and the relationship between them requires further studies.

Climatic Variation of Maximum Intensification Rate for Major Tropical Cyclones over the Western North Pacific

To analyze the dependence of intensification rates of tropical cyclones (TCs) on the variation of environmental conditions, an index is proposed here to measure the lifetime maximum intensification rates (LMIRs) for the Saffir–Simpson scale category 4–5 TCs over the western North Pacific. To quantitatively describe the intensification rate of major TCs, the LMIR is defined as the maximum acceleration in the sustained-wind-speed over a 24-h period of an overwater TC. This new index, LMIR, is generally independent of the indices for RI frequency. The results show that the Pacific Decadal Oscillation (PDO) modulates the inter-annual relationship between the LMIR and El Niño/Southern Oscillation (ENSO). The PDO’s modulation on the ENSO’s effect on the LMIR is explored here by considering the relationship between the LMIR and the environmental conditions in different PDO phases. While the ENSO’s effect on the LMIR for the warm PDO phase is generally by affecting the variations of upper ocean heat content, ENSO mainly influences the variations of zonal wind and vertical wind shear for the cold PDO phase. Our results suggest that fast translating TCs tend to attain strong intensification during the warm PDO phase, while a warm subsurface condition may permit slow-translating TCs also to become strongly intensified during the cooling PDO phase. These findings have an important implication for both prediction of RI and the long-term projection of TC activities in the western North Pacific.

Influence of Pacific Decadal Oscillation on global precipitation extremes

While the influence of the Pacific Decadal Oscillation (PDO) on precipitation has been extensively studied, its influence on precipitation extremes remains not well-known. Based on a series of generalized extreme value modeling experiments, this paper demonstrates some distinct regional patterns of the PDO’s influence on precipitation extremes worldwide. In eastern China, the well-known ‘southern flood (drought) and northern drought (flood)’ pattern in summer corresponds well to the positive (negative) phase of the PDO. In Australia, there tends to be a tripole pattern, with positive correlation between precipitation extremes and the PDO in the central region, and negative correlation in both eastern and western Australia. The precipitation extremes in northwestern Europe and western Russia roughly hold positive correlation with the PDO. These regional patterns of the PDO’s influences are explained via comparative analyses of the atmospheric circulation conditions between cold and warm PDO phases. Certain precipitation extremes tend to be missed or happen more than once during different phases of the PDO at more than 2/3 of stations in a typical region. The cold phase tends to exert more consistent influences than the warm phase in these typical regions. These findings not only indicate different risks of extreme precipitation for the typical regions during different phases of the PDO, but also have important implications for the near-term projection of variable regional climate extremes under global warming.

Five centuries of reconstructed streamflow in Athabasca River Basin, Canada: Non-stationarity and teleconnection to climate patterns

Given the challenge to estimate representative long-term natural variability of streamflow from limited observed data, a hierarchical, multilevel Bayesian regression (HBR) was developed to reconstruct the 1489–2006 annual streamflow data at six Athabasca River Basin (ARB) gauging stations based on 14 tree ring chronologies. Seven nested models were developed to maximize the applications of available tree ring predictors. Based on results of goodness-of-fit tests, the HBR developed was skillful and reliable in reconstructing the streamflow of ARB. From five centuries of reconstructed streamflow for ARB, five or six abrupt change points are detected. The streamflow time series obtained from a backward moving, 46-year window for six gauging sites in ARB vary significantly over five centuries (1489–2006) and at times could exceed the 90% and/or 95% confidence intervals, denoting significant non-stationarities. Apparently changes in the mean state and the lag-1 autocorrelation of reconstructed streamflow across the gauging sites can be similar or radically different from each other. These nonstationary features imply that the default stationary assumption is not applicable in ARB. Further, the reconstructed streamflow shows statistically significant oscillations at interannual, interdecadal and multidecadal time scales and are teleconnected to climate patterns such as El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO) and Atlantic Multi-decadal Oscillation (AMO). A composite analysis shows that La Niña (El Niño), cold (warm) PDO, and cold (warm) AMO events are typically associated with increased (decreased) streamflow anomalies of ARB. The reconstructed streamflow data provides us the full range of streamflow variability and recurrence characteristics of extremes spanned over five centuries from which it is useful for us to evaluate and manage the current water systems of ARB more effectively and a better risk analysis of future droughts of ARB.

Longshore drift produced by climate-modulated monsoons and typhoons in the South China Sea

Abstract Monsoons and typhoons impact the tropical coastal zones through their signature on winds and waves, leading to increased vulnerability – through erosion, marine submersion and flooding – of an ever growing coastal population. This study addresses wave-driven coastal impacts in the South China Sea (SCS), particularly along the Vietnam coast. Using 38 years of offshore wave fields from ERA-Interim reanalysis, we assess the seasonal and interannual variability of wave patterns, storminess, and wave-induced alongshore sediment transport (longshore drift). Results suggest a large seasonal coastal impact due to high-energy, northeasterly waves generated by winter monsoon winds. The net annual sediment transport is southward with significant interannual and decadal variations (20% standard deviation with particular years at 40% of the mean), presenting strong correlations with the El Nino Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) regime changes. The regime shift of 1998, from a warm to a cold PDO phase characterized by more La Nina events, is associated with an increase in winter wave energy and thus higher sediment transport (about 10%). The typhoon activity in the SCS is partly associated with that of the Pacific Northwest basin, but as a large semi-enclosed sea, presents local differences. It is positively correlated with La Nina in summer and with El Nino Modoki in winter. The summer correlation to ENSO phases is opposite to that of the whole Northwest Pacific due to competing local and remote mechanisms driving cyclone formation and trajectory. For the same reason, the effects of ENSO and PDO phases are opposite in the SCS: during the last cold PDO phase, typhoon frequency was reduced by 20%, with significant net impact on estimated sediment transport. The typhoon contribution to sediment transport is a 15% reinforcement of southward transport due essentially to winter activity. If winter monsoon and typhoons appear to work together on average, their low-frequency variability are out-of-phase. This is particularly clear at decadal scale, as cold PDO phases seem favorable to strong winter monsoon waves but detrimental to SCS typhoon numbers. These results confirm that regional climate variability (together with human factors affecting river sediment supply, coastal management of beach-dune systems, land subsidence, mangrove deforestation) is an essential part of coastal vulnerability that needs to be better assessed.

The Pacific decadal oscillation modulates the relation of ENSO with the rainfall variability in coast of Ecuador

AbstractWorldwide and likewise in Ecuador, the 1982–1983 and 1997–1998 El Niño Southern Oscillation (ENSO) events had devastating effects in the economic and human dimension. Thus, scientists and decision markers look for a deeper knowledge about ENSO and its phases El Niño (EN)/La Niña (LN). Recent research highlights the changing nature of ENSO under opposite conditions of the Pacific Decadal Oscillation (PDO), making the assessment of the ENSO–PDO relation in Ecuador urgent. This study explores the time‐frequency characteristics of rainfall in the coast of Ecuador from January to April (PC‐JA) and evaluates the influence of PDO in the relation of ENSO with PC‐JA. For this, wavelet analysis was used to asses this nonstationary problem, Five long‐term (1964–2014) ground stations were used. The main results indicate that during the warm PDO period 1980–2000, the high wavelet coherence (ca. 0.9) implies a strong coupling between ENSO and PC‐JA. For cold PDO periods, prior to 1980 and after 2005, such coupling weakens with coherence about 0.5. This might indicate that PDO influence the relation between ENSO and rainfall in the coast of Ecuador. This coupling, during warm PDO, enhances high rainfall when in phase with EN, and drought conditions in LN events. The weak coupling of ENSO‐PC‐JA during cold PDO produces high rainfall amounts in Niño Neutral conditions and droughts during Neutral and LN. To account for ENSO flavours variability, the wavelet coherence between PC‐JA and the two ENSO uncorrelated indices E and C from Takahashi et al. was studied. Interestingly, we show that PDO warm phase influences the relation of Eastern Pacific related E index with PC‐JA from 2 to 8 years periods, and that the orthogonal Central Pacific related C index is not affected. These results raise questions about the validity of ENSO indices for contrasting PDO phases.

The Influence of the Atlantic Multidecadal Oscillation on the Choco Low-Level Jet and Precipitation in Colombia

This study examines the influence of the Atlantic Multidecadal Oscillation (AMO) on the Choco Low-level Jet (CJ) variations during the 1983–2016 period. Considering the September–November (SON) 925 hPa zonal wind index in the CJ core, a significant breakpoint occurs in 1997 with larger values after 1997. The changes in the CJ and Caribbean Low-Level Jet (CLLJ), and their related ocean-atmospheric patterns and impacts on precipitation over Colombia were analyzed considering separately the 1983–1996 and 1998–2016 periods, which overlap the cold and warm AMO phases, respectively. During the 1998–2016 period, the negative sea surface temperature (SST) anomalies in the tropical Pacific Ocean and the positive ones in the Caribbean Sea and Tropical North Atlantic (TNA) strengthen the CJ and weaken the CLLJ, and moisture is transported into Central and Western Colombia increasing the rainfall there. Our results indicate that part of the CJ strengthening after 1997 was due to a higher percentage of intense CJ events coinciding with La Niña events during the warm AMO and cold Pacific Decadal Oscillation (PDO) background. However, the AMO-related SST and sea level pressure (SLP) variations in the TNA seem to be more crucial in modulating the CJ and CLLJ intensities, such that CJ is weakened (intensified) and CLLJ is intensified (weakened) before (after) 1997. As far as we know, the relations of the CJ and CLLJ intensities to the AMO phases were not examined before and might be useful for modeling studies.

Decadal Changes in Interannual Dependence of the Bay of Bengal Summer Monsoon Onset on ENSO Modulated by the Pacific Decadal Oscillation

Interannual variations of the Bay of Bengal summer monsoon (BOBSM) onset in association with El Nino-Southern Oscillation (ENSO) are reexamined using NCEP1, JRA-55 and ERA20C atmospheric and Hadley sea surface temperature (SST) reanalysis datasets over the period 1900–2017. Decadal changes exist in the dependence of the BOBSM onset on ENSO, varying with the Pacific Decadal Oscillation (PDO). A higher correlation between the BOBSM onset and ENSO arises during the warm PDO epochs, with distinct late (early) onsets following El Nino (La Nina) events. In contrast, less significant correlations occur during the cold PDO epochs. The mechanism for the PDO modulating the ENSO-BOBSM onset relationship is through the variations in SST anomaly (SSTA) patterns. During the warm PDO epochs, the superimpositions of the PDO-related and ENSO-related SSTAs lead to the SSTA distribution of an El Nino (La Nina) event exhibiting significant positive (negative) SSTAs over the tropical central-eastern Pacific and Indian Ocean along with negative (positive) SSTAs, especially over the tropical western Pacific (TWP), forming a strong zonal interoceanic SSTA gradient between the TWP and tropical Indian Ocean. Significant anomalous lower tropospheric easterlies (westerlies) together with upper-tropospheric westerlies (easterlies) are thus induced over the BOB, favoring an abnormally late (early) BOBSM onset. During the cold PDO epochs, however, the superimpositions of PDO-related SSTAs with El Nino-related (La Nina-related) SSTAs lead to insignificant SSTAs over the TWP and a weak zonal SSTA gradient, without distinct circulation anomalies over the BOB favoring early or late BOBSM onsets.

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.

Rainfall variability and vegetation recovery in rocky desertification areas recorded in recently-deposited stalagmites from Guilin, South China

An actively growing stalagmite (sample DY-2) from Maomaotou Big Cave in Guilin, south China was dated using a combined approach of 230Th/U, 210Pb and AMS 14C dating techniques. The post-bomb 14C curve shows that the DY-2 was deposited during 1964–2009 CE and a fast growth rate of ∼2.25 mm/a was determined for this 9.3 cm-long stalagmite. The 210Pb dating of the top 7 mm part of another stalagmite sample PL4 from Panlong Cave located in the same area as the Maomaotou Big Cave constrained a depositional interval during 1830–2009 CE with a much slower growth rate of ∼0.04 mm/a. The δ18O variations of DY-2 and PL4 are highly comparable on decadal time-scales within age uncertainties, and both reveal a good coherence with local instrumental rainfall and historic dry-wet index records. These two δ18O records suggest that rainfall variability in the region of Guilin is modulated by Pacific Decadal Oscillation (PDO) and El Niño-Southern Oscillation (ENSO), as the latter can affect the East Asian Summer Monsoon (EASM) and Western Pacific Subtropical High (WPSH). On annual-to-decadal scales, the increase of EASM intensity and WPSH as well as the shift of negative (cold) PDO and La Niña phases would enhance the summer rainfall in the Guilin region, as revealed by the lower stalagmite δ18O values. While weakening EASM and WPSH with positive (warm) PDO and El Niño phases tend to lead to a dry climate in this region of south China. In addition, the stalagmite δ13C and Dead Carbon Proportion (DCP) records show gradually decreasing trends over the last several decades. According to the findings of synchronous increase in both soil pCO2 and dissolved inorganic carbon (DIC) concentrations in seepage waters from the area of Guilin, the variation of stalagmite δ13C and DCP may reflect a history of vegetation recovery in such a rocky desertification area like Guilin, where the area of rocky desertification has been reducing since the 1990s due to afforestation policies and measures.

Analysing the influences of ENSO and PDO on water discharge from the Yangtze River into the sea

AbstractThe El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) are two important climate oscillations that affect hydrological processes at global and regional scales. However, few studies have attempted to identify their single and combined influences on water discharge variability at multiple timescales. In this study, we examine temporal variation in water discharge from the Yangtze River into the sea and explore the influence of the ENSO and the PDO on multiscale variations in water discharge over the last century. The results of the wavelet transform analysis of the water discharge series show significant periodic variations at the interannual timescale of 2 to 8 years and the decadal timescale of 15 to 17 years. Water discharge tended to be higher during the La Niña–PDO cold phase and lower during the El Niño–PDO warm phase. The results of the cross wavelet spectrum and wavelet coherence analyses confirm the relationship between the interannual (i.e., 2 to 8 years) and decadal (i.e., 15 to 17 years) periodicities in water discharge with the ENSO and the PDO, respectively. As an important large‐scale climate background, the PDO can modulate the influence of the ENSO on water discharge variability. In general, the warm PDO enhances the influence of El Niño events, and the cold PDO enhances the influence of La Niña events. Our study is helpful in understanding the influencing mechanism of climate change on hydrological processes and provides an important scientific guideline for water resource prediction and management.