Cold Phase Of 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.

Interannual Variations of the North Equatorial Current Across the Pacific Ocean

AbstractThe interannual variation of the North Equatorial Current (NEC) across the North Pacific Ocean and the modulation of the Pacific Decadal Oscillation (PDO) are investigated based on both observations and reduced gravity model experiments. Results reveal that the interannual variation of the NEC is highly related to El Niño‐Southern Oscillation (ENSO) with unsynchronized variations both in zonal and meridional structure. The tropical and subtropical gyre branches of the NEC (NEC‐S and NEC‐N) behave antiphase variation during El Niño events with the transport of the former being enhanced and the latter being weakened across the Pacific. The NEC axis in the western/eastern Pacific shifts northward/southward during El Niño events, consistent with the meridional shift of the zero‐line of wind stress curl. Model experiments indicate that Rossby waves mainly modulate the NEC near the western boundary and the NEC‐N, while Ekman pumping modulates NEC‐S in the western basin. Meanwhile, the relationship between the interannual variation of the NEC and ENSO events is significantly modulated by different PDO phases. The PDO cold phase favors a much closer connection between the NEC and the Niño 3.4 index. The dynamics lie in that PDO‐related wind stress curl and sea level anomalies are consistent with that of El Niño, favoring the enhanced correlation between NEC and El Niño. The reverse is true for the warm phase. These results will contribute to a better understanding of the behavior of the tropical Pacific Ocean circulation and its role in climate change.

Changes in ENSO Modulation of the Distribution of Rapidly Intensifying Tropical Cyclones over the Western North Pacific in Boreal Autumn

Abstract This work explores the modulation of the Pacific decadal oscillation (PDO) on the relationship between the occurrence position of rapid intensification (RI) events of tropical cyclones (TCs) over the western North Pacific (WNP) in boreal autumn and El Niño–Southern Oscillation (ENSO). From the warm to cold phase of the PDO, the occurrence position of WNP RI events experiences a significant westward shift of 5.5° in El Niño years and a significant northward shift of 4.5° in La Niña years. The strengthening of thermodynamic conditions west of 160°N plays a dominant role in the westward shift of RI events in El Niño years, and the northward shift in La Niña years is associated with the expansion of areas with warm sea surface temperature, high tropical cyclone heat potential and midlevel relative humidity, strengthening of relative vorticity north of 20°N, and weakening of dynamic conditions within 10°–20°N. During the PDO cold phase, the descending branch of the Walker circulation over the western Pacific is weak and shifts west of 140°E in El Niño years, whereas it is much stronger in La Niña years. In addition, the Hadley circulation over the WNP shows little change during El Niño, but the ascending branch around 10°N expands to 20°N during La Niña. These trends reflect the changing responses of the WNP environment to ENSO variation and are consistent with the changing distribution of WNP RI events. Moreover, during the PDO cold phases, SST over the north Indian Ocean is much warmer, and anomalous anticyclonic circulation occurs in the WNP in boreal spring (summer and autumn) during the developing phase of El Niño (La Niña) years, which may also contribute to strengthening the thermodynamic conditions over the WNP.

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.

Decadal variation of the relationship between the previous autumn IOD and the South China Sea summer monsoon

Using reanalysis data from the ECMWF and Hadley Centre of the UK Meteorological Office, the present study analyzes the relationship between Indian Ocean Dipole (IOD) in the previous autumn (September–November) and South China Sea Summer (June–August) Monsoon (SCSSM) from 1951 to 2021. It is found that the relationship between the two indices was significantly enhanced from the late 1970s to the beginning of the 21st century, with a significant resonance cycle of 2–4a. The previous autumn IOD forced the Gill pattern in the upper troposphere by changing the Sea Surface Temperature Anomaly (SSTA) of the following summer in the Tropical Indian Ocean (TIO), resulting in the anticyclonic circulation anomaly in the Philippine Sea. It eventually alters the southwest monsoon near the South China Sea and the Philippine Sea. However, for a decade or more, as the Pacific Decadal Oscillation (PDO) phase turns from warm to cold and the relationship between El Niño–Southern Oscillation (ENSO) and IOD weakens, the impact of the previous autumn IOD on Indian Ocean SSTA in the summer of the following year weakens, resulting in a decrease in the correlation between the previous autumn IOD and SCSSM. The impact of the previous autumn’s Indian Ocean SSTA on the following year’s SCSSM is more considered in terms of the eastern Bay of Bengal and the Somali Sea basin. In addition, during the warm phase of PDO, the IOD of the previous autumn can cause abnormal summer precipitation in South China by strengthening or weakening the Western Pacific Subtropical High (WPSH) of the following summer, but there is no significant effect in the cold phase of PDO.

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.

Seasonality of precipitation recorded in a modern (1907–2008) annually laminated stalagmite from central China

Seasonality of precipitation in the Asian monsoon region has significant impacts on social and economic development. Here, we analyzed a seasonally resolved δ18O series of an annually laminated stalagmite from Xianglong Cave to assess its potential as a proxy for seasonal precipitation. The δ18O values reveal fabric-correlated annual cycles, with lower δ18O values occurring in the white porous layer (WPL) during summer monsoon (SM) season, whereas higher values occurring in the dark compacted layer (DCL) during non-summer monsoon (NSM) season. We calculated the seasonal amplitude of δ18O (∆18O) using the highest value minus the lowest value in an annual cycle. Comparisons suggest that the ∆18O series can record precipitation seasonality, with lower ∆18O values corresponding to increased SM/NSM rainfall ratios and higher values corresponding to reduced SM/NSM rainfall ratios. Our reconstruction suggests increased precipitation seasonality contrast (i.e., increasing SM/NSM rainfall ratio with more SM rainfall and/or less NSM rainfall) during 1914–1919, 1935–1942, 1958–1962, 1979–1985, and 1999–2005 (A.D.), with a probably 27-year cycle. The seasonality of precipitation in central China correlates well with the Pacific Decadal Oscillation (PDO) on a decadal scale. Increased SM/NSM rainfall ratio was observed during the warm phase of PDO, and decreased SM/NSM rainfall ratio was observed during the cold phase of PDO.

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.

Impact of Pacific Decadal Oscillation in relation to QBO on Indian summer monsoon rainfall

The relationship between Indian Summer Monsoon (ISM) rainfall (June to September) and Pacific Decadal Oscillation (PDO) Index for the concurrent and succeeding seasons over all India and its three specific regions, i.e., northeast, northwest, and peninsular region has been examined from the years 1953 to 2016. The information is shared for the full series as well as by grouping the seasonal rainfall of India and PDO data according to the different phases (westerly/easterly) of quasi-biennial oscillation (QBO) at the 50 hPa level. The results suggest that for full series, the concurrent season July–August–September (JAS) shows higher degree of association as compared with June–July–August (JJA). When the seasonal rainfall and the PDO index data are stratified according to the phase of QBO, correlation coefficient (CC) values are showing a strong inverse association during the easterly phase of QBO over all India and the northwest region of India while the CC value is reduced during the westerly phase. For the northeast Indian region, concurrent CC values are showing high (low) during the westerly (easterly) phase of QBO. In addition, the warm PDO events are persistent during the summer monsoon period in association of easterly (westerly) phase of QBO causes drought (flood) events over India. However, the cold phase of PDO follows the easterly and westerly phase of QBO where the indication of drought and flood events is not persistent.

Intrusion of Kuroshio Helps to Diminish Coastal Hypoxia in the Coast of Northern South China Sea

Since half century ago, the number and area of dead zones (dissolved oxygen (DO) < 2 mg L-1 or 30% saturation) in the coastal oceans have increased dramatically. As is widely recognized the increased terrestrial nutrient and organic matter inputs are the two main factors causing the eutrophication of many coastal oceans. Here we show with decadal observed time series data from stations off the Pearl River Estuary and in the northern South China Sea (nSCS) that a strong intrusion into the nSCS of the West Philippine Sea (WPS) seawater in the form of Kuroshio branch occurred during the warm phase of the Pacific Decadal Oscillation (PDO) around 2003–2004 and 2015–2016 (also a strong El Nino event). Consequently, the DO concentration increased but NO3- and PO43- concentrations decreased in the subsurface layers of the nSCS. The WPS seawater was observed to reach the hypoxic area off the Pearl River Estuary in 2003–2004. Likely, due to the oxygen supply carried by the Kuroshio, little hypoxia developed. Yet, anoxic condition developed in the cold phase of PDO or strong La Nina years with weak Kuroshio intrusions.

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.

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 Tropical Indian Ocean decadal sea level response to the Pacific Decadal Oscillation forcing

The tropical Indian Ocean sea level displayed decadal variations in response to Pacific Decadal Oscillation (PDO). Contrasting patterns of decadal oscillation in sea level is found during the opposite phases of PDO especially in the thermocline ridge region of the Indian Ocean (TRIO; 50°E–80°E; 15°S–5°S). Epochal mean sea level rise is observed over the TRIO region during the cold phase of PDO (1958–1977), whereas epochal mean sea level fall is observed during the warm phase of PDO (1978–2002). Analysis reveals that the decadal variability in the sea level pattern in the TRIO region is in accordance with the PDO phase shifts and is primarily caused by changes in the surface forcing over the Indian Ocean as a response to PDO. The changes in the large scale Walker circulation over the tropical Indian Ocean region during the different phases of PDO support our hypothesis. The winds and wind stress curl variations associated with these large scale circulation changes are primarily inducing the observed regional decadal sea level variability over TRIO. The decadal forcing through Indonesian Through Flow (ITF; oceanic channel) however did not show any significant impact on the TRIO sea level variability. Further Ocean General Circulation Model (OGCM) sensitivity experiments are carried out to understand the mechanisms and the possible contribution of the Pacific Ocean through oceanic pathways, in the decadal variability of the TRIO sea level. It is noted that wave propagation from the Pacific Ocean to the Indian Ocean through ITF region has contributed to the sea level variations in the eastern Indian Ocean. But on the decadal time scale, tropical Indian Ocean/TRIO sea level is unaffected by decadal variability in the ITF. Moreover, the ITF contribution to the decadal sea level variability in the Indian Ocean is found to be significant only in the region south of 20°S.

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.

Pacific decadal oscillation impact on East China precipitation and its imprint in new geological documents

This paper briefly introduces the history of the study of the Pacific decadal oscillation (PDO) and explores the relationship between the PDO and sediment grain size from two typical sediment cores from the lower Changjiang (Yangtze River) and Huanghe (Yellow River) estuaries. It is found that the median grain sizes of both cores exhibit relatively high correlation with the PDO. This is because the PDO causes interdecadal variability of precipitation in the East Asia Monsoon region, thus changing the hydrodynamics in both the Changjiang and Huanghe catchments, eventually resulting in variation of sediment grain size. Our analysis also revealed that during different phases of the PDO, the sediment grain size of the Changjiang and Huanghe estuaries showed different variations in cold and warm PDO phases. This is related to movement of the precipitation center driven by the shift in different PDO phases. Moreover, we compared more high resolution geological proxies with the PDO, including stalagmites and tree rings, in East China over the past century. The results indicate that variations of studied geological proxies are generally well correlated with the PDO but have some differences. Finally, longer variations of sediment grain sizes in the Changjiang and Huanghe estuaries are compared with a reconstructed PDO over the last 200 years; sediment grain sizes were still correlated with the PDO, implying that sediment grain size may be used as a new proxy for studying the long-term behavior of the PDO. This result supports previous knowledge of the PDO impact on East China climate evolution and offers a new proxy for further PDO study. Our study will improve paleoenvironment reconstruction in East China on a decadal time scale and benefit future climatic predictions.

Modulation of the PDO to the relationship between moderate ENSO events and the winter climate over North America

ABSTRACTThe modulation of the Pacific Decadal Oscillation (PDO) to the relationship between the El Niño‐Southern Oscillation (ENSO) events and the winter climate over North America (NA) was investigated using both observational data and a simple general circulation model. ENSO events are divided into strong and moderate events according to their amplitudes. These ENSO events are further categorized into two groups according to the phases of the PDO. The present study focuses on the impact of moderate ENSO events on the wintertime climate over NA. The results show that a moderate El Niño signal is strong and stable and can significantly influence the wintertime climate variability over NA during a cold PDO phase, whereas climate anomalies are weak and are not significantly associated with moderate El Niño events during a positive PDO phase. Possible mechanisms accounting for the modulation of the impacts of moderate El Niño events on the wintertime climate over NA are analysed. Two factors may contribute to the different impacts of moderate El Niño events during warm and cold PDO phases. First, the different characteristics of the tropical Pacific sea surface temperature anomalies (SSTAs) associated with moderate El Niño events. Second, the difference in the climatological mean state between the warm and cold phases of PDO years. Our numerical experiments suggest that the cold PDO mean state is more favourable to a wavetrain‐like atmospheric response to an El Niño‐like tropical Pacific forcing over the North American sector than the warm PDO mean state is.

The influence of large-scale climate phenomena on precipitation in the Ordos Basin, China

Large-scale atmospheric circulations significantly affect regional precipitation patterns. However, it is not well known whether and how these phenomena affect regional precipitation distribution in northern China. This paper reported the individual and coupled effects of El Nino–Southern Oscillation (ENSO), Indian summer monsoon (ISM), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO) on annual precipitation for the Ordos Basin, an arid and semi-arid basin, currently with major industries of coal, fossil oil, natural gas, and halite in north central China. Our results showed that ENSO and ISM exerted substantial impact on annual precipitation while the impact of PDO and AMO was relatively limited. There were 24 and 15 out of 33 stations showing significant differences (p < 0.1) in annual precipitation (from 1950 to 2013) for ENSO and ISM, respectively. The median precipitation across the basin during El Nino years was 21.49 % higher than that during La Nina years and 17.28 % higher during the positive phase of ISM years compared to the negative phase of the ISM years. The impacts of ENSO and ISM on precipitation were enhanced during a PDO cold phase but weakened in a PDO warm phase. The impact of ENSO was still enhanced by an AMO warm phase. The effects of climatic phenomena on precipitation showed a strong spatial difference in the Ordos Basin. The impact of ENSO was mostly evident around the edges of the basin while the impact of ISM decreases from south to north. The deserts (i.e., Hobq Desert and Mu Us Sandy Land) in the center of the basin were less affected by these large-scale climatic phenomena. An improved understanding of such relationships would be helpful in water resource planning and disaster management for the Ordos Basin.