Pacific Decadal Oscillation Phase Research Articles

Global teleconnections in droughts caused by oceanic and atmospheric circulation patterns

Long-duration droughts are usually tied to persistent local or remote forcings; for example, persistent droughts over California are frequently observed along with the ‘ridiculously resilient ridge’ over the West Coast. It is now evident that some oceanic forcings (e.g. El Niño–Southern Oscillation) have global reaches and affect multiple regions concurrently during their progression. Here, we show robust significant temporal concordancy of persistent droughts in many regions, revealing multiple teleconnections (distant regions experiencing droughts concurrently), such as the ‘Western North America–Mediterranean (WNA–MED)’ and the ‘Southeast Asia–Southern Africa (SEA–SAF)’ teleconnections. Composite pressure and sea surface temperature anomalies during concurrent droughts in WNA and the MED reveal a persistent weather regime that resembles the positive phase of Arctic Oscillation and negative phase of Pacific Decadal Oscillation. During concordant droughts of SEA and SAF, composite pressure anomalies remarkably resemble the El Niño pattern, which we infer as the leading cause of the teleconnection. The insights gained here offer a new dimension to understanding droughts and improving their long-term predictability.

Interannual-decadal variation in satellite-derived surface chlorophyll-a concentration in the Bohai Sea over the past 16 years

Chlorophyll-a (Chl-a) concentration is an important indicator of the state of marine ecosystems. In the current study, the interannual-decadal variation in the Bohai Sea's surface Chl-a concentration and the underlying dynamics are investigated based on 16 years (August 2002–February 2019) of satellite ocean colour observations. The Chl-a concentration shows a spatially coherent increasing trend from 2003 to 2011 and a decreasing trend from 2012 to 2018 in both the Bohai and North Yellow Seas. Both decadal trends are clearer in the Bohai Sea than in the Yellow Sea and in coastal waters than in offshore waters. The decreasing trend is more obvious than the increasing trend. The low-frequency variations are mainly controlled by sea surface temperature (SST) and rainfall variations and eventually related to the Pacific Decadal Oscillation (PDO). During positive PDO phases, warm SST anomalies exist in the Bohai and Yellow Seas and negative rainfall anomalies exist over the sea and surrounding lands. These anomalies would decrease the surface layer dissolved inorganic nitrogen concentration through suppressing the upward turbulent nutrient diffusion and reducing land-sourced nutrient fluxes into the ocean, leading to negative Chl-a concentration anomalies. During negative PDO phases, the opposite situation occurs. Human activities may make a secondary contribution through enhancing/reducing the anthropogenic nitrogen emission before/after 2011. This study emphasizes the natural climate variability in modulating low-frequency variations in surface Chl-a concentrations in the Bohai and Yellow Seas in recent decades and is useful for marine resource planning and marine ecological management in the Bohai Sea in the future. • Surface chlorophyll-a concentration in the Bohai Sea has interannual-decadal variation. • Spatially coherent upward trend over 2003–2011 and downward trend over 2012–2018. • Downward trends are more obvious than upward trends. • Chlorophyll-a concentration variation is related to the Pacific Decadal Oscillation.

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.

Changes in the role of Pacific decadal oscillation on sea ice extent variability across the mid-1990s

Characteristics of sea ice extent (SIE) have been rapidly changing in the Pacific Arctic sector (PAS) in recent years. The SIE variability in PAS during the late spring and early summer (i.e., April–May–June, AMJ) plays a key role in determining the SIE during the following fall when SIE is at a minimum. We find that the Pacific Decadal Oscillation (PDO), which is the most dominant variability of sea surface temperature (SST) on the low-frequency timescales, differently influences the SIE in PAS during AMJ before and after the mid-1990s. While a positive phase of PDO during the previous winter acts to increases SIE during AMJ before the mid-1990s, it acts to decrease SIE during AMJ after the mid-1990s. Further analysis indicates that atmospheric circulation associated with PDO differently influences the variability of SIE in the PAS during AMJ by modulating poleward moisture transport across the Alaska or the Far East Asia peninsula. This results in the change in the relationship of PDO and SIE in the PAS before and after the mid-1990s.

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.

Trans-Pacific multidecadal changes of habitat patterns of two squid species

Trans-Pacific changes in the habitat patterns of neon flying squid (Ommastrephes bartramii) in the Northwest Pacific Ocean and jumbo flying squid (Dosidicus gigas) in the Southeast Pacific Ocean during 1950–2015 were examined on a decadal scale using a habitat suitability index (HSI) model. The HSI model involved two environmental drivers for the occurrence of O. bartramii and D. gigas: sea surface temperature (SST) and sea surface height anomaly (SSHA). Significant seesaw-like patterns of environmental variability and habitat changes, consistent with the regime shift of positive and negative Pacific Decadal Oscillation (PDO), were observed for the two squid species. In a negative PDO phase, SST anomaly and SSHA increased on the fishing grounds of O. bartramii. The areas of suitable habitat of O. bartramii contracted markedly due to the decrease in suitable SST and SSHA. In contrast, cooling SST and decreasing SSHA on the fishing grounds of D. gigas during a negative PDO phase led to the formation of more suitable SST and SSHA, yielding expansive suitable habitats for D. gigas. Moreover, the most favorable SST for O. bartramii and D. gigas moved northward in the negative PDO regime, and the suitable habitat for both squids shifted into the northern regions as well. When the climate regime shifted into a positive PDO phase, the responses of the two squids were opposite to those in the negative PDO phase. Our findings indicate that interannual-to-decadal variability in habitat patterns of O. bartramii and D. gigas can be detected from climate regime shifts in the Pacific Ocean.

Effects of the Pacific Decadal Oscillation on the Characteristics of Two Types of El Niño under Possible Climate Change

The effect of the Pacific Decadal Oscillation (PDO) phase on the intensity and localization of anomalies in sea surface temperature and meteorological parameters associated with two types of El Nino is analyzed. The analysis is based on reanalysis and CMIP5 model data for modern and future climate (RCP 8.5 scenario). It is found that El Nino does not exhibit robust intensification during the positive PDO phase, as mentioned in the previous studies, but the PDO influence is highly dependent on the El Nino type. The canonical El Nino is intensified during the positive PDO phase, while, on the contrary, El Nino Modoki weakens. In future climate the interaction between PDO and El Nino is modified, but the significant intermodel spread does not allow an unambiguous determination of trends.

CP El Niño and PDO Variability Affect Summer Precipitation over East China

The summer precipitation produced by the East Asian summer monsoon (EASM) is significantly affecting agriculture and socioeconomics. Based on the Precipitation Reconstruction dataset in East China from 1950 to 2017, we investigate the spatiotemporal variations of summer precipitation, influencing environmental factors and their relation with the EASM and the Pacific Decadal Oscillation (PDO) in both central Pacific (CP) El Niño developing and decaying years. Results indicate the following: (1) The evolutions of CP El Niño events modulate the summer precipitation anomalies in East China. In the cool PDO phase, CP El Niño causes enhanced precipitation anomalies in the decaying years but less precipitation anomalies in the developing years, and vice versa for the warm PDO phase. (2) Atmospheric circulation anomalies drive the moisture transportation and combine the motion of western Pacific subtropical high resulting in the variation of precipitation patterns. Anomalous cyclone over the western North Pacific and the sustained Western Pacific Subtropical High (WPSH) are favorable for the increment of summer precipitation. (3) The different CP El Niño-EASM relationship is caused by the influences of PDO on the evolution of CP El Niño. CP El Niño develops slowly (decays rapidly) and is associated with rapidly developing (slowly decaying) anomalous warming in the north Indian Ocean during the developing (decaying) years.

Climatic oscillations effect on the yellowfin tuna (Thunnus albacares) Spanish captures in the Indian Ocean

AbstractThe yellowfin tuna (Thunnus albacares) (YFT) is among the eight marine species with the highest catches globally. The Spanish purse seine freezer fleet operating in the Indian Ocean is one of the most important YFT fishing fleets in the world. The South Oscillation Index (SOI), Pacific Decadal Oscillation (PDO), and Indian Ocean Dipole (IOD) are interrelated, and have combined effects in the Indian Ocean. Moreover, Madden–Julian Oscillation (MJO) is the dominant component of intraseasonal variability in the tropical Indian and Pacific oceans where the sea surface is warm. The main aim of present study is to understand the effect of these four climatic oscillations on Spanish purse seine YFT catches in the Indian Ocean. The ultimate goal is to estimate the specific time lag of the effect of each climatic oscillation on the YFT catches for management purposes. To estimate this, we adjusted different General Additive Models between the response variable (corrected YFT catches per unit of effort per year), compared to a combination of SOI, PDO, IOD, and MJO lagged up to 8 years. Our results suggest that there is a lagged effect modulated mainly by PDO‐SOI, which could be related to a good recruitment, larval survival, or improved spawning. Thus, negative PDO phase (or positive SOI phase) lagged between 3 and 6 years could favor future stock abundance, while positive PDO phase (or negative SOI phase) lagged 3 or 6 years could negatively affect future stock abundance.

Chemistry of the Karst Sarabkalan Spring, Iran, and Controls of PDO and ENSO Climate Indices on It.

The Sarabkalan Spring serves as a primary water supply to irrigation and domestic use in the Sirvan Region, Iran. As it has a highly variable discharge, understanding its teleconnections with large-scale climate variability is crucial. In this study, we first characterize the springshed and its corresponding karst aquifer system using genetic algorithm analysis on the spring discharge, water balance calculations, temporal variations of physicochemical parameters, and stable isotopes along with considering its geological settings. Then, the large-scale climate indices teleconnections with precipitation and spring discharge are studied using wavelet analysis. Results reveal that the springshed contains two karst subaquifers resulting from geological and morphological settings. Unlike most developed karst systems in Zagros, which show one peak, the spring has two principal flow peaks over most hydrological years where the second peak occurs over the dry season. It takes ∼99 d (from lag correlation over 2008-2019) and ∼145 d (from δ18 O measurements over 2018-2019) for rain water to reach the Sarabkalan Spring. Moreover, intense precipitations would result in an increase in discharge and a decrease in electrical conductivity, Ca + Mg, HCO3 , SO4 , Cl, ionic strength, and δ18 O of the spring because of the developed karst conduit(s). It is further found that a positive Pacific Decadal Oscillation phase coupled with El Niño causes an increase in both the precipitation and spring discharge, signifying the influence of the atmospheric circulations of the Pacific Ocean on the spring behavior.

Interdecadal Variations in Extreme High–Temperature Events over Southern China in the Early 2000s and the Influence of the Pacific Decadal Oscillation

This study documents a sudden interdecadal variation in the frequency of extreme high–temperature events (FEHE) over southern China during summer in the early 2000s, which is characterized by a relatively small (large) FEHE during 1991–2000 (2003–2018). The composite analysis on the extreme high–temperature events (EHEs) over southern China indicates that the occurrence of EHEs is mainly influenced by increased downward surface net shortwave radiation, which is induced by the cloud–forced radiation anomalies associated with reduced cloud; the reduced cloud is attributed to anomalous descent motion and decreased water vapor content in the troposphere. Compared to the situation during 1991–2000, anomalous descent motion and decreased atmospheric water vapor content occurred over southern China in summer during 2003–2018, providing a more favorable climatic condition for EHEs. This interdecadal variation is associated with the strengthened Pacific Walker circulation after 2003. The Pacific decadal oscillation (PDO) is suggested to be an important driver for the above interdecadal variation, which shifted from a positive phase towards a negative phase after 2003. Numerical experiments demonstrate that a negative phase of PDO may induce a strengthened Walker circulation and anomalous atmospheric descent motion as well as water vapor divergence over Southern China.

The combined influence of ENSO and PDO on the spring UTLS ozone variability in South America

Ozone in the upper troposphere–lower stratosphere (UTLS) is primarily regulated by tropospheric dynamics. Understanding mechanisms driving ozone variability at the UTLS is crucial to evaluate the transport of mass to and from the lower stratosphere. The El Niño-Southern Oscillation (ENSO) is the primary coupled mode acting on interannual timescales modulating tropospheric circulation worldwide. ENSO teleconnections can depend on the phases of the Pacific Decadal Oscillation (PDO) and on the characteristics of the warming over central and eastern tropical Pacific. This study investigates the role of ENSO on UTLS ozone variability with focus on South America and examines patterns of teleconnections in the two recent warm (1980–1997) and cool (1998–2012) PDO phases. The dominant mode of ozone variability is identified by applying a principal component analysis (PCA) to modern-era retrospective analysis for research and applications, Version 2 (MERRA-2) ozone data from September–November (SON). SON is the season with the largest UTLS ozone variance over South America. The first mode resembles a Rossby wave train across South America with spatial patterns dependent on PDO phase. We show that the ENSO teleconnections and respective influences on SON UTLS ozone are stronger during the cool PDO when ENSO and PDO are mostly in phase. Additionally, the strength of the ENSO teleconnection appears to depend on patterns of SST anomalies over tropical Pacific. The decadal variability in the ENSO-PDO relationships and teleconnections with the Southern Hemisphere resulted in a shift in upper tropospheric circulation in tropical and subtropical regions of South America.

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.

Ocean–atmosphere coupled Pacific Decadal variability simulated by a climate model

Currently, the mechanisms for Pacific Decadal Oscillation (PDO) are still disputed, and in particular the atmosphere response to the ocean in the mid-latitude remains a key uncertainty. In this study, we investigate two potential feedbacks—a local positive and a delayed negative—for the PDO based on a long-term control simulation using the ECHAM5/MPI-OM coupled model, which is selected because of reproduces well the variability of PDO. The positive feedback is as follows. In the PDO positive phase, the meridional sea surface temperature (SST) gradient is intensified and this strengthens the lower level atmospheric baroclinicity in the mid-latitudes, leading to the enhancement of Aleutian low and zonal wind. These atmospheric changes reinforce the meridional SST temperature gradient through the divergence of ocean surface currents. The increased heat flux loss over the anomalously warm water and decreased heat flux loss over the anomalously cold water in turn reinforce the lower atmospheric meridional temperature gradient, baroclinicity and atmospheric circulation anomalies, forming a local positive feedback for the PDO. The delayed negative feedback arises, because the intensified meridional SST gradient also generates an anticyclonic wind stress in the central North Pacific, warming the upper ocean by Ekman convergence. The warm upper ocean anomalies then propagate westward and are transported to the mid-latitudes in the western North Pacific by the western boundary current. This finally reduces the meridional SST gradient, 18 years after the peak PDO phase. These results demonstrate the significant contributions of the meridional SST gradient to the PDO’s evolution.

A Statistical Comparison of the Westerly Wind Bursts between the Positive and Negative Phases of the PDO

To understand the diversity of the El Nino-Southern Oscillation (ENSO) under the background of Pacific decadal oscillation (PDO) during recent decades, characteristics of westerly wind bursts (WWBs) during positive and negative phases of the PDO were analyzed. It is shown that, during the ENSO developing period, the El Nino evolution may be affected by stronger or more frequent WWBs in the positive PDO phase than in the negative PDO phase. The sustained effects of atmospheric dynamics on the equatorial ocean can be indicated by the accumulated WWB strength, which contains most WWB characteristics, including the accumulated days, occurrence frequency, strength, and spatial range of WWBs. The synoptic/climate systems that are directly related to WWBs show a wider spatial distribution in the positive PDO phase than in the negative PDO phase.

Large‐Scale and Local Climatic Controls on Large Herbivore Productivity: Implications for Adaptive Rangeland Management

Rangeland ecosystems worldwide are characterized by a high degree of uncertainty in precipitation, both within and across years. Such uncertainty creates challenges for livestock managers seeking to match herbivore numbers with forage availability to prevent vegetation degradation and optimize livestock production. Here, we assess variation in annual large herbivore production (LHP, kg/ha) across multiple herbivore densities over a 78-yr period (1940-2018) in a semiarid rangeland ecosystem (shortgrass steppe of eastern Colorado, USA) that has experienced several phase changes in global-level sea surface temperature (SST) anomalies, as measured by the Pacific Decadal Oscillation (PDO) and the El Nino-Southern Oscillation (ENSO). We examined the influence of prevailing PDO phase, magnitude of late winter (February-April) ENSO, prior growing-season precipitation (prior April to prior September) and precipitation during the six months (prior October to current April) preceding the growing season on LHP. All of these are known prior to the start of the growing season in the shortgrass steppe and could potentially be used by livestock managers to adjust herbivore densities. Annual LHP was greater during warm PDO irrespective of herbivore density, while variance in LHP increased by 69% (moderate density) and 91% (high density) under cold-phase compared to warm-phase PDO. No differences in LHP attributed to PDO phase were observed with low herbivore density. ENSO effects on LHP, specifically La Nina, were more pronounced during cold-phase PDO years. High herbivore density increased LHP at a greater rate than at moderate and low densities with increasing fall and winter precipitation. Differential gain, a weighted measure of LHP under higher relative to lower herbivore densities, was sensitive to prevailing PDO phase, ENSO magnitude, and precipitation amounts from the prior growing season and current fall-winter season. Temporal hierarchical approaches using PDO, ENSO, and local-scale precipitation can enhance decision-making for flexible herbivore densities. Herbivore densities could be increased above recommended levels with lowered risk of negative returns for managers during warm-phase PDO to result in greater LHP and less variability. Conversely, during cold-phase PDO, managers should be cognizant of the additional influences of ENSO and prior fall-winter precipitation, which can help predict when to reduce herbivore densities and minimize risk of forage shortages.

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.

Trends and decadal oscillations of oxygen and nutrients at 50 to 300 m depth in the equatorial and North Pacific

Abstract. A strong oxygen-deficient layer is located in the upper layers of the tropical Pacific Ocean and deeper in the North Pacific. Processes related to climate change (upper-ocean warming, reduced ventilation) are expected to change ocean oxygen and nutrient inventories. In most ocean basins, a decrease in oxygen (“deoxygenation”) and an increase in nutrients have been observed in subsurface layers. Deoxygenation trends are not linear and there could be multiple influences on oxygen and nutrient trends and variability. Here oxygen and nutrient time series since 1950 in the Pacific Ocean were investigated at 50 to 300 m depth, as this layer provides critical pelagic habitat for biological communities. In addition to trends related to ocean warming the oxygen and nutrient trends show a strong influence of the Pacific Decadal Oscillation (PDO) in the tropical and the eastern Pacific, and the North Pacific Gyre Oscillation (NPGO) in particular in the North Pacific. In the Oyashio Region the PDO, the NPGO, the North Pacific Index (NPI) and an 18.6-year nodal tidal cycle overlay the long-term trend. In most eastern Pacific regions oxygen increases and nutrients decrease in the 50 to 300 m layer during the negative PDO phase, with opposite trends during the positive PDO phase. The PDO index encapsulates the major mode of sea surface temperature variability in the Pacific, and oxygen and nutrients trends throughout the basin can be described in the context of the PDO phases. El Niño and La Niña years often influence the oxygen and nutrient distribution during the event in the eastern tropical Pacific but do not have a multi-year influence on the trends.

An extreme sea level event along the northwest coast of the South China sea in 2011–2012

An extreme sea-level (ESL) event in 2011–2012 and its associated mechanisms are investigated using long-term tide gauge records and satellite altimeter data. The composite sea level anomaly (SLA) along the northwest coast of the SCS is up to 85.2 mm in 2012, reaching the highest since 1975. The ESL event in 2011–2012 is a combined effect of the mean sea-level rise, La Niña event and negative Pacific Decadal Oscillation (PDO) phase. The three factors work on different time scales. The long-term rising rate is 3.1 mm/yr from 1975 to 2015, the linear estimate of the mean sea level is 45.1 mm in 2012, accounting for about 52.9% of the total ESL (85.2 mm). After removing the linear trend, La Niña has a 28.2% contribution of the highest SLA, the negative PDO phase has a 51.3% contribution, and they jointly give rise to the ESL event on interannual to decadal time scales. The dynamic sea-level variation is the dominant contributor, explaining 72.8% of the observed sea-level differences between 2010 and 2012. The sea level pressure contributes 34.4%, and the steric effect contributes 7.7% of the observed results.

South America Climate During the 1970–2001 Pacific Decadal Oscillation Phases Based on Different Reanalysis Datasets

In this paper circulation and precipitation climate patterns over the South America domain obtained from NCEP-NCAR reanalysis I, JRA-55 and ERA-40 reanalysis datasets were intercompared within the period 1970-2001 in order to verify their climatic association with the Pacific Decadal Oscillation, PDO. Although all three datasets show similar climatic patterns during each PDO phase, they show distinct intensities for circulation and precipitation variables. The mean low frequency flow propagation over the South Pacific during negative and positive PDO phases reaches central-eastern South America (SA) with opposite signals, contributing to modulate the local climate. During the negative PDO phases, the northerly flow over South America is intensified, while during the positive phase it is weakened. NCEP-NCAR reanalysis I, in comparison with JRA-55 and ERA-40 values, always indicates the strongest signal for circulation and precipitation variables. NCEP-NCAR reanalysis I meridional wind at the low-level jet area is about 5% and 12% larger than the respective JRA-55 and the ERA-40 values. For the positive and negative PDO phases, only NCEP-NCAR and ERA-40 high level zonal wind anomalies show, respectively, divergent (positive PDO) and convergent (negative PDO) patterns close to 140o W at equatorial latitudes of the Pacific Ocean. This one indicates distinct representations of the positioning of Walker cells in the three reanalysis datasets. GPCC dataset, NCEP-NCAR, JRA-55 and ERA-40 reanalysis, show, in general, the dipole precipitation pattern over eastern of South America, with signs of changes between the central-eastern and southeastern areas during PDO reversal phases. Over the central-eastern South American region, NCEP-NCAR precipitation is, on average, 57 % larger than GPCC values, and, respectively, 35% and 46% larger than JRA-55 and ERA-40 values. Observed patterns of anomalies for different PDO phases appear in all reanalysis data sets, leading us to conclude that the physical meaning is independent from the datasets. Even considering the JRA-55 climate signal over the SA associated with the PDO influence as the weakest one, the ability of JRA-55 to calculate monthly precipitation provided the highest linear correlation coefficient values in tropical areas, leading us to consider this data set as the most reliable for the studied area.