Negative Pacific Decadal Oscillation Phase Research Articles

Suppressed CO2 Outgassing by an Enhanced Biological Pump in the Eastern Tropical Pacific

AbstractThe Eastern Tropical Pacific (ETP) is the largest oceanic source of carbon dioxide (CO2) to the atmosphere. Sinking particle fluxes at a depth of 4,950 m (50 m above the seafloor) in the ETP were monitored from 2003 to 2013, during which the Pacific decadal oscillation (PDO) shifted from a positive to negative phase. We show a disproportionate increase in the efficiency of the biological pump in this region relative to the increase in primary production that occurred during La Niña years following the shift of the PDO in 2008. Biogenic carbon export from the surface mixed layer was estimated from the observed particulate organic carbon and inorganic carbon fluxes at a depth of 4,950 m and from empirical equations of the vertical attenuation of carbon flux. Enhanced biological carbon export accounted for 2.3–5.5 mol C m–2 year–1 during the La Niña events. Despite a large uncertainty associated with these estimates, we propose that CO2 outgassing was largely suppressed by an enhanced biological pump during the La Niña events in the negative PDO phase.

Decadal Variability of the Barrier Layer and Forcing Mechanism in the Bay of Bengal

AbstractUsing Simple Ocean Data Assimilation data set for 1951–2010, we analyze the decadal variability of barrier layer thickness (BLT) and its forcing mechanisms in the Bay of Bengal (BoB). The decadal variability of BLT shows a close connection with the Pacific Decadal Oscillation (PDO). The PDO‐induced spatial distribution of BLT is similar to its first Empirical Orthogonal Function mode, showing a meridional dipolar pattern. PDO can exert an influence on the decadal variations in the Walker circulation to indirectly modulate decadal variations in BLT. Weakened southwesterly winds over the BoB associated with a positive PDO phase enhance shoreward Ekman transport along the eastern and northern boundary of the BoB and therefore deepen isothermal layer depth, which further induces positive BLT anomalies along the eastern and northern coast of the BoB. On the other hand, the weakened winds decrease oceanic heat loss; meanwhile, the enhanced upwelling driven by positive wind stress curl anomalies shoals the isothermal layer depth and thus decreases the BLT over the southeastern BoB. The opposite is true for the negative PDO phase. Using an ocean mixed layer model, we diagnose the relative contribution of different forcing terms to BLT variations on decadal timescales. It is found that horizontal advection and freshwater flux‐induced entrainment dominate the decadal BLT variability in the northern BoB, whereas heat and freshwater flux‐induced entrainment play a dominant role in the southern BoB.

An assessment of the impact of Pacific Decadal Oscillation on autumn droughts in North China based on the Palmer drought severity index

AbstractThis study assesses the impact of the Pacific Decadal Oscillation (PDO) on the severity of autumn droughts in North China (ADNC) during the period of 1962–2013. The ADNC are defined using the Palmer drought severity index (PDSI). The results show that the ADNC are more severe during negative PDO phases than during positive PDO phases. During the negative PDO phase, both the autumn precipitation and the Penman–Monteith reference evaporation (PET_pm) play roles in the significant severity of the ADNC. In contrast, during positive PDO phases, the droughts over North China are impacted to a greater degree by the climate conditions in previous seasons. Regression analysis shows that the atmospheric circulation anomalies associated with ADNC are quite different over North China under different PDO phases. Specifically, during negative PDO phases, anomalous low‐pressure systems are observed over the East Asia‐western Japan area, indicating a strengthened East Asian trough (EAT). Anomalous northerly winds bring cold and dry air from high‐latitude inland regions to North China, leading to severe ADNC during this period. Meanwhile, significant descending motion is observed in the upstream region of the EAT and the area of North China, accompanied by reduced cloudiness and reduced precipitation over these regions. In contrast, during positive PDO phases, the ADNC‐associated atmospheric circulation anomalies over North China are quite weak. Further analysis shows that during the negative phase of PDO years, the El Niño Southern Oscillation can impact the severity of ADNC through the centre of the Pacific‐East Asian teleconnection pattern over northeastern Asia.

Phytoplankton productivity and community structure variations over the last 160 years in the East China Sea coast in response to natural and human-induced environmental changes

Eutrophication has caused drastic changes to the marine ecosystem of the East China Sea during the past decades. However, there is relatively sparse evidence of historical changes, as well as the explicit effects of climatic changes and anthropogenic activities on the primary productivity of marine coastal ecosystems. In this study, surface and core sediments from the Zhejiang-Fujian coastal mud area, East China Sea coast, were analyzed using the bulk and molecular biomarkers. The results showed that ecosystem changes were characterized by increased phytoplankton productivity and a fluctuant transition from blooms mostly dominated by diatoms to red tide events dominated by dinoflagellates. Variations from the early 1850s to the early 2010s can be divided into a nature-dominated period (the early 1850s–1960s) and a human-impacted period (1960s–the early 2010s). Particularly, natural forcing such as heavy floods (e.g. 1998, 1954, and 1931) in the whole of the Yangtze River catchment, variations in the intensity of East Asia Monsoon, and strengthened or weakened Kuroshio intrusion/positive or negative Pacific Decadal Oscillation phase in the coastal mud area have substantially affected the phytoplankton productivity and community structure during the nature-dominated period. In contrast, changes in nutrient supply and compositions were more apparent during the human-impacted period, which could have been because of increased fertilizer usage, discharges of industrial wastewater and domestic sewage, and large-scale human projects (e.g. Danjiangkou Reservoir and Three Gorges Dam) in the Yangtze River drainage area, leading to significant phytoplankton productivity and community structure variations in the coastal mud area system of East China Sea.

Climatic Effects of China Large-Scale Urbanization on East Asian Summer Monsoon under Different Phases of Pacific Decadal Oscillation

The effects of urbanization over eastern China on the East Asian summer monsoon (EASM) under different sea surface temperature background are compared using a Community Atmosphere Model (CAM5.1). Experiments of urbanization investigated by comparing two climate simulations with and without urban land cover under both positive and negative phases of Pacific Decadal Oscillation (PDO) show the spatial distribution of precipitation with ‘southern flood and northern drought’ and weakening status of EASM. The climate effect of urbanization in eastern China is significantly different from north to south. Anomalous vertical ascending motion due to the role of urbanization in the south of 30° N have induced an increase in convective available potential energy (CAPE) and precipitation increase over southern China. At the same time, the downward vertical motion occurs in the north of 30° N which cause warming over northern China. Due to the anti-cyclonic anomalies in the upper and lower layers of the north, the monsoon circulation is weakened which can reduce the precipitation. However, urbanization impact under various phases of PDO show different effect. In the 1956–1970 urbanization experiments of negative PDO phase, the downward vertical motion and anti-cyclonic anomalies in the north of 30° N are also weaker than that of positive phase of PDO in 1982–1996. In terms of this situation, the urbanization experiments of negative phase of PDO reveal that the range of the warming area over the north of 40° N is small, and the warming intensity is weak, but the precipitation change is more obvious compared with the background of positive phase of PDO.

Linear respective roles of El Niño–Southern Oscillation and East Asian winter monsoon in the formation of the western North Pacific anticyclone

Based on reanalysis data, we investigate the respective roles of the East Asian winter monsoon (EAWM) and El Niño–Southern Oscillation (ENSO) in building an anomalous western North Pacific anticyclone (WNPAC) in winter to the succeeding summer. The isolated ENSO and EAWM parts (termed ENSOres and EAWMres, respectively) are acquired by removing the ENSO–EAWM interdependent part through linear regression. The results suggest that both ENSOres and EAWMres can produce a WNPAC in winter. The ENSOres‐related WNPAC dominantly covers the southern part of the western North Pacific, whereas the EAWMres‐related WNPAC over the northern part of the western North Pacific. In the subsequent spring and summer, only ENSOres yields a WNPAC, while EAWMres has no effect on the atmospheric circulation over the western North Pacific. Additionally, the Pacific Decadal Oscillation (PDO) can modulate the influence of ENSOres and EAWMres on the WNPAC. There tends to be a much stronger WNPAC associated with ENSOres (EAWMres) during positive (negative) PDO phases than during negative (positive) ones.

Decadal variations of blocking and storm tracks in centennial reanalyses

Extreme weather events in the mid-latitudes are often associated with weather systems, such as extratropical cyclones or blocks. However little is known about the decadal variation in the frequency of blocks and the activity of storm tracks and their relation to the two most important low-frequency oceanic modes of variability, the Atlantic multidecadal oscillation (AMO) and the Pacific decadal oscillation (PDO). To address this question, first the representation of blocks, storm tracks and Alpine circulation types is assessed in recently released centennial reanalyses during the 20th century.While interannual variability is represented similarly in all centennial reanalyses, the trends in particular for storm track activity before 1950 differ substantially between the NOAA 20th century reanalyses (20CR and 20CRv2c) and ECMWF centennial reanalyses (ERA-20C and CERA-20C). Therefore, the analysis of the connection between the AMO and PDO and the mid-latitudinal circulation is restricted to the period 1950–2010. All centennial reanalyses agree on the anomaly patterns associated with the different AMO/PDO phases. We find no significant connections except a southward (northward) shift of the storm track over the North Pacific as well as the North Atlantic during the positive (negative) PDO phase. The ERA-20CM atmosphere-only model ensemble captures this relationship, indicating that the ocean forces the atmosphere in the North Pacific. For studies investigating periods earlier than 1950, we recommend using several reanalysis products to ensure the robustness of results.

Decadal Variability of Great Lakes Ice Cover in Response to AMO and PDO, 1963–2017

AbstractIn this study, decadal variability of ice cover in the Great Lakes is investigated using historical airborne and satellite measurements from 1963 to 2017. It was found that Great Lakes ice cover has 1) a linear relationship with the Atlantic multidecadal oscillation (AMO), similar to the relationship of lake ice cover with the North Atlantic Oscillation (NAO), but with stronger impact than NAO; 2) a quadratic relationship with the Pacific decadal oscillation (PDO), which is similar to the relationship of lake ice cover to Niño-3.4, but with opposite curvature; and 3) decadal variability with a positive (warming) trend in AMO contributes to the decreasing trend in lake ice cover. Composite analyses show that during the positive (negative) phase of AMO, the Great Lakes experience a warm (cold) anomaly in surface air temperature (SAT) and lake surface temperature (LST), leading to less (more) ice cover. During the positive (negative) phase of PDO, the Great Lakes experience a cold (warm) anomaly in SAT and LST, leading to more (less) ice cover. Based on these statistical relationships, the original multiple variable regression model established using the indices of NAO and Niño-3.4 only was improved by adding both AMO and PDO, as well as their interference (interacting or competing) mechanism. With the AMO and PDO added, the correlation between the model and observation increases to 0.69, compared to 0.48 using NAO and Niño-3.4 only. When November lake surface temperature was further added to the regression model, the prediction skill of the coming winter ice cover increased even more.

Interdecadal changes in the asymmetric impacts of ENSO on wintertime rainfall over China and atmospheric circulations over western North Pacific

Based on data diagnoses, in this study the interdecadal changes in asymmetric impacts of ENSO on wintertime East Asian climate are investigated. It is found that the Pacific Decadal Oscillation (PDO) can significantly modulate the asymmetry in the responses of the East Asian climate to El Nino and La Nina events. In positive PDO phase the response is asymmetric with a strong anomalous anticyclone over western North Pacific (WNP) and significant positive rainfall anomalies over southern China during El Nino winters, but a weak anomalous cyclone over WNP and insignificant negative rainfall anomalies over southern China during La Nina winters. However, such asymmetric responses do not appear in negative PDO phase, with comparable amplitudes of anomalous circulations over WNP and rainfall over southern China in El Nino and La Nina winters. Further analyses reveal that the warm background of the tropical Pacific in positive PDO phase causes significant difference in the amplitudes of convection anomalies over tropical western Pacific, resulting in asymmetric responses of the WNP circulation and rainfall over southern China to El Nino and La Nina events. Nevertheless, the cold background in negative PDO phase reduces the amplitude difference between El Nino and La Nina winters. A comparable convection anomalies appear in tropical western Pacific in El Nino and La Nina winters and the asymmetric responses do not occur.

Decadal Relationship between the Stratospheric Arctic Vortex and Pacific Decadal Oscillation

Abstract Using reanalysis datasets and numerical simulations, the relationship between the stratospheric Arctic vortex (SAV) and the Pacific decadal oscillation (PDO) on decadal time scales was investigated. A significant in-phase relationship between the PDO and SAV on decadal time scales during 1950–2014 is found, that is, the North Pacific sea surface temperature (SST) cooling (warming) associated with the positive (negative) PDO phases is closely related to the strengthening (weakening) of the SAV. This decadal relationship between the North Pacific SST and SAV is different from their relationship on subdecadal time scales. Observational and modeling results both demonstrate that the decadal variation in the SAV is strongly affected by the North Pacific SSTs related to the PDO via modifying the upward propagation of planetary wavenumber-1 waves from the troposphere to the stratosphere. The decreased SSTs over the North Pacific tend to result in a deepened Aleutian low along with a strengthened jet stream over the North Pacific, which excites a weakened western Pacific pattern and a strengthened Pacific–North American pattern. These tropospheric circulation anomalies are in accordance with the decreased refractive index (RI) at middle and high latitudes in the northern stratosphere during the positive PDO phase. The increased RI at high latitudes in the upper troposphere impedes the planetary wavenumber-1 wave from propagating into the stratosphere, and in turn strengthens the SAV. The responses of the RI to the PDO are mainly contributions of the changes in the meridional gradient of the zonal-mean potential vorticity via alteration of the baroclinic term .

ENSO Frequency Asymmetry and the Pacific Decadal Oscillation in Observations and 19 CMIP5 Models

Using observational data and the pre-industrial simulations of 19 models from the Coupled Model Intercomparison Project Phase 5 (CMIP5), the El Nino (EN) and La Nina (LN) events in positive and negative Pacific Decadal Oscillation (PDO) phases are examined. In the observational data, with EN (LN) events the positive (negative) SST anomaly in the equatorial eastern Pacific is much stronger in positive (negative) PDO phases than in negative (positive) phases. Meanwhile, the models cannot reasonably reproduce this difference. Besides, the modulation of ENSO frequency asymmetry by the PDO is explored. Results show that, in the observational data, EN is 300% more (58% less) frequent than LN in positive (negative) PDO phases, which is significant at the 99% confidence level using the Monte Carlo test. Most of the CMIP5 models exhibit results that are consistent with the observational data.

Potential impact of the Pacific Decadal Oscillation and sea surface temperature in the tropical Indian Ocean\u2013Western Pacific on the variability of typhoon landfall on the China coast

The landfall activity of typhoons (TYs) along the coast of China during July–August–September (JAS) shows significant interdecadal variation during 1965–2010. We identify three sub-periods of TY landfall activity in JAS along the China coast in this period, with more TY landfall during 1965–1978 (Period I) and 1998–2010 (Period III), and less during 1982–1995 (Period II). We find that the interdecadal variation might be related to the combined effects of Pacific Decadal Oscillation (PDO) phase changes and sea surface temperature (SST) variation in the tropical Indian Ocean and Western Pacific (IO–WP). During the negative PDO phase in Periods I and III, a cyclonic anomaly is located in the western North Pacific (WNP), inducing easterly flow in its northern part, which favors TY landfall along the eastern China coast. Warm SST anomalies over the tropical IO–WP during Period III induce an anomalous anticyclonic circulation in the WNP through both the Gill-pattern response to the warm SST in the tropical IO and the anomalous meridional circulation induced by the warm SST in the tropical WNP. As a result, the northern South China Sea and WNP (10°–20° N) are dominated by southeasterly flow, which favors TYs making landfall on both the southern and eastern China coast. With both landfalling-favorable conditions satisfied, there are significantly more TYs making landfall along the China coast during Period III than during Period I, which shows cool SST anomalies in the tropical IO–WP.

Impacts of oceanographic factors on interannual variability of the winter-spring cohort of neon flying squid abundance in the Northwest Pacific Ocean

The neon flying squid Ommastrephes bartramii is an economically important species in the Northwest Pacific Ocean. The life cycle of O. bartramii is highly susceptible to climatic and oceanic factors. In this study, we have examined the impacts of climate variability and local biophysical environments on the interannual variability of the abundance of the western winter-spring cohort of O. bartramii over the period of 1995–2011. The results showed that the squid had experienced alternant positive and negative Pacific Decadal Oscillation (PDO) over the past 17 years during which five El Nino and eight La Nina events occurred. The catch per unit effort (CPUE) was positively correlated with the PDO index (PDOI) at a one-year time lag. An abnormally warm temperature during the La Nina years over the positive PDO phase provided favorable oceanographic conditions for the habitats of O. bartramii, whereas a lower temperature on the fishing ground during the El Nino years over the negative PDO phase generally corresponded to a low CPUE. The same correlation was also found between CPUE and Chl a concentration anomaly. A possible explanation was proposed that the CPUE was likely related to the climateinduced variability of the large-scale circulation in the Northwest Pacific Ocean: high squid abundance often occurred in a year with a significant northward meander of the Kuroshio Current. The Kuroshio Current advected the warmer and food-rich waters into the fishing ground, and multiple meso-scale eddies arising from current instability enhanced the food retention on the fishing ground, all of which were favorable for the life stage development of the western squid stocks. Our results help better understand the potential process that the climatic and oceanographic factors affect the abundance of the winter-spring cohort of O. bartramii in the Northwest Pacific Ocean.

Modulation of Pacific Decadal Oscillation on the relationship of El Niño with southern China rainfall during early boreal winter

AbstractThe modulation of Pacific Decadal Oscillation (PDO) on the relationship of El Niño with southern China rainfall during early boreal winter (November–December) is investigated in this study. When El Niño occurs in positive phase of PDO, significantly positive rainfall anomalies only appears over the southwestern part of southern China. When El Niño occurs in negative phase of PDO, pronounced positive rainfall anomalies are observed over almost the whole southern China. Further analyses revealed that the anomalous Philippine Sea anticyclone (PSAC) and local meridional circulation play an important role in modulating the relationship of El Niño with southern China rainfall by PDO. During the negative PDO phase, under the combined influence of tropical Pacific and Indian Ocean dipole sea surface temperature anomalies pattern, the anomalous PSAC over the western North Pacific and rising motion associated with local meridional circulation over southern China provide sufficient water vapor and dynamical conditions for the formation of rainfall. During positive PDO phase, however, PSAC and local meridional circulation are only influenced by tropical Pacific and they become weak, not favoring the formation of southern China rainfall.

Variations of monsoonal rain and vegetation during the past millennium in Tiangui Mountain, North China reflected by stalagmite δ18O and δ13C records from Zhenzhu Cave

An 11-cm long stalagmite (ZZ12) collected from Zhenzhu cave, Tiangui Mountain, northwest of the North China Plain (NCP) has been measured by 230Th/U, AMS 14C and 210Pb dating methods. Only AMS 14C dating yielded the chronology successfully, while 230Th/U did not due to low uranium content and the young age of the stalagmite. The top layers in the stalagmite contain “nuclear bomb carbon” signal, and the AMS 14C dates of charcoal and carbonate samples at the bottom are similar. Based on a series of AMS 14C dates which contain minimal dead carbon influence (DCI) in the stalagmite, the age model of the stalagmite has been constructed, indicating that the stalagmite was 1970 years old. A total of 470 subsamples from the upper 51 mm (since 1180 CE) of the stalagmite have been analyzed for δ18O and δ13C. The δ18O record of stalagmite ZZ12 reflects changes in the summer monsoonal rainfall in the NCP, being comparable with the local and north-central China dry-wet index. The driest periods were found in 14th-15th centuries in the Little Ice Age (LIA) and since 1970s, perhaps due to weakening of East Asian summer monsoon. The δ13C of ZZ12 mainly recorded vegetation changes in Tiangui Mountain and revealed deforestation in two major periods: 1500–1600 CE and 1830–1940 CE caused by local human activities. Spectral analysis of the δ18O record suggests that the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) may both play important roles in changes of multi-decadal monsoonal rainfall in the NCP, with more rainfall responding to the negative PDO, La Niño-like, and positive AMO phases.

Combined Impacts of PDO and Two Types of La Niña on Climate Anomalies in Europe

Combined impacts of the Pacific decadal oscillation (PDO) and two types of La Niña on climate anomalies in Europe are studied. Particularly, the conjunction of the negative PDO phase and two different types of La Niña events favors strong and significant North Atlantic Oscillation (NAO) pattern anomalies with opposite polarity. For the central Pacific (CP) La Niña, a clear positive NAO signal can be detected, which is accompanied by positive surface air temperature (SAT) anomaly and a dipolar structure of precipitation anomalies in Europe. In addition, a typical negative Pacific–North America (PNA) teleconnection pattern forms, including a high pressure anomaly over the southeastern United States, which may contribute to the development and maintenance of the NAO anomaly by strengthening the baroclinicity and the local eddy–mean flow interaction. However, for the eastern Pacific (EP) La Niña, a zonal wave train in the high latitudes can be observed, which is quite different from the typical PNA structure. Here, an anomalous anticyclone over southern Greenland supports a negative NAO pattern through the local eddy–mean flow interaction and the associated vorticity advection. Hence, reversed SAT and precipitation anomalies occur over Europe. Further analyses indicate that the wave trains emanating from the North Pacific and the synoptic eddy–mean flow interaction play essential roles in forming the anomalous NAO phases. The different wave trains for the CP and EP La Niña events may be attributed to the differences in the location and intensity of anomalous convection induced by different types of SST anomaly as well as by the corresponding background westerly wind anomalies in the upper troposphere.

Increased Drought and Pluvial Risk over California due to Changing Oceanic Conditions

Abstract This study evaluates wintertime drought and pluvial risk over California through a Bayesian analysis of the upper and lower quartile of PRISM-based precipitation from 1901 to 2015. Risk is evaluated for different time windows to estimate the impact of interannual and decadal-to-multidecadal Pacific and Atlantic variability [positive and negative phases of ENSO, Pacific decadal oscillation (PDO), and Atlantic multidecadal oscillation (AMO)]. The impact of increasing trends in global sea surface temperature (SST) on drought and pluvial risk is also examined with idealized experimental runs from three climate models [GFDL Atmospheric Model version 2.1 (AM2.1), CCM3, and GFS]. The results show that the influence of oceanic conditions on drought risk in California is significant but has changed with higher risk in the last half century, especially in Southern California. The influence of oceanic conditions on pluvial risk has also been significant, especially during the warm phase of the Pacific Ocean, but increases over the last century are small compared to drought. Results from the idealized climate model experiments show that natural variability likely played a major role in the observed changes in risk, with the global SST increasing trend possibly tempering the increases regionally but not significantly over California. Despite evolving preferential oceanic conditions for a pluvial event during the 2015/16 winter (positive phase of ENSO and PDO), California received an 11% winter precipitation surplus, which was not sufficient for drought recovery. The seasonal and longer-term outlook for negative phases of ENSO and PDO implies that drought risk will be elevated in Southern California for the next decade.

ENSO and East Asian winter monsoon relationship modulation associated with the anomalous northwest Pacific anticyclone

Using observational datasets and numerical model experiments, the mechanism on the slowly varying change in the relationship between the El Nino-Southern Oscillation (ENSO) and the East Asian winter monsoon (EAWM) is investigated. The decadal-window (11-, 15-, and 21-year) moving correlations show a significant change in the boreal wintertime ENSO–EAWM relationship between two sub-periods of 1976‒1992 and 1997‒2013. Such recent change in ENSO–EAWM relationship is mainly attributed to the changes in the intensity and zonal location of the anomalous lower-tropospheric northwest Pacific anticyclone (NWP-AC). NWP-AC commonly develops near the region of the Philippine Sea during the ENSO’s peak phase and plays an important role of bridging the tropical convection and mid-latitude teleconnection. On one hand, the intensity of the NWP-AC is influenced by the interdecadal variation in a linkage between ENSO and the Indian Ocean sea surface temperature (SST) variability, referring that a strong connection between the Pacific and Indian Oceans results in the strengthening of NWP-AC response to ENSO. On the other hand, the zonal displacement of the NWP-AC is associated with the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO). That is, the tropical Pacific mean state (i.e., zonal SST gradient between climatologically warm western Pacific and cold eastern Pacific)—strengthened by either the negative PDO phase or the positive AMO phase—drives the anomalous ENSO-induced convection to be shifted to the west. With this westward shift, the zonal center of the NWP-AC also migrates westward over the Philippine Islands and exerts stronger connection between ENSO and EAWM. In contrast, the relaxed zonal SST contrast associated with either the positive PDO phase or the negative AMO phase tends to exhibit weaker ENSO–EAWM relationship via both of eastward shifted zonal centers of the anomalous ENSO-induced convection and the NWP-AC. Finally, a series of the numerical experiments conducted by an atmospheric general circulation model supports the observational findings.

High-Resolution Simulations of Decadal Climate Variability Impacts on Water Yield in the Missouri River Basin with the Soil and Water Assessment Tool (SWAT)

ABSTRACT The Missouri River basin (MRB) is the largest river basin in the United States and is one of the most important agricultural regions in the world. Three decadal climate variability (DCV) phenomena—the Pacific decadal oscillation (PDO), the tropical Atlantic sea surface temperature (SST) gradient variability (TAG), and the west Pacific warm pool SST variability (WPWP)—substantially influence hydrometeorology in the MRB. The authors report on a simulation study with the Soil and Water Assessment Tool (SWAT) to estimate impacts on water availability in response to realistic values of PDO, TAG, and WPWP indices in approximately 13 500 hydrologic unit areas covering the MRB. SWAT, driven by hydrometeorological anomalies associated with positive and negative phases of PDO and TAG, indicated major impacts on water yields and streamflows, as much as ±40% of the average in many locations. Impacts of the WPWP index were smaller. Consistent with observations during 1949–2010, SWAT showed water flow increases of as much as 80% of the average, causing very wet periods when the positive phase of the PDO and the negative phase of the TAG at extreme amplitudes were superposed. Water flows decreased by a similar amount, resulting in severe to extreme droughts when the negative phase of the PDO and the positive phase of the TAG at extreme amplitudes were superposed. Thus, the combined and cumulative effects of these DCV phenomena on water flows, droughts, and wet periods in the MRB can be dramatic, with important consequences for all water-consuming sectors as well as for feedbacks to the climate system.

The Influence of the Pacific Decadal Oscillation on Annual Floods in the Rivers of Western Canada

AbstractWe analyzed annual peak flow series from 127 naturally flowing or naturalized streamflow gauges across western Canada to examine the impact of the Pacific Decadal Oscillation (PDO) on annual flood risk, which has been previously unexamined in detail. Using Spearman's rank correlation ρ and permutation tests on quantile‐quantile plots, we show that higher magnitude floods are more likely during the negative phase of the PDO than during the positive phase (shown at 38% of the stations by Spearman's rank correlations and at 51% of the stations according to the permutation tests). Flood frequency analysis (FFA) stratified according to PDO phase suggests that higher magnitude floods may also occur more frequently during the negative PDO phase than during the positive phase. Our results hold throughout much of this region, with the upper Fraser River Basin, the Columbia River Basin, and the North Saskatchewan River Basin particularly subject to this effect. Our results add to other researchers' work questioning the wholesale validity of the key assumption in FFA that the annual peak flow series at a site is independently and identically distributed. Hence, knowledge of large‐scale climate state should be considered prior to the design and construction of infrastructure.