Pacific Decadal Oscillation Phase Research Articles

Nonlinear reconstruction of global climate leading modes on decadal scales

A nonlinear decomposition method is applied to the analysis of global sea surface temperature (SST) time series in different epochs related to the Pacific Decadal Oscillation (PDO) since the end of 19th century to present time. This method allows one to extract an optimal (small) number of global nonlinear teleconnection patterns associated with distinct dominant time scales from the original high-dimensional spatially extended data set. In particular, it enables us to reveal ENSO teleconnection patterns corresponding to different PDO cycles during the last 145 years, to uncover four climate shifts connected with PDO phase changes and to reconstruct the corresponding global PDO patterns. We find that SST teleconnections between the ENSO region, extra-tropical Pacific regions and the Indian ocean became fundamentally nonlinear since the second half of 20th century.

Oscillations and trends of river discharge in the southern Central Andes and linkages with climate variability

This study analyzes the discharge variability of small to medium drainage basins (102–104 km2) in the southern Central Andes of NW Argentina. The Hilbert-Huang Transform (HHT) was applied to evaluate non-stationary oscillatory modes of variability and trends, based on four time series of monthly-normalized discharge anomaly between 1940 and 2015. Statistically significant trends reveal increasing discharge during the past decades and document an intensification of the hydrological cycle during this period. An Ensemble Empirical Mode Decomposition (EEMD) analysis revealed that discharge variability in this region can be best described by five quasi-periodic statistically significant oscillatory modes, with mean periods varying from 1 to ∼20 y. Moreover, we show that discharge variability is most likely linked to the phases of the Pacific Decadal Oscillation (PDO) at multi-decadal timescales (∼20 y) and, to a lesser degree, to the Tropical South Atlantic SST anomaly (TSA) variability at shorter timescales (∼2–5 y). Previous studies highlighted a rapid increase in discharge in the southern Central Andes during the 1970s, inferred to have been associated with the global 1976–77 climate shift. Our results suggest that the rapid discharge increase in the NW Argentine Andes coincides with the periodic enhancement of discharge, which is mainly linked to a negative to positive transition of the PDO phase and TSA variability associated with a long-term increasing trend. We therefore suggest that variations in discharge in this region are largely driven by both natural variability and the effects of global climate change. We furthermore posit that the links between atmospheric and hydrologic processes result from a combination of forcings that operate on different spatiotemporal scales.

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.

Variability and trends in runoff in the rivers of British Columbia's Coast and Insular Mountains

AbstractThis study examines the 1914–2015 runoff trends and variability for 136 rivers draining British Columbia's Coast and Insular Mountains. Rivers are partitioned into eastward and westward flowing rivers based on flow direction from the Coast Mountains. Thus, eastward and westward runoff trends and influence of topography on runoff are explored. Our findings indicate that rivers flowing eastward to the Nechako and Chilcotin plateaus contribute the lowest annual runoff compared to westward rivers where runoff is high. Low interannual runoff variability is evident in westward rivers and their alpine watersheds, whereas eastward rivers exhibit high interannual runoff variability. On Vancouver Island, some of the rivers with the highest annual runoff exhibit high interannual variability. A significant (p < .05) negative correlation exists between mean annual runoff (Rm) and latitude, gauged area, mean elevation, and its corresponding coefficient of variation. However, a significant positive correlation was found between the glacierized area of mountainous regions and Rm. The mean coefficient of variation in annual runoff is significantly negatively correlated with latitude and glacierized area, but significantly positively correlated with longitude. Annual and seasonal runoff trend analyses of each river were performed for an early (1936–2015), a middle (1966–2015), and a late (1986–2015) period using the Mann–Kendall test. Trend analyses revealed a shift towards more positive detectable (signal‐to‐noise ratio > 1) trends in annual and seasonal runoff from the middle to the late period across the study domain. Most positive detectable seasonal runoff trends in the middle period occur in spring in glacierized westward rivers located >1,200 m, whereas in the late period, they all occur in fall and are regionally coherent around Vancouver Island and south coastal BC. Rivers draining eastward exhibit more positive trends over 1986–2015 compared to westward rivers. This study provides crucial information on the hydrology of mountain watersheds across British Columbia's coast in response to Pacific Decadal Oscillation phase changes, the elevational amplification of regional climate change, and their influences on precipitation and glacier retreat.

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.

Interannual climate variability drives regional fires in west central British Columbia, Canada

We investigated the influence of climate variability on forest fire occurrence at eight sites in west central British Columbia, Canada. Forty-six local fire years affecting a single site and 16 moderate fire years affecting two or more sites were identified (1600–1900 A.D.). Existing fire history data were incorporated to identify 17 regionally synchronous fire years (fires that affected ≥3 sites). Interannual and multidecadal relationships between fire occurrence and the Palmer Drought Severity Index (PDSI), El Nino–Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and the Pacific North American (PNA) pattern were examined, in addition to the effects of additive positive phases of ENSO and PDO. We examined multiple reconstructions of ENSO, PDO, and PNA and utilized three methodological approaches to characterize climate-fire relationships. We found that the influence of interannual climate, expressed as PDSI, increasingly synchronized the occurrence of fires when examined from local to regional scales. An association between local fires and positive antecedent moisture conditions suggests moisture-driven fine fuel development and the proximity of some sites to grasslands likely function as key determinants of local-scale fire activity. The relationships between regional fires and ENSO, PDO, and PNA suggest that large-scale patterns of climate variability exert a weak and/or inconsistent influence over fire activity in west central British Columbia between 1700 and 1900 A.D. Although inconsistent among reconstructions of climate patterns, we identified a significant relationship between regional fires and large-scale climate patterns when ENSO and PDO were both in positive phases.

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.

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.

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.

Accumulation rate in a tropical Andean glacier as a proxy for northern Amazon precipitation

Andean tropical glaciers have shown a clear shrinkage throughout the last few decades. However, it is unclear how this general retreat is associated with variations in rainfall patterns in the Amazon basin. To investigate this question, we compared the annual net accumulation variations in the Bolivian Cordillera Real (Andes), which is derived from an ice core from the Nevado Illimani (16° 37′ S, 67° 46′ W), covering the period 1960–1999 using the Amazonian Rainfall Index, Northern Atlantic Index (TNA), Multivariate ENSO Index (MEI), and Pacific Decadal Oscillation (PDO). The accumulation rate at the Nevado Illimani ice core decreased by almost 25% after 1980, from 1.02 w.eq. a−1 (water equivalent per year) in the 1961–1981 period to 0.76 w.eq. a−1 in the 1981–1999 period. The Northern Amazonian Rainfall (NAR) index best reflects changes in accumulation rates in the Bolivian ice core. Our proposal is based on two observations: (1) This area shows reduced rainfall associated with a more frequent and intense El Nino (during the positive phase of the MEI). The opposite (more rain) is true during La Nina phases. (2) Comparisons of the ice core record and NAR, PDO, and MEI indexes showed similar trends for the early 1980s, represented by a decrease in the accumulation rates and its standard deviations, probably indicating the same causality. The general changes observed by early 1980s coincided with the beginning of a PDO warm phase. This was followed by an increase in the Amazonian and tropical Andean precipitation from 1999, coinciding with a new PDO phase. However, this increase did not result in an expansion of the Zongo Glacier area.

The structure and large-scale organization of extreme cold waves over the conterminous United States

Extreme cold waves (ECWs) occurring over the conterminous United States (US) are studied through a systematic identification and documentation of their local synoptic structures, associated large-scale meteorological patterns (LMPs), and forcing mechanisms external to the US. Focusing on the boreal cool season (November–March) for 1950‒2005, a hierarchical cluster analysis identifies three ECW patterns, respectively characterized by cold surface air temperature anomalies over the upper midwest (UM), northwestern (NW), and southeastern (SE) US. Locally, ECWs are synoptically organized by anomalous high pressure and northerly flow. At larger scales, the UM LMP features a zonal dipole in the mid-tropospheric height field over North America, while the NW and SE LMPs each include a zonal wave train extending from the North Pacific across North America into the North Atlantic. The Community Climate System Model version 4 (CCSM4) in general simulates the three ECW patterns quite well and successfully reproduces the observed enhancements in the frequency of their associated LMPs. La Nina and the cool phase of the Pacific Decadal Oscillation (PDO) favor the occurrence of NW ECWs, while the warm PDO phase, low Arctic sea ice extent and high Eurasian snow cover extent (SCE) are associated with elevated SE-ECW frequency. Additionally, high Eurasian SCE is linked to increases in the occurrence likelihood of UM ECWs.

Evidence for a climate-driven hydrologic regime shift in the Canadian Columbia Basin

Water resources from the Columbia River Basin are intensely used for domestic, agricultural, industrial and hydroelectric generation needs. Water availability in the Pacific Northwest is influenced by several ocean–atmosphere modes of climate variability that occur in the Pacific Ocean. Climate change has the potential to alter these relationships and influence both the volume and timing of streamflow in the snowmelt-dominated tributaries to the Columbia River. Here, the historical influences of climate variability and recent climate warming on the volume and timing of streamflow for 40 tributary streams in the Columbia River Basin of Canada were evaluated. Regional relationships were found between streamflow and several Pacific Ocean climate indices, including the already established relationships with the Pacific Decadal Oscillation (PDO) and El Nino/Southern Oscillation (ENSO). However, in recent decades the statistical relationship between streamflow and climate indices has become weaker, which has implications for managers using these indices as decision-making tools. A comparison of the average annual streamflow for the cool PDO phase, which occurred from 1947 to 1976, to the more recent cool phase from 1999 to 2011 indicates a 11% decline across the Canadian portion of the basin. Removing the influence of these climate indices on historical streamflow reveals decreases in the residual streamflow beginning sometime in the 1980s. The potential role of increased temperatures on streamflow was investigated, and statistically significant relationships between decreased streamflow and increased temperatures in the summer months were found, particularly with the number of days over 18°C. The results suggest that climate change may be altering the historical relationship between climate indices and streamflow in the Canadian portion of the Columbia Basin.

Late Holocene hydroclimatic change at Cienega Amarilla, west-central New Mexico, USA

AbstractA late Holocene carbonate spring mound and associated wetland deposits at Cienega Amarilla, New Mexico, contain a 4000-yr record of geomorphic, paleoenvironmental, and hydroclimatic change on the southern Colorado Plateau. Forty-four14C dates support a century-scale chronostratigraphic framework. Pollen, plant macrofossil, mollusk, ostracode, and soil analyses indicate rapid spring mound growth and wetland expansion beginning ~2300 cal yr BP, followed by a pronounced decline in groundwater discharge (GWD) between ~1500 and 1000 cal yr BP. The isotopic composition of Cienega Amarilla springwater suggests GWD is driven primarily by winter precipitation. Historical climate data indicate that El Niño and warm Pacific Decadal Oscillation (PDO) conditions foster wetter-than-average winters in the Cienega Amarilla area, whereas dry winters are associated with La Niña conditions regardless of PDO phase. The ~2300–1500 cal yr BP Cienega Amarilla pluvial appears to represent an interval of peak, late Holocene cool-season precipitation that implies unusually strong or persistent El Niño–like and warm PDO–like conditions in the Pacific. Other southwestern paleoenvironmental records corroborate atypically wet conditions during this interval, and pluvial conditions related to increased winter precipitation likely fostered significant prehistoric cultural changes throughout the region, including increased sedentism, population, and dependence on agriculture.

Pacific‐Atlantic Ocean influence on wildfires in northeast China (1774 to 2010)

AbstractIdentification of effects that climate teleconnections, such as El Niño–Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and North Atlantic Oscillation (NAO), have on wildfires is difficult because of short and incomplete records in many areas of the world. We developed the first multicentury wildfire chronologies for northeast China from fire‐scarred trees. Regional wildfires occurred every 7 years from the 1700s to 1947, after which fire suppression policies were implemented. Regional wildfires occurred predominately during drought years and were associated with positive phases of ENSO and PDO and negative NAO. Twentieth century meteorological records show that this contingent combination of +ENSO/+PDO/−NAO is linked to low humidity, low precipitation, and high temperature during or before late spring fire seasons. Climate and wildfires in northeast China may be predictable based on teleconnection phases, although future wildfires may be more severe due to effects of climate change and the legacy of fire suppression.

Decadal to multi-decadal scale variability of Indian summer monsoon rainfall in the coupled ocean-atmosphere-chemistry climate model SOCOL-MPIOM

The present study is an effort to deepen the understanding of Indian summer monsoon rainfall (ISMR) on decadal to multi-decadal timescales. We use ensemble simulations for the period AD 1600–2000 carried out by the coupled Atmosphere-Ocean-Chemistry-Climate Model (AOCCM) SOCOL-MPIOM. Firstly, the SOCOL-MPIOM is evaluated using observational and reanalyses datasets. The model is able to realistically simulate the ISMR as well as relevant patterns of sea surface temperature and atmospheric circulation. Further, the influence of Atlantic Multi-decadal Oscillation (AMO), Pacific Decadal Oscillation (PDO), and El Niño Southern Oscillation (ENSO) variability on ISMR is realistically simulated. Secondly, we investigate the impact of internal climate variability and external climate forcings on ISMR on decadal to multi-decadal timescales over the past 400 years. The results show that AMO, PDO, and Total Solar Irradiance (TSI) play a considerable role in controlling the wet and dry decades of ISMR. Resembling observational findings most of the dry decades of ISMR occur during a negative phase of AMO and a simultaneous positive phase of PDO. The observational and simulated datasets reveal that on decadal to multi-decadal timescales the ISMR has consistent negative correlation with PDO whereas its correlation with AMO and TSI is not stationary over time.

Multi-decadal variations of the South Indian Ocean subsurface temperature influenced by Pacific Decadal Oscillation

Over recent decades, a strong subsurface cooling trend in the South Indian Ocean (SIO) occurred, despite a continuous sea surface warming. Previous studies suggest this long-term (around 1960–2000) cooling trend is mainly driven by remote Pacific atmospheric forcing or local Indian Ocean (IO) forcing. This study reveals that the dominant driver of the SIO subsurface cooling trend in different periods is closely related to the phase of Pacific Decadal Oscillation (PDO). Our results suggest that the local IO wind forcing is responsible for the majority of the subsurface cooling trend and overwhelms a weak warming trend induced by the remote tropical Pacific wind forcing during the negative-phase period of PDO during 1960–76. However, this situation reverses during the PDO positive-phase period during 1977–98. Our analysis suggests that the PDO strengthens/weakens the tropical Pacific trade winds during negative/positive phase periods. Furthermore, the multi-decadal variations in the western Pacific induced by PDO impact the SIO subsurface temperature via baroclinic Rossby waves.

Novel forest decline triggered by multiple interactions among climate, an introduced pathogen and bark beetles.

Novel forest decline is increasing due to global environmental change, yet the causal factors and their interactions remain poorly understood. Using tree ring analyses, we show how climate and multiple biotic factors caused the decline of whitebark pine (Pinus albicaulis) in 16 stands in the southern Canadian Rockies. In our study area, 72% of whitebark pines were dead and 18% had partially dead crowns. Tree mortality peaked in the 1970s; however, the annual basal area increment of disturbed trees began to decline significantly in the late 1940s. Growth decline persisted up to 30years before trees died from mountain pine beetle (Dendroctonus ponderosae), Ips spp. bark beetles or non-native blister rust pathogen (Cronartium ribicola). Climate-growth relations varied over time and differed among the healthy and disturbed subpopulations of whitebark pine. Prior to the 1940s, cool temperatures limited the growth of all subpopulations. Growth of live, healthy trees became limited by drought during the cool phase (1947 -1976) of the Pacific Decadal Oscillation (PDO) and then reverted to positive correlations with temperature during the subsequent warm PDO phase. In the 1940s, the climate-growth relations of the disturbed subpopulations diverged from the live, healthy trees with trees ultimately killed by mountain pine beetle diverging the most. We propose that multiple factors interacted over several decades to cause unprecedented rates of whitebark pine mortality. Climatic variation during the cool PDO phase caused drought stress that may have predisposed trees to blister rust. Subsequent decline in snowpack and warming temperatures likely incited further climatic stress and with blister rust reduced tree resistance to bark beetles. Ultimately, bark beetles and blister rust contributed to tree death. Our findings suggest the complexity of whitebark pine decline and the importance of considering multiway drought-disease-insect interactions over various timescales when interpreting forest decline.

Establishment of Nothofagus pumilio at Upper Treelines Across a Precipitation Gradient in the Northern Patagonian Andes

ABSTRACTTrees at upper treelines are exposed to more extreme environmental conditions than those at lower elevations. Climate changes at the upper treeline facilitate the establishment or intensify the mortality of trees and, consequently, affect species distributions. The structure and density of individuals of Nothofagus pumilio above the upper treeline, together with their temporal patterns of establishment, were determined in three sites located along a west-east precipitation gradient across the Patagonian Andes. Patterns of tree establishment were compared to regional variations in temperature and precipitation, as well as to indexes of atmospheric circulation that modulate northern Patagonian climate. Mesic and dry sites along the moisture gradient have a lower density of newly established trees; however, individuals show larger basal diameters and greater annual growth rates, heights, and number of branches than those established in humid sites. In wet areas, the high density of individuals reflects the higher rates of N. pumilio establishment and survival. At drier treelines, low snow persistence, associated with longer growing seasons, appears to be related to the larger size of individuals. At all sites, patterns of tree establishment are characterized by an abrupt increase in recruitment starting in the mid-1970s and a marked decrease in the late 1990s. The onset of tree establishment above the treeline coincides with an increase in regional spring-summer temperature in the year 1977, concurrent with the negative-to-positive shift in the phase of the Pacific Decadal Oscillation (PDO). In contrast, the decrease in N. pumilio establishment since the late 1990s coincides with an opposite shift (positive to negative) in the PDO. This recent change in the PDO phase did not significantly modify the mean values but increased the interannual variability of the spring-summer temperatures in the region. Changes in the PDO, which encompasses complex variations in environmental conditions at the upper treeline, are more closely related to N. pumilio establishment than are variations in temperature or precipitation alone. In addition, the distinction between the effects of changes in mean values versus the effects of climate variability is crucial for properly predicting forest responses to climate changes.

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 PDO-related climate variability impacts on Midwestern United States crop yields.

An analysis of crop yields for the state of Missouri was completed to determine if an interannual or multidecadal variability existed as a result of the El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). Corn and soybean yields were recorded in kilograms per hectare for each of the six climate regions of Missouri. An analysis using the Mokhov "method of cycles" demonstrated interannual, interdecadal, and multidecadal variations in crop yields. Cross-spectral analysis was used to determine which region was most impacted by ENSO and PDO influenced seasonal (April-September) temperature and precipitation. Interannual (multidecadal) variations found in the spectral analysis represent a relationship to ENSO (PDO) phase, while interdecadal variations represent a possible interaction between ENSO and PDO. Average crop yields were then calculated for each combination of ENSO and PDO phase, displaying a pronounced increase in corn and soybean yields when ENSO is warm and PDO is positive. Climate regions 1, 2, 4, and 6 displayed significant differences (p value of 0.10 or less) in yields between El Niño and La Niña years, representing 55-70% of Missouri soybean and corn productivity, respectively. Final results give the opportunity to produce seasonal predictions of corn and soybean yields, specific to each climate region in Missouri, based on the combination of ENSO and PDO phases.