Positive Pacific Decadal Oscillation Phase Research Articles

Baseflow response to climate variability induced droughts in the Apalachicola–Chattahoochee–Flint River Basin, U.S.A.

Droughts have been a major factor leading to the Tri-State Water Wars in the southeastern United States. One of the primary issues related to the conflict is the reduction in baseflow levels in the Flint River during droughts. This affects the availability of freshwater resources to support the endangered mussel species in the Flint and Apalachicola Rivers and threatens the shellfish industry in the Apalachicola Bay. Study of large-scale climate phenomena as well as the interactions of interannual with decadal and multidecadal oceanic–atmospheric phenomena can provide valuable information regarding regional climatic conditions such as droughts and their impact on water resources. This study was conducted to quantify the impacts of climate variability cycles on baseflow levels in the Flint River. The individual and coupled impacts of the El Nino-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), Atlantic Multidecadal Oscillation (AMO), and North Atlantic Oscillation (NAO) on baseflow were quantified. The non-parametric Joint Rank Fit (JRFit) procedure was used to provide a robust test of the significance of interactions between the phases of ENSO–PDO, ENSO–AMO and ENSO–NAO baseflows. Simple-main effect comparisons were also performed using the JRFit model to estimate significant difference between the positive and negative phase baseflows of PDO, AMO and NAO associated with El Nino or La Nina phases. The results indicate that the phases of ENSO, AMO and NAO significantly affect baseflows in the Flint River. Interaction tests showed that the PDO and AMO phases modulate ENSO phase baseflows. La Nina associated with positive phases of PDO and AMO resulted in greater decrease in baseflow levels of approximately 28% and 33%, respectively. However, La Nina associated with negative phase of AMO showed above normal baseflows. The results illustrate the importance of coupled analyses of climate variability by providing a better understanding of the severity of droughts and their impact on baseflows. The results obtained from this study can be used by water managers in the region as a guide for the issuance of drought severity-based water restrictions.

Connection between weak stratospheric vortex events and the Pacific Decadal Oscillation

We investigate the possible impacts of the Pacific Decadal Oscillation (PDO) on the occurrence of weak stratospheric polar vortex (WSV) events in the Northern Hemisphere winter. WSV events, which are defined when polar-cap geopotential height anomalies at 50 hPa fall below the 10th percentile in winter, are observed more frequently during positive PDO phases than during negative PDO phases. Additionally, tropospheric wave forcings that drive WSV events are remarkably different between the two phases of the PDO. During the positive PDO phase, the vertical propagation of wavenumber-one waves plays a predominant role with a rather minor contribution of wavenumber-two waves. This contrasts with the negative PDO phase when the WSV events are primarily caused by wavenumber-two waves. This difference is partly related to the PDO-induced tropospheric circulation anomalies over the North Pacific whose zonal wavenumber-one component constructively (destructively) interferes with climatological planetary-scale waves during positive (negative) PDO winters. The predominant wavenumber-two wave forcings during the negative PDO phase are likely related to the enhanced tropospheric eddy activity over Alaska that results from the poleward shift of the Pacific jet in response to the negative PDO.

Modulation of Rossby waves on the Pacific North Equatorial Current bifurcation associated with the 1976 climate regime shift

AbstractSimulated current velocity and long‐term reanalysis wind data are used to investigate interannual variations in the bifurcation of the Pacific North Equatorial Current (NEC) after the 1976 climate regime shift. Wind stress curl anomaly (WSCA) in the region of 10°N–15°N and 160°E–170°E generates Rossby waves and affects the NEC bifurcation along the Philippine coast. From 1976 to 1992, following a regime shift to the positive Pacific Decadal Oscillation (PDO) phase, PDO and El Niño‐Southern Oscillation (ENSO) match each other in strength and have a neutralized effect on the WSCA. From 1993 to 2009, WSCA matches PDO well, and its correlation with ENSO is lower. Using a linear regression model, we show that the influence of PDO has nearly 13 times weight over that of ENSO. Prior to the 1976 regime shift, WSCA is closely related to ENSO from 1961 to 1975, and it does not correlate significantly with PDO. Our analysis results show that Rossby waves are preferentially generated in either the negative PDO phase when the ENSO signal dominates, or in the positive PDO phase when the ENSO signal is overshadowed. In the phase when the positive PDO counteracts with the ENSO signal, neither ENSO nor PDO has a significant influence on Rossby wave generations through the WSCA.

A Climatology of 500-hPa Closed Lows in the Northeastern Pacific Ocean, 1948–2011

AbstractThe northeastern Pacific Ocean is a preferential location for the formation of closed low pressure systems. These slow-moving, quasi-barotropic systems influence vertical stability and sustain a moist environment, giving them the potential to produce or affect sustained precipitation episodes along the west coast of the United States. They can remain motionless or change direction and speed more than once and thus often pose difficult forecast challenges. This study creates an objective climatological description of 500-hPa closed lows to assess their impacts on precipitation in the western United States and to explore interannual variability and preferred tracks. Geopotential height at 500 hPa from the NCEP–NCAR global reanalysis dataset was used at 6-h and 2.5° × 2.5° resolution for the period 1948–2011. Closed lows displayed seasonality and preferential durations. Time series for seasonal and annual event counts were found to exhibit strong interannual variability. Composites of the tracks of landfalling closed lows revealed preferential tracks as the features move inland over the western United States. Correlations of seasonal event totals for closed lows with ENSO indices, the Pacific decadal oscillation (PDO), and the Pacific–North American (PNA) pattern suggested an above-average number of events during the warm phase of ENSO and positive PDO and PNA phases. Precipitation at 30 U.S. Cooperative Observer stations was attributed to closed-low events, suggesting 20%–60% of annual precipitation along the West Coast may be associated with closed lows.

Linkage between the Pacific Decadal Oscillation and the low frequency variability of the Pacific Subtropical Cell

AbstractThe decadal variability of Pacific Subtropical Cell (STC) and its linkages with the Pacific Decadal Oscillation (PDO) are investigated in the present study based on a Simple Ocean Data Assimilation (SODA 2.2.4). It is found that, on decadal time scales, the western boundary and interior pycnocline transports are anticorrelated and the variation of the interior component is more significant, which is consistent with previous studies. The decadal variability of STC in the Northern Hemisphere is found to be strongly associated with PDO. Associated with a positive (negative) phase of PDO, the relaxation (acceleration) of the northeast trades slows down (spins up) the STC within a few years through baroclinic adjustment in conjunction with the subduction of the cold (warm) mixed‐layer anomalies in the extratropics. The cold (warm) water is then injected into the thermocline and advected further southwestward to the tropics along the isopycnal surfaces, leading to the slowdown (spin‐up) of STC due to zonal pressure gradient change at low latitude. Along with the STC weakening (strengthening), a significant warming (cold) anomaly appears in the tropics and it is advected to the midlatitude by the Kuroshio and North Pacific currents, thus feeding back to the atmosphere over the North Pacific. In contrast to the Northern Hemisphere, it is found the STC in the south only passively responds to the PDO. The mechanism found here highlights the role of the STC advection of extratropical anomalies to the tropics and horizontal gyre advection of the tropical anomalies to the extratropics in decadal variability of the STC and PDO.

Modulation of the Pacific Decadal Oscillation on the summer precipitation over East China: a comparison of observations to 600-years control run of Bergen Climate Model

Observations show that the summer precipitation over East China often goes through decadal variations of opposite sign over North China and the Yangtze River valley (YRV), such as the “southern flood and northern drought” pattern that occurred during the late 1970s–1990s. In this study it is shown that a modulation of the Pacific Decadal Oscillation (PDO) on the summer precipitation pattern over East China during the last century is partly responsible for this characteristic precipitation pattern. During positive PDO phases, the warm winter sea surface temperatures (SSTs) in the eastern subtropical Pacific along the western coast of North American propagate to the tropics in the following summer due to weakened oceanic meridional circulation and the existence of a coupled wind–evaporation–SST feedback mechanism, resulting in a warming in the eastern tropical Pacific Ocean (5°N–20°N, 160°W–120°W) in summer. This in turn causes a zonal anomalous circulation over the subtropical–tropical Pacific Ocean that induces a strengthened western Pacific subtropical high (WPSH) and thus more moisture over the YRV region. The end result of these events is that the summer precipitation is increased over the YRV region while it is decreased over North China. The suggested mechanism is found both in the observations and in a 600-years fully coupled pre-industrial multi-century control simulations with Bergen Climate Model. The intensification of the WPSH due to the warming in the eastern tropical Pacific Ocean was also examined in idealized SSTA-forced AGCM experiments.

The climate of Myanmar: evidence for effects of the Pacific Decadal Oscillation

ABSTRACTWe show evidence for the influence of the Pacific Decadal Oscillation (PDO) on Myanmar's monsoonal hydroclimate using both instrumental and 20th century reanalysis data, and a tree‐ring width chronology from Myanmar's central Dry Zone. The ‘regime shifts’ identified in the instrumental PDO for the past century are clearly evident in the Myanmar teak. The teak record and PDO index correlate most significantly and positively during December–May, at r = 0.41 (0.002, n = 109). We generated composite climate anomalies for southern Asia and adjacent ocean areas during negative and positive PDO phases and above/below average teak growth for the May–September wet monsoon season. They show that negative (positive) PDO phases correspond to dry (wet) conditions, due to reduced (enhanced) moisture flux into central Myanmar. Multitaper Method (MTM) and Singular Spectrum Analysis (SSA) spectral analyses reveal considerable multidecadal variability over the past several centuries of the teak chronology, consistent with the PDO.

Multidecadal Variability of North China Aridity and Its Relationship to PDO during 1900–2010

Abstract North China has undergone a severe drying trend since the 1950s, but whether this trend is natural variability or anthropogenic change remains unknown due to the short data length. This study extends the analysis of dry–wet changes in north China to 1900–2010 on the basis of self-calibrated Palmer drought severity index (PDSI) data. The ensemble empirical mode decomposition method is used to detect multidecadal variability. A transition from significant wetting to significant drying is detected around 1959/60. Approximately 70% of the drying trend during 1960–90 originates from 50–70-yr multidecadal variability related to Pacific decadal oscillation (PDO) phase changes. The PDSI in north China is significantly negatively correlated with the PDO index, particularly at the 50–70-yr time scale, and is also stable during 1900–2010. Composite differences between two positive PDO phases (1922–45 and 1977–2002) and one negative PDO phase (1946–76) for summer exhibit an anomalous Pacific–Japan/East Asian–Pacific patternlike teleconnection, which may develop locally in response to the PDO-associated warm sea surface temperature anomalies in the tropical Indo-Pacific Ocean and meridionally extends from the tropical western Pacific to north China along the East Asian coast. North China is dominated by an anomalous high pressure system at mid–low levels and an anticyclone at 850 hPa, which are favorable for dry conditions. In addition, a weakened land–sea thermal contrast in East Asia from a negative to a positive PDO phase also plays a role in the dry conditions in north China by weakening the East Asian summer monsoon.

Coral Luminescence Identifies the Pacific Decadal Oscillation as a Primary Driver of River Runoff Variability Impacting the Southern Great Barrier Reef

The Pacific Decadal Oscillation (PDO) is a large-scale climatic phenomenon modulating ocean-atmosphere variability on decadal time scales. While precipitation and river flow variability in the Great Barrier Reef (GBR) catchments are sensitive to PDO phases, the extent to which the PDO influences coral reefs is poorly understood. Here, six Porites coral cores were used to produce a composite record of coral luminescence variability (runoff proxy) and identify drivers of terrestrial influence on the Keppel reefs, southern GBR. We found that coral skeletal luminescence effectively captured seasonal, inter-annual and decadal variability of river discharge and rainfall from the Fitzroy River catchment. Most importantly, although the influence of El Niño-Southern Oscillation (ENSO) events was evident in the luminescence records, the variability in the coral luminescence composite record was significantly explained by the PDO. Negative luminescence anomalies (reduced runoff) were associated with El Niño years during positive PDO phases while positive luminescence anomalies (increased runoff) coincided with strong/moderate La Niña years during negative PDO phases. This study provides clear evidence that not only ENSO but also the PDO have significantly affected runoff regimes at the Keppel reefs for at least a century, and suggests that upcoming hydrological disturbances and ecological responses in the southern GBR region will be mediated by the future evolution of these sources of climate variability.

Modulation of PDO on the predictability of the interannual variability of early summer rainfall over south China

AbstractThis study investigates the modulation of the Pacific Decadal Oscillation (PDO) on the predictability of interannual early summer south China rainfall (SCR) using high‐quality station rainfall data. Of particular interest is the difference in impact between negative and positive phases of the PDO on the predictability of interannual early summer SCR. A clear difference in the correlation between the interannual early summer SCR and the preceding sea surface temperature (SST) over the Pacific Ocean appears in negative and positive phases of the PDO. In the negative PDO phase, the correlation between interannual early summer SCR and SST is dominated by a pattern with significant negative correlations in the subtropical western North Pacific and southeast Pacific and significant positive correlations in the tropical central Pacific. However, in the positive PDO phase, significant positive correlations are observed in the tropical eastern Pacific. It is found that, for each PDO phase, the preceding SST anomalies in some regions in the Pacific may act as predictors of the interannual early summer SCR. As such, a two‐regime regression model for the relationship between interannual early summer SCR and preceding SST anomalies is established based on the negative and positive PDO phases using respective multiple linear regression models. Results suggest that the interannual early SCR is more predictable in PDO positive phase than in negative phase. It offers a support for the argument that a segmented statistical forecasting approach associated with the decadal modulation effect of the coupled ocean atmospheric mode should be adopted to forecast the early summer SCR.

Changes in the spatial structure of strong and moderate El Niño events under global warming

In this study, we examine how the spatial structures of strong and moderate El Niño events have changed due to global warming using the Coupled Model Intercomparison Project Phase 5 (CMIP5) coupled general circulation models (CGCMs) based on a historical run and a Representative Concentration Pathways (RCP)4.5 run. The centre of the maximum anomalous sea surface temperature (SST) in the moderate El Niño has shifted significantly westward from the historical run to the RCP4.5 run; however, this is not seen with the strong El Niño events. Further analysis indicates that the centre of the maximum anomalous SST in the moderate El Niño with a negative Pacific Decadal Oscillation (PDO) phase shifted westward around the international dateline from the historical run to the RCP4.5 run. This is in contrast to the moderate El Niño, which shows a positive PDO phase. We also briefly discuss a possible mechanism on the change in the spatial structure of moderate El Niño with a negative phase of PDO from the historical run to the RCP4.5 run.

Combined effect of El Niño-Southern Oscillation and Pacific Decadal Oscillation on the East Asian winter monsoon

Using long-term observational data and numerical model experiments, the combined effect of the El Nino-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) on the variability of the East Asian winter monsoon is examined. In the observations, it is found that when the ENSO and PDO are in-phase combinations (i.e., El Nino/positive PDO phase and La Nina/negative PDO phase), a negative relationship between ENSO and East Asian winter monsoon is significantly intensified. In other words, when El Nino (La Nina) occurs with positive (negative) PDO phase, anomalous warm (cold) temperatures are dominant over the East Asian winter continent. On the other hand, there are no significant temperature anomalies when the ENSO and PDO are out-of-phase combinations (i.e., El Nino/negative PDO phase and La Nina/positive PDO phase). Further analyses indicate that the anticyclone over the western North Pacific including the East Asian marginal seas plays an essential role in modulating the intensity of the East Asian winter monsoon under the changes of ENSO–PDO phase relationship. Long-lasting high pressure and warm sea surface temperature anomalies during the late fall/winter and following spring over the western North Pacific, which appear as the El Nino occurs with positive PDO phase, can lead to a weakened East Asian winter monsoon by transporting warm and wet conditions into the East Asian continent through the southerly wind anomalies along the western flank of the anomalous high pressure, and vice versa as the La Nina occurs with negative PDO phase. In contrast, the anomalous high pressure over the western North Pacific does not show a prominent change under the out-of-phase combinations of ENSO and PDO. Numerical model experiments confirm the observational results, accompanying dominant warm temperature anomalies over East Asia via strong anticyclonic circulation anomalies near the Philippine Sea as the El Nino occurs with positive PDO phase, whereas such warming is weakened as the El Nino occurs with negative PDO phase. This result supports the argument that the changes in the East Asian winter monsoon intensity with ENSO are largely affected by the strength of the anticyclone over the western North Pacific, which significantly changes according to the ENSO–PDO phase relationship.

Madagascar corals reveal a multidecadal signature of rainfall and river runoff since 1708

Abstract. Pacific Ocean sea surface temperatures (SST) influence rainfall variability on multidecadal and interdecadal timescales in concert with the Pacific Decadal Oscillation (PDO) and Interdecadal Pacific Oscillation (IPO). Rainfall variations in locations such as Australia and North America are therefore linked to phase changes in the PDO. Furthermore, studies have suggested teleconnections exist between the western Indian Ocean and Pacific Decadal Variability (PDV), similar to those observed on interannual timescales related to the El Niño Southern Oscillation (ENSO). However, as instrumental records of rainfall are too short and sparse to confidently assess multidecadal climatic teleconnections, here we present four coral climate archives from Madagascar spanning up to the past 300 yr (1708–2008) to assess such decadal variability. Using spectral luminescence scanning to reconstruct past changes in river runoff, we identify significant multidecadal and interdecadal frequencies in the coral records, which before 1900 are coherent with Asian-based PDO reconstructions. This multidecadal relationship with the Asian-based PDO reconstructions points to an unidentified teleconnection mechanism that affects Madagascar rainfall/runoff, most likely triggered by multidecadal changes in North Pacific SST, influencing the Asian Monsoon circulation. In the 20th century we decouple human deforestation effects from rainfall-induced soil erosion by pairing luminescence with coral geochemistry. Positive PDO phases are associated with increased Indian Ocean temperatures and runoff/rainfall in eastern Madagascar, while precipitation in southern Africa and eastern Australia declines. Consequently, the negative PDO phase that started in 1998 may contribute to reduced rainfall over eastern Madagascar and increased precipitation in southern Africa and eastern Australia. We conclude that multidecadal rainfall variability in Madagascar and the western Indian Ocean needs to be taken into account when considering water resource management under a future warming climate.

Interdecadal Linkages Between the Pacific Decadal Oscillation and Interhemispheric Air Mass Oscillation and Their Possible Connections with East Asian Monsoon

Abstract:The Pacific decadal oscillation (PDO) recently emerged in the literature as a robust signal in the Northern Hemisphere climate variability. Many studies reported that the relationships between PDO and East Asian monsoon (EAM) and climate variability in China are significant. Their possible mechanisms are, however, still unclear. Using the observational NCEP/NCAR reanalysis and Chinese station data during the period of 1969–2008, this study investigates the interdecadal relationship between Pacific decadal oscillation (PDO) and interhemispheric air mass imbalance or oscillation (IHO) between the Northern and Southern Hemispheres. The possible connection of PDO and IHO with both East Asian monsoon and climate variability in China are also assessed. It is found that the interdecadal components (11∼38 years) of PDO, IHO, and EAM contribute large variance to low frequency variations, and they are well‐matched with each other on (inter) decadal timescale. In particular, their negative phases mainly appeared in the 1970 s and late 1990 s, while positive phase in period from 1980 s to mid 1990 s. Decadal change of global mean air columnar temperature may be the key factor for the notable difference between PDO and IHO from mid 1970 s to mid 1990 s. The spatial distributions of PDO and IHO associated surface air temperature and surface pressure anomalies exhibit high similarity and large scale characteristics, indicative of their intimate linkage with air mass redistribution over the global domain especially over 30°S–50°N. The PDO associated columnar integral of velocity potential anomalies that maintain the air mass redistribution show a dipole pattern with air mass flux emanating mainly from the eastern hemisphere to the Pacific regions in positive PDO phase. This contributes to hemispherical and land‐sea mass exchange and redistribution, and also leads to the decadal displacement of both upward and downward branch of Walker circulation. In positive phase of PDO, an anomalous anticyclone is found in the Mongolian region in both boreal summer and winter seasons, inducing significant anomalous northerlies in the eastern China, and hence intensifying (weakening) the east Asian winter (summer) monsoon. Consequently, the interdecadal components of temperature and precipitation at most stations in east China are simultaneously correlated with the Pacific decadal oscillation index significantly.

Seasonal Variation and Spatial Patterns of the Atmospheric Component of the Pacific Decadal Oscillation

Abstract The Pacific decadal oscillation (PDO) is an El Niño–Southern Oscillation (ENSO)-like climate oscillation that varies on multidecadal and higher-frequency scales, with a sea surface temperature (SST) dipole in the Pacific. This study addresses the seasonality, vertical structure, and across-variable relationships of the local North Pacific and downstream North American atmospheric signal of the PDO. The PDO-based composite difference fields of 500-mb geopotential height, surface air temperature, sea level pressure, and precipitation vary not only across seasons, but also from one calendar month to another within a season, although month-to-month continuity is apparent. The most significant signals occur in western North America and in the southeastern United States, where a positive PDO is associated with negative heights, consistent with underlying temperatures in the winter. In summer, a negative precipitation signal in the southeastern United States associated with a positive PDO phase is consistent with a ridge over the region. When an annual harmonic is fit to the 12 monthly surface air temperature differences at each grid point, the PDO temperature signal peaks in winter in most of North America, while a peak in summer occurs in the southeastern United States. Approximately 25% of the variance of the PDO index is accounted for by ENSO. Atmospheric composite differences based on a residual (ENSO linearly removed) PDO index have many similarities to those of the full PDO signal.

Roles of ENSO and PDO in the Link of the East Asian Winter Monsoon to the following Summer Monsoon

Abstract The present study investigates the roles of El Niño–Southern Oscillation (ENSO) and the Pacific decadal oscillation (PDO) in the relationship between the East Asian winter monsoon (EAWM) and the following East Asian summer monsoon (EASM). The variability of the EAWM is divided into an ENSO-related part named EAWMEN and an ENSO-unrelated part named EAWMres. Corresponding to a weak EAWMEN, an anomalous low-level anticyclone forms over the western North Pacific (WNP) and persists from winter to the following summer. This anticyclone enhances southerlies over the coast of East Asia in summer. Hence, a weak EAWMEN tends to be followed by a strong EASM and vice versa. As such, a link is established between the EAWMEN and the EASM. The persistence of this WNP anticyclone may be mainly attributed to the sea surface temperature anomalies associated with the ENSO-related EAWM part in the tropical Indian Ocean and the extratropical North Pacific. In contrast, corresponding to a weak EAWMres, the anomalous WNP anticyclone is only seen in winter, and there is no obvious relationship between the EAWMres and the following EASM. Therefore, the observed EAWM–EASM relationship is dominated by the winter monsoon variability associated with ENSO. It is found that the EAWMEN–EASM relationship is modulated by the PDO. There tends to be a much stronger EASM after a weak EAWMEN during the positive PDO phases than during the negative PDO phases.

Forcing of hydroclimatic variability in the northwestern Great Plains since AD 1406

The 20th century hydroclimatology of northwestern North America has been linked to naturally recurring large-scale climate patterns such as the Pacific Decadal Oscillation (PDO) and the El Niño-Southern Oscillation (ENSO). Few observed hydroclimatic records from this region exceed in length the ∼60-year periodicity of the lower frequency climate oscillations; however, tree-ring proxy data from semi-arid western North America document natural hydroclimate variation over centennial to millennial scales. We reconstructed the summer Palmer Drought Severity Index (PDSI) over the western Canadian Prairies, providing a record of drought for the past 600 years. This long reconstruction was examined for information about the severity, intensity, and duration of positive (wet) and negative (dry) summer moisture anomalies during the different phases of the PDO and the ENSO, as reconstructed by other researchers from independent proxy datasets. As well, by comparing our PDSI reconstruction to other researchers' independent regional summer temperature reconstruction, we were able to identify warm versus cold droughts, and warm versus cold wet periods. The reconstruction was also compared to 500 hPa geopotential heights. These results suggest that summer moisture conditions are associated with the North Pacific Ocean and tropical Pacific Ocean variability. Summer droughts generally have occurred during the positive PDO phase; however, ENSO conditions have varied between El Niño and La Niña. The most severe droughts, such as those of AD 1858–1872 and AD 1930–1941, were commonly associated with higher summer temperatures, the positive phase of the PDO, and increased ENSO variability.

Changes in the linear relationship of ENSO–PDO under the global warming

AbstractWe examine changes in El Niño and Southern Oscillation (ENSO)/Pacific Decadal Oscillation (PDO) relationship under the global warming using coupled climate models participated in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). The temporal structure for the ENSO–PDO relationship is changed remarkably. The relationship between ENSO and PDO during the boreal winter (December, January and February) becomes stronger so that there would be more frequent in phase occurrence of ENSO and PDO (i.e. El Niño—a positive phase of PDO or La Niña—a negative phase of PDO). As PDO could constructively interfere with the ENSO‐related climate when ENSO and PDO are in phase, in the future one may expect stronger climate signal because of ENSO in the midlatitude. The IPCC AR4 model also shows that the Pacific North America‐like pattern is slightly shifted eastward and much stronger. We also discuss the possible reason for these changes. Copyright © 2012 Royal Meteorological Society

Variation of the North Atlantic subtropical high western ridge and its implication to Southeastern US summer precipitation

Variations of the North Atlantic subtropical high (NASH) western ridge and their implication to the Southeastern United States (SE US) summer precipitation were analyzed for the years 1948–2007. The results show that the movement of the NASH western ridge regulates both moisture transport and vertical motion over the SE US, especially in the last three decades, during which the ridge moved westward towards the American continent. When the NASH western ridge is located southwest (SW) of its mean climate position, excessive summer precipitation is observed due to an enhanced moisture transport. In contrast, when the western ridge is located in the northwest (NW), a precipitation deficit prevails as downward motion dominates the region. Composite analysis indicates that SW ridging results mainly from the NASH center’s intensification; whereas NW ridging is likely caused by stationary wave propagation from the eastern Pacific/US western coast. In recent decades, both the SW and NW ridge positions have been observed to increase in frequency. Our results suggest that the increase in the SW ridging consistently follows the NASH’s intensification associated with anthropogenic forcing as projected by coupled climate models. However, the increased frequency of NW ridging tends to follow the positive Pacific decadal oscillation (PDO) index. Thus, the enhanced variability in the SE US summer precipitation in recent decades might be a combined result of anthropogenic forcing and internal variability of the climate system. Results suggest that, as anthropogenic forcing continues to increase, the SE US will experience more frequent wet summers and an increase in the frequency of dry summers during positive PDO phases.

Anti‐correlated variability in subduction rate of the western and eastern North Pacific Oceans identified by an eddy‐resolving ocean GCM

Analysis of results from an eddy‐resolving ocean general circulation model has revealed the existence of an anti‐correlated variability in subduction rate of the western and eastern North Pacific Oceans, consistent with the leading mode of Pacific Decadal Oscillation (PDO). A composite analysis shows that during the positive phase of PDO, the winter mixed layer depth (MLD) generally gets deeper in the west and shallower in the east, being closely related to the variability in surface heat flux stemming from an enhanced Aleutian Low, and the situation during the negative phase of PDO is reversed. Changes in MLD are primarily responsible for the anti‐correlated variability in subduction rate, though changes in subtropical gyre circulation also play a role. The result also provides useful evidence for the interior and western boundary pathways, through which variability in eastern subtropical mode water can be conveyed into the equatorial region.