Pacific Teleconnection Pattern Research Articles

An Arctic‐Tibetan Connection on Subseasonal to Seasonal Time Scale

AbstractRecent research indicates the great potentials of springtime land surface temperature (LST) as a new source of predictability to improve the subseasonal to seasonal climate prediction. In this study, we explore the initial cause of the springtime large‐scale LST in Tibetan Plateau (TP) and disentangle its close connection with the February wave activities from the Arctic region. Our Maximum Covariance Analysis show that the spring LST in TP is significantly coupled with the regional snow cover in the preceding months. The latter is further strongly coupled with the February atmospheric circulation and wave activities in mid‐to‐high latitudes. When the atmospheric circulation is in a combined pattern of Arctic Oscillation and West Pacific teleconnection pattern, wave trains from the Arctic can propagate and reach the TP through northern and southern pathways. This brings dynamical and moisture conditions for the TP snowfall and builds a bridge for Arctic‐Tibetan connection.

On the Role of the Eastern Pacific Teleconnection in ENSO Impacts on Wintertime Weather over East Asia and North America

Previous studies have mainly focused on the influence of El Niño–Southern Oscillation (ENSO) on seasonal-mean conditions over East Asia and North America. This study, instead, proposes an ENSO pathway that influences the weather events over East Asia and North America, in which the eastern Pacific teleconnection pattern (EP) plays an important role. On the one hand, the EP pattern can induce significant surface temperature anomalies over East Asia during its development and mature stages, with the positive (negative) EPs causing colder (warmer) than normal weather events. Besides, the frequency of occurrence of EPs is significantly modulated by ENSO, with 50% of the positive EPs occurring in La Niña winters, and 47% of the negative EPs occurring in El Niño winters. As a result, in El Niño winters, more negative and fewer positive EPs tend to occur, and thus more warm and fewer cold weather events are expected in East Asia. For La Niña winters, the reverse is true. On the other hand, for the EP pattern without its canonical convection pattern (referred to as the nonconvective EP), extremely cold anomalies over the northern United States and western Canada are induced in its negative phase. Moreover, when there are positive sea surface temperature anomalies in the central equatorial Pacific, the frequency of occurrence of negative nonconvective EPs is 2.0 times greater than the climatological value, and thus an enhanced likelihood of extremely cold spells over North America may be expected.

Terrestrial evaporation response to modes of climate variability

Large-scale modes of climate variability (or teleconnection patterns), such as the El Niño Southern Oscillation and the North Atlantic Oscillation, affect local weather worldwide. However, the response of terrestrial water and energy fluxes to these modes of variability is still poorly understood. Here, we analyse the response of evaporation to 16 teleconnection patterns, using a simple supervised learning framework and global observation-based datasets of evaporation and its key climatic drivers. Our results show that the month-to-month variability in terrestrial evaporation is strongly affected by (coupled) oscillations in sea-surface temperature and air pressure: in specific hotspot regions, up to 40% of the evaporation dynamics can be explained by climate indices describing the fundamental modes of climate variability. While the El Niño Southern Oscillation affects the dynamics in land evaporation worldwide, other phenomena such as the East Pacific–North Pacific teleconnection pattern are more dominant at regional scales. Most modes of climate variability affect terrestrial evaporation by inducing changes in the atmospheric demand for water. However, anomalies in precipitation associated to particular teleconnections are crucial for the evaporation in water-limited regimes, as well as in forested regions where interception loss forms a substantial fraction of total evaporation. Our results highlight the need to consider the concurrent impact of these teleconnections to accurately predict the fate of the terrestrial branch of the hydrological cycle, and provide observational evidence to help improve the representation of surface fluxes in Earth system models.

Abrupt ice-age shifts in southern westerly winds and Antarctic climate forced from the north.

The mid-latitude westerly winds of the Southern Hemisphere play a central role in the global climate system via Southern Ocean upwelling1, carbon exchange with the deep ocean2, Agulhas leakage (transport of Indian Ocean waters into the Atlantic)3 and possibly Antarctic ice-sheet stability4. Meridional shifts of the Southern Hemisphere westerly winds have been hypothesized to occur5,6 in parallel with the well-documented shifts of the intertropical convergence zone7 in response to Dansgaard-Oeschger (DO) events- abrupt North Atlantic climate change events of the last ice age. Shifting moisture pathways to West Antarctica8 are consistent with this view but may represent a Pacific teleconnection pattern forced from the tropics9. The full response of the Southern Hemisphere atmospheric circulation to the DO cycle and its impact on Antarctic temperature remain unclear10. Here we use five ice cores synchronized via volcanic markers to show that the Antarctic temperature response to the DO cycle can be understood as the superposition of two modes: a spatially homogeneous oceanic 'bipolar seesaw' mode that lags behind Northern Hemisphere climate by about 200 years, and a spatially heterogeneous atmospheric mode that is synchronous with abrupt events in the Northern Hemisphere. Temperature anomalies of the atmospheric mode are similar to those associated with present-day Southern Annular Mode variability, rather than the Pacific-South American pattern. Moreover, deuterium-excess records suggest a zonally coherent migration of the Southern Hemisphere westerly winds over all ocean basins in phase with Northern Hemisphere climate. Our work provides a simple conceptual framework for understanding circum-Antarctic temperature variations forced by abrupt Northern Hemisphere climate change. We provide observational evidence of abrupt shifts in the Southern Hemisphere westerly winds, which have previously documented1-3 ramifications for global ocean circulation and atmospheric carbon dioxide. These coupled changes highlight the necessity of a global, rather than a purely North Atlantic, perspective on the DO cycle.

Predictable and Unpredictable Components of the Summer East Asia–Pacific Teleconnection Pattern

The East Asia–Pacific (EAP) teleconnection pattern is the dominant mode of circulation variability during boreal summer over the western North Pacific and East Asia, extending from the tropics to high latitudes. However, much of this pattern is absent in multi-model ensemble mean forecasts, characterized by very weak circulation anomalies in the mid and high latitudes. This study focuses on the absence of the EAP pattern in the extratropics, using state-of-the-art coupled seasonal forecast systems. The results indicate that the extratropical circulation is much less predictable, and lies in the large spread among different ensemble members, implying a large contribution from atmospheric internal variability. However, the tropical–mid-latitude teleconnections are also relatively weaker in models than observations, which also contributes to the failure of prediction of the extratropical circulation. Further results indicate that the extratropical EAP pattern varies closely with the anomalous surface temperatures in eastern Russia, which also show low predictability. This unpredictable circulation–surface temperature connection associated with the EAP pattern can also modulate the East Asian rainband.

Interannual variation in the number and severity of autumnal haze days in the Beijing–Tianjin–Hebei region and associated atmospheric circulation anomalies

In contrast with the considerable number of studies on the variability of winter haze pollution over eastern China, there have been few studies of autumnal haze in this region. The interannual variability of autumnal (September–November) haze days in the Beijing–Tianjin–Hebei region (AHDBTH) is significantly correlated with the Scandinavia–central Siberia–Western Pacific (SCSWP) teleconnection pattern, a quasi-barotropic atmospheric circulation anomaly. The SCSWP teleconnection pattern may therefore be a useful indicator for forecasting AHDBTH. The positive phase of the SCSWP teleconnection pattern enhances the anomalous anticyclonic circulation centered over the Western Pacific via the southeastward propagating Rossby wave train emanating from Scandinavia. The positive SCSWP teleconnection pattern, together with the northward shift of the East Asian jet stream induced by low-level atmospheric baroclinicity, could provide the favorable local meteorological circumstances (i.e., suppressed cold air activity, higher surface temperatures and low-level atmospheric inversion, and lower planetary boundary layer height and decreased surface wind speed) required to explain the recent increased incidence of AHDBTH.

A key process of the nonstationary relationship between ENSO and the Western Pacific teleconnection pattern

Recent studies have discovered an intriguing nonstationary relationship between El Ninõ–Southern Oscillation (ENSO) and the Western Pacific (WP) teleconnection pattern, one of the most prominent winter atmospheric circulation patterns in the North Pacific, with a regime-dependent interdecadal modulation of significant and insignificant correlations. However, the physical process underlying the observed nonstationary ENSO-WP relationship is a puzzle and remains to be elucidated, which is also essential for clarifying the still-debated nontrivial issue on whether the WP is directly forced by ENSO or by midlatitude storm tracks-driven intrinsic processes. Based on empirical orthogonal function (EOF) analysis of the upper-tropospheric teleconnection patterns and associated Rossby wave sources (RWS), we show that the nonstationarity in question is due to the regime-dependent constructive or destructive interference in meridional overturning circulation between the two leading EOFs of RWS best correlated with ENSO and WP, respectively. The observed insignificant correlation between ENSO and the WP after the 1988 regime shift can be explained by interrupted teleconnection between the tropics and high latitudes due to the collapse of the subtropical bridge pillar in the jet entrance region, consequence of the destructive interference. This suggested interference mechanism related to the regime-dependent upper-level RWS fields has significant implications for resolving the puzzle that hinders better understanding of decadal regime behaviors of the climate system in the North Pacific.

Potential Influence of the East Asia–Pacific Teleconnection Pattern on Persistent Precipitation in South China: Implications of Atypical Yangtze River Valley Cases

Abstract In this study, cases of the East Asia–Pacific (EAP) teleconnection pattern not responsible for persistent precipitation processes in the Yangtze River valley (YRV) have been investigated. The results suggest that such a type of EAP pattern has some linkage with persistent precipitation processes in south China (SC) with the following properties: 1) in response to the negative SSTAs and anticyclone near the Philippines, the meridional energy propagates from the low latitudes over the north of the Philippines; 2) the western Pacific subtropical high (WPSH) then intensifies and extends westward; 3) a meridional triple structure of the EAP teleconnection pattern is established; 4) at the same time, the cyclonic circulation over northeastern China introduces cold and dry air to the lower latitudes, merging with the water vapor into SC and leading to heavy precipitation from the fringe of the WPSH, the South China Sea, and the Bay of Bengal and the combination of systems persists for at least 3 days, leading to the persistent precipitation processes in SC; and 5) compared with the EAP teleconnection responsible for the precipitation in YRV, the positions of the three centers in the mid- and low latitudes are more southerly located than the YRV EAP centers. Further study indicates that the ocean surface heat conditions in the areas near the Philippines seem to be important in affecting the EAP teleconnection pattern for persistent precipitation processes in SC. Finally, all of the cases with persistent precipitation in SC during 1961–2010 linked with the EAP pattern have been investigated; the results are consistent with the above conclusions.

Observed and Simulated Spring and Summer Dryness in the United States: The Impact of the Pacific Sea Surface Temperature and Beyond

AbstractThis study characterizes major modes of variability in the spring and summer U.S. dryness, as measured by the seasonal total number of dry (no‐precipitation) days, and assesses the impact of the Pacific sea surface temperature. The most severe spring and summer dry conditions typically occur in the western United States. Maximum covariance analysis reveals that the Pacific Decadal Oscillation (PDO) is the primary driver of interannual variability in the U.S. dryness. El Niño–Southern Oscillation (ENSO) also contributes in part (especially during spring). Beyond the PDO and ENSO impact, interannual variations in spring dryness exhibit a meridional dipole structure, while variations during summer are related to a northwest‐southeast (NW‐SE) oriented dipole. The spring meridional dipole is associated with circulation anomalies resembling the West Pacific teleconnection pattern, while the summer NW‐SE dipole is a downstream manifestation of a quasi‐stationary wave train. A parallel analysis of Coupled Model Intercomparison Project's fifth phase models' historical simulations demonstrates that such models generally capture the relation between U.S. dryness and the PDO, albeit with varying degrees of accuracy. The models also show reasonable skill in simulating the residual meridional dipole in spring dryness variability but have difficulty representing the NW‐SE oriented dipole occurring during summer. The model shortcomings isolated here largely arise from a misrepresentation of the corresponding large‐scale circulation and moisture transport anomalies. These model biases suggest that great challenges exist in our ongoing pursuit of reliable projections of the U.S. hydroclimate variability.

Influence of the May Southern annular mode on the South China Sea summer monsoon

The possible impact of the May Southern Hemisphere (SH) annular mode (SAM) on the following South China Sea (SCS) summer monsoon (SCSSM) is examined. A close inverse relationship between the two is revealed in the observations. The simultaneous South Pacific dipole (SPD), a dipole-like sea surface temperature anomaly pattern in the South Pacific, acts as the “oceanic bridge” to preserve the May SAM signal and prolong it into June–September. Observational evidence and numerical simulations both demonstrate that the SPD communicates its large thermal inertia signal to the atmosphere, regulating the Southern Pacific Subtropical Jet (SPSJ) variability over eastern Australia. Corresponding to the adjustment of circulation associated with the SPSJ is a prominent tripolar cross-Pacific teleconnection pattern stretching from the SH middle–high latitudes into the NH East Asia coastal region, referred to as the South–North Pacific (SNP) teleconnection pattern. Wave ray tracing analysis manifests that the SNP acts as the “atmospheric bridge” to propagate the related wave energy across the equator and into the Maritime Continent and SCS monsoon region, modulating the vertical motion and middle–lower tropospheric flows, and favoring the out-of-phase variation of the SCSSM. Therefore, the “coupled oceanic-atmospheric bridge” process and the related Rossby wave energy transmission are possible mechanisms for the significant influence of the May SAM on the variability of the following SCSSM. Therefore, the May SAM provides a fresh insight into the prediction of the SCSSM from the perspective of the SH high latitudes.

Modes of interannual variability in northern hemisphere winter atmospheric circulation in CMIP5 models: evaluation, projection and role of external forcing

Models from the coupled model intercomparison project phase 5 (CMIP5) dataset are evaluated for their ability to simulate the dominant slow modes of interannual variability in the Northern Hemisphere atmospheric circulation 500 hPa geopotential height in the twentieth century. A multi-model ensemble of the best 13 models has then been used to identify the leading modes of interannual variability in components related to (1) intraseasonal processes; (2) slowly-varying internal dynamics; and (3) the slowly-varying response to external changes in radiative forcing. Modes in the intraseasonal component are related to intraseasonal variability in the North Atlantic, North Pacific and North American, and Eurasian regions and are little affected by the larger radiative forcing of the Representative Concentration Pathways 8.5 (RCP8.5) scenario. The leading modes in the slow-internal component are related to the El Nino-Southern Oscillation, Pacific North American or Tropical Northern Hemisphere teleconnection, the North Atlantic Oscillation, and the Western Pacific teleconnection pattern. While the structure of these slow-internal modes is little affected by the larger radiative forcing of the RCP8.5 scenario, their explained variance increases in the warmer climate. The leading mode in the slow-external component has a significant trend and is shown to be related predominantly to the climate change trend in the well mixed greenhouse gas concentration during the historical period. This mode is associated with increasing height in the 500 hPa pressure level. A secondary influence on this mode is the radiative forcing due to stratospheric aerosols associated with volcanic eruptions. The second slow-external mode is shown to be also related to radiative forcing due to stratospheric aerosols. Under RCP8.5 there is only one slow-external mode related to greenhouse gas forcing with a trend over four times the historical trend.

Combined effect of the East Atlantic/West Russia and Western Pacific teleconnections on the East Asian winter monsoon

This study investigates the individual effects of the East Atlantic/West Russia (EATL/WRUS) and Western Pacific (WP) teleconnection patterns and their combined effect on the East Asian winter monsoon (EAWM). The contributions of the respective EATL/WRUS and WP teleconnection patterns to the EAWM are revealed by removing the dependence on the Arctic Oscillation (AO) and the El Nino-Southern Oscillation (ENSO) using a linear regression, which are named as N_EATL/WRUS and N_WP, respectively. This is because the EATL/WRUS (WP) is closely linked to the Arctic (tropics) region. A significant increase (decrease) in temperature over East Asia (EA) corresponding to a weak (strong) EAWM is associated with the N_EATL/WRUS and N_WP teleconnection patterns during the positive (negative) phases. In order to examine impacts of these two teleconnections on the EAWM, three types of effects are reconstructed on the basis of ± 0.5 standard deviation: 1) Combined effect, 2) N_EATL/WRUS effect, and 3) N_WP effect. The positive N_EATL/WRUS teleconnection induces to a weakened Siberian High and a shallow EA trough at the mid-troposphere through wave propagation, leading to the weak EAWM. During the positive N_WP pattern, warm air from the tropics flows toward the EA along western flank of an anomalous anticyclone over the North Pacific that is relevant to the meridional shift of the Aleutian Low. When the two mid-latitude teleconnections have the in-phase combination, the increase in temperature over EA appears to be more pronounced than the individual effects by transporting warm air from tropics via strong southeasterly wind anomalies induced by anomalous zonal pressure gradient between the Siberian High and Aleutian Low. Therefore, the impact of the mid-latitude teleconnections on the EAWM becomes robust and linearly superimposed, unlike a nonlinear in-phase combined effect of the AO and ENSO.

Different impact of central Pacific and eastern Pacific El Ni\xf1o on the duration of sudden stratospheric warming

The NCEP–NCAR reanalysis dataset and the HadISST dataset (1959–2014) are used to analyze the impact of two types of El Niño events, i.e., eastern Pacific El Niño (EP-El Niño) and central Pacific El Niño (CP-El Niño) events, on the duration of major and minor sudden stratospheric warmings (SSWs) in Northern Hemisphere winter (November to February). Although the frequency of major and minor SSWs during different types of El Niño shows no distinct differences, the duration of both major and minor SSWs during CP-El Niño is shorter than that during EP-El Niño. The spatial distribution of geopotential height anomalies preceding major SSWs resembles the western Pacific (WP) teleconnection pattern, while the spatial distribution of geopotential height anomalies preceding minor SSWs bears similarity to the Pacific–North America (PNA) teleconnection pattern. An enhancement of the strength of both wavenumber 1 and wavenumber 2 is found before major SSWs. Before minor SSWs, wavenumber 1 is also strengthened, but wavenumber 2 is weakened. The analysis also reveals that EP-El Niño tends to induce positive phases of PNA and WP teleconnections, while CP-El Niño induces negative-phase WP teleconnection. As the positive phases of the PNA and WP teleconnections are related to the strengthening of wavenumber 1, EP-El Niño causes an enhancement of wavenumber 1 in the high-latitude upper troposphere and an enhancement of the upward wave flux in the high-latitude stratosphere, accompanied by a negative anomaly in Eliassen–Palm flux divergence in the subpolar stratosphere, which accounts for the longer SSW duration during EP-El Niño than during CP-El Niño.

Upper-ocean thermal variability controlled by ocean dynamics in the Kuroshio-Oyashio Extension region

To understand the upper-ocean thermal variability in the Kuroshio−Oyashio Extension (KOE) region, the upper 400 m heat budget in the western North Pacific is analyzed for the 1981 − 2013 period using outputs from a high resolution (1/12°) ocean general circulation model. Winter heat storage rate on interannual to decadal time scales is mainly determined by oceanic heat advection rather than by net air-sea heat flux. The role of heat advection becomes particularly prominent and widely spread over the entire western North Pacific after the 1990 regime shift in association with the reduced variability of surface heat flux caused by weakened SST variability. The net heat flux acts to dampen temperature anomalies caused by the ocean dynamics. The ocean dynamics causing the upper-ocean heat storage rate is principally associated with the meridional shift of the Oyashio Extension front, which is significantly correlated with both the West Pacific and Pacific-North America teleconnection patterns.

Impacts of the leading modes of tropical Indian Ocean sea surface temperature anomaly on sub-seasonal evolution of the circulation and rainfall over East Asia during boreal spring and summer

The two leading modes of the interannual variability of the tropical Indian Ocean (TIO) sea surface temperature (SST) anomaly are the Indian Ocean basin mode (IOBM) and the Indian Ocean dipole mode (IODM) from March to August. In this paper, the relationship between the TIO SST anomaly and the sub-seasonal evolution of the circulation and rainfall over East Asia during boreal spring and summer is investigated by using correlation analysis and composite analysis based on multi-source observation data from 1979 to 2013, together with numerical simulations from an atmospheric general circulation model. The results indicate that the impacts of the IOBM on the circulation and rainfall over East Asia vary remarkably from spring to summer. The anomalous anticyclone over the tropical Northwest Pacific induced by the warm IOBM is closely linked with the Pacific–Japan or East Asia–Pacific teleconnection pattern, which persists from March to August. In the upper troposphere over East Asia, the warm phase of the IOBM generates a significant anticyclonic response from March to May. In June and July, however, the circulation response is characterized by enhanced subtropical westerly flow. A distinct anomalous cyclone is found in August. Overall, the IOBM can exert significant influence on the western North Pacific subtropical high, the South Asian high, and the East Asian jet, which collectively modulate the precipitation anomaly over East Asia. In contrast, the effects of the IODM on the climate anomaly over East Asia are relatively weak in boreal spring and summer. Therefore, studying the impacts of the TIO SST anomaly on the climate anomaly in East Asia should take full account of the different sub-seasonal response during boreal spring and summer.

Vertical Structure and Energetics of the Western Pacific Teleconnection Pattern

AbstractThe western Pacific (WP) pattern, characterized by north–south dipolar anomalies in pressure over the Far East and western North Pacific, is known as one of the dominant teleconnection patterns in the wintertime Northern Hemisphere. Composite analysis reveals that monthly height anomalies exhibit baroclinic structure with their phase lines tilting southwestward with height in the lower troposphere. The anomalies can thus yield not only a poleward heat flux across the climatological thermal gradient across the strong Pacific jet but also a westward heat flux across the climatological thermal gradient between the North Pacific and the cooler Asian continent. The resultant baroclinic conversion of available potential energy (APE) from the climatological-mean flow contributes most efficiently to the APE maintenance of the monthly WP pattern, acting against strong thermal damping effects by anomalous heat exchanges with the underlying ocean and anomalous precipitation in the subtropics and by the effect of anomalous eddy heat flux under modulated storm-track activity. Kinetic energy (KE) of the pattern is maintained through barotropic feedback forcing associated with modulated activity of transient eddies and the conversion from the climatological-mean westerlies, both of which act against frictional damping. The net feedback forcing by transient eddies is therefore not particularly efficient. The present study suggests that the WP pattern has a characteristic of a dynamical mode that can maintain itself through efficient energy conversion from the climatological-mean fields even without external forcing, including remote influence from the tropics.

An east‐west contrast of upper ocean heat content variation south of the subpolar front in the East/Japan Sea

AbstractLong‐term variability of nonseasonal ocean heat content (OHC) in the upper 500 m in the East/Japan Sea (EJS) exhibits a distinct east‐west contrast during the recent 30 years. The contrasting OHC variations are revisited and investigated more in detail by analyzing two observational data sets, the gridded data from 1976 to 2007 and in situ data from 1976 to 2011 in the southwestern EJS that covers the zone of western boundary current. The OHC variability shows in‐phase and predominant decadal variation in both east and west regions before 1995, but uncorrelated and predominant interannual variations after 1995. Heaving effects due to major branches of warm currents in the EJS, the East Korea Warm Current (EKWC) in the western part and other two branches in the eastern part, mainly contribute to the OHC variations. The heaving effect in the western EJS is shown to be associated with changes in winter wind‐stress curl field in the northern EJS. Weakening of the subpolar gyre due to weakening of positive wind‐stress curl in the Japan Basin related with wintertime Western Pacific teleconnection pattern and Siberian High appears to enhance the northward penetration of the EKWC resulting in an increase of OHC in the western EJS. The heaving effect in the eastern EJS is significantly correlated with the Siberian High, but the causative mechanism is inconclusive. This study also demonstrates the importance of using highly resolved data sets for areas affected by strong and narrow boundary currents in computing and understanding the OHC variability.

Teleconnected ocean forcing of Western North American droughts and pluvials during the last millennium

Abstract Western North America (WNA) is rich in hydroclimate reconstructions, yet questions remain about the causes of decadal-to-multidecadal hydroclimate variability. Teleconnection patterns preserved in annually-resolved tree-ring reconstructed drought maps, and anomalies in a global network of proxy sea surface temperature (SST) reconstructions, were used to reassess the evidence linking ocean forcing to WNA hydroclimate variability over the past millennium. Potential forcing mechanisms of the Medieval Climate Anomaly (MCA) and individual drought and pluvial events—including two multidecadal-length MCA pluvials—were evaluated. We show strong teleconnection patterns occurred during the driest (wettest) years within persistent droughts (pluvials), implicating SSTs as a potent hydroclimate forcing mechanism. The role of the SSTs on longer timescales is more complex. Pacific teleconnection patterns show little long-term change, whereas low-resolution SST reconstructions vary over decades to centuries. While weaker than the tropical Pacific teleconnections, North Atlantic teleconnection patterns and SST reconstructions also show links to WNA droughts and pluvials, and may in part account for longer-term WNA hydroclimate changes. Nonetheless, evidence linking WNA hydroclimate to SSTs still remains sparse and nuanced—especially over long-timescales with a broader range of hydroclimatic variability than characterized during the 20th century.

Low-frequency oscillations of the East Asia–Pacific teleconnection pattern and their impacts on persistent heavy precipitation in the Yangtze–Huai River valley

Based on the daily reanalysis data from NCEP–NCAR and daily precipitation data from the China National Meteorological Information Center, an ensemble empirical mode decomposition method is employed to extract the predominant oscillation modes of the East Asia–Pacific (EAP) teleconnection pattern. The influences of these low-frequency modes on persistent heavy precipitation in the Yangtze–Huai River (YHR) valley are investigated. The results indicate that the EAP pattern and rainfall in YHR valley both exhibit remarkable 10–30- and 30–60-day oscillations. The impacts of the EAP pattern on the YHR persistent heavy precipitation can be found on both the 10–30- and 30–60-day timescales—the 10–30-day scale for most cases. Composite analysis indicates that, on the 10–30-day timescale, formation of the EAP pattern in the lower and middle troposphere is determined by convective systems near the tropical western Pacific; whereas in the middle troposphere, the phase transition is jointly contributed by both the dispersion of zonal wave energies at higher latitudes and convective systems over the South China Sea. In the context of the 10–30-day EAP pattern, the anomalously abundant moisture is transported by an anomalous subtropical anticyclone system, and strong moisture convergence results from that anomalous anticyclone system and a cyclonic system in the midlatitude East Asia. Such a combination of systems persists for at least three days, contributing to the formation of persistent heavy precipitation in the YHR valley.

Climate indices strongly influence old-growth forest carbon exchange

We present a decade and a half (1998–2013) of carbon dioxide fluxes from an old-growth stand in the American Pacific Northwest to identify ecosystem-level responses to Pacific teleconnection patterns, including the El Niño/Southern Oscillation (ENSO). This study provides the longest, continuous record of old-growth eddy flux data to date from one of the longest running Fluxnet stations in the world. From 1998 to 2013, average annual net ecosystem exchange (FNEE) at Wind River AmeriFlux was −32 ± 84 g C m−2 yr−1 indicating that the late seral forest is on average a small net sink of atmospheric carbon. However, interannual variability is high (>300 g C m−2 yr−1) and shows that the stand switches from net carbon sink to source in response to climate drivers associated with ENSO. The old-growth forest is a much stronger sink during La Niña years (mean FNEE = −90 g C m−2 yr−1) than during El Niño when the stand turns carbon neutral or into a small net carbon source (mean FNEE = +17 g C m−2 yr−1). Forest inventory data dating back to the 1930s show a similar correlation with the lower frequency Pacific North American (PNA) and Pacific Decadal Oscillation (PDO) whereby higher aboveground net primary productivity (FANPP) is associated with cool phases of both the PNA and PDO. These measurements add evidence that carbon exchange in old-growth stands may be more sensitive to climate variability across shorter time scales than once thought.