Pacific Teleconnection Pattern Research Articles

Impact of May–June Antarctic Oscillation on July–August Heat-Drought Weather in Yangtze River Basin

Investigating the physical mechanism behind the formation of summer heat-drought weather (HDW) in the Yangtze River Basin (YRB) holds significant importance for predicting summer precipitation and temperature patterns in the region as well as disaster mitigation and prevention. This study focuses on spatiotemporal patterns of July–August (JA) HDW in the YRB from 1979 to 2022, which is linked partially to the preceding May–June (MJ) Antarctic Oscillation (AAO). Key findings are summarized as follows: (1) The MJ AAO displays a marked positive correlation with the JA HDW index (HDWI) in the southern part of upper YRB (UYRB), while showing a negative correlation in the area extending from the Han River to the western lower reaches of the YRB (LYRB); (2) The signal of MJ AAO persists into late JA through a specific pattern of Sea Surface Temperature anomalies in the Southern Ocean (SOSST). This, in turn, modulates the atmospheric circulation over East Asia; (3) The SST anomalies in the South Atlantic initiate Rossby waves that cross the equator, splitting into two branches. One branch propagates from the Somali-Tropical Indian Ocean, maintaining a negative-phased East Asia–Pacific (EAP) teleconnection pattern. This enhances the moisture flow from the Pacific towards the middle and lower reaches of the Yangtze River Basin (MYRB-LYRB). The other branch propagates northward, crossing the Somali region, and induces a positive geopotential height anomaly over Urals-West Asia. This reduces the southwesterlies towards the UYRB, thereby contributing to HDW variabilities in the region. (4) Partial Least Squares Regression (PLSR) demonstrated predictive capability for JA HDW in the YRB for 2022, based on Southern Ocean SST.

What Determines the East Asian Winter Temperature during El Niño?—Role of the Early Onset El Niño and Tropical Indian Ocean Warming

Abstract Atmospheric teleconnections from the Pacific El Niño are key to determining the East Asian winter climate. Using the database for policy decision-making for future climate change (d4PDF) large-ensemble simulations, the present study investigates a mechanism for the warm and cold East Asian winters during El Niño with a focus on atmospheric teleconnections triggered by anomalous sea surface temperature (SST) patterns in the tropical Indo-Pacific. Our results show that the western Pacific (WP) teleconnection pattern plays a primary role in the warm winters in East Asia. The WP pattern tends to appear in years when both an early El Niño and the positive phase of the Indian Ocean dipole (IOD) mode develop in boreal autumn. In those years, the tropical Indian Ocean (TIO) strongly warms in the following winter, forming a distinct zonal contrast in precipitation anomalies over the tropical Indo-Pacific through a reduced Walker circulation. The Rossby wave source anomalies indicate that the WP pattern is associated with the weakened Indo-Pacific Walker circulation. By contrast, the WP pattern does not dominate in the cold winters due to the absence of strong TIO warming. The present study proposes a mechanism that promotes the excitation of the WP pattern through the upper-troposphere divergence in East Asia associated with the Walker circulation modulated by the tropical Indo-Pacific interbasin interaction. Significance Statement The East Asian winter temperature variability is controlled not only by the strong atmospheric internal variability in the midlatitudes and high latitudes but also by remote forcing from the tropical ocean. Our study investigates how El Niño exerts diverse impacts on the East Asian winter temperature, depending on where atmospheric convection intensifies in the tropical Pacific Ocean and the Indian Ocean. Our results show that an intense warming of the tropical Indian Ocean and the early development of El Niño are the major factors for warm winters in East Asia. Given that a precursor of the intense Indian Ocean warming appears in boreal autumn, our findings should contribute to the improvement of seasonal prediction for the East Asian winter climate.

Tree-Ring Inferred Drought Variations in the Source Region of the Yangtze, Yellow, and Mekong Rivers over the Past Five Centuries

The climate in the source region of the Yangtze River, Yellow River, and Mekong River is of great research interest because of its sensitivity to global change and its importance in regulating water resources to densely populated and vast areas downstream. A five-century long record of spring (May–June) for the Palmer Drought Severity Index (PDSI) was reconstructed for this region using tree-ring width chronologies of Qilian juniper (Juniperus przewalskii Kom.) from five high-elevation sites. The reconstruction explained 46% variance in the PDSI during the instrumental period 1955–2005. The reconstructed PDSI showed that the occurrence of dry extremes became frequent during the last century relative to the previous four centuries. The standard deviation of the reconstructed PDSI in the 100-year window showed that the recent century held apparent high values of standard deviation in the long-term context. Sustained droughts occurred in periods 1582–1631, 1737–1757, 1772–1791, 1869–1891, 1916–1939, and 1952–1982, whereas relatively wet intervals were observed in 1505–1527, 1543–1564, 1712–1736, 1792–1816, 1852–1868, 1892–1915, and 1983–2008. Notably, in the context of the past five centuries, the study region showed an increased inter-annual variability in the recent century, suggesting an intensified hydroclimatic activity possibly associated with global warming. Moreover, through diagnostic analysis of atmospheric circulation, we found that the negative phase East Asian–Pacific teleconnection pattern may be likely to trigger drought in the study region.

Interannual covariation of the thermal conditions of the southeast asian low-latitude highlands and summer East Asia–Pacific teleconnection pattern

Interannual covariation of the thermal conditions of the southeast asian low-latitude highlands and summer East Asia–Pacific teleconnection pattern

Weakened western Pacific teleconnection pattern caused a decrease in spring persistent rainfall in north of 26°N over Southeast China

AbstractThe spring persistent rainfall (SPR) is a unique synoptic and climatic phenomenon before the onset of the Asian summer monsoon over Southeast China (SEC). The warming climate has caused notable interdecadal changes of various climate systems, such as the Asian summer monsoon. However, it is still unclear whether there is a regime shift within the SPR band. To fill this knowledge gap, the boreal spring (March–May) rainfall data across SEC from the Climate Research Unit TS 4.05 are used herein to reveal its spatio‐temporal variations for 1951–2019 with the empirical orthogonal function (EOF) analysis. The EOF1 pattern exhibits a spatial monotonic variability of the SPR over SEC, which is jointly impacted by the El Niño‐Southern Oscillation and the Arctic Oscillation. The EOF2 pattern, a south–north dipole pattern, highlights a marked decrease (increase) of the SPR approximately to north (south) of the 26°N. We find that the decrease in the northern SPR band is closely modulated by western Pacific teleconnection pattern (WPP). Recent weakening WPP is coincident with a decline of the East Asian subtropical jet over this area, causing anomalous descending motions and thus accounts for persistent decline of the SPR over north of 26°N.

Significant Northward Jump of the Western Pacific Subtropical High: The Interannual Variability and Mechanisms

AbstractThe intensity and position of the western Pacific subtropical high (WPSH) have crucial effects on climate and disaster events in East Asia during summer. The WPSH significant northward jump (SNJ) events are the main manifestation of the seasonal evolution of WPSH, which are important for the precipitation over East Asia. Using the daily reanalysis data sets from year 1979 to 2020, this study further defines the early and late SNJ events of WPSH on the interannual timescale, which are connected separately with the tropical, midlatitude subseasonal signals and the local air–sea interaction. However, the mechanisms of the WPSH‐SNJ events are different in the anomalous early and late years. In the early SNJ years, the subseasonal signals from the midlevel East Asia–Pacific teleconnection pattern or the low‐level boreal summer intraseasonal oscillation cause the positive 500 hPa geopotential height anomalies, which contributes to the significant WPSH northward jump in the first pentad of July. However, the above factors are unable to cause the WPSH‐SNJ in the late years. Until the second pentad of August, the collaborative effects between midhigh latitudes wave trains over high levels and cold sea surface temperature (SST) anomalies in the core region lead to the barotropic geopotential height anomalies and the lagged northward jump of WPSH. It seems that the meridional position of WPSH has a complex interannual variability, which is modulated by the solar radiation, the atmospheric disturbances at different levels, and the SST anomalies at the same time.

The Relationship between the Atmospheric Heat Source over Tibetan Plateau and the Westerly-Monsoon Evolution in August and Its Physical Mechanism

In this study, the relationship between the East Asian subtropical westerly jet (EASWJ) and the East Asian summer monsoon (EASM) (westerly monsoon) and the correlation with the atmospheric heat source (AHS) on the Tibetan plateau (TP), especially the possible connection of the sudden enhancement of the correlation in August were analyzed. The results show that there is a significant correlation between the EASWJ and the EASM from June to October in terms of both intra-annual variability and interannual fluctuations, and the correlation between the AHS over TP and the EASWJ and the EASM during the same period is significantly enhanced in August. The synthetic analysis indicated that when the AHS was strong, a positive anomaly of a horizontal temperature gradient appeared over TP, which was conducive to the southward shift of the high-altitude temperature gradient center, resulting in the southward position of the axis of the 200 hPa westerly jet, and an upward and downward inclined westerly anomaly zone appeared from the south slope of TP to the main body and its north slope. Meanwhile, the East Asia–Pacific (EAP) teleconnection pattern with a negative phase appeared at 500 hPa, and TP to western Japan was located in the negative value area of the wave train. The AHS was conducive to the enhancement of the EAP negative phase, which was not conducive to the further northward transportation of water vapor by the EASM. On the contrary, when the AHS on TP was weak, the position of the westerly jet was northward and the EAP positive phase enhanced, contributing to the further northward transport of water vapor from the EASM.

Record-breaking rainfall over the middle reaches of the Yangtze River in August 2021: Sub-seasonal perspective and its predictability

An extremely heavy rainfall cluster was observed over the middle reaches of the Yangtze River (MYR) in August 2021, breaking the historical record since 1981 and causing severe floods. The controlling circulation regime and possible predictability sources of the un-expected rainfall are still unclear, especially from a sub-seasonal perspective. Our results show that the successive heavy rainfall events in August 2021 had significant intraseasonal oscillations for 10–24 d and 30–60 d, and they were synergistically influenced by tropical and extra-tropical circulation regime. Above all, the East Asia–Pacific teleconnection pattern (EAP) turned into a negative phase in late July and maintained throughout August, providing favorable background conditions for the northward transport of tropical water vapor and the southward intrusion of cold air from mid-high latitudes. The 30–60-d oscillation of precipitation was dominated by the first mode of boreal summer intraseasonal oscillation (BSISO1, with 30–60-d period), which strengthened the tropical moisture transport, while the quasi-biweekly oscillation of the western Pacific subtropical high and the anomalous mid-latitude cyclone of EAP intensified the precipitation on the scale of 10–24 d. In addition, the second mode of BSISO (BSISO2, with 10–30-d period) may also intensify the 10–24-d precipitation in late August. The forecast leading time of the S2S models for the persistent heavy rainfall in August 2021 was basically 1–2 weeks, while the EAP and BSISO1 were revealed as the main predictable sources of the abnormal rainfall events. These results highlight the importance of superposition of different intraseasonal oscillations in forming the extreme rainfall event and demonstrate a potential chance to enhance the prediction skill of extreme event if the ensemble members in S2S models could be reasonably selected according to their performances on key predictability sources.

Joint effect of East Asia–Pacific and Scandinavian teleconnections on summer precipitation in Southwest China

Based on the summer precipitation data from 328 meteorological stations in Southwest China and the monthly mean reanalysis data from the National Centers for Environmental Prediction/National Center for Atmospheric Research, the relationships of summer precipitation in Southwest China with the East Asia–Pacific (EAP) teleconnection pattern and the Scandinavian (SCA) teleconnection pattern are explored by using correlation analysis and composite analysis. The results show that the two teleconnections are significantly negatively correlated with the summer precipitation in Southwest China in the same period. The EAP teleconnection pattern and the SCA teleconnection pattern can affect the summer precipitation in Southwest China by affecting the atmospheric circulation situation in the middle and high latitudes, the north–south and east–west movement of the WPSH, and the water vapor transport in southwest China. In particular, the EAP and SCA teleconnections jointly affect the summer precipitation in Southwest China, which can be divided into four categories of configurations based on their phases, that is, (I) positive EAP + positive SCA, (II) negative EAP + negative SCA, (III) positive EAP + negative SCA, and (IV) negative EAP + positive SCA. The distribution of summer precipitation in Southwest China varies under different categories of configurations. Category I (II) corresponds to an opposite distribution pattern of precipitation anomalies between the north and south of Southwest China. In the configuration of category I(II), the cold air from high latitudes is weaker (stronger), the WPSH is more north(south) and more west(east) than the climatological mean, and the water vapor mainly comes from the Western Pacific (Bay of Bengal), thus resulting in category I(II), the opposite distribution of precipitation. While for configuration of category III(IV), no similar symmetrical distribution of precipitation anomalies can be found, and no obvious other distribution characteristics as well.

Skill assessment of Saudi-KAU and C3S models in prediction of spring season rainfall over the Arabian Peninsula

A skillful prediction of precipitation has great value, particularly for regions that suffer from water stress. In this study, we assess the potential predictability and skill of the Copernicus Climate Change Service (C3S) and Saudi-KAU models in their simulation of precipitation over the Arabian Peninsula during spring (March–May) for the period 1993–2016. For this purpose, data from individual models as well as the multi-model ensemble (MME) is used. The prediction data for MAM precipitation initialized at Feb (Lead 1), Jan (Lead 2), and Dec (Lead 3), were obtained from the 5 C3S and Saudi-KAU coupled global climate model. The potential predictability was computed by evaluating the signal to noise ratio and the theoretical limit of correlation skill, while the prediction skill was estimated from the temporal anomaly correlation co-efficient. The results show that the Saudi-KAU, CMCC, and UKMO models have slightly higher potential predictability of about 0.25, 0.35, and 0.25 respectively, as compared to other models. It is also observed that individual models as well as their MME show a high (low) potential predictability over southwestern (northern) regions of the Peninsula. Moreover, the Saudi-KAU, CMCC, and MME show a reasonably good correlation skill (0.68, 0.59, and 0.57) while the SEAS model displays lower skill (0.14) for spring precipitation. All model simulations reveal a decrease in prediction skill for longer lead times. On the other hand, the individual models and their MME successfully reproduced the Pacific (i.e. ENSO) teleconnection patterns while displaying lower skill over the tropical Atlantic Ocean. The results indicate that the model biases have negative impacts on potential predictability and prediction skill over the Arabian Peninsula during the spring season.

The strengthening relationship between summer rainfall over North China and PDO since the mid-2000s

The interdecadal change of the summer (July–August) rainfall over North China (SRNC) is of great concern to the climate research community. Based on the sliding correlation and interdecadal difference analysis, the interdecadal change of SRNC was studied. Results show a significantly wetter period of SRNC since the mid-2000s, which was associated with strengthening negative relationship between SRNC and PDO. PDO affected the SRNC through the jet stream in the upper-level and the East Asia–Pacific (EAP) teleconnection pattern. In the negative phase of the PDO since the mid-2000s, the contractive upper-level jet stream in the west of North China resulted in the abnormal updraft over the southern region of North China. Meanwhile, the positive phase of the EAP teleconnection pattern brought warm and wet southeast flow entering North China from its eastern boundary, both bringing more rainfall in North China. The abnormal water vapor transport from the western Pacific was very different from that accompanying the East Asian summer monsoon.

A New Norm for Seasonal Sea Ice Advance Predictability in the Chukchi Sea: Rising Influence of Ocean Heat Advection

Abstract Predictability of seasonal sea ice advance in the Chukchi Sea has been investigated in the context of ocean heat transport from the Bering Strait; however, the underlying physical processes have yet to be fully clarified. Using the Pan-Arctic Ice–Ocean Modeling and Assimilation System (PIOMAS) reanalysis product (1979–2016), we examined seasonal predictability of sea ice advance in early winter (November–December) and its source using canonical correlation analysis. It was found that 2-month leading (September–October) surface heat flux and ocean heat advection is the major predictor for interannual variability of sea ice advance. Surface heat flux is related to the atmospheric cooling process, which has influenced sea ice area in the southeastern Chukchi Sea particularly in the 1980s and 1990s. Anomalous surface heat flux is induced by strong northeasterly winds related to the east Pacific/North Pacific teleconnection pattern. Ocean heat advection, which is related to fluctuation of volume transport in the Bering Strait, leads to decrease in the sea ice area in the northwestern Chukchi Sea. Diagnostic analysis revealed that interannual variability of the Bering Strait volume transport is governed by arrested topographic waves (ATWs) forced by southeasterly wind stress along the shelf of the East Siberian Sea. The contribution of ocean heat flux to sea ice advance has increased since the 2000s; therefore, it is suggested that the major factor influencing interannual variability of sea ice advance in early winter has shifted from atmospheric cooling to ocean heat advection processes. Significance Statement Predictability of sea ice advance in the marginal Arctic seas in early winter is a crucial issue regarding future projections of the midlatitude winter climate and marine ecosystem. This study examined seasonal predictability of sea ice advance in the Chukchi Sea in early winter using a statistical technique and historical model simulation data. We identified that atmospheric cooling and ocean heat transport are the two main predictors of sea ice advance, and that the impact of the latter has become amplified since the 2000s. Our new finding suggests that the precise information on wind-driven ocean currents and temperatures is crucial for the skillful prediction of interannual variability of sea ice advance under present and future climatic regimes.

Changing Impacts of Tropical Cyclones on East and Southeast Asian Inland Regions in the Past and a Globally Warmed Future Climate

The impacts of the western North Pacific (WNP) tropical cyclone (TC) on East and Southeast Asian inland regions are analyzed. Here, based on a stringent TC selecting criterion, robust increase of TC-related inland impacts between 1979 and 2016 over East and Southeast Asian regions have been detected. The storms sustained for 2–9 h longer and penetrated 30–190 km further inland, as revealed from different best track datasets. The most significant increase of the TC inland impacts occurred over Hanoi and South China. The physical mechanism that affects TC-related inland impacts is shortly discussed. First, the increasing TC inland impacts just occur in the WNP region, but it is not a global effect. Second, besides the significant WNP warming effects on the enhanced TC landfall intensity and TC inland impacts, it is suggested that the weakening of the upper-level Asian Pacific teleconnection pattern since 1970s may also play an important role, which may reduce the climatic 200 hPa anti-cyclonic wind flows over the Asian region, weakening the wind shear near the Philippine Sea, and may eventually intensify the TC intensity when the TCs across the basin. Moreover, the TC inland impacts in the warming future are projected based on a high-resolution (20 km) global model according to the Representative Concentration Pathway 8.5 scenario. By the end of the 21st century, TC mean landfall intensity will increase by 2 m/s (6%). The stronger storms will sustain 4.9 h (56%) longer and penetrate 92.4 km (50%) farther inland, thereby almost doubling the destructive power delivered to Asian inland regions. More inland locations will therefore be exposed to severe storm–related hazards in the future due to warmer climate. Long-term planning to enhance disaster preparedness and resilience in these regions is called for.

Is there any improvement in simulation of wintertime Western Pacific teleconnection pattern and associated climate anomalies in CMIP6 comparing with CMIP5 models?

AbstractThe western Pacific pattern (WP) is one of the most important atmospheric teleconnections over the Northern Hemisphere (NH) in boreal winter, which plays key roles in regulating weather and climate variations over many parts of the NH. This study evaluates ability of the coupled models participated in CMIP5 and CMIP6 in capturing the spatial pattern, dominant frequency, and associated climate anomalies of the winter WP. Ensemble means of the CMIP5 and CMIP6 models well capture spatial structures of the WP, with slightly higher skills for the CMIP6. However, the northern (southern) centre of the WP is shifted westward (eastward) relative to the observations, and the strength of the northern centre is overestimated in most CMIP5 and CMIP6 models. CMIP6 shows an improvement in simulating the dominant periodicity of the WP. WP-related climatic anomalies in most parts of the NH can be well simulated. However, there exists a large spread across the models in simulating surface air temperature (SAT) anomalies in Russian Far East and Northwest North America, which is attributable to the diversity of the intensity of the WP’s northern lobe. Most CMIP5 and CMIP6 models largely overestimate the WP-related precipitation anomalies over Siberia, which is partly due to the overestimation of mean precipitation there. Furthermore, most models simulate a close relation of the WP and Arctic Oscillation (AO), which does not exist in observation. The CMIP5 and CMIP6 models with weak WP-AO relations have better ability than the models with strong WP-AO relations in capturing the WP-related SAT and precipitation anomalies over the NH, especially over Eurasia.

Connectivity of climatic variations
 in the North Atlantic and North Pacific

Based on the analysis of changes in the spatial structure of climatic variations in the Northern Hemisphere before and after the climatic regime shift in the 1980s, the modes of interaction between climatic processes in the North Atlantic and North Pacific have been identified. The first (“western”) mode prevailed until the late 1980s, reflected the impact of the North Pacific climatic processes on the North Atlantic climate as a result of interaction of two mutually independent Pacific teleconnection patterns (Pacific/North American and Tropical/Northern Hemisphere patterns) with the West Atlantic pattern. The pronounced eastward shift of the North Atlantic Oscillation (NAO) centers from the late 1970s resulted in establishment of the “eastern” mode of interaction between the aquatories under consideration. The climatic changes originated in the North Atlantic basin propagated in the western half of the North Pacific via the system of atmospheric teleconnection patterns over Eurasia (“atmospheric bridge”). The establishment of the “eastern” mode became obviously one of the reasons of sharp warming of surface waters in the western and central areas of the North Pacific from the end of the 1980s, which favored the beginning of a new “salmon epoch” in its northwestern part. Along with the synchronous relationships between the Eurasian atmospheric modes and North Pacific sea surface temperature anomalies, an asynchronous response in the ocean to longitudinal shifts in position of the NAO centers, was found. The atmospheric signal associated with its southern center propagated eastward along the equatorial zone and appeared in the southwestern sector of the North Pacific 5–6 years later.

Plausible causes of the interdecadal change of the North Pacific teleconnection pattern in boreal spring around the late 1990s

An interdecadal change of the North Pacific teleconnection pattern in the upper troposphere in boreal spring was noted around the late 1990s. During the period of 1979–1998, a meridional dipole pattern with alternative cyclone and anticyclone to the south and the north of 45° N over North Pacific, was identified as the leading mode by applying the empirical orthogonal function (EOF) technique onto the 300-hPa geopotential height anomaly after removing its zonal average. This pattern is similar to the upper-tropospheric structure of North Pacific Oscillation (NPO)/western Pacific (WP) pattern. During the period of 1999–2018, the meridional dipole is replaced by a northeast–southwest orientated teleconnection with centers of activity over the subtropical central Pacific and northeastern North Pacific, resembling a Pacific-North America (PNA) teleconnection. That is, the upper-level atmospheric teleconnection shifted from a NPO/WP-like pattern to a PNA-like pattern in 1998/1999. For the plausible causes of this interdecadal change, the effect of synoptic high-frequency (HF, 2–8 day) eddy and tropical convection over the central Pacific are emphasized. Before 1998/1999, the NPO/WP-like teleconnection could be ascribed to the vorticity forcing of the HF transient eddy activity over North Pacific between 40° and 60° N, with the role of tropical heating ignorable. After 1998/1999, the spatial distribution of HF transient eddy activity shifted significantly from a north–south pattern to a northeast–southwest pattern, sustaining a PNA-like teleconnection through the transportation of the eddy vorticity fluxes, while the vorticity forcing of the subseasonal low-frequency (LF, 10–90 day) transient eddy activity plays a role in the maintenance of the teleconnection lobe over the subtropical central Pacific. In addition, the tropical convection anomaly centers shifted towards the equatorial central Pacific and Maritime Continent, which can induce a poleward-propagating Rossby wave train, and amplify the PNA-like teleconnection pattern.

The Role of Tropical Mean-State Biases in Modeled Winter Northern Hemisphere El Niño Teleconnections

AbstractThe role of tropical mean-state biases in El Niño–Southern Oscillation teleconnections in the winter Northern Hemisphere is examined in coupled general circulation models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). The main North Pacific teleconnection pattern, defined here by the strengths of the anomalous Kuroshio anticyclone and North Pacific cyclone, is linked to two anomalous Rossby wave sources that occur during El Niño: a negative source over East Asia and a positive source to the west of the North Pacific. Errors in the teleconnection pattern in models are associated with spatial biases in mean atmospheric ascent and descent and the strength of the corresponding forcing of Rossby waves via suppressed or enhanced El Niño precipitation responses in the tropical western North Pacific (WNP) and the equatorial central Pacific (CP). The WNP El Niño precipitation response is most strongly linked to the strength of the Kuroshio anticyclone and the CP El Niño precipitation response is most strongly linked to the strength of the North Pacific cyclone. The mean state and corresponding El Niño precipitation response can have seemingly distinct biases. A bias in the WNP does not necessarily correspond to a bias in the CP, suggesting that improvement of biases in both tropical WNP and equatorial CP regions should be considered for an accurate teleconnection pattern.

West Pacific teleconnection pattern in dynamical seasonal predictions: how is it connected to the Atlantic atmospheric mean bias?

This study examines the West Pacific (WP) pattern, one of the primary modes of low-frequency variability during boreal winter, isolated in 11 global climate model seasonal hindcasts. WP mode separation are verified using three metrics: selected mode number, explained variance, and pattern correlation coefficient of the loading vector. When it comes to the pattern reproducibility, it turns out that the WP is tightly linked to the Atlantic jet and stationary wave. To diagnose details of mean biases in terms of WP reproducibility, atmospheric mean fields are composited for two groups: one group replicates well the key feature, a north–south dipole pattern, while the other manifests considerable displacements and magnitude disparities. The group with a lower pattern correspondence to the observation shows noticeable biases with the southeastward-shifted Atlantic dipole of the stationary wave and downstream-expanded Atlantic jet, which is attributed to the increase in meridional gradient of near surface temperature and resultant enhanced local baroclinicity. Magnified jet tail in the eastern North Atlantic (NA) can intensify the barotropic energy conversion thereby the wave energy actively passes toward both east and west. The excessive wave energy over the Arabian Peninsula and the central North Pacific can be favorable for the formation of a ridge, therefore it possibly leads to disorganized WP patterns at both ends of NA basin. A series of analysis suggests the importance of a realistic winter climatology simulation over NA for better WP representation through the interaction between different scales as well as different basins.

Midlatitude atmospheric transient eddy feedbacks influenced ENSO-associated wintertime Pacific teleconnection patterns in two PDO phases

The El Nino-Southern Oscillation (ENSO)-associated wintertime atmospheric teleconnection patterns in two Pacific decadal oscillation (PDO) phases are investigated using ERA-20C reanalysis data for 1950–2010. A strengthened ENSO-associated Pacific-North American (PNA) teleconnection pattern presents in PDO positive phase, while a West Pacific (WP) pattern over Northwestern Pacific and a squeezed PNA pattern coexist when ENSO occurs in PDO negative phase. The dynamical role of atmospheric transient eddy feedbacks to the teleconnection patterns are highlighted in the present study. When ENSO occurs in PDO positive phase, the uniform strengthened westerly jet anomalies downstream of the climatological main body of jet accompany with energetic transient eddy anomalies over Northeastern Pacific. The transient eddy feedbacks largely enhance and favor the strengthened PNA pattern. When ENSO occurs in PDO negative phase, the strengthened westerly jet anomalies appear to separate into two parts, one locating north of the climatological main body of jet and the other at the downstream. The accompanied transient eddy anomalies also split into two parts. Under such conditions, the transient eddy feedbacks are limited over Northeastern Pacific and favor a weak PNA pattern. However, the transient eddy anomalies over Northwestern Pacific strengthen, and the feedbacks also strengthen and largely contribute to the WP pattern. Moreover, the transient eddy anomalies over Northwestern Pacific seem to be anchored along the anomalously poleward strengthened oceanic subarctic frontal zone (SAFZ) in PDO negative phase. The enhanced atmospheric baroclinicity anomalies, coupled with the strengthened SAFZ, energize atmospheric transient eddy anomalies, and work as the potential maintenance in shaping the WP pattern.

The Role of Circulation and Its Changes in Present and Future Atmospheric Rivers over Western North America

AbstractPerformance in simulating atmospheric rivers (ARs) over western North America based on AR frequency and landfall latitude is evaluated for 10 models from phase 5 of the Coupled Model Intercomparison Project among which the CanESM2 model performs well. ARs are classified into southern, northern, and middle types using self-organizing maps in the ERA-Interim reanalysis and CanESM2. The southern type is associated with the development and eastward movement of anomalous lower pressure over the subtropical eastern Pacific, while the northern type is linked with the eastward movement of anomalous cyclonic circulation stimulated by warm sea surface temperatures over the subtropical western Pacific. The middle type is connected with the negative phase of North Pacific Oscillation–west Pacific teleconnection pattern. CanESM2 is further used to investigate projected AR changes at the end of the twenty-first century under the representative concentration pathway 8.5 scenario. AR definitions usually reference fixed integrated water vapor or integrated water vapor transport thresholds. AR changes under such definitions reflect both thermodynamic and dynamic influences. We therefore also use a modified AR definition that isolates change from dynamic influences only. The total AR frequency doubles compared to the historical period, with the middle AR type contributing the largest increases along the coasts of Vancouver Island and California. Atmospheric circulation (dynamic) changes decrease northern AR type frequency while increasing middle AR type frequency, indicating that future changes of circulation patterns modify the direct effect of warming on AR frequency, which would increase ARs (relative to fixed thresholds) almost everywhere along the North American coastline.