Pacific North American Research Articles

Global Atmospheric Oscillation: An Integrity of ENSO and Extratropical Teleconnections

Many researchers have recognized the long distance teleconnections between El Nino–Southern Oscillation (ENSO) events and some other processes in the global climate system happening in areas often very far from the equatorial zone of the Pacific Ocean. In this paper we investigate similar links between ENSO and extratropical processes taken in their whole set. For this goal we compute differences between the sea-level pressure and the near-surface air (sea surface) temperature global fields which correspond to either El Nino or La Nina events observed since the end of the XIX century and up to the beginning of the XXI century. As a result, we establish that an integrity exists of ENSO and the extratropical teleconnections such as North Atlantic Oscillation (NAO), Arctic Oscillation (AO), the Northern Hemisphere annular (NHA) mode as well as the Pacific–North American (PNA) pattern, and their analogs in the Southern Hemisphere in the interannual timescale. Named this integrity the Global Atmospheric Oscillation (GAO), we define two representative indices of GAO, and investigate the temporal dynamics of these indices. This investigation reveals that GAO’s extratropical components may be real irrespective of ENSO while the latter accompanies GAO in all cases. Moreover, in view of a general eastward propagation of the GAO as a spatial structure, some its extratropical components show changes of their features prior the El Nino (La Nina) begins to form. Using these features as fingerprints of the forthcoming evolution of ENSO, we define another GAO index for the El Nino prediction with the lead-time of about 1 year. At last, we establish that some of the CMIP5-models reproduce GAO reasonably well.

A Comparison of North American Surface Temperature and Temperature Extreme Anomalies in Association with Various Atmospheric Teleconnection Patterns

The atmospheric teleconnection pattern reflects large-scale variations in the atmospheric wave and jet stream, and has pronounced impacts on climate mean and extremes over various regions. This study compares those patterns that have significant circulation anomalies over the North Pacific–North American–North Atlantic sector, which directly influence surface temperature and temperature extremes over North America. We analyze the pattern associated anomalies of surface temperature and warm and cold extremes over North America, during the northern winter and summer seasons. In particular, we assess the robustness of the regional temperature and temperature extreme anomaly patterns by evaluating the field significance of these anomalies over North America, and quantify the percentages of North American temperature and temperature extreme variances explained by these patterns. The surface temperature anomalies in association with the Pacific–North American pattern (PNA), Tropical–Northern Hemisphere pattern (TNH), North Pacific pattern (NP), North Atlantic Oscillation (NAO), Arctic Oscillation (AO), Western Pacific pattern (WP), circumglobal teleconnection (CGT), and Asian–Bering–North American (ABNA) patterns are similar to those reported in previous studies based on various datasets, indicating the robustness of the results. During winter, the temperature anomaly patterns considered are field significant at the 5% level over North America, except the WP-related one. These pattern associated anomalies explained about 5–15% of the total interannual temperature variance over North America, with relatively high percentages for the ABNA and PNA patterns, and low for the WP pattern. The pattern associated warm and cold extreme anomalies resemble the corresponding surface mean temperature anomaly patterns, with differences mainly in magnitude of the anomalies. Most of the anomalous extreme patterns are field significant at the 5% level, except the WP-related patterns. These extreme anomalies explain about 5–20% of the total interannual variance over North America. During summer, the pattern-related circulation and surface temperature anomalies are weaker than those in winter. Nevertheless, all of the pattern associated temperature anomalies are of field significance at the 5% level over North America, except the PNA-related one, and explain about 5–10% of the interannual variance. In addition, the temperature extreme anomalies, in association with the circulation patterns, are comparable in summer and winter. Over North America, the NP-, WP-, ABNA-, and CGT-associated anomalies of warm extremes are field significant at the 5% level and explain about 5–15% of the interannual variance. Most of the pattern associated cold extreme anomalies are field significant at the 5% level, except the PNA and NAO related anomalies, and also explain about 5–15% of the interannual variance over North America.

Variability of extreme precipitation over Texas and its relation with climatic cycles

Many hydrometeorological studies have evaluated the impact of climate variability on hydrologic extremes. Recent studies have shown that the varying state of climatic cycles has intensified the regional hydrologic cycle within a wide range of geographical regions in the state of Texas. These climatic cycles define numerous sea surface temperature and pressure anomalies which lead to heavy precipitation in a region. The objective of this paper is to quantify the impact of five major Atlantic and Pacific Ocean related climatic cycles, including (i) Atlantic Multidecadal Oscillation (AMO), (ii) North Atlantic Oscillation (NAO), (iii) Pacific Decadal Oscillation (PDO), (iv) Pacific North American Pattern (PNA), and (v) Southern Oscillation Index (SOI), on maximum daily precipitation within a year in various climate regions of Texas, using a weighted correlation approach incorporating Leave-One-Out Test (LOOT). The uncertainty in the estimated correlation coefficient is factored in by determining the sample correlation coefficient at the 95% confidence interval. The influence of these global scale climatic cycles on the regional hydrologic cycle is found to be governed by the integrated hydrometeorological properties of weather stations, including (i) station elevation, (ii) average temperature, and (iii) average total precipitation, in the months of extremes. Results of this study will help regional water boards prepare for extreme hydrometeorological events in a changing climate.

Systematic decomposition of the MJO and its Northern Hemispheric extratropical response into Rossby and inertio‐gravity components

The Madden–Julian Oscillation (MJO) is the dominant form of intraseasonal variability in the Tropics. The MJO is a complex convectively coupled phenomenon, which is still poorly represented in the current generation of climate models, and our understanding of its essential dynamics and its influence on the midlatitude circulation is still incomplete. Here, we use a normal‐mode decomposition method to decompose the MJO systematically into Kelvin, inertio‐gravity (IG), and Rossby‐wave components in the ERA‐Interim reanalysis data for the period 1980–2015 to provide a climatology of the eight MJO phases for the Kelvin, IG, and Rossby‐wave components.Our analysis shows that the Rossby modes provide a larger contribution to the magnitude of the MJO in terms of geopotential height and winds than the Kelvin wave and IG modes. Moreover, the kinetic energy associated with the Rossby modes of the MJO accounts for about 93% of the kinetic energy. Our decomposition also shows that the Kelvin wave is the dominant mode in the unbalanced wave part, which is flanked by Rossby waves on both sides of the Equator, consistent with previous studies. The extratropical response to the MJO also consists of both IG and Rossby‐wave components in the Northern Hemisphere (NH). The midlatitude MJO response is also linked to well‐known teleconnection patterns like the North Atlantic Oscillation and the Pacific–North American pattern. The transient NH atmospheric response is fast, of the order of 5–7 days. While the extratropical response is dominated by Rossby waves, IG waves also show a prominent response in the NH.

Multi-trace-element sea surface temperature coral reconstruction for the southern Mozambique Channel reveals teleconnections with the tropical Atlantic

Abstract. Here we report seasonally resolved sea surface temperatures for the southern Mozambique Channel in the SW Indian Ocean based on multi-trace-element temperature proxy records preserved in two Porites sp. coral cores. Particularly, we assess the suitability of both separate and combined Sr∕Ca and Li∕Mg proxies for improved multielement SST reconstructions. Overall, geochemical records from Europa Island Porites sp. highlight the potential of Sr∕Ca and Li∕Mg ratios as high-resolution climate proxies but also show significant differences in their response at this Indian Ocean subtropical reef site. Our reconstruction from 1970 to 2013 using the Sr∕Ca SST proxy reveals a warming trend of 0.58±0.1 ∘C in close agreement with instrumental data (0.47±0.07 ∘C) over the last 42 years (1970–2013). In contrast, the Li∕Mg showed unrealistically large warming trends, most probably caused by uncertainties around different uptake mechanisms of the trace elements Li and Mg and uncertainties in their temperature calibration. In our study, Sr∕Ca is superior to Li∕Mg to quantify absolute SST and relative changes in SST. However, spatial correlations between the combined detrended Sr∕Ca and Li∕Mg proxies compared to instrumental SST at Europa revealed robust correlations with local climate variability in the Mozambique Channel and teleconnections to regions in the Indian Ocean and southeastern Pacific where surface wind variability appeared to dominate the underlying pattern of SST variability. The strongest correlation was found between our Europa SST reconstruction and instrumental SST records from the northern tropical Atlantic. Only a weak correlation was found with ENSO, with recent warm anomalies in the geochemical proxies coinciding with strong El Niño or La Niña. We identified the Pacific–North American (PNA) atmospheric pattern, which develops in the Pacific in response to ENSO, and the tropical North Atlantic SST as the most likely causes of the observed teleconnections with the Mozambique Channel SST at Europa.

Two Types of the Western Pacific Pattern, Their Climate Impacts, and the ENSO Modulations

The western Pacific (WP) pattern is a major teleconnection pattern that influences the wintertime Northern Hemisphere climate variations. Based on daily NCEP–NCAR reanalysis data, this study examines the climate impacts and the El Niño–Southern Oscillation (ENSO) modulation of two types of the WP pattern. The result shows that the WP patterns may arise from precursory disturbances over Asia and the North Pacific or from the Pacific–North American (PNA) pattern of the same polarity as or opposite polarity to that of the WP patterns. Among these WP patterns, the WP patterns that arise from the PNA pattern of the same polarity are most influential on North American near-surface and polar stratospheric air temperatures; furthermore, their frequency of occurrence, amplitude, and duration can be affected by ENSO phases: the positive WP patterns occur more frequently with larger amplitude and longer duration in El Niño than in La Niña; and the negative WP patterns occur less frequently with smaller amplitude and shorter duration in El Niño than in La Niña. The above findings suggest that the PNA pattern plays a crucial role in the climate impacts and the ENSO modulation of the WP patterns.

Spatiotemporal Changes in Precipitation Extremes over Canada and Their Teleconnections to Large-Scale Climate Patterns

AbstractIn the past few decades, there have been more extreme climate events occurring worldwide, including Canada, which has also suffered from many extreme precipitation events. In this paper, trend analysis, probability distribution functions, principal component analysis, and wavelet analysis were used to investigate the spatial and temporal patterns of extreme precipitation events of Canada. Ten extreme precipitation indices were calculated using long-term daily precipitation data (1950–2012) from 164 Canadian gauging stations. Several large-scale climate patterns such as El Niño–Southern Oscillation (ENSO), Pacific decadal oscillation (PDO), Pacific–North American (PNA), and North Atlantic Oscillation (NAO) were selected to analyze the relationships between extreme precipitation and climate indices. Convective available potential energy (CAPE), specific humidity, and surface temperature were employed to investigate potential causes of trends in extreme precipitation. The results reveal statistically significant positive trends for most extreme precipitation indices, which means that extreme precipitation of Canada has generally become more severe since the mid-twentieth century. The majority of indices display more increasing trends along the southern border of Canada while decreasing trends dominated the central Canadian Prairies. In addition, strong teleconnections are found between extreme precipitation and climate indices, but the effects of climate patterns differ from region to region. Furthermore, complex interactions of climate patterns with synoptic atmospheric circulations can also affect precipitation variability, and changes to the summer and winter extreme precipitation could be explained more by the thermodynamic impact and the combined thermodynamic and dynamic effects, respectively. The seasonal CAPE, specific humidity, and temperature are correlated to Canadian extreme precipitation, but the correlations are season dependent, which could be positive or negative.

A New Perspective toward Cataloging Northern Hemisphere Rossby Wave Breaking on the Dynamic Tropopause

Abstract Rossby wave breaking (RWB) events are a common feature on the dynamic tropopause and act to modulate synoptic-scale jet dynamics. These events are characterized on the dynamic tropopause by an irreversible overturning of isentropes and are coupled to troposphere-deep vertical motions and geopotential height anomalies. Prior climatologies have focused on the poleward streamer, the equatorward streamer, or the reversal in potential temperature gradient between the streamers, resulting in differences in the frequencies of RWB. Here, a new approach toward cataloging these events that captures both streamers is applied to the National Centers for Environmental Prediction Reanalysis-2 dataset for 1979–2011. Anticyclonic RWB (AWB) events are found to be nearly twice as frequent as cyclonic RWB (CWB) events. Seasonal decompositions of the annual mean find AWB to be most common in summer (40% occurrence), which is likely due to the Asian monsoon, while CWB is most frequent in winter (22.5%) and is likely due to the equatorward shift in mean baroclinicity. Trends in RWB from 1980 to 2010 illustrate a westward shift in North Pacific AWB during winter and summer (up to 0.4% yr−1), while CWB in the North Pacific increases in winter and spring (up to 0.2% yr−1). These changes are hypothesized to be associated with localized changes in the two-way interaction between the jet and RWB. The interannual variability of AWB and CWB is also explored, and a notable modality to the frequency of RWB is found that may be attributable to known low-frequency modes of variability including the Arctic Oscillation, the North Atlantic Oscillation, and the Pacific–North American pattern.

High-frequency climate oscillations drive ice-off variability for Northern Hemisphere lakes and rivers

This study examines the climatic drivers of ice-off dates for lakes and rivers across the Northern Hemisphere. Most lakes and rivers have trended toward earlier ice-off dates over the last century, as would be expected from long-term climate change. However, we also identify modes of climate variability that significantly impact the short-term behavior of ice-off time series. In particular, the North Atlantic Oscillation (NAO), Pacific-North American Pattern (PNA), and to a lesser degree the El Nino-Southern Oscillation (ENSO) explain a substantial fraction of the interannual variance in melt dates, while the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) generally do not. Furthermore, the spatial pattern of the early or late ice-off dates associated with the NAO, PNA, and ENSO matches a priori expectations due to the known surface temperature patterns associated with these oscillations. In all regions, the strongest correlation to ice-off is with one of the high-frequency modes—the NAO or PNA, suggesting that short-term weather variations play a stronger role than lower-frequency climate variability (ENSO, PDO, AMO) in driving ice-off.

100 Years of Progress in Understanding the Dynamics of Coupled Atmosphere–Ocean Variability

AbstractIn situ observation networks and reanalyses products of the state of the atmosphere and upper ocean show well-defined, large-scale patterns of coupled climate variability on time scales ranging from seasons to several decades. We summarize these phenomena and their physics, which have been revealed by analysis of observations, by experimentation with uncoupled and coupled atmosphere and ocean models with a hierarchy of complexity, and by theoretical developments. We start with a discussion of the seasonal cycle in the equatorial tropical Pacific and Atlantic Oceans, which are clearly affected by coupling between the atmosphere and the ocean. We then discuss the tropical phenomena that only exist because of the coupling between the atmosphere and the ocean: the Pacific and Atlantic meridional modes, the El Niño–Southern Oscillation (ENSO) in the Pacific, and a phenomenon analogous to ENSO in the Atlantic. For ENSO, we further discuss the sources of irregularity and asymmetry between warm and cold phases of ENSO, and the response of ENSO to forcing. Fundamental to variability on all time scales in the midlatitudes of the Northern Hemisphere are preferred patterns of uncoupled atmospheric variability that exist independent of any changes in the state of the ocean, land, or distribution of sea ice. These patterns include the North Atlantic Oscillation (NAO), the North Pacific Oscillation (NPO), and the Pacific–North American (PNA) pattern; they are most active in wintertime, with a temporal spectrum that is nearly white. Stochastic variability in the NPO, PNA, and NAO force the ocean on days to interannual times scales by way of turbulent heat exchange and Ekman transport, and on decadal and longer time scales by way of wind stress forcing. The PNA is partially responsible for the Pacific decadal oscillation; the NAO is responsible for an analogous phenomenon in the North Atlantic subpolar gyre. In models, stochastic forcing by the NAO also gives rise to variability in the strength of the Atlantic meridional overturning circulation (AMOC) that is partially responsible for multidecadal anomalies in the North Atlantic climate known as the Atlantic multidecadal oscillation (AMO); observations do not yet exist to adequately determine the physics of the AMO. We review the progress that has been made in the past 50 years in understanding each of these phenomena and the implications for short-term (seasonal-to-interannual) climate forecasts. We end with a brief discussion of advances of things that are on the horizon, under the rug, and over the rainbow.

The Consistency of MJO Teleconnection Patterns: An Explanation Using Linear Rossby Wave Theory

AbstractThe Madden–Julian oscillation (MJO) excites strong variations in extratropical atmospheric circulations that have important implications for subseasonal-to-seasonal (S2S) prediction. A previous study showed that particular MJO phases are characterized by a consistent modulation of geopotential heights in the North Pacific and adjacent regions across different MJO events, and demonstrated that this consistency is beneficial for extended numerical weather forecasts (i.e., lead times of two weeks to one month). In this study, we examine the physical mechanisms that lead some MJO phases to have more consistent teleconnections than others using a linear baroclinic model. The results show that MJO phases 2, 3, 6, and 7 consistently generate Pacific–North American (PNA)-like patterns on S2S time scales while other phases do not. A Rossby wave source analysis is applied and shows that a dipole-like pattern of Rossby wave source on each side of the subtropical jet can increase the pattern consistency of teleconnections due to the constructive interference of similar teleconnection signals. On the other hand, symmetric patterns of Rossby wave source can dramatically reduce the pattern consistency due to destructive interference. A dipole-like Rossby wave source pattern is present most frequently when tropical heating is found in the Indian Ocean or the Pacific warm pool, and a symmetric Rossby wave source is present most frequently when tropical heating is located over the Maritime Continent. Thus, the MJO phase-dependent pattern consistency of teleconnections is a special case of this mechanism.

Modification of the wintertime Pacific\u2013North American pattern related North American climate anomalies by the Asian\u2013Bering\u2013North American teleconnection

The Pacific–North American (PNA) pattern is closely related to North American climate. In this study, we demonstrate that the PNA associated anomalies of North American surface temperatures and temperature extremes are modified by the extratropical Asian–Bering–North American (ABNA) teleconnection. The temperature and temperature extreme anomalies in association with the combination of the PNA and ABNA patterns are most prominent over the Canadian Prairies, and can be approximated by a linear superposition of the circulation and temperature responses to the two patterns. The various temperature responses result from the corresponding atmospheric circulation anomalies, which are associated with sea surface temperature and precipitation anomalies in the tropical eastern-central Pacific and snow cover anomalies in the Siberian region. In particular, pronounced circulation anomalies are apparent over the Great Lakes/Hudson Bay, and around the Aleutian Islands with opposite sign for the in-phase PNA and ABNA combination. This induces anomalous temperature advection and precipitation, as well as the heating associated variations of vertical motion and clouds, leading to prominent surface temperature anomalies over the Canadian Prairies directly through the variation of the surface energy budget. For the out-of-phase PNA and ABNA combination, the anomalous circulation center tends to be situated along the west coast of North America. This induces weak circulation anomalies over north-central North America and brings weak thermal advection and precipitation anomalies there, leading to insignificant temperature anomalies over the Canadian Prairies. Both the ABNA and PNA can be skillfully predicted on the seasonal time scale, as well as their pattern related North American temperature anomalies. Hence it is important to consider both tropical and extratropical sources of predictability in improving North American climate prediction skill on seasonal to interannual time scales.

The influence of atmospheric circulation patterns on cold air outbreaks in the eastern United States

In this paper, we build upon previous literature in directly addressing the temporal relationship between the stratospheric and tropospheric polar vortex (PV), sea level pressure (SLP), and resultant cold air outbreaks (CAO). An atmospheric and teleconnection analysis was conducted on 49 predefined CAOs across the eastern United States from 1948 to 2016. Clusters of SLP, 100 and 10‐mb geopotential height anomalies were mapped utilizing self‐organizing maps (SOMs) to understand the surface, tropospheric PV, and stratospheric PV patterns preceding CAOs. The Arctic Oscillation (AO), North Atlantic Oscillation (NAO), and Pacific–North American (PNA) teleconnections were used as variables to explain the magnitude and location of mid‐latitude Arctic air displacement. Persistently negative SLP anomalies across the Arctic and North Atlantic were evident 1–2 weeks prior to the CAOs throughout the winter. The tropospheric and stratospheric PV were found to be persistently weak/weakening prior to mid‐winter CAOs and predominantly strong and off‐centred prior to early and late season CAOs. Negative phases of the AO and NAO were favoured prior to CAOs, while the PNA was found to be less applicable. This method of CAO and synoptic pattern characterization benefits from a continuous pattern representation and provides insight as to how specific teleconnections and atmospheric patterns lead to CAOs in the eastern United States.

An Intraseasonal Mode of Atmospheric Variability Relevant to the U.S. Hydroclimate in Boreal Summer: Dynamic Origin and East Asia Connection

Intraseasonal modes of atmospheric variability over the Northern Hemisphere (NH) midlatitudes in boreal summer are identified via an empirical orthogonal function (EOF) analysis of the daily 10–90-day bandpass-filtered 250-hPa streamfunction for the period of 1950–2016. The first two EOF modes are characterized, respectively, by (i) a single-signed streamfunction anomaly that extends across the NH and (ii) a regional dipole structure with centers over the Aleutian Islands and northeastern Pacific. The third EOF mode (EOF-3) is a quasi-stationary wave train over the Pacific–North American sector with an equivalent barotropic structure in the vertical. EOF-3 is associated with a northwest–southeast oriented anomalous precipitation dipole over the United States. A nonmodal instability analysis of the boreal summer climatological flow in terms of the 250-hPa streamfunction reveals that one of the top “optimal mode” disturbances mimicking the EOF-3 structure grows from an initial precursor disturbance over East Asia through extracting kinetic energy from background flow and attains its maximum amplitude in around nine days. An additional lag regression analysis illustrates that anomalous latent heating associated with cloud and precipitation formation over East Asia is responsible for generating the precursor disturbance for the EOF-3-like optimal mode. This result suggests the existence of an important connection between the hydrological cycles of East Asia and North America, which is dynamically intrinsic to the boreal summer upper-tropospheric flow. Knowledge of such a connection will help us better understand and model hydroclimate variability over these two continents.

Dynamic and thermodynamic changes conducive to the increased occurrence of extreme spring fire weather over western Canada under possible anthropogenic climate change

On May 2016 an extreme large wildfire affected Fort McMurray of Canada, leading to the largest wildfire evacuation and the costliest natural disaster in Canadian history. This wildfire was caused by extremely warm and dry weather conditions in spring. Here we investigate thermodynamic and dynamic (atmospheric circulation) conditions, and teleconnections conducive to extreme wildfire climate of western Canada since 1871. Results show that the extreme wildfire was very likely an outcome of anthropogenic effects that increase the occurrence of a persistent upper ridge associated with a warm and dry weather over western Canada. Changes in dynamic conditions decreased temperature and increased precipitation, while changes in thermodynamic conditions increased temperature and decreased precipitation. Thus the observed increase in temperature and decrease in precipitation on 26 April–15 May over western Canada were caused by changes in thermodynamic conditions. Although the Pacific North American (PNA) pattern was teleconnected with the occurrence of certain synoptic circulation patterns over western Canada, changes in the occurrence of the synoptic circulation pattern associated with the extreme wildfire cannot be explained by increased occurrences of the positive phase of PNA. The El Niño-Southern Oscillation and the Pacific Decadal Oscillation have not been found to have contributed to wildfire weather in western Canada. The spring warming and drying trends since 1871 over western Canada cannot be attributed to changes in common teleconnections.

Atmospheric rivers impacting Northern California and their modulation by a variable climate

Understanding the role of climate variability in modulating the behavior of land-falling atmospheric rivers (ARs) is important for seasonal and subseasonal predictability for water resource management and flood control. We examine daily activity of ARs targeting the Northern California coast over six decades using observations of synoptic-scale circulation, high-resolution precipitation, and a long-term AR detection catalog to quantify distinct types of land-falling ARs categorized by their circulation features. We demonstrate how dramatically different atmospheric states evolve into landfalling ARs along distinct pathways that are modulated by interannual (El Niño/Southern Oscillation (ENSO) and the Pacific Decadal Oscillation) and subseasonal (Arctic Oscillation, Pacific North American Pattern, Western Pacific Oscillation, and the Eastern Pacific Oscillation) modes of large-scale climate variability. Different configurations of climate variability modes are shown to favor ARs having different characteristics in terms of synoptic evolution, integrated vapor transport and landfall orientation resulting in different patterns of precipitation over the landscape. In particular, our results show that while ENSO plays an important role in modulating the synoptic evolution of ARs and their orientation at landfall, subseasonal regional climate modes, which also influence landfall orientation as well as the position of the storm track, appear to be more influential than ENSO in modulating precipitation variability in California. This could have implications for seasonal to subseasonal (S2S) forecasting. Finally, we examine AR activity over the most recent and highly anomalous winter 2016–2017 and show how the unprecedented wet conditions in Northern California were at least partly due to the persistence of ARs characterized by a southward storm track and southerly orientation, which represent the type of ARs associated with heavy rainfall in California, and which are associated with the negative phase of subseasonal regional teleconnection patterns.

Understanding Anomalous Synoptic Eddy Vorticity Forcing in Pacific–North American Teleconnection Pattern Events

Abstract Utilizing a decomposition of anomalous eddy vorticity forcing (EVF) proposed by Song in 2016 and a modified Geophysical Fluid Dynamics Laboratory (GFDL) dynamical core atmospheric model, this study provides a different understanding of physical mechanisms that are responsible for the formation of the anomalous synoptic EVF (SEVF) associated with Pacific–North American teleconnection pattern (PNA) events. A series of short-term control experiments (CEs) and initial-value modified experiments (IVMEs) is conducted. In each case of CEs, there are no obvious PNA-like circulation anomalies. IVMEs are exactly the same as CEs except that appropriate small perturbations are introduced into the initial-value fields of CEs. The modified initial-value fields led to a gradual development of the PNA-like flow anomalies in IVMEs. Based on these numerical results, deformations of the synoptic eddy due to the emergence of the PNA pattern can be easily acquired by subtracting the synoptic eddy in CEs from the synoptic eddy in IVMEs . The anomalous SEVF associated with the PNA events in the model can be decomposed into ensembles of two linear and interaction terms (EVF1 and EVF2) and a nonlinear self-interaction term (EVF3). It is demonstrated that the physical essence of the anomalous SEVF associated with the PNA events is a competition result between EVF1 plus EVF2 and EVF3. Results also indicate that the different signs of SEVF associated with the positive and negative PNA events are not necessarily related to the different tilts of the synoptic eddy.

The interannual variability of wind energy resources across China and its relationship to large‐scale circulation changes

This study investigates the interannual variability of wind energy resources across China and how it changes with season by applying empirical orthogonal function (EOF) analyses to gridded wind data from the Climate Forecast System Reanalysis (CFSR) from January 1979 through December 2011. The first EOF mode (EOF1) represents between 22% variance for winter and 29% for summer. Spatially, the variation is largely consistent across China for summer and autumn and almost opposite between north and south for spring and winter, and the strongest variation in all seasons is found over Inner Mongolia and Tibet. The second EOF mode (EOF2) represents between 13% variance for autumn and 16% for spring, and is largely dominated by a sharp contrast between Inner Mongolia and Tibet for all seasons. The EOF1 appears to be linked statistically to the pacific decadal oscillation for summer and autumn and to the Pacific North American pattern for spring and winter, while the EOF2 seems to be connected to the Arctic Oscillation for spring and winter and to an interdecadal variability for summer and autumn. The anomalous wind fields associated with these large‐scale circulation patterns modify the climatological wind fields in different ways that lead to an increase or a decrease of the 80‐m winds in different regions of China.

Multiscale characteristics of Jing-Jin-Ji’s seasonal precipitation and their teleconnection with large-scale climate indices

Using wavelet theory, Mann-Kendall non-parametric test, principal component analysis, and correlation analysis, the study investigated the change characteristics and periodic variation of seasonal precipitation and their relationships with seven large-scale global climate indices including Nino 3.4, Southern Oscillation Index (SOI), Pacific Decadal Oscillation (PDO), Pacific North American Index (PNA), North Atlantic Oscillation (NAO), Arctic Oscillation (AO), and Eastern Asia/Western Russia (EA/WR), for the period of 1959–2016 in Jing-Jin-Ji metropolitan region. The results showed the following: (1) The change characteristics of Jing-Jin-Ji’s precipitation varied for different seasons. Only JJA precipitation showed statistically significant decreasing trend at 5% significance level. Precipitation in other three seasons had increasing trends. The trend of annual precipitation was mainly dominated by JJA precipitation. The results of fractal characteristics of precipitation indicated that the future trend of JJA precipitation would likely increase, but the future trends of annual precipitation and precipitation in other three seasons would likely decrease. The inapparent periodic variation of seasonal precipitation was revealed using adjacent-average smoothing method. (2) Wavelet theory was used to investigate the periodic variation of seasonal precipitation and seven large-scale climate indices. In general, the oscillations varied for different seasons and periods. The real part of the Morlet wavelet amplitude exhibited similar cycles using wavelet power spectrum. However, it is possible to estimate the future trends of seasonal precipitation according to the nested periodic positive and negative cycles. The periods of seven large-scale climate indices were scattered. (3) The PC1 of seasonal precipitation and climate indices had isolated coherence, which varied for different climate indices and periods. The phases between them also varied for different periods. As for different climate indices, the EA/WR had strong positive impacts on the seasonal precipitation at relatively large period, such as 8–16 years. The effects of climate indices on the seasonal precipitation varied for different PCs and climate indices. However, the results of correlation analysis were consistent with those of wavelet coherence and phase difference. The corrections varied for different time lags between climate indices and seasonal precipitation. The results should be beneficial to reveal the historical and future trends of seasonal precipitation in Jing-Jin-Ji metropolitan region. The linear and nonlinear relationships between seasonal precipitation and climate indices indicated that it might be possible to predict the seasonal precipitation using the climate indices as potential predictors. However, the mechanisms of climate change impacts on the precipitation need to be further elucidated, which are recommended in our future study.

ENSO influence over the Pacific North American sector: uncertainty due to atmospheric internal variability

The relative influence of El Nino Southern Oscillation (ENSO) forced response versus internally generated atmospheric variability or noise on the upper tropospheric Pacific North American circulation is investigated. The analysis is performed on the boreal winter (December–January–February) 200 hPa circulation and the associated precipitation based on observational records and modeling experiments. The model experiment includes an ocean eddy-resolving coupled general circulation model (CGCM) and an atmospheric noise reduced ocean eddy-resolving CGCM. The noise reduction technique is the interactive ensemble approach, adopted to reduce the effects of internal atmospheric dynamics noise at the air-sea interface. Tropical rainfall anomalies associated with ENSO forces a teleconnection pattern that is a combination of the so-called Pacific North American (PNA) pattern, themed here as state-dependent atmospheric noise, and a pattern distinct from the PNA, themed here as ENSO-signal. The ENSO signal has a meridional structure in the streamfunction associated with significant poleward Rossby wave flux emanating from the eastern Pacific. Conversely, the PNA teleconnection pattern is zonally oriented, with most of the wave flux in the zonal direction from the Pacific towards North America. The mid-latitude ENSO forced response is asymmetric between warm and cold events. This asymmetry is strongly dependent on the amplitude of atmospheric noise. It is shown that the ENSO forced response is masked by atmospheric noise, with the latter being 3–10 times larger in amplitude. We show that the PNA pattern was positive during the 2015–2016 boreal winter and prevented the large 2015–2016 El Nino event from alleviating the persistent drought in the western US.