Pacific Storm Track Activity Research Articles

Strengthening Relationship between the AO and the Occurrence Frequency of Arctic Daily Warming since the 1980s

Abstract Recently, the occurrence of Arctic winter daily warming events has attracted substantial attention from the media and scholars alike. Herein, these events are investigated further, particularly in respect of their relation with the Arctic Oscillation (AO). We define a Pacific pattern–Arctic rapid tropospheric daily warming (Pacific-RTDW) event, when the Arctic winter daily warming is triggered by a warm humid air mass that is transported by storms from the North Pacific into the Arctic. An anomalous northwest–southeast dipole pattern at 500-hPa geopotential is identified spanning from the Arctic to the North Pacific and associated with AO. The dipole pattern could favor the occurrence of Pacific-RTDW events because it induces a strong southerly in the middle troposphere, which mid-high-latitude storms may follow into the Arctic. In particular, the correlation between AO and the occurrence frequency of Pacific-RTDW events has strengthened since the early 1980s. This shift is attributed to the intensified Pacific storm-track (PST) activity intensity, which is possibly related to the phase transition of the Pacific decadal oscillation (PDO) and has induced stronger synoptic-scale eddy feedbacks to low-frequency flows since this period. Accordingly, stronger feedbacks of the synoptic-scale eddies could provide a favorable condition for the formation of such a dipole pattern associated with the AO, bolstering the connection between AO and Pacific-RTDW events. In contrast, the eddy feedback to the flow is not strong enough and is, therefore, unable to provide favorable conditions before this period due to the comparatively weak PST intensity. Significance Statement The Arctic winter daily warming event may lead to more severe cold air outbreaks invading the Northern Hemisphere midlatitudes. We explored the climatic conditions favorable for the occurrence of Arctic winter daily warming events, thereby revealing that the correlated atmospheric circulation features a northwest–southeast dipole pattern. Additionally, since the early 1980s, the intensified Pacific storm-track activity is believed to have contributed to strengthening of the correlation between the Arctic Oscillation and the occurrence frequency of Arctic winter daily warming events. The findings here are expected to assist in prediction of future Arctic winter daily warming events.

Distinct Roles of Cyclones and Anticyclones in Setting the Midwinter Minimum of the North Pacific Eddy Activity: A Lagrangian Perspective

Abstract The North Pacific storm-track activity is suppressed substantially under the excessively strong westerlies to form a distinct minimum in midwinter, which seems inconsistent with linear baroclinic instability theory. This “midwinter minimum” of the storm-track activity has been intensively investigated for decades as a test case for storm-track dynamics. However, the mechanisms controlling it are yet to be fully unveiled and are still under debate. Here we investigate the detailed seasonal evolution of the climatological density of surface migratory anticyclones over the North Pacific, in comparison with its counterpart for cyclones, based on a Lagrangian tracking algorithm. We demonstrate that the frequency of surface cyclones over the North Pacific maximizes in midwinter, whereas that of anticyclones exhibits a distinct midwinter minimum under the upstream influence, especially from the Japan Sea region. In midwinter, it is only on such a rare occasion that prominent weakening of the East Asian winter monsoon allows a migratory surface anticyclone to form over the Japan Sea, despite the unfavorable climatological-mean conditions due to persistent monsoonal cold-air outbreaks and the excessively strong upper-tropospheric westerlies. The midwinter minimum of the North Pacific anticyclone density suggests that anticyclones are likely the key to understanding the midwinter minimum of the North Pacific storm-track activity as measured by Eulerian eddy statistics.

The Impact of Tropical Pacific SST Biases on the S2S Forecast Skill over North America in the UFS Global Coupled Model

Abstract The impact of tropical Pacific sea surface temperature (SST) biases on the deterministic skill of the Unified Forecast System (UFS) coupled model Prototype 5 is evaluated during weeks 1–4 of the forecast. The evaluation is limited to the contiguous United States (CONUS) and two seasons: boreal summer (June–September) and winter (December–March). The tropical SST in the UFS model is warmer than in observations and bias patterns show seasonal dependence especially in the central and western Pacific. During boreal summer, the bias is located north of the equator whereas in winter, the bias is located in the Southern Hemisphere. A regression analysis indicates a significant relationship between these SST biases and the biases in the surface temperature and precipitation over the CONUS along with midtroposphere large-scale circulation and North Pacific storm-track activity. The SST biases affect the biases in other fields from week 1 of the forecast and the impact becomes stronger as the lead time increases to week 4. The impact of SST biases on the biases in other fields show a qualitative relationship to the patterns of forecast errors of the fields.

Response of the North Pacific Storm Track Activity in the Cold Season to Multi-scale Oceanic Variations of Kuroshio Extension System: A Statistical Assessment

Response of the North Pacific Storm Track Activity in the Cold Season to Multi-scale Oceanic Variations of Kuroshio Extension System: A Statistical Assessment

Energetics of Transient Eddies Related to the Midwinter Minimum of the North Pacific Storm-Track Activity

Abstract Storm-track activity over the North Pacific (NP) climatologically exhibits a clear minimum in midwinter, when the westerly jet speed sharply maximizes. This counterintuitive phenomenon, referred to as the “midwinter minimum (MWM),” has been investigated from various perspectives, but the mechanisms are still to be unrevealed. Toward better understanding of this phenomenon, the present study delineates the detailed seasonal evolution of climatological-mean Eulerian statistics and energetics of migratory eddies along the NP storm track over 60 years. As a comprehensive investigation of the mechanisms for the MWM, this study has revealed that the net eddy conversion/generation rate normalized by the eddy total energy, which is independent of eddy amplitude, is indeed reduced in midwinter. The reduction from early winter occurs mainly due to the decreased effectiveness of the baroclinic energy conversion through seasonally weakened temperature fluctuations and the resultant poleward eddy heat flux. The reduced net normalized conversion/generation rate in midwinter is also found to arise in part from the seasonally enhanced kinetic energy conversion from eddies into the strongly diffluent Pacific jet around its exit. The seasonality of the net energy influx also contributes especially to the spring recovery of the net normalized conversion/generation rate. The midwinter reduction in the normalized rates of both the net energy conversion/generation and baroclinic energy conversion was more pronounced in the period before the late 1980s, during which the MWM of the storm-track activity was climatologically more prominent.

Rapid Increase of Explosive Cyclone Activity over the Midwinter North Pacific in the Late 1980s

Abstract Long-term changes in the activity of explosively developing “bomb” cyclones over the wintertime North Pacific are investigated by using a particular version of a global atmospheric reanalysis dataset into which only conventional observations have been assimilated. Bomb cyclones in January are found to increase rapidly around 1987 in the midlatitude central North Pacific. Some of the increased bomb cyclones formed over the East China Sea and then moved along the southern coast of Japan before developing explosively in the central North Pacific. The enhanced cyclone activity is found to be concomitant with rapid warming and moistening over the subtropical western Pacific and the South and East China Seas under the weakened monsoonal northerlies, leading to the enhancement of the lower-tropospheric Eady growth rate and equivalent potential temperature gradient, setting a condition favorable for cyclone formation in the area upstream of the North Pacific storm track. Along the storm track, poleward moisture transport in the warm sector of a cyclone and associated precipitation along the warm and cold fronts tended to increase and thereby enhance its explosive development. After the transition around 1987, a bomb cyclone has become more likely to develop without a strong upper-level cyclonic vortex propagating from Eurasia than in the earlier period. The increased bomb cyclone activity in January is found to contribute to the diminished midwinter minimum of the North Pacific storm track activity after the mid-1980s.

Change in the variability in the Western Pacific pattern during boreal winter: roles of tropical Pacific sea surface temperature anomalies and North Pacific storm track activity

Using multiple reanalysis datasets, this study reveals that the variability in the western Pacific pattern (WP) in boreal winter has shown notable changes during recent decades. The variability in the winter WP exhibited a marked weakening trend before the early 2000s and increased slightly thereafter. Two epochs with the highest and lowest WP variabilities are selected for a comparative analysis. Winter WP-related meridional dipole atmospheric anomalies over the North Pacific were stronger and had a broader range during the high-variability epoch than during the low-variability epoch. Correspondingly, the winter WP had larger impacts on surface temperature variations over Eurasia and North America during the high-variability epoch than that during the low-variability epoch. We find that the shift in the winter WP variability is closely related to changes in the connection between the winter WP and the El Niño-Southern Oscillation (ENSO) and to changes in the amplitude of the North Pacific storm track. Specifically, ENSO had a closer connection with the WP during the high-variability epoch, when the amplitude of the North Pacific storm track was also stronger. During the high-variability epoch, the extratropical atmospheric anomalies generated by the tropical ENSO shifted westward and projected more on the WP-related atmospheric anomalies, thus contributing to an increase in WP variability. In addition, the larger amplitude of the North Pacific storm track that occurred during the high-variability epoch led to the stronger feedback of synoptic-scale eddies to the mean flow and contributed to stronger WP variability. Further analysis indicates that the change in the connection of ENSO with the WP may be partly related to the zonal shift of the sea surface temperature anomaly in the tropical Pacific associated with ENSO.

Northern Hemisphere winter storm track trends since 1959 derived from multiple reanalysis datasets

In this study, a comprehensive comparison of Northern Hemisphere winter storm track trend since 1959 derived from multiple reanalysis datasets and rawinsonde observations has been conducted. In addition, trends in terms of variance and cyclone track statistics have been compared. Previous studies, based largely on the National Center for Environmental Prediction–National Center for Atmospheric Research Reanalysis (NNR), have suggested that both the Pacific and Atlantic storm tracks have significantly intensified between the 1950s and 1990s. Comparison with trends derived from rawinsonde observations suggest that the trends derived from NNR are significantly biased high, while those from the European Center for Medium Range Weather Forecasts 40-year Reanalysis and the Japanese 55-year Reanalysis are much less biased but still too high. Those from the two twentieth century reanalysis datasets are most consistent with observations but may exhibit slight biases of opposite signs. Between 1959 and 2010, Pacific storm track activity has likely increased by 10 % or more, while Atlantic storm track activity has likely increased by <10 %. Our analysis suggests that trends in Pacific and Atlantic basin wide storm track activity prior to the 1950s derived from the two twentieth century reanalysis datasets are unlikely to be reliable due to changes in density of surface observations. Nevertheless, these datasets may provide useful information on interannual variability, especially over the Atlantic.

Role of the Tibetan Plateau on the Annual Variation of Mean Atmospheric Circulation and Storm-Track Activity*

Abstract This study reexamines how the Tibetan Plateau (TP) modulates the annual variation of atmospheric circulation and storm-track activity based on the Meteorological Research Institute's atmosphere–ocean coupled model experiments with a progressive TP uplift from 0% to 100% of the present height. Three major roles of the TP on atmospheric circulation and storm-track activity are identified. First, consistent with a previous finding, the TP tends to intensify the upper-level jet and enhance baroclinicity in the North Pacific Ocean but significantly weaken storm-track activity over the TP, East Asia, and the western North Pacific during the cold season. Second, the TP amplifies stationary waves that are closely linked to transient eddies. In particular, the TP enhances the Siberian high and the Aleutian low, which together contribute to the strengthening of the East Asian winter monsoon circulation and the weakening of storm-track activity. Third, the TP significantly modulates the subseasonal variability of the Pacific storm-track (PST) activity. In particular, the TP tends to suppress PST activity during midwinter despite the fact that it strengthens baroclinicity along the Pacific jet. The midwinter suppression of PST activity, which is well reproduced in a control run with a realistic TP, gradually disappears as the TP height decreases. Major factors for the midwinter suppression of the PST associated with the TP include the 1) destructive effect of an excessively strong jet leading to an inefficiency of barotropic energy conversion, 2) reduction of baroclinicity over the northern part of the TP, and 3) subseasonally varying SST change and resulting moist static energy.

Warming of Western North Pacific Ocean and Energetics of Transient Eddy Activity

Abstract This study examines the increase in the sea surface temperature (SST) in the western North Pacific Ocean (WNPO) during December–February for the period 1959–2008. The relationship of this SST increase with significant interdecadal changes in the baroclinicity and the energetics of transient eddy activity is also examined. These results show that the interannual variations of the WNPO SST and atmospheric conditions including baroclinicity and turbulent heat flux are responses to the upstream atmospheric forcings associated with the East Asian winter monsoon (EAWM). For the interdecadal variations, the intensified baroclinicity downstream of the Pacific storm-track activity is responsible for an increase in the baroclinic energy conversion (BCEC) from the mean available potential energy (MAPE) to the eddy available potential energy (EAPE) to the east of 180°. This in turn increases the BCEC from the EAPE to the eddy kinetic energy (EKE) over this region. The BCEC and generation of EAPE by diabatic heating are enhanced to the east of 180° as a result of the intensified baroclinicity. This could be responsible for the development of transient eddy activity downstream of the Pacific storm track over the North Pacific.

Is Pacific Storm-Track Activity Correlated with the Strength of Upstream Wave Seeding?

Abstract In this paper, the relationship between upstream seeding over north Asia and downstream storm-track activity over the North Pacific in midwinter and spring/fall has been analyzed using 45 years of variance and feature-tracking statistics. It is shown that for each season, interannual variations in upstream seeding and downstream storm-track activity are largely uncorrelated. Moreover, during midwinter months in which the upstream seeding from north Asia is about as strong as that during a typical spring/fall month, the downstream storm track in central Pacific is still significantly weaker during midwinter than that during spring/fall. In addition, during cool seasons in which the midwinter suppression is more pronounced in the upstream seeding region, the suppression is not significantly enhanced in the downstream Pacific storm track. A recent study suggested that reduced upstream seeding from north Asia is the main “source” of the midwinter suppression of the Pacific storm track. Results presented in this study suggest that it is unlikely that the weakness in upstream seeding is the primary cause of the midwinter suppression.

Comments on “The Source of the Midwinter Suppression in Storminess over the North Pacific”

In a recent paper, Penny et al. employed feature tracking to investigate why there is a relative minimum in storminess during winter within the Pacific storm track. They concluded that reduced upstream seeding, especially seeding from northern Asia, is the main “source” of the midwinter suppression of the Pacific storm track. Results presented here show that during midwinter months when upstream seeding is as strong as that in spring/fall, the Pacific storm track is not significantly stronger than average and is still much weaker than that in spring/fall, suggesting that the strength of upstream seeding cannot be the primary cause of the midwinter suppression of Pacific storm-track activity.

Interdecadal changes in the storm track activity over the North Pacific and North Atlantic

Analysis of NCEP-NCAR I reanalysis data of 1948–2009 and ECMWF ERA-40 reanalysis data of 1958–2001 reveals several significant interdecadal changes in the storm track activity and mean flow-transient eddy interaction in the extratropics of Northern Hemisphere. First, the most remarkable transition in the North Pacific storm track (PST) and the North Atlantic storm track (AST) activities during the boreal cold season (from November to March) occurred around early-to-mid 1970s with the characteristics of global intensification that has been noticed in previous studies. Second, the PST activity in midwinter underwent decadal change from a weak regime in the early 1980s to a strong regime in the late 1980s. Third, during recent decade, the PST intensity has been enhanced in early spring whereas the AST intensity has been weakened in midwinter. Finally, interdecadal change has been also noted in the relationship between the PST and AST activities and between the storm track activity and climate indices. The variability of storm track activity is well correlated with the Pacific Decadal Oscillation and North Atlantic Oscillation prior to the early 1980s, but this relationship has disappeared afterward and a significant linkage between the PST and AST activity has also been decoupled. For a better understanding of the mid-1970s’ shift in storm track activity and mean flow-transient eddy interaction, further investigation is made by analyzing local barotropic and baroclinic energetics. The intensification of global storm track activity after the mid-1970s is mainly associated with the enhancement of mean meridional temperature gradient resulting in favorable condition for baroclinic eddy growth. Consistent with the change in storm track activity, the baroclinic energy conversion is significantly increased in the North Pacific and North Atlantic. The intensification of the PST and AST activity, in turn, helps to reinforce the changes in the middle-to-upper tropospheric circulation but acts to interfere with the changes in the low-tropospheric temperature field.

A comparison of climatological subseasonal variations in the wintertime storm track activity between the North Pacific and Atlantic: local energetics and moisture effect

Distinct differences of the storm track–jet relationship over the North Pacific and North Atlantic are investigated in terms of barotropic and baroclinic energetics using NCEP-2 reanalysis data for the period of 1979–2008. From fall to midwinter the Pacific storm track (PST) activity weakens following the southward shift of the Pacific jet, whereas the Atlantic storm track (AST) activity remains steady in position and intensifies regardless of the slight southward shift of the Atlantic jet. This study is devoted to seeking for the factors that can contribute to this conspicuous difference between the two storm tracks on climatological subseasonal variation by analyzing eddy properties and local energetics. Different eddy properties over the two oceans lead to different contribution of barotropic energy conversion to the initiation of storm tracks. In the North Atlantic, meridionally elongated eddies gain kinetic energy efficiently from stretching deformation of the mean flow in the jet entrance. On the other hand, the term associated with shearing deformation is important for the initiation of PST. Analysis of baroclinic energetics reveals that the intensification of the AST activity in midwinter is mainly attributed to coincidence between location of maximum poleward and upward eddy heat fluxes and that of the largest meridional temperature gradient over slight upstream of the AST. The relatively large amount of precipitable water and meridional eddy moisture flux along baroclinic energy conversion axis likely provides a more favorable environment for baroclinic eddy growth over the North Atlantic than over the North Pacific. In the meantime, the midwinter minimum of the PST activity is attributable to the southward shift of the Pacific jet stream that leads to discrepancy between core region of poleward and upward heat fluxes and that of meridional thermal gradient. Weakening of eddy-mean flow interaction due to eddy shape and reduction of moist effect are also responsible for the weakening of storm track activities in midwinter when the strongest baroclinicity exists over the North Pacific.

Assessing the Increasing Trend in Northern Hemisphere Winter Storm Track Activity Using Surface Ship Observations and a Statistical Storm Track Model

Abstract Recent studies, based largely on analyses of reanalysis datasets, suggest that the Northern Hemisphere winter storm track activity has increased significantly during the second half of the twentieth century. In this study, this increasing trend, in terms of filtered mean sea level pressure (MSLP) variance statistics, is assessed using surface ship observations and a statistical storm track model. MSLP observations made by ships, archived as part of the reanalysis project conducted by the National Centers for Environmental Prediction–National Center for Atmospheric Research, have been analyzed. Observational errors are estimated by comparing reports of nearly collocated observations. Consistent with previous studies, the observational errors of ship pressure observations are found to be very large during the late 1960s and early 1970s. Without correcting for observational errors, the storm track activity over the Pacific, computed based on ship observations, is found to be decreasing with time, while the upward trend in the Atlantic is much smaller than that found in the reanalysis data. Even after corrections have been made to account for secular changes in observational error statistics, the ship-based trend in the Pacific is still found to be much smaller than that found in the reanalysis, while over the Atlantic, the corrected ship-based trend is consistent with that found in the reanalysis. The robustness of the results is tested by application of data trimming based on the reanalysis products. Ship observations that are different from the reanalysis by more than a prescribed limit are removed before the statistics are computed. As the prescribed limit is reduced from 30 to 2.5 hPa, the ship-based storm track activity becomes increasingly consistent with that based on the reanalysis. However, even when the smallest limit is used, the trends computed from the ship observations are still smaller than those computed from the reanalysis, strongly suggesting that the trends in the reanalysis are biased high. Nevertheless, the results suggest that decadal-scale variability of the Atlantic storm track activity is not very sensitive to the trimming limit, while results for the Pacific storm track are not as robust. As an independent corroboration of the ship observation results, a statistical model is used to test whether the storm track trend found in the reanalysis is dynamically consistent with observed mean flow change. Five hundred winters of GCM simulations are used to construct a linear model based on canonical correlation analysis (CCA), using monthly mean distribution of MSLP anomalies as a predictor to hindcast monthly mean MSLP variance. The Atlantic storm track in the CCA model hindcast is well correlated with the storm track in the reanalysis on both interannual and decadal time scales, with the hindcast trend being 82% of that found in the reanalysis. Over the Pacific, the CCA hindcast does not perform as well, and the hindcast trend is only 32% of that found in the reanalysis. The results of this study suggest that the actual trend in Pacific storm track activity is probably only about 20%–60% of that found in the reanalysis, while over the Atlantic, the actual trend is likely to be about 70%–80% of that found in the reanalysis. Two new basinwide storm track indices, which should contain less bias in the secular trends, have been defined based mainly on ship observations.

Using Mean Flow Change as a Proxy to Infer Interdecadal Storm Track Variability

Abstract Several recent studies, based mainly on analyses of reanalysis data, have suggested that the Northern Hemisphere storm tracks have intensified during the second half of the twentieth century. However, comparisons with rawinsonde observations over land areas suggest that eddy variance/covariance statistics may contain spurious jumps that had led to an exaggeration of the storm track secular trend. In this study, storm track variations are inferred from mean flow anomalies using canonical correlation analysis (CCA). A CCA model relating storm track anomalies to mean flow anomalies is derived using recent, more reliable data. The model is then applied to infer storm track anamalies using mean flow anomalies for earlier periods, when storm track analyses are deemed more suspect (or even nonexistent). Results of the CCA analyses suggest that the strong secular trend observed over the Atlantic basin and Europe is consistent with mean flow anomalies, while CCA predictions based on mean flow changes suggest only a much weaker trend in the Pacific storm track activity than that present in the reanalysis data. Over the regions where comparisons with rawinsonde data can be made, the interdecadal trends inferred by the CCA model are quite consistent with rawinsonde data over the Pacific storm track entrance and exit regions, as well as the Atlantic storm track exit regions, but are inconsistent with rawinsonde observations and reanalysis data over northeastern North America, where CCA predictions are generally poor. A CCA hindcast based on Trenberth and Paolino mean sea level pressure data as a predictor shows no indication that the secular increase in storm track intensity extends further back prior to 1960, suggesting that during the entire twentieth century, storm track activity was weakest during the 1960s.

Midwinter Suppression of the Pacific Storm Track Activity as Seen in Aircraft Observations

Abstract Unassimilated observational data, from the years 1979 to 1993, are analyzed and compared to the NCEP–NCAR reanalysis data to confirm the magnitude of the seasonal cycle in Pacific storm track activity. Such a comparison is necessary since recent publications have indicated that the reanalysis data may contain biases and spurious jumps that may affect climate signals, and eddy variance/covariance statistics are particularly sensitive to model biases, as well as changes in data coverage and quality. High-pass-filtered variance in the 250-hPa meridional wind component is taken to be the indicator of upper-tropospheric storm track intensity. Rawinsonde observations located mainly over the land masses, and aircraft observations over the oceans, both display seasonal cycles very similar to that seen in the reanalysis data. Both the midwinter suppression in Pacific storm track activity, as well as the more recent finding that the Pacific storm track is significantly stronger during the late 1980s and ea...

Interannual and Decadal Modulations Recently Observed in the Pacific Storm Track Activity and East Asian Winter Monsoon

Abstract Interannual variability of the North Pacific storm track observed over 17 recent winters is documented. The local storm track activity is measured by a meridional flux of sensible heat associated with the lower-tropospheric subweekly fluctuations. The interannual variability in the heat flux over the northwestern (NW) Pacific is found to be strongest in midwinter. The first empirical orthogonal function of the interannual variability in midwinter captures the decadal tendency toward the enhanced storm track activity in midwinter over the NW Pacific, in association with the decadal weakening of the east Asian winter monsoon (Siberian high) and the Aleutian low that occurred in the late 1980s. The most marked signature of this enhancement is that the midwinter minimum in the storm track activity, which had been apparent in the early to mid-1980s, almost disappeared afterward. As opposed to linear theory of baroclinic instability, the enhanced activity occurred despite the weakening of the Pacific j...