Eddies in the Northwest Subtropical Pacific and Their Possible Effects on the South China Sea

Based on an analysis of drifter data from the World Ocean Circulation Experiment during 1979-1998, the sizes of the eddies in the North subtropical Pacific are determined from the radii of curvature of the drifter paths calculated by using a non-linear curve fitting method. To support the drifter data results, Sea Surface Height from the TOPEX/POSEIDON and ERS2 satellite data are analyzed in connection with the drifter paths. It is found that the eddies in the North Pacific (18°-23°N and 125°-150°E) move westward at an average speed of approximately 0.098 ms-1 and their average radius is 176 km,with radii ranging from 98 km to 298 km. During the nineteen-year period, only 4 out of approximately 200 drifters (2%)actually entered the South China Sea from the area adjacent to the Luzon Strait (18°-22°N and 121°-125°E) in the winter. It is also found that eddies from the interior of the North Pacific are unlikely to enter the South China Sea through the Luzon Strait.

Different Configurations of Interannual Variability of the Western North Pacific Subtropical High and East Asian Westerly Jet in Summer

This study investigates the circulation and precipitation anomalies associated with different configurations of the western North Pacific subtropical high(WNPSH)and the East Asian westerly jet(EAJ)in summer on interannual timescales.The in-phase configuration of the WNPSH and EAJ is characterized by the westward(eastward)extension of the WNPSH and the southward(northward)shift of the EAJ,which is consistent with the general correspondence between their variations.The out-of-phase configuration includes the residual cases.We find that the in-phase configuration manifests itself as a typical meridional teleconnection.For instance,there is an anticyclonic(cyclonic)anomaly over the tropical western North Pacific and a cyclonic(anticyclonic)anomaly over the mid-latitudes of East Asia in the lower troposphere.These circulation anomalies are more conducive to rainfall anomalies over the Yangtze River basin and south Japan than are the individual WNPSH or EAJ.By contrast,for the out-of-phase configuration,the mid-latitude cyclonic(anticyclonic)anomaly is absent,and the lower-tropospheric circulation anomalies feature an anticyclonic(cyclonic)anomaly with a large meridional extension.Correspondingly,significant rainfall anomalies move northward to North China and the northern Korean Peninsula.Further results indicate that the out-of-phase configuration is associated with the developing phase of ENSO,with strong and significant sea surface temperature(SST)anomalies in the tropical central and eastern Pacific which occur simultaneously during summer and persist into the following winter.This is sharply different from the in-phase configuration,for which the tropical SSTs are not a necessity.

Impacts of Two Types of Northward Jumps of the East Asian Upper-tropospheric Jet Stream in Midsummer on Rainfall in Eastern China

The East Asian upper-tropospheric jet stream （EAJS） typically jumps north of 45~N in midsummer. These annual northward jumps are mostly classified into two dominant types： the first type corresponds to the enhanced westerly to the north of the EAJS＇s axis （type A）, while the second type is related to the weakened westerly within the EAJS＇s axis （type B）. In this study, the impacts of these two types of northward jumps on rainfall in eastern China are investigated. Our results show that rainfall significantly increases in northern Northeast China and decreases in the Yellow River-Huaihe River valleys, as well as in North China, during the type A jump. As a result of the type B jump, rainfall is enhanced in North China and suppressed in the Yangtze River valley. The changes in rainfall in eastern China during these two types of northward jumps are mainly caused by the northward shifts of the ascending air flow that is directly related to the EAJS. Concurrent with the type A （B） jump, the EAJS-related ascending branch moves from the Yangtze-Huai River valley to northern Northeast （North） China when the EAJS＇s axis jumps from 40~N to 55~N （50~N）. Meanwhile, the type A jump also strengthens the Northeast Asian low in the lower troposphere, leading to more moisture transport to northern Northeast China. The type B jump, however, induces a northwestward extension of the lower-tropospheric western North Pacific subtropical high and more moisture transport to North China.

Responses of the East Asian Jet Stream to the North Pacific Subtropical Front in Spring

This study concerns atmospheric responses to the North Pacific subtropical front （NPSTF） in boreal spring over the period 1982-2014. Statistical results show that a strong NPSTF in spring can significantly enhance the East Asian jet stream （EAJS）. Both transient eddy activity and the atmospheric heat source play important roles in this process. The enhanced atmospheric temperature gradient due to a strong NPSTF increases atmospheric baroclinicity, resulting in an intensification of transient eddy and convection activities. On the one hand, the enhanced transient eddy activities can excite an anomalous cyclonic circulation with a quasi-baraotropical structure in the troposphere to the north of the NPSTF. Accordingly, the related westerly wind anomalies around 30°N can intensify the component of the EAJS over the Northeast Pacific. On the other hand, an enhanced atmospheric heat source over the NPSTF, which is related to increased rainfall, acts to excite an anomalous cyclonic circulation system in the troposphere to the northwest of the NPSTF, which can explain the enhanced component of the EAJS over the Northwest Pacific. The two mechanisms may combine to enhance the EAJS.

Decadal Change in the Influence of the Western North Pacific Subtropical High on Summer Rainfall over the Yangtze River Basin in the Late 1970s

It is well known that on the interannual timescale,the westward extension of the western North Pacific subtropical high(WNPSH)results in enhanced rainfall over the Yangtze River basin(YRB)in summer,and vice versa.This study identifies that this correspondence experiences a decadal change in the late 1970s.That is,the WNPSH significantly affects YRB precipitation(YRBP)after the late 1970s(P2)but not before the late 1970s(P1).It is found that enhanced interannual variability of the WNPSH favors its effect on YRB rainfall in P2.On the other hand,after removing the strong WNPSH cases in P2 and making the WNPSH variability equivalent to that in P1,the WNPSH can still significantly affect YRB rainfall,suggesting that the WNPSH variability is not the only factor that affects the WNPSH-YRBP relationship.Further results indicate that the change in basic state of thermal conditions in the tropical WNP provides a favorable background for the enhanced WNPSH-YRBP relationship.In P2,the lower-tropospheric atmosphere in the tropical WNP gets warmer and wetter,and thus the meridional gradient of climatological equivalent potential temperature over the YRB is enhanced.As a result,the WNPSH-related circulation anomalies can more effectively induce YRB rainfall anomalies through affecting the meridional gradient of equivalent potential temperature over the YRB.

Subtropical Mode Water in the Northwestern Pacific

Based on the in situ XBT and other data sets, by analyzing the seasonal cycle of the mixed layer depth (MLD) and using the conservative potential vorticity (PV) as a tool, a clear description of the formation process of the North Pacific Subtropical Mode Water (NPSTMW) is presented for explaining the well known 'Stommel Demon'. The forming of NPSTMW reflects well the ventilation process of the isotherms of the permanent thermocline. The formation process can be divided into the 'ventilation' phase and the 'formation' phase. In the first phase (October-March), with large heat losses at the sea surface from October, the mixed layer deepens and correspondingly, the water mass with low PV emerges and sinks. After continual cooling from October to March, the mixed layer reaches its maximum value ( >300 m) in March. Then, in the second phase (April-June), the mixed layer shoals rapidly from April, a large part of the low PV water mass is sheltered from further air-sea interaction by the emerging seasonal thermocline, and thus forms new NPSTMW. Further analysis indicates that the formation region of warm NPSTMW (17-18℃) is limited between 140°-150°E, while the relatively cold NPSTMW (16-17℃) originates in a wider longitude range (140°-170°E).Climate features of NPSTMW are presented with the use of climatological Levitus (1994 a, b) dataset. It is shown that NPSTMW lies in the region of (130°-170°E, 22°-34°N) with core temperature ranging from about 16-19℃ and potential density around 25-25.8σθ NPSTMW has a three-dimensional structure lying below the seasonal thermocline (about 100 m deep) and reaches almost to 350m depths.

Simulation of the western North Pacific subtropical high in El Ni?o decaying summers by CMIP5 AGCMs

The performances of CMIP5 atmospheric general circulation models （AGCMs） in simulating the western North Pacific subtropical high （WNPSH） in El Nino decaying summers are examined in this study. Results show that most models can reproduce the spatial pattern of both climatological and anomalous circulation associated with the WNPSH in El Nino decaying summers. Most CMIP5 AGCMs can capture the westward shift of the WNPSH in El Nino decaying summers compared with the climatological location. With respect to the sub-seasonal variation of the WNPSH, the performances of these AGCMs in reproducing the northward jump of the WNPSH are better than simulating the eastward retreat of the WNPSH from July to August. Twenty-one out of twenty-two （20 out of 22） models can reasonably reproduce the northward jump of the WNPSH in El Nino decaying summers （climatology）, while only 7 out of 22 （8 out of 22） AGCMs can reasonably reproduce the eastward retreat of the WNPSH in El Nino decaying summers （climatology）. In addition, there is a close connection between the climatological WNPSH location bias and that in El Nino decaying summers.

Interdecadal Change in the Interannual Variation of the Western Edge of the Western North Pacific Subtropical High During Early Summer and the Influence of Tropical Sea Surface Temperature

This study reveals that the interannual variability of the western edge of the western North Pacific(WNP)subtropical high(WNPSH)in early summer experienced an interdecadal decrease around 1990.Correspondingly,the zonal movement of the WNPSH and the zonal extension of the high-pressure anomaly over the WNP(WNPHA)in abnormal years possess smaller ranges after 1990.The different influences of the tropical SSTAs are important for this interdecadal change,which exhibit slow El Nino decaying pattern before 1990 while rapid transformation from El Nino to La Nina after 1990.The early summer tropical SSTAs and the relevant atmospheric circulation anomalies present obvious interdecadal differences.Before 1990,the warm SSTAs over the northern Indian Ocean and southern South China Sea favor the WNPHA through eastward-propagating Kelvin wave and meridional-vertical circulation,respectively.Meanwhile,the warm SSTA over the tropical central Pacific induces anomalous ascent to its northwest through the Gill response,which could strengthen the anomalous descent over the WNP through meridional-vertical circulation and further favor the eastward extension of the WNPHA to central Pacific.After 1990,the warm SSTAs over the Maritime Continent and northern Indian Ocean cause the WNPHA through meridional-vertical and zonal-vertical circulation,respectively.Overall,the anomalous warm SSTs and ascent and the resultant anomalous descent over the WNP are located more westward and southward after 1990 than before 1990.Consequently,the WNPHA features narrower zonal range and less eastward extension after 1990,corresponding to the interdecadal decease in the interannual variability of the western edge of the WNPSH.On the other hand,the dominant oscillation period of ENSO experienced an interdecadal reduction around 1990,contributing to the change of the El Nino SSTA associated with the anomalous WNPSH from slow decaying type to rapid transformation type.

Contribution of Mesoscale Eddies to the Subduction and Transport of North Pacific Eastern Subtropical Mode Water

This study investigates the contribution of mesoscale eddies to the subduction and transport of North Pacific Eastern Subtropical Mode Water(ESTMW)using the high-frequency output of an eddy-resolved ocean model spanning the period 1994–2010.Results show that the subduction induced by mesoscale eddies accounts for about 31%of the total subduction of ESTMW formation.The volume of ESTMW trapped by anticyclonic eddies is slightly larger than that trapped by cyclonic eddies.The ESTMW trapped by all eddies in May reaches up to about 2.8×1013m3,which is approximately 16%of the total ESTMW volume.The eddy-trapped ESTMW moves primarily westward,with its meridional integration at 18°–30°N reaching about 0.17Sv,which is approximately 18%of the total zonal ESTMW transport in this direction,at 140°W.This study highlights the important role of eddies in carrying ESTMW westward over the northeastern Pacific Ocean.

ON THE DIVISION OF NORTH BOUNDARY OF SUBTROPICALZONE ACCORDING TO THE COMPOSITIONS AND PROPERTIES OF SOIL HUMUS

In this paper predecessors′achievements about the division between subtropical zone and warm temperate zone on the south slope of Funiu Mountain are firstly summarized, and the cause why these viewpoints about the division are different also has been presented. Seven soil profiles at different heights above sea level are dug along the south slope of Funiu Mountain. Many compositions and properties of soil humus have been analyzed in laboratory. A comprehensive study has been made about the division according to the compositions and properties of soil humus with mathematical method. During the analysis process eight indexes have been used, such as altitude, organic carbon, humic acid (HA), fulvic acid (FA), the ratio of humic acid and fulvic acid (HA/FA), two extinction coefficients (E4,E6), and their ratio (E4/E6).The result indicates that the boundary is at about 1000 meters above sea level.

Indices of strength and location for the North Pacific Subtropical and Subpolar Gyres

The adjustment of the North Pacific Subtropical and Subpolar Gyres towards changes in wind stress leads to different time-scale variabilities, which plays a significant role in climate changes. Based on the Sim- ple Ocean Data Assimilation （SODA） and Global Ocean Data Assimilation System （GODAS） datasets, the variations of the Subtropical and Subpolar Gyres are diagnosed using ＂three-dimension Ocean Circulation Diagnostic Method＂, and established three types of index series describe the strength, meridional and depth center of the Subtropical and Subpolar Gyres. The above indices present the seasonal, interannual and in- terdecadal variabilities of the Subtropical and Subpolar Gyres, which proves well. Both the Gyres are the strongest in winter, but the Subtropical Gyre is the weakest in summer and the Subpolar Gyre is the weakest in autumn. The Subtropical Gyre moves northward from February to March, southward in October, and to the southernmost in around January, while the Subpolar Gyre moves northward in spring, southward in summer, northward again in autumn and reaching the extreme point in winter to the south. The common feature of the interannual and interdecadal variabilities is that the two gyres were weaker and to the north before 1976-1977, while they were stronger and to the south after 1976-1977. The Subpolar Gyre has made a paramount contribution to the variability on interdecadal scales. As is indicated with the Subpolar Gyre strength indices, there was an important shift from weak to strong around 1976-1977, and the correlation coefficient with the North Pacific Decadal Oscillation （PDO） indices was 0.45, which was far better than that between the Subtropical Gyre strength indices and the PDO. Tests show that influenced by small and mesoscale eddies, the magnitude of large-scale gyres strength is strongly dependent on data resolution. But seasonal interannual and interdecadal large-scale variabilities of the two gyres presented with indices is less affected by model resolution.

Projected changes in the western North Pacific subtropical high under six global warming targets

The summer western North Pacific subtropical high(WNPSH) has large influences on the East Asian summer climate. Many studies have focused on the projected changes in the WNPSH, but little is known about the changes under different global warming targets, such as 1.5℃ and 2.0℃. This study investigates the changes in the WNPSH under six global warming targets(1.5℃, 2.0℃, 2.5℃,3.0℃, 3.5℃, and 4.0℃) in both the mid-and lower troposphere, using the outputs of CMIP5 model in historical simulations and under Representative Concentration Pathway 8.5. The projected changes in the WNPSH, which is measured by multiple variables, show that it changes little under the 1.5℃ target in the mid-troposphere, but weakens and retreats approximately 2.5° in longitude under the 2.0℃ target. It tends to linearly weaken with warming greater than 2.5℃ and shifts eastward by approximately 6.0° in longitude by the 4.0℃ target. Meanwhile, the WNPSH intensifies and extends westward under the 1.5℃ target in the lower troposphere, but changes little with warming rising from 1.5℃ to 2.0℃. It is projected to extend westward by approximately2.0° in longitude by the 4.0℃ target.

Interannual Meridional Displacement of the East Asian Upper-tropospheric Jet Stream in Summer

On the interannual timescale, the meridional displacement of the East Asian upper-tropospheric jet stream (EAJS) is significantly associated with the rainfall anomalies in East Asia in summer. In this study, using the data from the National Centers for Environmental Prediction-Department of Energy (NCEP/DOE) reanalysis-2 from 1979 to 2002, the authors investigate the interannual variations of the EAJS's meridional displacement in summer and their associations with the variations of the South Asian high (SAH) and the western North Pacific subtropical high (WNPSH), which are dominant circulation features in the upper and lower troposhere, respectively. The result from an EOF analysis shows that the meridional displacement is the most remarkable feature of the interannual variations of the EAJS in each month of summer and in summer as a whole. A composite analysis indicates that the summer (June-July-August, JJA) EAJS index, which is intended to depict the interannual meridional displacement of the EAJS, is not appropriate because the anomalies of the zonal wind at 200 hPa (U200) in July and August only, rather than in June, significantly contribute to the summer EAJS index. Thus, the index for each month in summer is defined according to the location of the EAJS core in each month. Composite analyses based on the monthly indexes show that corresponding to the monthly equatorward displacement of the EAJS, the South Asian high (SAH) extends southeastward clearly in July and August, and the western North Pacific subtropical high (WNPSH) withdraws southward in June and August.

Intercomparison of the Impacts of Four Summer Teleconnections over Eurasia on East Asian Rainfall

East Asian summer climate is strongly affected by extratropical circulation disturbances.In this study,impacts of four atmospheric teleconnections over Eurasia on East Asian summer rainfall are investigated using National Centers for Environmental Prediction/National Center for Atmospheric Research （NCEP/NCAR） reanalysis data and Climatic Research Unit （CRU） land precipitation data during 1979-2009.The four teleconnections include the Scandinavian （SCA）,the Polar/Eurasian （PEU）,the East Atlantic/Western Russian （EAWR）,and the circumglobal teleconnection （CGT）.Moreover,the related changes of lower-tropospheric circulation are explored,specifically,the low pressure over northern East Asia （NEAL） and the subtropical high over the western North Pacific （WNPSH）.The results presented are in the positive phase.The PEU and SCA induce significant negative anomalies in North China rainfall （NCR）,while the CGT induces significant positive anomalies.In the past three decades,the PEU and SCA explain more than 20％ of the variance in NCR,twice that explained by the CGT,suggesting a more important role of the former two teleconnections in NCR variation than the latter one.Meanwhile,the PEU and SCA reduce rainfall in Northeast China and South Korea,respectively,and the CGT enhances rainfall in Japan.The rainfall responses are attributed to the SCA-induced northward shift of the NEAL,and PEU-induced northward shift and weakening of the NEAL,respectively.For the CGT,the dipole pattern of rainfall anomalies between North China and Japan is affected by both westward extension of the NEAL and northwestward expansion of the WNPSH.In addition,the EAWR leads to an increase of sporadic rainfall in South China as a result of the eastward retreat of the WNPSH.

Atmospheric Rivers and Mei-yu Rainfall in China:A Case Study of Summer 2020

Atmospheric rivers(ARs)are long,narrow,and transient filaments of strong horizontal water vapor transport that can lead to extreme precipitation.To investigate the relationship between ARs and mei-yu rainfall in China,the mei-yu season of 2020 in the Yangtze-Huaihe River basin is taken as an example.An adjusted AR-detection algorithm is applied on integrated water vapor transport(IVT)of the ERA5 reanalysis.The JRA-55 reanalysis and the data from Integrated Multi-satellite Retrievals for GPM(IMERG)are also utilized to study the impacts of ARs on mei-yu rainfall in 2020.The results reveal that ARs in East Asia have an average length of 5400 km,a width of 600 km,a length/width ratio of 9.3,and a northeastward orientation of 30°.ARs are modulated by the western North Pacific subtropical high.The IVT core is located at the south side of low pressure systems,moving eastward with a speed of 10°d−1.For the cross sections of ARs in the Yangtze-Huaihe River basin,75%of the total flux is concentrated below 4 km with low-level jets near AR cores.Moreover,ARs occur mainly in the mei-yu period with a frequency of 20%–60%.The intensity of AR-related precipitation is 6–12 times that of AR-unrelated precipitation,and AR-related precipitation contributes about 50%–80%to total mei-yu precipitation.As shown in this case study of summer 2020,ARs are an essential part of the mei-yu system and have great impacts on mei-yu rainfall.Thus,ARs should receive more attention in research and weather forecast practices.

Revisiting the Second EOF Mode of Interannual Variation in Summer Rainfall over East China

The second EOF（EOF2） mode of interannual variation in summer rainfall over East China is characterized by inverse rainfall changes between South China（SC） and the Yellow River-Huaihe River valleys（YH）.However,understanding of the EOF2 mode is still limited.In this study,the authors identify that the EOF2 mode physically depicts the latitudinal variation of the climatological summer-mean rainy belt along the Yangtze River valley（YRRB）,based on a 160-station rainfall dataset in China for the period 1951-2011.The latitudinal variation of the YRRB is mostly attributed to two different rainfall patterns：one reflects the seesaw（SS） rainfall changes between the YH and SC（SS pattern）,and the other features rainfall anomalies concentrated in SC only（SC pattern）.Corresponding to a southward shift of the YRRB,the SS pattern,with above-normal rainfall in SC and below-normal rainfall in the YH,is related to a cyclonic anomaly centered over the SC-East China Sea region,with a northerly anomaly blowing from the YH to SC;while the SC pattern,with above-normal rainfall in SC,is related to an anticyclonic anomaly over the western North Pacific（WNP）,corresponding to an enhanced southwest monsoon over SC.The cyclonic anomaly,related to the SS pattern,is induced by a near-barotropic eastward propagating wave train along the Asian upper-tropospheric westerly jet,originating from the mid-high latitudes of the North Atlantic.The anticyclonic anomaly,for the SC pattern,is related to suppressed rainfall in the WNP.

Mechanism of Regional Subseasonal Precipitation in the Strongest and Weakest East Asian Summer Monsoon Subseasonal Variation Years

Using the National Center for Environment Prediction Climate Forecast System Reanalysis coupled dataset during 1979–2010,we selected four subseasonal indexes from the 16 East Asian Summer Monsoon(EASM)indexes to characterize the subseasonal variability of the entire EASM system.The strongest(1996)and weakest(1998)years of the subseasonal variation were revealed based on these subseasonal EASM indexes.Furthermore,three rainfall concentration areas were defined in East Asia,and these areas were dissected by the atmospheric midlatitude jet stream axis and the position of the Western North Pacific Subtropical High(WNPSH).Then,the subseasonal effects of the WNPSH,the South Asian High(SAH),the Mongolian Cyclone(MC),and the Boreal Summer Intraseasonal Oscillation(BSISO)on each rainfall concentration area were studied in the strongest and weakest subseasonal variation years of the EASM.During the summer of 1998,the WNPSH and the SAH were stable in the more southern region,which not only blocked the northward progression of the BSISO but also caused the MC to advance southward.Therefore,the summer of 1998 was the weakest subseasonal variability of the EASM,but with significant subseasonal precipitation episodes in the northern and central rainfall areas.However,in 1996,the BSISO repeatedly spread northward in the south rainfall area because of the weak intensities and northern positions of the WNPSH and the SAH,which caused significant subseasonal precipitation episodes.In addition,MC was blocked to the north of approximately 42°N with a weak subseasonal rainfall.

Large-scale Circulation Control of the Occurrence of Low-level Turbulence at Hong Kong International Airport

This study identifies the atmospheric circulation features that are favorable for the occurrence of low-level turbulence at Hong Kong International Airport [below 1600 feet （around 500 m）]. By using LIDAR data at the airport, turbulence and nonturbulence cases are selected. It is found that the occurrence of turbulence is significantly related to the strength of the southerly wind at 850 hPa over the South China coast. On the other hand, the east-west wihd at this height demonstrates a weak relation to the occurrence. This suggests that turbulence is generated by flow passing Lantau Island from the south. The southerly wind also transports moisture from the South China Sea to Hong Kong, reducing local stability. This is favorable for the development of strong turbulence. It is also noted that the strong southerly wind during the occurrence of low-level turbulence is contributed by an anomalous zonal gradient of geopotential in the lower troposphere over the South China Sea. This gradient is caused by the combination of variations at different timescales. These are the passage of synoptic extratropical cyclones and anticyclones and the intraseasonal variation in the western North Pacific subtropical high. The seasonal variation in geopotential east of the Tibetan Plateau leads to a seasonal change in meridional wind, by which the frequency of low-level turbulence is maximized in spring and minimized in autumn.

The Key Oceanic Regions Responsible for the Interannual Variability of the Western North Pacific Subtropical High and Associated Mechanisms

The western North Pacific subtropical high （WNPSH） is an important circulation system that impacts the East Asian summer climate. The interannual variability of the WNPSH is modulated by tropical air-sea interaction. In order to make it clear which oceanic regions are crucial to the interannual variability of the WNPSH, the research progresses in this regard in the past decade are reviewed. Based on the review, it is recognized that five oceanic regions are responsible for the interannual variability of the WNPSH in summer, including the equatorial central-eastern Pacific Ocean, tropical Indian Ocean, subtropical western North Pacific, the vicinity of the maritime continent, and the tropical Atlantic Ocean. The mechanisms how the sea surface temperature anomalies （SSTAs） in these regions affect the WNPSH are elaborated. The formation mechanisms for the SSTAs in these five regions are discussed. Strengths and weaknesses of the climate models in simulating and predicting the WNPSH are also documented. Finally, key scientific problems deserving further studies are proposed.

Atmospheric eddy anomalies associated with the wintertime North Pacific subtropical front strength and their influences on the seasonal-mean atmosphere

This study investigates transient eddy activity anomalies in the mid-latitude upper troposphere associated with intensity variability of the wintertime North Pacific subtropical front. Our results show that the meridional gradient of air temperature and baroclinic instability in the mid-latitude atmosphere become stronger as the subtropical front intensifies, and the mid-latitude westerly jet accelerates with barotropic structure. We further divide the mid-latitude atmospheric eddy activities into high-(2–7 days) and low-frequency(10–90 days) eddy activities according to their life periods. We find that, when the oceanic subtropical front intensifies, the high-frequency atmospheric eddy activity in the mid-latitudes strengthens while the low-frequency eddy activity weakens. The stronger high-frequency eddy activity tends to moderate the air temperature gradient and baroclinicity in the mid-latitudes. High-frequency eddy anomalies accelerate the westerly jet on the northern side and downstream of the westerly jet, and enhance the jet with equivalent barotropic structure. In contrast, the weaker low-frequency eddy activity has a negative contribution to zonal wind speed tendency and attenuates the zonal homogenization of the jet. The anomalous thermodynamic forcing of the low-frequency eddy activity helps maintain the meridional gradient of air temperature in the mid-troposphere.