East Asian Subtropical Jet Stream Research Articles

Enhanced Intensity of Interannual Variability of May Drought in Northeast China After 2000 and Its Connection With the Barents Sea Ice

AbstractUsing the standardized precipitation evapotranspiration index, this article demonstrates that the intensity of interannual variability (IIV) of May drought in Northeast China has enhanced remarkably after entering into the 21st century. Relative to 1980–1999, the percentage increase of IIV reaches 70% during 2000–2016. Such an enhancement is well supported by the intensification of IIVs in the East Asian polar front jet stream (EAPJ), East Asian subtropical jet stream (EASJ), and lower‐level anticyclonic circulation anomaly over Northeast China. Remarkable impact from the Barents Sea ice in April is also highlighted. With respect to the former period, a substantial increase occurs as well in the IIV of April Barents Sea ice during 2000–2016. Due to larger IIV in the latter period, the decline of April Barents Sea ice can exert evident effects on heat fluxes, consequently resulting in a significant increase of surface air temperature (SAT) in situ in May. The increased SAT further induces a wave train propagating southeastward to the East Asian region from the Barents Sea, which leads to a strengthened EAPJ and a weakened EASJ in the upper level, concomitant with an anomalous anticyclonic circulation covering Northeast China in the lower level. These atmospheric circulation anomalies link to the drought variability over Northeast China. Thus, the increase in the IIV of April Barents Sea ice favors the enhanced IIV of May drought in Northeast China.

Increasing peak intensity of tropical cyclones passing through the Korean Peninsula

An increasing trend along with an abrupt increase in the peak intensity of tropical cyclones (TCs) passing through the Korean Peninsula (KP) are significantly detected from 1981 to 2020 and since 2003, respectively. Here we present observational evidence that such trend and shift are largely attributed with the increased passages of intense TCs in the KP during the mature boreal autumn (i.e., September–October, SO) and linked with the recent shift of the Pacific Decadal Oscillation (PDO) to its negative phase. A negative PDO during SO is related to environmental changes that are favorable for more intense TC passages in the KP including a weakened East Asian subtropical jet stream, weaker vertical wind shear, warmer subtropical sea surface temperature, and stronger low-level relative vorticity. Such findings are expected to provide new insights on understanding regional TC variability and ultimately, contribute to long range TC prediction initiatives in the KP region.

Increasing activity of tropical cyclones in East Asia during the mature boreal autumn linked to long-term climate variability

In recent years, the East Asian (EA) region experienced escalated cost of damages associated with tropical cyclones (TC) during the mature boreal autumn (i.e., September−October). Questions arise whether such increased TC-associated cost of damages are indicative of increasing activity of TCs in EA, particularly during the mature boreal autumn. Here we show evidence of significantly increasing activity of TCs in EA from 1981 to 2019 that is mainly accompanied by an abrupt regime shift in TC passage frequency since 1998. Such trend and shift are robustly linked to the shift of the Pacific Decadal Oscillation (PDO) to its negative phase in the mid-1990s. Characterized by warm sea surface temperature anomalies in the subtropical North Pacific, a negative PDO phase is related to the weakening of the East Asian subtropical jetstream and the westward displacement of the WNP subtropical high, which initiates a favorable steering environment for increased TC passages into the EA region. Contrasting environmental patterns are associated in a positive PDO phase. Considering the prominence of EA in the global economy, our paper contributes additional insights on long-term tropical cyclone risk reduction and management in the region.

Frequency Associations between East Asian Jet Streams and the Temperature over the Barents–Kara Sea Region/Arctic Oscillation in Winter

The frequency associations between jet streams over East Asia and the Arctic key temperature at 2 m (AKT2m) in the Barents–Kara Sea region (40°–75° E, 66°–82° N) and the Arctic Oscillation in winter are investigated using continuous wavelet transform, cross-wavelet transform, and wavelet coherence. The cross-wavelet transforms between the AKT2m/Arctic Oscillation and the East Asian polar front jet stream (EAPJ) suggest that the EAPJ is closely related to the AKT2m and Arctic Oscillation on an interannual (3–5-year band) timescale, but the variation in the phase angle denotes a complex frequency connection between the EAPJ and Arctic Oscillation. The squared wavelet coherence suggests that weakening of the EAPJ is associated with the rise in AKT2m during the period of abrupt climate change in East Asia. The EAPJ contains more forced components from the Arctic than the East Asian subtropical jet stream. By comparison, the relationship between AKT2m and the EAPJ is closer than that between the Arctic Oscillation and EAPJ, especially during the period of abrupt climate change in East Asia. This suggests that the EAPJ serves as a bridge for Arctic warming to affect the weather and climate over East Asia in winter. By contrast, the Arctic Oscillation does not play an important part, although it also contains information about the Arctic.

The East Asian Subtropical Jet Stream and Atlantic Tropical Cyclones

AbstractAtlantic tropical cyclones (TCs) can cause significant societal and economic impacts, as 2019's Dorian serves to remind us of these storms' destructiveness. Decades of effort to understand and predict Atlantic TC activity have improved seasonal forecast skill, but large uncertainties still remain, in part due to an incomplete understanding of the drivers of TC variability. Here we identify an association between the East Asian Subtropical Jet Stream (EASJ) during July–October and the frequency of Atlantic TCs (wind speed ≥34 knot) and hurricanes (wind speed ≥64 knot) during August–November based on observations for 1980–2018. This strong association is tied to the impacts of EASJ on a stationary Rossby wave train emanating from East Asia and the tropical Pacific to the North Atlantic, leading to changes in vertical wind shear in the Atlantic Main Development Region (80–20°W, 10–20°N).

Variable correspondence between western North Pacific tropical cyclone frequency and East Asian subtropical jet stream during boreal summer: A tropical Pacific sea surface temperature perspective

Fluctuations in the summer relationship between western North Pacific tropical cyclone (TC) frequency (TCF) and East Asian subtropical jet stream (EAJS) are investigated using observations and composite analysis. Here TCF is divided into four categories to assess how different tropical Pacific sea surface temperature (SST) anomalies modulate TCF‐EAJS associations. Results show that high TCF would weaken (strengthen) the south (north) part of EAJS, but with different locally poleward shifts during La Niña‐ and El Niño‐like events. In contrast, during low TCF, a clearly southward shifted, strengthened EAJS occurs only when the western Pacific SST cools, but no significant change is observed when the western Pacific SST warms up. Further analysis shows that the possible reason why TCF has diverse linkages to EAJS is that the TC genesis locations and TC tracks are quite different under different tropical Pacific SST anomalies, even for quasi‐constant TCF. Potential mechanisms responsible for the changing TCF‐EAJS associations can be physically explained by the exceptional configurations of tropospheric meridional temperature gradient anomaly and the deformed‐and‐displaced Pacific–Japan‐like wavetrain, which is mainly due to non‐negligible differences in large‐scale oceanic and atmospheric environments that modulate the TC genesis locations and TC tracks. This study highlights the importance of a tropical SST perspective when analysing TCF‐EAJS associations.

High‐frequency daily temperature variability in China and its relationship to large‐scale circulation

ABSTRACTTwo measures of intra‐seasonal variability, indicated respectively by standard deviations (SD) and day‐to‐day (DTD) fluctuations denoted by absolute differences between adjacent 2‐day periods, as well as their relationships with large‐scale circulation patterns were investigated in China during 1962–2008 on the basis of homogenized daily temperature records from 549 local stations and reanalysis data. Our results show that both the SD and DTD of daily minimum temperatures (Tmin) in summer as well as the minimum and maximum temperatures in winter have been decreasing, while the daily maximum temperature (Tmax) variability in summer is fluctuating more, especially over southern China. In summer, an attribution analysis indicates that the intensity of the Western Pacific Subtropical High (WPSH) and high‐level East Asian Subtropical Jet stream (EASJ) are positively correlated with both SD and DTD, but the correlation coefficients are generally greater with the SD than with the DTD of the daily maximum temperature, Tmax. In contrast, the location of the EASJ shows the opposite correlation pattern, with intensity regarding the correlation with both SD and DTD. In winter, the Arctic Oscillation (AO) is negatively correlated with both the SD and DTD of the daily minimum temperature, but its intra‐seasonal variability exhibits good agreement with the SD of the Tmin. The Siberian High acts differently with respect to the SD and DTD of the Tmin, demonstrating a regionally consistent positive correlation with the SD. Overall, the large‐scale circulation can well explain the intra‐seasonal SD, but DTD fluctuations may be more local and impacted by local conditions, such as changes in the temperature itself, the land surface, and so on.

Atmospheric responses to idealized urban land surface forcing in eastern China during the boreal spring

AbstractIn contrast to the impacts of anthropogenic aerosols and greenhouse gases, little is known about the impact of urban land surface forcing (ULSF) on large‐scale atmospheric circulation. This study explores atmospheric responses to idealized ULSF in eastern China during the boreal spring using the Community Atmosphere Model version 5.1 coupled with the Community Land Model version 4. Results show that the ULSF leads to an increased air temperature in northern China both near the surface and in the lower troposphere. Related to a strong thermal feedback loop, a middle‐upper tropospheric cooling is found in eastern China while a relatively strong warming occurs in the middle‐high latitudes, which acts to enhance the meridional temperature gradient to the north of the source region and then shifts the East Asian subtropical jet stream (EASJ) southward. A weakened southwesterly in the lower troposphere in southern China slows down moisture transportation to northern China, and the southward shifted EASJ induces strong anomalous sinking motion to the north of the Yangtze River Valley (YRV). The associated changes in moisture and vertical airflow result in moisture divergence along the YRV and convergence in southern China. Thus, the spring rain belt is shifted southward, as characterized by below‐normal rainfall extending from the Huai River Valley to South Korea and above‐normal rainfall from southern China to the south coast of Japan. In addition, analysis of the upper tropospheric wave activity signifies that large‐scale atmospheric responses due to the ULSF also exert an important influence on local climate.

Meridional position biases of East Asian subtropical jet stream in CMIP5 models and their relationship with ocean model resolutions

ABSTRACTThe East Asian subtropical jet (EASJ) stream in each of the 20 models from the Coupled Model Intercomparison Project Phase 5 (CMIP5) is evaluated against the ERA‐interim product. The multi‐model ensemble mean (MME) simulates a reasonably realistic climatology of the EASJ; however, the MME EASJs are shifted southward in all seasons in comparison with the observations. Upon closer inspection, it is found that only in the coupled general circulation models (CGCMs) with relatively coarse ocean model resolutions are the simulated EASJs shifted southward remarkably in all four seasons; for those CGCMs with relatively high ocean model resolutions, the simulated EASJs are shifted northward, indicating that the simulated meridional location of the EASJ is closely related to the horizontal resolution in the ocean model. Based on the singular value decomposition (SVD) and composite analyses, the simulated meridional location of the EASJ is closely associated with the sea surface temperature (SST) over the Kuroshio–Oyashio Extension (KOE) region, with southward (northward) shift of the EASJ corresponding to low (high) SSTs. The CGCMs with relatively low (high) ocean model resolutions generally simulate negative (positive) SST biases over the KOE region with an SST decrease (increase) of about −3 K (+2 K) in comparison with the MME. Through a mixed‐layer heat budget analysis, it is revealed that the SST differences between the models with coarse and fine ocean components are mainly attributed to the ocean heat transport effect.

Climatological study of the total ozone field around the subtropical jet stream

AbstractA study of the average distribution of total ozone was accomplished for the period from October 1982 to September 1983 in the area 20–50°N, 90–160°E, i.e. the entrance region of the East Asian subtropical jet stream. Total ozone data were obtained from the Total Ozone Mapping Spectrometer on the Nimbus‐7 satellite.Normally, the subtropical jet stream (STJ) marks a steep gradient in total ozone. From the monthly mean study of total ozone and wind fields at the 200 hPa surface, it was found that in winter the total ozone contours are almost parallel to the wind directions; the horizontal gradient in total ozone increases as the wind speed strengthens. In winter and spring, a sharp gradient in total ozone is found on the cyclonic side of the STJ because of the strong Hadley circulation. As the Hadley cell ceases in summer, the jet weakens and horizontal gradients in total ozone become much smaller, disappearing almost entirely by early autumn.