Sea Surface Temperature Tripole Research Articles

The Impact of Interactive Ocean Dynamics on Atlantic Sea Surface Temperature Variability

Abstract The impact of interactive ocean dynamics on internal variations of Atlantic sea surface temperature (SST) is investigated by comparing preindustrial control simulations of a fully coupled atmosphere–ocean–ice model to the same atmosphere–ice model with the ocean replaced by a motionless slab layer (henceforth slab ocean model). Differences in SST variability between the two models are diagnosed by an optimization technique that finds components whose variance differs as much as possible. This technique reveals that Atlantic SST variability differs significantly between the two models. The two components with the most extreme enhancement of SST variance in the slab ocean model resemble the tripole SST pattern associated with the North Atlantic Oscillation (NAO) and the Atlantic multidecadal variability (AMV) pattern. This result supports previous claims that ocean dynamics are not necessary for the AMV, although ocean dynamics lead to slight increases in the memory of both the AMV and the NAO tripole. The component with the most extreme enhancement of SST variance in the fully coupled model resembles the Atlantic Niño pattern, confirming the ability of our technique to isolate physical modes known to require ocean dynamics. The second component with more variance in the fully coupled model is a mode of subpolar SST variability. Both the reemergence of SST anomalies and changes in ocean heat transport lead to increased SST variance and memory in the subpolar Atlantic. Despite large differences in the mean and variability of SST, atmospheric variability is quite similar between the two models, confirming that most atmospheric variability is generated by internal atmospheric dynamics.

Joint influence of the North Atlantic sea surface temperature and the Barents sea-ice concentration on the dipole pattern of Eurasian surface air temperature in March

In recent decades, an increase in the frequency of extreme events in early spring across Eurasia has resulted in significant impacts on crop yields and socio-economic development. However, the variations of March surface air temperature (SAT) over Eurasia and mechanisms are still unclear. This study aims to investigate the dominant mode of the March surface air temperature (SAT) over Eurasia during 1979–2022, and its relationship with sea surface temperature (SST) and sea ice concentration (SIC). The results reveal the prevalence of a dipole mode of the Eurasian SAT anomalies (SATAs) in March characterized by positive SATAs in northern (50°N–80°N) Eurasia and negative ones in southern (10°N–45°N) region. Further analyses demonstrate that this dipole mode of Eurasian SATAs shows significant relationships with a positive phase of the North Atlantic tripole SST anomalies (NAT SSTAs), as well as with the decline of Barents Sea SIC anomalies (SICAs). Associated with the NAT SSTAs and the Barents Sea SICAs during March, two Rossby wave trains propagate towards Eurasia through different ways, one is propagating eastwards from the mid-latitude of the North Atlantic to Eurasia, and another one spreads from the polar region to Eurasia. Modulated by these two wave trains, anomalous near-surface southwesterly (northeasterly) wind induces more (less) warm air towards northern Eurasia, while more (less) cloud emerges in southern Eurasia and causes less (more) solar radiation reaching southern Eurasia, leading to the dipole mode of the Eurasian SATAs in March. These results are further confirmed by numerical experiments from the version 5 of the Community Atmosphere Model forced by the above mentioned SSTAs and SICAs.

A hybrid statistical downscaling prediction model with timescale decomposition for summer precipitation over Hainan Island, China

AbstractBy observing well‐known sea surface temperature (SST) indices and outputs from Climate Forecast System version 2 (CFSv2) hindcasts and predictions, in this study, we developed a hybrid statistical downscaling prediction model (HSDPM) based on a timescale decomposition approach, which effectively improved the forecasting ability of summer (June–August) precipitation over Hainan Island (HNSP) in China; this precipitation presented multiple obvious timescale variabilities. We found that the interannual variability of the HNSP is closely related to the observed North Atlantic tripole SST anomalies in the preceding winter and the summer sea level pressure over the South China Sea by CFSv2 predicted in March. On the other hand, the interdecadal variability in the HNSP is linked to the prewinter Pacific Interdecadal Oscillation (PDO) according to the observations, the CFSv2 predictions of the summer surface air temperature over the tropical central‐east Pacific. On this basis, both the interannual and interdecadal components of the HNSP are effectively predicted using the corresponding predictors via multiple regression; thus, the HSDPM is ultimately established by combining the above two components. Compared with the original CFSv2 model, the HSDPM model achieves a considerable improvement in performance in predicting the HNSP. Specifically, the temporal correlation coefficient, ratio of root‐mean‐square error and anomaly sign consistency rate between the observed and HSDPM‐predicted HNSPs are 0.70, 17.4% and 80.5%, respectively, which are significantly greater than the above three forecasting index values of 0.23, 38.3% and 48.8%, respectively, obtained from the original CFSv2 predictions. The application of the HSDPM may be beneficial for drought and flood prevention and mitigation in Hainan Island.

Inter-model spread in East Asian Summer Monsoon onset timing linked to the tropical Pacific and Indian oceans' mean state in CMIP6 models

A major challenge for Earth system climate models in simulating the onset of the East Asian summer monsoon (EASM) is the wide inter-model spread of outputs, which causes considerable uncertainty in projections. Based on 31 models of the Coupled Model Inter-comparison Project Phase 6 (CMIP6), this study investigated the inter-model spread of EASM onset biases and its physical connection with simulated atmospheric circulation and tropical Pacific mean state biases. The inter-model leading mode of EASM onset biases exhibits a meridional seesaw pattern, with inverse patterns for South China and the Yangtze–Yellow River basins. The leading mode is closely related to the simulated meridional displacement of the East Asian subtropical westerly jet (EASWJ) and the Northwest Pacific subtropical anticyclone (WNPAC). The northward EASWJ and WNPAC respectively cause ascending or descending motions with moisture convergence or divergence in the Yangtze–Yellow River basins of South China. The leading mode can be attributed to simulated biases in the zonal tripole sea surface temperature (SST) in the tropical Pacific Ocean. Statistical analysis and numerical experiments based on CAM5.0 indicate that the SST bias stimulates “double-loop” zonal vertical circulation in the tropical Pacific and Indian oceans, influencing convection precipitation in the tropical central–western Pacific. The altered convection then triggers a Pacific–Japan-like pattern anomaly, forming meridional seesaw circulation and precipitation anomalies in East Asia. Such meridional seesaw circulation modulates the meridional displacement of the EASWJ and WNPAC, thus affecting EASM onset. Our results provide potential indicators of inter-model uncertainty of EASM onset.

A Zonal See‐Saw Variation of Tropical Cyclogenesis Over the Arabian Sea and Bay of Bengal‐South China Sea

AbstractThe variation of tropical cyclone (TC) genesis over the North Indian Ocean (NIO) and South China Sea (SCS) during the post‐monsoon season is explored during 1979–2020. A zonal see‐saw variation of TC genesis is detected between the Arabian Sea (AS) and Bay of Bengal (BOB) –SCS (BOBSCS for brevity). More (less) TCs over the AS tend to concur with fewer (more) TCs over the BOBSCS. Analyses of both the observations and the Coupled Model Intercomparison Project Phase 6 outputs show that tripole sea surface temperature (SST) anomalies in the tropical Indian and Pacific Oceans play a crucial role in the zonal see‐saw variation of TC genesis. Positive western Indian (WI) Ocean and tropical central and eastern Pacific (CEP) Ocean SST anomalies and negative western Pacific (WP) SST anomalies induce anomalous ascending and descending motions and high and low humidity transportation to the middle troposphere over the AS and BOBSCS, respectively. Meantime, anticyclonic vorticity anomalies appear in the BOBSCS region and negative anomalies of vertical wind shear are induced over the NIO and SCS. The combined positive contributions of upward motion, mid‐level humidity increase, and low vertical wind shear favor higher TC genesis over the AS, whereas the joint negative effects of downward motion, mid‐level water vapor decrease, and lower‐level anticyclonic vorticity anomalies result in less TC genesis over the BOBSCS. Numerical experiments show a critical role of the WI SST anomalies in the dipole pattern of vertical motion, which is crucial for the zonal dipole variation of TC genesis.

Influence of the Late-Winter North Atlantic Tripole Sea Surface Temperature Anomalies on Spring Land Surface Temperature in Mid-to-High Latitudes of Western Eurasia

Abstract The spring land surface temperature (LST) over western Eurasia, which is critical for ensuring food security, shows a clear interannual variability. Based on reanalysis data and numerical simulations, we investigated the potential influencing factors and the related mechanisms of spring LST variability in mid-to-high latitudes of western Eurasia (MHWEA). The results show that the North Atlantic tripole sea surface temperature anomalies (SSTAs) in February, which persist into spring, can significantly affect the spring LST variability over MHWEA. Analyses indicate that the positive phase of the North Atlantic tripole SSTAs pattern tends to increase the meridional SST gradient between positive SSTAs over the midlatitude North Atlantic and negative SSTAs over the south of Greenland, which strengthens the low-level atmospheric baroclinicity and thus induces more active transient eddy activities. Correspondingly, a Rossby wave train triggered by the eddy-mediated processes originates from the North Atlantic and propagates downstream, thereby causing anomalous anticyclonic circulation over MHWEA. Meanwhile, the westerly anomalies over the subpolar North Atlantic accelerate the polar front jet and provide a favorable thermodynamical condition for the tropospheric warming over the Barents–Kara Seas by bringing warm and moist oceanic air. The polar warming tends to weaken the poleward temperature gradient at mid-to-high latitudes and then decelerate the Eurasian midlatitude westerlies, thus dynamically contributing to the circulation changes that can affect spring LST over MHWEA. Model results suggest that the link can be generally reproduced. Therefore, the late-winter North Atlantic tripole SSTAs may act as a precursor for the prediction of spring LST over western Eurasia. Significance Statement The purpose of this study is to better understand the mechanisms underlying the interannual variability of the spring land surface temperature over western Eurasia, which is of great significance in ensuring food security. Here we show that the positive phase of the North Atlantic tripole sea surface temperature anomalies in February can modulate the tropospheric warming over the Barents–Kara Seas and further decelerate the Eurasian midlatitude westerlies in spring, which benefits local surface warming over western Eurasia. Our results provide a precursor for forecasting spring land surface temperature over western Eurasia.

Causes of the Interannual Variation of Summer Precipitation in Eastern Southwest China

Using ERA5 reanalysis data, we conducted an EOF analysis of summer precipitation in the eastern part of southwestern China (ESWC) over the past 60 years. Our study aimed to investigate the spatial distribution characteristics and interannual variability of summer precipitation in the ESWC, as well as to reveal the possible physical mechanisms influencing its interannual variability. The results indicate that, at the interannual scale, the first two modes of summer precipitation in the ESWC exhibit a uniform pattern and a north–south dipole pattern, respectively. The maximum and minimum time coefficients of the first mode correspond to the severe flood and drought events in 1998 and 2006, respectively, indicating that their time coefficients reflect the typical events well. The tri-pole sea surface temperature anomaly in the North Atlantic, along with the sea ice anomaly near the Barents Sea, induces a quasi-zonal wave train in the upper troposphere. This wave train propagates from the tropical and mid-latitude Atlantic, traverses the mid-high latitudes of Eurasia, and reaches the eastern part of China. The wave train induces cyclonic (anticyclonic) anomalies in the north (south) of the ESWC, facilitating the convergence of high-latitude dry and cold air with warm and moist southwesterly winds from the tropical ocean. This convergence promotes increased summer precipitation in the ESWC. We provided valuable insights into the interannual variability of summer precipitation in the ESWC, shedding light on the physical mechanisms responsible for these variations.

Influence of the Northwest Pacific tripole mode on the mid-summer precipitation in North China and the regulation by the North Atlantic

The North China mid-summer (July) precipitation (NCJP) contributes the largest proportion of total annual precipitation in North China, with significant interdecadal and interannual variability. The interannual variability of the NCJP was further investigated on the basis of a study of its interdecadal variability and found that a sea surface temperature (SST) pattern in July located in the northwest Pacific, defined here as the northwest Pacific SST tripole (NWPT), can significantly influence the interannual variability of the NCJP, and that this relationship is regulated by the decadal northern North Atlantic SST (NNASST). Diagnostic analysis and the linear baroclinic model experiment indicate that the positive (negative) NWPT in July can excite an anomalous anticyclone (cyclone) in the region centered on the Korean peninsula and an anomalous cyclone (anticyclone) in the northwest Pacific off southeast Japan, thereby strengthening (weakening) the NCJP. When the decadal NNASST is in a significantly positive phase, the positive geopotential height anomalies it excites in the northwestern region off North China are not favorable for the connection between the NWPT and the NCJP. When the decadal NNASST is in a negative or insignificantly positive phase, the July NWPT and the NCJP have a significant positive correlation on interannual timescale.

Decadal variability of extreme high temperature in mid- and high-latitude Asia and its associated North Atlantic air–sea interaction

The decadal variability of extreme high temperature (EHT) in the mid and highlatitudes of continental Asia (MHLCA) and associated mechanisms were investigated. Observational analysis indicated that, after removing global warming, the first leading mode of the EHT events showed a meridional dipole pattern and had significant decadal variability. During the periods 1980–1994 and 2012–2019 (1995–2011), EHT events were more frequent and stronger (less frequent and weaker) in western-central Siberia than normal, whereas they were less frequent and weaker (more frequent and stronger) in the wide area to the south of Lake Baikal. Further Observational and CESM (Community Earth System Model) results based analysis suggested that decadal change in air–sea interaction in the North Atlantic play an important role in shaping the decadal variability of EHT events in MHLCA. On decadal timescales, meridional negative–positive–negative sea surface temperature (SST) anomalies in the North Atlantic and their coupled positive North Atlantic Oscillation can trigger stronger wave activity flux and cause a significant anticyclonic–cyclonic teleconnection wave train in the troposphere over the mid and high latitudes of the Eurasian continent. As a result, an anomalous high-pressure center is evident in western-central Siberia, and thereby reducing total cloud cover and causing stronger solar heating. Thus, EHT events become more frequent and stronger. On the contrary, for the areas south of Lake Baikal, EHT events are weakened by corresponding cyclonic circulation anomalies. Additionally, the CAM5 (Community Atmosphere Model version 5) experiments suggested more important driving role of the decadal North Atlantic tripole SST anomalies in the abovementioned processes.

Potential remote forcing of North Atlantic SST tripole anomalies on the seesaw haze intensity between late winter months in the North China plain: A case study

AbstractThis study identified a prominent temporal seesaw haze intensity case that occurred between the late winter months of 2010 in the North China Plain (NCP), featuring considerably suppressed haze intensity in January and enhanced haze intensity in the adjacent month of February in 2011. We suggest that dramatic alternations of atmospheric and oceanic anomalies played fundamental roles in forming this seesaw haze intensity case, rather than changes in manmade emission anomalies. The suppressed haze intensity in January 2011 was tied to an equivalent barotropic cyclonic anomaly that dominated the NCP and its surroundings, which generated in situ haze‐suppressed meteorology characterized by strengthened lower‐level northerly anomalies with cold and dry conditions, as well as elevated boundary layer height and destabilized atmospheric stratification. In stark contrast, the enhanced haze intensity in February 2011 was connected to an equivalent barotropic anticyclonic anomaly, linking a haze‐favourable meteorology opposite to that in January 2011. The pronounced North Atlantic sea surface temperature (SST) tripole anomalies, with positive anomalies in the tropical and mid‐latitudinal North Atlantic and negative anomalies in the subtropical North Atlantic, made a significant contribution to the above‐mentioned seesaw haze intensity case. Diagnostic analyses suggested that the January North Atlantic SST tripole anomalies were linked to a significant negative North Atlantic Oscillation (NAO)‐like pattern, which acted as the source of the Rossby wave train to generate concurrent haze‐suppressed meteorology over the NCP. In February, although the NAO‐like pattern was drastically dampened, the enhanced barotropic cyclonic anomaly centred southeast of the Yamal Peninsula played a critical role in relaying the impact of January tripole SST anomalies, thus inducing concurrent haze‐favourable meteorology. Consequently, January North Atlantic SST tripole anomalies could exert an effective modulation effect on the generation of seesaw haze intensity. The proposed mechanism was further verified using the Community Earth System Model Large Ensemble Numerical Simulation (CESM‐LENS) datasets.

Evaluation on the Forecast Skills of Precipitation and Its Influencing Factors in the Flood Season in Liaoning Province of China

To clarify the precipitation forecast skills of climate forecast operations in the flood season in Liaoning Province of China, this study examines the forecast accuracies of China’s national and provincial operational climate prediction products and the self-developed objective prediction methods and climate model products by Shenyang Regional Climate Center (SRCC) in the flood season in Liaoning. Furthermore, the forecast accuracies of the main influencing factors on the precipitation in the flood season of Liaoning are assessed. The results show that the SRCC objective methods have a relatively high accuracy. The European Centre for Medium-Range Weather Forecasts (ECMWF) sub-seasonal forecast initialized at the sub-nearest time has the best performance in June. The National Climate Center (NCC) Climate System Model sub-seasonal forecast initialized at the sub-nearest time, and the ECMWF seasonal and sub-seasonal forecasts initialized at the nearest time, perform the best in July. The NCC sub-seasonal forecast initialized at the sub-nearest time has the best performance in August. For the accuracy of the SRCC objective method, the more significant the equatorial Middle East Pacific sea surface temperature (SST) anomaly is, the higher the evaluation score of the dynamic–analogue correction method is. The more significant the North Atlantic SST tripole is, the higher the score of the hybrid downscaling method is. For the forecast accuracy of the main influencing factors of precipitation, the tropical Atlantic SST and the north–south anti-phase SST in the northwest Pacific can well predict the locations of the southern vortex and the northern vortex in early summer, respectively. The warm (clod) SST in China offshore has a good forecast performance on the weak (strong) southerly wind in midsummer in Northeast China. The accuracy of using the SST in the Nino 1+2 areas to predict the north–south location of the western Pacific subtropical high is better than that of using Kuroshio SST. The accuracy of predicting northward-moving typhoons from July to September by using the SST in the west-wind-drift area is better than using the SST in the Nino 3 area. The above conclusions are of great significance for improving the short-term climate prediction in Liaoning.

The Summertime Circulation Types over Eurasia and Their Connections with the North Atlantic Oscillation Modulated by North Atlantic SST

ERA5 monthly averaged reanalysis data during 1979–2020 are used to analyze the anomalous characteristics of summertime circulation types over Eurasia and their connections with the North Atlantic Oscillation (NAO) modulated by North Atlantic sea surface temperature (SST). A circulation index (CI) is defined to describe the anomalous characteristics of summertime circulation types over the Eurasian mid-high latitude and classify the anomalous circulation into a double-ridge type (DR-type) and double-trough type (DT-type). The results show that these anomalous circulation types are closely related to the variation of the western Pacific subtropical high (WPSH), East Asian subtropical jet (EASJ), South Asia high (SAH) and summer precipitation anomalies in China. There is a significant negative correlation between summer NAO and circulation types over Eurasia. The positive CI is favorable for the southward movement of the EASJ and two positive height anomalies over the Ural Mountains and the Sea of Okhotsk, respectively. Accompanied by moisture convergence and a strong ascending motion over the middle and lower reaches of the Yangtze River Valley (MLYRV), the summer rainfall will be above normal. These patterns are reversed in positive NAO-index years. The connection between the NAO and circulation types over Eurasia is modulated by a tri-pole SST anomaly pattern over the North Atlantic, which may induce the NAO-like atmospheric circulation and strengthen the impacts of the NAO on Eurasian circulation types. A wave train from the North Atlantic to East Asia, which is aroused by the tri-pole SST anomaly pattern, is the potential mechanism for linking summer NAO and circulation types over Eurasia.

Persistence or Transition of the North Atlantic Oscillation Across Boreal Winter: Role of the North Atlantic Air‐Sea Coupling

AbstractClimate anomalies over North America and Europe are closely related to the North Atlantic Oscillation (NAO). The persistence, decay or switch of the NAO phase across the boreal winter determines whether the NAO‐related climate anomalies are sustained, weakened or reversed during the boreal winter. This study compares the spatiotemporal evolution of atmospheric wind and sea surface temperature (SST) anomalies over the North Atlantic corresponding to persistence and switch of the NAO phase across the boreal winter. It is revealed that the persistence of the NAO from early to late winter is associated with a strong coupling between the NAO and the North Atlantic tripole SST anomalies. The formation of the tripole SST anomalies is attributed to the wind‐related surface latent heat flux changes. The tripole SST anomalies feedback on the NAO through eddy‐mean flow interaction. In contrast, during the winters with the switch of the NAO phase, the North Atlantic SST anomalies follow the wind changes, but do not have obvious feedback on the NAO. Our analysis identifies a prominent change of the winter NAO evolution in the early 1990s. The NAO switches its phase from early to late winter during 1970s through the early 1990s, but tends to maintain its phase during the boreal winter after the early 1990s. This interdecadal change appears to be related to the mean SST change. The SST increase in both the tropical and mid‐latitude North Atlantic Ocean in the early 1990s enhances the feedback of the NAO‐induced tripole SST anomalies on the NAO.

Impact of March North Atlantic Oscillation on Indian Ocean Dipole: role of air–sea interaction over the Western North Pacific

We investigated the relationship between the North Atlantic Oscillation (NAO) and Indian Ocean Dipole (IOD), which has remained unknown to date. Reanalysis data and linear baroclinic model experiments were employed in our study. The results showed significant correlation between the March NAO and the boreal summer and autumn IOD, independent of the El Niño–Southern Oscillation signal, verified by partial correlation analysis. Air–sea interaction over the western North Pacific (WNP) is a significant aspect of the physical mechanism through which the March NAO affects the subsequent IOD. A strong positive March NAO induces equivalent barotropic cyclonic circulation over the WNP through a steady Rossby wave, accompanied by a local tripole sea surface temperature (SST) anomaly pattern. Facilitated by local air–sea positive feedback, the low-level cyclonic circulation and associated precipitation anomalies over the WNP persist from early spring to summer and shift equatorward. During May–June, the WNP anomalous cyclone strengthens the southeasterly wind and enhances cooling off Sumatra–Java through local meridional circulation. Such circulation ascends over the WNP and descends over the tropical southeastern Indian Ocean and Maritime Continent. Subsequently, wind–evaporation–SST and wind–thermocline–SST positive feedback in the tropical Indian Ocean contribute to IOD development. A diagnosis of ocean mixed-layer heat budget indicated that the ocean dynamic process associated with the NAO contributes more to IOD development than does atmospheric thermal forcing. Determining the influence mechanism of the March NAO on the subsequent IOD is considered useful in advancing the seasonal prediction of IOD.

Southeast China Extreme Drought Event in August 2019: Context of Coupling of Midlatitude and Tropical Systems

Abstract An extreme drought occurred over Southeast China (SEC) in August 2019. We demonstrate synergistic effects of midlatitude and tropical circulation on this extreme event and highlight the impacts of the coupling and locking of two cyclones at different latitudes, which are otherwise ignored. We propose the relaying roles of the Tibetan Plateau (TP) and western North Pacific in connection with the tropical convection and SEC precipitation. The equivalent-barotropic anticyclone over the TP and lower-tropospheric cyclone over the western North Pacific both resulted from the positive Indian Ocean dipole and El Niño Modoki. The equivalent-barotropic cyclone over Northeast China originated from the dispersion of Rossby waves upstream along the subtropical waveguide associated with the North Atlantic tripole sea surface temperature anomaly pattern and the Rossby wave response to the TP precipitation deficiency. Further, they jointly contributed to this drought by inducing strong northerly wind anomalies in the entire troposphere over East China. These anomalous northerly winds led to decreased warm moisture from the south and substantial sinking motions, which inhibited the occurrence of the SEC local convection and precipitation. The SEC precipitation is closely related to convection over the Maritime Continent from a climate perspective. This relationship is verified by observations, linear baroclinic model experiments, and general circulation model sensitivity experiments with and without the TP, in which precipitation anomalies over the southern TP and Philippine Sea play important bridge roles. The results will advance the prediction of the SEC extreme drought events.

Two Approaches of the Spring North Atlantic Sea Surface Temperature Affecting the Following July Precipitation over Central China: The Tropical and Extratropical Pathways

Abstract The spring tripole sea surface temperature (SST) anomalies in North Atlantic are an outstanding regional mode of interannual variability. Based on the observed and reanalyzed datasets during 1979–2019, this study reveals the relationship and linking mechanism between the spring tripole North Atlantic SST anomalies and the central China July precipitation (CCJP). Results show that the tripole SST anomalies, especially the warm SST anomalies in the tropical North Atlantic (TNA) and the subpolar North Atlantic (SNA), often cause surplus CCJP through the tropical and extratropical pathways. On the one hand, the spring TNA SST warming induces a pan-tropical climate response with the cooling in the central equatorial Pacific and the warming in the Indo-western Pacific until the following July through a series of air–sea interactions, helping maintain an anomalous anticyclone over the northwest Pacific and transport more warm humid flows to central China. On the other hand, the spring TNA and SNA SST warming persist into the following July and then emanate a wave train extending from the SNA throughout the Eurasian continent to East Asia, which induces an anomalous anticyclone over North China with its southeast flank transporting more cold air to central China. The warm humid flows from the south against the cold air from the north are conductive to the local ascending motion, favoring the increased CCJP. Our results highlight both the tropical and extratropical teleconnection pathways of the North Atlantic SST anomalies affecting the CCJP. This suggests an important seasonal predictor of the regional climate. Significance Statement July is the peak rainy month of central China, with heavy precipitation occurring frequently and often causing serious impacts on the local production and livelihood of millions of people. This study finds that the spring tripole sea surface temperature anomalies in North Atlantic induced by the North Atlantic Oscillation can exert significant impacts on the following July precipitation over central China through both the tropical and extratropical pathways. This improves our understanding of the causes of the surplus July precipitation over central China and has important implications for the seasonal predictability of the regional climate.

2022: An Unprecedentedly Rainy Early Summer in Northeast China

In the early summer (June) of 2022, the spatial mean precipitation in northeast China (NEC) was 62% higher than normal and broke the historical record since 1951. Based on the precipitation data of 245 meteorological stations in NEC and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis, this paper analyzes the role of large-scale circulation and sea-surface temperature (SST) associated with anomalous precipitation over NEC in June using singular value decomposition (SVD), correlation analysis, regression analysis, and composite analysis methods, and further investigates the possible cause of the abnormal precipitation in June 2022. Results show that the northeast China cold vortex (NCCV) accompanying the blocking high in the Okhotsk Sea (BHOS) has been the primary mid-to-high latitude atmospheric circulation pattern affecting NEC precipitation in June since 2001. This circulation pattern is closely related to the tripole SST pattern over the North Atlantic (NAT) in March. In June 2022, the NAT SST anomaly in March stimulates eastward-propagating wave energy, resulting in the downstream anomalous circulation pattern in which the NCCV cooperates with the BHOS in the mid-high latitudes of East Asia. Under this background atmospheric circulation favorable for precipitation, the Kuroshio region SST anomaly in June led to a more northward and stronger anomalous anticyclone in the northwestern Pacific through local air–sea interaction, which provides more sufficient water vapor for NEC, resulting in unprecedented precipitation in June 2022.

Causes of Interannual Variability of Summer Precipitation Intraseasonal Oscillation Intensity over Southwest China

AbstractThe interannual variability in summer precipitation intraseasonal oscillation intensity over southwest China (SWC) is investigated in this study. The results indicate that the 7–20-day period dominates the intraseasonal variability in summer SWC precipitation. The leading mode of summer SWC precipitation 7–20-day oscillation intensity (SPOI) is a north–south dipole pattern with prominent interannual variability. The atmospheric circulation anomalies from both tropics and mid- to high latitudes are responsible for the interannual variability in the dipole pattern. In the tropics, an enhanced local Hadley cell and an anomalous anticyclone over southern China and the northwest Pacific contribute to the north-positive–south-negative pattern of SPOI by inducing moisture convergence (divergence) over northern (southern) SWC in the background state. In the mid- to high latitudes, the 7–20-day Rossby wave trains along the subtropical jet are crucial for the 7–20-day precipitation over northern SWC. Further analyses suggest that the sea surface temperature (SST) anomalies over the Maritime Continent (MC) and the North Atlantic (NA) are associated with the SPOI dipole pattern. The MC SST warming causes convection anomalies over the tropical Indo-Pacific, consequently triggering a Matsuno–Gill-type atmospheric response conducive to the north-positive–south-negative pattern of SPOI. The NA SST tripole triggers a Rossby wave train across Eurasia that strengthens the East Asian westerly jet and enhances 7–20-day atmospheric variability, consequently favoring the variability of 7–20-day precipitation over northern SWC. Diagnoses of moisture and vorticity budgets further indicate the importance of the interaction between intraseasonal fluctuations and atmospheric background in the formation of the north–south difference in 7–20-day precipitation variability over SWC.

Interannual Impact of the North Atlantic Tripole SST Mode on the Surface Potential Vorticity Over the Tibetan Plateau During Boreal Summer

AbstractThe Tibetan Plateau (TP) is an important forcing in the global climate system. Compared to the extensive studies on the influence of the TP on surrounding climates, the variability of the surface potential vorticity (PV) over the TP (TPSPV) in response to other forcings is seldom discussed. In this article, the influence of the interannual mode of North Atlantic tripole sea surface temperature (NAT) on the TPSPV during boreal summer is investigated. Results show that the negative (positive) anomalous TPSPV is associated with anomalous unstable (stable) surface air, cyclonic (anticyclonic) surface circulation, and rising (sinking) motion, which favors the stronger (weaker) East Asian summer monsoon and weaker (stronger) Indian summer monsoon. Further analysis indicates that the negative TPSPV is significantly correlated with a positive NAT mode with a cold center in the middle and warm centers in the subtropic and subpolar regions. This relationship is further testified by numerical experiments. The results further demonstrate that the positive NAT forcing can stimulate a Rossby wave train aloft, which passes Scandinavia, Ural and arrives at the TP and eventually triggers a low‐level positive PV over the TP. Constrained by the PV‐θ mechanism, the low‐level positive PV favors a decrease in static stability near the surface and thus leads to a negative TPSPV. Under the positive feedback between TPSPV and vertical motion, the negative TPSPV could further strengthen. Therefore, the positive NAT phase could intensify the thermal and dynamic forcings of the TP in association with TPSPV during boreal summer.

Impacts of a Tripolar Sea Surface Temperature Pattern Over Tropical‐North Pacific on Interannual Variations of Spring Extreme Consecutive Dry Days Over Southern China

AbstractIt is well known that the El Niño‐Southern Oscillation (ENSO) has a profound impact on climate over southern China (SC). This study indicates that independent from the ENSO variability, a meridional tripolar sea surface temperature (SST) pattern over tropical‐North Pacific in February has a close relationship with interannual variations in the frequency of spring extreme consecutive dry days (extreme‐CDDs) over SC. Further analyses suggest that the tripolar SST pattern has a good persistence from February to the following spring and could influence the variations of SC extreme‐CDDs through two ways. On one hand, the extratropical SST anomalies of the tripolar pattern can cause the meridional shift of the North Pacific subarctic oceanic front, resulting in a meridional dipole atmospheric pattern over the region from the North Pacific to East Asia through eddy‐mean flow interaction. The meridional atmospheric pattern can influence the East Asian upper‐level jet and western Pacific subtropical high (WPSH), leading to anomalous vertical motion and moisture transport over SC. On the other hand, the SST anomalies of the tripolar pattern over the tropical western Pacific can excite an overturning circulation and influence the WPSH through the Matsuno‐Gill response, also contributing to anomalous vertical motion and moisture transport over SC. These physical processes are further confirmed by numerical simulations. Therefore, through the aforementioned two physical ways, the February tripolar SST pattern can influence the moisture and dynamical conditions associated with the occurrence of extreme‐CDDs, consequently providing a factor and predictor for the variations of SC spring extreme‐CDDs.