Thermocline fluctuations in the equatorial Pacific related to the two types of El Niño events

Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 30 (2017): 6611-6627, doi:10.1175/JCLI-D-16-0291.1.The interannual fluctuations of the equatorial thermocline are usually associated with El Niño activity, but the linkage between the thermocline modes and El Niño is still under debate. In the present study, a mode function decomposition method is applied to the equatorial Pacific thermocline, and the results show that the first two dominant modes (M1 and M2) identify two distinct characteristics of the equatorial Pacific thermocline. The M1 reflects a basinwide zonally tilted thermocline related to the eastern Pacific (EP) El Niño, with shoaling (deepening) in the western (eastern) equatorial Pacific. The M2 represents the central Pacific (CP) El Niño, characterized by a V-shaped equatorial Pacific thermocline (i.e., deep in the central equatorial Pacific and shallow on both the western and eastern boundaries). Furthermore, both modes are stable and significant on the interannual time scale, and manifest as the major feature of the thermocline fluctuations associated with the two types of El Niño events. As good proxies of EP and CP El Niño events, thermocline-based indices clearly reveal the inherent characteristics of subsurface ocean responses during the evolution of El Niño events, which are characterized by the remarkable zonal eastward propagation of equatorial subsurface ocean temperature anomalies, particularly during the CP El Niño. Further analysis of the mixed layer heat budget suggests that the air–sea interactions determine the establishment and development stages of the CP El Niño, while the thermocline feedback is vital for its further development. These results highlight the key influence of equatorial Pacific thermocline fluctuations in conjunction with the air–sea interactions, on the CP El Niño.This work is jointly supported by the Funds for Creative Research Groups of China (Grant 41521005), the Special Fund for Public Welfare Industry (GYHY201506013), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDA11010301), and the National Natural Science Foundation of China (Grants 41406033, 41475057, 41376024, 41676013) and the CAS/SAFEA International Partnership Program for Creative Research Teams.2018-01-2

Subseasonal swing of cold and warm extremes between Eurasia and North America in winter of 2020/21: initiation and physical process

Eurasia and North America experienced a robust subseasonal swing of surface air temperature (SAT) extremes in 2020/21 winter, featuring severe cold (warm) extremes over Eurasia before (after) 15 January and conversely over North America. This sharp subseasonal swing of intercontinental cold and warm extremes exerted considerable severe impacts on human activities and the global economy. Here we examined the initiation and physical process based on data analyses. Our results show annual cycle (AC) anomalies of SAT caused this subseasonal alternating of temperature extremes in two regions. The AC anomalies of SAT are regulated by the phase transition of the North-Pacific-Oscillation-like (NPO-like) circumglobal Rossby wave (CRW) train. Unprecedented warming sea surface temperature over midlatitude Northwest Atlantic in early winter initiated a positive phase of the NPO-like CRW train, via eddy-mediated physical processes and the resultant feedback of sea ice loss over the Barents-Kara Seas. While, the subsequent downward feedback of stratospheric processes resulted in the negative phase of the NPO-like CRW pattern in late winter. This work advances the understanding of the subseasonal predictability of SAT extremes from impacts of AC anomalies and intercontinental seesawing

Tropical Pacific cold tongue mode triggered by enhanced warm pool convection due to global warming

A cold tongue mode (CTM) formed in the 1980s as a La Niña-like stepwise response to recent global warming; however, a consensus has not been reached on the mechanism underlying the CTM formation. Here, we attribute the CTM to the enhanced deep convection of the warm pool regions over the western Pacific and south of North America. Increases in the sea surface temperatures in the two Pacific warm pool regions that occurred due to global warming exceeded the threshold of deep convection after the 1980s, which resulted in two opposite anomalous vertical circular circulation patterns and induced the CTM via the intensification, contraction, and westward shift of the Walker circulation and the uplift of the thermocline. Our results provide a novel explanation of the La Niña-like response under recent global warming

Remote linkage of record-breaking U.S. Tornado outbreaks to the tropical cyclone in western North Pacific in December 2021

The frequency of tornadoes usually peaks during spring to summer rather than winter in climatology. However, the United States (U.S.) experienced more than 200 tornadoes in December 2021, which broke the historical record and caused 87 fatalities. Historically, the frequency of tornadoes in December tends to increase under El Niño conditions. Our results show that the monthly large-scale weather regime conducive to these record-breaking tornado outbreaks under a La Niña condition is closely associated with Typhoon Nyatoh in the western North Pacific. As the tropical cyclone (TC) recurved into the mid-latitudes, its interaction with the extratropical flows has caused distortions in the Asian jet stream and the dramatic development of anomalous anticyclone west of the dateline, which in turn strongly regulated the response of the monthly atmospheric teleconnection to La Niña forcing. Accurate forecasts of the monthly mean circulation for December 2021 first appeared in the European Center for Medium-Range Weather Forecasts sub-seasonal to seasonal forecasts from 29 November, with a forecast skill closely related to that of Typhoon Nyatoh. Given most studies on the warm seasons with frequent tornadoes, the present results advance our understanding of the TC effect on the monthly atmospheric response to El Niño-Southern Oscillation forcing and its linkage to the tornado occurrence during boreal winter

Enhanced Subseasonal Variability of Spring Temperature Over Eastern China in 2022: Initial Role of Extremely Heavy Arctic Sea Ice in Previous Winter

Abstract In spring 2022, strong temperature whiplash events occurred in eastern China (EC), characterized by enhanced subseasonal variability in surface air temperature (SAT). Our results show that extreme temperature fluctuations are dominated by the amplified Eurasian wave train on a biweekly timescale as a possible response to increased sea ice concentrations (SICs) over the Barents‐Kara Seas (BK) in previous winter. Increased winter SICs weaken the upward planetary waves and enhance the stratospheric polar vortex. In spring, the downward propagation of enhanced polar vortex intensifies the polar front jet, increasing kinetic energy gain of the North Atlantic wave train and guiding it to travel farther eastward. The Eurasian wave train strengthens through an upstream development and enhanced local baroclinicity and enhances subseasonal SAT variability over EC. Therefore, the state of BK SICs in previous winter could be a useful signal for the enhanced likelihood of spring extreme temperature whiplash events

Subseasonal mode of cold and wet climate in South China during the cold season: a climatological view

The authors investigate the dominant mode of climatological intraseasonal oscillation (CISO) of surface air temperature (SAT) and rainfall in China, and discuss the linkage of cold and wet climate in South China (SC) with the Arctic circulation regime during the cold season (from November to March). Results show that a positive CISO displays a cold-dry climate in North China, whereas a cold-wet pattern prevails in SC with a quasi-30-day oscillation during the peak winter season. In SC, the intraseasonal variability of SAT plays a leading role, altering the cold-wet climate by the southward shift of a cold front. Evidence shows that the circulation regime related to the cold and wet climate in SC is mainly regulated by a pair of propagating ISO modes at the 500-hPa geopotential height in the negative phase of Arctic Oscillation. It is demonstrated that the local cyclonic wave activity enhances the southward movement of the Siberian high, favoring an unstable atmosphere and resulting in the cold-wet climate over SC. Therefore, the cold-air activity acts as a precursor for subseasonal rainfall forecasting in SC

Droughts near the northern fringe of the East Asian summer monsoon in China during 1470–2003

Historical annual dry-wet index for 1470-2003 combined with instrumental precipitation since 1951 were used to identify extremely dry years and events near the northern fringe of the East Asian summer monsoon in China-the Great Bend of the Yellow River (GBYR) region. In total, 49 drought years, of which 26 were severe, were identified. Composites of the dry-wet index under the drought years show an opposite wet pattern over the Southeast China. The longest drought event lasted for 6 years (1528-1533), the second longest one 4 years (1637-1640). The most severe 2-year-long drought occurred in 1928-1929, and the two driest single years were 1900 and 1965. These persistent and extreme drought events caused severe famines and huge losses of human lives. Wavelet transform applied to the dry-wet index indicates that the severe drought years are nested in several significant dry-wet variations across multiple timescales, i.e., the 65-85 year timescale during 1600- 1800, 40-55 year timescale before 1640 and 20-35 year timescale mainly from 1550 to 1640. These timescales of dry-wet variations are discussed in relation to those forcing such as cycles of solar radiation, oscillation in the thermohaline circulation and the Pacific Decadal Oscillation (PDO). Comparing 850 hPa winds in Asia in extremely dry and wet years, it was concluded that dry-wet variability in the GBYR region strongly depends upon whether the southerly monsoon flow can reach northern China.Environmental SciencesMeteorology & Atmospheric SciencesSCI(E)EI10ARTICLE1-2373-38311

Two Types of Interannual Variability of South China Sea Summer Monsoon Onset Related to the SST Anomalies before and after 1993/94

An advance in the timing of the onset of the South China Sea (SCS) summer monsoon (SCSSM) during the period 1980-2014 can be detected after 1993/94. In the present study, the interannual variability of the SCSSM onset is classified into two types for the periods before and after 1993/94, based on their different characteristics of vertical coupling between the upper-and lower-tropospheric circulation and the differences in their related sea surface temperature anomalies (SSTAs). On the interannual time scale, type-I SCSSM onset is characterized by anomalous low-level circulation over the northern SCS during 1980-93, whereas type-II SCSSM onset is associated with anomalies of upper-level circulation in the tropics during 1994-2014. The upper-tropospheric thermodynamic field and circulation structures over the SCS are distinct between the two types of SCSSM onset, and this investigation shows the importance of the role played by the spring SSTAs in the southern Indian Ocean (SIO) and that of ENSO events in type-I and type-II SCSSM onset, respectively. In the early episode, the warming SIO SSTAs can induce an anomalous low-level anticyclone over the northern SCS that affects local monsoonal convection and rainfall over land to its north, demonstrating a high sensitivity of subtropical systems in type-I SCSSM onset. However, in type-II SCSSM onset during the later episode, the winter warm ENSO events and subsequent warming in the tropical Indian Ocean can influence the SCSSM onset by modulating the spring tropical temperature and upper-level pumping effect over the SCS

Modulation of the cold tongue mode (CTM) in tropical cyclone genesis over the western North Pacific during 1975–2021

The cold tongue mode (CTM) in the tropical Pacific enhanced with the global warming in the recent decades. Here, we show that the change of oceanic thermal condition and atmospheric circulation plays a critical role in the tropical cyclones (TCs) genesis in the western North Pacific (WNP). The frequency of TCs in the WNP decreases with its northwest movement, when the CTM transitions from its negative phase in 1975–1997(P1) to positive phase in 1998–2021 (P2). Intercomparison of the TC activities between P1 and P2, we found that the main reason responsible for decreasing of TC genesis is from reducing of meridional and zonal heat transportation. And the northwest movement of TC position is driven by the heat increasing at the western and northern boundary areas. In addition, the northwest movement of TC is dominated by the atmospheric circulation associated with local Walker circulation and Hadley cell