The Connection Between the Hadley Circulation and Meridional Structure of Tropical SST during ENSO from 1950 to 1977

The connection between the meridional structure of tropical sea surface temperature (SST) and the Hadley circulation (HC) under the effect of ENSO (El Niño Southern Oscillation) from 1950 to 1977 is studied. We decompose the HC and zonal mean SST into equatorially symmetric (HES for HC, SES for SST) and asymmetric variations (HEA for HC, SEA for SST) to discuss the modulation of their connection by ENSO. During El Niño events from 1950 to 1977, the HC is less sensitive to the different SST meridional structures and expressed by response ratio. The ratio in La Niña and neutral events is around 4, which is equivalent to the result in the climatology. The reason for the decreased ratio during El Niño events is explored. The interdecadal variation in the linkage between the HC and tropical SST is due to a clear interdecadal shift in the impacts of ENSO on the tropical Indian Ocean (TIO) SST. For the period 1950–1977, when El Niño events occur, larger SST warming amplitude is observed over the northern TIO (0°–15°N, 50°–100°E). However, the southern TIO (15°S–0°, 50°–100°E) shows greater warming amplitude during 1980–2016. The anomalous SST variation over the TIO linked to El Niño events alters the meridional SST distribution, inducing anomalies in the meridional circulation. These results can help us to understand the interdecadal modulation by ENSO of the relationship between tropical SST and the HC.

Understanding the Increasing Hot Extremes over the Northern Extratropics Using Community Atmosphere Model

The past four decades have seen an increase of terrestrial hot extremes during summer in the northern extratropics, accompanied by the Northern Hemisphere (NH) sea surface temperature (SST) warming (mainly over 10°–70°N, 0°–360°) and CO2 concentration rising. This study aims to understand possible causes for the increasing hot extremes, which are defined on a daily basis. We conduct a series of numerical experiments using the Community Atmosphere Model version 5 model for two periods, 1979–1995 and 2002–2018. The experiment by changing the CO2 concentration only with the climatological SST shows less increase of hot extremes days than that observed, whereas that by changing the NH SST (over 10°–70°N, 0°–360°) with constant CO2 concentration strengthens the hot extremes change over mid-latitudes. The experiment with both SST and CO2 concentration changes shows hot extremes change closer to the observation compared to the single-change experiments, as well as more similar simulations of atmospheric circulations and feedbacks from cloud and radiative processes. Also discussed are roles of natural variability (e.g., Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation) and other factors (e.g., Arctic sea ice and tropical SST).

Trans-basin influence of southwest tropical Indian Ocean warming during early boreal summer

This study examines the climate response to a sea surface temperature (SST) warming imposed over the southwest Tropical Indian Ocean (TIO) in a coupled ocean-atmosphere model. The results indicate that the southwest TIO SST warming can remotely modulate the atmospheric circulation over the western North Pacific (WNP) via inter-basin air-sea interaction during early boreal summer. The southwest TIO SST warming induces a “C-shaped” wind response with northeasterly and northwesterly anomalies over the north and south TIO, respectively. The northeasterly wind anomalies contribute to the north TIO SST warming via a positive Wind-Evaporation-SST(WES) feedback after the Asian summer monsoon onset. In June, the easterly wind response extends into the WNP, inducing an SST cooling by WES feedback on the background trade winds. Both the north TIO SST warming and the WNP SST cooling contribute to an anomalous anticyclonic circulation (AAC) over the WNP. The north TIO SST warming, WNP SST cooling, and AAC constitute an inter-basin coupled mode called the Indo-western Pacific ocean capacitor (IPOC), and the southwest TIO SST warming could be a trigger for IPOC. While the summertime southwest TIO SST warming is often associated with antecedent El Niño, the warming in 2020 seems to be related to extreme Indian Ocean Dipole in 2019 fall. The strong southwest TIO SST warming seems to partly explain the strong summer AAC of 2020 over the WNP even without a strong antecedent El Niño.

A Contrast of Recent Changing Tendencies in Genesis Productivity of Tropical Cloud Clusters over the Western North Pacific in May and October

Tropical cloud clusters (TCCs) are embryos of tropical cyclones (TCs) and may have the potential to develop into TCs. The genesis productivity (GP) of TCCs is used to quantify the proportion of TCCs that can evolve into TCs. Recent studies have revealed a decrease in GP of western North Pacific (WNP) TCCs during the extended boreal summer (July–October) since 1998. Here, we show that the changing tendencies in GP of WNP TCCs have obvious seasonality. Although most months could see recent decreases in GP of WNP TCCs, with October experiencing the strongest decreasing trend, May is the only month with a significant recent increasing trend. The opposite changing tendencies in May and October could be attributed to different changes in low-level atmospheric circulation anomalies triggered by different sea surface temperature (SST) configurations across the tropical oceans. In May, stronger SST warming in the tropical western Pacific could prompt increased anomalous westerlies associated with anomalous cyclonic circulation, accompanied by the weakening of the WNP subtropical high and the strengthening of the WNP monsoon. Such changes in background atmospheric circulations could favor the enhancement of atmospheric eddy kinetic energy and barotropic energy conversions, resulting in a recent intensified GP of WNP TCCs in May. In October, stronger SST warming in the tropical Atlantic and Indian Oceans contributed to anomalous easterlies over the tropical WNP associated with anomalous anticyclonic circulation, giving rise to the suppressed atmospheric eddy kinetic energy and recent weakened GP of WNP TCCs. These results highlight the seasonality in recent changing tendencies in the GP of WNP TCCs and associated large-scale atmospheric-oceanic conditions.

The Role of Indian Ocean Warming on Extreme Rainfall in Central China During Early Summer 2020: Without El Niño Influence

This study investigates the role of water vapor transport and sea surface temperature (SST) warming in the tropical Indian Ocean (TIO) on the heavy rainfall in central China during boreal early summer. In the past four decades, four significant rainfall events, in 1983, 1998, 2016, and 2020, occured in central China and caused severe floods, in which the year 2020 has the most extreme event. All four events are associated with significant TIO SST warming, associated with a strong and westward extending anomalous anticyclone on the western North Pacific (WNPAC). The anomalous winds in the northwestern flank of the WNPAC bring excess water vapor into central China. The water vapor, mainly carried from the central tropical Pacific, converges in central China and result in heavy rainfall. A theory of regional ocean-atmosphere interaction can well explain the processes, called the Indo-Western Pacific Ocean Capacitor (IPOC) effect. The WNPAC is usually associated with strong El Niño-Southern Oscillation (ENSO), except for the 2020 case. The 2020 event is extraordinary, without ensuring El Niño occurred in the previous winter. In 2020, the significant TIO warming sustained the anomalous WNPAC, inducing the most significant extreme rainfall event in central China. This study reveals that the IPOC effect can dramatically influence the East Asian climate even without involving the ENSO in the Pacific.

Contrasting Impacts of Three Extreme El Niños on Double ITCZs over the Eastern Pacific Ocean

In the recent four decades, there were three record-breaking El Niño events: 1982/1983, 1997/1998, and 2015/2016 events. A double intertropical convergence zone (ITCZ) pattern distinctively emerges over the eastern Pacific Ocean during boreal spring. Based on reanalysis (ERA-Interim) during 1979–2018, this study examines how these three extreme El Niños modulate such double ITCZs. The 1982/1983 and 1997/1998 El Niños moved both northern and southern ITCZs equatorward to form an individual and broad equatorial ITCZ. In contrast, the regulation of 2015/2016 El Niño was unique with a strengthened southern ITCZ to form a symmetric double-ITCZ. The above differences can be attributed to the different meridional structures of sea surface temperatures (SSTs). For the 1982/1983 and 1997/1998 El Niños, there was a meridionally symmetric structure of SST warming with a maximum at the equator. While for 2015/2016 El Niño, there was a meridionally symmetric structure of SST warming with a minimum at the equator.

Rapid warming of sea surface temperature along the Kuroshio and the China coast in the East China Sea during the 20th century

It has been reported that the sea surface temperature (SST) trend of the East China Sea during the 20th century was a couple of times larger than the global mean SST trend. However, the detailed spatial structure of the SST trend in the East China Sea and its mechanism have not been understood. The present study examines the SST trend in the East China Sea from 1901 to 2010 using observational data and a Regional Ocean Modeling System (ROMS) with an eddy-resolving horizontal resolution. A comparison among two observational datasets and the model output reveal that enhanced SST warming occurred along the Kuroshio and along the coast of China over the continental shelf. In both regions, the SST trends were the largest in winter. The heat budget analysis using the model output indicates that the upper layer temperature rises in both regions were induced by the trend of ocean advection, which was balanced to the increasing of surface net heat release. In addition, the rapid SST warming along the Kuroshio was induced by the acceleration of the Kuroshio. Sensitivity experiments revealed that this acceleration was likely caused by the negative wind stress curl anomalies over the North Pacific. In contrast, the enhanced SST warming along the China coast resulted from the ocean circulation change over the continental shelf by local atmospheric forcing.

Antarctic Peninsula warm winters influenced by Tasman Sea temperatures

The Antarctic Peninsula of West Antarctica was one of the most rapidly warming regions on the Earth during the second half of the 20th century. Changes in the atmospheric circulation associated with remote tropical climate variabilities have been considered as leading drivers of the change in surface conditions in the region. However, the impacts of climate variabilities over the mid-latitudes of the Southern Hemisphere on this Antarctic warming have yet to be quantified. Here, through observation analysis and model experiments, we reveal that increases in winter sea surface temperature (SST) in the Tasman Sea modify Southern Ocean storm tracks. This, in turn, induces warming over the Antarctic Peninsula via planetary waves triggered in the Tasman Sea. We show that atmospheric response to SST warming over the Tasman Sea, even in the absence of anomalous tropical SST forcing, deepens the Amundsen Sea Low, leading to warm advection over the Antarctic Peninsula.