Above-normal Sea Surface Temperature Research Articles

Strong heatwaves with widespread urban-related hotspots over Africa in 2019

2019 was one of the hottest years in recent decades, with widespread heatwaves over many parts of the world, including Africa. However, as a developing and vulnerable region, the understanding of recent heatwave events in Africa is limited. Here, the authors incorporated different climate datasets, satellite observations, and population estimates to investigate patterns and hotspots of major heatwave events over Africa in 2019. Overall, 2019 was one of the years that experienced the strongest heatwaves in terms of intensity and duration since 1981 in Africa. Heatwave hotspots were clearly identified across western-coastal, northeastern, southern, and equatorial Africa, where major cities and human populations are located. The proportion of urban agglomerations (population) exposed to extreme (99th percentile) heatwaves in the Northern Hemisphere and Southern Hemisphere rose from 4% (5 million people) and 15% (17 million people), respectively, in the baseline period of 1981–2010 to 36% (43 million people) and 57% (53 million people), respectively, in 2019. Heatwave patterns and hotspots in 2019 were related to anomalous seasonal change in atmospheric circulation and above-normal sea surface temperature. Without adaptation to minimize susceptibility to the effects of heatwave events, the risks they pose in populated areas may increase rapidly in Africa.摘要2019 年是近几十年来最热的年份之一, 包括非洲在内的全球许多地区都受到大范围的热浪侵袭. 然而, 非洲作为脆弱的发展中地区, 我们对其近年热浪事件的了解非常有限. 本研究中, 我们结合了不同的气候数据集, 卫星观测资料和人口数据, 研究了 2019 年非洲地区主要热浪事件发生的时空特征和热点分布区. 总体而言, 2019 年是非洲地区自 1981 年以来热浪强度最强, 持续时间最久的年份之一. 在主要城市和人口所在的非洲西海岸, 东北部, 南部和赤道地区是热浪发生的热点区. 位于赤道以北的非洲地区, 暴露于极端 (第 99 个百分位) 热浪的城市人口比例从 1981–2010 年基准期的 4% (500 万人) 上升至2019 年的 36% (4300 万人). 位于赤道以南地区, 暴露于极端热浪的城市人口则从基准期的15% (1700 万人) 上升至57% (5300 万人). 2019 年的热浪时空特征和热点分布与大气环流的季节变化异常和海温的暖异常有关. 如果不及时采取适应措施以尽量减少人口对热浪事件影响的敏感性, 热浪对非洲人口稠密地区构成的风险可能会迅速增加.

Dynamical response of atmospheric circulation to below-normal East Sea sea surface temperatures associated with heavy snowfall in eastern Korea

Prior studies have shown that above-normal sea surface temperatures (SSTs) enhance snowfall over Korea. Here, we show that heavy snow is also associated with below-normal East Sea SSTs, and we investigate the dynamical response of the atmosphere to this surface condition using observations and numerical modeling. The results indicate that anomalous southeasterly/easterly winds are induced by heavy snowfall-related cold SST anomalies, and consequently, the moisture flux is converged. The existence of the southeasterly winds and the accompanied moisture flux convergence appear to be instrumental in producing the heavy snowfall events. The anomalous southeasterly/easterly winds associated with heavy snowfall-related cold SST anomalies reduce the climatological northwesterly/westerly winds, leading to relatively warm and wet conditions over the east coast of Korea that are favorable for forming and intensifying snowfall events in the region.

Extremely Active Tropical Cyclone Activities over the Western North Pacific and South China Sea in Summer 2018: Joint Effects of Decaying La Niña and Intraseasonal Oscillation

In summer 2018, a total of 18 tropical cyclones (TCs) formed in the western North Pacific (WNP) and South China Sea (SCS), among which 8 TCs landed in China, ranking respectively the second and the first highest since 1951. Most of these TCs travelled northwest to northward, bringing in heavy rainfall and strong winds in eastern China and Japan. The present study investigates the impacts of decaying La Nina and intraseasonal oscillation (ISO) on the extremely active TCs over the WNP and SCS in summer 2018 by use of correlation and composite analyses. It is found that the La Nina episode from October 2017 to March 2018 led to above-normal sea surface temperature (SST) over central-western Pacific, lower sea level pressure and 500-hPa geopotential height over WNP, and abnormally strong convective activities over the western Pacific in summer 2018. These preceding oceanic thermal conditions and their effects on circulation anomalies are favorable to TC genesis in summer. Detailed examination reveals that the monsoon trough was located further north and east, inducing more TCs in northern and eastern WNP; and the more east-ward WNP subtropical high as well as the significant wave train with a “ − + − +” height anomaly pattern over the midlatitude Eurasia-North Pacific region facilitated the northwest to northward TC tracks. Further analyses reveal that two successively active periods of Madden-Julian Oscillation (MJO) occurred in summer 2018 and the boreal summer intraseasonal oscillation (BSISO) was also active over WNP, propagating northward significantly, corresponding to the more northward TC tracks. The MJO was stagnant over the Maritime Continent to western Pacific, leading to notably enhanced convection in the lower troposphere and divergence in the upper troposphere, conducive to TC occurrences. In a word, the extremely active TC activities over the WNP and SCS in summer 2018 are closely linked with the decaying La Nina, and the MJO and BSISO; their joint effects result in increased TC occurrences and the TC tracks being shifted more northwest to northward than normal.

Relation of the South China Sea Precipitation Variability to Tropical Indo-Pacific SST Anomalies during Spring-to-Summer Transition

Abstract The present study investigates the relationship of South China Sea (SCS) precipitation to tropical Indo-Pacific sea surface temperature (SST) during April–June (AMJ), which is the transition season from spring to summer. It is revealed that SCS rainfall anomalies in AMJ are influenced by SST anomalies in the equatorial Pacific (EP), tropical Indian Ocean (TIO), and western North Pacific (WNP). Three types of SST-influenced cases are obtained based on different combinations of SST anomalies in the above three regions. When same-sign EP and TIO SST anomalies are accompanied by opposite WNP SST anomalies, both anomalous cross-equatorial flows from the southwestern TIO induced by negative SST anomalies there and an anomalous Walker circulation forced by negative EP SST anomalies contribute to enhanced convection over the SCS and the surrounding regions with additional contribution from positive WNP SST anomalies via a Rossby wave–type response. In the cases of combined effects of EP and WNP SST anomalies, above-normal SST in the WNP is a direct cause of above-normal SCS rainfall though the WNP SST anomalies are induced by EP SST forcing. In the cases of combined impacts of TIO and EP SST anomalies, the accompanying coastal Sumatra SST anomalies contribute to the SCS rainfall variability via an anomalous cross-equatorial vertical circulation. The negative SST anomalies near the Sumatra coast induce descent over the southeastern TIO and ascent over the SCS and WNP. Model experiments with an atmospheric model confirm the impacts of southern TIO and EP SST anomalies on AMJ rainfall variation over the SCS.

Impacts of Rapid Warm-To-Cold ENSO Transitions on Summer Monsoon Rainfall over the Northeastern Indian Subcontinent

In this study, the impacts of warm-to-cold El Nino-Southern Oscillation (ENSO) phase transitions on the summer rainfall over the northeastern Indian monsoon (NEIM) region were investigated. Extremely rapid warm-to-cold ENSO transitions were identified in 1983, 1988, and 1998. In these rapid ENSO transition years, during August, the NEIM rainfall tended to be higher than that in other years, leading to large river water discharge into Bangladesh in August and September through the Ganges and Brahmaputra. In summer (June, July, and August), the western North Pacific monsoon (WNPM) was significantly suppressed in association with the east-west sea surface temperature (SST) anomaly contrast between an above-normal SST over the Indian Ocean and a negative SST anomaly that developed over the central equatorial Pacific. The suppression of WNPM was highly likely to be a key factor linking rapid ENSO transition to the heavy NEIM rainfall in August. WNPM suppression caused a prominent lower tropospheric anticyclonic anomaly, which extended westward over the Indian Ocean and Indian subcontinent. Associated with this circulation anomaly, a lower tropospheric westerly anomaly developed over the Bengal Plain in August, which activated the NEIM rainfall. Thus, especially in August, WNPM suppression significantly correlated with heavy rainfall over a large area over the northeastern Indian subcontinent. In all three rapid warm-to-cold ENSO transition years, WNPM was extremely suppressed in summer. However, in the August of 1983, owing to atmospheric variations at intraseasonal time scales, WNPM suppression was damped, and both the lower tropospheric circulation anomaly and the NEIM rainfall activation were not clear. On the other hand, in 1988 and 1998, WNPM suppression enhanced the NEIM rainfall in August and increased flood risk in Bangladesh.

THE EFFECT OF THE 1997–1998 EL NIÑO ON RHINOCEROS AUKLETS ON PROTECTION ISLAND, WASHINGTON

Abstract The 1997–1998 El Nino event was one of the most severe warm episodes the Pacific Ocean experienced during the 20th century. In Washington, it resulted in above-normal sea surface temperatures and sea levels during 1997–1998 on the state's outer coast and in the Strait of Juan de Fuca. Sea surface temperature anomalies of the two areas were highly correlated during 1995–2000. Rhinoceros Auklet (Cerorhinca monocerata) chick growth rates on Protection Island in the eastern Strait of Juan de Fuca were significantly lower (19%) during the El Nino years. Although chicks' peak weights were lowest during 1997, they did not significantly differ from non-El Nino years, suggesting that birds were able to compensate for smaller or less frequent chick feedings by extending the chicks' time in the burrow. Consequently, chicks were also significantly older at peak weight during this warm event, which likely led to prolonged fledging periods. However, the 1997–1998 El Nino did not affect other aspects of Rhinoce...

Coupled Atmosphere–Ocean Modeling of the El Niño of 1997–98

This study is based on a global coupled atmosphere–ocean model climate prediction that was designed to include 14 layers over the atmosphere and 17 layers within the ocean. In this model an 11-yr data assimilation includes physical initialization of the daily rainfall estimates. No flux corrections are included in the seasonal and annual forecasts of this coupled model. It is first shown that intraseasonal oscillation on the Madden–Julian timescale was an important feature during the onset of the El Niño of 1997. It is shown that this feature is retained in the model’s data assimilation and in the forecasts. The forecasts commence on 1 April 1997. The model forecasts showed an El Niño warming of the equatorial Pacific Ocean waters commencing with the excitation of a Kelvin wave. The Niño-3.4 region acquired above-normal sea surface temperature anomalies (SSTAs) by 15 May. The warm SSTs reached a peak by around January 1998. The El Niño made its demise by June 1998. The life cycle of the entire SSTA shows remarkable agreement to the observed anomalies over the Pacific Ocean. The subsurface temperature anomalies exhibit eastward propagating subsurface warm and cold water that are in phase with the El Niño and the La Niña features at the surface. Phenomenologically, this study is quite successful in showing the following. Velocity potential anomalies at the 200-hPa level are good indicators for long-lasting dry spells. In particular the authors have remarkable success in predicting the long-lasting dry spell over Florida (which resulted in major fires over Florida during June 1998, some 14 months into the forecast) and over Indonesia (which resulted in major fires over Indonesia during September and October 1997). This was by far the most promising result of the coupled modeling study. This study also enumerates several areas of the climate of 1997–98 that were not reasonably simulated at the present resolution of the coupled model. The model does not exhibit very high skill in prediction of precipitation anomalies over the Asian–Australian monsoon world, which is most likely due to the resolution and organization of convection issues. A realistic picture is shown of the North American monsoon system (the Mexico–Arizona monsoon) with wet conditions along 110°W, dry conditions along 95°W, and wet conditions along 80°W during the summers of 1997 and 1998. Furthermore, the model successfully shows a stronger North American monsoon system during the post–El Niño year 1998 compared to the El Niño year 1997. This is in accordance with the climatological and observational findings. California rainfall during January and February 1998, arising from the eastward passage of disturbances from the Pacific Ocean, was successfully simulated, although the rainfall amounts at the model resolution were roughly one-third of the observed peak estimates.

LONG-TERM VARIATIONS IN EQUATORIAL CIRCULATION AND RAINFALL

Abstract The large-scale air-sea interaction over the equatorial Pacific proposed by Bjerknes is investigated. It was found from a study of the Canton Island record that ocean temperature, rainfall, trade wind flow, and equivalent potential temperature are related and undergo long-term variations with periods in excess of a year. Similar changes occur in the high troposphere. Satellite cloud observations, however, indicate important longitudinal variations near the Equator. During the abnormal rainy season of 1965–66 at Canton Island, the amount of cloudiness remained low over the eastern equatorial Pacific despite above-normal sea-surface temperatures. This suggests a continuation of the widespread subsidence usually present over this region, which is apparently part of a large-scale semipermanent zonal circulation. Satellite observations further show that there are three major “centers of action” (standing eddies) in the vicinity of the Equator. Probably the major part of the condenstion heating necessa...