Unprecedented atmospheric circulations with extreme weather were observed in the extratropical Northern Hemisphere during the winter of 2013-2014. The anomalous circulations were the manifestation of the Pacific pattern or the North Pacific Oscillation/Western Pacific pattern but with extremely large amplitude. Simulation results suggest that the anomalous atmospheric circulations were constructively induced by anomalous sea surface temperature in the tropical Pacific and extratropical North Pacific, as well as the low sea ice concentration in the Arctic. Natural variability played a major role in inducing the anomaly pattern, whereas the anomalously warm sea surface temperature and low Arctic sea ice concentration in the Bering Sea contributed to the intensity. If the anthropogenic warming has a significant impact on causing the synchronization of the aforementioned anomalies in sea surface temperature and sea ice concentration and this trend continues, severe winters similar to that in 2013-2014 may occur more frequently in the future.
A sequence of monsoon surges struck Pakistan and Northwestern India during late July‐early August 2010. The unusually heavy monsoon rainfall resulted in record‐breaking floods, which affected 20 million people with a death toll of near 3000. Simultaneously, a long‐lived blocking high appeared over Europe and Russia in middle June and persisted for nearly two months. Extreme flooding occurred when the southward penetration of extratropical potential vorticity in the deep trough east of the European blocking and the tropical monsoon surges arrived concurrently in Pakistan. This study demonstrates that the interaction between the tropical monsoon surges and the extratropical disturbances downstream of the European blocking was the key factor leading to the severe flooding in Pakistan. The 2010 La Niña condition contributed indirectly to the flooding by inducing a low‐level easterly anomaly in South and Southeast Asia, which weakened eastward moisture transport and helped enhance moisture transport (convergence) to (in) the Northern Arabian Sea and Pakistan.
[1] The differences in the temporal evolution and spatial characteristics of the Indian Ocean Dipole (IOD) between positive and negative events with and without ENSO have been investigated using observations for the period . To document such differences is particularly important for climate forecasts over far east Asia, since distinctly different monsoon activities over China, Korea, and Japan for different types of IOD are found in the composite maps of precipitation anomalies. The composite map of SST and wind during various stages of IOD and the ocean mixed layer heat budget showed that the IOD with and without ENSO has a large difference in its temporal evolutions and their triggering mechanisms. In both negative and positive IOD events without ENSO, the wind anomaly in the eastern Indian Ocean seems to be responsible for the formation of sea surface temperature anomalies, while the anomaly in the western Indian Ocean seems to be the oceanic dynamical response to the anomaly in the east. During the ENSO years, the temporal and spatial contrast of the asymmetry of the IOD evolution is smaller, and the SST anomaly is driven by the anomalies in incoming radiation due to changes in cloudiness caused by the ENSO associated anomalous atmospheric circulations and not by the local wind anomalies.
The western North Pacific subtropical high (WNPSH) in boreal summer shows a remarkable enhancement after the early 1980s. Whereas the sea surface temperature (SST) in the North Indian Ocean (NIO) and the equatorial eastern Pacific had been noted to have remarkable local or remote effects on enhancing the WNPSH, the influence of the Atlantic SST, so far, is hardly explored. This article reports a new finding: enhanced relationship between the tropical Atlantic (TA)-SST and the WNPSH after the early 1980s. Regression study suggests that the warm TA-SST produced a zonally overturning circulation anomaly, with descending over the equatorial central Pacific and ascending over the tropical Atlantic/eastern Pacific. The anomalous descending over the equatorial central Pacific likely induced low-level anticyclonic anomaly to the west and therefore enhanced the WNPSH. One implication of this new finding is for predictability. The well-known "spring predictability barrier" (i.e., the influence of El Niño-Southern Oscillation (ENSO) falls dramatically during boreal spring) does not apply to the TA-SST/WNPSH relationship. The TA-SST shows consistently high correlation starting from boreal spring when the ENSO influence continues declining. The TA-SST extends the predictability of the WNPSH in boreal summer approximately one season earlier to boreal spring.
The physical mechanism for the amplitude asymmetry of SST anomalies (SSTA) between the positive and negative phases of the Indian Ocean dipole (IOD) is investigated, using Simple Ocean Data Assimilation (SODA) and NCAR-NCEP data. It is found that a strong negative skewness appears in the IOD east pole (IODE) in the mature phase [September-November (SON)], while the skewness in the IOD west pole is insignificant. Thus, the IOD asymmetry is primarily caused by the negative skewness in IODE.A mixed-layer heat budget analysis indicates that the following two air-sea feedback processes are responsible for the negative skewness. The first is attributed to the asymmetry of the wind stress-ocean advection-SST feedback. During the IOD developing stage [June-September (JJAS)], the ocean linear advection tends to enhance the mixed-layer temperature tendency, while nonlinear advection tends to cool the ocean in both the positive and negative events, thus contributing to the negative skewness in IODE. The second process is attributed to the asymmetry of the SST-cloud-radiation (SCR) feedback. For a positive IODE, the negative SCR feedback continues with the increase of warm SSTA. For a negative IODE, the same negative SCR feedback works when the amplitude of SSTA is small. After reaching a critical value, the cold SSTA may completely suppress the mean convection and lead to cloud free conditions; a further drop of the cold SSTA does not lead to additional thermal damping so that the cold SSTA may grow faster. A wind-evaporation-SST feedback may further amplify the asymmetry induced by the aforementioned nonlinear advection and SCR feedback processes.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.