Ming-Ying Lee

Compounding effects of warm sea surface temperature and reduced sea ice on the extreme circulation over the extratropical North Pacific and North America during the 2013–2014 boreal winter

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.

Topographic Effects on the Eastward Propagation and Initiation of the Madden-Julian Oscillation

Abstract This study investigates the relationship between deep convection (and heating anomaly) in the Madden–Julian oscillation (MJO) and the tropical topography. The eastward propagation of the deep heating anomalies is confined to two regions: the Indian Ocean and the western Pacific warm pool. Superimposed on the eastward propagation is a series of quasi-stationary deep heating anomalies that occur sequentially and discretely downstream in a leapfrog manner in the central Indian Ocean, the Maritime Continent, tropical South America, and tropical Africa. The deep heating anomaly, usually preceded by near-surface moisture convergence and shallow heating anomalies, tends to occur on the windward side of the tropical topography in these regions (except the central Indian Ocean) under the prevailing surface easterly anomaly of the MJO. It is suggested that the lifting and frictional effects of the tropical topography and landmass induce the near-surface moisture convergence anomaly, which in turn triggers ...

Impacts of central Pacific and eastern Pacific El Niños on tropical cyclone tracks over the western North Pacific

[1]xa0This study examines the different impacts of two types of El Ninos, the eastern Pacific El Nino (EP-EN) and the central Pacific El Nino (CP-EN), on tropical cyclone (TC) tracks over the western North Pacific (WNP) based on observational data. Whereas TC tracks between CP-EN and EP-EN show a small difference in boreal summer (JJA), they do exhibit a great difference in boreal autumn (SON), that is, TCs recurve northward at a further westward location near the coastline of East Asia during CP-EN. As a consequence, more TCs make landfall to Taiwan and South China during CP-EN. A further observational analysis indicates that the westward shift of the subtropical high and associated steering flow during CP-EN is a key factor that causes the difference in the TC tracks in autumn. Numerical experiments further suggest that the difference of local SST in the WNP between CP-EN and EP-EN accounts for the distinctive differences in the local Hadley circulation, the subtropical high and the TC steering flow.

Tropical SST forcing on the anomalous WNP subtropical high during July–August 2010 and the record-high SST in the tropical Atlantic

Abstract In summer of 2010, the western North Pacific subtropical high (WNPSH) was extremely strong and exhibited unusual westward extension, which resulted in record-breaking warmth in Japan and considerably below-normal and westward-shifted tropical cyclone activity in the western North Pacific (WNP). Although a moderate La Niña occurred, the sea surface temperature (SST) in the northern Indian Ocean (NIO) and tropical Atlantic (TA) was considerably high. In this study, we argued that the La Niña cold SST alone was not sufficient to maintain the strong WNPSH of 2010, and that the unusually warm SSTs in the NIO and TA markedly contributed to the enhancement and westward shift of the WNPSH in the boreal summer of that year. We focused on the effects of sea surface temperature anomalies in the tropical Atlantic (TA-SSTAs), which have been seldom explored and are poorly understood compared with the effects of SSTAs in the tropical Pacific and NIO. The warm TA-SST forced a westward-extending overturning circulation, with a sinking branch over the central Pacific Ocean, which produced a remote response similar to the La Niña condition and enhanced the WNPSH. The warm TA-SST also induced the cyclonic anomaly in the tropical eastern North Pacific, a distinct phenomenon not observed in a canonical La Niña event. Furthermore, we demonstrated that the anomalous near-surface circulation associated with the negative North Atlantic Oscillation might play a more dominant role than that of the 2009 El Niño in inducing the record-high SST in the TA in 2010.

Extratropical Forcing Triggered the 2015 Madden–Julian Oscillation–El Niño Event

In this paper, we report the triggering effect of extratropical perturbation on the onset of an atypical Madden–Julian Oscillation (MJO) and onset of the 2015–16 El Niño in March 2015. The MJO exhibited several unique characteristics: the effect of extratropical forcing, atypical genesis location and timing in the equatorial western Pacific, and the extremity of amplitudes in many aspects. The southward-penetrating northerly associated with the extratropical disturbances in the extratropical western North Pacific contributed to triggering the deep convection and westerly wind burst (WWB) and onset of the MJO over the anomalously warm tropical western Pacific in early March. The persisting strong WWB forced downwelling Kelvin wave-like oceanic perturbation that propagated eastward and led to the onset of the 2015–16 El Niño. The proposed novel extratropical forcing mechanism explaining the unique extratropics–MJO–El Niño association, based on both data diagnostics and numerical experiments, warrants further attention for a more detailed understanding of the onset of the MJO and its potential effect on El Niño.

Accelerated increase in the Arctic tropospheric warming events surpassing stratospheric warming events during winter

In January 2016, a robust reversal of the Arctic Oscillation took place associated with a rapid tropospheric warming in the Arctic region; this was followed by the occurrence of a classic sudden stratospheric warming in March. The succession of these two distinct Arctic warming events provides a stimulating opportunity to examine their characteristics in terms of similarities and differences. Historical cases of these two types of Arctic warming were identified and validated based upon tropical linkages with the Madden-Julian Oscillation and El Nino as documented in previous studies. The analysis indicates a recent and seemingly accelerated increase in the tropospheric warming type versus a flat trend in stratospheric warming type. The shorter duration and more rapid transition of tropospheric warming events may connect to the documented increase in midlatitude weather extremes, more so than the route of stratospheric warming type. Forced simulations with an atmospheric general circulation model suggest that the reduced Arctic sea ice contributes to the observed increase in the tropospheric warming events and associated remarkable strengthening of the cold Siberian high manifest in 2016.

Compounding factors causing the unusual absence of tropical cyclones in the western North Pacific during August 2014

The western North Pacific (WNP) is a region where tropical cyclones (TCs) occur most frequently. However, no TCs occurred in August, climatologically the month of most frequent TC-genesis, in 2014. Such absence of TC activity in August was approximately 3σ below the climatological mean (5.1) and occurred for the first time during 1945–2014. This study investigates the large-scale factors responsible for the absence. While July-August 2014 was a period of weak El Nino condition, rainfall in the eastern North Pacific was unusually high, exceeding the 95th percentile for the period 1979–2014. We demonstrate that the unusually positive sea surface temperature anomaly in the eastern North Pacific (ENP) resulted in such high rainfall amount. Diabatic heating associated with the unusual rainfall resulted in a Walker circulation anomaly in the Pacific, which in turn led to anomalous downward motion in the West Pacific and suppressed TC genesis in the WNP. Moreover, an eastward propagating intraseasonal oscillation (ISO) was identified in the Indo-Pacific in August. The dry-phase ISO induced subsidence anomaly in the WNP. The combination of the dry ISO phase in the WNP and a west-west overturning circulation anomaly triggered by the warm SST in the ENP led to extremely dry and warm conditions and the absence of TCs in the WNP. Both empirical diagnostics and numerical simulations confirm this result.

Distinct influences of the ENSO-like and PMM-like SST anomaly on the mean TC genesis location in the western North Pacific: The 2015 summer as an extreme example

AbstractThis study reports the different effects of tropical and subtropical sea surface temperature anomalies (SSTAs) on the mean tropical cyclone (TC) genesis location in the western North Pacifi...