Chi-Cherng Hong

Causes of the El Niño and La Niña Amplitude Asymmetry in the Equatorial Eastern Pacific

Abstract The amplitude asymmetry between El Nino and La Nina is investigated by diagnosing the mixed-layer heat budget during the ENSO developing phase by using the three ocean assimilation products: Simple Ocean Data Assimilation (SODA) 2.0.2, SODA 1.4.2, and the Global Ocean Data Assimilation System (GODAS). It is found that the nonlinear zonal and meridional ocean temperature advections are essential to cause the asymmetry in the far eastern Pacific, whereas the vertical nonlinear advection has the opposite effect. The zonal current anomaly is dominated by the geostrophic current in association with the thermocline depth variation. The meridional current anomaly is primarily attributed to the Ekman current driven by wind stress forcing. The resulting induced anomalous horizontal currents lead to warm nonlinear advection during both El Nino and La Nina episodes and thus strengthen (weaken) the El Nino (La Nina) amplitude. The convergence (divergence) of the anomalous geostrophic mixed-layer currents dur...

Asymmetry of the Indian Ocean Dipole. Part I: Observational Analysis*

Abstract 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 se...

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.

The Extreme Cold Anomaly over Southeast Asia in February 2008: Roles of ISO and ENSO*

A record-breaking, long-persisting extreme cold anomaly (ECA) over Southeast Asia, accompanied by an intraseasonal convection over the Maritime Continent, is identified during the La Nina mature phase in February 2008. The cause of the ECA, in particular the role of the intraseasonal oscillation (ISO) and El Nino-Southern Oscillation (ENSO) on the ECA, is investigated by diagnosing observations and con- ducting numerical experiments. The ECA is associated with an enhanced prolonged Siberian high (SH) and a persistent northerly anomaly over Southeast Asia. In contrast to conventional cold surges, which are characterized by a synoptic time scale (less than 10 days), the northerly anomaly associated with the ECA persisted for a month or so. The onset of the northerly anomalyis concurrent with a phase change of an ISO over Sumatra. Unlike the normal ISO that continues its eastward journey, the convection associated with this ISO stationed there during all of February 2008. Numerical experiments with an anomaly atmospheric GCM suggest that the ISO heating over the Maritime Continent is responsible for initiating and maintaining the northerly anomaly. The westward progression of the La Nina is crucial for blocking the ISO. The circulation and SST anomalies associated with the La Nina moved westward at a speed of about 158 longitude per month. By early February, the suppressed convective anomaly had moved to the far western Pacific. The westward shift of the cold episode prevented the ISO from moving farther eastward. In addition to its blocking effect, the La Nina also enhanced the heating over the Maritime Continent through the anomalous Walker circulation. Therefore, it is the combined effect of the ISO and ENSO that maintained a prolonged positive heating anomaly, which resulted in a persistent northerly anomaly and thus the ECA.

Asymmetry of the Indian Ocean Dipole. Part II: Model Diagnosis*

In this second part of a two-part paper, the mechanism for the amplitude asymmetry of SST anomalies (SSTA) between positive and negative Indian Ocean dipole (IOD) events is investigated through the diagnosis of coupled model simulations. Same as the observed in Part I, a significant negative skewness appears in the IOD east pole (IODE) in September–November (SON), whereas there is no significant skewness in the IOD west pole (IODW). A sensitivity experiment shows that the negative skewness in IODE appears even in the case when the ENSO is absent. The diagnosis of the model mixed layer heat budget reveals that the negative skewness is primarily induced by the nonlinear ocean temperature advection and the asymmetry of the cloud–radiation–SST feedback, consistent with the observation (Part I). However, the simulated latent heat flux anomaly is greatly underestimated in IODE during the IOD developing stage [June–September (JJAS)]. As a result, the net surface heat flux acts as strong thermal damping. The underestimation of the latent heat flux anomaly in the IODE is probably caused by the westward shift of along-coast wind anomalies off Sumatra.

Asymmetry of the Indian Ocean Basinwide SST Anomalies: Roles of ENSO and IOD*

Abstract A basinwide warming (cooling) in the Indian Ocean is observed following the El Nino (La Nina) mature phase, with the amplitude of the warming being significantly larger than the cooling. A composite analysis reveals that the amplitude asymmetry (positive skewness) between the warm and cold Indian Ocean basinwide sea surface temperature anomaly pattern (IOB) appears only when ENSO is concurrent with the Indian Ocean dipole (IOD). The amplitude asymmetry becomes insignificant during the ENSO-only and the IOD-only events. The physical mechanism for the amplitude asymmetry is investigated by analyzing the mixed layer heat budget based on the Simple Ocean Data Assimilation (SODA) 2.0.2 data. It is found that the positive skewness in the IOD west pole (IODW) is mainly caused by the asymmetry of ocean temperature advection, whereas the positive skewness in the IOD east pole (IODE) is caused by the asymmetry of the surface heat flux anomaly (primarily shortwave radiation) in response to the ENSO remote f...

Enhanced relationship between the tropical Atlantic SST and the summertime western North Pacific subtropical high after the early 1980s

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 Nino–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.

Influence of climate regime shift on the interdecadal change in tropical cyclone activity over the Pacific Basin during the middle to late 1990s

Abstract In this study, a new interpretation is proposed for the abrupt decrease in tropical cyclone (TC) activity in the western North Pacific (WNP) after the late 1990s. We hypothesize that this abrupt change constitutes a part of the phenomenon of interdecadal change in TC activity in the Pacific Basin, including the WNP, western South Pacific (WSP), and eastern North Pacific. Our analysis revealed that the climate-regime shift (CRS) in the Pacific during the middle to late 1990s resulted in a La Niña-like mean state, which was responsible for the interdecadal change in TC activity in the late 1990s. Analyses of the TC genesis potential index and numerical experiments revealed that the decline in TC activity in both the WNP and WSP was primarily attributable to the increase of vertical wind shear in the central Pacific due to the La Niña-like associated cold sea surface temperature (SST). Conversely, the La Niña-like associated warm SST in the western Pacific produced anomalous vertical transport of water vapor, increasing moisture levels in the mid-troposphere and TC activity in the western WNP. Furthermore, the CRS modified the mean TC genesis position and shifted the steering flow to the west, resulting in the increased frequency of TC landfalls in Taiwan, southeastern China, and northern Australia after the late 1990s.

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.