Sang-Wook Yeh

Changes in the Tropical Pacific SST Trend from CMIP3 to CMIP5 and Its Implication of ENSO

This study assesses the changes in the tropical Pacific Ocean sea surface temperature (SST) trend and ENSO amplitude by comparing a historical run of the World Climate Research Programme Coupled Model Intercomparison Project (CMIP) phase-5 multimodel ensemble dataset (CMIP5) and the CMIP phase-3 dataset (CMIP3). The results indicate that the magnitude of the SST trend in the tropical Pacific basin has been significantly reduced from CMIP3 to CMIP5, which may be associated with the overestimation of the response to natural forcing and aerosols by including Earth system models in CMIP5. Moreover, the patterns

Combined effect of El Niño-Southern Oscillation and Pacific Decadal Oscillation on the East Asian winter monsoon

Using long-term observational data and numerical model experiments, the combined effect of the El Niño-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) on the variability of the East Asian winter monsoon is examined. In the observations, it is found that when the ENSO and PDO are in-phase combinations (i.e., El Niño/positive PDO phase and La Niña/negative PDO phase), a negative relationship between ENSO and East Asian winter monsoon is significantly intensified. In other words, when El Niño (La Niña) occurs with positive (negative) PDO phase, anomalous warm (cold) temperatures are dominant over the East Asian winter continent. On the other hand, there are no significant temperature anomalies when the ENSO and PDO are out-of-phase combinations (i.e., El Niño/negative PDO phase and La Niña/positive PDO phase). Further analyses indicate that the anticyclone over the western North Pacific including the East Asian marginal seas plays an essential role in modulating the intensity of the East Asian winter monsoon under the changes of ENSO–PDO phase relationship. Long-lasting high pressure and warm sea surface temperature anomalies during the late fall/winter and following spring over the western North Pacific, which appear as the El Niño occurs with positive PDO phase, can lead to a weakened East Asian winter monsoon by transporting warm and wet conditions into the East Asian continent through the southerly wind anomalies along the western flank of the anomalous high pressure, and vice versa as the La Niña occurs with negative PDO phase. In contrast, the anomalous high pressure over the western North Pacific does not show a prominent change under the out-of-phase combinations of ENSO and PDO. Numerical model experiments confirm the observational results, accompanying dominant warm temperature anomalies over East Asia via strong anticyclonic circulation anomalies near the Philippine Sea as the El Niño occurs with positive PDO phase, whereas such warming is weakened as the El Niño occurs with negative PDO phase. This result supports the argument that the changes in the East Asian winter monsoon intensity with ENSO are largely affected by the strength of the anticyclone over the western North Pacific, which significantly changes according to the ENSO–PDO phase relationship.

The North Pacific Climate Transitions of the Winters of 1976/77 and 1988/89

Abstract This paper examines characteristic changes in North Pacific sea surface temperature (SST) variability during the boreal winter (December–February) for two subperiods (1956–88 and 1977–2009) during which the 1976/77 and the 1988/89 climate transitions occurred. It is found that the Pacific decadal oscillation (PDO)-like SST variability plays a dominant role in the 1976/77 climate transition, while both the North Pacific Gyre Oscillation (NPGO)-like and PDO-like SST variability contribute to the 1988/89 climate transition. Furthermore, the leading mode changes from PDO-like SST variability during the period 1956–88 to NPGO-like SST variability during the period 1977–2009, indicative of an enhancement of NPGO-like SST variability since 1988. Changes in sea level pressure across the 1976/77 climate transition project strongly onto the Aleutian low pressure system. But sea level pressure changes across the 1988/89 climate transition project primarily onto the North Pacific Oscillation, which is associ...

Influence of the Pacific Decadal Oscillation on the Relationship between El Niño and the Northeast Asian Summer Monsoon

Abstract The influence of the Pacific decadal oscillation (PDO) on the relationship between El Nino and the northeast Asian summer monsoon (NEASM) is examined using observational datasets for the period of 1979–2007. When El Nino occurs during the boreal winter (December–February), the amount of rainfall over northeast Asia is usually above normal during the following summer (June–August). This relationship between El Nino and the NEASM is intensified when El Nino and the PDO are in phase during the previous winter. However, when El Nino and the PDO are out of phase, the relationship is weakened. The authors argue that the PDO can constructively or destructively interfere with the summer rainfall response over northeast Asia to El Nino. They follow the hypothesis that the summer rainfall over northeast Asia could be separated into two components, that is, the tropics-related component and the extratropics-related component. Then they argue that the PDO could modulate the relationship between El Nino and t...

The Influence of ENSO on Decadal Variations in the Relationship between the East Asian and Western North Pacific Summer Monsoons

Abstract A recent study suggested that the relationship between the East Asian summer monsoon (EASM) and the western North Pacific summer monsoon (WNPSM) experienced a decadal change around 1993–94. Based on a longer-term integration of a hybrid coupled model, the present study investigates decadal variations in the relationship between the EASM and the WNPSM. Apparent decadal variations in the above relationship have been identified in the model simulation. The authors have analyzed the spatial pattern and variability during strong and weak EASM–WNPSM correlation periods. The purpose of this study is to understand potential reasons for decadal variations in the relationship between the two submonsoons. It is found that the precipitation variability associated with the WNPSM (ENSO) is enhanced over the East Asia and western North Pacific regions during periods when the EASM–WNPSM relationship is strong (weak). The large variability in precipitation associated with the WNPSM during strong periods strengthe...

On the Relationship between the North Pacific Climate Variability and the Central Pacific El Niño

AbstractThis study examined connections between the North Pacific climate variability and occurrence of the central Pacific (CP) El Nino for the period from 1950 to 2012. A composite analysis indicated that the relationship between the North Pacific sea surface temperature (SST), along with its overlying atmospheric circulation, and the CP El Nino during the developing and mature phases was changed when the occurrence frequency of the CP El Nino significantly increased after 1990. Empirical orthogonal function (EOF) and singular value decomposition (SVD) analyses of variability in the tropical Pacific and its relationship to the North Pacific show that the North Pacific anomalous SST and the atmospheric variability are more closely associated with the occurrence of the CP El Nino after 1990 than before 1990. There were noticeable differences in terms of the atmospheric variability conditions over the North Pacific, such as the North Pacific Oscillation (NPO)-like atmospheric variability during the spring ...

Rectification of ENSO variability by interdecadal changes in the equatorial background mean state in a CGCM simulation

The link between the changes in equatorial background stratification and El Nino-Southern Oscillation (ENSO) modulation is investigated using a simulation from a 260-yr-long coupled general circulation model (CGCM). The work focuses on the role of nonlinearities associated with equatorial wave dynamics. As a first step, the low-frequency change in mean stratification is diagnosed and documented from the shallow-water parameters derived from a vertical mode decomposition of the CGCM. The parameters vary differently according to the baroclinic mode order, which may explain why a flattening thermocline does not necessarily lead to reduced ENSO activity. Estimations of baroclinic mode contributions to zonal current anomalies indicate that the decadal variability projects differently for the baroclinic modes as compared to the interannual variability. In particular, the high-order modes associated with decadal variability have a more pronounced signature in the western Pacific, whereas that associated with interannual variability (i.e., ENSO) shows more energy in the eastern Pacific. In the light of the results of the CGCM vertical mode decomposition, an intermediate coupled model (ICM) is used to test whether the nonlinearities associated with the changes in the baroclinic mode energy distribution can lead to coherent ENSO modulation. The results indicate that rectification of the interannual variability (ENSO time scales) by the interdecadal variability associated with changes in the oceanic mean states takes place in the ICM. The rectified effect results mostly in an increased variability and skewness of the zonal advection, which tends to produce a zonal seesaw of the sea surface temperature anomaly. A tropical mechanism for producing ENSO modulation is then proposed that reconciles both the rectified effect resulting from nonlinearities associated with equatorial wave dynamics and the tropical decadal mode of thermocline depth arising from Ekman-pumping anomalies located in the central South Pacific.

The central Pacific as the export region of the El Niño‐Southern Oscillation sea surface temperature anomaly to Antarctic sea ice

[1] In the mature season of El Nino, Rossby waves do not easily propagate into the polar region, and the seasonal climatology of sea ice is minimal. Austral summer is a barrier to the persistent Antarctic dipole pattern (ADP) in sea ice. The sea surface temperature (SST) anomaly of central Pacific type El Nino (CP‐El Nino) in the central Pacific contributes to a strong Rossby wave response and weakening of the polar‐front jet that yields strong ADP in austral spring just before the ADP barrier. The strong ADP produces intensive sea‐ice‐air feedback, which allows the ADP anomaly to breach the barrier. In the conventional El Nino (EP‐El Nino) events, the upper‐level structure cannot contribute to the strong anomalous high pressure. In EP‐El Nino events, the anomalous high in the Bellingshausen Sea is replaced by an anomalous low after the austral autumn following the mature season, whereas the anomalous high pressure persists up to the austral winter in the CP‐El Nino. In the CP‐El Nino, the ADP persists until austral winter after the mature season of El Nino, whereas, in the EP‐El Nino, it does not persist after austral summer. The central Pacific cold SST anomaly of La Nina together with the seasonal SST climatology prolongs the opposite phase of the ADP anomaly up to the austral winter. Consequently, the tropical climate anomaly is exported to extratropics at the central Pacific in the Southern Hemisphere.

Two types of strong Northeast Asian summer monsoon.

The characteristics of a strong northeast Asian summer monsoon (NEASM) with and without (A and B type, respectively) a basinwide warming in the Indian Ocean during the preceding winter are examined for the period of 1979-2006. In the case of the A type, strong El Nino-like sea surface temperature (SST) decays very rapidly from the preceding winter (December-February) to the following summer (June-August), which may be due to a feedback process of the warm Indian Ocean. In addition, the A-type strong NEASM is more associated with a weak western North Pacific summer monsoon than the B-type strong NEASM. On the other hand, for the B type an El Nino-like SST during the preceding winter is a persistent influence into the following summer. A striking difference can be found in the atmospheric teleconnection pattern from the tropics to the midlatitudes over the Indo-Pacific region, that is, the Pacific-Japan-like pattern versus a pronounced Rossby wave train pattern. This may result from the difference in location of the maximum center of rainfall anomalies over the tropical northwestern Pacific between the two types of strong NEASM. The authors argue that Indian Ocean basin warming plays a role in modifying the convective system over the subtropical western Pacific, resulting in changes in atmospheric teleconnections between the two types of strong NEASM. The weak NEASM, in which the anomalous rainfall pattern resembles that of the A-type strong NEASM except for the sign, is also discussed.

Critical Role of Northern Off-Equatorial Sea Surface Temperature Forcing Associated with Central Pacific El Niño in More Frequent Tropical Cyclone Movements toward East Asia

AbstractObservational records reveal that the number of tropical cyclones (TCs) approaching East Asia in July–October is positively correlated with sea surface temperatures (SSTs) in the equatorial and northern off-equatorial central Pacific (CP) oceans, indicating the significant impact of CP El Nino (CP-EN). Through experiments using a Weather Research and Forecast (WRF) model–based regional climate model, this study demonstrates that it is northern off-equatorial CP warming, rather than equatorial CP warming, that effectively induces local anomalous steering flows pertinent to the observed increase in TC activity over East Asia during CP-EN. Sensitivity experiments, in which the prescribed CP-EN-related SST anomaly is confined near the equator, do not capture the observed TC increase over East Asia, whereas those including the off-equatorial region successfully reproduce observed atmospheric and TC variabilities. The off-equatorial CP SST anomaly acts to expand the anomalous cyclonic response in the Ph...