Jong-Seong Kug

Two Types of El Niño Events: Cold Tongue El Niño and Warm Pool El Niño

Abstract In this study, two types of El Nino events are classified based on spatial patterns of the sea surface temperature (SST) anomaly. One is the cold tongue (CT) El Nino, which can be regarded as the conventional El Nino, and the other the warm pool (WP) El Nino. The CT El Nino is characterized by relatively large SST anomalies in the Nino-3 region (5°S–5°N, 150°–90°W), while the WP El Nino is associated with SST anomalies mostly confined to the Nino-4 region (5°S–5°N, 160°E–150°W). In addition, spatial patterns of many atmospheric and oceanic variables are also distinctively different for the two types of El Nino events. Furthermore, the difference in the transition mechanism between the two types of El Nino is clearly identified. That is, the discharge process of the equatorial heat content associated with the WP El Nino is not efficient owing to the spatial structure of SST anomaly; as a result, it cannot trigger a cold event. It is also demonstrated that zonal advective feedback (i.e., zonal adve...

Interactive Feedback between ENSO and the Indian Ocean

Abstract A feedback process of the Indian Ocean SST on ENSO is investigated by using observed data and atmospheric GCM. It is suggested that warming in the Indian Ocean produces an easterly wind stress anomaly over Indonesia and the western edge of the Pacific during the mature phase of El Nino. The anomalous easterly wind in the western Pacific during El Nino helps a rapid termination of El Nino and a fast transition to La Nina by generating upwelling Kelvin waves. Thus, warming in the Indian Ocean, which is a part of the El Nino signal, operates as a negative feedback mechanism to ENSO. This Indian Ocean feedback appears to operate mostly for relatively strong El Ninos and results in a La Nina one year after the mature phase of the El Nino. This 1-yr period of phase transition implies a possible role of Indian Ocean–ENSO coupling in the biennial tendency of the ENSO. Atmospheric GCM experiments show that Indian Ocean SST forcing is mostly responsible for the easterly wind anomalies in the western Pacific.

Warm Pool and Cold Tongue El Niño Events as Simulated by the GFDL 2.1 Coupled GCM

Recent studies report that two types of El Nino events have been observed. One is the cold tongue (CT) El Nino, which is characterized by relatively large sea surface temperature (SST) anomalies in the eastern Pacific, and the other is the warm pool (WP) El Nino, in which SST anomalies are confined to the central Pacific. Here, both types of El Nino events are analyzed in a long-term coupled GCM simulation. The present model simulates the major observed features of both types of El Nino, incorporating the distinctive patterns of each oceanic and atmospheric variable. It is also demonstrated that each type of El Nino has quite distinct dynamic processes, which control their evolutions. The CT El Nino exhibits strong equatorial heat discharge poleward and thus the dynamical feedbacks control the phase transition from a warm event to a cold event. On the other hand, the discharge process in the WP El Nino is weak because of its spatial distribution of ocean dynamic field. The positive SST anomaly of WP El Nino is thermally damped through the intensified evap- orative cooling.

How well do current climate models simulate two types of El Nino

In this study, we evaluate the fidelity of current climate models in simulating the two types of El Nino events using the pre-industrial output in CMIP3 archives. It is shown that a few climate models simulate the two types of El Nino events to some extent, while most of the models have serious systematic problems in simulating distinctive patterns of sea-surface temperature (SST) and precipitation anomaly associated with the two types of El Nino; that is, they tend to simulate a single type of El Nino. It is shown that the ability of climate models in simulating the two types of El Nino is related to the sensitivity of the atmospheric responses to the SST anomaly patterns. Models whose convective location is shifted to the east (west) as the SST anomaly center moves to the east (west) tends to simulate the two types of El Nino events successfully. On the other hand, models whose location of convective anomaly is confined over the western or central Pacific tends to simulate only the single type of El Nino event. It is also shown that the confinement of the convective anomaly over the western or central Pacific is closely linked to the dry bias and the associated cold bias over the eastern Pacific. That is, because positive El Nino SST anomalies over the eastern Pacific cannot increase local convection effectively when the total SSTs are still too cold due to a cold bias. This implies that the realistic simulation of climatology, especially over the equatorial eastern Pacific, is essential to the successful simulation of the two types of El Nino.

Propagating versus Nonpropagating Madden–Julian Oscillation Events

AbstractBasinwide convective anomalies over the Indian Ocean (IO) associated with the Madden–Julian oscillation (MJO) sometimes propagate eastward and reach the west Pacific (WP), but sometimes do not. Long-term observations and reanalysis products are used to investigate the difference between the propagating and nonpropagating MJO events. IO convection onset events associated with the MJO are grouped into three categories based on the strengths of the simultaneous dry anomalies over the eastern Maritime Continent and WP. The IO convection anomaly preferentially makes eastward propagation and reaches the WP when the dry anomaly is stronger.Analysis of the column-integrated moist static energy (MSE) budget shows that horizontal advection moistens the atmosphere to the east of the positive MSE anomaly associated with the active convection over the IO and is of sufficient magnitude to explain the eastward propagation of the positive MSE anomaly. Interpretation is complicated, however, by lack of closure in ...

Role of the ENSO–Indian Ocean coupling on ENSO variability in a coupled GCM

warming are only confined to the IO, and it has little impact on the tropical Pacific. During the boreal winter when the El Nino is in its mature phase, the anomalous warming extends eastward to cover the entire tropical IO, and the anomalous easterlies extend into the Western Pacific (WP). The anom- alous easterlies in the WP generate oceanic upwelling Kelvin waves, which propagate eastward and accelerate the decay of the warm SST in the eastern Pacific. As a result, the El Nino is rapidly terminated, and a La Nina develops within one year from the El Nino mature phase. They concluded that the interaction between ENSO and the IO may generate a biennial tendency for ENSO. (4) The samples used for the composite analysis by KK06 are rather limited, due to the relatively short length of the observational data. In this study, we intended to analyze a longer (200-yr) data set obtained from the simulation of a SINTEX-F (Scale INTeraction EXperiment-FRCGC) cou- pled GCM in order to verify the ENSO-IO feedback hypoth- esis.ItturnsoutthatthefeedbackprocessbetweenENSOand the IO is quite plausible not only from observations but also from the state-of-the-art coupled GCM.

Recent recovery of the Siberian High intensity

This study highlights the fast recovery of the wintertime Siberian High intensity (SHI) over the last two decades. The SHI showed a marked weakening trend from the 1970s to 1980s, leading to unprecedented low SHI in the early 1990s according to most observational data sets. This salient declining SHI trend, however, was sharply replaced by a fast recovery over the last two decades. Since the declining SHI trend has been considered as one of the plausible consequences of climate warming, the recent SHI recovery seemingly contradicts the continuous progression of climate warming in the Northern Hemisphere. We suggest that alleviated surface warming and decreased atmospheric stability in the central Siberia region, associated with an increase in Eurasian snow cover, in the recent two decades contributed to this rather unexpected SHI recovery. The prominent SHI change, however, is not reproduced by general circulation model (GCM) simulations used in the IPCC AR4. The GCMs indicate the steady weakening of the SHI for the entire 21st century, which is found to be associated with a decreasing Eurasian snow cover in the simulations. An improvement in predicting the future climate change in regional scale is desirable.

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...

El Niño–La Niña Asymmetry in the Coupled Model Intercomparison Project Simulations*

The El Nino–La Nina asymmetry was estimated in the 10 different models participating in the Coupled Model Intercomparison Project (CMIP). Large differences in the “asymmetricity” (a variance-weighted skewness) of SST anomalies are found between models and observations. Most of the coupled models underestimate the nonlinearity and only a few exhibit the positively skewed SST anomalies over the tropical eastern Pacific as seen in the observation. A significant association between the nonlinear dynamical heating (NDH) and asymmetricity in the model–ENSO indices is found, inferring that asymmetricity is caused mainly by NDH. Among the 10 models, one coupled GCM simulates the asymmetricity of the tropical SST realistically, and its simulation manifests a strong relationship between the intensity and the propagating feature of ENSO—the strong ENSO events moving eastward and the weak ENSO events moving westward—which is consistent with the observation. Interestingly, the coupled general circulation models, of which the ocean model is based on the one used by Bryan and Cox, commonly showed the reasonably positive skewed ENSO. The decadal changes in the skewness, variance, and NDH of the model-simulated ENSO are also observed. These three quantities over the tropical eastern Pacific are significantly correlated to each other, indicating that the decadal change in ENSO variability is closely related to the nonlinear process of ENSO. It is also found that these decadal changes in ENSO variability are related to the decadal variation in the tropical Pacific SST, implying that the decadal change in the El Nino–La Nina asymmetry could manifest itself as a rectified change in the background state.