Yukiko Imada

Convective Control of ENSO Simulated in MIROC

Abstract The high sensitivity of the El Nino–Southern Oscillation (ENSO) to cumulus convection is examined by means of a series of climate simulations using an updated version of the Model for Interdisciplinary Research on Climate (MIROC), called MIROC5. Given that the preindustrial control run using MIROC5 shows a realistic ENSO, the integration is repeated with four different values of the parameter, λ, which affects the efficiency of the entrainment rate in cumuli. The ENSO amplitude is found to be proportional to λ−1 and to vary from 0.6 to 1.6 K. A comparison of four experiments reveals the mechanisms for which the cumulus convections control behavior of ENSO in MIROC as follows. Efficient entrainment due to a large λ increases congestus clouds over the intertropical convergence zone (ITCZ) and reduces the vertical temperature gradient over the eastern Pacific, resulting in a wetter ITCZ and drier cold tongue via accelerated meridional circulation. The dry cold tongue then shifts the atmospheric resp...

Over 5,000 Years of Ensemble Future Climate Simulations by 60-km Global and 20-km Regional Atmospheric Models

AbstractAn unprecedentedly large ensemble of climate simulations with a 60-km atmospheric general circulation model and dynamical downscaling with a 20-km regional climate model has been performed to obtain probabilistic future projections of low-frequency local-scale events. The climate of the latter half of the twentieth century, the climate 4 K warmer than the preindustrial climate, and the climate of the latter half of the twentieth century without historical trends associated with the anthropogenic effect are each simulated for more than 5,000 years. From large ensemble simulations, probabilistic future changes in extreme events are available directly without using any statistical models. The atmospheric models are highly skillful in representing localized extreme events, such as heavy precipitation and tropical cyclones. Moreover, mean climate changes in the models are consistent with those in phase 5 of the Coupled Model Intercomparison Project (CMIP5) ensembles. Therefore, the results enable the a...

Predictability of Two Types of El Niño Assessed Using an Extended Seasonal Prediction System by MIROC

AbstractPredictability of El Nino–Southern Oscillation (ENSO) is examined using ensemble hindcasts made with a seasonal prediction system based on the atmosphere and ocean general circulation model, the Model for Interdisciplinary Research on Climate, version 5 (MIROC5). Particular attention is paid to differences in predictive skill in terms of the prediction error for two prominent types of El Nino: the conventional eastern Pacific (EP) El Nino and the central Pacific (CP) El Nino, the latter having a maximum warming around the date line. Although the system adopts ocean anomaly assimilation for the initialization process, it maintains a significant ability to predict ENSO with a lead time of more than half a year. This is partly due to the fact that the system is little affected by the “spring prediction barrier,” because increases in the error have little dependence on the thermocline variability. Composite analyses of each type of El Nino reveal that, compared to EP El Ninos, the ability to predict C...

Control of Decadal and Bidecadal Climate Variability in the Tropical Pacific by the Off-Equatorial South Pacific Ocean

AbstractDelayed negative feedback processes determining intrinsic decadal and bidecadal time scales for the tropical variability in the Pacific are investigated based on climate model experiments. By comparing a control run driven by preindustrial forcing and partial blocking runs driven by the same forcing but with ocean temperature and salinity restored to climatology in selected regions, subsurface oceanic signals of South Pacific origin are shown to precede SST variability in the Nino-3.4 region. Using a linear reduced-gravity ocean model driven only by wind stress changes and an offline tracer model, oceanic wave adjustment triggered by changes of wind stress curl in the South Pacific extratropics is suggested to be essential for the decadal component of the equatorial SST, while slower isopycnal advection of subsurface temperature anomalies from the formation region of South Pacific Eastern Subtropical Mode Water controls the bidecadal component. The intrinsic time scales of the tropical variability...

Parameterization of Tropical Instability Waves and Examination of Their Impact on ENSO Characteristics

AbstractThe impact of tropical instability waves (TIWs) on El Nino–Southern Oscillation (ENSO) characteristics is investigated by introducing a new parameterization of TIWs into an atmosphere–ocean general circulation model (AOGCM), the Model for Interdisciplinary Research on Climate (MIROC), with a medium-resolution (~1.4°) ocean model (known as MIROCmedres). Because this resolution is not sufficient to reproduce eddies at the spatial scale of TIWs, this approach isolates TIW effects from other factors that can affect ENSO characteristics. The parameterization scheme represents the effect of baroclinic eddy heat transport by TIWs. A 100-yr integration reveals a significant role of TIWs in observed ENSO asymmetry. Asymmetric heat transport associated with TIWs that are active (inactive) during La Nina (El Nino) generates a significant asymmetric negative feedback to ENSO and explains the observed asymmetric feature of a stronger-amplitude El Nino and weaker-amplitude La Nina. Furthermore, the parameterize...

Impact of the Atmospheric Mean State on Tropical Instability Wave Activity

AbstractThe features of simulated tropical instability waves (TIWs) in the Pacific Ocean are compared between atmospheric models of two different resolutions coupled with a uniform oceanic model. Results show that TIWs are more active in the high-resolution model, even though it includes atmospheric negative feedback. Such negative feedback is not identified in the low-resolution atmospheric model because of the absence of atmospheric responses. Comparison of the energetics between the two models shows that the large TIW activity in the higher-resolution model is due to the difference in barotropic energy sources near the surface. A high-resolution atmosphere results in a tighter intertropical convergence zone and associated stronger wind curl and shear. This causes a stronger surface current shear between the South Equatorial Current (SEC) and North Equatorial Counter Current (NECC), which is one of the main sources of TIW kinetic energy. These results indicate the important role of the atmospheric mean ...

South Pacific influence on the termination of El Nino in 2014

The El Niño/Southern Oscillation (ENSO) is the dominant mode of climate variability affecting worldwide extreme weather events; therefore, improving ENSO prediction is an important issue. In this regard, a peculiar time evolution of ENSO in 2014 posed a challenge to the climate science community. Despite the observance of several precursors for a strong El Niño to develop during the summer and autumn, cold sea surface temperature (SST) anomalies appeared unexpectedly to the south of the equatorial cold tongue, which prevented development of an El Niño event in the late summer. Several hypotheses have been raised to explain the unmaterialized El Niño in 2014, but complete understanding of processes responsible for terminating this event has not yet been obtained. Here we show, using observations and extended seasonal prediction experiments with a climate model, that cold off-equatorial subsurface water in the South Pacific Ocean penetrated into the equatorial region along the slanted isopycnal surface via the mean advection, and it prevented the El Niño evolution in 2014. The negative subsurface temperature anomalies in the off-equatorial South Pacific Ocean were persistent throughout the last decade, and additional numerical simulations indicated that they contributed to the suppression of El Niño events during the 2000s.

Predictability of Persistent Thailand Rainfall during the Mature Monsoon Season in 2011 Using Statistical Downscaling of CGCM Seasonal Prediction

AbstractPredictability of above-normal rainfall over Thailand during the rainy season of 2011 was investigated with a one-tier seasonal prediction system based on an atmosphere–ocean coupled general circulation model (CGCM) combined with a statistical downscaling method. The statistical relationship was derived using singular value decomposition analysis (SVDA) between observed regional rainfall and the hindcast of tropical sea surface temperature (SST) from the seasonal prediction system, which has an ability to forecast oceanic variability for lead times up to several months. The downscaled product of 2011 local rainfall was obtained by combining rainfall patterns derived from significant modes of SVDA. This method has the advantage in terms of flexibility that phenomenon-based statistical relationships, such as teleconnections associated with El Nino–Southern Oscillation (ENSO), Indian Ocean dipole (IOD), or the newly recognized central Pacific El Nino, are considered separately in each SVDA mode. The ...

A Persistent Japanese Heat Wave in Early August 2015: Roles of Natural Variability and Human-Induced Warming

Introduction. A prolonged heat wave hit Japan in early August 2015. Daily maximum surface air temperature (SAT) exceeded 35°C for eight consecutive days and the 8-day mean anomaly was greater than 4°C at several observation sites, causing over 10 000 people to suffer from heatstroke. This heat wave was particularly unusual because an ongoing extreme El Niño of 2015 was expected to lead to a cooler summer in Japan. A primary cause of this heat wave was an intraseasonal tropical disturbance (Fig. 21.1). A tropical cyclone (TC), TC1513, was generated in the western North Pacific (WNP) during the convectively active phase of the intraseasonal oscillation (ISO) in the western Pacific (Fig. 21.1c and Supplemental Fig. S21.3c), followed by another TC (TC1514). Li and Zhou (2013) demonstrated that the two major components of the ISO, the 30–60 day Madden–Julian oscillation (MJO, Madden and Julian 1971) and the 10–20 day quasi-biweekly oscillation (QBWO, Chen and Sui 2010) can affect the genesis and intensity of TCs in the WNP during the summer. It appears that the TCs in this study likewise formed in association with the ISO. The diabatic heating associated with the TCrelated precipitation induced a Rossby wave train, which is characterized by cyclonic circulation and anticyclonic circulation anomalies in the WNP and East Asia, respectively (Figs. 21.1b,d). This pattern is similar to the so-called Pacific–Japan (PJ) teleconnection pattern (Nitta 1987; Wakabayashi and Kawamura 2004) in July and August, accompanying a meridional tripolar pattern in precipitation, vorticity, and temperature anomalies (Supplemental Fig. S21.1a). Several studies have also reported that TCs can generate the PJ pattern over the WNP (Kawamura and Ogasawara 2006; Yamada and Kawamura 2007). In early August 2015, the positive PJ pattern gave rise to the abnormally persistent hot and dry days over Japan. The El Niño conditions climatologically had a cooling impact on Japan in July–August (Supplemental Fig. S21.1a), yet the 2015 summer was still unexpectedly hot (Supplemental Fig. S21.1b). Anthropogenic warming can change the likelihood of specific extreme events, although the odds of an event occurring may vary from year to year depending on the regional sea surface temperature (SST) pattern (Christidis and Stott 2014). In this study, we investigate the possible influences of the intraseasonal variability, the 2015 strong El Niño, and anthropogenic warming on the Japanese heat wave in August 2015 using an atmospheric general circulation model (AGCM).