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Jing-Jia Luo and Toshio Yamagata

Abstract

Using outputs of a high-resolution ocean general circulation model, upper-ocean heat content budget and mixed layer heat budget are analyzed to investigate the reason for the 1988–89 decadal warming event in the northern North Pacific. The model reproduces realistic upper-ocean temperature changes in comparison with observational data. This analysis suggests that the horizontal mean geostrophic advection of anomalous temperature is the main contributor to the heat content increase around 1988–89, and surface heat flux forcing is the main contributor to increasing mixed layer temperature. The anomalous geostrophic advection of mean temperature plays a negative role in the increase of both the upper-ocean heat content and mixed layer temperature in midlatitudes around 1988–89. Another negative contribution to the mixed layer temperature increase is provided by the Ekman advection. In the Kuroshio Extension region, the warm upper-ocean heat content anomaly appears in 1987–88, in which the mean geostrophic advection also plays a dominant role. South of Japan the decadal warming appears even earlier, around 1985–86. The anomalous Kuroshio transport shows a decadal decreasing trend since the early 1980s and therefore cannot explain the late 1980s warming event in midlatitudes. The 1988–89 event is found to be closely linked with the decadal change of the Kuroshio path south of Japan. It is found that subtropical Rossby waves may influence the decadal temperature changes south of Japan.

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Jing-Jia Luo and Toshio Yamagata

Abstract

Using outputs of a high-resolution ocean general circulation model, upper-ocean heat content budget and mixed layer heat budget are analyzed to investigate the reason for the 1988–89 decadal warming event in the northern North Pacific. The model reproduces realistic upper-ocean temperature changes in comparison with observational data. This analysis suggests that the horizontal mean geostrophic advection of anomalous temperature is the main contributor to the heat content increase around 1988–89, and surface heat flux forcing is the main contributor to increasing mixed layer temperature. The anomalous geostrophic advection of mean temperature plays a negative role in the increase of both the upper-ocean heat content and mixed layer temperature in midlatitudes around 1988–89. Another negative contribution to the mixed layer temperature increase is provided by the Ekman advection. In the Kuroshio Extension region, the warm upper-ocean heat content anomaly appears in 1987–88, in which the mean geostrophic advection also plays a dominant role. South of Japan the decadal warming appears even earlier, around 1985–86. The anomalous Kuroshio transport shows a decadal decreasing trend since the early 1980s and therefore cannot explain the late 1980s warming event in midlatitudes. The 1988–89 event is found to be closely linked with the decadal change of the Kuroshio path south of Japan. It is found that subtropical Rossby waves may influence the decadal temperature changes south of Japan.

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Chen Li, Jing-Jia Luo, and Shuanglin Li

Abstract

The impacts of different types of El Niño–Southern Oscillation (ENSO) on the interannual negative correlation (seesaw) between the Somali cross-equatorial flow (CEF) and the Maritime Continent (MC) CEF during boreal summer (June–August) are investigated using the ECMWF twentieth-century reanalysis (ERA-20C) dataset and numerical experiments with a global atmospheric model [the Met Office Unified Model global atmosphere, version 6 (UM-GA6)]. The results suggest that ENSO plays a prominent role in governing the CEF-seesaw relation. A high positive correlation (0.86) exists between the MC CEF and Niño-3.4 index and also in the case of eastern Pacific (EP) El Niño, central Pacific (CP) El Niño, EP La Niña, and CP La Niña events. In contrast, a negative correlation (−0.35) exists between the Somali CEF and Niño-3.4 index, and this negative relation is significant only in the EP El Niño years. Further, the variation of the MC CEF is highly correlated with the local north–south sea surface temperature (SST) gradient, while the variation of the Somali CEF displays little relation with the local SST gradient. The Somali CEF may be remotely influenced by ENSO. The model results confirm that the EP El Niño plays a major role in causing the weakened Somali CEF via modifying the Walker cell. However, the impact of the EP El Niño on the Somali CEF differs with different seasonal background. It is also found that the interannual CEF seesaw displays a multidecadal change before and after the 1950s, which is linked with the multidecadal strengthening of the intensity of the EP ENSO.

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Jing-Jia Luo, Sebastien Masson, Swadhin Behera, and Toshio Yamagata

Abstract

The Indian Ocean Dipole (IOD) has profound socioeconomic impacts on not only the countries surrounding the Indian Ocean but also various parts of the world. A forecast system is developed based on a relatively high-resolution coupled ocean–atmosphere GCM with only sea surface temperature (SST) information assimilated. Retrospective ensemble forecasts of the IOD index for the past two decades show skillful scores with up to a 3–4-month lead and a winter prediction barrier associated with its intrinsic strong seasonal phase locking. Prediction skills of the SST anomalies in both the eastern and western Indian Ocean are higher than those of the IOD index; this is because of the influences of ENSO, which is highly predictable. The model predicts the extreme positive IOD event in 1994 at a 2–3-season lead. The strong 1997 cold signal in the eastern pole, however, is not well predicted owing to errors in model initial subsurface conditions. The real-time forecast system with more ensembles successfully predicted the weak negative IOD event in the 2005 boreal fall and La Niña condition in the 2005/06 winter. Recent experimental real-time forecasts showed that a positive IOD event would appear in the 2006 summer and fall accompanied by a possible weak El Niño condition in the equatorial Pacific.

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Lisuo Hu, Jing-Jia Luo, Gang Huang, and Matthew C. Wheeler

Abstract

Central Africa (CA) is identified as a location of a large positive trend of the occurrence of heat waves (HWs) during 1979–2016, appearing to result mostly from a regime shift around the year 2000. Therefore, we study the evolution of synoptic features associated with the occurrence of HW events in CA. It is found that the HW-related circulation is typically characterized by an anomalous convergence in the upper troposphere but there are important differences for HW events occurring in the south region of CA (CA_S) versus the north region (CA_N). For the occurrence of the HW events in CA_S, the anomalous subsidence associated with upper troposphere anomalous convergence is the dominant factor for their occurrence and magnitude: the strong subsidence leads to warming through greater solar insolation. The HW events in CA_S are also accompanied by an anomalous surface anticyclone in the north with anomalous northerly flow transporting heat into the CA_S region. In contrast, although the HW events in CA_N are also associated with upper troposphere anomalous convergence, the intensity of the convergence is weak with small solar insolation. Instead, the anomalous warm advection is the main factor for determining the magnitude of the HW events in CA_N, induced by the prevailing northerly winds acting on the anomalous temperature gradient. Thus, the synoptic features associated with HW events in the CA_N and CA_S are quite different despite their nearby locations. The discovered dominant factors for the HW events in CA can be used to improve the forecast skill.

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Tomoki Tozuka, Jing-Jia Luo, Sebastien Masson, and Toshio Yamagata

Abstract

Using outputs from the SINTEX-F1 coupled GCM, the thermodynamics of ENSO events and its relation with the seasonal cycle are investigated. Simulated El Niño events are first classified into four groups depending on during which season the Niño-3.4 sea surface temperature anomaly (SSTA) index (5°S–5°N, 120°–170°W) reaches its peak. Although the heat content of the tropical Pacific decreases for all four types, the tropical Pacific loses about twice as much during an El Niño that peaks during winter compared with one that peaks during summer. The surface heat flux, the southward heat transport at 15°S, and the Indonesian Throughflow heat transport contribute constructively to this remarkable seasonal difference. It is shown that the Indonesian Throughflow supplies anomalous heat from the Indian Ocean, especially during the summer El Niño–like event. Changes in the basic state provided by the seasonal cycle cause differences in the atmospheric response to the SSTA, which in turn lead to the difference between the surface heat flux and the meridional heat transport anomaly.

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Jing-Jia Luo, Swadhin K. Behera, Yukio Masumoto, and Toshio Yamagata

Abstract

Surface air temperature (SAT) over the globe, particularly the Northern Hemisphere continents, has rapidly risen over the last 2–3 decades, leading to an abrupt shift toward a warmer climate state after 1997/98. Whether the terrestrial warming might be caused by local response to increasing greenhouse gas (GHG) concentrations or by sea surface temperature (SST) rise is recently in dispute. The SST warming itself may be driven by both the increasing GHG forcing and slowly varying natural processes. Besides, whether the recent global warming might affect seasonal-to-interannual climate predictability is an important issue to be explored. Based on the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) climate prediction system in which only observed SSTs are assimilated for coupled model initialization, the present study shows that the historical SST rise plays a key role in driving the intensified terrestrial warming over the globe. The SST warming trend, while negligible for short lead predictions, has substantial impact on the climate predictability at long lead times (>1 yr) particularly in the extratropics. The tropical climate predictability, however, is little influenced by global warming. Given a perfect warming trend and/or a perfect model, global SAT and precipitation could be predicted beyond two years in advance with an anomaly correlation skill above ∼0.6.

Without assimilating ocean subsurface observations, model initial conditions show a strong spurious cooling drift of subsurface temperature; this is caused by large negative surface heat flux damping arising from the SST-nudging initialization. The spurious subsurface cooling drift acts to weaken the initial SST warming trend during model forecasts, leading to even negative trends of global SAT and precipitation at long lead times and hence deteriorating the global climate predictability. Concerning the important influence of the subsurface temperature on the global SAT trend, future efforts are required to develop a good scheme for assimilating subsurface information particularly in the extratropical oceans.

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Chi-Cherng Hong, Tim Li, and Jing-Jia Luo

Abstract

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.

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Tomoki Tozuka, Jing-Jia Luo, Sebastien Masson, and Toshio Yamagata

Abstract

The decadal variation in the tropical Indian Ocean is investigated using outputs from a 200-yr integration of the Scale Interaction Experiment-Frontier Research Center for Global Change (SINTEX-F1) ocean–atmosphere coupled model. The first EOF mode of the decadal bandpass- (9–35 yr) filtered sea surface temperature anomaly (SSTA) represents a basinwide mode and is closely related with the Pacific ENSO-like decadal variability. The second EOF mode shows a clear east–west SSTA dipole pattern similar to that of the interannual Indian Ocean dipole (IOD) and may be termed the decadal IOD. However, it is demonstrated that the decadal air–sea interaction in the Tropics can be a statistical artifact; it should be interpreted more correctly as decadal modulation of interannual IOD events (i.e., asymmetric or skewed occurrence of positive and negative events). Heat budget analysis has revealed that the occurrence of IOD events is governed by variations in the southward Ekman heat transport across 15°S and variations in the Indonesian Throughflow associated with the ENSO. The variations in the southward Ekman heat transport are related to the Mascarene high activities.

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Chaoxia Yuan, Junqi Liu, Jing-Jia Luo, and Zhaoyong Guan

Abstract

Because of the seasonal northward migration of the East Asian summer monsoon, the mean-state atmospheric circulation in South China (SC) is remarkably different between the early (May–June) and late (July–August) rainy seasons. This study presents distinct teleconnections between the SC precipitation in the two periods and the sea surface temperatures (SSTs) in the tropical oceans. In the early rainy season when the major monsoon rain belt is located in SC, the increased local precipitation is related to the tropical Indian Ocean Basin warming. The basin warming induces an anomalous anticyclone in the South China Sea–western North Pacific (SCS-WNP). The related southwesterly anomalies transport more moisture to SC and lead to more moisture convergence and precipitation there. In the late rainy season when the major monsoon rain belt migrates northward to the Yangtze River valley, the precipitation increase in SC can be caused by the dipole SST anomalies in the tropical Pacific with the cold anomalies near the Maritime Continent and warm ones near the date line. The dipole SST anomalies generate an anomalous cyclone in the WNP with its center more northward than that of the anomalous anticyclone in the early rainy season. The related northeasterly anomalies along its northwestern flank reduce the climatological northward transport of moisture flux out of SC, and increase the moisture convergence and precipitation there. The distinct teleconnections between the SC precipitation and the tropical SSTs in the early and late rainy seasons can be well reproduced in the sensitivity experiments by an atmospheric general circulation model.

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