Yushi Morioka

On the Growth and Decay of the Subtropical Dipole Mode in the South Atlantic

AbstractUsing observational data and outputs from an ocean general circulation model, the growth and decay of the South Atlantic subtropical dipole (SASD) are studied. The SASD is the most dominant mode of interannual variability in the South Atlantic Ocean, and its sea surface temperature (SST) anomaly shows a dipole pattern that is oriented in the northeast–southwest direction. The positive (negative) pole develops because the warming of the mixed layer by the contribution from the climatological shortwave radiation is enhanced (suppressed) when the mixed layer is thinner (thicker) than normal. The mixed layer depth anomaly over the positive (negative) pole is due to the suppressed (enhanced) latent heat flux loss associated with the southward migration and strengthening of the subtropical high. During the decay phase, since the temperature difference between the mixed layer and the entrained water becomes anomalously large (small) as a result of the positive (negative) mixed layer temperature anomaly, ...

Subtropical Dipole Modes Simulated in a Coupled General Circulation Model

The growth and decay mechanisms of subtropical dipole modes in the southern Indian and South Atlantic Oceans and their impacts on southern African rainfall are investigated using results from a coupled general circulationmodeloriginallydevelopedforpredictingtropicalclimatevariations.Thesecond(most)dominant mode of interannual sea surface temperature (SST) variations in the southern Indian (South Atlantic) Ocean represents a northeast‐southwest oriented dipole, now called subtropical dipole mode. The positive (negative) SST interannualanomalypole startsto grow in australspring and reachesits peakin February.In austral late spring, the suppressed (enhanced) latent heat flux loss associated with the variations in the subtropical high causes a thinner (thicker) than normal mixed layer thickness that, in turn, enhances (reduces) the warming of the mixed layer by the climatological shortwave radiation. The positive (negative) pole gradually decays in austral fall because the mixed layercooling by the entrainmentis enhanced(reduced),mostlyowing to the larger (smaller) temperature difference between the mixed layer and the entrained water. The increased (decreased) latent heat loss due to the warmer (colder) SST also contributes to the decay of the positive (negative) pole. Although furtherverification using longerobservational data is required,the present coupled model suggests that the South Atlantic subtropical dipole may play a more important role in rainfall variations over the southern African region than the Indian Ocean subtropical dipole.

Impact of Mascarene High variability on the East African ‘short rains’

The interannual variability of East African ‘short rains’ (EASR) and its link with the Mascarene High (MH) variation are explored, using observations and reanalysis data. Correlation and composite analyses for flood and drought events reveal that the EASR variability is strongly linked to the MH zonal displacement, in particular, the zonal movement of the MH eastern ridge. When the MH eastern ridge is anomalously displaced to the west (east) of its normal position, the south east (SE) trade winds over the South Indian Ocean (SIO) anomalously strengthen (weaken). This enhances (reduces) the relatively cool and dry SE trade winds and induces cold (warm) sea surface temperature anomaly in the SIO. As a result, convection over the western equatorial SIO is suppressed (enhanced) and leads to rainfall deficits (excess) over East Africa. Droughts in East Africa are associated with a westward migration of the MH eastern ridge, while the relationship is less clear for flood events and their link to an eastward migration of the MH. Therefore, the zonal migration of the MH eastern ridge provides a novel indicator for the EASR extremes especially droughts. This revelation has immense social application for rainfall forecast over East Africa where rainfall deficits have become more prevalent against the background of deteriorating conventional forecasts for EASR droughts.

How is the Indian Ocean Subtropical Dipole excited

Based on experiments using a coupled general circulation model which resolves tropical ocean–atmosphere coupled phenomena such as El Niño/Southern Oscillation (ENSO) and the Indian Ocean Dipole, forcing mechanisms of the Indian Ocean subtropical dipole (IOSD) are investigated. In the control experiment, as in the observation, several types of the IOSD are generated by the variations in the Mascarene High during austral summer and characterized by a dipole pattern of sea surface temperature (SST) anomalies in the northeastern and southwestern parts of the southern Indian Ocean. In another experiment, where the SST outside the southern Indian Ocean is nudged toward the monthly climatology of the simulated SST, one type of the IOSD occurs, but it is less frequent and associated with the zonal wavenumber four pattern of equivalently barotropic geopotential height anomalies in high latitudes, suggesting an interesting link with the Antarctic Circumpolar Wave. This indicates that, even without the atmospheric teleconnection from tropical coupled climate modes, the IOSD may develop in association with the atmospheric variability in high latitudes of the Southern Hemisphere. In the other experiment, where only the southern Indian Ocean and the tropical Pacific are freely interactive with the atmosphere, two types of both positive and negative IOSD occur. Since the occurrence frequency of the IOSD significantly increases as compared to the second experiment, this result confirms that the atmospheric teleconnection from ocean-atmosphere coupled modes in the tropical Pacific such as ENSO may also induce the variations in the Mascarene High that generate the IOSD. The present research, even within the realm of model studies, shows clearly that the predictability of the IOSD in mid-latitudes is related to both low and high-latitudes climate variations.

Role of Tropical SST Variability on the Formation of Subtropical Dipoles

AbstractInterannual variations of sea surface temperature (SST) in the midlatitudes of the Southern Hemisphere play an important role in the rainfall variability over the surrounding countries by modulating synoptic-scale atmospheric disturbances. These are frequently associated with a northeast–southwest-oriented dipole of positive and negative SST anomalies in each oceanic basin, referred to as a subtropical dipole. This study investigates the role of tropical SST variability on the generation of subtropical dipoles by conducting SST-nudging experiments using a coupled general circulation model. In the experiments where the simulated SST in each tropical basin is nudged to the climatology of the observed SST, the subtropical dipoles tend to occur as frequently as the case in which the simulated SST is allowed to freely interact with the atmosphere. It is found that without the tropical SST variability, the zonally elongated atmospheric mode in the mid- to high latitudes, called the Antarctic Oscillation...

Role of climate variability in the heatstroke death rates of Kanto region in Japan

The death toll by heatstroke in Japan, especially in Kanto region, has sharply increased since 1994 together with large interannual variability. The surface air temperature and humidity observed during boreal summers of 1980–2010 were examined to understand the role of climate in the death toll. The extremely hot days, when the daily maximum temperature exceeds 35°C, are more strongly associated with the death toll than the conventional Wet Bulb Globe Temperature index. The extremely hot days tend to be associated with El Niño/Southern Oscillation or the Indian Ocean Dipole, suggesting a potential link with tropical climate variability to the heatstroke related deaths. Also, the influence of these climate modes on the death toll has strengthened since 1994 probably related to global warming. It is possible to develop early warning systems based on seasonal climate predictions since recent climate models show excellent predictability skills for those climate modes.

Potential Sources of Decadal Climate Variability over Southern Africa

AbstractPotential sources of decadal climate variability over southern Africa are examined by conducting in-depth analysis of available datasets and coupled general circulation model (CGCM) experiments. The observational data in recent decades show a bidecadal variability noticeable in the southern African rainfall with its positive phase of peak during 1999/2000. It is found that the rainfall variability is related to anomalous moisture advection from the southwestern Indian Ocean, where the anomalous sea level pressure (SLP) develops. The SLP anomaly is accompanied by anomalous sea surface temperature (SST). Both SLP and SST anomalies slowly propagate eastward from the South Atlantic to the southwestern Indian Ocean. The analysis of mixed layer temperature tendency reveals that the SST anomaly in the southwestern Indian Ocean is mainly due to eastward advection of the SST anomaly by the Antarctic Circumpolar Current. The eastward propagation of SLP and SST anomalies are also confirmed in the 270-yr outp...

Generation Mechanism of the South Pacific Subtropical Dipole

AbstractDistinct pattern of interannual variability in sea surface temperature (SST) in the South Pacific [i.e., the South Pacific subtropical dipole (SPSD)] is examined using outputs from a coupled general circulation model. The SPSD appears as the second empirical orthogonal function (EOF) mode of the SST anomalies in the South Pacific and is associated with a northeast–southwest-oriented dipole of positive and negative SST anomalies in the central basin. The positive and negative SST anomaly poles start to develop during austral spring, reach their peak during austral summer, and gradually decay afterward. Close examination of mixed-layer heat balance yields that the SST anomaly poles develop mainly because warming of the mixed layer by shortwave radiation is modulated by the anomalous mixed-layer thickness. Over the positive (negative) pole, the mixed layer becomes thinner (thicker) than normal and acts to enhance (reduce) the warming of the mixed layer by climatological shortwave radiation. This thin...

A Regional Climate Mode Discovered in the North Atlantic: Dakar Niño/Niña.

The interrannual variability of coastal sea surface temperature (SST) anomalies confined off Senegal is explored from a new viewpoint of the ocean-land-atmosphere interaction. The phenomenon may be classified into “coastal Niño/Niña” in the North Atlantic as discussed recently in the Northeastern Pacific and Southeastern Indian Oceans. The interannual variability of the regional mixed-layer temperature anomaly that evolves in boreal late fall and peaks in spring is associated with the alongshore wind anomaly, mixed-layer depth anomaly and cross-shore atmospheric pressure gradient anomaly, suggesting the existence of ocean-land-atmosphere coupled processes. The coupled warm (cold) event is named Dakar Niño (Niña). The oceanic aspect of the Dakar Niño (Niña) may be basically explained by anomalous warming (cooling) of the anomalously thin (thick) mixed-layer, which absorbs shortwave surface heat flux. In the case of Dakar Niña, however, enhancement of the entrainment at the bottom of the mixed-layer is not negligible.

Local SST Impacts on the Summertime Mascarene High Variability

AbstractThe interannual variations in the summertime Mascarene high have great impacts on the southern African climate as well as the sea surface temperature (SST) in the southern Indian Ocean. A set of coupled general circulation model (CGCM) experiments are performed to examine a role of the interannual SST variability in the southern Indian Ocean on the summertime Mascarene high variability. The dominant interannual variability in the summertime Mascarene high shows the strengthening (weakening) in its southern part throughout the austral summer (December–February). However, in the experiment where the interannual SST variability in the southern Indian Ocean is suppressed, the strengthening (weakening) of the Mascarene high in its southern part does not persist until February. Also, the Mascarene high variability and its associated SST anomalies in December and January are found to increase (decrease) the southern African rainfall via more (less) moisture supply from the southern Indian Ocean. The Masc...