Search Results

You are looking at 1 - 10 of 15 items for

  • Author or Editor: Motoki Nagura x
  • All content x
Clear All Modify Search
Motoki Nagura

Abstract

This study investigates spreading and generation of spiciness anomalies of the Subantarctic mode water (SAMW) located on 26.6 to 26.8 σθ in the south Indian Ocean, using in-situ hydrographic observations, satellite measurements, reanalysis datasets and numerical model output. The amplitude of spiciness anomalies is about 0.03 psu or 0.13°C and tends to be large along the streamline of the subtropical gyre, whose upstream end is the outcrop region south of Australia. The speed of spreading is comparable with that of the mean current, and it takes about a decade for a spiciness anomaly in the outcrop region to spread into the interior up to Madagascar Island. In the outcrop region, interannual variability in mixed layer temperature and salinity tends to be density compensating, which indicates that Eulerian temperature or salinity changes account for the generation of isopycnal spiciness anomalies. It is known that wintertime temperature and salinity in the surface mixed layer determine the temperature/salinity relationship of a subducted water mass. Considering this, the mixed layer heat budget in the outcrop region is estimated based on the concept of effective mixed layer depth, the result of which shows the primary contribution from horizontal advection. The contributions from Ekman and geostrophic currents are comparable. Ekman flow advection is caused by zonal wind stress anomalies and the resulting meridional Ekman current anomalies, as is pointed out by a previous study. Geostrophic velocity is decomposed into large-scale and mesoscale variability, both of which significantly contribute to horizontal advection.

Restricted access
Motoki Nagura and Masanori Konda

Abstract

The relationship between the interannual variation of the surface wind in the north Indian Ocean (0°–30°N, 30°–100°E) and El Niño–Southern Oscillation (ENSO) during boreal summer is investigated. The association of the surface wind with the sea surface temperature (SST) in the north Indian Ocean is evaluated. The NCEP–NCAR reanalysis, NOAA outgoing longwave radiation (OLR), and Reynolds SST data are used. The June–August mean of the surface wind anomaly over the north Indian Ocean is decomposed by EOF analysis, and two dominant modes are extracted. The first (second) mode shows the corresponding variation with the ENSO events maturing in the subsequent (previous) winter. The first mode has a large amplitude during the 1990s, while the amplitude of the second mode is large mainly during the 1980s. Such contrast of the amplitude of the two modes results in the temporal change of the surface wind–ENSO relationships between the two decades. The temporal characteristics of the first and second modes are consistent with those of convective variability in the eastern Indian Ocean and the Philippine Sea, respectively.

The local thermal forcings associated with these two contrastive modes are compared with the time change of the SST anomaly. The thermal forcings are evaluated in terms of the latent heat flux and the Ekman heat transport. The thermal forcing of the first mode is consistent with a meridionally antisymmetric pattern of the SST anomaly during the 1990s, while that of the second mode is correlated with the basinwide SST anomaly during the 1980s. This result suggests that the temporal change is also found in the north Indian Ocean SST anomaly.

Full access
Motoki Nagura and Shinya Kouketsu

Abstract

This study investigates an isopycnal temperature/salinity T/S, or spiciness, anomaly in the upper south Indian Ocean for the period from 2004 to 2015 using observations and reanalyses. Spiciness anomalies at about 15°S on 24–26σ θ are focused on, whose standard deviation is about 0.1 psu in salinity and 0.25°C in temperature, and they have a contribution to isobaric temperature variability comparable to thermocline heave. A plausible generation region of these anomalies is the southeastern Indian Ocean, where the 25σ θ surface outcrops in southern winter, and the anticyclonic subtropical gyre advects subducted water equatorward. Unlike the Pacific and Atlantic, spiciness anomalies in the upper south Indian Ocean are not T/S changes in mode water, and meridional variations in SST and sea surface salinity in their generation region are not density compensating. It is possible that this peculiarity is owing to freshwater originating from the Indonesian Seas. The production of spiciness anomalies is estimated from surface heat and freshwater fluxes and the surface T/S relationship in the outcrop region, based on several assumptions including the dominance of surface fluxes in the surface T/S budget and effective mixed layer depth proposed by Deser et al. The result agrees well with isopycnal salinity anomalies at the outcrop line, which indicates that spiciness anomalies are generated by local surface fluxes. It is suggested that the Ningaloo Niño and El Niño–Southern Oscillation lead to interannual variability in surface heat flux in the southeastern Indian Ocean and contribute to the generation of spiciness anomalies.

Full access
Motoki Nagura and Masanori Konda

Abstract

The seasonal development of the sea surface temperature (SST) anomaly in the Indian Ocean is investigated in relation to El Niño–Southern Oscillation (ENSO), using NOAA optimally interpolated SST and NCEP reanalysis data. The result shows that the onset season of El Niño affects the seasonal development of surface wind anomalies over the equatorial eastern Indian Ocean (EEIO); these surface wind anomalies, in turn, determine whether the SST anomaly in the EEIO evolves into the eastern pole of the dipole pattern. In years when the dipole pattern develops, surface zonal wind anomalies over the EEIO switch from westerly to easterly in spring as La Niña switches to El Niño. The seasonal zonal wind over the EEIO also switches from westerly to easterly in spring, and the anomalous wind strengthens seasonal wind from winter to summer. Stronger winds and resultant thermal forcings produce the negative SST anomaly in the EEIO in winter, and its amplitude increases in summer. The SST anomaly becomes the eastern pole of the dipole pattern in fall. In contrast, if the change from La Niña to El Niño is delayed until late summer/fall or if La Niña persists throughout the year, a westerly anomaly persists from winter to summer over the EEIO. The persistent westerly anomaly strengthens the wintertime climatological westerlies and weakens the summertime easterlies. Therefore, negative SST anomalies are produced in the EEIO in winter, but the amplitude decreases in summer, and the eastern pole is not present in fall. The above explanation also applies to onset years of La Niña if the signs of the anomalies are reversed.

Full access
Motoki Nagura and Yukio Masumoto

Abstract

A wake due to islands in background zonal flow has been observed in the equatorial Pacific Ocean. This study detects and examines a wake due to the Maldives in the eastward Wyrtki jet in the Indian Ocean. Observations by acoustic Doppler current profilers deployed east of the Maldives show semiannual variability in cross-equatorial currents, which cannot be explained by annual monsoonal wind forcing. Output from a high-resolution ocean general circulation model (OGCM) shows that the semiannual current variability is a part of a stationary wavelike pattern of meridional currents, which appears east of the Maldives concurrently with the eastward Wyrtki jet. Idealized numerical experiments are conducted using a 1.5-layer model, in which an equatorial jet driven by wind forcing or steady inflow impinges islands that are similar to the Maldives in shape. The results show the meandering of the equatorial eastward jet east of the model islands, and the resulting cross-equatorial currents have a similar pattern compared to those in the OGCM simulation. The momentum budget analysis obtained from the OGCM simulation and the layer model experiments shows a significant contribution of momentum advection to the generation of the wake. Also, the layer model experiments exhibit that the wake is essentially stationary; its zonal wavelength becomes larger when the eastward jet is stronger, and the wake is absent when the equatorial jet is westward. The similarity of the wake in the equatorial jet to stationary damped Rossby waves in the quasigeostrophic barotropic ocean model is discussed.

Full access
Motoki Nagura and Michael J. McPhaden

Abstract

The number of in situ observations in the Indian Ocean has dramatically increased over the past 15 years thanks to the implementation of the Argo profiling float program. This study estimates the mean circulation in the Indian Ocean using hydrographic observations obtained from both Argo and conductivity–temperature–depth (CTD) observations. Absolute velocity at the Argo float parking depth is used so there is no need to assume a level of no motion. Results reveal previously unknown features in addition to well-known currents and water masses. Some newly identified features include the lack of an interior pathway to the equator from the southern Indian Ocean in the pycnocline, indicating that water parcels must transit through the western boundary to reach the equator. High potential vorticity (PV) intrudes from the western coast of Australia in the depth range of the Subantarctic Mode Water, which leads to a structure similar to a PV barrier. The subtropical anticyclonic gyre retreats poleward with depth, as happens in the subtropical Atlantic and Pacific. An eastward flow was found in the eastern basin along 15°S at the depth of the Antarctic Intermediate Water—a feature expected from property distributions but never before detected in velocity estimates. Meridional mass transport indicates about 10 Sv (1 Sv ≡ 106 m3 s−1) southward flow at 6°S and 18 Sv northward flow at 20°S, which results in meridional convergence of currents and thermocline depression at about 16°–20°S. These estimated absolute velocities agree well with those of an ocean reanalysis, which lends credibility to the strictly databased analysis.

Full access
Motoki Nagura and Michael J. McPhaden

Abstract

Zonal propagation of zonal velocity along the equator in the Indian Ocean and its relationship with wind forcing are investigated with a focus on seasonal time scales using in situ observations from four acoustic Doppler current profilers (ADCPs) and an ocean reanalysis dataset. The results show that the zonal phase speed of zonal currents varies depending on season and depth in a very complicated way in relation to surface wind forcing. Surface layer zonal velocity propagates to the west in northern spring but to the east in fall in response to zonally propagating surface zonal winds, while in the pycnocline zonal phase speed is related to wind-forced ocean wave dynamics. In the western half of the analysis domain (78°–83°E), zonal phase speed in the pycnocline is eastward all year, which is attributed to the radiation of Kelvin waves forced in the western basin. In the eastern half of the domain (80°–90°E), zonal phase speed is westward at 50- to 100-m depths in northern fall, but eastward above and below, most likely due to Rossby waves generated at the eastern boundary.

Full access
Tomoki Tozuka, Motoki Nagura, and Toshio Yamagata

Abstract

The sea surface temperature (SST) in the western Arabian Sea upwelling region is known to influence the amount of precipitation associated with the Indian summer monsoon. Thus, understanding what determines the SST in this region is an important issue. Using outputs from an ocean general circulation model with and without strong damping in the eastern equatorial Indian Ocean, this study examines how the reflection of semiannual Kelvin waves at the eastern boundary of the Indian Ocean may influence the western Arabian Sea upwelling region. The downwelling Kelvin waves generated in boreal spring are reflected at the eastern boundary and reach the western equatorial Indian Ocean as reflected Rossby waves about 6 months later. The resulting westward current along the equator in the western equatorial Indian Ocean transports warmer water to the western Arabian Sea upwelling region. Thus, the SST in this region becomes colder especially in boreal fall without the reflected Rossby waves. These results are further supported by the analysis of the mixed layer temperature balance. Surprisingly, vertical processes do not contribute to the SST difference, even though the thermocline becomes shallower without the downwelling Rossby waves. This is because the mixed layer is shoaling rapidly from September to November, and there is basically no entrainment of water from below. In contrast, the reflected Rossby waves do not have large impacts on the SST in other seasons mainly because the zonal SST gradient is not as strong and/or the amplitude of Rossby waves is weaker.

Full access
Motoki Nagura, Yukio Masumoto, and Takanori Horii

Abstract

This study examines heat advection due to mixed Rossby gravity waves in the equatorial Indian Ocean using moored buoy observations at (0°, 80.5°E) and (0°, 90°E) and an ocean general circulation model (OGCM) output. Variability associated with mixed Rossby gravity waves is defined as that at periods of 10–30 days, where both observations and the OGCM results show high energy in meridional velocity and meridional gradient of temperature. The 10–30-day variability in meridional velocity causes convergence of heat flux onto the equator, the net effect of which amounts to 2.5°C month−1 warming at the depth of the thermocline. Detailed analysis shows that the wave structure manifested in temperature and velocity is tilted in the xz plane, which causes the phase lag between meridional velocity and meridional temperature gradient to be a half cycle on the equator and results in sizable thermocline warming. An experiment with a linear continuously stratified model shows that the contributions of many baroclinic modes, and the right zonal wavelength of wind forcing, are essential in generating the correct wave structure. It is also shown that contributions of mixed Rossby gravity waves to cross-equatorial heat transport are negligible, as temperature variability associated with this wave mode has a node on the equator.

Full access
Motoki Nagura, J. P. McCreary, and H. Annamalai

Abstract

This study investigates biases of the climatological mean state of the northern Arabian Sea (NAS) in 31 coupled ocean–atmosphere models. The focus is to understand the cause of the large biases in the depth of the 20°C isotherm that occur in many of them. Other prominent biases are the depth and temperature of Persian Gulf water (PGW) and the wintertime mixed-layer thickness (MLT) along the northern boundary.

For models that lack a Persian Gulf (group 1), is determined by the wintertime MLT bias through the formation of an Arabian Sea high-salinity water mass (ASHSW) that is too deep. For models with a Persian Gulf (group 2), if > MLT (group 2B), PGW remains mostly trapped to the western boundary and, again, directly controls . If MLT (group 2A), PGW spreads into the NAS and impacts because > 20°C; nevertheless still influences indirectly through its impact on .

The thick wintertime mixed layer is driven primarily by surface cooling during the fall. Nevertheless, variations in ΔMLT among the models are more strongly linked to biases in the density stratification (jump) across the bottom of the mixed layer than to biases. The jump is in turn determined primarily by sea surface salinity biases (ΔSSS) advected into the NAS by the West India Coastal Current, and the source of ΔSSS is the rainfall deficit associated with the models’ weak summer monsoon. Ultimately, then, ΔD20 is linked to this deficit.

Full access