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Ming Feng and Susan Wijffels

Abstract

Satellite altimeter data reveal that the strongest intraseasonal variability in the southeast Indian Ocean occurs in the South Equatorial Current (SEC) during the second half of the year. The length scale of the variability is 100–150 km, with a westward phase speed of 15–19 cm s−1 and dominant periods between 40 and 80 days. A continuously stratified quasigeostrophic model is solved to analyze the baroclinic stability of the climatological SEC. Large growth rate of the instability waves (with e-folding timescale of less than 50 days) can be found east of the active region of intraseasonal variability during the July–September season, when the SEC and the Pacific to Indian Ocean throughflow are at their strongest. Geostrophic current shear in the upper 200-m ocean is crucial for the growth of the instability. The results suggest that the baroclinic instability draws most of its energy from the available potential energy associated with the throughflow, and, to a lesser degree, by local Ekman pumping. The predicted characteristics of the most unstable mode are consistent with both altimeter and profiling float observations. From all the available evidence, baroclinic instability seems to be the main cause of intraseasonal variability in the SEC. However, more field observations are necessary to properly address the possibility of combined barotropic/baroclinic instability.

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Ming Feng, Humio Mitsudera, and Yasushi Yoshikawa

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Four years of mooring array measurements in Tokara Strait, south of Kyushu, Japan, from 1992 to 1996 are used to analyze the structure and temporal variability of the Kuroshio Current. The mean Kuroshio current in Tokara Strait shows a nearly permanent subsurface double-core structure, possibly due to topographic blockage effects.

The dominant variations of the Kuroshio in Tokara Strait are separated into long-term variability (typically 100-day period) and short-term variability (10 days to 1 month) according to spectrum and wavelet analysis. The long-term variability has a large horizontal scale across the strait, with a striking twofold banded structure in spatial correlations. This is due to the north–south Kuroshio axis shift that advects the double-core structure of the mean current. The axis shift can be indexed with the northeastward current velocity at the northernmost station; a composite analysis using this index shows well-defined northward and southward axis shift structures of the Kuroshio current. From the composite of the TOPEX/Poseidon sea level anomaly in terms of this index, the Kuroshio axis shift and the current structure change are associated with a dipole-shape sea level anomaly east of Tokara Strait. On the other hand, the short-term variability of high kinetic energy only has a small horizontal scale within the northern part of the current, which is related to frontal variability.

There exists a deep southwestward undercurrent below 600 m in the northern part of Tokara Strait, flowing along the isobaths. The undercurrent becomes stronger during the northward shift of the Kuroshio axis, while it almost disappears during the southward shift.

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Yuhong Zhang, Yan Du, and Ming Feng

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In this study, multiple time scale variability of the salinity dipole mode in the tropical Indian Ocean (S-IOD) is revealed based on the 57-yr Ocean Reanalysis System 4 (ORAS4) sea surface salinity (SSS) reanalysis product and associated observations. On the interannual time scale, S-IOD is highly correlated with strong Indian Ocean dipole (IOD) and ENSO variability, with ocean advection forced by wind anomalies along the equator and precipitation anomalies in the southeastern tropical Indian Ocean (IO) dominating the SSS variations in the northern and southern poles of the S-IOD, respectively. S-IOD variability is also associated with the decadal modulation of the Indo-Pacific Walker circulation, with a stronger signature at its southern pole. Decadal variations of the equatorial IO winds and precipitations in the central IO force zonal ocean advection anomalies that contribute to the SSS variability in the northern pole of S-IOD on the decadal time scale. Meanwhile, oceanic dynamics dominates the SSS variability in the southern pole of S-IOD off Western Australia. Anomalous ocean advection transports the fresher water from low latitudes to the region off Western Australia, with additional contributions from the Indonesian Throughflow. Furthermore, the southern pole of S-IOD is associated with the thermocline variability originated from the tropical northwestern Pacific through the waveguide in the Indonesian Seas, forced by decadal Pacific climate variability. A deepening (shoaling) thermocline strengthens (weakens) the southward advection of surface freshwater into the southern pole of S-IOD and contributes to the high (low) SSS signatures off Western Australia.

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Ming Feng, Roger Lukas, and Peter Hacker

Abstract

During the TOGA COARE Intensive Observing Period, an energetic, surface-intensified, submesoscale cyclonic eddy was observed in the near-equatorial western Pacific warm pool. The eddy appeared to have been generated as part of the spindown of a strong eastward surface jet forced by the December 1992 westerly wind burst. Because of its potential impacts on the long-term heat, salt, and momentum budgets of the warm pool, the authors provide a thorough description of the evolution of the surface jet and the development of the eddy in the present study. Both the isopycnal surface fit and the zeroth-order dynamic balance confirm the existence of the eddy. Surface layer convergence and northward inertial motion are suggested to be the main causes of the negative eddy vorticity, and it is likely that the eddy drew its energy from the decaying surface jet. This study indicates that in the near-equatorial region the inertial motion has a decreasing meridional spatial scale with time, (βt)−1, due to the β effect, which increases the Rossby number of the decaying jet and generates the nonlinearity.

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Jau-Ming Chen and Ching-Feng Shih

Abstract

Tropical cyclones (TCs) of a particular track type move northward along the open oceans to the east of Taiwan and later pass over or near northern Taiwan. Their northward movement may be associated with intensified monsoon southwesterly flows from the northern South China Sea (SCS) toward Taiwan. Prolonged heavy rainfall then occurs in western Taiwan across the landfall and postlandfall periods, leading to severe floods. Characteristics of this TC–southwesterly flow association and related large-scale regulatory processes of intraseasonal oscillations (ISOs) are studied. For summers from 1958 to 2009, 16 out of 108 TCs affecting Taiwan exhibit the aforementioned northward-moving track. Among them, four TCs (25%) concur with enhanced southwesterly flows. Intensified moisture supplies from the SCS result in strong moisture convergence and prolonged heavy rainfall in western Taiwan. Both 30–60- and 10–24-day ISOs make positive contributions to the TC–southwesterly flow association. Both ISOs exhibit the northward progress of a meridional circulation pair from the tropics toward Taiwan. During landfall and the ensuing few days, Taiwan is surrounded by a cyclonic anomaly to the north and an anticyclonic anomaly to the south of these two ISOs. The appearance of anomalous southwesterly–westerly flows acts to prolong heavy rainfall in western Taiwan after the departure of a TC. The TC–southwesterly flow association tends to occur during the minimum phase of the 30–60-day ISO featuring a cyclonic anomaly in the vicinity of Taiwan but in various phases of the 10–24-day ISO. Rainfall in western Taiwan increases when these two ISOs simultaneously exhibit a cyclonic anomaly to the north of Taiwan.

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Jau-Ming Chen, Pei-Hua Tan, and Ching-Feng Shih

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Tropical cyclones (TCs) moving northwestward/westward across northern Taiwan are defined to have a type-2 track. This study aims to analyze heavy rainfall associated with type-2 TCs in Taiwan, focusing on the modulation processes of the intraseasonal oscillation (ISO). During 1958–2011, 21 summer type-2 TCs are separated into three rainfall types: strong, moderate, and weak. For the strong rainfall type, both 30–60-day and 10–24-day ISOs propagate northwestward across Taiwan. During landfall and the ensuing two days, both ISOs exhibit a cyclonic anomaly centering northwest of Taiwan that causes anomalous westerly flows (or enhance seasonal southwesterly flows) from the South China Sea (SCS) onto Taiwan. Persistent moisture supply and strong moisture convergence result in prolonged heavy rainfall on the windward side over western Taiwan. TCs with weak rainfall are accompanied by a northward-propagating 30–60-day ISO from the tropical western Pacific toward Japan and a westward-propagating 10–24-day ISO along 20°N latitude. During the landfall stage both ISOs have a cyclonic anomaly with its center south of Taiwan. Major anomalous westerly flows are displaced southward across the central SCS, leading to a weak moisture supply and rainfall in Taiwan. The moderate rainfall type features a 30–60-day (10–24 day) ISO resembling that of the weak (strong) rainfall type. The amount of rainfall thus ranges between the strong and weak rainfall types. Major processes regulating the rainfall of type-2 TCs relate to the intensity of the moisture supply associated with anomalous westerly flows from the SCS onto Taiwan, which is jointly modulated by 30–60-day and 10–24-day ISOs.

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Shaojun Zheng, Ming Feng, Yan Du, Xuhua Cheng, and Jiaxun Li

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This study documents the spatial distributions and temporal variations of anticyclonic eddies with identified radii ≥100 km in the equatorial eastern tropical Pacific Ocean [viz., tropical instability vortices (TIVs)] using Lagrangian surface drifters. The TIVs identified from Lagrangian surface drifters are distributed in a band along 5°N and are closely associated with latitudinal barotropically unstable shear between the westward South Equatorial Current (SEC) and the eastward North Equatorial Countercurrent (NECC). Fewer TIVs are identified from February to June when the shear between the SEC and NECC is weak, whereas more TIVs are found from July to January when the shear is enhanced. The number of identified TIVs also exhibits substantial interannual variability, with fewer TIVs identified during El Niño events and more TIVs found during La Niña events. This relationship is likely associated with the interannual variations of the zonal circulation in the equatorial Pacific modulated by El Niño–Southern Oscillation (ENSO).

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Ming Feng, Susan Wijffels, Stuart Godfrey, and Gary Meyers

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The Leeuwin Current is a poleward-flowing eastern boundary current off the western Australian coast, and alongshore momentum balance in the current has been hypothesized to comprise a southward pressure gradient force balanced by northward wind and bottom stresses. This alongshore momentum balance is revisited using a high-resolution upper-ocean climatology to determine the alongshore pressure gradient and altimeter and mooring observations to derive an eddy-induced Reynolds stress. Results show that north of the Abrolhos Islands (situated near the shelf break between 28.2° and 29.3°S), the alongshore momentum balance is between the pressure gradient and wind stress. South of the Abrolhos Islands, the Leeuwin Current is highly unstable and strong eddy kinetic energy is observed offshore of the current axis. The alongshore momentum balance on the offshore side of the current reveals an increased alongshore pressure gradient, weakened alongshore wind stress, and a significant Reynolds stress exerted by mesoscale eddies. The eddy Reynolds stress has a −0.5 Sv (Sv ≡ 106 m3 s−1) correction to the Indonesian Throughflow transport estimate from Godfrey’s island rule. The mesoscale eddies draw energy from the mean current through mixed barotropic and baroclinic instability, and the pressure gradient work overcomes the negative wind work to supply energy for the instability process. Hence the anomalous large-scale pressure gradient in the eastern Indian Ocean drives the strongest eddy kinetic energy level among all the midlatitude eastern boundary currents.

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Jie Ma, Ming Feng, Bernadette M. Sloyan, and Jian Lan

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In this study, low-frequency variability of the meridional temperature transport in the Indian Ocean is examined using a mesoscale-eddy-resolving global ocean circulation model for the period 1979–2014. The dominant empirical orthogonal function (EOF) of the meridional temperature transport is found to be highly influenced by Pacific El Niño–Southern Oscillation (ENSO) through both oceanic and atmospheric waveguides, with the southward temperature transport being stronger during La Niña and weaker during El Niño. A dynamical decomposition of the meridional streamfunction and temperature transport shows that the relative importance of different dynamic modes varies with latitude; these modes act together to contribute to the coherent ENSO response. The Ekman mode explains a larger part of low-frequency variability in overturning and temperature transport north of the equator. Between 25° and 3°S, variations associated with vertical shear mode are of greater importance. The external mode has an important contribution between 30° and 25°S where the western boundary currents impinge on topography. South of 25°S, the variability of the external mode contribution has significant negative correlations with the vertical shear mode, suggesting that the large variability of external mode depends on the joint effects of baroclinicity and topography, such that hydrographic sections alone may not be suitable for deducing changes in the meridional temperature transport at these latitudes.

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Jau-Ming Chen, Tim Li, and Ching-Feng Shih

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The authors investigate persistence characteristics of sea surface temperature (SST) in the South China Sea (SCS) in association with El Niño–Southern Oscillation (ENSO). It is found that a persistence barrier exists around October and November. This fall persistence barrier (FPB) is well recognized in the developing phase of strong ENSO cases, but becomes vague in weak ENSO and normal (non-ENSO) cases. During a strong El Niño developing year, salient features of the SCS SST anomaly (SSTA) associated with the FPB include a sign reversal between summer and winter and a rapid warming during fall. One possible cause of these SST changes, as well as the occurrence of the FPB, is the development and evolution of a low-level anomalous anticyclone (LAAC). The analyses show that the LAAC emerges in the northern Indian Ocean in early northern fall, moves eastward into the SCS during fall, and eventually anchors in the Philippine Sea in northern winter. This provides a new scenario for the generation of the anomalous Philippine Sea anticyclone previously studied. Its eastward movement appears to result from an east–west asymmetry, relative to the anticyclonic circulation center, of divergent flow and associated atmospheric vertical motion/moisture fields. The eastward passage of the LAAC across the SCS warms the underlying SST first via increased absorption of solar heating in October as it suppresses convective activities in situ, and next via decreased evaporative cooling in November and December as the total wind speed is weakened by the outer flows of the eastward-displacing LAAC. As such, the SCS SST changes quickly from a cold to a warm anomaly during fall, resulting in an abrupt change in anomaly patterns and the occurrence of the FPB. Analyses also suggest that the LAAC development during fall is relatively independent from the preceding Indian summer monsoon and the longitudinal propagation features of the ENSO-related Pacific SSTA. The aforementioned ocean–atmosphere anomalies contain an opposite polarity in a strong La Niña event. The low-level circulation anomaly weakens in intensity during weak ENSO cases and simply disappears during normal cases. As a result, the SCS FPB becomes indiscernible in these cases.

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