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Masamichi Inoue

Abstract

Recently collected hydrographic data show that each of the four water mass zones previously identified in Drake Passage is characterized by a distinctly different vertical profile of Brunt-Väisälä frequency. Stratification in Drake Passage is rather weak, resulting in small values of the first baroclinic radius of deformation varying from 17.3 km for the Subantarctic Zone in the north to 7.7 km for the Continental Zone in the south.

Using current meter records collected in 1979 at nine moorings, the vertical structure of the low-frequency currents is described in terms of dynamic normal and empirical modes. Dominance of the barotropic and first baroclinic modes was evident irrespective of mooring location, accounting for typically 83–98% of record variance. The first empirical mode, which explains more than 90% of record variance at most moorings, is surface-intensified, and appears to be a superposition of the barotropic and first baroclinic modes due to the observed modal coupling between the barotropic and first baroclinic modes.

Current variability in Drake Passage is characterized by red spectra with time scales of 20–50 days. At mooring NT in the northern passage, however, currents vary more quickly with time scales of 12–15 days while the vertical structure is richer probably due to topographic waves trapped near the shelf break or generated by an upstream ridge. The second empirical mode is bottom-trapped with short time scales of 7–20 days, indicating the possible effects of bottom topography. At mooring MS5, the bottom-trapped mode in the cross-passage direction is significant and has a time scale comparable to that of the surface-intensified mode, indicating that those two modes are related probably due to current-topography interaction in the rough bottom area of the central passage.

The effect of various stratification profiles on baroclinic instability were considered. For the stratification profiles and mean currents characteristic of Drake Passage, the currents appear to be quite unstable. The zonal wave length of the fastest growing wave varies from 183 km in the Subantarctic Zone to 91 km in the Continental Zone, consistent with the observed eddy scales. Barotropic and first baroclinic modes dominate the fastest growing waves, explaining the observed dominance of the two modes and the modal coupling.

The major conclusion of this study is that the vertical structure of the low-frequency currents at Drake Passage appears to be a manifestation of very active baroclinic instabilities.

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Masamichi Inoue and Susan E. Welsh

Abstract

A basin-scale fine-resolution primitive equation reduced-gravity model forced by the climatological monthly wind of Hellerman and Rosenstein is used to study seasonal variability of the wind-driven upper-layer circulation in the Indo–Pacific region. The model domain is limited to the tropical Pacific Ocean and the eastern Indian Ocean. The use of the open boundary conditions allows the model to account for a net transport of mass from the Pacific to the Indian Ocean. An active eddy field is modeled east of Mindanao in the strong shear zone formed between the North Equatorial Current and the North Equatorial Countercurrent. This eddy field gives rise to the energetic shorter time scale variations in the western Pacific and appears to be responsible for considerable variability in surface currents noted previously. A distinct current bifurcation at the southern coast of New Britain in the Solomon Sea is modeled. The western branch of the resulting flow exits through Vitiaz Strait, while the eastern branch exits through Solomon Strait and St. George's Channel. The modeled annual cycle of the total Indonesian throughflow ranges from 0.5 Sv (Sv ≡ 106 m3 s−1) toward the Pacific in February to 18.1 Sv toward the Indian Ocean in July–October with a mean of 9.8 Sv. Among the three passages carrying the throughflow, Lombok appears to be the most dominant, accounting for nearly 40% of the total. Timor is a close second accounting for 37% of the total, while Savu carries the rest. Seasonal variability in transport through each of the three straits correlates well with pressure gradient between the two basins representing the flow path for each strait. The bulk of the water feeding the throughflow comes through Makassar Strait, with a significant additional contribution coining from the north through the Banda Sea during the peak season. Seasonal variability in the Indonesian throughflow appears to be dominated by the sea level lowering in the eastern Indian Ocean resulting from the seasonal development of coastal upwelling centered around south of Java. This coastal upwelling is in response to the seasonal development of the southeast trade winds during the boreal summer.

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Masamichi Inoue and James J. O'Brien

Abstract

A dynamical forecast model which has been applied to the onset of the 1982/83 El Niño is applied to the decay of this event. The timing of the decay is well predicted, illustrating the flexibility of the dynamical forecast model which could handle an unusual El Niño, i.e., the 1982/83 event with significant wind changes outside a well-recognized site for usual El Niño related wind changes. The results suggest the need to include zonal winds from the entire equatorial Pacific. It appears that the dynamical forecast model based on a linear numerical model forced by ship winds can be used to forecast the timing of the onset and decay of a major El Niño.

The evolution of the 1982/83 El Niño is described using the dynamical model forced by the observed wind. The equatorial Pacific Ocean response during this event is basically that to an eastward translating zonal band of westerly wind anomalies. The observed double peaks in the sea-level record in the eastern Pacific in early 1983 appear to be due to the observed amplitude modulation of the wind anomalies east of 140°W, confirming the previous findings of Tang and Weisberg. It appears that the first Kelvin wave pulse generated in the western Pacific in early 1982 was reflected as a Rossby wave from the eastern boundary. The propagation of this Rossby wave into the central Pacific in July 1982 coincides with the dramatic intensification of the westerly wind anomalies in that region. This suggest a possible air-sea interaction leading to the major onset of this El Niño.

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Masamichi Inoue and James J. O'Brien

Abstract

The feasibility of a forecasting scheme for predicting the onset of a major El Ni-o in the ocean is demonstrated using the linear numerical model of Busalacchi and O'Brien and the interannual components of the shipboard observed wind for 1961-82. The model upper layer thickness anomaly in the eastern Pacific, which was used as the predictand, was estimated after three months of steady wind integration. A lag time of three months is used to permit the propagation of a large El Niflo-type Kelvin wave across the Pacific Ocean. If the necessary wind changes required to generate a large El Niflo type Kelvin wave have already taken place in the western and/or central Pacific, El Niflo could be predicted one to three months in advancefrom knowledge of the wind field alone. Starting from November 1963, three-month steady-wind integrations were performed for the wind condition of each of the seven months (November to May), for each of the 15 years extending from 1964 to 1978. This period includes four El Ni-o years. The probability distribution function of the three-month running mean of the upper layer thickness anomaly in the eastern Pacific was estimated separately for the El Nifio and the non-El Nub groups using "bootstrap" estimates. The separation of the two probability distribution functions allows for the establishment of a criterion for forecasting El Nifio. An independent wind data set for the period 1979-82, which includes the onset of the 1982/83 El Nub, was used to test the feasibility of the forecasting scheme. If the null hypothesis is that a sample year is a non-El Niflo year and based on a forecast criterion of false positive error (false alarm rate) ~ 0.01, which corresponds to false negative error ~ 0.52-0.86 (which corresponds to a probability of detecting the occurrence of an El Niflo ~ 0.14-0.48), the 1982/83 El Niflo would be forecast to be underway following the analysis of the April 1982 surface wind field. Since the objective is to establish a forecasting scheme for predicting the onset of a major El Niflo, the forecast scheme is chosen to be one of low risk and low power in prediction performance.

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Eric Wolanski, Peter Ridd, and Masamichi Inoue

Abstract

A five-month field study of the circulation in the Torres Strait was carried out. Baroclinic effects were negligible. The Arafura Sea and the Coral Sea forced a different tide on either side of Torres Strait, resulting in fluctuations of sea level difference of up to 6 m on either side of the Strait. The tidal dynamics in the Strait were controlled by a local balance between the acceleration, the sea level slope, and the bottom friction. Only 30% of the semidiurnal tidal wave was transmitted through Torres Strait. There were also fluctuations of the high-frequency sea level residuals (up to 0.8 m peak to trough) which appeared to be related to complex flows both through the Strait and across the Strait. Low-frequency sea level fluctuations were incoherent on either side of the Strait, and resulted in fluctuations of the low-frequency sea level differences on either side of the Strait of typically 0.3 m. These sea level gradients and the local wind forcing generated low-frequency current fluctuations through the Strait. These currents were small, being ≤0.1 m s−1, because of the effect of friction which, at low-frequencies, was greatly enhanced by the nonlinear interaction between tidal and low-frequency currents. As a result, the Strait was also fairly impervious to long waves and there was only a negligible (for oceanic budget calculations) low-frequency transport through the Strait. The net current was only 0.01 m s−1 during the 5 months of observations, corresponding to a through-strait current of 10−2 sverdrups.

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Masamichi Inoue, Itsuki C. Handoh, and Grant R. Bigg

Abstract

Tropical cyclogenesis critically depends on the presence of warm water at the sea surface. For the North Atlantic basin as a whole, the tropical storm season starts in May, peaks in September, and then declines, generally following the seasonal warming and cooling of sea surface temperature. In the Caribbean, in contrast, there is a distinct bimodal distribution in the number of tropical storms formed, with peaks in June and October separated by a significant minimum in July. The timing of the observed minimum in tropical cyclogenesis appears to be related to the strengthening of the easterly trade winds over the Caribbean associated with the onset of the so-called veranillo, or midsummer drought (MSD), previously recognized over south-central Mexico, Central America, and parts of the Caribbean. It appears that the observed minimum in cyclogenesis is caused by a combination of environmental factors related to the strengthening of the easterly trade winds across the Caribbean Basin. The strengthening easterly trade winds and their associated changes in wind stress curl give rise to enhanced upwelling in the southwestern Caribbean. This appears to trigger an enhanced local atmosphere–ocean coupling, giving rise to very unfavorable conditions in several environmental variables including cooler sea surface temperature (SST), higher sea level pressure (SLP), increase in outgoing longwave radiation (OLR), and decrease in precipitable water content (PRW). Moreover, strengthening trade winds result in increases in tropospheric vertical wind shear (VSH). Except for OLR, these environmental variables become least favorable for southwestern Caribbean cyclogenesis in July. In contrast, the transition from weak to intense convective activity in the eastern Pacific results in weaker trade winds in the Caribbean in October. The resulting westerly wind anomalies lead to weakening upwelling, warmer SST, enhanced convection, and moist air coupled with weaker VSH in the southwestern Caribbean. All variables, except OLR, then become most favorable for cyclogenesis. In the rest of the Caribbean, some of the conditions, primarily SST related, are not fully met. Nevertheless, the southwestern Caribbean appears to dominate the rest of the Caribbean in terms of setting the stage for either favorable or unfavorable conditions for cyclogenesis in the whole Caribbean Basin. Therefore, ocean–atmosphere interaction over the southwestern Caribbean appears to play an integral role in both suppressing and enhancing tropical cyclogenesis in the Caribbean on an annual basis.

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Warren B. White, Stephen E. Pazan, and Masamichi Inoue

Abstract

The redistribution of observed upper-ocean heat content in the western tropical North Pacific for the four-year period 1979–82 was shown by Pazan et al. to provide a qualitative hindcast capability for the 1982–83 ENSO event. A related study (Inoue et al.) demonstrated that nearly 50% of the observed heat content redistribution in the western tropical North Pacific for the four-year period 1979–82 could be simulated by a linear, upper-ocean, numerical model driven by observed wind stress estimates. In this study, the redistribution of numerical model heat content as evidenced in model dynamic height in the western Pacific during the 22-year period 1964–85 is examined for its ability to hindcast and forecast ENSO events in this period. Complex EOF analysis is applied to the onset phase of ENSO events occurring in 1968–69, 1972–73, 1976–77, and 1982–83; it is used to determine the characteristic redistribution of heat content (dynamic height) prior to the mature phase of ENSO events. The first complex EOF explained 53% of the interannual variance of the numerical model anomalous dynamic height in the 22-year model data records. This analysis finds model dynamic height in the Northern Hemisphere to be characterized by wind-driven westward propagating, baroclinic Rossby wave activity, having a relatively stable period of 3–4 years over the 22-year period. The complex time series associated with these first spatial eigenfunctions are used to construct observed and model hindcast indices that yield high values one year prior to the mature phase of ENSO events of the period. They do not peak when ENSO events do not occur. These indices achieve these high values due to the incidence upon the Philippine coast in fall/winter of a positive anomaly in dynamic height propagating from the east at nondispersive (Rossby) long-wave speeds. The model dynamic height data have the advantage over the observed dynamic height of being available in near-real time (i.e., within a month of the present), which makes it useful in providing a near-real time forecast of future ENSO events. Application of this model forecast index to the upcoming year is discussed.

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