Julian P. McCreary

The monsoon circulation of the Indian Ocean

In this paper, we review observations, theory and model results on the monsoon circulation of the Indian Ocean. We begin with a general overview, discussing wind-stress forcing fields and their anomalies, climatological distributions of stratification, mixed-layer depths, altimetric sea-level distributions, and seasonal circulation patterns (Section 2). The three main monsoon circulation sections deal with the equatorial regime (Section 3), the Somali Current and western Arabian Sea (Section 4), and the Bay of Bengal, seasonally reversing monsoon currents south of India and Sri Lanka, and the eastern and central Arabian Sea (Section 5). For the equatorial regime, we discuss equatorial jets and undercurrents, their interactions with the eastern and western boundaries, and intraseasonal and vertically propagating signals. In the Somali Current section, we describe the oceans responses to the summer and winter monsoon winds, and outline the modelling efforts that have been carried out to understand them. In the Bay of Bengal section, we present observational and modeling evidence showing the importance of remote forcing from the east, which to a large extent originates along the equator. In the following three sections, we review the southern-hemisphere subtropical regime and its associated boundary currents (Section 6), the Indonesian Throughflow (Section 7), the Red Sea and Persian Gulf circulations (Section 8), and discuss aspects of their interactions with other Indian-Ocean circulations. Next, we describe the Indian Oceans deep and shallow meridional overturning cells (Section 9). Model results show large seasonal variability of the meridional overturning streamfunction and heat flux, and we discuss possible physical mechanisms behind this variability. While the monsoon-driven variability of the deep cell is mostly a sloshing motion affecting heat storage, interesting water-mass transformations and monsoonal reversals occur in the shallow cross-equatorial cell. In the mean, the shallow cell connects the subduction areas in the southern subtropics and parts of the Indonesian Throughflow waters with the upwelling areas of the northern hemisphere via the cross-equatorial Somali Current. Its near-surface branch includes a shallow equatorial roll that is seasonally reversing. We close by looking at coupled ocean-climate anomalies, in particular the large events that were observed in the tropical and subtropical Indian Ocean in 1993/94 and 1997/98. These events have been interpreted as an independent Indian-Ocean climate mode by some investigators and as an ENSO-forced anomaly by others.

Structure and Mechanisms of South Indian Ocean Climate Variability

A unique open-ocean upwelling exists in the tropical South Indian Ocean (SIO), a result of the negative wind curl between the southeasterly trades and equatorial westerlies, raising the thermocline in the west. Analysis of in situ measurements and a model-assimilated dataset reveals a strong influence of subsurface thermocline variability on sea surface temperature (SST) in this upwelling zone. El Nino-Southern Oscillation (ENSO) is found to be the dominant forcing for the SIO thermocline variability, with SST variability off Sumatra, Indonesia, also making a significant contribution. When either an El Nino or Sumatra cooling event takes place, anomalous easterlies appear in the equatorial Indian Ocean, forcing a westward-propagating downwelling Rossby wave in the SIO. In phase with this dynamic Rossby wave, there is a pronounced copropagation of SST. Moreover, a positive precipitation anomaly is found over, or just to the south of, the Rossby wave-induced positive SST anomaly, resulting in a cyclonic circulation in the surface wind field that appears to feedback onto the SST anomaly. Finally, this downwelling Rossby wave also increases tropical cyclone activity in the SIO through its SST effect. This coupled Rossby wave thus offers potential predictability for SST and tropical cyclones in the western SIO. These results suggest that models that allow for the existence of upwelling and Rossby wave dynamics will have better seasonal forecasts than ones that use a slab ocean mixed layer. The lagged-correlation analysis shows that SST anomalies off Java, Indonesia, tend to precede those off Sumatra by a season, a time lead that may further increase the Indian Ocean predictability.

Meridional Circulation Cells and the Source Waters of the Pacific Equatorial Undercurrent

Abstract A 3½-layer model is used to study the meridional circulation cells that provide the source waters of the Pacific Equatorial Undercurrent (EUC). Its three active layers represent tropical, thermocline, and upper-intermediate waters, respectively, and across-interface flow between the layers parameterizes the processes of upwelling, subduction, and diapycnal mixing. Solutions are driven by climatological winds in a domain resembling the Pacific basin from 35°S to 55°N. An additional forcing mechanism is a specified inflow into layer 3 across the open southern boundary and a compensating outflow from layers 1 and 2 along the western boundary just north of the equator; the resulting circulation simulates the Pacific interocean circulation (IOC), in which intermediate water enters the South Pacific and the same amount of thermocline and tropical waters exit via the Indonesian Throughflow. Five meridional cells contribute to the EUC in the main-run solution: north and south Subtropical Cells (STCs), no...

Influence of the ITCZ on the Flow of Thermocline Water from the Subtropical to the Equatorial Pacific Ocean

Abstract The flow of thermocline water from the subtropical to the equatorial Pacific Ocean is investigated using a 2½-layer numerical model. In this system, the lower of the two active layers represents the thermocline region of the ocean, and the upper layer simulates the near-surface region including the mixed layer. Water is allowed to move between the layers via an across-interface velocity that parameterizes the processes of upwelling and subtropical subduction. Solutions are obtained in a basin that resembles the Pacific basin, and they are forced by Hellerman and Rosenstein winds. The primary result is that the intertropical convergence zone (ITCZ) creates a potential vorticity barrier that inhibits the direct flow of lower-layer (thermocline) water from the subtropical North Pacific to the equator. Lower-layer water must first flow to the western boundary north of the ITCZ and only then can it move equatorward in a western boundary current to join the Equatorial Undercurrent. Another result is th...

Meridional Asymmetry and Energetics of Tropical Instability Waves

Abstract One of the striking features of tropical instability waves (TIWs) is that they appear to be more prominent north of the equator. A linearized, 2½-layer ocean model is used to investigate effects of various asymmetric background states on structures of equatorial, unstable waves. Our results suggest that the meridional asymmetry of TIWs is due to asymmetries of the two branches of the South Equatorial Current (SEC) and of the equatorial, sea surface temperature front; it is not due to the presence of the North Equatorial Countercurrent. Energetics analyses indicate that frontal instability associated with the equatorial, SST front, as well as barotropic instability due to shear associated with the SEC, are energy sources for the model TIWS.

Dynamics of the Pacific Subsurface Countercurrents

A hierarchy of models, varying from 2 -layer to 4 -layer systems, is used to explore the dynamics of the 11 22 Pacific Subsurface Countercurrents, commonly referred to as ‘‘Tsuchiya Jets’’ (TJs). The TJs are eastward currents located on either side of the equator at depths from 200 to 500 m and at latitudes varying from about 28 to 78 north and south of the equator, and they carry about 14 Sv of lower-thermocline (upper-intermediate) water throughout the tropical Pacific. Solutions are found in idealized and realistic basins and are obtained both analytically and numerically. They are forced by winds and by a prescribed Pacific interocean circulation (IOC) with transport M (usually 10 Sv), representing the outflow of water in the Indonesian passages and a compensating inflow from the Antarctic Circumpolar Current. Analytic solutions to the 2 -layer model suggest that the TJs are geostrophic currents along arrested fronts. 1 2 Such fronts are generated when Rossby wave characteristics, carrying information about oceanic density structure away from boundaries, converge or intersect in the interior ocean. They indicate that the southern and northern TJs are driven by upwelling along the South American coast and in the ITCZ band, respectively, that the northern TJ is strengthened by a recirculation gyre that extends across the basin, and that TJ pathways are sensitive to stratification parameters. Numerical solutions to the 2 -layer and 4 -layer models confirm the analytic results, 11 22 demonstrate that the northern TJ is strengthened considerably by unstable waves along the eastward branch of the recirculation gyre, show that the TJs are an important branch of the Pacific IOC, and illustrate the sensitivity of TJ pathways to vertical-mixing parameterizations and the structure of the driving wind. In a solution to the 2 -layer model with M 5 0, the southern TJ vanishes but the northern one remains, being 1 2 maintained by the unstable waves. In contrast, both TJs vanish in the M 5 0 solution to the 4 -layer model,

Dynamics of Biweekly Oscillations in the Equatorial Indian Ocean

Variability of the wind field over the equatorial Indian Ocean is spread throughout the intraseasonal (10–60 day) band. In contrast, variability of the near-surface field in the eastern, equatorial ocean is concentrated at biweekly frequencies and is largely composed of Yanai waves. The excitation of this biweekly variability is investigated using an oceanic GCM and both analytic and numerical versions of a linear, continuously stratified (LCS) model in which solutions are represented as expansions in baroclinic modes. Solutions are forced by Quick Scatterometer (QuikSCAT) winds (the model control runs) and by idealized winds having the form of a propagating wave with frequency and wavenumber kw. The GCM and LCS control runs are remarkably similar in the biweekly band, indicating that the dynamics of biweekly variability are fundamentally linear and wind driven. The biweekly response is composed of local (nonradiating) and remote (Yanai wave) parts, with the former spread roughly uniformly along the equator and the latter strengthening to the east. Test runs to the numerical models separately forced by the x and y components of the QuikSCAT winds demonstrate that both forcings contribute to the biweekly signal, the response forced by y being somewhat stronger. Without mixing, the analytic spectrum for Yanai waves forced by idealized winds has a narrowband (resonant) response for each baroclinic mode: Spectral peaks occur whenever the wavenumber of the Yanai wave for mode n is sufficiently close to kw and they shift from biweekly to lower frequencies with increasing modenumber n. With mixing, the higher-order modes are damped so that the largest ocean response is restricted to Yanai waves in the biweekly band. Thus, in the LCS model, resonance and mixing act together to account for the ocean’s favoring the biweekly band. Because of the GCM’s complexity, it cannot be confirmed that vertical mixing also damps its higher-order modes; other possible processes are nonlinear interactions with near-surface currents, and the model’s low vertical resolution below the thermocline. Test runs to the LCS model show that Yanai waves from several modes superpose to form a beam (wave packet) that carries energy downward as well as eastward. Reflections of such beams from the near-surface pycnocline and bottom act to maintain near-surface energy levels, accounting for the eastward intensification of the near-surface, equatorial field in the control runs.

On the Dynamics of the Throughflow from the Pacific into the Indian Ocean

Abstract The circulation forced by an inflow of water through an eastern ocean boundary is investigated using two linear, viscid, and continuously stratified models. One of the models has a flat bottom, and solutions are obtained analytically; the other has a continental shelf, and solutions are found numerically. Without vertical mixing all the inflow continues across the ocean. With vertical mixing, however, part of it bends poleward to generate a coastal circulation. The presence of a shelf displaces the coastal currents offshore, but otherwise changes their structure and magnitude very little. Solutions suggest that the southward bending of the throughflow from the Pacific into the Indian Ocean may contribute to the Leeuwin Current off western Australia, but that it is not the dominant mechanism for driving the circulation there.

The Relationship between Oscillating Subtropical Wind Stress and Equatorial Temperature

Abstract An earlier study showed that an atmosphere–ocean model of the Pacific develops a midlatitude oscillation that produces decadal sea surface temperature (SST) variability on the equator. The authors use the ocean component of this model to understand better how subtropical wind stress oscillations can cause such SST variability. The model ocean consists of three active layers that correspond to the mixed layer, the thermocline, and intermediate water, all lying above a motionless abyss. For a steady wind, the model develops a subtropical cell (STC) in which northward surface Ekman transport subducts, flows equatorward within the thermocline, and returns to the surface at the equator. Analytic results predict the models equatorial temperature, given some knowledge of the circulation and external forcing. A prescribed subtropical wind stress anomaly perturbs the strength of the STC, which in turn modifies equatorial upwelling and equatorial SST. The transient response to a switched-on wind perturbat...

Influence of Equatorial Dynamics on the Pacific North Equatorial Countercurrent

The Pacific North Equatorial Countercurrent (NECC) is generally not well simulated in numerical models. In this study, the causes of this problem are investigated by comparing model solutions to observed NECC estimates. The ocean model is a general circulation model of intermediate complexity. Solutions are forced by climatological and interannual wind stresses, t 5 (t x, t y), from Florida State University and the European Centre for MediumRange Weather Forecasts. Estimates of the observed NECC structure and transport are prepared from expendable bathythermograph data and from the ocean analysis product of NOAA/National Centers for Environmental Prediction. In solutions forced by climatological winds, the NECC develops a discontinuity in the central Pacific that is not present in the observations. The character of the error suggests that it arises from the near-equatorial (5 8S‐ 58N) zonal wind stress, t x, being relatively too strong compared to the y derivative of the wind stress curl term, (curlt )y, associated with the intertropical convergence zone. This is confirmed in solutions forced by interannual winds, which exhibit a wide range of responses from being very similar to the observed NECC to being extremely poor, the latter occurring when near-equatorial t x is relatively too strong. Results show further that the model NECC transport is determined mainly by the strength of (curlt )y, but that its structure depends on near-equatorial t x; thus, NECC physics involves equatorial as well as Sverdrup dynamics. Only when the two forcing features are properly prescribed do solutions develop a NECC with both realistic spatial structure and transport.