Abstract

The adjustment between two interconnecting basins of stratified fluid, resulting from the formation of a pool of dense water in one, is investigated with an 18-level numerical general circulation model (CYCM). It is found that basic features of the adjustment, i.e., (i) the generation of low-frequency, coastally trapped Kelvin waves, which carry information about the alongshore pressure gradient, created by the denser water, toward the dividing ridge; (ii) the subsequent generation of barotropic double Kelvin waves by the JEBAR (Joint Effect of Baroclinicity and Relief) effect, which propagate infinitely rapidly along the ridge setting up an anticyclonic gyre; (iii) the generation of further coastally trapped Kelvin waves at the opposite coast due to an alongshore pressure gradient, created by upwelling at the ridge edge associated with the closure of the barotropic gym; (iv) the generation of a coastally confined cyclonic gyre at the opposite ridge ~ by the JEBAR effect and viscous and diffusive processes as the coastally trapped Kelvin waves crow off the ridge, are reproduced by an equivalent linear, f-plane, two-layer model with the topography confined to the lower layer. Features, which arise from the increased number of vertical degrees of freedom in the GCM, are found to include (i) a further sheltering of the upper levels from the presence of the midge due to energy in the coastally trapped Kelvin waves being redistributed between various vertical modes; (ii) a baroclinic adjustment at the ridge face to eradicate any horizontal density gradients there, which takes the form of a Kelvin-type wave trapped against the face not previously described; and (iii) penetration of the circulation below the top of the ridge in the second basin due to vertical viscous and diffusive effects, as well as the dispersion of coastally trapped Kelvin waves of differing vertical modes.

First-order estimates of the magnitude of the baroclinic signals propagating on and away from the ridge are found by consideration of continuity of man flux. Assuming that the fields may be decomposed into local vertical normal modes, a simple model with uniform stratification is used to derive analytic estimates of the relative amplitudes of the vertical modes which compare favorably with those calculated from the GCM. Assuming that the energy of the coastally trapped Kelvin wave initially incident on the ridge is chiefly in the first baroclinic mode, then it is found that the higher the ridge, the low the energy scattered into the first-order mode propagating across the ridge (that in the higher-order modes remains small), and the greater the energy scattered into a first-order mode propagating along the face of the ridge and along the opposite coast. Also, a higher ridge results in more energy being scattered into higher-order mode, coastally trapped Kelvin waves propagating in the second basin. The magnitude of the barotropic response is found to be such as to cancel the vertical average of the baroclinic flow over the ridge face associated with the incident coastally trapped Kelvin waves for the anticyclonic gyre, and dependent on viscous and diffusive processes for the cyclonic gyre.

As well as ridges of top-hat cross section and differing heights, ridges with a pyramid cross section, i.e., “sloping” sides and with an island (cf. Iceland) on top, are considered.

The implications of features of the adjustment for modeling the Atlantic thermohaline circulation and the transport of passive tracers am discussed, as these highlight aspects of the formulation of topography in Gems, which merit further consideration in the general development of GCMs.

Abstract

The modifications to the structure and propagation characteristics of the double Kelvin wave due to various parameterizations of dissipative effects; namely, a lateral eddy viscosity and diffusion with coefficients A_M , A_H , and Rayleigh friction and Newtonian cooling with coefficients r_M , r_H , are examined. Under the assumption of geostrophy in the alongstep direction, and A_M = A_H , r_M = r_H , analytic expressions may be obtained and asymptotic limits derived.

Abstract

Whether seasonally phased-locked persistence and predictability barriers, similar to the boreal spring barriers found for El Niño–Southern Oscillation (ENSO), exist for the tropical Indian Ocean sector climate is investigated using observations and hindcasts from two coupled ocean–atmosphere dynamical ensemble forecast systems: the National Centers for Environmental Prediction (NCEP) Coupled Forecast System (CFS) for 1990–2003, and the NASA Seasonal-to-Interannual Prediction Project (NSIPP) system for 1993–2002. The potential predictability of the climate is also assessed under the “perfect model/ensemble” assumption.

Lagged correlations of the indices calculated over the east and west poles of the Indian Ocean dipole mode (IDM) index show weak sea surface temperature anomaly (SSTA) persistence barriers in boreal spring at both poles, but the major decline in correlation at the east pole occurs in boreal midwinter for all start months with an almost immediate recovery, albeit negative correlations, until summer approaches. Processes responsible for the change in sign of SSTAs associated with a major IDM event effect a similar change on much weaker SSTAs. At the west pole, a major decline occurs at the end of boreal summer for fall and winter starts when the thermocline deepens with the seasonal cycle and coupling between the ocean and atmosphere is weak.

A decline in skillful prediction of SSTA at the east pole over boreal winter is also found in the hindcasts, but the relatively large thermocline depth anomalies are skillfully predicted through this time and skill in SSTA prediction returns. A predictability barrier at the onset of the boreal summer monsoon is found at both IDM poles with some return of skill in late fall. Potential predictability calculations suggest that this barrier may be overcome at the west pole with improvements to the forecast systems, but not at the east pole for forecasts initiated in boreal winter unless the ocean is initialized with a memory of fall conditions.

Abstract

An analytical model of the mean wind-driven circulation of the North Atlantic and Caribbean Sea is constructed based on linear dynamics and assumed existence of a level of no motion above all topography. The circulation around each island is calculated using the island rule, which is extended to describe an arbitrary length chain of overlapping islands. Frictional effects in the intervening straits are included by assuming a linear dependence on strait transport. Asymptotic expansions in the limit of strong and weak friction show that the transport streamfunction on an island boundary is dependent on wind stress over latitudes spanning the whole length of the island chain and spanning just immediately adjacent islands, respectively. The powerfulness of the method in enabling the wind stress bands, which determine a particular strait transport, to be readily identified, is demonstrated by a brief explanation of transport similarities and differences in earlier numerical models forced by various climatological wind stress products.

In the absence of frictional effects outside western boundary layers, some weaker strait transports are in the wrong direction (e.g., Santaren Channel) and others are too large (e.g., Old Bahama Channel). Also, there is no western boundary current to the east of Abaco Island. Including frictional effects in the straits enables many of these discrepancies to be resolved. Sensitivity in strait transport to friction parameter is explored for the Caribbean island chain. Transport reversal in the minor passages around the Bahama Banks and Windward Passage as the friction parameter increased is noted. The separation latitude of the western boundary currents on Cuba's east coast moves southward as the friction parameter increases from zero, so making the Great Inagua Passage transport a better proxy for the Windward Passage transport. Major discrepancies with observations, namely, eastward instead of westward flow in Grenada Passage, a southward instead of northward Guyana Current, and hence a Caribbean circulation and Florida Current fed wholly by water masses of North Atlantic origin, cannot be resolved. However, they are simply overcome by extending the model to three layers with the wind-driven and upper limb of the thermohaline circulation confined to the top layer, and the lower limb of the thermohaline circulation to the bottom layer. If it is assumed that over the latitudes of the Caribbean there is no significant upwelling/downwelling between the layers, then the thermohaline-driven circulation is effectively a western boundary current, and all of the results for the analytical wind-driven-only model carry over, but with the value of the upper-layer transport streamfunction on the boundary of the American continent set to the magnitude of the thermohaline circulation rather than that on Africa. Exploration of strait transport sensitivity to friction parameter gives that realistic transports through the passages of the Windward Islands are only obtained if the friction coefficient in these passages is an order of magnitude larger than that in the western passages. Windward Passage transport reverses from south to north for a smaller value of the friction parameter than in the absence of the thermohaline circulation; Anegada and Mona Passages are robust inflow passages for the Caribbean Sea. South Atlantic water masses enter the Caribbean Sea through the passages from Grenada Passage to Martinique Passage. As the friction coefficient in the Windward Islands passages increases from zero, South Atlantic water mass is partially deflected northward along the outer arc of the islands and enters the Caribbean Sea through the passages up to Anegada Passage. The model suggests that for realistic friction parameters, South Atlantic water masses are unlikely to be found in the more western passages, or in the western boundary current skirting the edge of the Bahama Banks.

Abstract

A simple analytical model of the closure of the wind-driven gyres in the tropical west Pacific is developed to illustrate the role of Halmahera and variations in the Pacific wind stress on the water mass composition of the flow from the Pacific to the Indian Ocean through the Indonesian archipelago—that is, the Indonesian Throughflow. The model is expressed in terms of the streamfunction, ψ, for the depth-integrated velocity field. There are three principal dynamical states, and therefore composition regimes. In the absence of Halmahera, the throughflow is composed of water wholly originating from the South Pacific (SP) if the streamfunction at the interior edge of the western boundary layer at the latitude of the northern tip of Papua New Guinea (PNG), ψ _N , is greater than that on the Asian continent, taken as zero, which is typically the situation in the Northern Hemisphere late summer. The throughflow is composed of water of wholly North Pacific origin (NP) if ψ _N ≤ ψ _A , the value of the streamfunction on Australia–PNG, which corresponds to winter conditions. The throughflow is fed by both the fresh Mindanao Current and the saltier South Equatorial Current (SEC) otherwise. If Halmahera is taken into account, the criteria determining the composition regimes are modified. Its presence results in a fresher throughflow, 30% NP–70% SP compared with 100% SP, as ψ typically decreases northwards in the equatorial west Pacific. The result of averaging over perturbations in ψ _N , ψ _A is also considered.

Estimates of the throughflow composition from observed hydrography imply a predominantly NP source. A possible explanation is nonlinear retroflection of the SEC into the North Equatorial Countercurrent (NECC), which would lead to the throughflow being fed by the Mindanao Current for much of the year. However, the simple model predicts that a large amount of water of NP origin enters the Indonesian basins before exiting to feed the NECC, even when the throughflow is predicted to be wholly SP. Mixing within the basins would modify the composition of the NECC and throughflow. Different west Pacific states and their effect on the composition are investigated further with a flat-bottomed, homogeneous numerical general circulation model with a simplified geometry and climatological monthly mean forcing.

Abstract

The effect of interannual variability in wind stress on the transport between the Pacific and Indian Oceans through the Indonesian archipelago is investigated using a multilevel, numerical, general circulation model (GCM). The experiments are conducted in a spinup, anomaly mode: the GCM is initially at rest with a horizontally uniform stratification typical of tropical oceans. The wind stress anomalies are derived from the European Centre for Medium-Range Weather Forecasts 1000-mb winds for 1985–89.

The modeled variability is sensitive to the specification of bottom topography and archipelago geometry. Two model configurations are considered: (i) flat-bottomed with the option of sills within a simplified, idealized archipelago and (ii) realistic bottom topography and a complex archipelago.

In the first model, there is northward depth-integrated transport anomaly lasting 2½ years with phase 6 months ahead of the collapse of the equatorial easterlies prior to the 1986–87 El Niño. The peak transport is about 5 Sv (Sv ≡ 10⁶ m³ s⁻¹), which is reduced to 3 Sv if shallow sills are present in the archipelago. There follows a southward transport anomaly, peaking at about 6 Sv during the ensuing U Nina. The baroclinic transport is consistent with cold and warm equatorial Rossby waves partially scattered into the archipelago during El Niño and La Nina, respectively. The reduction in depth-integrated throughflow generated by the reduction in southern midlatitude westerlies over 1985–86 results in a distinct signal in the baroclinic transport, and similarly in their increase again over 1988. Contrasting models with open and wholly blocked archipelagos yields heat content differences, measured as the temperature averaged over the upper 300 m, of typically O(0.05°C) in the west equatorial Pacific and O(0.3°C) in the Pacific western boundary layers and Indian Ocean. This suggests that throughflow variations could have implications for the timing of ENSO.

In contrast, the second model has a very weak throughflow with peak-to-peak amplitude of only 3 Sv. Its archipelago is a relatively poor transmitter of equatorial waves, and its f/H contours are such that the depth-integrated throughflow is driven in part by the southerly extent of the South Pacific midlatitude wind stress anomalies. However, interestingly, the model still exhibits a decrease in throughflow in excess of 1 Sv prior to the onset of the 1986–87 El Niño due to variations in the southern midlatitude wind stress over the Pacific, which is also accompanied by a signature in the baroclinic transport.

Also, contrasting homogeneous and stratified versions of the models demonstrates the importance of JEBAR over direct wind stress forcing of the depth-integrated throughflow. A closed form analytical expression for the transport in terms of an integral over a closed path formed by uniquely defined f/H contours, an island rule, is derived and used to help interpret the results.

Abstract

A numerical ocean general circulation model is used to investigate the early stages in the adjustment to equilibrium of an ocean initially at rest with imposed uniform meridional potential temperature gradients, which yield density gradients representative of those observed in the North Atlantic. The main feature of the adjustment during the early stages (the first year) is the formation and decay of a “subtropical” (warm core) and a “subpolar” (cold core) density gyre. The gyres are formed by the irreversible winding-up of the initially zonal isotherms by first baroclinic mode, coastally-trapped, dissipative Kelvin waves. This phase and the north–south asymmetry arising from the variation in viscous–diffusive Kelvin wave properties with latitude were discussed in Part I of this series.

Within a month β-effects become significant, especially in the evolution of the southern gyre, which develops a distinct east–west asymmetry through western intensification and long planetary wave propagation of boundary information from the east. In the north, penetration of boundary information into the interior is attributed to diffusive effects, because the maximum allowable planetary wave frequency is lower than that of the Kelvin wave signal. This phase in the adjustment is described in Part A of the present paper. A simple model based on the quasi-geostrophic potential vorticity equation for a single vertical mode is used to explain and quantitatively assess the effects of finite spatial resolution and viscous and diffusive processes on the Kelvin wave–planetary wave dynamics.

The longshore temperature gradients at the middle depths of the ocean are considerably reduced by first baroclinic mode Kelvin wave propagation. The adjustment then enters another phase characterized by the destruction of the density gyres by either planetary wave or diffusive processes, there being no further significant Kelvin wave propagation. This phase is described and investigated in Part B, using the simple model developed in Part A with modified boundary conditions.

Abstract

The effect of interannual variations in the Pacific wind stress on the barotropic and baroclinic components of the flow between the Pacific and Indian Oceans through the Indonesian Archipelago, the Indo-Pacific Throughflow, is investigated using a numerical ocean general circulation model (GCM) with a simplified geometry. In agreement with the modified Island Rule, variations in the depth-integrated throughflow are generated by zonal wind stress variations over the Pacific at the latitudes of the tips of the Australian continent, assuming the Pacific basin is flat bottomed. Wind stress variations at other latitudes generate variations in the depth-integrated transport only if they produce a depth-integrated pressure drop along the oceanic eastern boundary through the archipelago. From the Island Rule, alongshore wind stress variations on the west coasts of Australia and South America produce direct variations, but the observed signal is weak at interannual periods. Baroclinic variations in the throughflow are generated by baroclinic waves entering the archipelago, or by interaction between the barotropic component and the sills within the archipelago.

Shallow sills within the archipelago are found to only partially block the throughflow. The dynamical constraint, that quasi-steady flow is parallel to f/H contours, is relaxed by weak friction; the reduction in throughflow is only 30% for sills blocking 70% of the water column at 9°S. In the absence of sills, the throughflow response to a southern midlatitude wind stress anomaly is purely barotropic at interannual periods. Sills within the archipelago induce a baroclinic adjustment resulting in a surface trapping of the transport, which is in phase with the wind stress variations. Also, the depth-integrated pressure gradient through the archipelago produced by the topographic upwelling and downwelling is always directed to reduce the magnitude of the throughflow. For equatorial wind stress variations, the associated equatorial baroclinic Rossby waves are partially scattered into the archipelago. Sills within the archipelago block the transmitted equatorial Rossby waves, which would enhance the throughflow except that the baroclinic response to the topographic upwelling and downwelling negates the effect for part of the forcing cycle. Nonlinearity in the eastern equatorial response to an oscillating equatorial wind stress anomaly results in a mean throughflow from the Pacific to Indian Ocean. The combined effect of equatorial and southern midlatitude wind stresses, as typified by climatological mean values, yields a throughflow that is reduced by the inclusion of sills within the archipelago.

Finally, in a comparison with the response in a GCM with a wholly blocked archipelago, the heat content anomaly (measured as the temperature averaged over the upper 300 m) in the equatorial Pacific is similar. However, the heat content anomaly in the archipelago and Indian Ocean is typically five to ten times larger than the equatorial anomaly difference between GCMs with an open/partially open and blocked archipelago. This is attributable to the difference in widths between equatorial and coastal baroclinic waveguides. The result suggests that the effect of variations in the throughflow on the Southern Oscillation is most likely felt in the archipelago and Indian Ocean.

Abstract

Simple theory gives that the depth-integrated flow between the Pacific and Indian Oceans, on interannual timescales and longer, is driven by the integral of the wind stress along a line from the northern tip of Papua–New Guinea across the equatorial Pacific, south along the South American west coast, westward across the South Pacific at the latitude of the southern tip of Australia, and northward along the west coast of Australia. Evaluation of this integral using ECMWF/JMA 1000-mb wind data for the decade 1980–89 yields an interdecadal and interannual signal. The interannual signal peaks in 1981 and 1985, then decreases sharply through the ensuing ENSO events. The variations are chiefly attributable to variations in the integral of wind stress across the midlatitude line in the South Pacific. The presence of the shallow sills within the Indonesian seas will partially block the midlatitude contribution, but local baroclinic adjustment over the sills will reduce the blocking effect and produce a corresponding interannual variation in upper-layer transport through the seas. Two mechanisms by which variations in throughflow magnitude could contribute to warm water pileup in the west Pacific are proposed.

Abstract

Prospects for forecasting Indian dipole mode (IDM) events with lead times of a season or more are examined using the NASA Seasonal-to-Interannual Prediction Project (NSIPP) coupled-model forecast system. The mean climatology of the system over the sector is reasonable, as determined from an almost-century-long run without data assimilation. However, the system presents biases, for example, too cool sea surface temperatures (SSTs), too shallow thermocline, and too strong southeasterlies along the Sumatra–Java coast in the east, and too warm SSTs, too deep thermocline, and too weak extension of the southeast trades into the Findlater jet in the west. These suggest coupling between the ocean and atmosphere is stronger, and the SST–clouds–shortwave radiation negative feedback less effective, than observed in the east with the opposite holding true in the west. Also, the negative zonal gradient in SST in the eastern equatorial basin, in contrast with the positive observed, suggests that equatorial Kelvin and Rossby coupled modes may have a different character from observed. Biases identified in the seasonal cycle, which may affect the strength and timing of IDM events, include a delayed onset of the boreal summer monsoon in the west, and a prolonged boreal summer monsoon in the east.

Eight major positive IDM events occur during the almost-century-long run over a range of El Niño–Southern Oscillation phases with a tendency to occur post–El Niño/pre–La Niña. Consistent with the identified air–sea interaction biases, the cold (warm) anomaly at the east (west) pole tends to be stronger (weaker) than observed. Also, the cold anomaly extends much farther westward and is more equatorially trapped than observed; its slow westward propagation and the structure of the associated fields is reminiscent of an unstable, coupled Rossby mode with SST governed by lateral advection due to the westward displacement of the convective anomaly from the heat source. Otherwise, the life cycle of the eight-event composite is similar in seasonal phase locking and mechanisms of evolution and decay to the canonical event.

For the decade from 1993 to the present, there were major positive IDM events in 1994 and 1997/98. Monthly mean SST anomalies over the western pole are well hindcast by the ocean component of the NSIPP system forced by observed surface fluxes with SST damped to observed values, and in which subsurface temperature data available in real time are assimilated; these data are very sparse over the Indian Ocean. Over the eastern pole, the SST anomalies are well hindcast except for the 1997/98 event, when it is too cool. The ensemble mean hindcast of the zonal surface wind anomaly of the central basin by the atmosphere component of the NSIPP system forced by observed SST is too weak during both events. These hindcasts provide initial conditions for the coupled system forecasts. Forecast ensembles for the decade 1993 onward, generated by the coupled system, give monthly mean SST anomalies averaged over the east and west poles of the IDM in agreement with observations at lead times of three months. The cool anomaly at the eastern pole is slightly too large in 1997/98, and the onset of the warm anomaly in 1997 is delayed by a month or so; its peak and decay are correctly timed. At lead times of six months, there is a significant deterioration in the forecast at the eastern pole with either false positive or negative alarms generated annually in boreal fall; that at the western pole remains good. These results are very encouraging and suggest that major IDM events have the potential to be forecast a season or more in advance.