A numerical model is used to mechanistically simulate the oceans’ seasonal cross-equatorial heat transport, and the results of Oort and Vonder Haar (1976). The basic process of Ekman pumping and drift is found to be able to account for a large amount of the cross-equatorial flux. Increased easterly wind stress in the winter hemisphere causes Ekman surface drift poleward, while decreased easterly stress allows a reduction in the poleward drift in the summer hemisphere. When the annual mean flow is removed, a net flow at the surface from summer to winter hemispheres is noted. The addition of planetary and gravity waves to this model does not alter the net cross-equatorial flow, although the planetary waves are clearly seen. On comparison with Oort and Vonder Haar (1976), this adiabatic advective redistribution of heat is seen to be plausible up to 10–20°N, beyond which other dynamics and thermodynamics are indicated.
A simple coupled model is used to examine decadal variations in El Niño–Southern Oscillation (ENSO) prediction skill and predictability. Without any external forcing, the coupled model produces regular ENSO-like variability with a 5-yr period. Superimposed on the 5-yr oscillation is a relatively weak decadal amplitude modulation with a 20-yr period. External uncoupled atmospheric “weather noise” that is determined from observations is introduced into the coupled model. Including the weather noise leads to irregularity in the ENSO events, shifts the dominant period to 4 yr, and amplifies the decadal signal. The decadal signal results without any external prescribed changes to the mean climate of the model. Using the coupled simulation with weather noise as initial conditions and for verification, a large ensemble of prediction experiments were made. The forecast skill and predictability were examined and shown to have a strong decadal dependence. During decades when the amplitude of the interannual variability is large, the forecast skill is relatively high and the limit of predictability is relatively long. Conversely, during decades when the amplitude of the interannual variability is low, the forecast skill is relatively low and the limit of predictability is relatively short. During decades when the predictability is high, the delayed oscillator mechanism drives the sea surface temperature anomaly (SSTA), and during decades when the predictability is low, the atmospheric noise strongly influences the SSTA. Additional experiments indicate that the relative effectiveness of the delayed oscillator mechanism versus the external noise forcing in determining interannual SSTA variability is strongly influenced by much slower timescale (decadal) variations in the state of the coupled model.
The impact of atmospheric internal variability on tropical instability wave (TIW) activity in the eastern equatorial Pacific is examined. To diagnose the atmospheric internal variability, two simulations were performed with a state‐of‐the‐art coupled general circulation model that uses an eddy permitting ocean component model. Standard coupling procedures are implemented in the control simulation. In the experimental simulation, the so‐called interactive ensemble coupling is used, which systematically reduces the contribution of internal atmospheric dynamics to the air‐sea fluxes of heat, momentum and fresh water. In the eastern equatorial Pacific, the reduction of the atmospheric internal variability leads to an enhancement of the available potential energy and higher exchanges from mean to eddy potential energy. The perturbations in the available potential energy and the eddy potential energy contribute to the enhancement in the TIW activity through the increased eddy kinetic energy. Due to the negative correlation between the atmospheric internal variability and TIW activity, the covariance between the momentum flux at the air‐sea interface and the ocean surface currents as well as heat flux at the air‐sea interface and the sea surface temperatures were nearly conserved west of 120°W between the control and the experimental simulations.
A linear quasi-geostrophic stability analysis was performed. The stability is dependent on the shears between the first and second layers and between the second layer and the abyss. By computing the maximum growth rate as a function of the shears, the possibility of a shallow baroclinic instability is discovered. This stability is largely dependent on the shear between layers 1 and 2, and quite independent of the speed in layer 2. The scaling of the shears shown is applicable to 4 deg latitude. As in Philander's study, the stability region increases strongly as the equator is approached. This probably explains the advance of the thermal front out to the 4 to 5 deg latitude zone without development of eddies and their rather sudden development at 80 days.
Results are presented from a 35-year integration of a coupled ocean-atmosphere model. Both ocean and atmosphere are two-level, nonlinear primitive equations models. The global atmospheric model is forced by a steady, zonally symmetric Newtonian heating. The ocean model is solved in a rectangular tropical basin. Heat fluxes between ocean and atmosphere are linear in air-sea temperature differences, and the interfacial stress is proportional to lower-level atmospheric winds. The coupled models produce ENSO-like variability on time scales of 3 to 5 years. Since there is no external time-dependent forcing, these are self-sustained vacillations of the nonlinear system. It is argued that the energetics of the vacillations is that of unstable coupled modes and that the time scale is crucially dependent on the effects of ocean waves propagating in a closed basin.
The annual mean and seasonal cycle in latent heating over the Indian Ocean are investigated using a simple, analytical ocean model and a 3D, numerical, ocean model coupled to a prescribed atmosphere, which permits interaction through sea surface temperature (SST). The role of oceanic divergence in determining the seasonal cycle in evaporation rate is reexamined from the viewpoint that the amount of rainfall over India during the southwest monsoon is a function of the amount of water evaporated over the “monsoon streamtube” as well as orographically induced convective activity. Analysis of Comprehensive Ocean–Atmosphere Dataset (COADS) shows that nearly 90% of the water vapor available to precipitate over India during the southwest monsoon results from the annual mean evaporation field. The seasonal change in direction of airflow, which opens up a pathway from the southern Indian Ocean to the Arabian Sea, rather than the change in evaporation rate is key to explaining the climatological cycle, though the change in latent heating due to seasonal variations is similar to that needed to account for observed interannual-to-interdecadal variability in monsoon rainfall. The simple model shows that net oceanic heat advection is not required to sustain vigorous evaporation over the southern tropical Indian Ocean; its importance lies in ensuring that the maximum evaporation occurs during boreal summer. Also shown with the simple model is that evaporation over the Arabian Sea cannot increase sufficiently to make up for the loss of water vapor accumulated over the southern Indian Ocean should there be a change in circulation such that the Southern Ocean is no longer part of the monsoon streamtube. Analytical, periodic solutions of the linearized heat balance equation for the simple model are presented under the assumption that the residual of net surface heat flux minus rate of change of heat content (DIV) is considered to be an external periodic forcing independent of SST to first order. These solutions, expressed as functions of the amplitude and phase of DIV, lie in two regimes. The first regime is characterized by increases (decreases) in the amplitude of DIV resulting in an increase (decrease) in the amplitude of the solution. In contrast, in the second regime, the amplitude of the solution decreases (increases) as the amplitude of DIV increases (decreases). It is noteworthy that the regime boundaries for SST and latent heating do not necessarily coincide. For the present climate, as determined from COADS, the southern Indian Ocean’s annual harmonics of latent heating and SST lie in the second regime near the border, and so their tendencies are sensitive to the nature of the perturbation to the harmonic in DIV. The southern Indian Ocean’s semiannual harmonic of latent heating lies in the first regime, and so its tendency is robust to the nature of the perturbation to the harmonic in DIV; that of SST lies in the second regime near the border. Contrasting runs of the 3D numerical model, in which the Indonesian throughflow differs by less than 4 × 106 m3 s−1 in the annual mean and less than ±2 × 106 m3 s−1 in seasonal variability, provides new estimates for its potential role in the Indian Ocean heat balance. Net surface heat flux differences of over 20 W m−2 are found along the length and breadth of the southwest monsoon streamtube: particularly noteworthy regions are over the Somali jet and to the east of Madagascar. These changes can be explained in part by the changes in oceanic meridional transport generated by the throughflow as well as by its heat input. Spatial resolution and upper ocean physics are sufficient for the throughflow to retain its zonal jet character across the Indian Ocean and so inhibit meridional overturning. Significantly, its presence reduces the amount of heat imported into the Southern Ocean from the Arabian Sea during boreal summer, so making SSTs in the Arabian Sea higher.
Dynamics readjustment of a stratified ocean model to wind perturbations leads to variations in sea surface temperature (SST) related to the early phases of the observed interannual warming of the tropical Pacific known as El Niño. The role that the atmosphere plays in determining the extent and strength of the SST warming is examined through numerical experiments with varying parameterizations for the atmospheric thermal response to SST anomalies. A prior specification of the atmospheric temperature (even as a function of space and time) amounts to assuming infinite heat capacity for the atmosphere. A zero-heat capacity atmospheric model is constructed, in which the surface air temperature is balanced between the SST and a radiative equilibrium temperature. In the latter model, SST perturbations are damped through radiative relaxation from the atmosphere, rather than through direct cooling to the atmosphere. This greatly increases the lifetime of SST anomalies and increases their areal extent. The effect that the atmospheric parameterization has on an upper ocean model for El Niño is examined. The model tests are conducted by imposing wind perturbation on simple mean states driven by constant winds. Westerly wind perturbations in the western part of the model basin excite Kelvin waves that propagate to the east. Under southerly mean winds, this Kelvin wave propagates to the east without any signal in the SST, but large SST anomalies are generated upon reflection of the Rossby waves. Much weaker changes in the southerly winds near the eastern coast produce SST anomalies that mimic those generated by the westerly wind changes. Such a counter-example to remotely forced Kelvin wave theories for El Niño also arises when the southerly stress anomaly is held off the coast by 200 km. Sea-level changes associated with the westerly and southerly wind perturbations are markedly different. The rapid adjustment of the atmosphere to the ocean appears to be a necessary conditions for successful simulations of the El Niño warming.
