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Roxana C. Wajsowicz and Paul S. Schopf

Abstract

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.

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Ben P. Kirtman and Paul S. Schopf

Abstract

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.

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Paul S. Schopf and Robert J. Burgman

Abstract

A simple mechanism is offered that accounts for a change in the long-term (decadal scale) mean of ocean temperatures as the El Niño–Southern Oscillation (ENSO) amplitude changes. It is intended as an illustration of a kinematic effect of oscillating a nonlinear temperature profile with finite-amplitude excursions that will cause the Eulerian time mean temperature to rise (fall) where the curvature of the temperature is positive (negative) as the amplitude of the oscillations increases. This mechanism is found to be able to mimic observed changes in the mean sea surface temperatures in the Pacific between the 1920s, 1960s, and 1990s due to the changing ENSO amplitude. The effects alter both the calculated mean surface temperatures and the time mean temperatures at depth. It also results in a skewness of the temperature distribution that shares many properties with the observed SST. In this model, the time-local gradients of temperature never change if referenced to a single isotherm (i.e., the Lagrangian description is one of DT/Dt = 0). This implies that changes in the amplitude of ENSO will have no influence on the stability of the underlying system, and that the simple Eulerian decadal mean temperature structure has no predictive value. This is in direct contrast to recent work that ascribes a change in ENSO statistics as due to a change in the background state.

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Bohua Huang, Paul S. Schopf, and J. Shukla

Abstract

The tropical Atlantic variability is composed of three major patterns of significant importance for variability and predictability of climate in the Atlantic sector. They are the southern tropical Atlantic (STA) pattern with anomalous sea surface temperature (SST) fluctuations expanding from the Angolan coast to the central equatorial ocean, the northern tropical Atlantic (NTA) pattern centered near the northern African coast, and the southern subtropical Atlantic (SSA) pattern in the open subtropical ocean.

Previous studies have suggested that both the regional air–sea coupling and remote forcing from outside the basin may affect the formation of these patterns and their variability. A specially designed global coupled ocean– atmosphere general circulation model, which eliminates air–sea feedback outside the Atlantic, reproduces the major features of these observed patterns realistically. This suggests that these patterns originate from air–sea coupling within the Atlantic Ocean or by the oceanic responses to atmospheric internal forcing, in which there is no anomalous forcing external to the Atlantic Ocean. The effect of the Pacific El Niño–Southern Oscillation (ENSO) seems to modulate their temporal evolution through influencing atmospheric planetary waves propagating into the basin.

One of the problems of the model simulation is that the STA pattern as represented by the SST fluctuations centered at the Angolan coast is weak in the equatorial waveguide. Unlike the observations, the model SST fluctuations around the equator are largely unconnected with the changes in the southeastern part of the ocean. This lack of connection between these two parts of the tropical ocean is related to a model systematic bias of excessive southward shift of the model intertropical convergence zone to around 10°S in boreal spring. In the coupled model, the air–sea feedback forms an artifical “warm pool” to the south of the equator extending from the Brazilian coast nearly to the eastern boundary. This warm pool blocks the connection between the fluctuations in the equatorial and the southern part of the ocean. Due to this systematic bias, this model did not simulate the STA pattern adequately.

Several sensitivity experiments have been conducted to further examine the mechanisms of the anomalous SST patterns. The results demonstrate that both the NTA and SSA patterns are mainly associated with the thermodynamic air–sea interactions, while the STA pattern is likely more closely associated with the dynamical response of the equatorial and tropical ocean to the surface wind forcing. Moreover, results from a simulation with a time-independent correction term of the surface heat flux show that the simulated STA mode can be significantly strengthened and have a more realistic spatial structure if the model mean SST errors are reduced.

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Robert J. Burgman, Paul S. Schopf, and Ben P. Kirtman

Abstract

Decadal variations in the amplitude of El Niño and the Southern Oscillation have been the subject of great interest in the literature for the past decade. One theory suggests that ENSO is best described as a stable system driven by linear dynamics and that stochastic atmospheric forcing is responsible for the development and modulation of ENSO on interannual as well as decadal time scales. Another theory suggests that ENSO is driven by strong nonlinear coupled feedbacks between the ocean and atmosphere and low frequency changes in ENSO amplitude are driven by decadal changes in the tropical Pacific mean state. Unfortunately, the observed record is too short to collect reliable statistics for such low frequency behavior. A hybrid coupled model composed of a simple statistical atmosphere coupled to the Poseidon isopycnal ocean model has been developed for the study of ENSO decadal variability. The model simulates realistic ENSO variability on interannual and decadal time scales with negligible climate drift over 1000 years. Through analysis and experimentation the authors show that low frequency changes in the atmospheric “weather noise” drive changes in the tropical Pacific mean state leading to changes in the amplitude of ENSO on decadal time scales. Additional model simulations suggest that, while predictability is limited by the presence of atmospheric noise, there are extended periods when the coupled instability, strengthened by changes in the mean state, is insensitive to noise on interannual time scales.

The relationship between decadal modulation of ENSO and mean state changes resides somewhere between the linear damped stochastically forced theory and the strongly unstable theory. Unlike the strongly unstable system, changes in ENSO amplitude on longer time scales are determined by the stochastic forcing. The stochastic forcing is not necessary in this model to sustain ENSO; however, its presence is crucial for low frequency changes in the mean state of the tropical Pacific. The strong relationship between the mean state and ENSO amplitude modulation in the model is in opposition to the linear damped stochastically forced theory. The fact that changes in the tropical Pacific mean state lead directly to changes in ENSO amplitude and predictability has positive implications for predictability.

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Anna Borovikov, Michele M. Rienecker, and Paul S. Schopf

Abstract

The surface heat budget in the equatorial Pacific Ocean was investigated through ocean model simulations, both the climatological cycle and the case of the 1994–95 warm event. The dominant processes governing the seasonal cycle of sea surface temperature (SST) vary significantly across the basin. In the western Pacific the annual cycle of SST is primarily in response to net surface heat flux. In the central basin the magnitude of the zonal advection term is comparable to that of the net surface heat flux. In the eastern basin the role of zonal advection is reduced and the vertical mixing and advection are more important. The model estimate of the vertical mixing contribution to the mixed layer heat budget compared well with estimates obtained by analysis of observations using the same diagnostic vertical mixing scheme. During 1994–95 the largest positive SST anomaly was observed in the midbasin and was related initially to reduced latent heat flux due to weak surface winds and later to anomalous zonal advection. In the eastern Pacific where winds were not significantly anomalous throughout 1994–95, only a moderate warm surface anomaly was detected. This is in contrast to strong El Niño events where the SST anomaly is largest in the eastern basin. Overall, the balances inferred from the model forced by Special Sensor Microwave/Imager winds are consistent with the balances derived using tropical atmosphere–ocean moorings data and Reynolds SST.

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Dudley B. Chelton, Steven K. Esbensen, Michael G. Schlax, Nicolai Thum, Michael H. Freilich, Frank J. Wentz, Chelle L. Gentemann, Michael J. McPhaden, and Paul S. Schopf

Abstract

Satellite measurements of surface wind stress from the QuikSCAT scatterometer and sea surface temperature (SST) from the Tropical Rainfall Measuring Mission Microwave Imager are analyzed for the three-month period 21 July–20 October 1999 to investigate ocean–atmosphere coupling in the eastern tropical Pacific. Oceanic tropical instability waves (TIWs) with periods of 20–40 days and wavelengths of 1000–2000 km perturb the SST fronts that bracket both sides of the equatorial cold tongue, which is centered near 1°S to the east of 130°W. These perturbations are characterized by cusp-shaped features that propagate systematically westward on both sides of the equator. The space–time structures of these SST perturbations are reproduced with remarkable detail in the surface wind stress field. The wind stress divergence is shown to be linearly related to the downwind component of the SST gradient with a response on the south side of the cold tongue that is about twice that on the north side. The wind stress curl is linearly related to the crosswind component of the SST gradient with a response that is approximately half that of the wind stress divergence response to the downwind SST gradient. The perturbed SST and wind stress fields propagate synchronously westward with the TIWs. This close coupling between SST and wind stress supports the Wallace et al. hypothesis that surface winds vary in response to SST modification of atmospheric boundary layer stability.

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