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Wenju Cai

Abstract

The author describes a series of mechanistic experiments showing the generation of interdecadal variability in the Bryan-Cox ocean general circulation model driven by a constant two-dimensional freshwater or heat flux field alone. The model ocean has a flat bottom and idealized model geometry of size comparable to the North and South Atlantic.

A set of experiments examines the variability of saline circulation. Four spinups are carried out: (a) under a restoring boundary condition on salinity alone (run RS; (b) under a restoring boundary condition on salinity and a wind forcing (run RSW); (c) under restoring boundary conditions on both temperature and salinity without wind forcing (run RST); and (d) under the same forcings as (c) but in the presence of the wind forcing (run RSTW). Four fields of surface freshwater flux are diagnosed from the steady state of each spinup. Four cases are then run, each under a diagnosed freshwater flux field. Except in the case under the field diagnosed from run RS, internal variability takes place. The internal variability is induced by a mismatch between the freshwater transport implied by the surface freshwater flux forcing and the oceanic freshwater transport. Parallel experiments are carried out to study the internal variability driven by a constant two-dimensional heat flux field alone. Internal oscillations again develop as a result of a mismatch between the atmospheric and oceanic heat transport. In a coupled atmosphere-ocean system the atmospheric freshwater (or heat) transport needs not always match the oceanic freshwater (or heat) transport. This may play a role in the generation of the variability in the coupled system.

The mismatch mechanism can operate in a system forced by a Haney restoration for surface temperature and a flux condition of salinity (mixed boundary conditions). A positive feedback mechanism associated with mixed boundary conditions misrepresents the role of the thermal and saline forcings. This can lead to destruction of the thermally driven circulation feature and yields solutions similar to those without thermal forcing, that is, with a persistent oscillation.

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Wenju Cai

Abstract

Ocean general circulation models can be run using Haney boundary conditions (BCs) for both temperature and salinity, or using mixed boundary conditions, which consists of a Haney BC for temperature and a flux BC for salinity. A switch from Haney BCs to mixed BCs often causes the model North Atlantic Deep Water Formation (NADWF) to either collapse or intensify. Recently, Tziperman et al. found that the collapse was due to an unrealistic freshwater flux field diagnosed from a spinup using a too short relaxation timescale for salinity. They replaced the unrealistic freshwater flux with a more realistic freshwater flux diagnosed from a spinup using a longer relaxation timescale for salinity and found that NADWF stabilized. In this study, the author shows that mixed BCs are not suitable for studying the stability of the present ocean climate, regardless whether a realistic freshwater flux is realistic or not. Further, the instability associated with mixed BCs is due more to the use of a Haney BC for temperature than to an unrealistic freshwater flux. This is shown in a series of numerical experiments using a global Bryan–Cox ocean general circulation model. In these experiments, although a more realistic freshwater flux is used, NADWF is still very sensitive to a perturbation in high-latitude freshwater flux and to an enhancement of the implied hydrological cycle. This is because a Haney BC for temperature, when used with a flux BC for salinity, promotes a positive feedback between surface salinity and overturning. When the Haney BC for temperature is replaced by a Schopf BC, the overturning circulation associated with NADWF is quite stable.

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Evan Weller and Wenju Cai

Abstract

An assessment of how well climate models simulate the Indian Ocean dipole (IOD) is undertaken using 20 coupled models that have partaken in phase 5 of the Coupled Model Intercomparison Project (CMIP5). Compared with models in phase 3 (CMIP3), no substantial improvement is evident in the simulation of the IOD pattern and/or amplitude during austral spring [September–November (SON)]. The majority of models in CMIP5 generate a larger variance of sea surface temperature (SST) in the Sumatra–Java upwelling region and an IOD amplitude that is far greater than is observed. Although the relationship between precipitation and tropical Indian Ocean SSTs is well simulated, future projections of SON rainfall changes over IOD-influenced regions are intrinsically linked to the IOD amplitude and its rainfall teleconnection in the model present-day climate. The diversity of the simulated IOD amplitudes in models in CMIP5 (and CMIP3), which tend to be overly large, results in a wide range of future modeled SON rainfall trends over IOD-influenced regions. The results herein highlight the importance of realistically simulating the present-day IOD properties and suggest that caution should be exercised in interpreting climate projections in the IOD-affected regions.

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Wenju Cai and Yun Qiu

Abstract

A well-known feature of the Indian Ocean dipole (IOD) is its positive skewness, with cold sea surface temperature (SST) anomalies over the east pole (IODE) exhibiting a larger amplitude than warm SST anomalies. Several mechanisms have been proposed for this asymmetry, but because of a lack of observations the role of various processes remains contentious. Using Argo profiles and other newly available data, the authors provide an observation-based assessment of the IOD skewness. First, the role of a nonlinear dynamical heating process is reaffirmed, which reinforces IODE cold anomalies but damps IODE warm anomalies. This reinforcing effect is greater than the damping effect, further contributing to the skewness. Second, the existence of a thermocline–temperature feedback asymmetry, whereby IODE cold anomalies induced by a shoaling thermocline are greater than warm anomalies associated with a deepening thermocline, is the primary forcing of the IOD skewness. This thermocline–temperature feedback asymmetry is a part of the nonlinear Bjerknes-like positive feedback loop involving winds, SST, and the thermocline, all displaying a consistent asymmetry with a stronger response when IODE SST is anomalously cold. The asymmetry is enhanced by a nonlinear barrier layer response, with a greater thinning associated with IODE cold anomalies than a thickening associated with IODE warm anomalies. Finally, in response to IODE cool anomalies, rainfall and evaporative heat loss diminish and incoming shortwave radiation increases, which results in damping the cool SST anomalies. The damping increases with IODE cold anomalies. Thus, the IOD skewness is generated in spite of a greater damping effect of the SST–cloud–radiation feedback process.

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Evan Weller and Wenju Cai

Abstract

Recent studies have shown that the impact of the Indian Ocean dipole (IOD) on southern Australia occurs via equivalent barotropic Rossby wave trains triggered by convective heating in the tropical Indian Ocean. Furthermore, the El Niño–Southern Oscillation (ENSO) influence on southern Australian climate is exerted through the same pathway during austral spring. It is also noted that positive phase [positive IOD (pIOD) and El Niño] events have a much larger impact associated with their respective skewness. These phenomena play a significant role in the region's rainfall reduction in recent decades, and it is essential that climate models used for future projections simulate these features. Here, the authors demonstrate that climate models do indeed simulate a greater climatic impact on Australia for pIOD events than for negative IOD (nIOD) events, but this asymmetric impact is distorted by an exaggerated influence of La Niña emanating from the Pacific. The distortion results from biases in the Pacific in two respects. First, the tropical and extratropical response to La Niña is situated unrealistically too far westward and hence too close to Australia, leading to an overly strong impact on southeast Australia that shows up through the nIOD–La Niña coherence. Second, the majority of models simulate a positive sea surface temperature skewness in the eastern Pacific that is too weak, overestimating the impact of La Niña relative to that of El Niño. As such, the impact of the positive asymmetry in the IOD only becomes apparent when the impact of ENSO is removed. This model bias needs to be taken into account when analyzing projections of regional Australian climate change.

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Wenju Cai and Tim Cowan

Abstract

Simulations by the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) models on the Southern Hemisphere (SH) circulation are assessed over the period 1950–99, focusing on the seasonality of the trend and the level of its congruency with the southern annular mode (SAM) in terms of surface zonal wind stress. It is found that, as a group, the models realistically produce the seasonality of the trend, which is strongest in the SH summer season, December–February (DJF). The modeled DJF trend is principally congruent with the modeled SAM trend, as in observations. The majority of models produce a statistically significant positive trend, with decreasing westerlies in the midlatitudes and increasing westerlies in the high latitudes. The trend pattern from an all-experiment mean achieves highest correlation with that from the National Centers for Environmental Prediction (NCEP) data. A total of 48 out of the 71 experiments were run with ozone-depletion forcing, which offers an opportunity to assess the importance of ozone depletion in driving the late-twentieth-century trends. The AR4 model ensemble that contains an ozone-depletion forcing produces an averaged trend that is comparable to the trend from the NCEP outputs corrected by station-based observations. The trend is largely generated after the mid-1970s. Without ozone depletion the trend is less than half of that in the corrected NCEP, although the errors in the observed trend are large. The impact on oceanic circulation is inferred from wind stress curl in the group with ozone-depletion forcing. The result shows an intensification of the southern midlatitude supergyre circulation, including a strengthening East Australian Current flowing through the Tasman Sea. Thus, ozone depletion also plays an important role in the subtropical gyre circulation change over the past decades.

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Wenju Cai and Tim Cowan

Abstract

Since the 1950s annual rainfall over southeastern Australia (SEA) has decreased considerably with a maximum decline in the austral autumn season (March–May), particularly from 1980 onward. The understanding of SEA autumn rainfall variability, the causes, and associated mechanisms for the autumn reduction remain elusive. As such, a new plausible mechanism for SEA autumn rainfall variability is described, and the dynamics for the reduction are hypothesized. First, there is no recent coherence between SEA autumn rainfall and the southern annular mode, discounting it as a possible driver of the autumn rainfall reduction. Second, weak trends in the subtropical ridge intensity cannot explain the recent autumn rainfall reduction across SEA, even though a significant relationship exists between the ridge and rainfall in April and May. With a collapse in the relationship between the autumn subtropical ridge intensity and position in recent decades, a strengthening in the influence of the postmonsoonal winds from north of Australia has emerged, as evident by a strong post-1980 coherence with SEA mean sea level pressure and rainfall. From mid to late autumn, there has been a replacement of a relative wet climate in SEA with a drier climate from northern latitudes, representing a climate shift that has contributed to the rainfall reduction. The maximum baroclinicity, as indicated by Eady growth rates, has shifted poleward. An associated poleward shift of the dominant process controlling SEA autumn rainfall has further enhanced the reduction, particularly across southern SEA. This observed change over the past few decades is consistent with a poleward shift of the ocean and atmosphere circulation.

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Anthony C. Hirst and Wenju Cai

Abstract

The sensitivity of a coarse-resolution model of the World Ocean to parameterization of subgrid-scale mixing is examined. The model is based on the GFDL code. Results are presented from a series of model runs where the subsurface mixing parameterization is sequentially upgraded toward a more physical representation. The surface forcing is the same for all principal model runs and features a strong relaxation of surface temperature and salinity toward perpetual wintertime observed values. One model version is rerun with a full annual cycle of surface forcing and verifies that use of the perpetual winter surface relaxation introduces only minor biases in the essential characteristics of the solution.

Runs 1 and 2 feature the diffusivity tensor in the traditional horizontal/vertical orientation, and examines the effect of different vertical diffusivity profiles on the solution. Results are compared with those of previous studies. In both cases, the water mass properties (espcially the salinity Acids) are rather poor. In runs 3–5, a standard parameterization is introduced that allows for enhanced diffusion along the isopycnal surfaces. Each of these runs feature a different prescribed profile of isopycnal diffusivity, though with the same profile of vertical diffusivity as for run 2. Introduction of isopycnal mixing considerably improves the water mass structure, in particular by freshening and cooling water at intermediate depths toward realistic levels. However, the vertical stratification and density fields are little changed from run 2. likewise. the current structure and meridional overturning are little changed. Thus isopycnal mixing has a major effect upon the temperature and salinity fields, but very minor effect on the ocean dynamics. Isopycnal mixing is found to modestly increase poleward oceanic heat transport in the midlatitudes via enhanced quasi-horizontal mixing of warm salty subtropical and cold fresh subpolar waters.

In run 6, the isopycnal diffusivity of run 4 is retained, but the vertical diffusivity is instead allowed to vary as the inverse of the local Brunt-Väisälä frequency. However, the resulting solution is little changed from that of run 4. Reasons for this small change are discussed. Also discussed are the impact of numerical problems associated with the use of realistically small vertical diffusivity, and problems inherent in deep water formation in coarse-resolution models.

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Wenju Cai and P. H. Whetton

Abstract

Recently, Cai and Whetton provided modeling evidence that the greenhouse warming pattern has undergone a systematic change from a pattern with maximum warming in subtropical and mid- to high latitudes to one that is El Niño–like from the 1960s onward. They suggest that the mechanism for the change is the transmission of the large extratropical warming to the equatorial east Pacific via modeled tropical–extratropical Pacific circulation pathways. The present study addresses several associated issues. How is the systematic change manifested in empirical orthogonal functions? How do the meridional heat balances respond to the systematic change? Does the proposed mechanism operate in the absence of greenhouse forcing? It is shown that the warming signals are represented by two empirical orthogonal functions, the first of these reflecting a long-term trend in the period considered, and the second showing the change in trend from the 1960s onward. Consistent with the time-varying warming pattern, the relative importance of various heat exchange processes in the tropical Pacific Ocean also undergoes systematic changes. Prior to the 1960s, advective heat flux from the extratropics is the heat source for warming the tropical subthermocline (80–270 m). This subthermocline warming weakens the thermocline and reduces the diffusive heat transfer down through the subthermocline. From the 1960s onward, as substantial subthermocline warming upwells, the El Niño–like pattern develops, strengthening the thermocline; consequently, the downward diffusive heat transfer to the subthermocline enhances reversing the trend prior to the 1960s, and eventually becomes the dominant source for subthermocline heating. The dynamical process, whereby extratropical anomalies are transmitted to the Tropics, operates in a run without external forcing, in association with a mode of ENSO-like interdecadal oscillation. In the equatorial central-eastern Pacific, the associated anomalies upwell and initiate an ocean–atmosphere feedback that changes the equatorial west–east sea surface temperature gradient and easterly winds, reinforcing the upwelled anomalies. The commonality of the modeled interannual ENSO cycles and the interdecadal ENSO-like variability is also discussed.

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Wenju Cai and Richard J. Greatbatch

Abstract

Recent observational study on the compensation for the North Atlantic Deep Water (NADW) outflow suggested that the compensation flow loops into the south Indian Ocean, whereby the compensating water gains heat and salt before returning to the South Atlantic. A question arises as to whether the heat and salt gain from the south Indian Ocean plays a significant role in determining the thermohaline circulation associated with the NADW formation. Many low-resolution ocean general circulation models (OGCMs) for coupled atmosphere–ocean studies fail to produce an Agulhas leakage. The consequence of this missing leakage in these climate models remains unclear. This study examines the role played by the Agulhas leakage in the compensating process for the NADW outflow, and assesses the feasibility of low-resolution ocean climate models. The authors do this in a series of numerical experiments using the Bryan–Cox global OGCM coupled to Schopf's zero heat capacity atmospheric model.

The model confirms that in the presence of the Agulhas leakage, the compensating route includes a loop extending into the southwestern Indian Ocean. Part of the compensating water flows to the Indian Ocean through this loop, and returns with Indian Ocean water to the South Atlantic via the Agulhas leakage. All of the compensating water flows with the Benguela Current. A small branch of the Benguela Current then breaks away from the main stream at about 15°S and heads for the North Atlantic.

The Agulhas leakage decreases only slightly when the NADW formation is suppressed. Most of the reduction occurs in the intermediate water. A comparison of model runs suggests that the contribution by the Indian Ocean water is less than 35% of the total compensating water leaving the South Atlantic and that the majority of the Indian Ocean contribution is intermediate water. Most of the South Atlantic area gains heat from the atmosphere, and the northward heat transport in the South Atlantic associated with the compensation can be sustained by this heat gain. The heat gain accompanies a conversion of intermediate water into surface water, providing the surface water source for the compensating water.

The southwestern Indian Ocean loop of the compensating flow provides a pathway whereby the compensating water may gain heat and salt from the Indian Ocean. In an experiment where the loop is suppressed, the Atlantic water cools and freshens. However, the cooling and freshening process hardly changes the density field, leading to an almost identical rate of the NADW formation and strength of the NADW outflow, with or without the Agulhas leakage.

That the majority of the compensating water, whether through the leakage or the Drake Passage, is intermediate water clears the way for the use of low-resolution OGCMs in climatic studies in terms of the compensation for the NADW outflow.

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