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Dake Chen and Xiaojun Yuan

Abstract

A linear Markov model has been developed to simulated and predict the short-term climate change in the Antarctic, with particular emphasis on sea ice variability. Seven atmospheric variables along with sea ice were chosen to define the state of the Antarctic climate, and the multivariate empirical orthogonal functions of these variables were used as the building blocks of the model. The predictive skill of the model was evaluated in a cross-validated fashion, and a series of sensitivity experiments was carried out. In both hindcast and forecast experiments, the model showed considerable skill in predicting the anomalous Antarctic sea ice concentration up to 1 yr in advance, especially in austral winter and in the Antarctic dipole regions. The success of the model is attributed to the domination of the Antarctic climate variability by a few distinctive modes in the coupled air–sea–ice system and to the model's ability to detect these modes. This model is presently being used for the experimental seasonal forecasting of Antarctic sea ice, and a current prediction example is presented.

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Xiaohui Xie and Dake Chen

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Two sets of mooring data were collected at two sites (MA and MB) along a cross-slope section on the northeastern continental slope in the South China Sea (SCS). These data are used to investigate evolution and energy decay of low-mode semidiurnal (M 2) internal tides on a subcritical slope with respect to M 2. At the deep portion of the slope (~1250 m; MA), the M 2 internal tides show upward energy propagation, while vertically-standing M 2 internal tides are often observed at shallow MB (~845 m). A two-dimensional linear internal tide model with realistic topography and stratification reproduces the observations, suggesting that low-mode M 2 internal tides incident on subcritical slopes evolve into vertically-propagating internal waves due to topographic scattering, propagate upward to the boundary and reflect from the sea surface. The reflection point largely depends on the phase between the modal components of the incoming flux.

In the near-surface reflection region, two kinds of nonlinear effects are observed to decay energy of the incoming internal tides. One is the resonant parametric subharmonic instability which transfers M 2 internal tides to diurnal subharmonics M 1 (=M 2/2), but the instability is found to mainly depend on the incident waves. The other one is the non-resonant wave-wave interaction, producing two higher-harmonic M 4 (=2M 2) rays with opposite vertical propagation. A strong westward mean flow is observed in the interacting region, with amplitude comparable to that of the incident waves. This mean flow also appears to be generated by the nonlinear reflection of the M 2 internal tides.

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Tao Lian and Dake Chen

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As an effective eigen method for phenomenon identification and space reduction, empirical orthogonal function (EOF) analysis is widely used in climate research. However, because of its orthorgonality constraint, EOF analysis has a tendency to produce unphysical modes. Previous studies have shown that the drawbacks of EOF analysis could be partly alleviated by rotated EOF (REOF) analysis, but such studies are always based on specific cases. This paper provides a thorough statistical evaluation of REOF analysis by comparing its ability with that of EOF analysis in reproducing a large number of randomly selected stationary modes of variability. The synthetic experiments indicate that REOF analysis is overwhelmingly a better choice in terms of accuracy and effectiveness, especially for picking up localized patterns. When applied to the tropical Pacific sea surface temperature variability, REOF and EOF analyses show obvious discrepancies, with the former making much better physical sense. This challenges the validity of the so-called sea surface temperature cooling mode and the spatial structure of “El Niño Modoki,” both of which are recently identified by EOF analysis. At any rate, one has to be cautious when claiming new discoveries of climate modes based on EOF analysis alone.

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Chen Chen, Mark A. Cane, Andrew T. Wittenberg, and Dake Chen

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Focusing on ENSO seasonal phase locking, diversity in peak location, and propagation direction, as well as the El Niño–La Niña asymmetry in amplitude, duration, and transition, a set of empirical probabilistic diagnostics (EPD) is introduced to investigate how the ENSO behaviors reflected in SST may change in a warming climate.

EPD is first applied to estimate the natural variation of ENSO behaviors. In the observations El Niños and La Niñas mainly propagate westward and peak in boreal winter. El Niños occur more at the eastern Pacific whereas La Niñas prefer the central Pacific. In a preindustrial control simulation of the GFDL CM2.1 model, the El Niño–La Niña asymmetry is substantial. La Niña characteristics generally agree with observations but El Niño’s do not, typically propagating eastward and showing no obvious seasonal phase locking. So an alternative approach is using a stochastically forced simulation of a nonlinear data-driven model, which exhibits reasonably realistic ENSO behaviors and natural variation ranges.

EPD is then applied to assess the potential changes of ENSO behaviors in the twenty-first century using CMIP5 models. Other than the increasing SST climatology, projected changes in many aspects of ENSO reflected in SST anomalies are heavily model dependent and generally within the range of natural variation. Shifts favoring eastward-propagating El Niño and La Niña are the most robust. Given various model biases for the twentieth century and lack of sufficient model agreements for the twenty-first-century projection, whether the projected changes for ENSO behaviors would actually take place remains largely uncertain.

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Dake Chen, Hsien Wang Ou, and Changming Dong

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Internal tides near a midlatitude shelf–slope front are studied using an idealized numerical model, with emphasis on their structure, energetics, and mixing effects. It is found that the properties of internal tides are highly dependent on frontal configuration and tidal frequency. At a winter front, energetic internal tides are generated and arrested in the frontal zone; the cross-shelf flow tends to be surface (bottom) intensified by a large internal circulation cell at the diurnal (semidiurnal) frequency. At a summer front, the diurnal internal tide is still trapped, but a semidiurnal internal tide propagates out of the frontal zone in the offshore direction while arrested at the inshore boundary. The presence of the shelf–slope front enhances the generation of internal tides, and it also causes an amplification of the semidiurnal internal tide by trapping its energy in the frontal zone. This amplification is most prominent at the offshore boundary of the winter front and the inshore boundary of the summer front, where strong tidal refraction takes place. Internal tides can cause significant mixing and dispersion in the frontal zone, with the semidiurnal internal tide being most effective toward the frontal boundaries, and the diurnal internal tide more effective near the site of generation.

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Rong-Hua Zhang, Dake Chen, and Guihua Wang

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Satellite-based ocean color measurements indicate clear evidence for bioclimate interactions in the tropical Pacific associated with El Niño–Southern Oscillation (ENSO). Recent modeling studies have demonstrated that ocean biology can potentially affect the climate through the penetration depth of solar radiation in the upper ocean (Hp), a primary parameter in coupling biology to physics in the ocean. At present, interannual variability in Hp and its related bioclimate feedback effects have not been adequately represented in coupled ocean–atmosphere models. In this work, chlorophyll (Chl) concentration data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), available since 1997, are used to characterize interannual Hp variability in the tropical Pacific and to quantify its relationships with physical fields, including sea surface temperature (SST) and sea level (SL). It is found that interannual Hp variability is dominated by ENSO signals, with the largest variability located in the central basin near the date line and a coherent relationship with SST. A singular value decomposition (SVD) analysis is adopted to extract interannual covariability patterns between Hp and SST during the period 1997–2007. Their close relationships are then utilized to construct an empirical anomaly model for Hp, allowing for its prognostic estimate in terms of SST anomalies without explicit involvement of a marine ecosystem model. Validation and sensitivity experiments indicate that the empirical model can reasonably well capture interannual Hp responses to SST anomalies in association with ENSO. The derived empirical Hp model offers a simple and an effective way to parameterize and represent the effects of Chl containing biomass on penetrative solar radiation in the tropical Pacific, demonstrating the dynamical implication of remotely sensed Chl data for bioclimate coupling studies. Further improvements and applications of the empirical Hp model to climate modeling are discussed.

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Qiaoyan Wu, Ying Yan, and Dake Chen

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A linear Markov model has been developed to predict the short-term climate variability of the East Asian monsoon system, with emphasis on precipitation variability. Precipitation, sea level pressure, zonal and meridional winds at 850 mb, along with sea surface temperature and soil moisture, were chosen to define the state of the East Asian monsoon system, and the multivariate empirical orthogonal functions of these variables were used to construct the statistical Markov model. The forecast skill of the model was evaluated in a cross-validated fashion and a series of sensitivity experiments were conducted to further validate the model. In both hindcast and forecast experiments, the model showed considerable skill in predicting the precipitation anomaly a few months in advance, especially in boreal winter and spring. The prediction in boreal summer was relatively poor, though the model performance was better in an ENSO decaying summer than in an ENSO developing summer. Also, the prediction skill was better over the ocean than the land. The model's forecast ability is attributed to the domination of the East Asian monsoon climate variability by a few distinctive modes in the coupled atmosphere–ocean–land system, to the strong influence of ENSO on these modes, and to the Markov model's capability to capture these modes.

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Guihua Wang, Dake Chen, and Jilan Su

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Generation of mesoscale eddies in the eastern South China Sea (SCS) in winters during August 1999 to July 2002 is studied with a reduced-gravity model. It is found that the orographic wind jets associated with the northeast winter monsoon and the gaps in the mountainous island chain along the eastern boundary of the SCS can spin up cyclonic and anticyclonic eddies over the SCS. Results suggest that direct wind forcing could be an important generation mechanism for the rich eddy activity in the SCS, and that to simulate this mechanism the resolution of the wind forcing has to be high enough to resolve the local wind jets induced by orographic effects.

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William S. Kessler, Lewis M. Rothstein, and Dake Chen

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The annual onset of the east Pacific cold tongue is diagnosed in an ocean GCM simulation of the tropical Pacific. The model uses a mixed-layer scheme that explicitly simulates the processes of vertical exchange of heat and momentum with the deeper layers of the ocean; comparison with observations of temperature and currents shows that many important aspects of the model fields are realistic. As previous studies have found, the heat balance in the eastern tropical Pacific is notoriously complicated, and virtually every term in the balance plays a significant role at one time or another. However, despite many complications, the three-dimensional ocean advection terms in the cold tongue region tend to cancel each other in the annual cycle and, to first order, the variation of SST can be described as simply following the variation of net solar radiation at the sea surface (sun minus clouds). The cancellation is primarily between cooling due to equatorial upwelling and warming due to tropical instability waves, both of which are strongest in the second half of the year (when the winds are stronger). Even near the equator, where the ocean advection is relatively intense, the terms associated with cloudiness variations are among the largest contributions to the SST balance. The annual cycle of cloudiness transforms the semiannual solar cycle at the top of the atmosphere into a largely 1 cycle yr−1 variation of insolation at the sea surface. However, the annual cycle of cloudiness appears closely tied to SST in coupled feedbacks (positive for low stratus decks and negative for deep cumulus convection), so the annual cycle of SST cannot be fully diagnosed in an ocean-only modeling context as in the present study. Zonal advection was found to be a relatively small influence on annual equatorial cold tongue variations; in particular, there was little direct (oceanic) connection between the Peru coastal upwelling and equatorial annual cycles. Meridional advection driven by cross-equatorial winds has been conjectured as a key factor leading to the onset of the cold tongue. The results suggest that the SST changes due to this mechanism are modest, and if meridional advection is in fact a major influence, then it must be through interaction with another process (such as a coupled feedback with stratus cloudiness). At present, it is not possible to evaluate this feedback quantitatively.

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Hsien-Wang Ou, Chang-Ming Dong, and Dake Chen

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The horizontal property flux induced by tides is examined by both analytical and numerical models. It is found that this flux is highly heterogeneous in the vertical and may be directed up the mean gradient near the bottom. This countergradient tidal flux is a consequence of differing boundary conditions satisfied by velocity and property fields, and hence a robust feature. The corresponding tidal diffusivity is substantial where tides are strong and hence potentially important in the mean property balance.

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