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Shiling Peng and John Fyfe

Abstract

Monthly variability of atmosphere-ocean interactions in the midlatitude North Atlantic during the winter months (November–April) is examined. Composite and singular value decomposition (SVD) analyses are applied to the observed sea level pressure (SLP) and sea surface temperature (SST) anomalies of each winter month for the period 1950–1987. The SLP anomaly composites (i.e., SLP differences between selected warm and cold SST months) are constructed based on the averaged SST anomalies over the RM region (60°–40°W and 50°–40°N). These composites shift from a positive monopole pattern in early winter to a dipole pattern in midwinter and then back to a monopole pattern in late winter.

A complementary SVD analysis reveals that the first SVD mode is dipole structured and especially dominant in midwinter. The second SVD mode is monopole featured and more dominant in early and late winter than in midwinter. By examining the spatial distributions of the SVD modes and especially their similarities to the patterns derived from other model simulations, two coupling processes are suggested. The dipole mode is suggested to be related to an atmosphere driving the ocean process and the monopole mode to an ocean forcing the atmosphere process. The month-dependency and the statistical significance of the SVD modes are subjected to two Monte Carlo tests. The results are used to further explain the shifts in the SLP anomaly composites and to indirectly estimate the predominance of the proposed coupling processes during each winter month.

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Mingfang Ting and Shiling Peng

Abstract

The differences between early and middle winter atmospheric responses to the sea surface temperature anomalies (SSTA) in the northwest Atlantic are examined using a linear baroclinic model. Using a global spectral model, Peng et al. found a positive height anomaly in the perpetual November and a negative height anomaly in the perpetual January experiments in response to a warm SSTA over the northwest Atlantic. These height anomalies are found to be associated with the reduced Atlantic jet stream in November and enhanced jet in January. Linear model diagnostics suggest that the difference in jet stream response may induce anomalous storm track eddy vorticity fluxes, which in turn maintain the different atmospheric responses under the early and middle winter conditions.

The different jet stream responses in November and January are further traced to the initial atmospheric response to a local heat source accompanying the warm SSTA. Under both the January and November conditions, the atmospheric response is dominated by an anticyclone downstream from the heat source at the jet stream level. The anticyclone is shifted northward in November, however, from its position in January. Combined with a northeast–southwest tilted jet stream in January and an east–west oriented, southward shifted November jet stream in the Atlantic, the above difference in the atmospheric responses to the initial heat source may lead to a reduced jet in November and an enhanced jet in January. The feedback between the anomalous storm track eddy vorticity fluxes and the anomaly flow induced by the heat source may further enhance the different equilibrium responses in the global spectral model.

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Shiling Peng and Walter A. Robinson

Abstract

The January and February responses of a GCM to an imposed extratropical Pacific SST anomaly are compared with the patterns of the model's internal variability. Relevant patterns of internal variability are diagnosed from ensembles of model control runs by regressing monthly mean geopotentials and temperatures against low-level temperatures in the vicinity of the SST anomaly and by EOF analysis. These patterns are found to play a significant role in determining the local and the remote responses to the SST anomaly. Different responses to the SST anomaly in the GCM's January and February climates are largely explained by the differences in the regression patterns and in the leading EOF. The GCM response may be considered as comprising a direct linear response to low-level heating, that is local to the forcing and baroclinic, and an eddy-forced component that closely resembles patterns of the model's internal variability—it is equivalent barotropic and extends over the entire hemisphere. The results suggest that for a warm SST anomaly over the Kuroshio Extension to induce an equivalent-barotropic ridge immediately east of the anomaly, the internal variability must have a well-defined center of action over the central Pacific. In this GCM, this is nearly true in February but not in January. Similar analyses are performed for the observed flow to determine the patterns of variability in nature and thereby to suggest the potential response to SST forcing. The natural variability in January and February has a strong large-scale center over the Pacific, which, according to the model results, should favor the development of an equivalent-barotropic ridge in response to a warm SST anomaly.

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Shiling Peng and Lawrence A. Mysak

Abstract

The spatial distributions of northern North Atlantic sea surface temperature and the high-latitude Northern Hemisphere sea level pressure anomalies averaged over six consecutive warm SST winters (1951–1956) and six consecutive cold SST winters (1971–1976) are examined. Three SLP anomaly difference (i.e., warm - cold winters) centers, significant at the 5% level, are observed over the northern North Atlantic, Europe, and western Siberia. This anomaly pattern is consistent in principle with what was identified in a related analyses by Palmer and Sun, who used composite data from selected winter months.

The SLP difference centers over the northern North Atlantic and western Siberia are in phase. The impact of the latter center upon the runoff from the underlying Oh and Yenisey rivers and especially the teleconnection between SST anomalies in the northern North Atlantic and runoff of those two rivers via the atmosphere are investigated. The temporal cross-correlation analyses of 50 years (1930–1979) of records of SST, precipitation, and runoff anomalies indicate that the winter SST anomalies in the northern North Atlantic are significantly correlated with the winter and following summer runoff fluctuations of the Ob and Yenisey rivers. Positive (negative) northern North Atlantic SST anomalies are related to less (more) precipitation, and hence, less (more) runoff, over western Siberia.

Discussions of possible physical mechanisms and process that lead to the above relationships are attempted. The analyses of spatial distributions of precipitation in the warm and cold SST winters suggest that precipitation fluctuations over Europe and western Siberia may be affected by shifts of cyclone tracks associated with the SST variations in the northern North Atlantic.

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Shiling Peng and Jeffrey S. Whitaker

Abstract

Previous GCM experiments demonstrated that a model atmosphere produces two different responses to a midlatitude warm SST anomaly over the Pacific under perpetual January and February conditions. To elucidate the mechanisms responsible for the different GCM responses and their dependence on the background flow, experiments with two idealized models are conducted. Experiments with a linear baroclinic model reveal that the GCM responses at equilibrium are primarily maintained by the anomalous eddy forcing. The anomalous flow induced directly by an idealized initial heat source exhibits little sensitivity to the background flow. Eddy feedbacks on the heating-induced anomalous flow are examined using a linear storm track model. The anomalous eddy forcing produced by the storm track model is sensitive to the basic state. The eddy forcing in January acts to shift the heating-induced upper-level ridge toward the northeast of the Gulf of Alaska, while in February it acts to reinforce the ridge. This suggests that the differences in the GCM responses are primarily associated with differences in the response of synoptic eddies to the presence of an anomalous ridge at the end of the Pacific storm track.

The idealized model experiments are also performed with the observed winter mean flow. The eddy feedbacks depend on the position of the heating relative to the storm track. With the heating centered over the western Pacific the eddy-driven anomalous flow reinforces the ridge over the Pacific, similar to that in GCM February, but much stronger. No such reinforcement by the transients is found with the heating shifted over the eastern Pacific. These results suggest that SST anomalies over the western Pacific perhaps play a more active role in midlatitude atmosphere–ocean interactions.

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Shiling Peng, Walter A. Robinson, and Shuanglin Li

Abstract

The response of an atmospheric general circulation model (GCM) to the North Atlantic SST tripole exhibits both symmetric and asymmetric components with respect to the sign of the SST anomaly. The symmetric part of the response is characterized by a North Atlantic Oscillation (NAO)–like dipole with an equivalent barotropic structure over the Atlantic. The asymmetry is manifested in a weaker and smaller-scale dipole response to the positive SST tripole in contrast to a stronger and more zonally elongated dipole response to the negative tripole. Mechanisms for developing and maintaining these GCM responses are elucidated through diagnostic experiments using a linear baroclinic model and a statistical storm track model based on GCM intrinsic variability.

The NAO-like symmetric response is primarily maintained by a dipolar anomalous eddy forcing that results from interactions between the heating-forced anomalous flow and the Atlantic storm track, as expected from an eddy-feedback mechanism. To account for the asymmetry of the responses about the sign of the SST tripole, a nonlinear eddy-feedback mechanism is proposed that extends the previous mechanism to include the nonlinear self-interaction of the heating-forced anomalous flow and its effects on transient eddy feedbacks. The results of idealized model experiments demonstrate that, due to its nonlinear self-interaction, the tripole heating induces a much weaker response in the positive phase than in the negative phase. Interactions of these nonlinear heating-forced anomalous flows with the Atlantic storm track result in asymmetric eddy vorticity forcings that in turn sustain asymmetric eddy-forced anomalous flows in the two cases.

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Shiling Peng, Walter A. Robinson, and Shuanglin Li
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Clara Deser, Robert A. Tomas, and Shiling Peng

Abstract

The objective of this study is to investigate the transient evolution of the wintertime atmospheric circulation response to imposed patterns of SST and sea ice extent anomalies in the North Atlantic sector using a large ensemble of experiments with the NCAR Community Climate Model version 3 (CCM3). The initial adjustment of the atmospheric circulation is characterized by an out-of-phase relationship between geopotential height anomalies in the lower and upper troposphere localized to the vicinity of the forcing. This initial baroclinic response reaches a maximum amplitude in ∼5–10 days, and persists for 2–3 weeks. Diagnostic results with a linear primitive equation model indicate that this initial response is forced by diabatic heating anomalies in the lower troposphere associated with surface heat flux anomalies generated by the imposed thermal forcing. Following the initial baroclinic stage of adjustment, the response becomes progressively more barotropic and increases in both spatial extent and magnitude. The equilibrium stage of adjustment is reached in 2–2.5 months, and is characterized by an equivalent barotropic structure that resembles the hemispheric North Atlantic Oscillation–Northern Annular Mode (NAO–NAM) pattern, the model’s leading internal mode of circulation variability over the Northern Hemisphere. The maximum amplitude of the equilibrium response is approximately 2–3 times larger than that of the initial response. The equilibrium response is primarily maintained by nonlinear transient eddy fluxes of vorticity (and, to a lesser extent, heat), with diabatic heating making a limited contribution in the vicinity of the forcing.

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Shiling Peng, Walter A. Robinson, and Martin P. Hoerling

Abstract

The atmospheric response to a midlatitude SST anomaly in the North Pacific and its dependence on background flow are examined in a GCM. Experiments are conducted using the same warm SST anomalies but two different model states: perpetual January and perpetual February. The atmospheric responses to the SST anomalies are statistically significant in both January and February but are completely different. The anomalous circulation in January is characterized by a trough decaying with height immediately downstream of the SST anomalies. In February, the anomalous circulation is dominated by a downstream ridge growing with height. The patterns of the anomalous heights in the two months are nearly orthogonal.

Vorticity and thermodynamic budgets are diagnosed to illustrate how the anomalous circulations are maintained. Over the SST anomalies, low-level convergence and ascent are observed in both months. In January the anomalous convergence is balanced by a residual due primarily to the forcing by submonthly transients. In February the convergence is balanced by the advection of planetary vorticity. Analysis of the thermodynamic budget indicates that the intensity of the mean meridional wind downstream of the SST anomalies plays a critical role in determining the nature of the responses in the two months. The “warm SST-ridge” type of response is favored when the background meridional flow is relatively weak. These results demonstrate that the atmospheric response to a midlatitude SST anomaly is strongly dependent on the background flow.

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Huei-Ping Huang, Andrew W. Robertson, Yochanan Kushnir, and Shiling Peng

Abstract

Hindcast experiments for the tropical Atlantic sea surface temperature (SST) gradient G1, defined as tropical North Atlantic SST anomaly minus tropical South Atlantic SST anomaly, are performed using an atmospheric general circulation model coupled to a mixed layer ocean over the Atlantic to quantify the contributions of the El Niño–Southern Oscillation (ENSO) forcing and the preconditioning in the Atlantic to G1 in boreal spring. The results confirm previous observational analyses that, in the years with a persistent ENSO SST anomaly from boreal winter to spring, the ENSO forcing plays a primary role in determining the tendency of G1 from winter to spring and the sign of G1 in late spring. In the hindcasts, the initial perturbations in Atlantic SST in boreal winter are found to generally persist beyond a season, leaving a secondary but nonnegligible contribution to the predicted Atlantic SST gradient in spring. For 1993/94, a neutral year with a large preexisting G1 in winter, the hindcast using the information of Atlantic preconditioning alone is found to reproduce the observed G1 in spring. The seasonal predictability in precipitation over South America is examined in the hindcast experiments. For the recent events that can be validated with high-quality observations, the hindcasts produced dryness in boreal spring 1983, wetness in spring 1996, and wetness in spring 1994 over northern Brazil that are qualitatively consistent with observations. An inclusion of the Atlantic preconditioning is found to help the prediction of South American rainfall in boreal spring. For the ENSO years, discrepancies remain between the hindcast and observed precipitation anomalies over northern and equatorial South America, an error that is partially attributed to the biased atmospheric response to ENSO forcing in the model. The hindcast of the 1993/94 neutral year does not suffer this error. It constitutes an intriguing example of useful seasonal forecast of G1 and South American rainfall anomalies without ENSO.

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