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Liping Zhang, Thomas L. Delworth, and Liwei Jia

Abstract

The average predictability time (APT) method is used to identify the most predictable components of decadal sea surface temperature (SST) variations over the Southern Ocean (SO) in a 4000-yr unforced control run of the GFDL CM2.1 model. The most predictable component shows significant predictive skill for periods as long as 20 years. The physical pattern of this variability has a uniform sign of SST anomalies over the SO, with maximum values over the Amundsen–Bellingshausen–Weddell Seas. Spectral analysis of the associated APT time series shows a broad peak on time scales of 70–120 years. This most predictable pattern is closely related to the mature phase of a mode of internal variability in the SO that is associated with fluctuations of deep ocean convection. The second most predictable component of SO SST is characterized by a dipole structure, with SST anomalies of one sign over the Weddell Sea and SST anomalies of the opposite sign over the Amundsen–Bellingshausen Seas. This component has significant predictive skill for periods as long as 6 years. This dipole mode is associated with a transition between phases of the dominant pattern of SO internal variability. The long time scales associated with variations in SO deep convection provide the source of the predictive skill of SO SST on decadal scales. These analyses suggest that if the SO deep convection in a numerical forecast model could be adequately initialized, the future evolution of SO SST and its associated climate impacts are potentially predictable.

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Jia Wang, L. A. Mysak, and R. G. Ingram

Abstract

Hibler's dynamic-thermodynamic sea ice model with viscous-plastic rheology is used to simulate the seasonal cycle of sea ice motion, thickness, compactness, and growth rate in Hudson Bay under monthly climatological atmospheric forcing and a prescribed ocean surface current field. The sea ice motion over most of the domain is driven mainly by the wind stress. Wintertime sea ice velocities are only of the order of 1–5 (× 10−4 m s−1) due to the nearly solid ice cover and the closed boundary constraint of Hudson Bay. However, the velocities rise to 0.10–0.20 m s−1 during the melting and freezing seasons when there is partial ice cover. The simulated thickness distribution in mid–April, the time of heaviest ice cover, ranges from 1.3 m in James Bay to 1.7 m in the northern part of Hudson Bay, which compares favorably with observations. The area-averaged growth rate, computed from the model is 1.5–0.5 cm day−1 from December to March, is negative in May (indicative of melting) and reaches its minimum value of −4.2 cm day−1 (maximum melting rate) in July. During autumn, the main freezing season, the growth rate ranges from 1 to 2 cm day−1. In the model, sea ice remains along the south shore of Hudson Bay in summer, as observed, even though the surface air temperatures are higher there than in central and northern Hudson Bay. A sensitivity experiment shows that this is mainly due to the pile-up of ice driven southward by the northwesterly winds. The simulated results for ice cover in other seasons also compare favorably with the observed climatology and with measurements from satellites. In particular, the model gives complete sea ice cover in winter and ice-free conditions in late summer. A series of sensitivity experiments in which the model parameters and external forcing are varied is also carried out.

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A. L. New, R. Bleck, Y. Jia, R. Marsh, M. Huddleston, and S. Barnard

Abstract

This paper describes a 30-yr spinup experiment of the North Atlantic Ocean with the Miami isopycnic-coordinate ocean model, which, when compared with previous experiments, possesses improved horizontal resolution, surface forcing functions, and bathymetry, and is extended to higher latitudes. Overall, there is a conversion of lighter to heavier water masses, and waters of densities 1027.95 and 1028.05 kg m−3 are produced in the Greenland-lceland Norwegian basin, and of density 1027.75 kg m−3 in the Labrador and Irminger basins. These water masses flow primarily southward. The main purpose of this present study, however, is to investigate the ventilation of the subtropical gyre. The role of Ekman pumping and lateral induction in driving the subduction process is examined and the relative importance of the latter is confirmed. The paper also illustrates how the mixed layer waters are drawn southward and westward into the ocean interior in a continuous spectrum of mode waters with densities ranging between 1026.40 and 1027.30 kg m−3. These are organized into a regular fashion by the model from a relatively disorganized initial state. The evolution of the model gyre during spinup is governed by mixed layer cooling in the central North Atlantic, which causes the ventilation patterns to move southwestward, the layers to rise, and surprisingly, to become warmer. This warming is explained by thermodynamic considerations. Finally, it is shown that the rate of change of potential vorticity following a fluid pathway in the subtropical gyre is governed by the diffusion of layer thickness, which represents subgrid-scale mixing processes in the model. This leads to increasing potential vorticity along pathways that ventilate from the thickest outcrop regions as fluid is diffused laterally and to decreasing potential vorticity along neighboring trajectories.

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Hiroyuki Murakami, Gabriel A. Vecchi, Thomas L. Delworth, Karen Paffendorf, Liwei Jia, Richard Gudgel, and Fanrong Zeng
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W. Zhang, G. A. Vecchi, H. Murakami, G. Villarini, and L. Jia

Abstract

This study investigates the association between the Pacific meridional mode (PMM) and tropical cyclone (TC) activity in the western North Pacific (WNP). It is found that the positive PMM phase favors the occurrence of TCs in the WNP while the negative PMM phase inhibits the occurrence of TCs there. Observed relationships are consistent with those from a long-term preindustrial control experiment (1000 yr) of a high-resolution TC-resolving Geophysical Fluid Dynamics Laboratory (GFDL) Forecast-Oriented Low Ocean Resolution (FLOR) coupled climate model. The diagnostic relationship between the PMM and TCs in observations and the model is further supported by sensitivity experiments with FLOR. The modulation of TC genesis by the PMM is primarily through the anomalous zonal vertical wind shear (ZVWS) changes in the WNP, especially in the southeastern WNP. The anomalous ZVWS can be attributed to the responses of the atmosphere to the anomalous warming in the northwestern part of the PMM pattern during the positive PMM phase, which resembles a classic Matsuno–Gill pattern. Such influences on TC genesis are strengthened by a cyclonic flow over the WNP. The significant relationship between TCs and the PMM identified here may provide a useful reference for seasonal forecasting of TCs and interpreting changes in TC activity in the WNP.

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Honghai Zhang, Thomas L. Delworth, Fanrong Zeng, Gabriel Vecchi, Karen Paffendorf, and Liwei Jia

Abstract

Observed austral summertime (November through April) rainfall in southeastern South America (SESA)—including northern Argentina, Uruguay, southern Brazil, and Paraguay—has exhibited substantial low-frequency variations with a multidecadal moistening trend during the twentieth century and a subsequent decadal drying trend during the current century. Understanding the mechanisms responsible for these variations is essential for predicting long-term rainfall changes. Here with a suite of attribution experiments using a pair of high-resolution global climate models, GFDL CM2.5 and FLOR-FA, the authors investigate the causes of these regional rainfall variations. Both models reproduce the twentieth-century moistening trend, albeit with a weaker magnitude than observed, in response to the radiative forcing associated with increasing greenhouse gases. The increasing greenhouse gases drive tropical expansion; consequently, the subtropical dry branch of Hadley cell moves away from SESA, leading to the rainfall increase. The amplitude discrepancy between the observed and simulated rainfall changes suggests a possible underestimation by the models of the atmospheric response to the radiative forcing, as well as an important role for low-frequency internal variability in the observed moistening trend. Over the current century, increasing greenhouse gases drive a continuous SESA rainfall increase in the models. However, the observed decadal rainfall decline is largely (~60%) reproduced in response to the observed Pacific trade wind strengthening, which is likely associated with natural Pacific decadal variability. These results suggest that the recent summertime rainfall decline in SESA is temporary and that the positive trend will resume in response to both increasing greenhouse gases and a return of Pacific trade winds to normal conditions.

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Gan Zhang, Hiroyuki Murakami, Xiaosong Yang, Kirsten L. Findell, Andrew T. Wittenberg, and Liwei Jia

Abstract

Climate models often show errors in simulating and predicting tropical cyclone (TC) activity, but the sources of these errors are not well understood. This study proposes an evaluation framework and analyzes three sets of experiments conducted using a seasonal prediction model developed at the Geophysical Fluid Dynamics Laboratory (GFDL). These experiments apply the nudging technique to the model integration and/or initialization to estimate possible improvements from nearly perfect model conditions. The results suggest that reducing sea surface temperature (SST) errors remains important for better predicting TC activity at long forecast leads—even in a flux-adjusted model with reduced climatological biases. Other error sources also contribute to biases in simulated TC activity, with notable manifestations on regional scales. A novel finding is that the coupling and initialization of the land and atmosphere components can affect seasonal TC prediction skill. Simulated year-to-year variations in June land conditions over North America show a significant lead correlation with the North Atlantic large-scale environment and TC activity. Improved land–atmosphere initialization appears to improve the Atlantic TC predictions initialized in some summer months. For short-lead predictions initialized in June, the potential skill improvements attributable to land–atmosphere initialization might be comparable to those achievable with perfect SST predictions. Overall, this study delineates the SST and non-oceanic error sources in predicting TC activity and highlights avenues for improving predictions. The nudging-based evaluation framework can be applied to other models and help improve predictions of other weather extremes.

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Xiaosong Yang, G. A. Vecchi, T. L. Delworth, K. Paffendorf, L. Jia, R. Gudgel, F. Zeng, and Seth D. Underwood
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Salvatore Pascale, Simona Bordoni, Sarah B. Kapnick, Gabriel A. Vecchi, Liwei Jia, Thomas L. Delworth, Seth Underwood, and Whit Anderson

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The impact of atmosphere and ocean horizontal resolution on the climatology of North American monsoon Gulf of California (GoC) moisture surges is examined in a suite of global circulation models (CM2.1, FLOR, CM2.5, CM2.6, and HiFLOR) developed at the Geophysical Fluid Dynamics Laboratory (GFDL). These models feature essentially the same physical parameterizations but differ in horizontal resolution in either the atmosphere (≃200, 50, and 25 km) or the ocean (≃1°, 0.25°, and 0.1°). Increasing horizontal atmospheric resolution from 200 to 50 km results in a drastic improvement in the model’s capability of accurately simulating surge events. The climatological near-surface flow and moisture and precipitation anomalies associated with GoC surges are overall satisfactorily simulated in all higher-resolution models. The number of surge events agrees well with reanalyses, but models tend to underestimate July–August surge-related precipitation and overestimate September surge-related rainfall in the southwestern United States. Large-scale controls supporting the development of GoC surges, such as tropical easterly waves (TEWs), tropical cyclones (TCs), and trans-Pacific Rossby wave trains (RWTs), are also well captured, although models tend to underestimate the TEW and TC magnitude and number. Near-surface GoC surge features and their large-scale forcings (TEWs, TCs, and RWTs) do not appear to be substantially affected by a finer representation of the GoC at higher ocean resolution. However, the substantial reduction of the eastern Pacific warm sea surface temperature bias through flux adjustment in the Forecast-Oriented Low Ocean Resolution (FLOR) model leads to an overall improvement of tropical–extratropical controls on GoC moisture surges and the seasonal cycle of precipitation in the southwestern United States.

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Liping Zhang, Thomas L. Delworth, Xiaosong Yang, Richard G. Gudgel, Liwei Jia, Gabriel A. Vecchi, and Fanrong Zeng

Abstract

This study explores the potential predictability of the Southern Ocean (SO) climate on decadal time scales as represented in the GFDL CM2.1 model using prognostic methods. Perfect model predictability experiments are conducted starting from 10 different initial states, showing potentially predictable variations of Antarctic bottom water (AABW) formation rates on time scales as long as 20 years. The associated Weddell Sea (WS) subsurface temperatures and Antarctic sea ice have potential predictability comparable to that of the AABW cell. The predictability of sea surface temperature (SST) variations over the WS and the SO is somewhat smaller, with predictable scales out to a decade. This reduced predictability is likely associated with stronger damping from air–sea interaction. As a complement to this perfect predictability study, the authors also make hindcasts of SO decadal variability using the GFDL CM2.1 decadal prediction system. Significant predictive skill for SO SST on multiyear time scales is found in the hindcast system. The success of the hindcasts, especially in reproducing observed surface cooling trends, is largely due to initializing the state of the AABW cell. A weak state of the AABW cell leads to cooler surface conditions and more extensive sea ice. Although there are considerable uncertainties regarding the observational data used to initialize the hindcasts, the consistency between the perfect model experiments and the decadal hindcasts at least gives some indication as to where and to what extent skillful decadal SO forecasts might be possible.

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