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Warren B. White
,
Shyh-Chin Chen
, and
Ray G. Peterson

Abstract

The Antarctic circumpolar wave (ACW) is a nominal 4-yr climate signal in the ocean–atmosphere system in the Southern Ocean, propagating eastward at an average speed of 6–8 cm s−1, composed of two waves taking approximately 8 years to circle the globe. The ACW is characterized by a persistent phase relationship between warm (cool) sea surface temperature (SST) anomalies and poleward (equatorward) meridional surface wind (MSW) anomalies. Recently, White and Chen demonstrated that SST anomalies in the Southern Ocean operate to induce anomalous vortex stretching in the lower troposphere that is balanced by the anomalous meridional advection of planetary vorticity, yielding MSW anomalies as observed. In the present study, the authors seek to understand how this atmospheric response to SST anomalies produces a positive feedback to the ocean (i.e., an anomalous SST tendency displaced eastward of SST anomalies) that both maintains the ACW against dissipation and accounts for its eastward propagation. To achieve this, we couple a global equilibrium climate model for the lower troposphere to a global heat budget model for the upper ocean. In the absence of coupling, the model Antarctic Circumpolar Current (ACC) advects SST anomalies from initial conditions to the east at speeds slower than observed, taking 12–14 years to circle the globe with amplitudes that become insignificant after 6–8 years. In the presence of coupling, eastward speeds of the model ACC are matched by those due to coupling, together yielding a model ACW of a nominal 4-yr period composed of two waves that circle the globe in approximately 8 years, as observed. Feedback from atmosphere to ocean works through the anomalous zonal surface wind response to SST anomalies, yielding poleward Ekman flow anomalies in phase with warm SST anomalies. As such, maintenance of the model ACW is achieved through a balance between anomalous meridional Ekman heat advection and anomalous sensible-plus-latent heat loss to the atmosphere. This balance requires the alignment of covarying SST and MSW anomalies to be tilted into the southwest–northeast direction, which accounts for the spiral structure observed in global SST and sea level pressure anomaly patterns around the Southern Ocean. Eastward coupling speeds of the model ACW derive from a beta effect in coupling that displaces a portion of the anomalous meridional Ekman heat advection, and its corresponding anomalous SST tendency, to the east of SST anomalies. Therefore, the ACW is an example of self-organization within the global ocean–atmosphere system, depending upon the spherical shape of the rotating earth for its propagation and the mean meridional SST gradient for its maintenance, and producing a net poleward eddy heat flux in the upper ocean that tends to reduce this mean gradient.

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Hann-Ming Henry Juang
,
Ching-Teng Lee
,
Yongxin Zhang
,
Yucheng Song
,
Ming-Chin Wu
,
Yi-Leng Chen
,
Kevin Kodama
, and
Shyh-Chin Chen

Abstract

The National Centers for Environmental Prediction regional spectral model and mesoscale spectral model (NCEP RSM/MSM) use a spectral computation on perturbation. The perturbation is defined as a deviation between RSM/MSM forecast value and their outer model or analysis value on model sigma-coordinate surfaces. The horizontal diffusion used in the models applies perturbation diffusion in spectral space on model sigma-coordinate surfaces. However, because of the large difference between RSM/MSM and their outer model or analysis terrains, the perturbation on sigma surfaces could be large over steep mountain areas as horizontal resolution increases. This large perturbation could introduce systematical error due to artificial vertical mixing from horizontal diffusion on sigma surface for variables with strong vertical stratification, such as temperature and humidity. This nonnegligible error would eventually ruin the forecast and simulation results over mountain areas in high-resolution modeling.

To avoid the erroneous vertical mixing on the systematic perturbation, a coordinate transformation is applied in deriving a horizontal diffusion on pressure surface from the variables provided on terrain-following sigma coordinates. Three cases are selected to illustrate the impact of the horizontal diffusion on pressure surfaces, which reduces or eliminates numerical errors of mesoscale modeling over mountain areas. These cases address concerns from all aspects, including unstable and stable synoptic conditions, moist and dry atmospheric settings, weather and climate integrations, hydrostatic and nonhydrostatic modeling, and island and continental orography.

After implementing the horizontal diffusion on pressure surfaces for temperature and humidity, the results show better rainfall and flow pattern simulations when compared to observations. Horizontal diffusion corrects the warming, moistening, excessive rainfall, and convergent flow patterns around high mountains under unstable and moist synoptic conditions and corrects the cooling, drying, and divergent flow patterns under stable and dry synoptic settings.

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Hann-Ming Henry Juang
,
Shyh-Chin Chen
,
Songyou Hong
,
Hideki Kanamaru
,
Thomas Reichler
,
Takeshi Enomoto
,
Dian Putrasahan
,
Bruce T. Anderson
,
Sasha Gershunov
,
Haiqin Li
,
Kei Yoshimura
,
Nikolaus Buenning
, and
Diane Boomer
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