Baroclinic-Barotropic Interaction in Teleconnections
Teleconnections are remote impacts from one location to another, which are transmitted through planetary-scale wave motions, e.g., Rossby waves. Teleconnections can be forced by tropical heat sources, for example, El Nino/Southern Oscillation (ENSO), and result in many societally important impacts in remote ocean basins and continents. It is thus important to understand the complex pathway and wave dynamics in teleconnections. This dissertation discusses the role of baroclinic-barotropic interaction in three different aspects of atmospheric teleconnections.
The first major question addressed is what mechanisms control the interhemispheric teleconnections from tropical heat sources. These are investigated using an intermediate complexity model [a Quasi-equilibrium Tropical Circulation Model (QTCM)] and a simple linear two-level model with dry dynamics. Illustrating the interhemispheric teleconnection process with an Atlantic Warm Pool principal case, the heat source directly excites a baroclinic response that spreads across the equator. Three processes involving baroclinic-barotropic interactions---shear advection, surface drag, and vertical advection---then force a cross-equatorial barotropic Rossby wave response. An analysis of these processes in QTCM simulations indicates that: (1) shear advection has a pattern that roughly coincides with the baroclinic signal in the tropics and subtropics; (2) surface drag has large amplitude and spatial extent, and can be very effective in forcing barotropic motions around the globe; (3) vertical advection has a significant contribution locally and remotely where large vertical motions and vertical shear occur. A simple model is modified to perform experiments in which each of these three mechanisms may be included or omitted. By adding surface drag and vertical advection, and comparing each to shear advection, the effects of the three mechanisms on the generation and propagation of the barotropic Rossby waves are shown to be qualitatively similar to the results in QTCM. It is also found that the moist processes included in the QTCM can feed back on the teleconnection process and alter the teleconnection pattern by enlarging the prescribed tropical heating in both intensity and geographical extent, and by inducing remote precipitation anomalies by interaction with the basic state.
The second major question addressed concerns the dynamics that maintain the sea level pressure (SLP) anomalies in the western Pacific that are an integral part of the classic ENSO pattern. Traditional studies of the ENSO response in the tropics assume a single deep baroclinic mode associated with the tropospheric temperature anomalies. However, the SLP anomalies in the western Pacific are spatially separated from the baroclinic signal in the NCEP-NCAR reanalysis, CMIP5 models, and QTCM. Separation of ENSO SLP anomalies in the tropical Pacific into baroclinic and barotropic components indicates that the barotropic component contributes throughout the tropics and constitutes the primary contribution in the western Pacific. To demonstrate the roles of baroclinic and barotropic modes in ENSO teleconnections within the tropics, a series of QTCM experiments is performed, where anomalies in the interactions between baroclinic and barotropic modes are suppressed over increasingly wider latitudinal bands in the tropical Pacific. If this suppression is done in the 15N-15S band, the pressure signals in the western Pacific are only partly removed, whereas if it is done in the 30N-30S band, the anomalies in the western Pacific are almost entirely removed. This suggests the following pathway: interactions with sea surface temperature (SST) anomalies create the baroclinic response in the central and eastern Pacific, but baroclinic-barotropic interactions, arising substantially in the subtropical Pacific, generate a barotropic response that yields the SLP anomalies in the western Pacific.
The third major question addressed is by what pathways the baroclinic response forced by El Nino SST anomalies gives rise to the barotropic component of ENSO teleconnections, focusing especially on the subtropical Pacific. This is examined by diagnosing from NCEP-NCAR reanalysis the interaction terms in a vertical mode decomposition that yields an effective Rossby wave source, and by analyzing the response of QTCM to the NCEP-NCAR effective Rossby wave source. Among the three barotropic Rossby wave source contributions (shear advection, vertical advection and surface drag), the leading contribution is from shear advection, and more specifically, the mean baroclinic zonal wind advecting the anomalous baroclinic zonal wind. Vertical advection is the smallest term, while surface drag tends to cancel and reinforce the shear advection in different regions through damping on baroclinic mode, which spins up a barotropic response. There are also non-trivial impacts of transients in the barotropic wind response to ENSO. Both tropical and subtropical baroclinic vorticity advection contribute to the barotropic component of the Pacific subtropical jet near coast of North America, where the resulting barotropic wind contribution approximately doubles the zonal jet anomaly at upper levels, relative to the baroclinic anomalies alone. In this view, the effective Rossby wave source in the subtropics simply arises from the basic-state baroclinic flow acting on the well-known baroclinic ENSO flow pattern that spreads from the deep tropics into the subtropics over a scale of equatorial radius of deformation. This is inseparably connected to the leading deep tropical Rossby wave source that arises from eastern Pacific climatological baroclinic winds advecting the tropical portion of the same ENSO flow pattern.