The Generation of Global Rotational Flow by Steady Idealized Tropical Divergence
01 Apr 1988-Journal of the Atmospheric Sciences (American Meteorological Society)-Vol. 45, Iss: 7, pp 1228-1251
TL;DR: In this paper, a vorticity equation model is used to diagnose the relationship between tropical convective heating and the upper tropospheric rotational wind field, and it is shown that the Rossby wave source can be very different from the simple −fD source often used.
Abstract: Tropical convective heating is balanced on the large scale by the adiabatic cooling of ascent. The horizontal divergence of the wind above this heating may be viewed as driving the upper tropospheric rotational wind field. A vorticity equation model is used to diagnose this relationship. It is shown that because of the advection of vorticity by the divergent component of the flow, the Rossby wave source can be very different from the simple −fD source often used. In particular, an equatorial region of divergence situated in easterly winds can lead to a Rossby wave source in the subtropical westerlies where it is extremely effective. This part of the source can be relatively insensitive to the longitudinal position of the equatorial divergence. A divergence field which is asymmetric about the equator can lead to a quite symmetric Rossby wave source. For a steady frictionless flow the Rossby wave source averaged over regions within closed streamfunction or absolute vorticity contours is, under cert...
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TL;DR: The authors reviewed the connection between ocean basins via the "atmospheric bridge" through an examination of previous work augmented by analyses of 50 years of data from the National Centers for Environmental Prediction- National Center for Atmospheric Research (NCEP-NCAR) reanalysis project and coupled atmospheric general circulation (AGCM)-mixed layer ocean model experiments.
Abstract: During El Nino-Southern Oscillation (ENSO) events, the atmospheric response to sea surface temperature (SST) anomalies in the equatorial Pacific influences ocean conditions over the remainder of the globe. This connection between ocean basins via the ''atmospheric bridge'' is reviewed through an examination of previous work augmented by analyses of 50 years of data from the National Centers for Environmental Prediction- National Center for Atmospheric Research (NCEP-NCAR) reanalysis project and coupled atmospheric general circulation (AGCM)-mixed layer ocean model experiments. Observational and modeling studies have now established a clear link between SST anomalies in the equatorial Pacific with those in the North Pacific, north tropical Atlantic, and Indian Oceans in boreal winter and spring. ENSO-related SST anomalies also appear to be robust in the western North Pacific during summer and in the Indian Ocean during fall. While surface heat fluxes are the key component of the atmospheric bridge driving SST anomalies, Ekman transport also creates SST anomalies in the central North Pacific although the full extent of its impact requires further study. The atmospheric bridge not only influences SSTs on interannual timescales but also affects mixed layer depth (MLD), salinity, the seasonal evolution of upper-ocean temperatures, and North Pacific SST variability at lower fre- quencies. The model results indicate that a significant fraction of the dominant pattern of low-frequency (.10 yr) SST variability in the North Pacific is associated with tropical forcing. AGCM experiments suggest that the oceanic feedback on the extratropical response to ENSO is complex, but of modest amplitude. Atmosphere- ocean coupling outside of the tropical Pacific slightly modifies the atmospheric circulation anomalies in the Pacific-North America (PNA) region but these modifications appear to depend on the seasonal cycle and air- sea interactions both within and beyond the North Pacific Ocean.
1,668 citations
Cites background from "The Generation of Global Rotational..."
...The dynamical link between the Tropics and extratropics involves the excitation of Rossby waves by both tropical convection (Hoskins and Karoly 1981) and the associated divergent outflow in regions of strong vorticity gradients (Sardeshmukh and Hoskins 1988)....
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TL;DR: A review of tropical-extratropical teleconnections with a focus on developments over the Tropical Oceans-Global Atmosphere (TOGA) decade and the current state of understanding can be found in this article.
Abstract: The primary focus of this review is tropical-extratropical interactions and especially the issues involved in determining the response of the extratropical atmosphere to tropical forcing associated with sea surface temperature (SST) anomalies. The review encompasses observations, empirical studies, theory and modeling of the extratropical teleconnections with a focus on developments over the Tropical Oceans-Global Atmosphere (TOGA) decade and the current state of understanding. In the tropical atmosphere, anomalous SSTs force anomalies in convection and large-scale overturning with subsidence in the descending branch of the local Hadley circulation. The resulting strong upper tropospheric divergence in the tropics and convergence in the subtropics act as a Rossby wave source. The climatological stationary planetary waves and associated jet streams, especially in the northern hemisphere, can make the total Rossby wave sources somewhat insensitive to the position of the tropical heating that induces them and thus can create preferred teleconnection response patterns, such as the Pacific-North American (PNA) pattern. However, a number of factors influence the dispersion and propagation of Rossby waves through the atmosphere, including zonal asymmetries in the climatological state, transients, and baroclinic and nonlinear effects. Internal midlatitude sources can amplify perturbations. Observations, modeling, and theory have clearly shown how storm tracks change in response to changes in quasi-stationary waves and how these changes generally feedback to maintain or strengthen the dominant perturbations through vorticity and momentum transports. The response of the extratropical atmosphere naturally induces changes in the underlying surface, so that there are changes in extratropical SSTs and changes in land surface hydrology and moisture availability that can feedback and influence the total response. Land surface processes are believed to be especially important in spring and summer. Anomalous SSTs and tropical forcing have tended to be strongest in the northern winter, and teleconnections in the southern hemisphere are weaker and more variable and thus more inclined to be masked by natural variability. Occasional strong forcing in seasons other than winter can produce strong and identifiable signals in the northern hemisphere and, because the noise of natural variability is less, the signal-to-noise ratio can be large. The relative importance of tropical versus extratropical SST forcings has been established through numerical experiments with atmospheric general circulation models (AGCMs). Predictability of anomalous circulation and associated surface temperature and precipitation in the extratropics is somewhat limited by the difficulty of finding a modest signal embedded in the high level of noise from natural variability in the extratropics, and the complexity and variety of the possible feedbacks. Accordingly, ensembles of AGCM runs and time averaging are needed to identify signals and make predictions. Strong anomalous tropical forcing provides opportunities for skillful forecasts, and the accuracy and usefulness of forecasts is expected to improve as the ability to forecast the anomalous SSTs improves, as models improve, and as the information available from the mean and the spread of ensemble forecasts is better utilized.
1,523 citations
Cites background from "The Generation of Global Rotational..."
...V((• + f) can be an important contributor to the Rossby wave forcing [Sardeshmukh and Hoskins, 1988]....
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...Furthermore, the mean convergence associated with these poleward winds tends to produce a subtropical source of vorticity in the winter subtropical upper troposphere through its stretching effect on perturbation vorticity, thus making the influence of the heating in the winter hemisphere ven stronger [Sardeshmukh and Hoskins, 1988]....
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TL;DR: The Tropical Tropopause Layer (TTL) as discussed by the authors is a 3D model of the troposphere, and it has been shown that the transition from troposphere to stratosphere occurs in a layer, rather than at a sharp "tropopause".
Abstract: [1] Observations of temperature, winds, and atmospheric trace gases suggest that the transition from troposphere to stratosphere occurs in a layer, rather than at a sharp “tropopause.” In the tropics, this layer is often called the “tropical tropopause layer” (TTL). We present an overview of observations in the TTL and discuss the radiative, dynamical, and chemical processes that lead to its time-varying, three-dimensional structure. We present a synthesis definition with a bottom at 150 hPa, 355 K, 14 km (pressure, potential temperature, and altitude) and a top at 70 hPa, 425 K, 18.5 km. Laterally, the TTL is bounded by the position of the subtropical jets. We highlight recent progress in understanding of the TTL but emphasize that a number of processes, notably deep, possibly overshooting convection, remain not well understood. The TTL acts in many ways as a “gate” to the stratosphere, and understanding all relevant processes is of great importance for reliable predictions of future stratospheric ozone and climate.
881 citations
Cites background from "The Generation of Global Rotational..."
...…anticyclone in the Southern Hemisphere during boreal summer reveals cross-equatorial coupling of dynamics in the tropical upper troposphere and in the TTL [e.g., Sardeshmukh and Hoskins, 1988], and in general, the circulation in the TTL is more symmetric about the equator than in the troposphere....
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TL;DR: In this paper, a 56-yr NCEP-NCAR reanalysis data revealed a recurrent circumglobal teleconnection (CGT) pattern in the summertime midlatitude circulation of the Northern Hemisphere.
Abstract: Analysis of the 56-yr NCEP–NCAR reanalysis data reveals a recurrent circumglobal teleconnection (CGT) pattern in the summertime midlatitude circulation of the Northern Hemisphere. This pattern represents the second leading empirical orthogonal function of interannual variability of the uppertropospheric circulation. The CGT, having a zonal wavenumber-5 structure, is primarily positioned within a waveguide that is associated with the westerly jet stream. The spatial phases of CGT tend to lock to preferred longitudes. The geographically phase-locked patterns bear close similarity during June, August, and September, but the pattern in July shows shorter wavelengths in the North Pacific–North America sector. The CGT is accompanied by significant rainfall and surface air temperature anomalies in the continental regions of western Europe, European Russia, India, east Asia, and North America. This implies that the CGT may be a source of climate variability and predictability in the above-mentioned midlatitude regions. The CGT has significant correlations with the Indian summer monsoon (ISM) and El Nino–Southern Oscillation (ENSO). However, in normal ISM years the CGT–ENSO correlation disappears; on the other hand, in the absence of El Nino or La Nina, the CGT–ISM correlation remains significant. It is suggested that the ISM acts as a “conductor” connecting the CGT and ENSO. When the interaction between the ISM and ENSO is active, ENSO may influence northern China via the ISM and the CGT. Additionally, the variability of the CGT has no significant association with the Arctic Oscillation and the variability of the western North Pacific summer monsoon. The circulation of the wave train shows a barotropic structure everywhere except the cell located to the northwest of India, where a baroclinic circulation structure dominates. Two possible scenarios are proposed. The abnormal ISM may excite an anomalous west-central Asian high and downstream Rossby wave train extending to the North Pacific and North America. On the other hand, a wave train that is excited in the jet exit region of the North Atlantic may affect the westcentral Asian high and, thus, the intensity of the ISM. It is hypothesized that the interaction between the global wave train and the ISM heat source may be instrumental in maintaining the boreal summer CGT.
857 citations
TL;DR: In this paper, the authors analyzed the global response to tropical Pacific sea surface temperature forcing will be inherently nonlinear, based on the thermodynamic control on deep convection, and found evidence for an appreciable 358 longitude phase shift between the warm and cold event circulation composites, and two wave trains appear to have different tropical origins.
Abstract: The paradigm of an atmospheric system varying linearly with respect to extreme phases of the El Nino- Southern Oscillation is questioned. It is argued that the global response to tropical Pacific sea surface temperature forcing will be inherently nonlinear. A physical basis for this intrinsic nonlinearity is the thermodynamic control on deep convection. Climate statistics for warm and cold events of the tropical Pacific are analyzed separately for the northern winter periods during 1950-96. Composite analysis of 500-mb heights reveal planetary-scale teleconnection patterns, as noted in earlier studies. A new result is the evidence for an appreciable 358 longitude phase shift between the warm and cold event circulation composites, and the two wave trains appear to have different tropical origins. A large nonlinear component in North American surface climate anomalies is also found, which is consistent with such a phase shift in teleconnections. In the Tropics, rainfall anomalies also show evidence of nonlinear behavior. The maximum rain anomalies along the equator are located east of the date line during warm events, but west of the date line during cold events. The interpretation of this behavior is complicated, however, by the fact that composite warm event SST anomalies are not the exact inverse of their cold event counterparts. Idealized atmospheric general circulation model (AGCM) experiments are performed in order to test the question of whether the observed nonlinearity is an intrinsic property of the atmospheric system. The model is forced with a composite SST anomaly that undergoes a realistic seasonally varying ENSO life cycle, as described by E. Rasmusson and T. Carpenter. Both positive and negative phases of the SST anomaly are used, and a 40-member ensemble of warm and cold event model simulations is conducted. A nonlinear climate response in the AGCM is found that closely resembles the observed composites, including a shift in the equatorial positions of the maxmium rain responses and a phase shift of teleconnection patterns in the upper troposphere. Barotropic model experiments indicate that the inherent nonlinearity in the tropical rain response may itself be responsible for the phase shift in the extratropical teleconnection patterns.
793 citations
Cites background or methods from "The Generation of Global Rotational..."
...The forcing associated with divergence is expressed as a source S9 (Held and Kang 1987; Sardeshmukh and Hoskins 1988), given by...
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...This uses eigenvector reconstruction to estimate SST values at ocean points on a 28 latitude–longitude grid and is described in Smith et al. (1996). For the period since 1982, the combination of in situ measurements and satellite observations is adequate to resolve the global SST field....
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