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Observations of Coupling between Surface Wind Stress and Sea Surface Temperature in the Eastern Tropical Pacific
Bring Back 1962-63 posted a topic in Research PortalObservations of Coupling between Surface Wind Stress and Sea Surface Temperature in the Eastern Tropical Pacific Authors: Dudley B. Chelton, Steven K. Esbensen, Michael G. Schlax, Nicolai Thum and Michael H. Freilich First Published: December 27th, 1999 Published on line: April 1st, 2001 Abstract: Satellite measurements of surface wind stress from the QuikSCAT scatterometer and sea surface temperature (SST) from the Tropical Rainfall Measuring Mission Microwave Imager are analyzed for the three-month period 21 July–20 October 1999 to investigate ocean–atmosphere coupling in the eastern tropical Pacific. Oceanic tropical instability waves (TIWs) with periods of 20–40 days and wavelengths of 1000–2000 km perturb the SST fronts that bracket both sides of the equatorial cold tongue, which is centered near 1°S to the east of 130°W. These perturbations are characterized by cusp-shaped features that propagate systematically westward on both sides of the equator. The space–time structures of these SST perturbations are reproduced with remarkable detail in the surface wind stress field. The wind stress divergence is shown to be linearly related to the downwind component of the SST gradient with a response on the south side of the cold tongue that is about twice that on the north side. The wind stress curl is linearly related to the crosswind component of the SST gradient with a response that is approximately half that of the wind stress divergence response to the downwind SST gradient. The perturbed SST and wind stress fields propagate synchronously westward with the TIWs. This close coupling between SST and wind stress supports the Wallace et al. hypothesis that surface winds vary in response to SST modification of atmospheric boundary layer stability. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/1520-0442(2001)014<1479%3AOOCBSW>2.0.CO%3B2
Bring Back 1962-63 posted a topic in Research PortalNarrowing of the ITCZ in a warming climate: Physical mechanisms First published: 22 October 2016 Authors: Michael P. Byrne and Tapio Schneider First Published: October 22nd, 2016 Published on line: June 14th, 2016 Abstract: The Intertropical Convergence Zone (ITCZ) narrows in response to global warming in both observations and climate models. However, a physical understanding of this narrowing is lacking. Here we show that the narrowing of the ITCZ in simulations of future climate is related to changes in the moist static energy (MSE) budget. MSE advection by the mean circulation and MSE divergence by transient eddies tend to narrow the ITCZ, while changes in net energy input to the atmosphere and the gross moist stability tend to widen the ITCZ. The narrowing tendency arises because the meridional MSE gradient strengthens with warming, whereas the largest widening tendency is due to increasing shortwave heating of the atmosphere. The magnitude of the ITCZ narrowing depends strongly on the gross moist stability and clouds, emphasizing the need to better understand these fundamental processes in the tropical atmosphere. Link to Paper: https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1002/2016GL070396
Bring Back 1962-63 posted a topic in Research PortalEnergetic Constraints on the Width of the Intertropical Convergence Zone Authors: Michael P. Byrne and Tapio Schneider First Published: February 9th, 2016 Published on line: June 14th, 2016 Abstract: The intertropical convergence zone (ITCZ) has been the focus of considerable research in recent years, with much of this work concerned with how the latitude of maximum tropical precipitation responds to natural climate variability and to radiative forcing. The width of the ITCZ, however, has received little attention despite its importance for regional climate and for understanding the general circulation of the atmosphere. This paper investigates the ITCZ width in simulations with an idealized general circulation model over a wide range of climates. The ITCZ, defined as the tropical region where there is time-mean ascent, displays rich behavior as the climate varies, widening with warming in cool climates, narrowing in temperate climates, and maintaining a relatively constant width in hot climates. The mass and energy budgets of the Hadley circulation are used to derive expressions for the area of the ITCZ relative to the area of the neighboring descent region, and for the sensitivity of the ITCZ area to changes in climate. The ITCZ width depends primarily on four quantities: the net energy input to the tropical atmosphere, the advection of moist static energy by the Hadley circulation, the transport of moist static energy by transient eddies, and the gross moist stability. Different processes are important for the ITCZ width in different climates, with changes in gross moist stability generally having a weak influence relative to the other processes. The results are likely to be useful for analyzing the ITCZ width in complex climate models and for understanding past and future climate change in the tropics. Link to Paper: https://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-15-0767.1
Bring Back 1962-63 posted a topic in Research PortalAtmospheric Dynamics Feedback: Concept, Simulations, and Climate Implications Authors: Michael P. Byrne and Tapio Schneider First Published: January 12th, 2018 Published on line: March 26th, 2018 Abstract: The regional climate response to radiative forcing is largely controlled by changes in the atmospheric circulation. It has been suggested that global climate sensitivity also depends on the circulation response, an effect called the “atmospheric dynamics feedback.” Using a technique to isolate the influence of changes in atmospheric circulation on top-of-the-atmosphere radiation, the authors calculate the atmospheric dynamics feedback in coupled climate models. Large-scale circulation changes contribute substantially to all-sky and cloud feedbacks in the tropics but are relatively less important at higher latitudes. Globally averaged, the atmospheric dynamics feedback is positive and amplifies the near-surface temperature response to climate change by an average of 8% in simulations with coupled models. A constraint related to the atmospheric mass budget results in the dynamics feedback being small on large scales relative to feedbacks associated with thermodynamic processes. Idealized-forcing simulations suggest that circulation changes at high latitudes are potentially more effective at influencing global temperature than circulation changes at low latitudes, and the implications for past and future climate change are discussed. Link to Paper: https://pdfs.semanticscholar.org/8f29/8a8c3782fc69b576804e2ad9d037daf1df3f.pdf