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Abraham H. Oort and James J. Yienger

Abstract

Based on a 26-yr set of daily global upper-air wind data for the period January 1964–December 1989, the interannual variability in the strength of the tropical Hadley cells is investigated. Although several measures of the intensity of the zonal-mean cells are discussed, the main focus is on the maximum in the streamfunction in the northern and southern Tropics. The streamfunction was computed from observed monthly mean latitude versus pressure cross sections of the zonal-mean meridional wind component. Significant seasonal variations are found in the strength, latitude, and height of the maximum streamfunction for both Hadley cells. Significant correlations are also observed between the Hadley cells and the El Niño-Southern Oscillation phenomenon. During the extreme seasons, only one “winter” Hadley cell dominates the Tropics, with the rising branch in the summer hemisphere and the sinking branch in the winter hemisphere. Superimposed on this “normal” one-cell winter Hadley circulation in the Tropics are two strengthened direct (i.e., energy releasing) Hadley cells found during episodes of warm sea surface temperature anomalies in the eastern equatorial Pacific (El Niño) and weakened Hadley cells during episodes of cold anomalies. The anomalies in the strength of the Hadley cells are strongly and inversely correlated with the anomalies in the strength of the Walker oscillation.

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James E. Overland, Philip Turet, and Abraham H. Oort

Abstract

The authors investigate the climmological heating of the Arctic by the atmospheric moist static energy (MSE) flux from lower latitudes based on 25 years (November 1964–1989) of the GFDL dataset. During the five month winter period (NDJFM) the transport of sensible heat by transient eddies is the largest component (50%) at 70°N, followed by the transport of sensible but by standing eddies (25%), and the moist static energy flux by the mean meridional circulation (25%). The mean meridional circulation (MMC) changes from a Ferrel cell to a thermally direct circulation near 60°N; maximum horizontal velocities in the thermally direct circulation peak new 70°N. North of 60°N the sensible heat flux by the MMC is southward and opposes the greater northward transport of geopotential energy. The transport of energy is not uniform. Major pathways are the northward transport of positive anomalies through the Greenland and Barents Seas into the eastern Arctic and the southward transport of negative anomalies to the cast of the Siberian high. The Atlantic pathway in winter relates to transport by transient eddies, while the western Siberian flux relates to the standing eddy pattern. Interannual variability of northward MSE is concentrated in these two regions. The western Arctic Ocean from about 30° to 60°W receives about 50 W m−2 less energy flux convergence than the eastern Arctic. This result compares well with the observed minimum January surface air temperatures in the Canadian Basin of the western Arctic and implies that the greater observed ice thickness in this region may have a thermodynamic as well as a dynamic origin.

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James Abraham, J. Walter Strapp, Christopher Fogarty, and Mengistu Wolde

In order to better understand the behavior and impacts of tropical cyclones undergoing extratropical transition (ET), the Meteorological Service of Canada (MSC) conducted a test flight into Hurricane Michael. Between 16 and 19 October 2000 the transition of Hurricane Michael from a hurricane to an intense extratropical storm was investigated using a Canadian research aircraft instrumented for storm research. This paper presents the various data collected from the flight with a detailed description of the storm structure at the time when Michael was in the midst of ET.

Hurricane Michael was moving rapidly to the northeast, approximately 300 km southeast of Nova Scotia, Canada, during the time of the aircraft mission. A period of rapid intensification had also occurred during this time as the system moved north of the warm Gulf Stream waters and merged with a baroclinic low pressure system moving offshore of Nova Scotia. Consequently, the hurricane was sampled near the period of its lowest surface pressure and maximum surface winds. It is estimated that the aircraft passed approximately 10 km south of the estimated 42.7°N, 59.7°W position of the surface low pressure center at about 1645 UTC 19 October. Sixteen dropsondes were deployed in a single traverse from northwest to east of the storm center, and then back westbound south of the center. Winds were found to be highest on the southeast side of the hurricane where the storm movement adds to the hurricane rotational flow. A southwesterly jet with winds exceeding 70 m s−1 was observed between 500 and 2000 m approximately 85 km southeast of the center. This low-level jet was much deeper than the usual lowlevel maximum winds found in hurricanes. Michael was observed to have an elevated warm core similar to purely tropical systems, but low-altitude humidity appeared to be eroded by entrainment of dry midlatitude air surrounding the storm, which is typically observed during the ET process.

A cloud-profiling 35-GHz radar provided data on the distribution of precipitation across the system, and cloud microphysical probes measured cloud water contents, particle phases, and spectra. Although a wide variety of liquid, mixed phase, and deep glaciated clouds were observed, the glaciated cloud encountered on the northwest side of the center, associated with the most significant precipitation area, was relatively stratiform in nature, with a broad area of high ice water content reaching 1.5 g m−3, and very high concentrations of small ice particles.

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Steven C. Hardiman, Ian A. Boutle, Andrew C. Bushell, Neal Butchart, Mike J. P. Cullen, Paul R. Field, Kalli Furtado, James C. Manners, Sean F. Milton, Cyril Morcrette, Fiona M. O’Connor, Ben J. Shipway, Chris Smith, David N. Walters, Martin R. Willett, Keith D. Williams, Nigel Wood, N. Luke Abraham, James Keeble, Amanda C. Maycock, John Thuburn, and Matthew T. Woodhouse

Abstract

A warm bias in tropical tropopause temperature is found in the Met Office Unified Model (MetUM), in common with most models from phase 5 of CMIP (CMIP5). Key dynamical, microphysical, and radiative processes influencing the tropical tropopause temperature and lower-stratospheric water vapor concentrations in climate models are investigated using the MetUM. A series of sensitivity experiments are run to separate the effects of vertical advection, ice optical and microphysical properties, convection, cirrus clouds, and atmospheric composition on simulated tropopause temperature and lower-stratospheric water vapor concentrations in the tropics. The numerical accuracy of the vertical advection, determined in the MetUM by the choice of interpolation and conservation schemes used, is found to be particularly important. Microphysical and radiative processes are found to influence stratospheric water vapor both through modifying the tropical tropopause temperature and through modifying upper-tropospheric water vapor concentrations, allowing more water vapor to be advected into the stratosphere. The representation of any of the processes discussed can act to significantly reduce biases in tropical tropopause temperature and stratospheric water vapor in a physical way, thereby improving climate simulations.

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