Principal Component Analysis of the Summertime Winds over the Gulf of California: A Gulf Surge Index

A principal component analysis of the summertime near-surface Quick Scatterometer (QuikSCAT) winds is used to identify the leading mode of synoptic-scale variability of the low-level flow along the Gulf of California during the North American monsoon season. A gulf surge mode emerges from this analysis as the leading EOF, with the corresponding principal component time series interpretable as an objective index for gulf surge occurrence. This index is used as a reference time series for regression analysis and compositing meteorological fields of interest, to explore the relationship between gulf surges and precipitation over the core and marginal regions of the monsoon, as well as the manifestation of these transient events in the large-scale circulation. It is found that, although seemingly mesoscale features confined over the Gulf of California, gulf surges are intimately linked to patterns of large-scale variability of the eastern Pacific ITCZ and greatly contribute to the definition of the northward extent of the monsoonal rains

An optimized algebraic basis for molecular potentials

The computation of vibrational spectra of diatomic molecules through the exact diagonalization of algebraically determined matrixes based on powers of Morse coordinates is made substantially more efficient by choosing a properly adapted quantum-mechanical basis, specifically tuned to the molecular potential. A substantial improvement is achieved while still retaining the full advantage of the simplicity and numerical light-weightedness of an algebraic approach. In the scheme we propose, the basis is parameterized by two quantities which can be adjusted to best suit the molecular potential through a simple minimization procedure.Comment: 29 pages, 4 tables and 4 figures, latex. Sumbitted to J. Phys. Chem

Eddy-Mediated Regime Transitions in the Seasonal Cycle of a Hadley Circulation and Implications for Monsoon Dynamics

In a simulation of seasonal cycles with an idealized general circulation model without a hydrologic cycle and with zonally symmetric boundary conditions, the Hadley cells undergo transitions between two regimes distinguishable according to whether large-scale eddy momentum fluxes strongly or weakly influence the strength of a cell. The center of the summer and equinox Hadley cell lies in a latitude zone of upper-level westerlies and significant eddy momentum flux divergence; the influence of eddy momentum fluxes on the strength of the cell is strong. The center of the cross-equatorial winter Hadley cell lies in a latitude zone of upper-level easterlies and is shielded from the energy-containing midlatitude eddies; the influence of eddy momentum fluxes on the strength of the cell is weak. Mediated by feedbacks between eddy fluxes, mean zonal winds at upper levels, and the mean meridional circulation, the dominant balance in the zonal momentum equation at the center of a Hadley cell shifts at the transitions between the regimes, from eddies dominating the momentum flux divergence in the summer and equinox cell to the mean meridional circulation dominating in the winter cell. At the transitions, a feedback involving changes in the strength of the lower-level temperature advection and in the latitude of the boundary between the winter and summer cell is responsible for changes in the strength of the cross-equatorial winter cell. The transitions resemble the onset and end of monsoons, for example, in the shift in the dominant zonal momentum balance, rapid shifts in the latitudes of maximum meridional mass flux and of maximum convergence at lower levels, rapid changes in strength of the upward mass flux, and changes in direction and strength of the zonal wind at upper and lower levels. In the monsoonal regime, the maximum upward mass flux occurs in an off-equatorial convergence zone located where the balance of the meridional geopotential gradient in the planetary boundary layer shifts from nonlinear frictional to geostrophic. Similar dynamic mechanisms as at the regime transitions in the simulation—mechanisms that can act irrespective of land–sea contrasts and other inhomogeneities in lower boundary conditions—may be implicated in large-scale monsoon dynamics in Earth’s atmosphere

In the Driver's Seat: Rico and Education

The Rain in Cumulus over the Ocean (RICO) field campaign carried out a wide array of educational activities, including a major first in a field project—a complete mission, including research flights, planned and executed entirely by students. This article describes the educational opportunities provided to the 24 graduate and 9 undergraduate students who participated in RICO

Axisymmetric constraints on cross-equatorial Hadley cell extent

We consider the relevance of known constraints from each of Hide's theorem, the angular momentum conserving (AMC) model, and the equal-area model on the extent of cross-equatorial Hadley cells. These theories respectively posit that a Hadley circulation must span: all latitudes where the radiative convective equilibrium (RCE) absolute angular momentum ($M_\mathrm{rce}$) satisfies $M_\mathrm{rce}>\Omega a^2$ or $M_\mathrm{rce}<0$ or where the RCE absolute vorticity ($\eta_\mathrm{rce}$) satisfies $f\eta_\mathrm{rce}<0$; all latitudes where the RCE zonal wind exceeds the AMC zonal wind; and over a range such that depth-averaged potential temperature is continuous and that energy is conserved. The AMC model requires knowledge of the ascent latitude $\varphi_\mathrm{a}$, which need not equal the RCE forcing maximum latitude $\varphi_\mathrm{m}$. Whatever the value of $\varphi_\mathrm{a}$, we demonstrate that an AMC cell must extend at least as far into the winter hemisphere as the summer hemisphere. The equal-area model predicts $\varphi_\mathrm{a}$, always placing it poleward of $\varphi_\mathrm{m}$. As $\varphi_\mathrm{m}$ is moved poleward (at a given thermal Rossby number), the equal-area predicted Hadley circulation becomes implausibly large, while both $\varphi_\mathrm{m}$ and $\varphi_\mathrm{a}$ become increasingly displaced poleward of the minimal cell extent based on Hide's theorem (i.e. of supercritical forcing). In an idealized dry general circulation model, cross-equatorial Hadley cells are generated, some spanning nearly pole-to-pole. All homogenize angular momentum imperfectly, are roughly symmetric in extent about the equator, and appear in extent controlled by the span of supercritical forcing.Comment: 18 pages, 9 figures, publishe