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Leonard M. Druyan
and
Timothy M. Hall

Abstract

Ensembles of three simulations each, forced by June–September 1987 and 1988 sea surface temperatures, respectively, were made with a new version of the general circulation model of the National Aeronautics and Space Administration/Goddard Institute for Space Studies. Time series of 6-h meridional winds at about 780 mb over West Africa were spectrally analyzed to detect African wave disturbances, whose properties for the two ensembles are compared and contrasted. The realistically simulated, stronger 1988 tropical easterly jet and the associated stronger upper-tropospheric divergence are components of interannual differences in the SST-forced planetary circulation, which correspond to higher amplitudes of African wave activity and concomitant excesses in 1988 Sahel rainfall rates. Results do not show, however, that most of the heavier precipitation was spatially organized by African wave structures. The excess rainfall is associated with stronger mean southerly circulation in the lower troposphere, which carried more moisture into the Sahel. Nevertheless, because waves modulate winds, convergence, humidity, and precipitation, the study suggests that they serve as a teleconnection mechanism, whereby extreme Pacific Ocean SST anomalies are able to influence climate variability in Africa's Sahel.

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Leonard M. Druyan
and
Timothy M. Hall

Abstract

Simulations made with the general circulation model of the NASA/Goddard Institute for Space Studies (GISS GCM) run at 4° latitude by 5° longitude horizontal resolution are analyzed to determine the model's representation of African wave disturbances. Waves detected in the model's lower troposphere over northern Africa during the summer monsoon season exhibit realistic wavelengths of about 2200 km. However, power spectra of the meridional wind show that the waves propagate westward too slowly, with periods of 5–10 days, about twice the observed values. This sluggishness is most pronounced during August, consistent with simulated 600-mb zonal winds that are only about half the observed speeds of the midtropospheric jet. The modeled wave amplitudes are strongest over West Africa during the first half of the summer but decrease dramatically by September, contrary to observational evidence. Maximum amplitudes occur at realistic latitudes, 12°–20°N, but not as observed near the Atlantic coast. Spectral analyses suggest some wave modulation of precipitation in the 5–8-day band, and compositing shows that precipitation is slightly enhanced east of the wave trough, coincident with southerly winds. Extrema of low-level convergence west of the wave troughs, coinciding with northerly winds, were not preferred areas for simulated precipitation, probably because of the drying effect of this advection, as waves were generally north of the humid zone. The documentation of African wave disturbances in the GISS GCM is a first step toward considering wave influences in future GCM studies of Sahel drought.

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Timothy M. Hall
and
Stephen Jewson

Abstract

Two statistical methods for predicting the number of tropical cyclones (TCs) making landfall on sections of the North American coastline are compared. The first method—the “local model”—is derived exclusively from historical landfalls on the particular coastline section. The second method—the “track model”—involves statistical modeling of TC tracks from genesis to lysis, and is based on historical observations of such tracks. Identical scoring schemes are used for each model, derived from the out-of-sample likelihood of a Bayesian analysis of the Poisson landfall number distribution. The track model makes better landfall rate predictions on most coastal regions, when coastline sections at a scale of several hundred kilometers or smaller are considered. The reduction in sampling error due to the use of the much larger dataset more than offsets any bias in the track model. When larger coast sections are considered, there are more historical landfalls, and the local model scores better. This is the first clear justification for the use of track models for the assessment of TC landfall risk on regional and smaller scales.

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Emmi Yonekura
and
Timothy M. Hall

Abstract

A new statistical model for western North Pacific Ocean tropical cyclone genesis and tracks is developed and applied to estimate regionally resolved tropical cyclone landfall rates along the coasts of the Asian mainland, Japan, and the Philippines. The model is constructed on International Best Track Archive for Climate Stewardship (IBTrACS) 1945–2007 historical data for the western North Pacific. The model is evaluated in several ways, including comparing the stochastic spread in simulated landfall rates with historic landfall rates. Although certain biases have been detected, overall the model performs well on the diagnostic tests, for example, reproducing well the geographic distribution of landfall rates. Western North Pacific cyclogenesis is influenced by El Niño–Southern Oscillation (ENSO). This dependence is incorporated in the model’s genesis component to project the ENSO-genesis dependence onto landfall rates. There is a pronounced shift southeastward in cyclogenesis and a small but significant reduction in basinwide annual counts with increasing ENSO index value. On almost all regions of coast, landfall rates are significantly higher in a negative ENSO state (La Niña).

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Mark Holzer
and
Timothy M. Hall

Abstract

Transport in the atmosphere and in the ocean is the result of the complex action of time-dependent and often highly turbulent flow. A useful diagnostic that summarizes the rate at which fluid elements are transported from some region to a point (or the reverse) via a multiplicity of pathways and mechanisms is the probability density function (pdf) of transit times. The first moment of this pdf, often referred to as “mean age,” has become an important transport diagnostic commonly used by the observational community.

This paper explores how to probe the flow with passive tracers to extract transit-time pdf’s. As a foundation, the literal “tracer age” is defined as the elapsed time since tracer was injected into the flow, and the corresponding tracer-age distribution, Z, as the fractional tracer mass in a given interval of tracer age. The distribution, Z, has concrete physical interpretation for arbitrary sources, but is only equivalent to a tracer-independent transit-time pdf of the flow in special cases. The transit-time pdf is a propagator, G ′, of boundary conditions (the “age spectrum” of T. M. Hall and R. A. Plumb) applied over a control surface, Ω. The propagator G ′ is shown to be the flux into Ω resulting from a unit mass injected into the time-reversed flow. Through explicit construction of the transit-time pdf using the concept of tracer age, the special cases for which Z and G ′ coincide are established. This allows a direct physical demonstration of G ′, and its adjoint G , as the pdf’s of transit times since fluid at point r had last contact with Ω, and until fluid at r will have first contact with Ω, respectively. In the limit as Ω is shrunk to a point, point-to-point transit-time pdf’s are well defined, but their mean transit time and higher-order moments become infinite. Several concrete geophysical examples are considered to illustrate under what conditions characteristics of tracer-age and transit-time pdf’s can be inferred from observations in the atmosphere or the ocean.

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Emmi Yonekura
and
Timothy M. Hall

Abstract

Improvements on a statistical tropical cyclone (TC) track model in the western North Pacific Ocean are described. The goal of the model is to study the effect of El Niño–Southern Oscillation (ENSO) on East Asian TC landfall. The model is based on the International Best-Track Archive for Climate Stewardship (IBTrACS) database of TC observations for 1945–2007 and employs local regression of TC formation rates and track increments on the Niño-3.4 index and seasonally varying climate parameters. The main improvements are the inclusion of ENSO dependence in the track propagation and accounting for seasonality in both genesis and tracks. A comparison of simulations of the 1945–2007 period with observations concludes that the model updates improve the skill of this model in simulating TCs. Changes in TC genesis and tracks are analyzed separately and cumulatively in simulations of stationary extreme ENSO states. ENSO effects on regional (100-km scale) landfall are attributed to changes in genesis and tracks. The effect of ENSO on genesis is predominantly a shift in genesis location from the southeast in El Niño years to the northwest in La Niña years, resulting in higher landfall rates for the East Asian coast during La Niña. The effect of ENSO on track propagation varies seasonally and spatially. In the peak activity season (July–October), there are significant changes in mean tracks with ENSO. Landfall-rate changes from genesis– and track–ENSO effects in the Philippines cancel out, while coastal segments of Vietnam, China, the Korean Peninsula, and Japan show enhanced La Niña–year increases.

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Darryn W. Waugh
and
Timothy M. Hall

Abstract

The propagation of a range of tracer signals in a simple model of the deep western boundary current is examined. Analytical expressions are derived in certain limits for the transit-time distributions and the propagation times (tracer ages) of tracers with exponentially growing or periodic concentration histories at the boundary current’s origin. If mixing between the boundary current and the surrounding ocean is either very slow or very rapid, then all tracer signals propagate at the same rate. In contrast, for intermediate mixing rates tracer ages generally depend on the history of the tracer variations at the origin and range from the advective time along the current to the much larger mean age. Close agreement of the model with chlorofluorocarbon (CFC) and tritium observations in the North Atlantic deep western boundary current (DWBC) is obtained when the model is in the intermediate mixing regime, with current speed around 5 cm s−1 and mixing time scale around 1 yr. In this regime anomalies in temperature and salinity of decadal or shorter period will propagate downstream at roughly the current speed, which is much faster than the spreading rate inferred from CFC or tritium–helium ages (approximately 5 cm s−1 as compared with 2 cm s−1). This rapid propagation of anomalies is consistent with observations in the subpolar DWBC, but is at odds with inferences from measurements in the tropical DWBC. This suggests that observed tropical temperature and salinity anomalies are not simply propagated signals from the north. The sensitivity of the tracer spreading rates to tracer and mixing time scales in the model suggests that tight constraints on the flow and transport in real DWBCs may be obtained from simultaneous measurements of several different tracers—in particular, hydrographic anomalies and steadily increasing transient tracers.

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Thomas W. N. Haine
and
Timothy M. Hall

Abstract

A general theory to describe and understand advective and diffusive ocean transport is reported. It allows any passive tracer field with an atmospheric source to be constructed by superposing sea surface contributions with a generalized Green's function called the boundary propagator of the passive tracer equation. The boundary propagator has the interpretation of the joint water-mass and transit-time distribution from the sea surface. The theory thus includes the classical oceanographic idea of water-mass analysis and extends it to allow for a distribution of transit times from the sea surface. The joint water-mass and transit-time distribution contains complete information about the transport processes in the flow. It captures this information in a more accessible way than using velocity and diffusivity fields, however, at least for the case of sequestration and transport of dissolved material by the ocean circulation. The boundary propagator is thus the natural quantity to consider when discussing both steady-state and transient ocean tracers, including the inverse problem of interpreting tracer data in terms of ocean circulation. Two constraints on the shape of the transit-time distributions are derived. First, the asymptotic behavior for a steady, or time-averaged, circulation is exponential decay. Second, integrated over the whole ocean, the transit-time distribution from the sea surface cannot increase. The theory is illustrated using a one-dimensional advection–diffusion model, a box model, and a North Atlantic general circulation model.

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Timothy M. Hall
and
Thomas W. N. Haine

Abstract

The idealized age tracer is commonly used to diagnose transport in ocean models and to help interpret ocean measurements. In most studies only the steady-state distribution, the result of many centuries of model integration, has been presented and analyzed. However, in principle the transient solution provides more information about the transport. Here it is shown that this information can be readily interpreted in terms of the ventilation histories of water masses. A simple relationship is derived, valid for stationary transport, between the transient evolution, τ id(r, t), of the idealized age tracer and the “age spectrum,” G (r, t), the distribution of times t since a water mass was last ventilated. Namely, G (r, t) = −∂ tt τ id(r, t). Implications of the relationship are discussed, and the relationship is illustrated with an idealized model.

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Timothy M. Hall
,
James P. Kossin
,
Terence Thompson
, and
James McMahon

Abstract

We use a statistical tropical cyclone (TC) model, the North Atlantic Stochastic Hurricane Model (NASHM), in combination with sea surface temperature (SST) projections from climate models, to estimate regional changes in U.S. TC activity into the 2030s. NASHM is trained on historical variations in TC characteristics with two SST indices: global–tropical mean SST and the difference between tropical North Atlantic Ocean (NA) SST and the rest of the global tropics, often referred to as “relative SST.” Testing confirms the model’s ability to reproduce historical U.S. TC activity as well as to make skillful predictions. When NASHM is driven by SST projections into the 2030s, overall NA annual TC counts increase, and the fractional increase is the greatest at the highest wind intensities. However, an eastward anomaly in mean TC tracks and an eastward shift in TC formation region result in a geographically varied signal in U.S. coastal activity. Florida’s Gulf Coast is projected to see significant increases in TC activity relative to the long-term historical mean, and these increases are fractionally greatest at the highest intensities. By contrast, the northwestern U.S. Gulf Coast and the U.S. East Coast will see little change.

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