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Michael J. Bell

Abstract

The quasigeostrophic equations formulated using the Charney–Phillips vertical staggering of variables are well known to possess an analog of the form of conservation of potential vorticity. It is shown that a similar analog is enjoyed by the quasigeostrophic equations formulated using the modified Lorenz staggering of variables.

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Michael J. Bell

Abstract

The Sverdrup relationship when applied to the Southern Ocean suggests that some isopycnals that are deep in the eastern Pacific will shoal in the Atlantic. Cold waters surfacing in the South Atlantic at midlatitudes would be warmed by the atmosphere. The potential for water mass transformations in this region is studied by applying a three-layer planetary geostrophic model to a wide ocean basin driven by the Ekman upwelling typical of the Southern Ocean surface winds. The model uses a simple physically based parameterization of the entrainment of mass into the surface layer with zonally symmetric atmospheric surface fields to find steady-state subpolar gyre solutions. The solutions are found numerically by specifying suitable boundary conditions and integrating along characteristics. With reasonable parameter settings, transformations of more than 10 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) of water between layers are obtained. The water mass transformations are sensitive to the strength of the wind stress curl and the width of the basin and relatively insensitive to the parameterization of the surface heat fluxes. On the western side of the basin where the cold waters are near the surface, there is a large region where there is a local balance between the Ekman pumping and the exchange of mass between layers. Simple formulas are derived for the water mass transformation rates in terms of the difference between the maximum and minimum northward Ekman transports integrated across the basin and the depths of the isopycnal layers on the eastern boundary. The relevance of the model to the Southern Ocean and the Atlantic meridional overturning circulation is briefly discussed.

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Michael J. Bell

Abstract

The meridional overturning circulation (MOC) can be considered to consist of a downwelling limb in the Northern Hemisphere (NH) and an upwelling limb in the Southern Hemisphere (SH) that are connected via western boundary currents. Steady-state analytical gyre-scale solutions of the planetary geostrophic equations are derived for a downwelling limb driven in the NH solely by surface heat loss. In these solutions the rates of the water mass transformations between layers driven by the surface heat loss determine the strength of the downwelling limb. Simple expressions are obtained for these transformation rates that depend on the most southerly latitudes where heat loss occurs and the depths of the isopycnals on the eastern boundary. Previously derived expressions for the water mass transformation rates in subpolar gyres driven by the Ekman upwelling characteristic of the SH are also summarized. Explicit expressions for the MOC transport and the depths of isopycnals on the eastern boundary are then derived by equating the water mass transformations in the upwelling and downwelling limbs. The MOC obtained for a “single-basin” two-layer model is shown to be generally consistent with that obtained by Gnanadesikan. The model’s energetics are derived and discussed. In a world without a circumpolar channel in the SH, it is suggested that the upwelling limb would feed downwelling limbs in both hemispheres. In a world with two basins in the NH, if one of them has a strong halocline the model suggests that the MOC would be very weak in that basin.

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Michael A. Bell and Peter J. Lamb

Abstract

Since the late 1960s, the West African Sudan–Sahel zone (10°–18°N) has experienced persistent and often severe drought, which is among the most undisputed and largest regional climate changes in the last half-century. Previous documentation of the drought generally has used monthly, seasonal, and annual rainfall totals and departures, in a standard “climate” approach that overlooks the underlying weather system variability. Most Sudan–Sahel rainfall occurs during June–September and is delivered by westward-propagating, linear-type, mesoscale convective systems [disturbance lines (DLs)] that typically have much longer north–south (102–103 km) than east–west (10–102 km) dimensions. Here, a large set of daily rainfall data is analyzed to relate DL and regional climate variability on intraseasonal-to-multidecadal time scales for 1951–98. Rain gauge–based indices of DL frequency, size, and intensity are evaluated on a daily basis for four 440-km square “catchments” that extend across most of the West African Sudan–Sahel (18°W–4°E) and are then distilled into 1951–98 time series of 10-day and seasonal frequency/magnitude summary statistics. This approach is validated using Tropical Applications of Meteorology Using Satellite Data (TAMSAT) satellite IR cold cloud duration statistics for the same 1995–98 DLs.

Results obtained for all four catchments are remarkably similar on each time scale. Long-term (1951–98) average DL size/organization increases monotonically from early June to late August and then decreases strongly during September. In contrast, average DL intensity maximizes 10–30 days earlier than DL size/organization and is distributed more symmetrically within the rainy season for all catchments except the westernmost, where DL intensity tracks DL size/organization very closely. Intraseasonal and interannual DL variability is documented using sets of very deficient (8) and much more abundant (7) rainy seasons during 1951–98. The predominant mode of rainfall extremes involves near-season-long suppression or enhancement of the seasonal cycles of DL size/organization and intensity, especially during the late July–late August rainy season peak. Other extreme seasons result solely from peak season anomalies. On the multidecadal scale, the dramatic decline in seasonal rainfall totals from the early 1950s to the mid-1980s is shown to result from pronounced downtrends in DL size/organization and intensity. Surprisingly, this DL shrinking–fragmentation–weakening is not accompanied by increases in catchment rainless days (i.e., total DL absence). Like the seasonal rainfall totals, DL size/organization and intensity increase slightly after the mid-1980s.

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Michael J. Bell, Adam T. Blaker, and Joël J.-M. Hirschi

Abstract

Large-amplitude [±100 Sv (1 Sv ≡ 106 m3 s−1)], high-frequency oscillations in the Pacific Ocean’s meridional overturning circulation within 10° of the equator have been found in integrations of the NEMO ocean general circulation model. Part I of this paper showed that these oscillations are dominated by two bands of frequencies with periods close to 4 and 10 days and that they are driven by the winds within about 10° of the equator. This part shows that the oscillations can be well simulated by small-amplitude, wind-driven motions on a horizontally uniform, stably stratified state of rest. Its main novelty is that, by focusing on the zonally integrated linearized equations, it presents solutions for the motions in a basin with sloping side boundaries. The solutions are found using vertical normal modes and equatorial meridional modes representing Yanai and inertia–gravity waves. Simulations of 16-day-long segments of the time series for the Pacific of each of the first three meridional and vertical modes (nine modes in all) capture between 85% and 95% of the variance of matching time series segments diagnosed from the NEMO integrations. The best agreement is obtained by driving the solutions with the full wind forcing and the full pressure forces on the bathymetry. Similar results are obtained for the corresponding modes in the Atlantic and Indian Oceans. Slower variations in the same meridional and vertical modes of the MOC are also shown to be well simulated by a quasi-stationary solution driven by zonal wind and pressure forces.

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Jhordanne J. Jones, Michael M. Bell, and Philip J. Klotzbach

Abstract

Given recent insights into the role of anticyclonic Rossby wave breaking (AWB) in driving subseasonal and seasonal North Atlantic tropical cyclone (TC) activity, this study further examines tropical versus subtropical impacts on TC activity by considering large-scale influences on boreal summer tropical zonal vertical wind shear (VWS) variability, a key predictor of seasonal TC activity. Through an empirical orthogonal function analysis, it is shown that subtropical AWB activity drives the second mode of variability in tropical zonal VWS, while El Niño–Southern Oscillation (ENSO) primarily drives the leading mode of variability. Linear regressions of the four leading principal components against tropical North Atlantic zonal VWS and accumulated cyclone energy show that while the leading mode holds much of the regression strength, some improvement can be achieved with the addition of the second and third modes. Furthermore, an index of AWB-associated VWS anomalies, a proxy for AWB impacts on the large-scale environment, may be a better indicator of summertime VWS anomalies. The utilization of this index may be used to better understand AWB’s contribution to seasonal TC activity.

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Adam T. Blaker, Joël J.-M. Hirschi, Michael J. Bell, and Amy Bokota

Abstract

The great ocean conveyor presents a time-mean perspective on the interconnected network of major ocean currents. Zonally integrating the meridional velocities, either globally or across basin-scale domains, reduces the conveyor to a 2D projection widely known as the meridional overturning circulation (MOC). Recent model studies have shown the MOC to exhibit variability on near-inertial time scales, and also indicate a region of enhanced variability on the equator. We present an analysis of three integrations of a global configuration of a numerical ocean model, which show very large amplitude oscillations in the MOCs in the Atlantic, Indian, and Pacific Oceans confined to the equatorial region. The amplitude of these oscillations is proportional to the width of the ocean basin, typically about 100 (200) Sv (1 Sv ≡ 106 m3 s−1) in the Atlantic (Pacific). We show that these oscillations are driven by surface winds within 10°N/S of the equator, and their periods (typically 4–10 days) correspond to a small number of low-mode equatorially trapped planetary waves. Furthermore, the oscillations can be well reproduced by idealized wind-driven simulations linearized about a state of rest.

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Philip J. Klotzbach, Steven G. Bowen, Roger Pielke Jr., and Michael Bell

Abstract

Continental United States (CONUS) hurricane-related inflation-adjusted damage has increased significantly since 1900. However, since 1900 neither observed CONUS landfalling hurricane frequency nor intensity shows significant trends, including the devastating 2017 season.

Two large-scale climate modes that have been noted in prior research to significantly impact CONUS landfalling hurricane activity are El Niño–Southern Oscillation on interannual time scales and the Atlantic multidecadal oscillation on multidecadal time scales. La Niña seasons tend to be characterized by more CONUS hurricane landfalls than El Niño seasons, and positive Atlantic multidecadal oscillation phases tend to have more CONUS hurricane landfalls than negative phases.

Growth in coastal population and regional wealth are the overwhelming drivers of observed increases in hurricane-related damage. As the population and wealth of the United States has increased in coastal locations, it has invariably led to the growth in exposure and vulnerability of coastal property along the U.S. Gulf and East Coasts. Unfortunately, the risks associated with more people and vulnerable exposure came to fruition in Texas and Florida during the 2017 season following the landfalls of Hurricanes Harvey and Irma. Total economic damage from those two storms exceeded $125 billion. Growth in coastal population and exposure is likely to continue in the future, and when hurricane landfalls do occur, this will likely lead to greater damage costs than previously seen. Such a statement is made recognizing that the vast scope of damage from hurricanes often highlights the effectiveness (or lack thereof) of building codes, flood maps, infrastructure, and insurance in at-risk communities.

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Philip J. Klotzbach, Michael M. Bell, Steven G. Bowen, Ethan J. Gibney, Kenneth R. Knapp, and Carl J. Schreck III

Abstract

Atlantic hurricane seasons have a long history of causing significant financial impacts, with Harvey, Irma, Maria, Florence, and Michael combining to incur more than 345 billion USD in direct economic damage during 2017–2018. While Michael’s damage was primarily wind and storm surge-driven, Florence’s and Harvey’s damage was predominantly rainfall and inland flood-driven. Several revised scales have been proposed to replace the Saffir–Simpson Hurricane Wind Scale (SSHWS), which currently only categorizes the hurricane wind threat, while not explicitly handling the totality of storm impacts including storm surge and rainfall. However, most of these newly-proposed scales are not easily calculated in real-time, nor can they be reliably calculated historically. In particular, they depend on storm wind radii, which remain very uncertain. Herein, we analyze the relationship between normalized historical damage caused by continental United States (CONUS) landfalling hurricanes from 1900–2018 with both maximum sustained wind speed (V max) and minimum sea level pressure (MSLP). We show that MSLP is a more skillful predictor of normalized damage than V max, with a significantly higher rank correlation between normalized damage and MSLP (r rank = 0.77) than between normalized damage and V max (r rank = 0.66) for all CONUS landfalling hurricanes. MSLP has served as a much better predictor of hurricane damage in recent years than V max, with large hurricanes such as Ike (2008) and Sandy (2012) causing much more damage than anticipated from their SSHWS ranking. MSLP is also a more accurately-measured quantity than is V max, making it an ideal quantity for evaluating a hurricane’s potential damage.

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Philip J. Klotzbach, Carl J. Schreck III, Jennifer M. Collins, Michael M. Bell, Eric S. Blake, and David Roache

Abstract

The 2017 North Atlantic hurricane season was extremely active, with 17 named storms (1981–2010 median is 12.0), 10 hurricanes (median is 6.5), 6 major hurricanes (median is 2.0), and 245% of median accumulated cyclone energy (ACE) occurring. September 2017 generated more Atlantic named storm days, hurricane days, major hurricane days, and ACE than any other calendar month on record. The season was destructive, with Harvey and Irma devastating portions of the continental United States, while Irma and Maria brought catastrophic damage to Puerto Rico, Cuba, and many other Caribbean islands. Seasonal forecasts increased from calling for a slightly below-normal season in April to an above-normal season in August as large-scale environmental conditions became more favorable for an active hurricane season. During that time, the tropical Atlantic warmed anomalously while a potential El Niño decayed in the Pacific. Anomalously high SSTs prevailed across the tropical Atlantic, and vertical wind shear was anomalously weak, especially in the central tropical Atlantic, from late August to late September when several major hurricanes formed. Late-season hurricane activity was likely reduced by a convectively suppressed phase of the Madden–Julian oscillation. The large-scale steering flow was different from the average over the past decade with a strong subtropical high guiding hurricanes farther west across the Atlantic. The anomalously high tropical Atlantic SSTs and low vertical wind shear were comparable to other very active seasons since 1982.

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