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T. N. Krishnamurti, John Molinari, and Hua Lu Pan

Abstract

In this study we show that many of the observed features of the cross-equatorial low-level jet of the Arabian Sea, Indian Ocean and Somalia can be numerically simulated by including 1) the cast African and Madagascar mountains, 2) the beta effect and 3) a lateral forcing from the east around 75°E. This lateral forcing at 75°E is, in fact, a solution of another numerical model–one where the land-ocean contrast heating in the meridional direction is incorporated in much detail to simulate the zonally symmetric monsoons, essentially following Murakami et al. (1970). This zonally symmetric solution of a very long-term numerical integration from a state of rest exhibits many of the observed characteristics of the broad-scale monsoons at 75°E. This later solution is used as a lateral forcing for the low-level jet simulations over the Arabian Sea-Indian Ocean.

The numerical model presented here is a one-level primitive equation model with a detailed bottom topography and a one-degree latitude grid size.

Results of several controlled numerical experiments suppressing or including orography, the beta effect and the broad-scale lateral monsoon forcing at 75°E are discussed in this paper. When all the three above-mentioned parameters are included, features such as strong winds just downstream from the Madagascar mountains, an equatorial relative speed minimum, an intense jet off the Somali coast and a split of the jet over the northern Arabian Sea are simulated from an initial state of rest. The Ethiopian highland appears crucial for the simulation of the Somali coast strong winds; the Madagascar mountains are most important for the strong winds just downstream from Madagascar. The split in the jet over the Arabian Sea is analyzed as a barotropic instability problem. The beta effect is essential for the simulation of the observed geometry. Experiments with a weak broad-scale monsoon forcing at 75°E fall to produce strong winds over cast Africa. The implications of this forcing are analyzed in this paper and some relevant observations are presented.

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John Molinari, David Vollaro, Steven Skubis, and Michael Dickinson

Abstract

The genesis of Hurricane Hernan (1996) in the eastern Pacific was investigated using gridded analyses from the European Centre for Medium-Range Weather Forecasts and gridded outgoing longwave radiation. Hernan developed in association with a wave in the easterlies that could be tracked back to Africa in longitude–time plots of the filtered υ component of the wind (2–6-day period) at 700 mb. The wave crossed Central America near Lake Nicaragua with little change in its southwest–northeast tilt, but the most intense convection shifted from near the wave axis in the Caribbean to west of the wave axis in the Pacific. The wave intensified as it moved through a barotropically unstable background state (defined by a low-pass filter with a 20-day cutoff) in the western Caribbean and eastern Pacific. A surge in the southwesterly monsoons and enhanced convection along 10°N occurred to the west of the 700-mb wave in the Pacific and traveled with the wave. This had the effect of enhancing low-level vorticity over a wide region ahead of the 700-mb wave. Available evidence suggests that additional low-level vorticity was produced by enhanced flow from the north through the Isthmus of Tehuantepec as the 700-mb wave approached. Depression formation did not occur until 6–12 h after the 700-mb wave reached this region of large low-level vorticity in the Gulf of Tehuantepec.

Eastern Pacific SST and vertical wind shear magnitude are typically favorable for tropical cyclone development in Northern Hemisphere summer and early fall. Because the favorable mountain interaction and the surge in the low-level monsoons appear to relate directly to the wave in the easterlies, it is argued that the strength of such waves reaching Central America from the east is the single most important factor in whether subsequent eastern Pacific cyclogenesis occurs. Possible parallels with western Pacific cyclogenesis are discussed.

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John Molinari, Michaela Rosenmayer, David Vollaro, and Sarah D. Ditchek

Abstract

The NOAA G-IV aircraft routinely measures vertical aircraft acceleration from the inertial navigation system at 1 Hz. The data provide a measure of turbulence on a 250-m horizontal scale over a layer from 12.8- to 14.8-km elevation. Turbulence in this layer of tropical cyclones was largest by 35%–40% in the inner 200 km of radius and decreased monotonically outward to the 1000-km radius. Turbulence in major hurricanes exceeded that in weaker tropical cyclones. Turbulence data points were divided among three regions of the tropical cyclone: cirrus canopy; outside the cirrus canopy; and a transition zone between them. Without exception, turbulence was greater within the canopy and weaker outside the canopy. Nighttime turbulence exceeded daytime turbulence for all radii, especially within the cirrus canopy, implicating radiative forcing as a factor in turbulence generation. A case study of widespread turbulence in Hurricane Ivan (2004) showed that interactions between the hurricane outflow channel and westerlies to the north created a region of absolute vorticity of −6 × 10−5 s−1 in the upper troposphere. Outflow accelerated from the storm center into this inertially unstable region, and visible evidence for turbulence and transverse bands of cirrus appeared radially inward of the inertially unstable region. It is argued that both cloud-radiative forcing and the development of inertial instability within a narrow outflow layer were responsible for the turbulence. In contrast, a second case study (Isabel 2003) displayed strong near-core turbulence in the presence of large positive absolute vorticity and no local inertial instability. Peak turbulence occurred 100 km downwind of the eyewall convection.

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Robert L. Molinari, David Battisti, Kirk Bryan, and John Walsh

The Atlantic Climate Change Program (ACCP) is a component of NOAA's Climate and Global Change Program. ACCP is directed at determining the role of the thermohaline circulation of the Atlantic Ocean on global atmospheric climate. Efforts and progress in four ACCP elements are described. Advances include 1) descriptions of decadal and longer-term variability in the coupled ocean–atmosphere–ice system of the North Atlantic; 2) development of tools needed to perform long-term model runs of coupled simulations of North Atlantic air–sea interaction; 3) definition of mean and time-dependent characteristics of the thermohaline circulation; and 4) development of monitoring strategies for various elements of the thermohaline circulation.

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Elizabeth Johns, Rana A. Fine, and Robert L. Molinari

Abstract

In June–July 1990, hydrographic, chlorofluorocarbon (CFC), and velocity observations were taken along the western boundary of the North Atlantic south of the Blake Bahama Outer Ridge from 30° to 24°N between the northern Bahamas and 71°W. The deep flow in the region, associated with the deep western boundary current, forms a pattern of strong, narrow currents and cyclonic gyres close to the continental slope with broad, slower southward flow offshore. The CFCs reveal that the most recently “ventilated” water (i.e., having the highest CFC concentrations due to more recent contact with the atmosphere in the northern North Atlantic) is found along the western boundary in two distinct cores between potential temperatures 4°–6°C and 1.9°–2.4°C. Geostrophic transport streamlines were constructed for the deep flow, referenced using direct velocity observations at 26.5°N and assuming mass conservation between closed areas bounded by the hydrographic sections. The tracers and transports are used together to describe the deep circulation in the region, to determine the origins and pathways of the various flow components, to define the spatial scales and strengths of the deep currents and recirculation gyres, and to examine their relationship to bottom topography and their possible role in ventilating the interior. The close correspondence of the tracer distributions with the regional bottom topography implies that the major topographic features in this region strongly influence the deep circulation. The geostrophic transport for the narrow branch of current having the highest CFC concentration, which transits the region and continues equatorward adjacent to the western boundary, is 31 Sv (Sv ≡ 106 m3 s−1) below 6°C. A cyclonic gyre with one or more embedded gyres extends offshore of the narrow boundary current out to about 74°W, transporting 12 Sv of water with intermediate CFC concentrations. Farther offshore, a broad band of southward flow contributes an additional 16 Sv of water with considerably lower CFC concentrations. Thus there is a total deep (<6°C) equatorward transport through the study area along the western boundary of 47 Sv at 24°N. The layer containing the shallower CFC core (4°–6°C) appears to be less constrained by the bottom topography. Within this temperature layer, one current branch with high CFC and low salinity flows southward along the Blake Escarpment. However, there is another branch of flow within this layer that forms an extended zonal high CFC and high salinity distribution from the eastern to the western bounds of the study region. This second branch apparently originates in the Gulf Stream recirculation and carries the higher salinity influence of the Mediterranean Water.

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T. N. Krishnamurti, John Molinari, Hua-lu Pan, and Vince Wong

Abstract

In this paper we present many examples (based on 43 years of data) of a phenomenon of downstream amplification over the monsoonal belt. The specific finding here is the following sequence of events: 1) During northern summer pressure drops in the vicinity of the North Vietnam coast (near 20°N) as a typhoon or a tropical storm arrives; 2) during the ensuing week pressure rises over Indochina and Burma by some 5–7 mb; and 3) during the following week a monsoon disturbance forms near the northern part of the Bay of Bengal. On an x-t (or Hovmöller) diagram this sequence of low-high-low formation is similar to the downstream amplification phenomenon of the middle latitudes. The following are some interesting differences: over the middle latitudes the eastward propagating phase velocity is of the order of 10° longitude day−1, while the eastward propagating group velocity (the speed of propagation of the amplification) is around 30° longitude day−1. The tropical counterparts are westward propagating, and the phase and group velocity are, respectively, around 6° and 2° longitude day−1. In meteorological literature one frequently notes reference to in situ formation of monsoon depressions over the northern part of the Bay of Bengal. Our study illustrates the superposition of stationary long waves with progressive short waves, the latter arriving from the western Pacific. This result is contrary to this notion of in situ formation. In this paper we examine some aspects of this slowly westward propagating group velocity phenomenon.

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John Molinari, David M. Romps, David Vollaro, and Leon Nguyen

Abstract

Convective available potential energy (CAPE) and the vertical distribution of buoyancy were calculated for more than 2000 dropsonde soundings collected by the NOAA Gulfstream-IV aircraft. Calculations were done with and without the effects of condensate loading, entrainment, and the latent heat of fusion. CAPE showed larger values downshear than upshear within 400 km of the center, consistent with the observed variation of convective intensity. The larger downshear CAPE arose from (i) higher surface specific humidity, (ii) lower midtropospheric temperature, and, for entraining CAPE, (iii) larger free-tropospheric relative humidity.

Reversible CAPE had only one-half the magnitude of pseudoadiabatic CAPE. As shown previously, reversible CAPE with fusion closely resembled pseudoadiabatic CAPE without fusion. Entrainment had the most dramatic impact. Entraining CAPE was consistent with the observed radial distribution of convective intensity, displaying the largest values downshear at inner radii. Without entrainment, downshear CAPE was smallest in the core and increased outward to the 600-km radius.

The large number of sondes allowed the examination of soundings at the 90th percentile of conditional instability, which reflect the conditions leading to the most vigorous updrafts. Observations of convection in tropical cyclones prescribe the correct method for calculating this conditional instability. In particular, the abundance and distribution of vigorous deep convection is most accurately reflected by calculating CAPE with condensate retention and a fractional entrainment rate in the range of 5%–10% km−1.

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Carl J. Schreck III, John Molinari, and Anantha Aiyyer

Abstract

This study investigates the number of tropical cyclone formations that can be attributed to the enhanced convection from equatorial waves within each basin. Tropical depression (TD)-type disturbances (i.e., easterly waves) were the primary tropical cyclone precursors over the Northern Hemisphere basins, particularly the eastern North Pacific and the Atlantic. In the Southern Hemisphere, however, the number of storms attributed to TD-type disturbances and equatorial Rossby waves were roughly equivalent. Equatorward of 20°N, tropical cyclones formed without any equatorial wave precursor most often over the eastern North Pacific and least often over the western North Pacific.

The Madden–Julian oscillation (MJO) was an important tropical cyclone precursor over the north Indian, south Indian, and western North Pacific basins. The MJO also affected tropical cyclogenesis by modulating the amplitudes of higher-frequency waves. Each wave type reached the attribution threshold 1.5 times more often, and tropical cyclogenesis was 3 times more likely, within positive MJO-filtered rainfall anomalies than within negative anomalies. The greatest MJO modulation was observed for storms attributed to Kelvin waves over the north Indian Ocean.

The large rainfall rates associated with tropical cyclones can alter equatorial wave–filtered anomalies. This study quantifies the contamination over each basin. Tropical cyclones contributed more than 20% of the filtered variance for each wave type over large potions of every basin except the South Pacific. The largest contamination, exceeding 60%, occurred for the TD band near the Philippines. To mitigate the contamination, the tropical cyclone–related anomalies were removed before filtering in this study.

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Carl J. Schreck III, John Molinari, and Karen I. Mohr

Abstract

Tropical cyclogenesis is attributed to an equatorial wave when the filtered rainfall anomaly exceeds a threshold value at the genesis location. It is argued that 0 mm day−1 (simply requiring a positive anomaly) is too small a threshold because unrelated noise can produce a positive anomaly. A threshold of 6 mm day−1 is too large because two-thirds of storms would have no precursor disturbance. Between these extremes, consistent results are found for a range of thresholds from 2 to 4 mm day−1.

Roughly twice as many tropical cyclones are attributed to tropical depression (TD)-type disturbances as to equatorial Rossby waves, mixed Rossby–gravity waves, or Kelvin waves. The influence of the Madden–Julian oscillation (MJO) is even smaller. The use of variables such as vorticity and vertical wind shear in other studies gives a larger contribution for the MJO. It is suggested that its direct influence on the rainfall in forming tropical cyclones is less than for other variables.

The impacts of tropical cyclone–related precipitation anomalies are also presented. Tropical cyclones can contribute more than 20% of the warm-season rainfall and 50% of its total variance. The influence of tropical cyclones on the equatorial wave spectrum is generally small. The exception occurs in shorter-wavelength westward-propagating waves, for which tropical cyclones represent up to 27% of the variance. Tropical cyclones also significantly contaminate wave-filtered rainfall anomalies in their immediate vicinity. To mitigate this effect, the tropical cyclone–related anomalies were removed before filtering in this study.

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John Molinari, David Knight, Michael Dickinson, David Vollaro, and Steven Skubis

Abstract

A significant sign reversal in the meridional potential vorticity gradient was found during the summer of 1991 on the 310-K isentropic surface (near 700 mb) over the Caribbean Sea. The Charney–Stern necessary condition for instability of the mean flow is met in this region. It is speculated that the sign reversal permits either invigoration of African waves or actual generation of easterly waves in the Caribbean.

During the same season, a correlation existed between the strength of the negative potential vorticity gradient in the Caribbean and subsequent cyclogenesis in the eastern Pacific. The meridional PV gradient, convective heating measured by outgoing longwave radiation data, and eastern Pacific cyclogenesis all varied on the timescale of the Madden–Julian oscillation (MJO). It is hypothesized that upstream wave growth in the dynamically unstable region provides the connection between the MJO (or any other convective forcing) and the associated enhanced downstream tropical cyclogenesis.

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