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Maria Flatau and Duane E. Stevens

Abstract

The movement of a set of Lagrangian parcels in the two-dimensional pressure field associated with a tropical convective line is considered. The results are compared with observational characteristics of GATE quasi-two-dimensional convective lines. It is shown that for the composite, slow-moving GATE convective line it is necessary to account for horizontal pressure gradients in order to obtain realistic momentum flux. The role of vertical pressure gradients and initial conditions for an air parcel's movement is studied. Based on the results, some suggestions concerning proper formulation of parameterization of the convective momentum flux are made.

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Maria Flatau and Young-Joon Kim

Abstract

A tropical–polar connection and its seasonal dependence are examined using the real-time multivariate Madden–Julian oscillation (MJO) (RMM) index and daily indices for the annular modes, the Arctic Oscillation (AO) and the Antarctic Oscillation (AAO). On the intraseasonal time scale, the MJO appears to force the annular modes in both hemispheres. On this scale, during the cold season, the convection in the Indian Ocean precedes the increase of the AO/AAO. Interestingly, during the boreal winter (Southern Hemisphere warm season), strong MJOs in the Indian Ocean are related to a decrease of the AAO index, and AO/AAO tendencies are out of phase. On the longer time scales, a persistent AO/AAO anomaly appears to influence the convection in the tropical belt and impact the distribution of MJO-preferred phases. It is shown that this may be a result of the sea surface temperature (SST) changes related to a persistent AO, with cooling over the Indian Ocean and warming over Indonesia. In the Southern Hemisphere, the SST anomalies are to some extent also related to a persistent AAO pattern, but this relationship is much weaker and appears only during the Southern Hemisphere cold season. On the basis of these results, a mechanism involving the air–sea interaction in the tropics is suggested as a possible link between persistent AO and convective activity in the Indian Ocean and western Pacific.

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Maria Flatau and Duane E. Stevens

Abstract

The paper examines the role of the development of outflow-layer instabilities on the motion of tropical cyclones. The influence of barotropic instability is examined by comparing the time changes in the storm tracks with the frequencies of free, unstable barotropic modes. For intense vortices barotropic instability is shown to contribute to the slow (periods of a few days) trochoidal motion of a cyclone. The development of instability depends on the horizontal distribution and frequency of environmental forcing. The strongest response occurs when the frequency of the forcing matches the frequency of an unstable mode.

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Young-Joon Kim and Maria Flatau

Abstract

A very strong Arctic major sudden stratospheric warming (SSW) event occurred in late January 2009. The stratospheric temperature climbed abruptly and the zonal winds reversed direction, completely splitting the polar stratospheric vortex. A hindcast of this event is attempted by using the Navy Operational Global Atmospheric Prediction System (NOGAPS), which includes the full stratosphere with its top at around 65 km. As Part I of this study, extended-range (3 week) forecast experiments are performed using NOGAPS without the aid of data assimilation. A unified parameterization of orographic drag is designed by combining two parameterization schemes; one by Webster et al., and the other by Kim and Arakawa and Kim and Doyle. With the new unified orographic drag scheme implemented, NOGAPS is able to reproduce the salient features of this Arctic SSW event owing to enhanced planetary wave activity induced by more comprehensive subgrid-scale orographic drag processes. The impact of the SSW on the tropospheric circulation is also investigated in view of the Arctic Oscillation (AO) index, which calculated using 1000-hPa geopotential height. The NOGAPS with upgraded orographic drag physics better simulates the trend of the AO index as verified by the Met Office analysis, demonstrating its improved stratosphere–troposphere coupling. It is argued that the new model is more suitable for forecasting SSW events in the future and can serve as a tool for studying various stratospheric phenomena.

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Maria Flatau and Duane E. Stevens

Abstract

Measuremenits of the momentum transport in tropical convective lines suggest that horizontal momentum can be generated by the pressure low located near the center of the convective part of the line.

A simple convective parameterization is used to evaluate this effect. The parameterization is a version of the Fritsch and Chappell scheme, modified in order to evaluate the influence of the horizontal pressure gradients on momentum transport. The results suggest that in modeling of convective (particularly slow-moving) lines with 20 km resolution, subgrid horizontal pressure gradients should be taken into account. Sensitivity studies show that the magnitude of the calculated momentum flux strongly depends on the average vertical velocity and the vertical velocity in clouds.

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Maria Flatau, Piotr J. Flatau, Patricia Phoebus, and Pearn P. Niiler

Abstract

Existing theories of the Madden–Julian oscillation neglect the feedback between the modification of sea surface temperature by the convection and development of a convective cluster itself. The authors show that the convection-generated SST gradient plays an important role in cluster propagation and development. The relative importance of radiative and evaporative fluxes in SST regulation is also discussed. Various Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment and Central Equatorial Pacific Experiment observation platforms are used to estimate the effects of equatorial convection on SST changes during March 1993. The data include drifting buoys and TAO-buoy array measurements, combined with the Navy Operational Global Atmospheric Prediction System analyzed surface wind fields and Geostationary Meteorological Satellite cloud-top temperatures. It is shown that during the equatorial convection episode SST is decreasing under and to the west of the convective heat source due to the large wind velocities and solar flux reduction. To the east of the source, in the convergence region of a Kelvin wave, low wind speeds and high insolation cause the SST to increase.

The data are used to formulate an empirical relationship between wind speed and the 24-h SST change on the equator. Although formulated in terms of wind speed, this relationship implicitly includes radiative effects. This equation is then used in a global circulation model to examine the effect of SST feedback on the behavior of equatorial convection. A series of experiments is performed using an R15 general circulation model of the “aquaplanet” with a zonally symmetric SST distribution. In the case with fixed SSTs, equatorial wind fluctuations have the character of waves propagating around the globe with a phase speed of about 20 m s−1. When the effect of SST modification is included, the fluctuations slow down and become more organized. In addition, a 40–60-day peak appears in the spectral analysis of equatorial precipitation.

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Maria K. Flatau, Piotr J. Flatau, and Daniel Rudnick

Abstract

Double monsoon onset develops when the strong convection in the Bay of Bengal is accompanied by the monsoonlike circulation and appears in the Indian Ocean in early May, which is about 3 weeks earlier than the climatological date of the onset (1 Jun). The initial “bogus onset” is followed by the flow weakening or reversal and clear-sky and dry conditions over the monsoon region. The best example of such a phenomenon is the development of the summer monsoon in 1995, when monsoonlike perturbations that appeared in mid-May disappeared by the end of the month and were followed by a heat wave in India, delaying onset of the monsoon. The climatology of double onsets is analyzed, and it is shown that they are associated with delay of the monsoon rainfall over India. This analysis indicates that the development of bogus onsets depends on the timing of intraseasonal oscillation in the Indian Ocean and the propagation of convective episodes into the western Pacific. There is evidence that an SST evolution in the Bay of Bengal and the western Pacific plays an important role in this phenomenon. It is shown that in the case of the double monsoon onset it is possible to predict hot and dry conditions in India before the real monsoon onset. In the 32 yr of climatological data, six cases of double monsoon onset were identified.

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Maria Flatau, Lynne Talley, and David Musgrave

Abstract

Mass and heat budgets in the Gulf of Alaska during the 1991–94 El Niño are examined using hydrographic data from several cruises undertaken as part of the International North Pacific Ocean Climate program and the repeated Canadian hydrographic sections out to Ocean Weather Station Papa. The geostrophic ocean circulation resulted in convergence of heat into the region in spring 1992 and spring 1993. The advective heat convergence in spring 1992 corresponded to an average surface heat flux from the ocean to the atmosphere of about 74 W m−2 in comparison with only 30 W m−2 during spring 1993. The larger ocean heat loss to the atmosphere in 1992 followed a winter of large tropical SST anomalies and anomalously low pressure in the Aleutian low.

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Maria Flatau, Wayne H. Schubert, and Duane E. Stevens

Abstract

The numerical study presented here focuses on baroclinic processes that contribute to tropical cyclone (TC) propagation. A three-dimensional, semispectral, primitive equation model of baroclinic vortex was developed to study TC motion.

In a tilted vortex, interaction between upper- and lower-level vorticity anomalies leads to vortex propagation relative to the steering flow. On a, β plane, with no environmental flow, the vortex is tilted toward the south and the interaction between the layers reduces the westward movement of the vortex. The vortex tilting can also occur due to the vertical shear in the environmental wind. On an f plane, the interaction between the layers causes the northward movement of the vortex in westerly linear shear, and southward movement in easterly linear shear, with a meridional velocity of about 1 m s−1. Ibis velocity increases with increasing vortex intensity and vertical motion.

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Maria K. Flatau, Lynne Talley, and Pearn P. Niiler

Abstract

Changes in surface circulation in the subpolar North Atlantic are documented for the recent interannual switch in the North Atlantic Oscillation (NAO) index from positive values in the early 1990s to negative values in 1995/96. Data from Lagrangian drifters, which were deployed in the North Atlantic from 1992 to 1998, were used to compute the mean and varying surface currents. NCEP winds were used to calculate the Ekman component, allowing isolation of the geostrophic currents. The mean Ekman velocities are considerably smaller than the mean total velocities that resemble historical analyses. The northeastward flow of the North Atlantic Current is organized into three strong cores associated with topography: along the eastern boundary in Rockall Trough, in the Iceland Basin (the subpolar front), and on the western flank of the Reykjanes Ridge (Irminger Current). The last is isolated in this Eulerian mean from the rest of the North Atlantic Current by a region of weak velocities on the east side of the Reykjanes Ridge.

The drifter results during the two different NAO periods are compared with geostrophic flow changes calculated from the NASA/Pathfinder monthly gridded sea surface height (SSH) variability products and the Advanced Very High Resolution Radiometer (AVHRR) SST data. During the positive NAO years the northeastward flow in the North Atlantic Current appeared stronger and the circulation in the cyclonic gyre in the Irminger Basin became more intense. This was consistent with the geostrophic velocities calculated from altimetry data and surface temperature changes from AVHRR SST data, which show that during the positive NAO years, with stronger westerlies, the subpolar front was sharper and located farther east. SST gradients intensified in the North Atlantic Current, Irminger Basin, and east of the Shetland Islands during the positive NAO phase, associated with stronger currents. SST differences between positive and negative NAO years were consistent with changes in air–sea heat flux and the eastward shift of the subpolar front. SST advection, as diagnosed from the drifters, likely acted to reduce the SST differences.

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