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Lei Zhou and Raghu Murtugudde

Abstract

The possibility of interactions between oceanic and atmospheric oscillations with different temporal and spatial scales is examined with analytical solutions to idealized linear governing equations. With a reasonable choice for relevant parameters, the mesoscale oceanic features and the large-scale atmospheric oscillations can interact with each other and lead to unstable waves in the intraseasonal band in the specific coupled model presented in this study. This mechanism is different from the resonance mechanism, which requires similar temporal or spatial scales in the two media. Instead, this mechanism indicates that even in the cases in which the temporal and spatial scales are different but the dispersion relations (i.e., functions of frequency and wavenumber) of the oceanic and atmospheric oscillations are proximal, instabilities can still be generated due to the ocean–atmosphere coupling.

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Lei Zhou and Raghu Murtugudde

Abstract

Madden–Julian oscillations (MJOs) are the dominant mode of intraseasonal variability (ISV) in the atmosphere acting as a bridge between weather and climate. During boreal winter, many MJO events are detoured southward while propagating across the Maritime Continent. Although MJO simulations have been greatly improved in recent years, the mechanism and simulation of MJO detouring near the Maritime Continent are still a great scientific challenge. Several mechanisms have been proposed based on atmospheric dynamics and thermodynamics. In this study, the oceanic role in MJO detouring is diagnosed using observations and reanalysis products. It is found that warm sea surface temperature (SST) anomalies occur over the southeastern Indian Ocean that induce a cyclone in the lower troposphere. Due to the westerly background winds, westerly winds are strengthened (weakened) to the north (south) of warm SST anomalies. As a result, the latent heat flux (LHF) is enhanced, and convection is reinforced to the north of warm SST anomalies. In contrast, the LHF is reduced, and SSTs warm to the south of pre-existing warm SST anomalies. Hence, the warm SST anomalies and convection system shift the MJOs southward before they reach the Maritime Continent. The identification of the oceanic influence on the MJO detouring deepens our understanding of the mechanism of their detour and elicits the role of the ocean. It is expected to brighten the prospects for better simulation and forecast of MJOs over the Maritime Continent. The oceanic ISV in the southeastern Indian Ocean is subject to many forcings, such as intraseasonal atmospheric forcing, the Indonesian Throughflow, local oceanic instability, and coastal Kelvin waves along Sumatra. Determining the mechanism of ISV in the southeastern Indian Ocean requires further dedicated studies.

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Lei Zhou and Raghu Murtugudde

Abstract

The onset of the Indian summer monsoon (ISM) has a pronounced interannual variability, part of which originates from the large-scale circulation and its thermodynamic properties. While the northward-propagating intraseasonal variabilities (ISVs) are a prominent characteristic of the ISM, they tend to initiate an early onset by transferring moisture and momentum from the deep tropics to the Indian subcontinent. However, not all early onsets of ISM are attributable to strong ISVs and not all strong ISVs can lead to early ISM onsets. With a daily Indian monsoon index and a simple regression model, the onsets of ISM from 1982 to 2011 are separated into two groups. The years in which the early onsets of ISM are closely related to the northward-propagating ISVs are categorized as the ISVO years, and the other years in which the ISM onsets are not closely related to ISVs are categorized as non-ISVO years. The former category is the focus of this study. Before the onset of ISM in the ISVO years, the convective features are prominent, such as a cyclone over the Bay of Bengal (BoB) and the associated strong convection. The ocean–atmosphere interaction is found to be important for the northward-propagating ISVs before the ISM onset in the ISVO years. Evidence shows that warm SST anomalies drive the atmosphere and lead to atmospheric instability and convection. This reinforces the more recent view that the ocean does not just play a passive role in the northward-propagating ISVs. This process understanding helps shape the path to enhancing predictive understanding and monsoon prediction skills with obvious implications for the prediction of El Niño–Southern Oscillation.

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Jun Zhou, Hengchi Lei, and Lei Ji

Abstract

A new multilevel detection scheme for cloud tomography is developed. This scheme solves problems intrinsic to conventional single-level detection, such as the lateral boundary problem and the low accuracy of liquid water content (LWC) retrieval for clouds without distinct liquid water cores. Sensitivity studies show that the new multilevel detection scheme can significantly enhance the well posedness of the inverse problem and increases the accuracy of the retrieval. These improvements are achieved not only for clouds with distinct liquid water cores but also for clouds with weak or no liquid water cores, which are difficult to accurately reconstruct using a single-level detection scheme. The settlement of the lateral boundary problem also leads to a natural and easy way of solving the detection time limit problem in cloud tomography. By using a multi-aircraft flight (MAF) scheme, segmental retrieval can be applied to make the applicable scope of cloud tomography much broader. Considering the detection time limit and the cost in practice, the feasible flight scheme at present is MAF with two detection levels. Although only one detection level is added to the conventional single-level scheme, the accuracy of LWC retrieval can be improved by 1.4%–13.1%.

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Lei Zhou and In-Sik Kang

Abstract

Influences of convective momentum transport (CMT) on tropical waves are analytically studied with an idealized model, which captures the first-order baroclinic structure in the vertical. The CMT has significant influence on the mixed Rossby–gravity (MRG) waves, especially over the Indo-Pacific warm pool. The westward-propagating MRG wave with a small wavenumber becomes unstable because of the CMT. The convergence and geopotential are no longer in a quadrature phase relation, which is different from the classical MRG wave. As a result, there is a net source of mechanical energy within one wave period and there is an upscale momentum transfer that can have impacts on slow variabilities in the tropics, such as the Madden–Julian oscillation. The unstable MRG wave mimics the temporal and spatial features of the observed 2-day waves in tropics. Within this framework, the asymmetric structure of the MRG waves and the 2-day waves with respect to the equator are well captured by both the idealized model and observations. In addition, the CMT is found to be critical for determining the meridional scale of tropical waves. The meridional scale in the two-layer model is wider than the Rossby radius of deformation RL over the Indo-Pacific warm pool, but narrower than RL from the central to the eastern Pacific Ocean and over the Atlantic Ocean. Such variation is consistent with observations.

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Jun Zhou, Hengchi Lei, Lei Ji, and Tuanjie Hou

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A fast inverse algorithm based on the half-V cycle scheme (HV) of the multigrid technique is developed for cloud tomography. Fourier analysis shows that the slow convergence problem caused by the smoothing property of the iterative algorithm can be effectively alleviated in HV by performing iterations from the coarsest to the finest grid. In this way, the resolvable scales of information contained in observations can be retrieved efficiently on the coarser grid level and the unresolvable scales are left as errors on the finer grid level. Numerical simulations indicate that, compared with the previous algorithm without HV (NHV), HV can significantly reduce the runtime by 89%–96.9% while retaining a similar level of retrieval accuracy. For the currently feasible two-level flight scheme for a 20-km-wide target area, convergence can be accelerated from 407 s in NHV to 13 s in HV. This reduction in time would be multiplied several times if the target area were much wider; but then segmental retrieval would be required to avoid exceeding the time limit of cloud tomography. This improvement represents an important saving in terms of computing resources and ensures the real-time application of cloud tomography in a much wider range of fields.

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Lei Zhou, Raghu Murtugudde, and Markus Jochum

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The spatial and temporal features of intraseasonal oscillations in the southwestern Indian Ocean are studied by analyzing model simulations for the Indo-Pacific region. The intraseasonal oscillations have periods of 40–80 days with a wavelength of ∼650 km. They originate from the southeastern Indian Ocean and propagate westward as Rossby waves with a phase speed of ∼25 cm s−1 in boreal winter and spring. The baroclinic instability is the main driver for these intraseasonal oscillations. The first baroclinic mode dominates during most of the year, but during boreal winter and spring the second mode contributes significantly and often equally. Consequently, the intraseasonal oscillations are relatively strong in boreal winter and spring. Whether the atmospheric intraseasonal oscillations are also important for forcing the oceanic intraseasonal oscillations in the southwestern Indian Ocean needs further investigation.

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Lei Zhou, Raghu Murtugudde, and Markus Jochum

Abstract

The influence of the Indonesian Throughflow (ITF) on the dynamics and the thermodynamics in the southwestern Indian Ocean (SWIO) is studied by analyzing a forced ocean model simulation for the Indo-Pacific region. The warm ITF waters reach the subsurface SWIO from August to early December, with a detectable influence on weakening the vertical stratification and reducing the stability of the water column. As a dynamical consequence, baroclinic instabilities and oceanic intraseasonal variabilities (OISVs) are enhanced. The temporal and spatial scales of the OISVs are determined by the ITF-modified stratification. Thermodynamically, the ITF waters influence the subtle balance between the stratification and the mixing in the SWIO. As a result, from October to early December an unusual warm entrainment occurs, and the SSTs warm faster than just net surface heat flux–driven warming. In late December and January, the signature of the ITF is seen as a relatively slower warming of SSTs. A conceptual model for the processes by which the ITF impacts the SWIO is proposed.

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Jiwei Tian, Lei Zhou, and Xiaoqian Zhang

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Ten years of Ocean Topography Experiment (TOPEX)/Poseidon tidal data and an energy balance relation are used to estimate the mixing rate caused by M 2 internal tides in the upper ocean. The results indicate that latitudinal distribution of the mixing rate has a generally symmetrical structure with respect to the equator. The maxima are distributed around 28.9°N and 28.9°S and can be as high as 3.8 × 10−5 m2 s−1 in the Pacific, 5 × 10−5 m2 s−1 in the Atlantic, and 3.7 × 10−5 m2 s−1 in the Indian Oceans. The minimum, which is only 10% of those at 28.9°, is located near the equator. The data imply that midlatitudes are the key regions for internal tide mixing.

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Lei Zhou, Ruomei Ruan, and Raghu Murtugudde

Abstract

Madden-Julian Oscillations (MJOs) are a major component of tropical intraseasonal variabilities. There are two paths for MJOs across the Maritime Continent; one is a detoured route into the Southern Hemisphere and the other one is around the equator across the Maritime Continent. Here, it is shown that the detoured and non-detoured MJOs have significantly different impacts on the South Pacific convergence zone (SPCZ). The detoured MJOs trigger strong cross-equatorial meridional winds from the Northern Hemisphere into the Southern Hemisphere. The associated meridional moisture and energy transports due to the background states carried by the intraseasonal meridional winds are favorable for reinforcing the SPCZ. In contrast, the influences of non-detoured MJOs on either hemisphere or the meridional transports across the equator are much weaker. The detoured MJOs can extend their impacts to the surrounding regions by shedding Rossby waves. Due to different background vorticity during detoured MJOs in boreal winter, more ray paths of Rossby waves traverse the Maritime Continent connecting the southern Pacific Ocean and the eastern Indian Ocean, but far fewer Rossby wave paths traverse Australia. Further studies on such processes are expected to contribute to a better understanding of extreme climate and natural disasters on the rim of the southern Pacific and Indian Oceans.

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