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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

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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Lei Zhou and Raghu Murtugudde

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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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Markus Jochum and Raghu Murtugudde

Abstract

A numerical model of the tropical Pacific Ocean is used to investigate the processes that cause the horizontal temperature advection of tropical instability waves (TIWs). It is found that their temperature advection cannot be explained by the processes on which the mixing length paradigm is based. Horizontal mixing of temperature across the equatorial SST front does happen, but it is small relative to the “oscillatory” temperature advection of TIWs. The basic mechanism is that TIWs move water back and forth across a patch of large vertical entrainment. Outside this patch, the atmosphere heats the water and this heat is then transferred into the thermocline inside the patch. These patches of strong localized entrainment are due to equatorial Ekman divergence and due to thinning of the mixed layer in the TIW cyclones. The latter process is responsible for the zonal temperature advection, which is as large as the meridional temperature advection but has not yet been observed. Thus, in the previous observational literature the TIW contribution to the mixed layer heat budget may have been underestimated significantly.

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Markus Jochum and Raghu Murtugudde

Abstract

A 40-yr integration of an eddy-resolving numerical model of the tropical Indian Ocean is analyzed to quantify the interannual variability that is caused by the internal variability of ocean dynamics. It is found that along the equator in the western Indian Ocean internal variability contributes significantly to the observed interannual variability. This suggests that in this location the predictability of SST is limited to the persistence time of SST anomalies, which is approximately 100 days. Furthermore, a comparison with other sources of variability suggests that internal variability may play an important role in modifying the Indian monsoon or preconditioning the Indian Ocean dipole/zonal mode.

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Lei Zhou, Raghu Murtugudde, and Markus Jochum

Abstract

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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Manisha Ganeshan, Raghu Murtugudde, and John Strack

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Several warm season, late-afternoon precipitation events are simulated over the Chesapeake Bay watershed using the Weather Research and Forecasting (WRF) model at three different resolutions. The onset and peak of surface-based convection are predicted to occur prematurely when two popular cumulus parameterization schemes (Betts–Miller–Janjić and Kain–Fritsch) are used. Rainfall predictions are significantly improved with explicit convection. The early bias appears to be associated with the inadequacy in representing convective inhibition (CIN) or negative buoyancy in the trigger for moist convection. In particular, both schemes have weak constraints for the negative buoyancy above cloud base and below the level of free convection, leading to premature rainfall. Satellite-derived soundings suggest that, even with extremely favorable conditions, negative buoyancy in this layer may delay the onset of surface-based convection. Other factors, such as enhanced mixing due to overactive shallow convection, also appear to contribute to the early rainfall bias through the premature removal of CIN during the day.

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Vinu Valsala, Shamil Maksyutov, and Raghu Murtugudde

Abstract

Some of the possible interannual to interdecadal variabilities of the Indonesian Throughflow (ITF) source water pathways in the Pacific Ocean are identified from an ocean reanalysis product provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) under the name Ocean Reanalysis, version S3 (ORA-S3). The data were used in an offline mode to evolve adjoint pathways of a passive tracer, which is injected from the key channels of the Indonesian straits where the ITF enters into the Indian Ocean. The adjoint pathways are simulated using interannually varying circulations for 41 yr starting from December 2000 to January 1960 with reversed currents and other physical parameters (control run). A climatological run for the 41 yr is produced with the reversed currents and other physical parameters as a monthly climatology. The adjoint pathway variability is found by subtracting the climatological run from the control run. The empirical orthogonal function (EOF) analysis carried out over the monthly differences between the tracer concentrations of the control run and the climatological run shows that the ITF is largely supplied from the northwestern tropical Pacific during a normal year, whereas the supply from the south equatorial Pacific is dominant during El Niño–Southern Oscillation (ENSO) years at a lag of 6 months. The interannual variability of the ITF source water pathways in the Pacific is largely determined by the ENSO variability and they are confined to the tropical Pacific, whereas the corresponding interdecadal variability is controlled by the meridional overturning circulations in the tropical and subtropical Pacific. The adjoint pathways hint that the ITF volume transport may have interdecadal variability; they are closely related to the variability of the subtropical cells (STCs) in the Pacific Ocean and can be quantified using the tropical convergence changes. The ITF is just an active member of the recharge–discharge of tropical warm waters at all time scales, and its role in the coupled climate variability of the Indo-Pacific needs to be assessed in that context.

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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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Amy C. Clement, Richard Seager, and Raghu Murtugudde

Abstract

Tropical warm pools appear as the primary mode in the distribution of tropical sea surface temperature (SST). Most previous studies have focused on the role of atmospheric processes in homogenizing temperatures in the warm pool and establishing the observed statistical SST distribution. In this paper, a hierarchy of models is used to illustrate both oceanic and atmospheric mechanisms that contribute to the establishment of tropical warm pools. It is found that individual atmospheric processes have competing effects on the SST distribution: atmospheric heat transport tends to homogenize SST, while the spatial structure of atmospheric humidity and surface wind speeds tends to remove homogeneity. The latter effects dominate, and under atmosphere-only processes there is no warm pool. Ocean dynamics counter this effect by homogenizing SST, and it is argued that ocean dynamics is fundamental to the existence of the warm pool. Under easterly wind stress, the thermocline is deep in the west and shallow in the east. Because of this, poleward Ekman transport of water at the surface, compensated by equatorward geostrophic flow below and linked by equatorial upwelling, creates a cold tongue in the east but homogenizes SST in the west, creating a warm pool. High clouds may also homogenize the SST by reducing the surface solar radiation over the warmest water, but the strength of this feedback is quite uncertain. Implications for the role of these processes in climate change are discussed.

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