An official website of the United States government
Abstract Interactions between large-scale waves and the Hadley Cell are examined using a linear two-layer model on anf-plane. A linear meridional moisture gradient determines the strength of the idealized Hadley Cell. The trade winds are in thermal wind balance with a weak temperature gradient (WTG). The mean meridional moisture gradient is unstable to synoptic-scale (horizontal scale of ∼1000 km) moisture modes that are advected westward by the trade winds, reminiscent of oceanic tropical depression-like waves. Meridional moisture advection causes the moisture modes to grow from “moisture-vortex instability” (MVI), resulting in a poleward eddy moisture flux that flattens the zonal-mean meridional moisture gradient, thereby weakening the Hadley Cell. The amplification of waves at the expense of the zonal-mean meridional moisture gradient implies a downscale latent energy cascade. The eddy moisture flux is opposed by a regeneration of the meridional moisture gradient by the Hadley Cell. These Hadley Cell-moisture mode interactions are reminiscent of quasi-geostrophic interactions, except that wave activity is due to column moisture variance rather than potential vorticity variance. The interactions can result in predator-prey cycles in moisture mode activity and Hadley Cell strength that are akin to ITCZ breakdown. It is proposed that moisture modes are the tropical analog to midlatitude baroclinic waves. MVI is analogous to baroclinic instability, stirring latent energy in the same way that dry baroclinic eddies stir sensible heat. These results indicate that moisture modes stabilize the Hadley Cell, and may be as important as the latter in global energy transport.
more »
« less
The Stirring Tropics: The Ubiquity of Moisture Modes and Moisture-Vortex Instability
Abstract Observations of column water vapor in the tropics show significant variations in space and time, indicating that it is strongly influenced by the passage of weather systems. It is hypothesized that many of the influencing systems are moisture modes, systems whose thermodynamics are governed by moisture. On the basis of four objective criteria, results suggest that all oceanic convectively-coupled tropical depression-like waves (TD-waves) and equatorial Rossby waves are moisture modes. These modes occur where the horizontal column moisture gradient is steep and not where the column water vapor content is high. Despite geographical basic state differences, the moisture modes are driven by the same mechanisms across all basins. The moist static energy (MSE) anomalies propagate westward by horizontal moisture advection by the trade winds. Their growth is determined by the advection of background moisture by the anomalous meridional winds and anomalous radiative heating. Horizontal maps of column moisture and 850 hPa streamfunction show that convection is partially collocated with the low-level circulation in nearly all the waves. Both this structure and the process of growth indicate that the moisture modes grow from moisture-vortex instability. Lastly, space-time spectral analysis reveals that column moisture and low-level meridional winds are coherent and exhibit a phasing that is consistent with a poleward latent energy transport. Collectively, these results indicate that moisture modes are ubiquitous across the tropics. That they occur in regions of steep horizontal moisture gradients and grow from moisture-vortex instability suggests that these gradients are inherently unstable and are subject to continuous stirring.
more »
« less
- Award ID(s):
- 2236433
- PAR ID:
- 10487339
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of Climate
- ISSN:
- 0894-8755
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Caribbean easterly waves (CEWs) propagate in an environment that is distinct from that of other easterly waves since it exhibits substantial westerly vertical wind shear. In spite of this distinction, their structure, propagation, and growth have not received much attention. A linear regression analysis reveals that these systems exhibit features consistent with moisture modes that are destabilized by moisture–vortex instability. They exhibit large moisture fluctuations and are in weak temperature gradient (WTG) balance, and moist static energy (MSE) growth is partly driven by meridional mean MSE advection by the anomalous winds. However, its circulation tilts vertically against the mean shear, a feature that is often associated with baroclinic instability. To reconcile these differences, a linear stability analysis employing a moist two-layer model is performed using a basic state that resembles the Caribbean Sea during boreal summer. The unstable wave solution from this analysis exhibits a structure that resembles observed CEWs. Excluding the upper troposphere from the stability analysis has little impact on the propagation and growth of the wave, and its circulation still exhibits a westward tilt in height. Thus, baroclinic instability is not the main growth mechanism of CEWs despite their structural similarity to baroclinic waves. Instead, the instability is largely rooted in how the lower-tropospheric circulation interacts with water vapor, as expected from moisture mode theory. These results suggest that tilting against the shear should not be used as the sole diagnostic for baroclinic instability. Baroclinic instability is unlikely to be a primary driver of growth for most oceanic tropical-depression-type waves, in agreement with previous work. Significance StatementThe environment in which Caribbean easterly waves propagate has a vertical wind shear that is like that seen in the midlatitudes, with winds becoming more westerly with height. Furthermore, the center of low pressure of the waves shifts toward the west, as in deepening midlatitude weather systems. This wave structure and shear is different from easterly waves that occur in other regions. However, in spite of the similarity to midlatitude weather systems, we show that Caribbean easterly waves mostly grow from moisture transports in the lower atmosphere. Thus, in spite of the distinct environment and wave structure, Caribbean easterly waves are driven by the same processes as other tropical easterly waves. These results underscore the importance of water vapor in driving tropical circulations. They also indicate that the processes that govern the growth of midlatitude weather may be of less importance in the tropics, even in regions that suggest otherwise.more » « less
-
Abstract As one of the most prominent weather systems over the Indian subcontinent, the Indian summer monsoon low pressure systems (MLPSs) have been studied extensively over the past decades. However, the processes that govern the growth of the MLPSs are not well understood. To better understand these processes, we created an MLPS index using bandpass-filtered precipitation data. Lag regression maps and vertical cross sections are used to document the distribution of moisture, moist static energy (MSE), geopotential, and horizontal and vertical motions in these systems. It is shown that moisture governs the distribution of MSE and is in phase with precipitation, vertical motion, and geopotential during the MLPS cycle. Examination of the MSE budget reveals that longwave radiative heating maintains the MSE anomalies against dissipation from vertical MSE advection. These processes nearly cancel one another, and it is variations in horizontal MSE advection that are found to explain the growth and decay of the MSE anomalies. Horizontal MSE advection contributes to the growth of the MSE anomalies in MLPSs prior to the system attaining a maximum amplitude and contributes to decay thereafter. The horizontal MSE advection is largely due to meridional advection of mean state MSE by the anomalous winds, suggesting that the MSE anomalies undergo a moisture–vortex instability (MVI)-like growth. In contrast, perturbation kinetic energy (PKE) is generated through barotropic conversion. The structure, propagation, and energetics of the regressed MLPSs are consistent with both barotropic and moisture–vortex growth.more » « less
-
Abstract A linear two-layer model is used to elucidate the role of prognostic moisture on quasigeostrophic (QG) motions in the presence of a mean thermal wind (). Solutions to the basic equations reveal two instabilities that can explain the growth of moist QG systems. The well-documented baroclinic instability is characterized by growth at the synoptic scale (horizontal scale of ~1000 km) and systems that grow from this instability tilt against the shear. Moisture–vortex instability—an instability that occurs when moisture and lower-tropospheric vorticity exhibit an in-phase component—exists only when moisture is prognostic. The instability is also strongest at the synoptic scale, but systems that grow from it exhibit a vertically stacked structure. When moisture is prognostic andis easterly, baroclinic instability exhibits a pronounced weakening while moisture vortex instability is amplified. The strengthening of moisture–vortex instability at the expense of baroclinic instability is due to the baroclinic () component of the lower-tropospheric flow. In westward-propagating systems, lower-tropospheric westerlies associated with an easterlyadvect anomalous moisture and the associated convection toward the low-level vortex. The advected convection causes the vertical structure of the wave to shift away from one that favors baroclinic instability to one that favors moisture–vortex instability. On the other hand, a westerlyreinforces the phasing between moisture and vorticity necessary for baroclinic instability to occur. Based on these results, it is hypothesized that moisture–vortex instability is an important instability in humid regions of easterlysuch as the South Asian and West African monsoons.more » « less
-
Abstract A recent theory proposes that tropical depression (TD)‐type waves grow by flattening the mean meridional moisture gradient, consequently weakening the Hadley Cell through a poleward moisture flux. To evaluate this theory, we investigate the seasonality of TD‐type waves and their relation to the Hadley Cell in ERA5 and Coupled Model Intercomparison Project Phase 6 (CMIP6) models. On the basis of the theory, a Hadley Cell instability metric is defined whose variability is largely determined by the background meridional moisture gradient and the sensitivity of rainfall to moisture fluctuations. Results show that both TD‐type wave column moisture variance and eddy moisture fluxes peak when the Hadley Cell instability metric is a maximum. These conditions typically occur when the mean meridional precipitation gradient is strongest and the Hadley Cell is weak and narrow. CMIP6 models that exhibit higher Hadley Cell instability metric simulate stronger TD‐type wave activity in the Northern Hemisphere.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1175/JCLI-D-23-0145.1
