Convectively Coupled Kelvin Waves over Tropical Africa during the Boreal Summer: Structure and Variability

2008 ◽  
Vol 21 (24) ◽  
pp. 6649-6667 ◽  
Author(s):  
Ademe Mekonnen ◽  
Chris D. Thorncroft ◽  
Anantha R. Aiyyer ◽  
George N. Kiladis
Keyword(s):  
Indian Ocean ◽  
Wave Activity ◽  
Kelvin Waves ◽  
Sub Saharan Africa ◽  
Eastern Pacific ◽  
Boreal Summer ◽  
Kelvin Wave ◽  
Tropical Africa ◽  
Western Atlantic ◽  
Sst Anomalies

Abstract The structure and variability of convectively coupled Kelvin waves during the boreal summer are explored using satellite-observed brightness temperature data and ECMWF reanalyses. Kelvin wave activity is most prominent between the central and eastern Pacific, across Africa, and the Indian Ocean. Composite analysis shows that over sub-Saharan Africa Kelvin wave convection is peaked north of the equator, while the dynamical fields tend to be symmetric with respect to the equator. Convectively coupled Kelvin waves propagate faster over the Pacific and western Atlantic (∼24 m s−1), and slow down over tropical Africa (∼14 m s−1), consistent with stronger coupling between the dynamics and convection over tropical Africa. The Kelvin waves observed over Africa generally propagate into the region from anywhere between the eastern Pacific and the Atlantic, and decay over the eastern Indian Ocean basin. Results show marked interannual variability of Kelvin wave activity over Africa. Anomalously high Kelvin wave variance tends to occur during dry years, while low variance occurs during wet years. African Kelvin wave activity is positively correlated with SST anomalies in the equatorial east Pacific. The same warm SST anomalies that are favorable for enhanced Kelvin wave activity suppress the mean rainfall over tropical Africa via a more slowly varying teleconnection and associated subsidence. A brief analysis of an intense Kelvin wave in August 1987 (a dry year) shows a clear impact of the wave on convective development and daily rainfall over tropical Africa. This Kelvin wave was also associated with a series of easterly wave initiations over tropical Africa.

scholarly journals The Roles of Intraseasonal Kelvin Waves and Tropical Instability Waves in SST Variability along the Equatorial Pacific in an Isopycnal Ocean Model

2009 ◽  
Vol 22 (12) ◽  
pp. 3470-3487 ◽  
Author(s):  
Chuan Li Jiang ◽  
Lu Anne Thompson ◽  
Kathryn A. Kelly ◽  
Meghan F. Cronin
Keyword(s):  
Time Scales ◽  
Ocean Model ◽  
Kelvin Waves ◽  
Eastern Pacific ◽  
Kelvin Wave ◽  
Central Pacific ◽  
Instability Waves ◽  
Tropical Instability Waves ◽  
Zonal Advection ◽  
Sst Anomalies

Abstract The roles of intraseasonal Kelvin waves and tropical instability waves (TIWs) in the intraseasonal and low-frequency mixed-layer temperature budget were examined in an isopycnal ocean model forced by QuikSCAT winds from 2000 to 2004. Correlations between temperature tendency and other terms of the intraseasonal budget compare well with previous results using Tropical Atmosphere Ocean (TAO) observations: the net heat flux has the largest correlation in the western Pacific and zonal advection has the largest correlation in the central Pacific. In the central Pacific, the intraseasonal variations in zonal advection were due to both the zonal background velocity acting on the Kelvin wave temperature anomaly and the Kelvin wave’s anomalous velocity acting on the background temperature. In the eastern Pacific, three of the four temperature budget terms have comparable correlations. In particular, the vertical processes acting on the shallow thermocline cause large SST anomalies in phase with the intraseasonal thermocline anomalies. On intraseasonal time scales, the influence of individual composite upwelling and downwelling Kelvin waves cancel each other. However, because the intraseasonal SST anomalies increase to the east, a zonal gradient of SST is generated that is in phase with intraseasonal zonal velocity. Consequently, heat advection by the Kelvin waves rectifies into lower frequencies in the eastern Pacific. Rectification resulting from TIWs was also seen. The prevalence of intraseasonal Kelvin waves and the zonal structure of intraseasonal SST from 2002 to early 2004 suggested that they might be important in setting the eastern Pacific SST on interannual time scales.

scholarly journals A harmonic projection and least–squares method for quantifying Kelvin wave activity

2016 ◽  
Author(s):  
Andrew Delman ◽  
Janet Sprintall ◽  
Julie McClean ◽  
Lynne Talley
Keyword(s):  
Indian Ocean ◽  
Least Squares ◽  
Sea Level ◽  
Rossby Waves ◽  
Phase Speed ◽  
Wave Activity ◽  
Kelvin Waves ◽  
Three Dimensions ◽  
Kelvin Wave ◽  
Sea Level Anomaly

Abstract. A new method for isolating the equatorial and coastal Kelvin wave signal from alongtrack satellite altimetry data is presented and applied to sea level anomaly (SLA) observations in the tropical Indian Ocean. The method consists of sequential projections onto the SLA data, starting with meridional or cross-shore Kelvin wave profiles derived from shallow water theory (y-projections). Next, Fourier basis functions in x-t (along-waveguide distance and time respectively) space with the phase speed ranges of Kelvin and Rossby waves are projected onto the y-projections. After projections in all three dimensions have been carried out, least-squares methods are applied to optimize the non-orthogonal basis function coefficients and minimize the misfit of their along-waveguide forcing and dissipation. Lastly, the westward-propagating (Rossby wave-related) signals are removed, generating a Kelvin wave coefficient K that represents Kelvin wave activity. Along the Indian Ocean equatorial-coastal waveguide, Hovmöller diagrams of K show reduced high-wavenumber noise compared to analogous diagrams of pre-processed sea level anomaly. Results from a Monte Carlo simulation demonstrate that Kelvin wave signals generated a priori can be effectively recovered even when superimposed with strong Rossby waves; the signs of all but the weakest waves are diagnosed correctly in over 90 % of cases. When the method is applied to 21 years of satellite observations and the SLA signal associated with K is removed, the large residual in the equatorial SLA signal has a spatial distribution consistent with wind-forced Rossby waves. The equatorial SLA variability in the western part of the basin is poorly correlated with the SLA field associated with K, as the superimposed SLA profile of Rossby waves can distort the true origin locations of Kelvin waves in the raw SLA field. Therefore, this method offers improved tracking of Kelvin waves compared to the raw SLA dataset, and may provide the opportunity to study weakly nonlinear aspects of these waves by comparison with linear models.

scholarly journals Tropical temperature variability and Kelvin-wave activity in the UTLS from GPS RO measurements

2017 ◽  
Vol 17 (2) ◽  
pp. 793-806 ◽  
Author(s):  
Barbara Scherllin-Pirscher ◽  
William J. Randel ◽  
Joowan Kim
Keyword(s):  
Large Scale ◽  
Deep Convection ◽  
Wave Activity ◽  
Kelvin Waves ◽  
Temperature Variability ◽  
Stationary Waves ◽  
Kelvin Wave ◽  
Quasi Biennial Oscillation ◽  
Tropical Tropopause ◽  
Tropopause Region

Abstract. Tropical temperature variability over 10–30 km and associated Kelvin-wave activity are investigated using GPS radio occultation (RO) data from January 2002 to December 2014. RO data are a powerful tool for quantifying tropical temperature oscillations with short vertical wavelengths due to their high vertical resolution and high accuracy and precision. Gridded temperatures from GPS RO show the strongest variability in the tropical tropopause region (on average 3 K2). Large-scale zonal variability is dominated by transient sub-seasonal waves (2 K2), and about half of sub-seasonal variance is explained by eastward-traveling Kelvin waves with periods of 4 to 30 days (1 K2). Quasi-stationary waves associated with the annual cycle and interannual variability contribute about a third (1 K2) to total resolved zonal variance. Sub-seasonal waves, including Kelvin waves, are highly transient in time. Above 20 km, Kelvin waves are strongly modulated by the quasi-biennial oscillation (QBO) in stratospheric zonal winds, with enhanced wave activity during the westerly shear phase of the QBO. In the tropical tropopause region, however, peaks of Kelvin-wave activity are irregularly distributed in time. Several peaks coincide with maxima of zonal variance in tropospheric deep convection, but other episodes are not evidently related. Further investigations of convective forcing and atmospheric background conditions are needed to better understand variability near the tropopause.

Linking Equatorial African precipitation to Kelvin wave processes in the CP4-Africa convection-permitting regional climate simulation

2021 ◽  
Author(s):  
Godwin Ayesiga ◽  
Christopher Holloway ◽  
Charles Williams ◽  
Gui-Ying Yang ◽  
Rachel Stratton ◽  
...  
Keyword(s):  
Observational Studies ◽  
Model Simulation ◽  
Wave Activity ◽  
Kelvin Waves ◽  
Specific Humidity ◽  
Climate Simulation ◽  
Kelvin Wave ◽  
Low Level ◽  
Equatorial Africa ◽  
Precipitation Anomalies

<p>Synoptic timescale forecasts over Equatorial Africa are important for averting weather-and climate-related disasters and the resulting agricultural losses. Observational studies have shown that rainfall anomalies often propagate eastward across Equatorial Africa, and that there is a linkage between synoptic-scale eastward-propagating precipitation and Convectively Coupled Kelvin Waves (CCKWs) over this region. We explore the mechanisms in which CCKWs modulate the propagation of precipitation from West to East over Equatorial Africa. We examine the first Africa-wide climate simulation from a convection permitting model (CP4A) along with its global driving-model simulation (G25) and evaluate both against observations (TRMM) and ERA-Interim (ERA-I), with a focus on precipitation and Kelvin wave activity.</p><p>Lagged composites show that both simulations capture the eastward propagating precipitation signal seen in observational studies, though G25 has a weaker signal. Composite analysis on high-amplitude Kelvin waves further shows that both simulations capture the connection between the eastward propagating precipitation anomalies and Kelvin waves. In comparison to TRMM, however, the precipitation signal is weaker in G25, while CP4A is more realistic. As the Kelvin wave activity is also well represented in both simulations when compared to ERA-I, the weak precipitation signal in G25 may be partly associated with the weak coupling between the precipitation and Kelvin waves. We show that CCKWs modulate the eastward propagation of convection and precipitation across Equatorial Africa through at least two related physical processes. Firstly, an enhancement of the low-level westerlies leads to increased low-level convergence; secondly, westerly moisture flux anomalies amplify lower-to-mid-tropospheric specific humidity. Results show that both CP4A and G25 generally simulate the key horizontal features of CCKWs, with anomalous low-level westerlies in phase with positive precipitation anomalies. However, both models show a weakness in capturing the vertical profile of anomalous specific humidity, and the zonal-vertical circulation is too weak in G25 and incoherent in CP4A compared to ERA-I.</p><p>In both ERA-I and the simulations, Kelvin wave-induced convergence and the westward tilt with height of anomalous zonal winds and specific humidity tends to weaken to the east of the East African highlands. It appears that these highlands impede the coherent eastward propagation of the wave-precipitation coupled structure.</p>

scholarly journals A CGCM Study on the Interaction between IOD and ENSO

2006 ◽  
Vol 19 (9) ◽  
pp. 1688-1705 ◽  
Author(s):  
Swadhin K. Behera ◽  
Jing Jia Luo ◽  
Sebastien Masson ◽  
Suryachandra A. Rao ◽  
Hirofumi Sakuma ◽  
...  
Keyword(s):  
Indian Ocean ◽  
General Circulation ◽  
Southern Oscillation ◽  
Walker Circulation ◽  
Tropical Pacific ◽  
Boreal Summer ◽  
Enso Variability ◽  
The Indian Ocean ◽  
Sst Anomalies ◽  
The Tropical Pacific

Abstract An atmosphere–ocean coupled general circulation model known as the Scale Interaction Experiment Frontier version 1 (SINTEX-F1) model is used to understand the intrinsic variability of the Indian Ocean dipole (IOD). In addition to a globally coupled control experiment, a Pacific decoupled noENSO experiment has been conducted. In the latter, the El Niño–Southern Oscillation (ENSO) variability is suppressed by decoupling the tropical Pacific Ocean from the atmosphere. The ocean–atmosphere conditions related to the IOD are realistically simulated by both experiments including the characteristic east–west dipole in SST anomalies. This demonstrates that the dipole mode in the Indian Ocean is mainly determined by intrinsic processes within the basin. In the EOF analysis of SST anomalies from the noENSO experiment, the IOD takes the dominant seat instead of the basinwide monopole mode. Even the coupled feedback among anomalies of upper-ocean heat content, SST, wind, and Walker circulation over the Indian Ocean is reproduced. As in the observation, IOD peaks in boreal fall for both model experiments. In the absence of ENSO variability the interannual IOD variability is dominantly biennial. The ENSO variability is found to affect the periodicity, strength, and formation processes of the IOD in years of co-occurrences. The amplitudes of SST anomalies in the western pole of co-occurring IODs are aided by dynamical and thermodynamical modifications related to the ENSO-induced wind variability. Anomalous latent heat flux and vertical heat convergence associated with the modified Walker circulation contribute to the alteration of western anomalies. It is found that 42% of IOD events affected by changes in the Walker circulation are related to the tropical Pacific variabilities including ENSO. The formation is delayed until boreal summer for those IODs, which otherwise form in boreal spring as in the noENSO experiment.

scholarly journals Out-of-Phase Relationship between Boreal Spring and Summer Decadal Rainfall Changes in Southern China*

2014 ◽  
Vol 27 (3) ◽  
pp. 1083-1099 ◽  
Author(s):  
Zhiwei Zhu ◽  
Tim Li ◽  
Jinhai He
Keyword(s):  
Indian Ocean ◽  
Southern China ◽  
Phase Relationship ◽  
Hadley Cell ◽  
Eastern Pacific ◽  
Boreal Summer ◽  
Maritime Continent ◽  
Boreal Spring ◽  
Decadal Shift ◽  
Rainfall Change

Abstract A multivariate empirical orthogonal function (MV-EOF) analysis for 1979–2010 shows that low-level circulation and rainfall over East Asia experienced a significant decadal shift around the mid-1990s. During boreal spring (March–May), the first principal component (PC) of the MV-EOF exhibits a marked decadal change around the mid-1990s, while during boreal summer (June–August) the second PC shows a pronounced decadal shift around the same time. It is further noted that the decadal rainfall change over southern China experienced an out-of-phase relationship between boreal spring and summer; that is, from the pre-1994 to the post-1994 period, the rainfall tends to decrease in boreal spring but increase in boreal summer. A mechanism is put forward to explain the out-of-phase decadal rainfall change over southern China between boreal spring and summer. In boreal spring, the composite differences of SST between the latter and former decadal periods indicate a La Niña–like pattern with warming in the western Pacific and cooling in the eastern Pacific. This pattern leads the enhanced convection over the Maritime Continent, which may further induce anomalous subsidence and thus negative rainfall anomalies over southern China through the local Hadley circulation. In boreal summer, dominant decadal SST warming appears in the entire tropical Indian Ocean while the negative SST anomalies in eastern Pacific are much weaker. The warm SST anomaly over the Indian Ocean leads to suppressed convection over the Maritime Continent, which, through the local Hadley cell, favors the strengthening the East Asian monsoon trough and leads to a positive rainfall anomaly over southern China.

The Madden–Julian Oscillation’s Influence on Spring Rainy Season Precipitation over Equatorial West Africa*

2015 ◽  
Vol 28 (22) ◽  
pp. 8653-8672 ◽  
Author(s):  
Fisseha Berhane ◽  
Benjamin Zaitchik ◽  
Hamada S. Badr
Keyword(s):  
Indian Ocean ◽  
West Africa ◽  
Rainy Season ◽  
Regional Scale ◽  
Relative Strength ◽  
Boreal Summer ◽  
Kelvin Wave ◽  
Equatorial Wave ◽  
The Indian Ocean ◽  
Rain Rates

Abstract This paper characterizes the influence of the Madden–Julian oscillation (MJO) on spring rainy season (March–June) convection variability over equatorial West Africa (EWA) and investigates mechanisms of association. It is found that the MJO has a significant impact on convection and precipitation anomalies over the region. Over large portions of EWA, MJO impacts on rainfall constitute a difference on the order of 20%–50% from average daily rain rates for the season. This impact is primarily due to the direct influence of the eastward movement of the MJO convective core into EWA, which is associated with westerly low-level wind anomalies that advect moisture from the Atlantic Ocean to the region. In addition, equatorial Rossby and Kelvin waves triggered by MJO convection anomalies over the Indian Ocean have a significant and systematic influence on EWA spring rainy season precipitation. The Kelvin wave contribution and the relative strength of the direct MJO convective influence compared to that of equatorial wave activity differs from findings of studies that have examined MJO influence on EWA during boreal summer. In addition, MJO is found to influence precipitation extremes during spring rains in a manner that is not observed in summer. Importantly, in this analysis the influences of MJO convection and each of the MJO-associated convectively coupled equatorial waves frequently coincide, reaching EWA approximately 20 days after MJO convection initiates in the Indian Ocean. This coincident timing enhances the total MJO impact on the region, and it also implies that MJO events have potential for prediction of regional-scale convection and rainfall anomalies over EWA in this season.

Impact of Indian Ocean Sea Surface Temperature on Developing El Niño*

2005 ◽  
Vol 18 (2) ◽  
pp. 302-319 ◽  
Author(s):  
H. Annamalai ◽  
S. P. Xie ◽  
J. P. McCreary ◽  
R. Murtugudde
Keyword(s):  
Indian Ocean ◽  
South China Sea ◽  
South China ◽  
El Niño ◽  
El Nino ◽  
Sea Surface ◽  
Kelvin Wave ◽  
Climate Shift ◽  
East West ◽  
Sst Anomalies

Abstract Prior to the 1976–77 climate shift (1950–76), sea surface temperature (SST) anomalies in the tropical Indian Ocean consisted of a basinwide warming during boreal fall of the developing phase of most El Niños, whereas after the shift (1977–99) they had an east–west asymmetry—a consequence of El Niño being associated with the Indian Ocean Dipole/Zonal mode. In this study, the possible impact of these contrasting SST patterns on the ongoing El Niño is investigated, using atmospheric reanalysis products and solutions to both an atmospheric general circulation model (AGCM) and a simple atmospheric model (LBM), with the latter used to identify basic processes. Specifically, analyses of reanalysis products during the El Niño onset indicate that after the climate shift a low-level anticyclone over the South China Sea was shifted into the Bay of Bengal and that equatorial westerly anomalies in the Pacific Ocean were considerably stronger. The present study focuses on determining influence of Indian Ocean SST on these changes. A suite of AGCM experiments, each consisting of a 10-member ensemble, is carried out to assess the relative importance of remote (Pacific) versus local (Indian Ocean) SST anomalies in determining precipitation anomalies over the equatorial Indian Ocean. Solutions indicate that both local and remote SST anomalies are necessary for realistic simulations, with convection in the tropical west Pacific and the subsequent development of the South China Sea anticyclone being particularly sensitive to Indian Ocean SST anomalies. Prior to the climate shift, the basinwide Indian Ocean SST anomalies generate an atmospheric Kelvin wave associated with easterly flow over the equatorial west-central Pacific, thereby weakening the westerly anomalies associated with the developing El Niño. In contrast, after the shift, the east–west contrast in Indian Ocean SST anomalies does not generate a significant Kelvin wave response, and there is little effect on the El Niño–induced westerlies. The Linear Baroclinic Model (LBM) solutions confirm the AGCM’s results.

scholarly journals Tropical temperature variability and Kelvin wave activity in the UTLS from GPS RO measurements

2016 ◽  
Author(s):  
Barbara Scherllin-Pirscher ◽  
William J. Randel ◽  
Joowan Kim
Keyword(s):  
High Frequency ◽  
Large Scale ◽  
Wave Activity ◽  
Kelvin Waves ◽  
Temperature Variability ◽  
Kelvin Wave ◽  
Quasi Biennial Oscillation ◽  
Tropical Tropopause ◽  
Tropopause Region ◽  
High Frequency Waves

Abstract. Tropical temperature variability over 10–30 km and associated Kelvin wave activity is investigated using GPS radio occultation (RO) data from January 2002 to December 2014. RO data are a powerful tool to quantify tropical temperature oscillations with short vertical wavelengths due to their high vertical resolution and high accuracy and precision. Gridded temperatures from GPS RO show strongest variability in the tropical tropopause region (on average 3 K2). Large-scale zonal variability is dominated by transient high-frequency waves (2 K2) and about half of high-frequency variance is explained by eastward traveling Kelvin waves with periods of 7 to 30 days (1 K2). Quasi-stationary waves associated with the annual cycle and inter-annual variability contribute about a third (1 K2) to total resolved zonal variance. High-frequency waves, including Kelvin waves, are highly transient in time. Above 20 km, Kelvin waves are strongly modulated by the quasi-biennial oscillation (QBO) in stratospheric zonal winds, with enhanced wave activity during the westerly shear phase of the QBO. In the tropical tropopause region, however, peaks of Kelvin wave activity are irregularly distributed in time. Several peaks coincide with maxima of zonal variance in tropospheric deep convection, but other episodes are not evidently related. Further investigations of convective forcing and atmospheric background conditions are needed to better understand variability near the tropopause.

scholarly journals Moist Dynamical Linkage between the Equatorial Indian Ocean and the South Asian Monsoon Trough*

2010 ◽  
Vol 67 (3) ◽  
pp. 589-610 ◽  
Author(s):  
H. Annamalai
Keyword(s):  
Indian Ocean ◽  
Moisture Content ◽  
Equatorial Indian Ocean ◽  
Monsoon Trough ◽  
Boreal Summer ◽  
Coupled Models ◽  
Precipitation Anomalies ◽  
Anomalous Wind ◽  
Normal Rainfall ◽  
Sst Anomalies

Abstract During boreal summer, both the monsoon trough and the equatorial Indian Ocean (EIO) receive intense climatological precipitation. At various time scales, EIO sea surface temperature (SST) and/or precipitation variations interact with rainfall along the trough. For instance, during July–August in strong Indian Ocean dipole/zonal mode (IODZM) years, EIO experiences below-normal rainfall while regions along the monsoon trough receive above-normal rainfall. A lack of spatial coherency between SST and precipitation variations is noted in both regions. This paper posits the hypothesis that interaction between equatorial waves and moist physics is important in determining precipitation anomalies over these regions and in setting up the teleconnection. The hypothesis is tested using a linear baroclinic model (LBM). IODZM-related SST anomalies derived from multicentury integrations of the Geophysical Fluid Dynamics Laboratory coupled model (GFDL CM2.1) are used to force the LBM. Consistent with observations and CM2.1 composites of strong IODZM events, steady-state LBM solutions simulate zonally oriented negative (positive) precipitation anomalies over the EIO (along the monsoon trough). To identify the processes simulated in the LBM, moisture and moist static energy budgets are examined. Over both regions, analyses reveal that moisture advection contributes the most to the LBM budget, with advection of climatological moisture by the anomalous wind being the principal factor. Specifically, in response to cold SST anomalies in the EIO, moist stability due to surface fluxes increases, giving rise to below-normal rainfall. These conditions produce anomalous anticyclonic circulation as a Rossby wave response in the lower troposphere. Over the central-eastern EIO, this anomalous circulation advects climatological air of lower moisture content from the subtropics. In addition, advection of anomalous moisture by both climatological and anomalous wind results in anomalous dry conditions over the entire EIO. In contrast, anomalous divergent circulations that emanate from the EIO advect climatological air of higher moisture content from the equatorial region, amplifying rainfall along the monsoon trough. Consequently, the two regions are connected by a thermally driven overturning meridional circulation. Budget diagnostics performed with CM2.1 composites and the ECMWF interim reanalysis for observed IODZM events support the hypothesis. The results here imply that in coupled models, realistic representation of the basic state and details of the moist processes are necessary for successful monsoon prediction.

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