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.

scholarly journals Influence of Indian Ocean Dipole on Poleward Propagation of Boreal Summer Intraseasonal Oscillations

2008 ◽  
Vol 21 (21) ◽  
pp. 5437-5454 ◽  
Author(s):  
R. S. Ajayamohan ◽  
Suryachandra A. Rao ◽  
Toshio Yamagata
Keyword(s):  
Indian Ocean ◽  
Indian Ocean Dipole ◽  
Critical Role ◽  
Equatorial Indian Ocean ◽  
Summer Monsoon Rainfall ◽  
Specific Humidity ◽  
Boreal Summer ◽  
Intraseasonal Oscillations ◽  
Precipitation Anomalies

Abstract The influence of the Indian Ocean dipole (IOD) on the poleward propagation of boreal summer intraseasonal oscillations (BSISOs) is examined using observed datasets. This study finds that coherent (incoherent) poleward propagation of precipitation anomalies from 5°S to 25°N are observed during negative (positive) IOD years. Disorganized poleward propagation of BSISO in the south equatorial Indian Ocean is observed during positive IOD years. The rationale behind such an anomaly in the poleward propagation of BSISO in contrasting IOD years is identified based on the theory of northward-propagating BSISO, which suggests the influential role of air–sea interaction on the genesis and propagation of BSISO. It is found that the mean structure of moisture convergence and meridional specific humidity distribution undergoes radical changes in contrasting IOD years, which in turn influences the meridional propagation of BSISO. This study assumes significance, considering the critical role of BSISO in modulating the seasonal mean summer monsoon rainfall.

Subseasonal SST Variability in the Tropical Eastern North Pacific during Boreal Summer

2008 ◽  
Vol 21 (17) ◽  
pp. 4149-4167 ◽  
Author(s):  
Eric D. Maloney ◽  
Dudley B. Chelton ◽  
Steven K. Esbensen
Keyword(s):  
Costa Rica ◽  
Warm Pool ◽  
Boreal Summer ◽  
Sst Variability ◽  
East Pacific ◽  
Pacific Warm Pool ◽  
Northern Edge ◽  
Precipitation Anomalies ◽  
Sst Anomalies ◽  
Costa Rica Dome

Abstract Boreal summer intraseasonal (30–90-day time scale) sea surface temperature (SST) variability in the east Pacific warm pool is examined using Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) sea surface temperatures during 1998–2005. Intraseasonal SST variance maximizes at two locations in the warm pool: in the vicinity of 9°N, 92°W near the Costa Rica Dome and near the northern edge of the warm pool in the vicinity of 19°N, 108°W. Both locations exhibit a significant spectral peak at 50–60-day periods, time scales characteristic of the Madden–Julian oscillation (MJO). Complex empirical orthogonal function (CEOF) and spectra coherence analyses are used to show that boreal summer intraseasonal SST anomalies are coherent with precipitation anomalies across the east Pacific warm pool. Spatial variations of phase are modest across the warm pool, although evidence exists for the northward progression of intraseasonal SST and precipitation anomalies. Intraseasonal SSTs at the north edge of the warm pool lag those in the vicinity of the Costa Rica Dome by about 1 week. The MJO explains 30%–40% of the variance of intraseasonal SST anomalies in the east Pacific warm pool during boreal summer. Peak-to-peak SST variations of 0.8°–1.0°C occur during MJO events. SST is approximately in quadrature with MJO precipitation, with suppressed (enhanced) MJO precipitation anomalies leading positive (negative) SST anomalies by 7–10 days. Consistent with the CEOF and coherence analyses, MJO-related SST and precipitation anomalies near the Costa Rica Dome lead those at the northern edge of the warm pool by about 1 week.

scholarly journals Dominant Interannual Covariations of the East Asian–Australian Land Precipitation during Boreal Winter

2019 ◽  
Vol 32 (11) ◽  
pp. 3279-3296 ◽  
Author(s):  
Lin Liu ◽  
Jianping Guo ◽  
Wen Chen ◽  
Renguang Wu ◽  
Lin Wang ◽  
...  
Keyword(s):  
Indian Ocean ◽  
General Circulation ◽  
Southern Oscillation ◽  
East Asian ◽  
Circulation Model ◽  
Boreal Winter ◽  
Dipole Structure ◽  
Second Mode ◽  
Precipitation Anomalies ◽  
Sst Anomalies

AbstractThe present study applies the empirical orthogonal function (EOF) method to investigate the interannual covariations of East Asian–Australian land precipitation (EAALP) during boreal winter based on observational and reanalysis datasets. The first mode of EAALP variations is characterized by opposite-sign anomalies between East Asia (EA) and Australia (AUS). The second mode features an anomaly pattern over EA similar to the first mode, but with a southwest–northeast dipole structure over AUS. El Niño–Southern Oscillation (ENSO) is found to be a primary factor in modulating the interannual variations of land precipitation over EA and western AUS. By comparison, the Indian Ocean subtropical dipole mode (IOSD) plays an important role in the formation of precipitation anomalies over northeastern AUS, mainly through a zonal vertical circulation spanning from the southern Indian Ocean (SIO) to northern AUS. In addition, the ENSO-independent cold sea surface temperature (SST) anomalies in the western North Pacific (WNP) impact the formation of the second mode. Using the atmospheric general circulation model ECHAM5, three 40-yr numerical simulation experiments differing in specified SST forcings verify the impacts of the IOSD and WNP SST anomalies. Further composite analyses indicate that the dominant patterns of EAALP variability are largely determined by the out-of-phase and in-phase combinations of ENSO and IOSD. These results suggest that in addition to ENSO, IOSD should be considered as another crucial factor influencing the EAALP variability during the boreal winter, which has large implications for improved prediction of EAALP land precipitation on the interannual time scale.

scholarly journals Possible Impact of the Indian Ocean SST on the Northern Hemisphere Circulation during El Niño*

2007 ◽  
Vol 20 (13) ◽  
pp. 3164-3189 ◽  
Author(s):  
H. Annamalai ◽  
H. Okajima ◽  
M. Watanabe
Keyword(s):  
Indian Ocean ◽  
Northern Hemisphere ◽  
El Niño ◽  
El Nino ◽  
Tropical Indian Ocean ◽  
Tropical Pacific ◽  
The Indian Ocean ◽  
Precipitation Anomalies ◽  
Sst Anomalies ◽  
The Tropical Pacific

Abstract Two atmospheric general circulation models (AGCMs), differing in numerics and physical parameterizations, are employed to test the hypothesis that El Niño–induced sea surface temperature (SST) anomalies in the tropical Indian Ocean impact considerably the Northern Hemisphere extratropical circulation anomalies during boreal winter [January–March +1 (JFM +1)] of El Niño years. The hypothesis grew out of recent findings that ocean dynamics influence SST variations over the southwest Indian Ocean (SWIO), and these in turn impact local precipitation. A set of ensemble simulations with the AGCMs was carried out to assess the combined and individual effects of tropical Pacific and Indian Ocean SST anomalies on the extratropical circulation. To elucidate the dynamics responsible for the teleconnection, solutions were sought from a linear version of one of the AGCMs. Both AGCMs demonstrate that the observed precipitation anomalies over the SWIO are determined by local SST anomalies. Analysis of the circulation response shows that over the Pacific–North American (PNA) region, the 500-hPa height anomalies, forced by Indian Ocean SST anomalies, oppose and destructively interfere with those forced by tropical Pacific SST anomalies. The model results validated with reanalysis data show that compared to the runs where only the tropical Pacific SST anomalies are specified, the root-mean-square error of the height anomalies over the PNA region is significantly reduced in runs in which the SST anomalies in the Indian Ocean are prescribed in addition to those in the tropical Pacific. Among the ensemble members, both precipitation anomalies over the SWIO and the 500-hPa height over the PNA region show high potential predictability. The solutions from the linear model indicate that the Rossby wave packets involved in setting up the teleconnection between the SWIO and the PNA region have a propagation path that is quite different from the classical El Niño–PNA linkage. The results of idealized experiments indicate that the Northern Hemisphere extratropical response to Indian Ocean SST anomalies is significant and the effect of this response needs to be considered in understanding the PNA pattern during El Niño years. The results presented herein suggest that the tropical Indian Ocean plays an active role in climate variability and that accurate observation of SST there is of urgent need.

scholarly journals Weakening Trend of the Tropical Easterly Jet Stream of the Boreal Summer Monsoon Season 1950–2009

2013 ◽  
Vol 26 (23) ◽  
pp. 9408-9414 ◽  
Author(s):  
B. Abish ◽  
P. V. Joseph ◽  
Ola M. Johannessen
Keyword(s):  
Indian Ocean ◽  
Summer Monsoon ◽  
Monsoon Season ◽  
Equatorial Indian Ocean ◽  
Boreal Summer ◽  
Jet Stream ◽  
Moist Convection ◽  
Summer Monsoon Season ◽  
Upper Troposphere ◽  
Tropical Easterly Jet

Recent research has reported that the tropical easterly jet stream (TEJ) of the boreal summer monsoon season is weakening. The analysis herein using 60 yr (1950–2009) of data reveals that this weakening of the TEJ is due to the decreasing trend in the upper tropospheric meridional temperature gradient over the area covered by the TEJ. During this period, the upper troposphere over the equatorial Indian Ocean has warmed due to enhanced deep moist convection associated with the rapid warming of the equatorial Indian Ocean. At the same time, a cooling of the upper troposphere has taken place over the Northern Hemisphere subtropics including the Tibetan anticyclone. The simultaneous cooling of the subtropics and the equatorial heating has caused a decrease in the upper tropospheric meridional thermal gradient. The consequent reduction in the strength of the easterly thermal wind has resulted in the weakening of the TEJ.

Rain and Cloud Perspectives on the Seasonal Cycle over the Western Equatorial Indian Ocean

2020 ◽  
Vol 33 (3) ◽  
pp. 925-940
Author(s):  
Malcolm J. King ◽  
Christian Jakob
Keyword(s):  
Indian Ocean ◽  
Seasonal Cycle ◽  
Large Scale ◽  
Climate Models ◽  
Austral Summer ◽  
Equatorial Indian Ocean ◽  
Boreal Summer ◽  
Annual Peak ◽  
High Level ◽  
Cloud Regimes

AbstractConvection over the western equatorial Indian Ocean (WEIO) is strongly linked to precipitation over Africa and Australia but is poorly represented in current climate models, and its observed seasonal cycle is poorly understood. This study investigates the seasonal cycle of convection in the WEIO through rainfall and cloud measurements. Rainfall shows a single annual peak in early austral summer, but cloud proxies identify convective activity maxima in both boreal and austral summer. These diverging measures of convection during boreal summer are indicative of a reduction in the intensity of precipitation associated with a given cloud regime or cloud-top height during this time of year but an increase in the overall occurrence of high-top clouds and convectively active cloud regimes. The change in precipitation intensity associated with regimes is found to explain most of the changes in total precipitation during the period from May to November, whereas changes in the occurrence of convective regimes explains most of the changes throughout the rest of the year. The reduction in precipitation intensities associated with cloud regimes over the WEIO during boreal summer appears to be related to large-scale monsoon circulations, which suppress convection through forcing air descent in the midtroposphere and increase the apparent occurrence of convectively active cloud regimes through the advection of high-level cloud from monsoon-active areas toward the WEIO region.

scholarly journals Indian Ocean Variability and Its Association with ENSO in a Global Coupled Model

2005 ◽  
Vol 18 (17) ◽  
pp. 3634-3649 ◽  
Author(s):  
Aihong Zhong ◽  
Harry H. Hendon ◽  
Oscar Alves
Keyword(s):  
Indian Ocean ◽  
Positive Feedback ◽  
El Niño ◽  
El Nino ◽  
Equatorial Indian Ocean ◽  
Boreal Summer ◽  
Enso Events ◽  
Coupled Mode ◽  
Eastern Equatorial Indian Ocean ◽  
The Indian Ocean

Abstract The evolution of the Indian Ocean during El Niño–Southern Oscillation is investigated in a 100-yr integration of an Australian Bureau of Meteorology coupled seasonal forecast model. During El Niño, easterly anomalies are induced across the eastern equatorial Indian Ocean. These act to suppress the equatorial thermocline to the west and elevate it to the east and initially cool (warm) the sea surface temperature (SST) in the east (west). Subsequently, the entire Indian Ocean basin warms, mainly in response to the reduced latent heat flux and enhanced shortwave radiation that is associated with suppressed rainfall. This evolution can be partially explained by the excitation of an intrinsic coupled mode that involves a feedback between anomalous equatorial easterlies and zonal gradients in SST and rainfall. This positive feedback develops in the boreal summer and autumn seasons when the mean thermocline is shallow in the eastern equatorial Indian Ocean in response to trade southeasterlies. This positive feedback diminishes once the climatological surface winds become westerly at the onset of the Australian summer monsoon. ENSO is the leading mechanism that excites this coupled mode, but not all ENSO events are efficient at exciting it. During the typical El Niño (La Niña) event, easterly (westerly) anomalies are not induced until after boreal autumn, which is too late in the annual cycle to instigate strong dynamical coupling. Only those ENSO events that develop early (i.e., before boreal summer) instigate a strong coupled response in the Indian Ocean. The coupled mode can also be initiated in early boreal summer by an equatorward shift of the subtropical ridge in the southern Indian Ocean, which stems from uncoupled extratropical variability.

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 Tropical Atlantic SST Prediction with Coupled Ocean–Atmosphere GCMs

2006 ◽  
Vol 19 (23) ◽  
pp. 6047-6061 ◽  
Author(s):  
Timothy N. Stockdale ◽  
Magdalena A. Balmaseda ◽  
Arthur Vidard
Keyword(s):  
Initial Conditions ◽  
Coupled Model ◽  
Boreal Summer ◽  
Tropical Atlantic ◽  
Equatorial Atlantic ◽  
Seasonal Forecasts ◽  
Coupled Models ◽  
Equatorial Ocean ◽  
Sst Anomalies

Abstract Variations in tropical Atlantic SST are an important factor in seasonal forecasts in the region and beyond. An analysis is given of the capabilities of the latest generation of coupled GCM seasonal forecast systems to predict tropical Atlantic SST anomalies. Skill above that of persistence is demonstrated in both the northern tropical and equatorial Atlantic, but not farther south. The inability of the coupled models to correctly represent the mean seasonal cycle is a major problem in attempts to forecast equatorial SST anomalies in the boreal summer. Even when forced with observed SST, atmosphere models have significant failings in this area. The quality of ocean initial conditions for coupled model forecasts is also a cause for concern, and the adequacy of the near-equatorial ocean observing system is in doubt. A multimodel approach improves forecast skill only modestly, and large errors remain in the southern tropical Atlantic. There is still much scope for improving forecasts of tropical Atlantic SST.

scholarly journals Interdecadal weakening of the cross-equatorial flows over the Maritime Continent during the boreal summer in the mid-1990s: drivers and physical processes

2021 ◽  
Author(s):  
Xiaoxuan Zhao ◽  
Buwen Dong ◽  
Riyu Lu
Keyword(s):  
Dominant Role ◽  
Lower Troposphere ◽  
Hadley Circulation ◽  
Western Pacific ◽  
Boreal Summer ◽  
Maritime Continent ◽  
Coupled Models ◽  
The Cross ◽  
The Western Pacific ◽  
Sst Anomalies

AbstractIn this study, the cross-equatorial flows (CEF) on both high and low level (HCEF/LCEF) troposphere over the Maritime Continent (MC) in boreal summer are found to have experienced an interdecadal weakening in the mid-1990s based on both JRA55 and NCEP reanalyses. The outputs of 8 coupled models in CMIP6 are used to investigate drivers and the corresponding mechanisms. Model results show that the role of external forcing is weak in the interdecadal weakening of CEF. By contrast, the observed interdecadal weakening of both HCEF and LCEF can be largely explained by internal variability associated with a negative phase of the interdecadal Pacific Oscillation (IPO). Associated with negative IPO are anomalous divergence (convergence), enhanced precipitation over MC and anomalous cyclonic (anticyclonic) circulations, reduced precipitation over western North Pacific (WNP) in the upper (lower) troposphere. Sensitivity experiments based on MetUM-GA6 further manifest that this IPO phase transition can lead to the interdecadal weakening of CEF, in which the central tropical Pacific (CTP) sea surface temperature (SST) anomalies play a dominant role. The cold SST anomalies in CTP lead to reduced local convection and trigger enhanced convection over MC through changes in the Walker circulation. The enhanced convection over MC leads to a change in local Hadley circulation over the western Pacific sector. This change is characterized by anomalous ascents over MC, southerlies in the upper troposphere, descents and reduced precipitation over WNP and northerlies in the lower troposphere, leading to the weakening of CEF. Meanwhile, positive SST anomalies over MC associated with negative IPO also make a contribution to the weakening of CEF by inducing a change in the Hadley circulation in the western Pacific sector through similar processes.

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