scholarly journals Regional Heat Sources and the Active and Break Phases of Boreal Summer Intraseasonal (30–50 Day) Variability*

2005 ◽  
Vol 62 (8) ◽  
pp. 2726-2748 ◽  
Author(s):  
H. Annamalai ◽  
K. R. Sperber
Keyword(s):  
Life Cycle ◽  
Indian Ocean ◽  
Linear Model ◽  
Equatorial Indian Ocean ◽  
Boreal Summer ◽  
Heat Sources ◽  
West Pacific ◽  
Low Level ◽  
Cseof Analysis ◽  
Three Components

Abstract The boreal summer intraseasonal variability (BSISV) associated with the 30–50-day mode is represented by the coexistence of three components: poleward propagation of convection over the Indian and tropical west Pacific longitudes and eastward propagation along the equator. The hypothesis that the three components influence each other has been investigated using observed outgoing longwave radiation (OLR), NCEP–NCAR reanalysis, and solutions from an idealized linear model. The null hypothesis is that the three components are mutually independent. Cyclostationary EOF (CsEOF) analysis is applied on filtered OLR to extract the life cycle of the BSISV. The dominant CsEOF mode is significantly tied to the observed spatial rainfall pattern associated with the active/break phases over the Indian subcontinent. The components of the heating patterns from CsEOF analysis serve as prescribed forcings for the dry version of the linear model. This allows one to investigate the possible roles that the regional heat sources and sinks play in driving the large-scale monsoon circulation at various stages of the BSISV life cycle. To understand the interactive nature between convection and circulation, the moist version of the model is forced with intraseasonal SST anomalies. The linear models reproduce the major features of the BSISV seen in the reanalysis. The linear model suggests three new findings: (i) The circulation anomalies that develop as a Rossby wave response to suppressed convection over the equatorial Indian Ocean associated with the previous break phase of the BSISV results in low-level convergence and tropospheric moisture enhancement over the equatorial western Indian Ocean and helps trigger the next active phase of the BSISV. (ii) The development of convection over the tropical west Pacific forces descent anomalies to the west. This, in conjunction with the weakened cross-equatorial flow due to suppressed convective anomalies over the equatorial Indian Ocean, reduces the tropospheric moisture over the Arabian Sea and promotes westerly wind anomalies that do not recurve over India. As a result the low-level cyclonic vorticity shifts from India to Southeast Asia and break conditions are initiated over India. (iii) The circulation anomalies forced by equatorial Indian Ocean convective anomalies significantly influence the active/break phases over the tropical west Pacific. The model solutions support the hypothesis that the three components of the BSISV influence each other but do not imply that such an influence is responsible for the space–time evolution of the BSISV. Further, the applicability of the model results to the observed system is constrained by the assumption that linear interactions are sufficient to address the BSISV and that air–sea interaction and transient forcing are excluded.

scholarly journals Importance of seasonally resolved oceanic emissions for bromoform delivery from the tropical Indian Ocean and west Pacific to the stratosphere

2018 ◽  
Vol 18 (16) ◽  
pp. 11973-11990 ◽  
Author(s):  
Alina Fiehn ◽  
Birgit Quack ◽  
Irene Stemmler ◽  
Franziska Ziska ◽  
Kirstin Krüger
Keyword(s):  
Indian Ocean ◽  
Summer Monsoon ◽  
Tropical Indian Ocean ◽  
Boreal Summer ◽  
Boreal Winter ◽  
The West ◽  
West Pacific ◽  
Mixing Ratios ◽  
The Indian Ocean ◽  
Oceanic Emissions

Abstract. Oceanic very short-lived substances (VSLSs), such as bromoform (CHBr3), contribute to stratospheric halogen loading and, thus, to ozone depletion. However, the amount, timing, and region of bromine delivery to the stratosphere through one of the main entrance gates, the Indian summer monsoon circulation, are still uncertain. In this study, we created two bromoform emission inventories with monthly resolution for the tropical Indian Ocean and west Pacific based on new in situ bromoform measurements and novel ocean biogeochemistry modeling. The mass transport and atmospheric mixing ratios of bromoform were modeled for the year 2014 with the particle dispersion model FLEXPART driven by ERA-Interim reanalysis. We compare results between two emission scenarios: (1) monthly averaged and (2) annually averaged emissions. Both simulations reproduce the atmospheric distribution of bromoform from ship- and aircraft-based observations in the boundary layer and upper troposphere above the Indian Ocean reasonably well. Using monthly resolved emissions, the main oceanic source regions for the stratosphere include the Arabian Sea and Bay of Bengal in boreal summer and the tropical west Pacific Ocean in boreal winter. The main stratospheric injection in boreal summer occurs over the southern tip of India associated with the high local oceanic sources and strong convection of the summer monsoon. In boreal winter more bromoform is entrained over the west Pacific than over the Indian Ocean. The annually averaged stratospheric injection of bromoform is in the same range whether using monthly averaged or annually averaged emissions in our Lagrangian calculations. However, monthly averaged emissions result in the highest mixing ratios within the Asian monsoon anticyclone in boreal summer and above the central Indian Ocean in boreal winter, while annually averaged emissions display a maximum above the west Indian Ocean in boreal spring. In the Asian summer monsoon anticyclone bromoform atmospheric mixing ratios vary by up to 50 % between using monthly averaged and annually averaged oceanic emissions. Our results underline that the seasonal and regional stratospheric bromine injection from the tropical Indian Ocean and west Pacific critically depend on the seasonality and spatial distribution of the VSLS emissions.

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.

scholarly journals The Impact of Indian Ocean Mean-State Biases in Climate Models on the Representation of the East African Short Rains

2018 ◽  
Vol 31 (16) ◽  
pp. 6611-6631 ◽  
Author(s):  
Linda Hirons ◽  
Andrew Turner
Keyword(s):  
Indian Ocean ◽  
Wind Stress ◽  
Equatorial Indian Ocean ◽  
East African ◽  
Easterly Wind ◽  
Low Level ◽  
The Indian Ocean ◽  
Short Rains ◽  
The Impact ◽  
Mean State

The role of the Indian Ocean dipole (IOD) in controlling interannual variability in the East African short rains, from October to December, is examined in state-of-the-art models and in detail in one particular climate model. In observations, a wet short-rainy season is associated with the positive phase of the IOD and anomalous easterly low-level flow across the equatorial Indian Ocean. A model’s ability to capture the teleconnection to the positive IOD is closely related to its representation of the mean state. During the short-rains season, the observed low-level wind in the equatorial Indian Ocean is westerly. However, half of the models analyzed exhibit mean-state easterlies across the entire basin. Specifically, those models that exhibit mean-state low-level equatorial easterlies in the Indian Ocean, rather than the observed westerlies, are unable to capture the latitudinal structure of moisture advection into East Africa during a positive IOD. Furthermore, the associated anomalous easterly surface wind stress causes upwelling in the eastern Indian Ocean. This upwelling draws up cool subsurface waters, enhancing the zonal sea surface temperature gradient between west and east and strengthening the positive IOD pattern, further amplifying the easterly wind stress. This positive Bjerknes coupled feedback is stronger in easterly mean-state models, resulting in a wetter East African short-rain precipitation bias in those models.

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 Interannual Variability of Equatorial Eastern Indian Ocean Upwelling: Local versus Remote Forcing

2016 ◽  
Vol 46 (3) ◽  
pp. 789-807 ◽  
Author(s):  
Gengxin Chen ◽  
Weiqing Han ◽  
Yuanlong Li ◽  
Dongxiao Wang
Keyword(s):  
Indian Ocean ◽  
Interannual Variability ◽  
Southern Oscillation ◽  
Equatorial Indian Ocean ◽  
Eastern Indian Ocean ◽  
Boreal Summer ◽  
Thermocline Depth ◽  
Remote Forcing ◽  
Local Forcing ◽  
The Mean

AbstractThe equatorial eastern Indian Ocean (EIO) upwelling occurs in the Indian Ocean warm pool, differing from the equatorial Pacific and Atlantic upwelling that occurs in the cold tongue. By analyzing observations and performing ocean model experiments, this paper quantifies the remote versus local forcing in causing interannual variability of the equatorial EIO upwelling from 2001 to 2011 and elucidates the associated processes. For all seasons, interannual variability of thermocline depth in the EIO, as an indicator of upwelling, is dominated by remote forcing from equatorial Indian Ocean winds, which drive Kelvin waves that propagate along the equator and subsequently along the Sumatra–Java coasts. Upwelling has prominent signatures in sea surface temperature (SST) and chlorophyll-a concentration but only in boreal summer–fall (May–October). Local forcing plays a larger role than remote forcing in producing interannual SST anomaly (SSTA). During boreal summer–fall, when the mean thermocline is relatively shallow, SSTA is primarily driven by the upwelling process, with comparable contributions from remote and local forcing effects. In contrast, during boreal winter–spring (November–April), when the mean thermocline is relatively deep, SSTA is controlled by surface heat flux and decoupled from thermocline variability. Advection affects interannual SSTA in all cases. The remote and local winds that drive the interannual variability of the equatorial EIO upwelling are closely associated with Indian Ocean dipole events and to a lesser degree with El Niño–Southern Oscillation.

scholarly journals Importance of seasonally resolved oceanic emissions for bromoform delivery from the tropical Indian Ocean and west Pacific to the stratosphere

2018 ◽  
Author(s):  
Alina Fiehn ◽  
Birgit Quack ◽  
Irene Stemmler ◽  
Franziska Ziska ◽  
Kirstin Krüger
Keyword(s):  
Indian Ocean ◽  
Asian Monsoon ◽  
Tropical Indian Ocean ◽  
Boreal Summer ◽  
Boreal Winter ◽  
The West ◽  
West Pacific ◽  
Mixing Ratios ◽  
The Indian Ocean ◽  
Oceanic Emissions

Abstract. Oceanic very short-lived substances (VSLS), such as bromoform (CHBr3), contribute to stratospheric halogen loading and, thus, to ozone depletion. However, the amount, timing, and region of bromine delivery to the stratosphere through one of the main entrance gates, the Asian monsoon circulation, are still uncertain. In this study, we created two bromoform emission inventories with monthly resolution for the tropical Indian Ocean and west Pacific based on new in situ bromoform measurements and novel ocean biogeochemistry modeling. The mass transport and atmospheric mixing ratios of bromoform were modeled for the year 2014 with the particle dispersion model FLEXPART driven by ERA-Interim reanalysis. We compare results between two emission scenarios: (1) monthly and (2) annually averaged emissions. Both simulations reproduce the atmospheric distribution of bromoform from ship- and aircraft-based observations in the boundary layer and upper troposphere above the Indian Ocean well. Using monthly resolved emissions, main oceanic source regions for the stratosphere include the Arabian Sea and Bay of Bengal in boreal summer and the tropical west Pacific Ocean in boreal winter. The main stratospheric entrainment in boreal summer occurs over the southern tip of India associated with the high local oceanic sources and strong convection of the summer monsoon. In boreal winter more bromoform is entrained over the west Pacific than over the Indian Ocean. The annually averaged stratospheric entrainment of bromoform is in the same range whether using monthly or annually averaged emissions in our Lagrangian calculations. However, monthly averaged emissions result in highest mixing ratios within the Asian monsoon anticyclone in boreal summer and above the central Indian Ocean in boreal winter, while annually averaged emissions display a maximum above the west Indian Ocean in boreal spring. In the Asian summer monsoon anticyclone bromoform atmospheric mixing ratios vary up to 50 % between using monthly and annually averaged oceanic emissions. Our results underline that the seasonal and regional stratospheric bromine entrainment from the tropical Indian Ocean and west Pacific critically depends on the seasonality and spatial distribution of the VSLS emissions.

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.

Reinitiation of the Boreal Summer Intraseasonal Oscillation in the Tropical Indian Ocean*

2005 ◽  
Vol 18 (18) ◽  
pp. 3777-3795 ◽  
Author(s):  
Xian-An Jiang ◽  
Tim Li
Keyword(s):  
Indian Ocean ◽  
General Circulation ◽  
Intraseasonal Oscillation ◽  
Circulation Model ◽  
Equatorial Indian Ocean ◽  
Specific Humidity ◽  
Boreal Summer ◽  
Boreal Winter ◽  
Boreal Summer Intraseasonal Oscillation ◽  
Temperature Advection

Abstract The characteristic features of the boreal summer intraseasonal oscillation (BSISO) during its reinitiation period are studied using NCEP–NCAR reanalysis. Based on these observations and with the aid of an anomalous atmospheric general circulation model (AGCM), a possible mechanism responsible for the BSISO reinitiation is elucidated. The western equatorial Indian Ocean along the eastern African coast tends to be a key region for the phase transition of the BSISO from an enhanced to suppressed convective phase, or vise versa. The major precursory feature associated with reinitiation of suppressed convection is found in the divergence and reduced specific humidity in the boundary layer. Numerical experiments indicate that the low-level divergence is caused by the cold horizontal temperature advection and associated adiabatic warming (descending motion) in situ. The summer mean state is found to be important for the cold horizontal temperature advection through the modulation of a Gill-type response to an intraseasonal oscillation (ISO) heating in the eastern equatorial Indian Ocean. The results in this study suggest a self-sustained paradigm in the Indian Ocean for the BSISO; that is, the BSISO could be a basinwide phenomenon instead of a global circumstance system as hypothesized for the boreal winter ISO (i.e., the Madden–Julian oscillation).

scholarly journals The Intraseasonal Variability of the Indian Summer Monsoon Using TMI Sea Surface Temperatures and ECMWF Reanalysis

2008 ◽  
Vol 21 (11) ◽  
pp. 2519-2539 ◽  
Author(s):  
Nicholas P. Klingaman ◽  
Hilary Weller ◽  
Julia M. Slingo ◽  
Peter M. Inness
Keyword(s):  
Life Cycle ◽  
Indian Ocean ◽  
General Circulation ◽  
Function Analysis ◽  
Intraseasonal Variability ◽  
Equatorial Indian Ocean ◽  
Sea Surface ◽  
Sea Surface Temperatures ◽  
Surface Temperatures ◽  
Sst Anomalies

Abstract The northward-propagating intraseasonal (30–40 day) oscillation (NPISO) between active and break monsoon phases exerts a critical control on summer-season rainfall totals over India. Advances in diagnosing these events and comprehending the physical mechanisms behind them may hold the potential for improving their predictability. While previous studies have attempted to extract active and break events from reanalysis data to elucidate a composite life cycle, those studies have relied on first isolating the intraseasonal variability in the record (e.g., through bandpass filtering, removing harmonics, or empirical orthogonal function analysis). Additionally, the underlying physical processes that previous studies have proposed have varied, both among themselves and with studies using general circulation models. A simple index is defined for diagnosing NPISO events in observations and reanalysis, based on lag correlations between outgoing longwave radiation (OLR) over India and over the equatorial Indian Ocean. This index is the first to use unfiltered OLR observations and so does not specifically isolate intraseasonal periods. A composite NPISO life cycle based on this index is similar to previous composites in OLR and surface winds, demonstrating that the dominance of the intraseasonal variability in the monsoon climate system eliminates the need for more complex methods (e.g., time filtering or EOF analysis) to identify the NPISO. This study is also among the first to examine the NPISO using a long-period record of high-resolution sea surface temperatures (SSTs) from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager. Application of this index to those SSTs demonstrates that SST anomalies exist in near quadrature with convection, as suggested by recent coupled model studies. Analysis of the phase relationships between atmospheric fields and SSTs indicates that the atmosphere likely forced the SST anomalies. The results of this lag-correlation analysis suggest that the oscillation serves as its own most reliable—and perhaps only—predictor, and that signals preceding an NPISO event appear first over the Indian subcontinent, not the equatorial Indian Ocean where the events originate.

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