Search

This paper investigates how La Niña Modoki modulates the impacts of the warm Indian Ocean basin mode (IOBM) on the boreal summer climate and the genesis of tropical cyclones (TCs) over the northwest Pacific (NWP). The results showed that the influence of the Indian Ocean sea surface temperature (SST) on TC genesis is the primary mechanism during the boreal summer, while La Niña Modoki exerts a secondary influence. However, although the summertime index of the IOBM shows a high negative correlation with the number of TCs generated over the NWP, warm IOBM events without La Niña Modoki have only limited influences on the boreal summertime circulations and TC genesis. The present study showed that when warm IOBM events and La Niña Modoki coexisted, the average location of TC genesis shifted westward, and the annual number of generated TCs substantially decreased. La Niña Modoki–related cold sea surface temperature anomalies over the central Pacific further suppressed convective activities over the eastern NWP compared with warm IOBM events without La Niña Modoki. Upper-level convergence and enlarged tropospheric vertical wind shears both contributed to the weakening of the low-level relative vorticity in the coupled cases, leading to a suppressed NWP monsoon trough. Additionally, together with the weaker moisture supply, the impacts of warm IOBM cases were significantly enhanced under the modulation of La Niña Modoki, leading to poorer TC genesis conditions over the eastern NWP. In addition, the energy conversion processes in the aforementioned modulation showed that joint cases will provide fewer initial disturbance seedlings for TC genesis. These results are useful for further understanding the role of warm IOBM cases in TC genesis over the NWP. [ABSTRACT FROM AUTHOR]

Copyright of Advances in Atmospheric Sciences is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This study investigates extreme phytoplankton blooms in the southern tropical Indian Ocean (TIO) in January–April 2011 using the extended ocean color products and in situ data. The amplitude of the blooms was approximately 4 times higher than the climatological value and 2 times higher than that in 1998–1999. The anomalous enhancement of surface chlorophyll‐a concentration (Chla) was associated with the strong upwelling Rossby waves that forced by the extraordinary strong wind stress curl in the southeastern TIO during 2010–2011 La Niña, which was much stronger than that of 1998–1999 La Niña. The Rossby waves uplifted the thermocline, which was shoaled by more than 70 m relative to the climatology. The results of the nonlinear 1.5‐layer reduced‐gravity model further suggest that the thermocline variations are mainly due to the wind stress curl over the interior TIO. In the vertical direction, the Argo data show a distinct upward and westward propagation of subsurface cooling, indicating that the upwelling of cold nutrient‐rich waters leads to the abnormally high Chla. The strong upwelling processes are also well captured by the Moored Array for African‐Asian‐Australian Monsoon Analysis and Prediction observations at 8°S, 80.5°E. Based on the climatological in situ nitrate data, the Rossby wave‐induced nitrate supply and potential new production at the mooring site are estimated. The potential f ratio ranges from 50% to 87% when the water brought to the surface is assumed to have originated from 60 to 200 m, thus indicating the dominant role of nutrient supply by the upwelling processes. Plain Language Summary: Taking advantage of extended ocean color products and observation from Argo and Moored Array for African‐Asian‐Australian Monsoon Analysis and Prediction, this study investigates extreme phytoplankton blooms in the southern TIO during the 2010–2011 La Niña event. The amplitude of the blooms was approximately 4 times higher than the climatological value and 2 times higher than that in another La Niña year 1998–1999. The anomalous enhancement of surface chlorophyll‐a concentration was associated with the strong upwelling Rossby waves that forced by the extraordinary strong wind stress curl in the southeastern TIO during 2010–2011 La Niña. The in situ data reveal the dominant role of Rossby wave‐induced upwelling in triggering the extreme blooms in the southern TIO. Key Points: The extreme high chlorophyll‐a in the southern TIO in 2011 is identified from two‐decade satellite observationThe in situ data reveal the dominant role of Rossby wave‐induced upwelling in triggering the bloomsLa Niña‐induced wind stress curl in the southeastern TIO is much stronger in 2010–2011 than that in 1998–1999, accounting for the Rossby waves [ABSTRACT FROM AUTHOR]

The South‐East Madagascar Bloom occurs in an oligotrophic region of the southwest Indian Ocean. Phase locked to austral summer, this sporadic feature exhibits substantial temporal and spatial variability. Several studies, with different hypotheses, have focused on the initiation mechanism triggering the bloom, but none has been as yet clearly substantiated. With 19 years of ocean color data set available as well as in situ measurements (Argo profiles), the time is ripe to review this feature. The bloom is characterized in a novel manner, and a new bloom index is suggested, yielding 11 bloom years, including 3 major bloom years (1999, 2006, and 2008). Spatially, the bloom varies from a mean structure (22–32°S; 50–70°E) both zonally and meridionally. A colocation analysis of Argo profiles and chlorophyll‐a data revealed a bloom occurrence in a shallow‐stratified layer, with low‐salinity water in the surface layers. Additionally, a quantitative assessment of the previous hypotheses is performed and bloom occurrence is found to coincide with La Niña events and reduced upwelling intensity south of Madagascar. A stronger South‐East Madagascar Current during La Niña may support a detachment of the current from the coasts, dampening the upwelling south of Madagascar, and feeding low‐salinity waters into the Madagascar Basin, hence increasing stratification. Along with abundance of light, these provide the right conditions for a nitrogen‐fixing cyanobacterial phytoplankton bloom onset. Plain Language Summary: The South‐East Madagascar Bloom is one of the largest bloom in the world. It can play a major role in the fishing industry, as well as capturing carbon dioxide from the atmosphere. Hence, it needs to be better understood. In previous studies, several hypotheses have been put forward to explain the bloom but none have as yet been clearly substantiated. This study shows that the bloom occurs in a surface layer of low salinity, which tends to favor a specific type of phytoplankton, namely, cyanobacteria. The climate phenomenon, La Niña, also seems to co‐occur with the bloom, 10 out of 11 events. The input of nutrient‐rich waters to the surface south of Madagascar is weakened during the bloom, and this goes against what was previously thought. The present study gives a possible new reasoning as to why the bloom occurs. The current southeast of Madagascar detaches from the coast and brings low‐salinity and nutrient‐rich waters into the bloom region where enhanced photosynthesis occurs, hence causing the bloom. Key Points: A revisit of the South‐East Madagascar Bloom from updated observations is performedBloom occurs within low‐salinity surface waters and coincides with La Nina events as well as reduced coastal upwelling south of MadagascarLow‐salinity waters, increased stratification, and light provide good conditions for a cyanobacterial phytoplankton bloom [ABSTRACT FROM AUTHOR]

Understanding the interactions between the Pacific and other ocean basins during extreme phases of the El Niño-Southern Oscillation (ENSO) is necessary for explaining its global climate impacts. Here climate model experiments are used to highlight a mechanism by which the characteristic warming of the Indian Ocean during peak El Niño months can cool North Africa and South Asia, an area encompassing over three billion people. It is found that warming of the Indian Ocean during extreme El Niño events leads to broader upper tropospheric geopotential height anomalies than would otherwise occur. This weakens the extratropical Rossby wave response initiated in the tropics and leads to higher pressure and reduced cloud forcing over North Africa and South Asia. Reanalysis data provides empirical support for this mechanism, although it is likely only to be prominent during strong El Niño events when Indian Ocean warming tends to be larger. This dampening effect needs to be taken into account in understanding the climatic impact of extreme El Niño events, which are projected to increase under global warming. [ABSTRACT FROM AUTHOR]

In the present study the association between mean and interannual subsurface temperature bias over the equatorial Indian Ocean (EIO) is investigated during boreal summer (June through September; JJAS) in the National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFSv2) hindcast. Anomalously high subsurface warm bias (greater than 3 °C) over the eastern EIO (EEIO) region is noted in CFSv2 during summer, which is higher compared to other parts of the tropical Indian Ocean. Prominent eastward current bias in the upper 100 m over the EIO region induced by anomalous westerly winds is primarily responsible for subsurface temperature bias. The eastward currents transport warm water to the EEIO and is pushed down to subsurface due to downwelling. Thus biases in both horizontal and vertical currents over the EIO region support subsurface warm bias. The evolution of systematic subsurface warm bias in the model shows strong interannual variability. These maximum subsurface warming episodes over the EEIO are mainly associated with La Niña like forcing. Strong convergence of low level winds over the EEIO and Maritime continent enhanced the westerly wind bias over the EIO during maximum warming years. This low level convergence of wind is induced by the bias in the gradient in the mean sea level pressure with positive bias over western EIO and negative bias over EEIO and parts of western Pacific. Consequently, changes in the atmospheric circulation associated with La Niña like conditions affected the ocean dynamics by modulating the current bias thereby enhancing the subsurface warm bias over the EEIO. It is identified that EEIO subsurface warming is stronger when La Niña co-occurred with negative Indian Ocean Dipole events as compared to La Niña only years in the model. Ocean general circulation model (OGCM) experiments forced with CFSv2 winds clearly support our hypothesis that ocean dynamics influenced by westerly winds bias is primarily responsible for the strong subsurface warm bias over the EEIO. This study advocates the importance of understanding the ability of the models in representing the large scale air-sea interactions over the tropics and their impact on ocean biases for better monsoon forecast. [ABSTRACT FROM AUTHOR]

Indo-Pacific sea surface temperature dynamics play a prominent role in Asian summer monsoon variability. Two interactive climate modes of the Indo-Pacific-the El Niño/Southern Oscillation (ENSO) and the Indian Ocean dipole mode-modulate the amount of precipitation over India, in addition to precipitation over Africa, Indonesia, and Australia. However, this modulation is not spatially uniform. The precipitation in southern India is strongly forced by the Indian Ocean dipole mode and ENSO. In contrast, across northern India, encompassing the Ganges and Brahmaputra basins, the climate mode influence on precipitation is much less. Understanding the forcing of precipitation in these river basins is vital for food security and ecosystem services for over half a billion people. Using 28 years of remote sensing observations, we demonstrate that (i) the tropical west-east differential heating in the Indian Ocean influences the Ganges precipitation and (ii) the north-south differential heating in the Indian Ocean influences the Brahmaputra precipitation. The El Niño phase induces warming in the warm pool of the Indian Ocean and exerts more influence on Ganges precipitation than Brahmaputra precipitation. The analyses indicate that both the magnitude and position of the sea surface temperature anomalies in the Indian Ocean are important drivers for precipitation dynamics that can be effectively summarized using two new indices, one tuned for each basin. These new indices have the potential to aid forecasting of drought and flooding, to contextualize land cover and land use change, and to assess the regional impacts of climate change. [ABSTRACT FROM AUTHOR]

This study investigates the relative importance of tropical Indian Ocean warming (IOW) and equatorial central to eastern Pacific cooling (EPC) in sustaining an anomalous Western North Pacific anticyclone (WNPAC) during the transition from an El Niño in the preceding winter to a La Niña in the subsequent summer through a suite of numerical experiments. The numerical results indicate that the WNPAC is maintained by a combined effect of IOW and EPC during the La Niña developing years. The contribution of IOW in maintaining the WNPAC sustains from spring to early summer, but appears to weaken after that as IOW decays. The role of IOW is via an eastward-propagating Kelvin wave induced Ekman divergence mechanism. The decay of IOW is because of reduction in downward solar radiation associated with above normal precipitation in situ. As the cooling develops over central to eastern Pacific from spring to summer, EPC starts to contribute to the maintenance of the WNPAC during summer through stimulating a Rossby wave response to its northwest. In this study, we have identified that the cooling over the central to eastern Pacific plays an important role in sustaining the WNPAC during La Niña developing summers. This finding may help improve the prediction of the East Asian summer monsoon, which is closely associated with the WNPAC. [ABSTRACT FROM AUTHOR]

Vast amounts of energy are exchanged between the ocean, atmosphere, and space in association with El Niño-Southern Oscillation (ENSO). This study examines energy budgets of all tropical (30°S-30°N) ocean basins and the atmosphere separately using different, largely independent oceanic and atmospheric reanalyses to depict anomalous energy flows associated with ENSO in a consistent framework. It is found that variability of area-averaged ocean heat content (OHC) in the tropical Pacific to a large extent is modulated by energy flow through the ocean surface. While redistribution of OHC within the tropical Pacific is an integral part of ENSO dynamics, variability of ocean heat transport out of the tropical Pacific region is found to be mostly small. Noteworthy contributions arise from the Indonesian Throughflow (ITF), which is anticorrelated with ENSO at a few months lag, and from anomalous oceanic poleward heat export during the La Niña events in 1999 and 2008. Regression analysis reveals that atmospheric energy transport and radiation at the top of the atmosphere (RadTOA) almost perfectly balance the OHC changes and ITF variability associated with ENSO. Only a small fraction of El Niño-related heat lost by the Pacific Ocean through anomalous air-sea fluxes is radiated to space immediately, whereas the major part of the energy is transported away by the atmosphere. Ample changes in tropical atmospheric circulation lead to enhanced surface fluxes and, consequently, to an increase of OHC in the tropical Atlantic and Indian Ocean that almost fully compensates for tropical Pacific OHC loss. This signature of energy redistribution is robust across the employed datasets for all three tropical ocean basins and explains the small ENSO signal in global mean RadTOA. [ABSTRACT FROM AUTHOR]

We investigated inter-annual variability in the breeding performance of six tropical seabirds with different life histories. We further examined the extent to which presumed differences among years in food availability to seabirds were related to large-scale oceanic processes (El Niño Southern Oscillation and Indian Ocean Dipole) or local features around the colony (Sea Surface Temperature-SST, chlorophyll-a concentration, wind patterns, association with underwater predators). Two food shortages were recorded: (1) a short-term event operating at a local-scale, lacking any relationship with any measured oceanographic parameters and affecting only a few seabird species; (2) a generalized and protracted event, strikingly characterized by low SST, possibly induced by a La Niña and/or negative IOD event, and with detrimental impacts in the whole seabird community. Overall, species with inflexible feeding habits and fast chick growth rates were more affected by food shortage events than those with more plastic life-history traits. [ABSTRACT FROM AUTHOR]

El Niñño and La Niñña exhibit significant asymmetry not only in their spatial structure but also in their duration. Most El Niñños terminate rapidly after maturing near the end of the calendar year, whereas many La Niññas persist into the following year and often reintensify in boreal winter. Through atmospheric general circulation model experiments, it is shown that the nonlinear response of atmospheric deep convection to the polarity of equatorial Pacific sea surface temperature anomalies causes an asymmetric evolution of surface wind anomalies over the far western Pacific around the mature phase of El Niñño and La Niñña. Because of the eastward displacement of precipitation anomalies in the equatorial Pacific during El Niñño compared to La Niñña, surface winds in the western Pacific are more affected by SST forcing outside the equatorial Pacific, which acts to terminate the Pacific event. [ABSTRACT FROM AUTHOR]

The phase relationships of the western North Pacific (WNP) summer monsoon (WNPM) with the Australian monsoon (AM) and Indian monsoon (IM) are investigated using observational rainfall, SST, and NCEP reanalysis data for the period of 1979--2005. It is found that a strong WNPM often follows a strong AM but leads a weak AM, and a significant simultaneous negative correlation appears between WNPM and IM. The in-phase relationship from AM to the succeeding WNPM occurs often during the El Niño decaying phase when the warm eastern Pacific SST anomaly (SSTA) weakens AM through anomalous Walker circulation and the persistence of an anomalous WNP anticyclone from the boreal winter to summer leads to a weak WNPM. The out-of-phase relation from WNPM to the succeeding AM occurs either during the El Niño early onset year when the warm SSTA in June--August (JJA) is strong enough to force a low-level cyclonic flow anomaly in WNP and in December--February (DJF) the same warm SSTA forces a weak AM, or during the El Niño decaying phase when the persistence of the WNP anomalous anticyclone causes a weak WNPM and the transition of a warm to a cold episode causes a strong AM in DJF. The simultaneous negative correlation between WNPM and IM often appears either during the El Niño early onset years when the warm eastern Pacific SSTA induces the cyclonic wind shear that strengthens WNPM but suppresses convection over India, or during the El Niño decaying summer when a weak WNPM results from the persistence of the local anomalous anticyclone and a strong IM results from the El Niño-to-La Niña transition or a basin-wide Indian Ocean warming. [ABSTRACT FROM AUTHOR]

The Indian Ocean zonal dipole is a mode of variability in sea surface temperature that seriously affects the climate of many nations around the Indian Ocean rim, as well as the global climate system. It has been the subject of increasing research, and sometimes of scientific debate concerning its existence/nonexistence and dependence/independence on/from the El Niño–Southern Oscillation, since it was first clearly identified in Nature in 1999. Much of the debate occurred because people did not agree on what years are the El Niño or La Niña years, not to mention the newly defined years of the positive or negative dipole. A method that identifies when the positive or negative extrema of the El Niño–Southern Oscillation and Indian Ocean dipole occur is proposed, and this method is used to classify each year from 1876 to 1999. The method is statistical in nature, but has a strong basis on the oceanic physical mechanisms that control the variability of the near-equatorial Indo-Pacific basin. Early in the study it was found that some years could not be clearly classified due to strong decadal variation; these years also must be recognized, along with the reason for their ambiguity. The sensitivity of the classification of years is tested by calculating composite maps of the Indo-Pacific sea surface temperature anomaly and the probability of below median Australian rainfall for different categories of the El Niño–Indian Ocean relationship. [ABSTRACT FROM AUTHOR]