Ocean Warm Pool Research Articles

Emergence of decadal linkage between Western Australian coast and Western–central tropical Pacific

The impact of interbasin linkage on the weather/climate and ecosystems is significantly broader and profounder than that of only appearing in an individual basin. Here, we reveal that a decadal linkage of sea surface temperature (SST) has emerged between western Australian coast and western–central tropical Pacific since 1985, associated with continuous intensification of decadal variabilities (8–16 years). The rapid SST changes in both tropical Indian Ocean and Indo-Pacific warm pool in association to greenhouse gases and volcanoes are emerging factors resulting in enhanced decadal co-variabilities between these two regions since 1985. These SST changes induce enhanced convection variability over the Maritime Continent, leading to stronger easterlies in the western–central tropical Pacific during the warm phase off western Australian coast. The above changes bring about cooling in the western–central tropical Pacific and strengthened Leeuwin Current and anomalous cyclonic wind off western Australian coast, and ultimately resulting in enhanced coupling between these two regions. Our results suggest that enhanced decadal interbasin connections can offer further understanding of decadal changes under future warmer conditions.

Climate and sea-level fluctuations on the western coast of Bohai Bay during the 4.2 ka event: Multi-proxy evidence from the Nandagang area of Cangzhou, China

The 4.2 ka event was an important climatic event during the mid–late Holocene, with significant effects on the ecological environment and climatic evolution of different regions. Coastal wetlands, as a transitional ecosystem between land and sea, are sensitive to changes in climate and sea level; hence, research on coastal wetlands is important for reconstructing past climate and sea-level changes and for predicting future changes. However, little research has been conducted on the characteristics of the 4.2 ka event on the western coast of Bohai Bay in China. We analyzed the pollen assemblages (214 samples), grain size, and geochemical element contents of a mid–late Holocene sedimentary sequence from the Nandagang (NDG) wetland on Bohai Bay, with the chronology based on 13 AMS 14C dates. The results revealed the history of climate and sea-level changes on the western coast of Bohai Bay during the interval of 5000–3600 cal yr BP. High sedimentary arboreal pollen concentrations indicated that the NDG area had a relatively dense cover and humid climate during 5000–4450 cal yr BP. During the 380-year interval of 4450–4070 cal yr BP, corresponding to the 4.2 ka event, the decreased representation of arboreal pollen indicated a decrease in the regional forest cover and an arid climate. The internal structure of the 4.2 ka event recorded in core NDG showed a dry-wet-dry temporal structure during the intervals of 4450–4290 cal yr BP, 4290–4170 cal yr BP, and 4170–4070 cal yr BP, respectively, together with a relatively low sea level. Comparison of the relative sea level (RSL) record of the NDG wetland with similar records from other coastal regions in East Asia during the 4.2 ka event revealed contrasting trends. During 4070–3600 cal yr BP, the tree pollen concentration increased again, indicating that the forest vegetation coverage increased and the climate became wetter. Overall, our results suggest that the 4.2 ka event, as recorded in Bohai Bay, was related to the reduced influence of the western Pacific Ocean Warm Pool and the more frequent occurrence of the El Niño–Southern Oscillation.

Change and attribution of frost days and frost‐free periods in China

AbstractFrost events are a climatic phenomenon of great severity, exerting profound impacts on agricultural production and human endeavours. From 1961 to 2017, China experienced a reduction in frost days (FDs) and elongation of frost‐free periods (FFPs) across most regions. Many factors contribute to the variation of frost events, highlighting the urgent need to scrutinize the intrinsic factors driving these climatic frost events' inter‐annual and interdecadal fluctuations. This work analysed the temporal and spatial attributes of FDs and FFPs, using statistical methods to dissect daily minimum temperature data collected from ground observation stations from 1961 to 2017. The results reveal a declining trajectory in annual FDs and an ascending trend in FFPs. However, the interannual and interdecadal variations do not consistently align with the climatological course. The spatial distribution of the first four modes of EOF exhibits a persistent variation, north–south, monopole and east–west pattern for FDs and FFPs. In comparison, the modes of FFPs exhibit many complexities and diversities. Moreover, this study considers geographical location, altitude and proximity to the ocean, as well as atmospheric circulation factors, including circulation indices, ocean indices and other relevant indices, to elucidate their influence on frost events. In conjunction with the role of greenhouse gases in propelling global warming, we elucidate that the intensity of the Atlantic–European polar vortex, the area of the Indian Ocean warm pool, and the spatial positioning and extent of the subtropical high emerge as preeminent internal oscillations instigating interdecadal shifts in frost events. This study provides valuable scientific insights into the formation mechanisms and evolving patterns of frost events.

Intraseasonal and interannual variability of sea temperature in the Arabian Sea Warm Pool

Abstract. The Arabian Sea Warm Pool (ASWP) is a part of the Indian Ocean Warm Pool, formed in the Arabian Sea before the onset of the Indian summer monsoon. The ASWP has a significant impact on climate change in the Indian Peninsula and globally. In this study, we examined the intraseasonal and interannual variability of sea temperature in the ASWP using the latest Simple Ocean Data Assimilation (SODA) reanalysis dataset. We quantified the contributions of sea surface heat flux forcing, horizontal advection, and vertical entrainment to the sea temperature using the mixed-layer heat budget analysis method. We also used a lead–lag correlation method to examine the relationship between the interannual variability of the ASWP and various large-scale modes in the Indo-Pacific Ocean. We found that the ASWP formed in April and decayed in June; its formation and decay processes were asymmetrical, with the decay rate being twice as fast as the formation rate. During the ASWP development phase, the sea surface heat flux forcing had the largest impact on the mixed-layer temperature with a contribution of up to 85 %. Its impact was divided into the net surface heat flux (0.41–0.50 ∘C per 5 d) and the shortwave radiation loss penetrating the mixed layer (from −0.08 ∘C per 5 d to −0.17 ∘C per 5 d). During the decay phase, the cooling effect of the vertical entrainment on the temperature variation increased (from −0.05 ∘C per 5 d to −0.18 ∘C per 5 d) and dominated the temperature variation jointly with the sea surface heat flux forcing. We also found that the ASWP has strong interannual variability related to the basin warming of the Indian Ocean. The lead–lag correlation indicated that the ASWP had a good synchronous correlation with the Indian Ocean Dipole. The ASWP had the largest correlation coefficient at a lag of 5–7 months of the Niño3.4 index, showing the characteristics of modulation by the El Niño–Southern Oscillation (ENSO). When the El Niño (La Niña) event peaked in the winter of the previous year, the ASWP that occurred before the summer monsoon was more significant (insignificant) in the following year.

Climate Change and Potential Demise of the Indian Deserts

AbstractIn contrast to the “wet gets wetter and dry gets drier” paradigm, here, using observations and climate model simulations, we show that the mean rainfall over the semi‐arid northwest parts of India and Pakistan has increased by 10%–50% during 1901–2015 and is expected to increase by 50%–200% under moderate greenhouse gas (GHG) scenarios. The GHG forcing primarily drives the westward expansion of the Indian summer monsoon rainfall (ISMR) and is facilitated by a westward expansion of the Indian Ocean (IO) warm pool. Mechanistically, the westward expansion of ISMR is a consequence of the episodic genesis over IO and the northward propagation of an expanded Inter‐Tropical Convergence Zone on a sub‐seasonal time scale. While an adaptation strategy to increased hydrological disasters is a must, harnessing the augmented rainfall would lead to a substantial boost in food productivity, bringing transformative changes in the socio‐economic condition of people in the region.

Mid-Holocene expansion of the Indian Ocean warm pool documented in coral Sr/Ca records from Kenya

Proxy reconstructions suggest that mid-Holocene East African temperatures were warmer than today between 8 and 5 ka BP, but climate models cannot replicate this warming. Precessional forcing caused a shift of maximum insolation from boreal spring to fall in the mid-Holocene, which may have favored intense warming at the start of the warm season. Here, we use three Porites corals from Kenya that represent time windows from 6.55 to 5.87 ka BP to reconstruct past sea surface temperature (SST) seasonality from coral Sr/Ca ratios in the western Indian Ocean during the mid-Holocene. Although the Indian monsoon was reportedly stronger in the mid-Holocene, which should have amplified the seasonal cycle of SST in the western Indian Ocean, the corals suggest reduced seasonality (mean 3.2 °C) compared to the modern record (mean 4.3 °C). Warming in austral spring is followed by a prolonged period of warm SSTs, suggesting that an upper limit of tropical SSTs under mid-Holocene conditions was reached at the start of the warm season, and SSTs then remained stable. Similar changes are seen at the Seychelles. Bootstrap estimates suggest a reduction in SST seasonality of 1.3 ± 0.22 °C at Kenya and 1.7 ± 0.32 °C at the Seychelles. SST seasonality at Kenya corresponds to present-day SST seasonality at 55° E–60° E, while SST seasonality at the Seychelles corresponds to present day SST seasonality at ~ 65° E. This implies a significant westward expansion of the Indian Ocean warm pool. Furthermore, the coral data suggests that SST seasonality deviates from seasonal changes in orbital insolation due to ocean–atmosphere interactions.

Differential expansion speeds of Indo-Pacific warm pool and deep convection favoring pool under greenhouse warming

The Indo-Pacific warm pool (IPWP), which affects the global climate system through supporting tropical convection, has been reported to expand significantly under greenhouse warming. Although early research revealed that the sea surface temperature (SST) threshold for deep convection (σconv) increases with global warming, many latest relevant works were still conducted based on the traditional IPWP definition (e.g., static SST = 28 °C threshold, and is referred to as the oceanic warm pool, OWP28). Here, we claim that the OWP28 expansion differs from the deep convection favoring pool (DCFP) area change and may not reflect the direct impacts of Indo-Pacific warming on the climate system. Results show that, because of the long-term increase in σconv, the DCFP expands at a rate 2.6 times slower than the OWP28 from 1979 to 2020. The difference reaches 12–27 times from 2015–2100 under different emission scenarios, based on CMIP6 model simulations. While the OWP28 expands to the eastern Pacific, the DCFP will remain within the Indian Ocean and western Pacific Ocean regardless of emission levels. This study emphasizes the necessity of considering the response of the relationship between deep convection and SST to climate change when studying the long-term variability of the IPWP.

Changes in Extreme Precipitation on the Tibetan Plateau and Its Surroundings: Trends, Patterns, and Relationship with Ocean Oscillation Factors

The Tibetan Plateau is among the region’s most sensitive areas to global climate change. The observation data from 113 meteorological stations on the Tibetan Plateau and surrounding regions in China for 1971–2017 were used to analyze the periodic oscillations and trends in precipitation and extreme precipitation on multiple time scales to ensemble empirical mode decomposition. The relationship between extreme precipitation and sea-surface temperature (SST) anomalies was also explored. The results were as follows. (1) The timing of extreme-precipitation events in the highlands is consistent, with increased total precipitation and increased frequency, intensity, and extreme values of extreme precipitation. (2) Changes in temperature and precipitation are not completely synchronized. The total extreme precipitation, number of extreme-precipitation days, maximum single-day precipitation, and extreme single-day precipitation intensity all showed increases with fluctuations; the quasi-3-year oscillation contributes the most to the extreme precipitation. PRCPTOT is most strongly correlated with R10 and R95p. (3) The spatiotemporal patterns of the first and second empirical orthogonal function modes of the indices differed significantly and were not spatiotemporally uniform, but exhibited local clustering. (4) The Indian Ocean Warm Pool Strength and Western Pacific Warm Pool Strength indices were most highly correlated with each extreme-precipitation index, and the timings of the extreme-precipitation events lagged behind those of the SST anomalies. This study improves our understanding of extreme precipitation events in the context of climate warming and provides a basic analysis for the further assessment and prediction of extreme precipitation on the Tibetan Plateau and the surrounding ecologically fragile areas.

Indo-Pacific warm pool present warming attribution and future projection constraint

The Indo-Pacific warm pool (IPWP) has warmed and expanded substantially over the past decades, which has significantly affected the hydrological cycle and global climate system. It is unclear how the IPWP will change in the future under anthropogenic (ANT) forcing. Here, we quantify the human contribution to the observed IPWP warming/expansion and adjust the projected IPWP changes using an optimal fingerprinting method based on Coupled Model Intercomparison Project phase 6 (CMIP6) simulations. We find that more than 95% rapid warming and 85% expansion of the observed IPWP are detected and attributable to human influence. Furthermore, human activities affect IPWP warming through both greenhouse gases and ANT aerosols. The multiple model ensemble mean can capture the ANT warming trend and tends to underestimate the ANT warming trend. After using the observation constraint, the IPWP warming is projected to increase faster than that of the ensemble mean in the 21st Century, and the Indian Ocean warm pool is projected to expand more than previously expected. The rapid warming and expansion of IPWP over the rest of the 21st century will impact the climate system and the life of human beings.

Warming of the Indian Ocean and its impact on temporal and spatial dynamics of primary production

The Indian Ocean, the third largest among the world’s oceans, is experiencing unprecedented changes in sea surface temperature (SST). We present temporal and spatial dynamics of phytoplankton and their response to warming in the Indian Ocean (∼25°N to 30°S) during 1998–2019 using remote sensing data. Our study revealed that the area of the Indian Ocean Warm Pool (IOWP), defined as waters with SST values >28 °C, is significantly expanding in most regions, particularly in the most recent decade. The increase in IOWP area was greatest (∼74%) in the south-central basin. Furthermore, SST increased significantly in most areas of the Indian Ocean (10 out of 11 regions explored) over the 22-year study period with the highest increase of 0.7 °C observed in the south-central regions. Most other regions showed an average increase in temperature of 0.4–0.5 °C. At the same time, net primary production (NPP) showed large interannual variability in northern and central regions of the Indian Ocean, with slightly decreasing trends in a few northern regions. Overall, years of the first decade (1998–2008) showed more often cooler temperatures and higher productivity, except for a few years, whereas years of the last decade (2009–2019) showed more often warmer temperature and lower productivity, except in very recent years (2017–2019) when productivity was high. Mean Chl a concentrations increased in the last decade during the northeast monsoon period in the northwestern regions, suggesting increased NPP in December to March period as a future scenario in this highly productive area of the Indian Ocean. We also observed increasing SST in several major upwelling areas during the study period, whereas Chl a showed high interannual variability with no marked significant trends in most areas. Results from this study corroborate the importance of the southwest monsoon as a key driver of seasonal patterns in Chl a in major upwelling areas of the Indian Ocean.

Were springline carbonates in the Kurkur–Dungul area (southern Egypt) deposited during glacial periods?

The tufa deposits in the Kurkur–Dungul area, southern Egypt, date from marine isotope stage (MIS) 11 to MIS 1. Springs across the region were active during glacial periods (with sea-level below –50 m), reflecting changed atmospheric circulation over the Indian Ocean, as well as peak interglacial periods. During times of low sea-level, reduced Indonesian throughflow promoted formation of an Indian Ocean Warm Pool, and anomalous rainfall on its western margin. We suggest that Egypt lies at the intersection of westerly (‘maghrebian’) and easterly (‘mashriqian’) rainfall provinces, which show different timing with relation to orbital forcing and different source water regions. Tufa-growth periods are therefore not mechanistically linked to ‘humid periods’ or ‘sapropel events’ identified elsewhere. Stable isotope and T (Δ 47 ) data are also inconsistent with these spring systems being part of a larger system spanning northern Africa, and lack a clear interaction between northern hemisphere heating and mid-latitude rainfall. We also follow previous researchers in concluding that formation of springline deposit formation was probably delayed compared with rainfall, owing to aquifer flow distances. This delay is unlikely to be sufficient to explain why rainfall is out of phase with movements of the monsoon belts, but may complicate interpretation of these records. Supplementary material: A lithofacies description and supplementary figures and tables are available at https://doi.org/10.6084/m9.figshare.c.5246661

Why Indian summer monsoon circulation indices? Fidelity in representing rainfall variability and teleconnections

AbstractWith the recognition that the circulation is strongly coupled with the Indian summer monsoon rainfall (ISMR), a number of large‐scale circulation indices have been proposed to represent the Indian summer monsoon to aid diagnosis of teleconnections with ISMR and evaluation of simulations by climate models. The circulation indices, however, correlate poorly with ISMR raising doubts on the coupling between the two. In this study, we identify reasons behind the poor correlations and estimate a potential limit on this correlation between circulation indices and ISMR. One mechanistic reason for poor correlations between circulation indices and ISMR is that the period and variance explained by the oscillatory modes of circulation indices differ significantly from those of ISMR leading to their phases also differing between modes of circulation indices and corresponding modes of ISMR. Further, the wind anomaly patterns associated with wind indices project strongly on the climatological monsoon circulation unlike that associated with the ISMR, another reason for their poor correlation with ISMR. As a result, all circulation indices underestimate the negative correlation between June–September sea‐surface temperature (JJAS SST) over the Niño3.4 and ISMR and fail to simulate the lead–lag relationship between the two. A conservative estimate of the interannual variance explained by coupling of the JJAS rainfall over continental India and winds in the neighbourhood is strong, explaining about 55% variance distributed in three leading coupled modes. Therefore, the effectiveness of any circulation indices in explaining the ISMR variance is potentially limited to that associated with the dominant mode (∼30%). The local response of winds to deep convection over the waters of the Indian Ocean and western Pacific warm pool leads to a strong increasing trend in some of the circulation indices and adds “noise” modes contributing to the de‐correlation between circulation indices and ISMR.

MJO induced diurnal sea surface temperature variations off the northwest shelf of Australia observed from Himawari geostationary satellite

High frequency coupling between the tropical upper ocean and atmosphere at diurnal time scales is being recognised to be important for the behaviour of large scale climate modes such as the Madden Julian Oscillation (MJO). Sea surface temperature (SST) variability in the Eastern Indian Ocean warm pool and on the Northwest Shelf of Australia is strongly influenced by the MJO activity. In this study, we use the hourly Himawari-8 geostationary satellite SST data to understand the modulation of the amplitude of SST diurnal variations in the region by the MJO during two austral summer seasons of 2015–16 and 2016–17. The high temporal and spatial resolutions of the Himawari-8 SST data are able to capture the temporal and spatial variability of diurnal SST amplitude. Our results show that the MJO onset (active phase) is preceded by strong diurnal SST variations during the MJO suppressed phase. The amplitudes of the SST diurnal variations are mainly modulated by surface wind variability in the region, as denoted by their spatial and temporal correlations. The very strong El Niño event in 2015–16 may have significantly weakened the MJO activities in the region, resulting in prolonged MJO suppressed phase, high diurnal SST variation and mean SST. This study provides background knowledge for future research on how the diurnal SST variation in the region may influence the progression of the MJO events.

Coastal Upwelling Events, Salinity Stratification, and Barrier Layer Observed Along the Southwestern Coast of Sumatra

AbstractCoastal upwelling along the southwestern coast of Sumatra is a seasonal upwelling that occurs in areas of high sea surface temperature and abundant precipitation at the southeastern edge of the Indian Ocean warm pool. Based on observations from two Argo floats that drifted and stayed around Sumatra, we investigated ocean temperature and salinity variations during several coastal upwelling events observed in 2013–2017. The Argo floats observed the vertical structure of temperature and salinity every 10 days within 100 km from the southwestern coast of Sumatra. The observation data show intraseasonal‐scale subsurface temperature cooling events with significant upward displacements of the thermocline and high‐salinity water, led by anomalous local southesterly winds and equatorial easterly winds. During the coastal upwelling events, salinity stratification and a thick barrier layer related to local precipitation were also observed. Surface mixed layer temperature cooling were relatively small in contrast to the significant subsurface anomalies. It was found that during the coastal upwelling events, subsurface cold‐water upwelling signals did not necessarily reach the mixed layer when salinity stratification and a thick barrier layer were present. The implications of these observational results for understanding the local atmosphere ocean interaction, and hence the occurrence of the Indian Ocean Dipole, are discussed.

Seasonal and Interannual Variability of the Indo-Pacific Warm Pool and its Associated Climate Factors Based on Remote Sensing

With satellite observed Sea Surface Temperature (SST) accumulated for multiple decades, multi-time scale variabilities of the Indo-Pacific Warm Pool are examined and contrasted in this study by separating it into the Indian Ocean sector and the Pacific Ocean sector. Surface size, zonal center, meridional center, maximum SST and mean SST as the practical warm pool properties are chosen to investigate the warm pool variations for the period 1982–2018. On the seasonal time scale, the oscillation of the Indian Warm Pool is found much more vigorous than the Pacific Warm Pool on size and intensity, yet the interannual variabilities of the Indian Warm Pool and the Pacific Warm Pool are comparable. The Indian Warm Pool has weak interannual variations (3–5 years) and the Pacific Warm Pool has mighty interdecadal variations. The size, zonal movement and mean SST of the Indian Ocean Warm Pool (IW) are more accurate to depict the seasonal variability, and for the Pacific Ocean Warm Pool (PW), the size, zonal and meridional movements and maximum SST are more suitable. On the interannual scale, except for the meridional movements, all the other properties of the same basin have similar interannual variation signals. Following the correlation analysis, it turns out that the Indian Ocean basin-wide index (IOBW) is the most important contributor to the variabilities of both sectors. Lead-lag correlation results show that variation of the Pacific Ocean Warm Pool leads the IOBW and variation of the Indian Ocean Warm Pool is synchronous with the IOBW. This indicates that both sectors of the Indo-Pacific Warm Pool are significantly correlated with basin-wide warming or cooling.

The Role of Oceanic Processes in the Initiation of Boreal Winter Intraseasonal Oscillations Over the Indian Ocean

AbstractObservational analyses and ocean general circulation model experiments were performed to understand the influence of oceanic processes on warm sea surface temperature anomalies (SSTAs) prior to convection initiation of boreal winter intraseasonal oscillations (ISOs), including the Madden‐Julian Oscillation (MJO), in the tropical Indian Ocean. We found 39 strong ISOs that passed over the Indian Ocean Warm Pool (WP) region during the November–April season of the 2001–2012 period. Seventeen of them (44%) initiated over the Seychelles‐Chagos Thermocline Ridge (SCTR) before propagating eastward to the WP and Maritime Continent. Including only global‐scale MJOs, 71% (24%) of the remaining MJO events initiated from the SCTR (WP). Four (seven) SCTR (WP) ISO events were preceded by SSTAs that were strongly influenced by wind stress‐driven oceanic processes; all four (2/7) SCTR (WP) events were MJOs. Composite analyses of the four oceanic process influenced SCTR MJO events showed that wind stress‐driven oceanic processes contributed about 30% (between 0.1 and 0.2 °C) of the preconvection warming in the SCTR, which coincides with the Intertropical Convergence Zone with mean SST > 28 °C. Reduced upwelling and entrainment played a more important role than horizontal advection in elevating the +SSTAs. Case studies revealed that the two ocean process‐influenced primary MJO SCTR events were also associated with oceanic equatorial Rossby waves. Furthermore, results of a linear atmospheric mixed layer model indicated that boundary layer processes related to SSTAs contributed approximately half of the total convergence during MJO initiation, relative to the effect of the free troposphere, for the Rossby wave‐influenced events.

Temporal and Spatial Characteristics of Summer Extreme Precipitation in Eastern China and Possible Causalities

In the past decades, with the increasing frequency of extreme weather and climate events, the world has suffered huge losses. Based on NCEP/NCAR reanalysis data and China regional precipitation data provided by China Meteorological Administration, the extreme precipitation events in eastern China are defined by relative threshold method, and the temporal and spatial characteristics of summer extreme precipitation in eastern China from 1961 to 2016 are analyzed by empirical orthogonal function (EOF), and the reverse distribution of extreme precipitation in the middle and lower reaches of the Yangtze River and south China by Indian Ocean warm pool is revealed influence. The results show that the total amount and frequency of extreme precipitation in summer are concentrated in the Yangtze River Basin and south China. EOF1 decomposition of extreme precipitation reflects the interannual oscillation characteristics of reverse spatial distribution in the Yangtze River Basin and south China. The time series corresponding to EOF1 has significant interannual characteristics. The Pacific-Japan (PJ) teleconnection pattern is a circulation system that significantly affects the spatial-temporal pattern of extreme precipitation in southern China. When the PJ pattern is in the positive phase, the anticyclone controls the south China region, and restrains the convective activity, which results in the decrease of extreme precipitation. The anomalous southwest wind to the south of 30&#176N and the anomalous northerly wind to the north of 30&#176N converge in the middle and lower reaches of the Yangtze River. Combining with the sufficient water vapor carried by the anomalous southwest airflow at the edge of anticyclone, it is more conducive to the formation of extreme precipitation. The east propagating Kelvin wave in the warm pool of the Indian Ocean is an important reason for the formation of the PJ pattern and finally the formation of extreme precipitation anomalies in China.

Thermodynamic and dynamic effects of increased moisture sources over the Tropical Indian Ocean in recent decades

In the present work, the mechanisms for the changes in moisture sources (evaporation minus precipitation; EmP) during boreal summer (May–September) are explored over the tropical Indian Ocean during 1979–2016. We apply a moisture budget analysis to quantify the thermodynamic and dynamic effects. Our results show that the EmP in the tropical central-eastern and southwestern Indian Oceans experienced significant increasing trends during boreal summer. The increased EmP in the tropical central-eastern Indian Ocean is due to the enhanced dynamic divergence (account for approximately 51%), while a stronger dynamic advection contributes more moisture supply to the southwestern Indian Ocean (account for approximately 34%). We find that during recent decades, the enhanced east–west thermal gradient in the Pacific strengthens the Walker Circulation, which leads to a westward shift in convection over the Indian Ocean warm pool, resulting in weakened convection and ascent over the tropical central-eastern Indian Ocean. The weakened convection leads to an anomalous low-level atmospheric divergent circulation, which intensifies the dynamic divergence contributing to the enhanced EmP over the tropical central-eastern Indian Ocean. Additionally, the warming climate during recent decades also increases the land–sea thermal contrast in the vicinity of the Indian Ocean, which enhances the southeastern wind in the low-level troposphere and leads to an enhanced EmP over the southwestern Indian Ocean.

Stochastic Parameterization of Subgrid-Scale Velocity Enhancement of Sea Surface Fluxes

Abstract Subgrid-scale (SGS) velocity variations result in gridscale sea surface flux enhancements that must be parameterized in weather and climate models. Traditional parameterizations are deterministic in that they assign a unique value of the SGS velocity flux enhancement to any given configuration of the resolved state. In this study, we assess the statistics of SGS velocity flux enhancement over a range of averaging scales (as a proxy for varying model resolution) through systematic coarse-graining of a convection-permitting atmospheric model simulation over the Indian Ocean and west Pacific warm pool. Conditioning the statistics of the SGS velocity flux enhancement on 1) the fluxes associated with the resolved winds and 2) the precipitation rate, we find that the lack of a separation between “resolved” and “unresolved” scales results in a distribution of flux enhancements for each configuration of the resolved state. That is, the SGS velocity flux enhancement should be represented stochastically rather than deterministically. The spatial and temporal statistics of the SGS velocity flux enhancement are investigated by using basic descriptive statistics and through a fit to an anisotropic space–time covariance structure. Potential spatial inhomogeneities of the statistics of the SGS velocity flux enhancement are investigated through regional analysis, although because of the relatively short duration of the simulation (9 days) distinguishing true inhomogeneity from sampling variability is difficult. Perspectives for the implementation of such a stochastic parameterization in weather and climate models are discussed.

Seasonal variations in size and intensity of the Indo-western Pacific warm pool in different sectors

The present study investigates the roles of solar radiation and monsoon wind in annual and semiannual variations of size and intensity of the Indo-western Pacific warm pool. The Indo-western Pacific warm pool is divided into four sectors with the equator and 100°E (north of the equator) and 135°E (south of the equator) longitude as the demarcations. These sectors display different seasonal variations in size and intensity and different roles of surface heat flux and oceanic processes. The size of the North Indian Ocean warm pool displays a semiannual variation with two peaks in May and October, while that in the other three sectors shows an annual variation. These features are attributable to a dominant role of solar radiation in the four sectors and an additional effect of the Asian monsoon in the North Indian Ocean sector. The intensity of the warm pool displays semianunal variations, with two peaks in May and August in the North Indian Ocean sector, April and November in the South Indian Ocean sector, May–June and September in the North Pacific Ocean sector and December and April in the South Pacific Ocean sector. The semiannual feature is associated with two effects of monsoon winds: wind-induced ocean advection and convection-related cloud-radiation changes. These two changes induce a decrease of sea surface temperature during the monsoon period, leading to the formation of semiannual variations in the size and intensity of the warm pool.