Warm Pool Convection Research Articles

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.

Tropical Pacific cold tongue mode triggered by enhanced warm pool convection due to global warming

A cold tongue mode (CTM) formed in the 1980s as a La Niña-like stepwise response to recent global warming; however, a consensus has not been reached on the mechanism underlying the CTM formation. Here, we attribute the CTM to the enhanced deep convection of the warm pool regions over the western Pacific and south of North America. Increases in the sea surface temperatures in the two Pacific warm pool regions that occurred due to global warming exceeded the threshold of deep convection after the 1980s, which resulted in two opposite anomalous vertical circular circulation patterns and induced the CTM via the intensification, contraction, and westward shift of the Walker circulation and the uplift of the thermocline. Our results provide a novel explanation of the La Niña-like response under recent global warming.

A Mechanism for the Midwinter Minimum in North Pacific Storm‐Track Intensity From a Global Perspective

AbstractThe midwinter minimum in North Pacific storm‐track intensity is a perplexing phenomenon because the associated local baroclinity in the North Pacific is maximum during midwinter. Here, a new mechanism is proposed wherein the midwinter minimum occurs in part because global planetary‐scale waves consume the zonal available potential energy, limiting its availability for storm‐track eddy growth. During strong midwinter suppression years, the midwinter minimum is preceded by anomalously large planetary‐scale eddy kinetic energy and subsequent reduction in zonal available potential energy and global baroclinity. Consistent with previous studies, this large planetary‐scale eddy kinetic energy takes place after enhanced Pacific warm pool convection, which peaks during winter. These results indicate that the midwinter minimum is in part caused by heightened warm pool convection, which, through excitation of planetary‐scale waves, leads to a weaker storm‐track. This finding also helps explain the existence of the midwinter North Atlantic storm‐track minimum.

Vegetation evolution-based hydrological climate history since LGM in southern South China Sea

Abstract The reconstruction of vegetation evolution and climate changes at low latitudes since the Last Glacial Maximum (LGM) remains incomplete. In this study, we reconstructed the palynological history of vegetation changes since the LGM using a sediment core collected from the slope in the southern South China Sea (SCS). The chronology of the core was based on eight AMS14C dates, and 85 stratigraphic samples were palynologically analyzed. Our results denote that the vegetation reconstructed during the LGM in the southern SCS was characterized by a relatively high percentage of herb pollen and a modest decrease in the number of fern spores, suggesting a temperature decline and humidity change in this region. Further, the preservation of a rich diversity of pollen from the tropical lowland rainforest indicated that the rainforests might have covered large areas of the exposed Sunda Shelf during the LGM. The dynamic concentration of the Pinus pollen in the core, which is a proxy for the East Asian winter monsoon (EAWM), satisfactorily matched the changes in the Atlantic meridional overturning circulation and Australian-Indonesian summer monsoon, especially during the Heinrich Stadial 1. This suggests that the climate at the high latitudes of the Northern Hemisphere can drive the Intertropical Convergence Zone (ITCZ) southwards based on the enhanced EAWM. The southward-shifting ITCZ may change the hydrological climate of the equator and the Southern Hemisphere. The pteridophyte spores, which are good regional precipitation indicators, were positively correlated with the warm pool convection and inversely related with the EAWM, possibly indicating the considerable impact of the high-latitude climate in the Northern Hemisphere on precipitation in the tropical regions. Our results indicate that during the last deglaciation, the tropical hydrological changes were relatively moderate when compared with the drastic change observed at high latitudes and were attributed to the regulation of the El Nino-Southern Oscillation-like state. Our sequence of pollen and spores from the southern SCS can be used to observe the detailed dynamics of climatic evolution in tropical regions. The results of this study are essential to understand the complicated climate processes and the relation between high and low latitudes.

Decadal Variability of the Indian and Pacific Walker Cells since the 1960s: Do They Covary on Decadal Time Scales?

Previous studies have investigated the centennial and multidecadal trends of the Pacific and Indian Ocean Walker cells (WCs) during the past century, but have obtained no consensus owing to data uncertainties and weak signals of the long-term trends. This paper focuses on decadal variability (periods of one to few decades) by first documenting the variability of the WCs and warm-pool convection, and their covariability since the 1960s, using in situ and satellite observations and reanalysis products. The causes for the variability and covariability are then explored using a Bayesian dynamic linear model, which can extract nonstationary effects of climate modes. The warm-pool convection exhibits apparent decadal variability, generally covarying with the Indian and Pacific Ocean WCs during winter (November–April) with enhanced convection corresponding to intensified WCs, and the Indian–Pacific WCs covary. During summer (May–October), the warm-pool convection still highly covaries with the Pacific WC but does not covary with the Indian Ocean WC, and the Indian–Pacific WCs are uncorrelated. The wintertime coherent variability results from the vital influence of ENSO decadal variation, which reduces warm-pool convection and weakens the WCs during El Niño–like conditions. During summer, while ENSO decadal variability still dominates the Pacific WC, decadal variations of ENSO, the Indian Ocean dipole, Indian summer monsoon convection, and tropical Indian Ocean SST have comparable effects on the Indian Ocean WC overall, with monsoon convection having the largest effect since the 1990s. The complex causes for the Indian Ocean WC during summer result in its poor covariability with the Pacific WC and warm-pool convection.

Intraseasonal variation of the East Asian summer monsoon in La Niña years

Based on the composite result of six major La Niña events during 1979–2012, the authors reveal the intraseasonal variation of the East Asian summer monsoon (EASM) and summer rainfall in East Asia in La Niña years. Due to a higher SST over the western Pacific warm pool in the proceeding winter and spring, warm pool convection in summer is enhanced, leading to a cyclonic anomaly in the subtropical western Pacific. As a result, the western Pacific subtropical high is located more northeastward, and the seasonal march in East Asia is thus accelerated. This anomalous pattern tends to change with the seasonal march, with a maximum anomaly in July. Besides, there is less Mei-yu rainfall in the Yangtze River basin, with an earlier start and termination. The rainfall distribution in East Asia during La Niña years is characterized by a zonal pattern of less rainfall in eastern China and more rainfall over the oceanic region of the western Pacific. By comparison, a meridional pattern is found during El Niño years, with less rainfall in the tropics and more rainfall in the subtropics and midlatitudes. Therefore, the influence of La Niña on the EASM cannot be simply attributed to an anti-symmetric influence of El Niño.

Anomalous western Pacific subtropical high during late summer in weak La Niña years: Contrast between 1981 and 2013

Both 1981 and 2013 were weak La Nina years with a similar sea surface temperature (SST) anomaly in the tropical Pacific, yet the western Pacific subtropical high (WPSH) during August exhibited an opposite anomaly in the two years. A comparison indicates that, in the absence of a strong SST anomaly in the tropics, the cold advection from Eurasian high latitudes and the convection of the western Pacific warm pool play important roles in influencing the strength and position of the WPSH in August. In August 1981, the spatial pattern of 500 hPa geopotential height was characterized by a meridional circulation with a strong ridge in the Ural Mountains and a deep trough in Siberia, which provided favorable conditions for cold air invading into the lower latitudes. Accordingly, the geopotential height to the north of the WPSH was reduced by the cold advection anomaly from high latitudes, resulting in an eastward retreat of the WPSH. Moreover, an anomalous cyclonic circulation in the subtropical western Pacific, excited by enhanced warm pool convection, also contributed to the eastward retreat of the WPSH. By contrast, the influence from high latitudes was relatively weak in August 2013 due to a zonal circulation pattern over Eurasia, and the anomalous anticyclonic circulation induced by suppressed warm pool convection also facilitated the westward extension of the WPSH. Therefore, the combined effects of the high latitude and tropical circulations may contribute a persistent anomaly of the WPSH in late summer, despite the tropical SST anomaly being weak.

A high-resolution speleothem record of western equatorial Pacific rainfall: Implications for Holocene ENSO evolution

Abstract The El Nino-Southern Oscillation (ENSO) is the primary driver of interannual climate variability in the tropics and subtropics. Despite substantial progress in understanding ocean–atmosphere feedbacks that drive ENSO today, relatively little is known about its behavior on centennial and longer timescales. Paleoclimate records from lakes, corals, molluscs and deep-sea sediments generally suggest that ENSO variability was weaker during the mid-Holocene (4–6 kyr BP) than the late Holocene (0–4 kyr BP). However, discrepancies amongst the records preclude a clear timeline of Holocene ENSO evolution and therefore the attribution of ENSO variability to specific climate forcing mechanisms. Here we present δ 18 O results from a U–Th dated speleothem in Malaysian Borneo sampled at sub-annual resolution. The δ 18 O of Borneo rainfall is a robust proxy of regional convective intensity and precipitation amount, both of which are directly influenced by ENSO activity. Our estimates of stalagmite δ 18 O variance at ENSO periods (2–7 yr) show a significant reduction in interannual variability during the mid-Holocene (3240–3380 and 5160–5230 yr BP) relative to both the late Holocene (2390–2590 yr BP) and early Holocene (6590–6730 yr BP). The Borneo results are therefore inconsistent with lacustrine records of ENSO from the eastern equatorial Pacific that show little or no ENSO variance during the early Holocene. Instead, our results support coral, mollusc and foraminiferal records from the central and eastern equatorial Pacific that show a mid-Holocene minimum in ENSO variance. Reduced mid-Holocene interannual δ 18 O variability in Borneo coincides with an overall minimum in mean δ 18 O from 3.5 to 5.5 kyr BP. Persistent warm pool convection would tend to enhance the Walker circulation during the mid-Holocene, which likely contributed to reduced ENSO variance during this period. This finding implies that both convective intensity and interannual variability in Borneo are driven by coupled air-sea dynamics that are sensitive to precessional insolation forcing. Isolating the exact mechanisms that drive long-term ENSO evolution will require additional high-resolution paleoclimatic reconstructions and further investigation of Holocene tropical climate evolution using coupled climate models.

Stationary Wave Interference and Its Relation to Tropical Convection and Arctic Warming

Abstract The interference between transient eddies and climatological stationary eddies in the Northern Hemisphere is investigated. The amplitude and sign of the interference is represented by the stationary wave index (SWI), which is calculated by projecting the daily 300-hPa streamfunction anomaly field onto the 300-hPa climatological stationary wave. ERA-Interim data for the years 1979 to 2013 are used. The amplitude of the interference peaks during boreal winter. The evolution of outgoing longwave radiation, Arctic temperature, 300-hPa streamfunction, 10-hPa zonal wind, Arctic sea ice concentration, and the Arctic Oscillation (AO) index are examined for days of large SWI values during the winter. Constructive interference during winter tends to occur about one week after enhanced warm pool convection and is followed by an increase in Arctic surface air temperature along with a reduction of sea ice in the Barents and Kara Seas. The warming of the Arctic does occur without prior warm pool convection, but it is enhanced and prolonged when constructive interference occurs in concert with enhanced warm pool convection. This is followed two weeks later by a weakening of the stratospheric polar vortex and a decline of the AO. All of these associations are reversed in the case of destructive interference. Potential climate change implications are briefly discussed.

The ocean-atmosphere response to wind-induced thermocline changes in the tropical South Western Indian Ocean

In the Indian Ocean basin the sea surface temperatures (SSTs) are most sensitive to changes in the oceanic depth of the thermocline in the region of the Seychelles Dome. Observational studies have suggested that the strong SST variations in this region influence the atmospheric evolution around the basin, while its impact could extend far into the Pacific and the extra-tropics. Here we study the adjustments of the coupled atmosphere-ocean system to a winter shallow doming event using dedicated ensemble simulations with the state-of-the-art EC-Earth climate model. The doming creates an equatorial Kelvin wave and a pair of westward moving Rossby waves, leading to higher SST 1–2 months later in the Western equatorial Indian Ocean. Atmospheric convection is strengthened and the Walker circulation responds with reduced convection over Indonesia and cooling of the SST in that region. The Pacific warm pool convection shifts eastward and an oceanic Kelvin wave is triggered at thermocline depth. The wave leads to an SST warming in the East Equatorial Pacific 5–6 months after the initiation of the Seychelles Dome event. The atmosphere responds to this warming with weak anomalous atmospheric convection. The changes in the upper tropospheric divergence in this sequence of events create large-scale Rossby waves that propagate away from the tropics along the atmospheric waveguides. We suggest to repeat these types of experiments with other models to test the robustness of the results. We also suggest to create the doming event in June so that the East-Pacific warming occurs in November when the atmosphere is most sensitive to SST anomalies and El Nino could possibly be triggered by the doming event under suitable conditions.

A Comparative Study on Precipitation Climatology and Interannual Variability in the Lower Midlatitude East Asia and Central Asia

Abstract This study examines precipitation climatology and interannual variability in two regions in the lower midlatitude Asia to the east and west of the Tibetan Plateau, one located in monsoonal East Asia (the M region) and the other in semiarid central Asia (the W region). The focus is on the 5-month summer half year (May–September) for the M region and the winter half year (December–April) for the W region, corresponding to their respective rainy seasons. The main mechanism of moisture transport for the M region is the summer lower-tropospheric southerly winds, whereas the winter midtropospheric westerly circulation between 25° and 45°N is responsible for conducting moisture fluxes to the W region. It is further discovered that the winter precipitation series are positively correlated between the two regions (r = 0.47). There is also a weak cross-seasonal correlation between the winter W region precipitation and summer M region precipitation (r = 0.27). Winter westerly circulation over the W region is influenced by both the east Atlantic–western Russia and the polar–Eurasia extratropical teleconnection patterns, while El Niño–Southern Oscillation influences regional circulation patterns in both regions through teleconnections via the Indo-Pacific warm pool convection in winter and its lagged impact on the western North Pacific anticyclone over the Philippine Sea. In the meantime, responses of the regional winter circulation in the M region to the upstream westerly circulation intensity cause the correlation in winter precipitation between the two regions. Such linkages form the basis of the concurrent and cross-seasonal correlations in precipitation between the two remote regions.

Indo‐Pacific Warm Pool convection and ENSO since 1867 derived from Cambodian pine tree cellulose oxygen isotopes

The Indo‐Pacific Warm Pool (IPWP) is a major source of heat and moisture to the atmosphere and thus strongly influences the global climate. In this study, we investigate how moisture fluxes from the IPWP influence the stable isotope signature of precipitation over Southeast Asia by analyzing the oxygen isotopic composition (δ18O) of tree cellulose from southern Cambodia. The cellulose δ18O record, spanning AD 1867–2006, documents a regular seasonal cycle with an average amplitude of ∼4‰ that is primarily controlled by seasonal differences in the isotopic composition of precipitation. Using the outputs from an isotope‐enabled atmospheric model, we illustrate how theδ18O of precipitation at our site is predominantly controlled by the amount of rainout that occurs over the moisture source region, the IPWP. This is verified by strong correlations of our cellulose δ18O record with instrumental measurements of precipitation and outgoing longwave radiation over the IPWP, suggesting that the cellulose δ18O could be used to reconstruct the convection intensity over the IPWP. Spectral analysis of the cellulose δ18O reveals significant peaks at 2–7 years corresponding to ENSO frequencies. The variability of the cellulose δ18O record on the ENSO band exhibits characteristics that match existing coral δ18O records from the tropical Pacific, with reduced amplitude of variability in the 1920s through the 1960s, a period of weak ENSO activity.

Satellite and surface observations of Nauru Island clouds: Differences between El Nino and La Nina periods

Island cloud trails represent only a small fraction of ocean boundary layer clouds. However, they can be an important key to understanding how marine boundary layer clouds respond to perturbation. During the La Niña period of 1999–2001 the island cloud of Nauru demonstrated daytime persistence over 100 kilometers. Our results show that boundary layer clouds over the eastern margin of the warm pool are affected by regional subsidence and lack of CCN (Cloud Condensation Nuclei) during this active warm pool convection phase of the ENSO (El Nino Southern Oscillation). The increased persistence of boundary layer clouds during the day, as reflected by the island cloud trails, combined with the dissipation of boundary layer clouds at night contribute to cooling the ocean surface. During La Nina, boundary layer cloudiness around Nauru decreased by a factor of 1.9 and SSTs decreased by about 3°C.

Intraseasonal Variability over Tropical Africa during Northern Summer

The intraseasonal variability over Africa during northern summer was analyzed, using 25 years of NCEP– NCAR reanalysis and satellite data. The dominant pattern of variability was one of enhanced deep convection over the whole African monsoon region. It appeared to arise at least partly as a remote response to the intraseasonal (Madden–Julian) oscillation over the warm pool sector. Twenty days prior to the maximum in convection over Africa, there was no signal over Africa but convection was reduced over the equatorial warm pool. An equatorial Kelvin wave response to this change in warm pool convection propagated eastward and an equatorial Rossby wave response propagated westward and between them they completed a circuit of the equator and met up 20 days later over Africa, where the negative midtropospheric temperature anomalies in the Kelvin and Rossby waves favored deep convection. Over West Africa, the Kelvin wave component contained lower-tropospheric westerly anomalies that acted to increase the boundary layer monsoon flow and moisture supply. The westerly anomalies also increased the cyclonic shear on the equatorward flank of the African easterly jet, leading to enhanced African easterly wave and transient convective activity, which then contributed to the enhanced convection over Africa on the longer intraseasonal time scale. The implications of this intraseasonal mode for predictability over Africa are discussed.

The Vertical Structure of TOGA COARE Convection. Part II: Modulating Influences and Implications for Diabatic Heating

The temporal variability of western Pacific warm pool convection, especially its vertical structure, is examined in this study. Distributions of convective echo top heights and 30-dBZ contour heights have been produced from shipboard radar data collected during Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). Elevation and suppression of convective heights was primarily influenced by the phase of the intraseasonal oscillation (ISO), with heights being suppressed during convectively inactive and westerly wind burst (WWB) phases of the ISO. Echo top heights were greatest during the convective phases and post-WWB phases of the ISO. However, at least some very deep convection was always present within the area observed by radar, indicating that local conditions were favorable for deep convection, even when the large-scale environment was not capable of supporting widespread deep convection. In addition to the ISO, echo top and 30-dBZ contour heights were also influenced over shorter timescales by intrusions of dry subtropical air into the COARE Intensive Flux Array (IFA). Periods of convective suppression were also accompanied by upper-tropospheric drying. Convective diabatic heating profiles, computed from a combination of surface radar and sounding data, reveal that the shape of the monthly mean heating profiles varied over the four-month intensive observing period. Maximum heating occurred at the highest elevations during the convectively inactive phases of the ISO, and at the lowest elevations during the convectively active phases of the ISO. These variations are qualitatively consistent with higher (lower) convective available potential energy values and higher (lower) 30-dBZ contour heights above the freezing level during the strong surface easterly (westerly) phase of the ISO. Factors leading to widespread convective suppression despite the presence of a high environmental CAPE are also discussed.