Tropical Pacific Basin Research Articles

Inter‐Basin Versus Intra‐Basin Sea Surface Temperature Forcing of the Western North Pacific Subtropical High's Westward Extensions

AbstractZonal extensions of the Western Pacific subtropical high (WPSH) strongly modulate extreme rainfall activity and tropical cyclone (TC) landfall over the Western North Pacific (WNP) region. These zonal extensions are primarily forced on seasonal timescales by inter‐basin zonal sea surface temperature (SST) gradients. However, despite the presence of large‐scale zonal SST gradients, the WPSH response to SSTs varies from year to year. In this study, we force the atmosphere‐only NCAR Community Earth System Model version 2 simulations with two real‐world SST patterns, both featuring the large‐scale zonal SST gradient characteristic of decaying El Niño‐developing La Niña summers. For each of these patterns, we performed four experimental sets that tested the relative contributions of the tropical Indian Ocean, Pacific, and Atlantic basin SSTs to simulated westward extensions over the WNP during June–August. Our results indicate that the subtle differences between the two SST anomaly patterns belie two different mechanisms forcing the WPSH's westward extensions. In one SST anomaly pattern, extratropical North Pacific SST forcing suppresses the tropical Pacific zonal SST gradient forcing, resulting in tropical Atlantic and Indian Ocean SSTs being the dominant driver. The second SST anomaly pattern drives a similar westward extension as the first pattern, but the underlying SST gradient driving the WPSH points to intra‐basin forcing mechanisms originating in the Pacific. The results of this study have implications for understanding and predicting the impact of the WPSH's zonal variability on tropical cyclones and extreme rainfall over the WNP.

Plankton community structure in response to hydrothermal iron inputs along the Tonga-Kermadec arc

The Western Tropical South Pacific (WTSP) basin has been identified as a hotspot of atmospheric dinitrogen fixation due to the high dissolved iron ([DFe]) concentrations (up to 66 nM) in the photic layer linked with the release of shallow hydrothermal fluids along the Tonga-Kermadec arc. Yet, the effect of such hydrothermal fluids in structuring the plankton community remains poorly studied. During the TONGA cruise (November-December 2019), we collected micro- (20-200 μm) and meso-plankton (>200 μm) samples in the photic layer (0-200 m) along a west to east zonal transect crossing the Tonga volcanic arc, in particular two volcanoes associated with shallow hydrothermal vents (< 500 m) in the Lau Basin, and both sides of the arc represented by Melanesian waters and the South Pacific Gyre. Samples were analyzed by quantitative imaging (FlowCam and ZooScan) and then coupled with acoustic observations, allowing us to study the potential transfer of phytoplankton blooms to higher planktonic trophic levels. We show that micro- and meso-plankton exhibit high abundances and biomasses in the Lau Basin and, to some extent, in Melanesian waters, suggesting that shallow hydrothermal inputs sustain the planktonic food web, creating productive waters in this otherwise oligotrophic region. In terms of planktonic community structure, we identified major changes with high [DFe] inputs, promoting the development of a low diversity planktonic community dominated by diazotrophic cyanobacteria. Furthermore, in order to quantify the effect of the shallow hydrothermal vents on chlorophyll a concentrations, we used Lagrangian dispersal models. We show that chlorophyll a concentrations were significantly higher inside the Lagrangian plume, which came into contact with the two hydrothermal sites, confirming the profound impact of shallow hydrothermal vents on plankton production.

Tropical Pacific Surface Wind Energy Spectra and Coherence: Basinwide Observations and Their Observing System Implications

AbstractThe tropical Pacific moored-buoy array spacing was based on wind coherence scales observed from low-lying islands in the western-central tropical Pacific. Since the array was deployed across the full basin in the mid-1990s, winds from the array have proven critical to accurately monitoring for decadal-scale changes in tropical Pacific winds and identifying spurious trends in wind analysis products used to monitor for long-term change. The array observations have also greatly advanced our ability to diagnostically model (hindcast) and thereby better understand the observed development of central Pacific sea surface temperature anomaly development associated with El Niño and La Niña events, although the eastern equatorial Pacific is not yet accurately hindcast. The original array-design assumptions that the statistics calculated from the western-central Pacific island records are representative of open-ocean conditions and other regions of the tropical Pacific have not been thoroughly reexamined. We revisit these assumptions using the basinwide wind observations provided by the array and find that key wind statistics change across the tropical Pacific basin in ways that could not be determined from the original island wind study. The island results provided a best-case answer for mooring zonal spacing with minimally redundant coherence between adjacent buoys. Buoy-observed meridional coherence scales are longer than determined from the islands. Enhanced zonal sampling east of 140°W and west of 180° is needed to obtain minimal redundancy (optimal spacing). Reduced meridional sampling could still yield minimal redundancy for wind and wind stress fields over the ocean waveguide.

Assimilation of Allochthonous Matter by Estuarine Consumers During the 2015 El Niño Event

The El Nino phenomenon refers to a warming of the tropical Pacific basin whose meteorological effects influence the dynamics of aquatic ecosystems around the world. Prior studies have shown that strong El Nino events are highly correlated with high rainfall episodes and high freshwater discharge into subtropical estuaries, with subsequent changes in species composition, abundance, and diversity of their biota. In this work, we evaluated the hypothesis that riverine allochthonous matter associated with the strong 2015 El Nino event is assimilated by decapod crustaceans and fishes of a southwestern Atlantic estuary. We analyzed carbon (δ13C) and nitrogen (δ15N) stable isotope ratios of primary food sources and consumers in the estuary and of riverine allochthonous matter. Our findings revealed that higher water surplus and lower salinity associated with the 2015 El Nino coincided with an increase in the number of freshwater fish species and a decrease in the number of marine- and estuarine-dependent fishes inside the estuary. In addition, most estuarine consumers had lower average δ13C values during the wet period associated with the 2015 El Nino. This seemed to reflect the assimilation of 13C-depleted riverine matter, which according to Bayesian isotope mixing models ranged from 11% (adult resident decapod crustaceans) to 60% (adult resident fishes) during the wet season. Further studies are needed to evaluate the role of El Nino events on structuring food web organization in estuaries under the influence of this climatic phenomenon, which may become more frequent and intense in a global warming scenario.

Regulation of atmospheric circulation controlling the tropical Pacific precipitation change in response to CO2 increases

The spatial pattern of precipitation responses to CO2 concentration increases significantly influences global weather and climate variability by altering the location of tropical heating in a warmer climate. In this study, we analyze the Coupled Model Intercomparison Project Phase 5 (CMIP5) climate model projections of tropical Pacific rainfall response to quadrupled increase of CO2. We found that the precipitation changes to the CO2 concentration increase cannot be interpreted by a weakening or strengthening of large-scale east–west coupling across the tropical Pacific basin, i.e., Walker circulation. By calculating the water vapor transport, we suggest instead that different responses of the Walker and Hadley circulations to the increasing CO2 concentration shape the details of the spatial pattern of precipitation in the tropical Pacific. Therefore, more regionally perturbed circulations over the tropical Pacific, which is influenced by the mean state change in the tropical Pacific and the enhanced precipitation outside the tropical Pacific, lead to greater increases in precipitation in the western equatorial Pacific as compared to the eastern tropical Pacific in a warmer climate.

Rapid Weakening of Tropical Cyclones in Monsoon Gyres over the Tropical Western North Pacific

Abstract As one major source of forecasting errors in tropical cyclone intensity, rapid weakening of tropical cyclones [an intensity reduction of 20 kt (1 kt = 0.51 m s−1) or more over a 24-h period] over the tropical open ocean can result from the interaction between tropical cyclones and monsoon gyres. This study aims to examine rapid weakening events occurring in monsoon gyres in the tropical western North Pacific (WNP) basin during May–October 2000–14. Although less than one-third of rapid weakening events happened in the tropical WNP basin south of 25°N, more than 40% of them were associated with monsoon gyres. About 85% of rapid weakening events in monsoon gyres occurred in September and October. The rapid weakening events associated with monsoon gyres are usually observed near the center of monsoon gyres when tropical cyclone tracks make a sudden northward turn. The gyres can enlarge the outer size of tropical cyclones and tend to induce prolonged rapid weakening events with an average duration of 33.2 h. Large-scale environmental factors, including sea surface temperature changes, vertical wind shear, and midlevel environmental humidity, are not primary contributors to them, suggesting the possible effect of monsoon gyres on these rapid weakening events by modulating the tropical cyclone structure. This conclusion is conducive to improving operational forecasts of tropical cyclone intensity.

Land‐sea thermal contrast determines the trend of Walker circulation simulated in atmospheric general circulation models

AbstractStrengthening or weakening of the Walker circulation can highly influence the global weather and climate variability by altering the location and strength of tropical heating. Therefore, there is considerable interest in understanding the mechanisms that lead to the trends in the Walker circulation intensity. Conventional wisdom indicates that a strengthening or weakening of the Walker circulation is primarily controlled by inhomogeneous sea surface temperature (SST) patterns across the tropical Pacific basin. However, we show that Atmospheric Model Intercomparison Project climate model simulations with identical SST forcing have different Walker circulation trends that can be linked to differences in land surface temperatures. More prominently, stronger land‐sea thermal contrast leads to increases in the precipitation in South America as well as the sea level pressure in the eastern tropical Pacific through a local circulation, resulting in a strengthening of the Walker circulation trend. This implies that correctly simulating the land temperature in atmospheric models is crucial to simulating the intensity of the Walker circulation in the present climate as well as its future change.

On the distinct interannual variability of tropical cyclone activity over the eastern North Pacific

The tropical eastern North Pacific (ENP) basin exhibits very large interannual variability in the frequency of occurrence of tropical cyclones, presenting very active (more than 20 tropical cyclones per season) and very inactive years (only eigth tropical cyclones). The large-scale factors that may influence the distinct interannual variability are investigated in this study, by analyzing the composites of seven years of high activity and 10 years of low activity from 1965 to 2013. The results of composite analyses indicate that the low-level vorticity and mid-tropospheric relative humidity are mostly unfavorable for tropical cyclogenesis during active years. The sea surface temperature (SST) may play a small role modulating the occurrence of tropical cyclones, but the reduced vertical shear of the horizontal wind between 850 and 200 hPa is the main contributor to clyclogenesis during the active years, confirming earlier results by Camargo et al. (2007). We use an intensity model (Emanuel et al., 2006, 2008) to further investigate the key environmental factors affecting TC intensity, exploring the relative roles of changes in SST, vertical wind shear and TC tracks. The results indicate that the interannual variability in the frequency of major hurricanes (categories 3 through 5) is best simulated when the effects of both SST and vertical wind shear are combined. Furthermore, changes in TC tracks may play an important role in the intensity achieved. In particular, during the years with high activity, the location for cyclogenesis shifts eastward and more TCs have west-northwestward tracks, leading to longer lifetime and higher intensity, compared to years with low TC activity in the basin.

Using cytochrome P4501A1 expression in liver and blubber to understand effects of persistent organic pollutant exposure in stranded Pacific Island cetaceans.

Elevated levels of persistent organic pollutants (POPs) have been reported in tropical Pacific Island cetaceans and their environment. In addition, recent health concerns in cetacean populations have warranted investigation into potential physiological effects from POP exposure for this region. Cytochrome P450 1A1 (CYP1A1) is a candidate for examining such effects. This well-studied biomarker of exposure and effect was examined in stranded cetacean liver using immunoblot (n = 39, 16 species) and blubber using immunohistochemistry (n = 23, 10 species). Paired tissue samples allowed for CYP1A1 comparisons not only between species but also within each individual animal to examine differences between tissue types. Liver CYP1A1 expression correlated positively and significantly with blubber concentrations of all POP categories (n = 39, p < 0.050) except octachlorostyrene and pentachlorobenzene (p > 0.100). Among Stenella species, liver CYP1A1 tissue expression was correlated negatively with the sum of all blubber layer endothelial cell CYP1A1 expression (n = 14, p = 0.049). Overall, elevated expression of liver CYP1A1 confirms its use as a biomarker of POP exposure to cetaceans stranded in the tropical Pacific basin.

The impact of westerly wind bursts on the diversity and predictability of El Niño events: An ocean energetics perspective

AbstractIn this study, we apply ocean energetics as a diagnostic tool to investigate the impact of westerly wind bursts (WWBs) on the evolution, diversity, and predictability of El Niño events. Following Fedorov et al. (2014), we add an observed WWB to simulations within a comprehensive coupled model and explore changes in the available potential energy (APE) of the tropical Pacific basin. We find that WWB impacts strongly depend on the ocean initial state and can range from a Central Pacific (CP) to Eastern Pacific (EP) warming, which is closely reflected by the ocean energetics. Consequently, the APE can be used to quantify the diversity of El Niño events within this continuum—higher negative APE values typically correspond to EP events, lower values to CP events. We also find that a superimposed WWB enhances El Niño predictability even before the spring predictability barrier, if one uses the APE as a predictor.

The influence of El Niño on the Indian summer monsoon rainfall anomaly: a diagnostic study of the 1982/83 and 1997/98 events

The Indian summer monsoon of 1982 and 1997 depicts disparities, however, maximum sea surface temperature anomaly over Nino 3 region is observed in the following winter of both the years. The inter-annual variation of sea surface temperature anomaly shows maximum peak during 1982/83 and 1997/98 El Nino events. The inter-annual variation of multivariate ENSO index also supports the above observation. The analyses of the entire tropical Pacific basin including the equatorial region reveal an anomalous behavior of the mean sea level pressure (MSLP) and the convective activities. The observations further reveal that the negative anomaly in monsoon rainfall over India prevails throughout the monsoon season except for the month of August in 1982, while in the year 1997 the monsoon rainfall anomaly shows random variations. The comparison between the summer monsoon rainfall of 1982 and 1997 depicts that the magnitude of the positive anomaly is same in the month of August. The condition over tropical Pacific during 1982/83 and 1997/98 has been investigated through the variation of outgoing long wave radiation (OLR), MSLP and pressure vertical velocity. The time–longitude plots of OLR and MSLP reveal the changes in pressure distribution and convective pattern over the tropical equatorial Pacific. The zonal and meridional cross section of pressure vertical velocity over the tropical Pacific and tropical Indian Ocean facilitates to understand the strength of the vertical motion during the monsoons of 1982 and 1997.

Sensitivity of summer precipitation to tropical sea surface temperatures over East Asia in the GRIMs GMP

AbstractIn this study, uncoupled atmospheric general circulation model experiments are conducted to examine the sensitivity of tropical Ocean basins from the Indian Ocean to the tropical Pacific Ocean on the summer precipitation variability over East Asia. It is remarkable that the Indian Ocean basin sea surface temperature (SST) and the tropical Pacific basin SST act on summer precipitation variability over Northeast Asia and southern China quite differently. That is, SST warming in the Indian Ocean largely contributes to the increase in the amount of summer precipitation over East Asia, which is in contrast to the warming of the western tropical Pacific Ocean. Our further analysis indicates that an altered large‐scale atmospheric circulation over the western tropical Pacific contributes to contrasting atmospheric motion over East Asia due to the tropics‐East Asia teleconnections, which results in changes in the amount of summer precipitation due to the warming of the Indian and western tropical Pacific Oceans.

Tropical Indo‐Pacific Ocean chlorophyll response to MJO forcing

Previous studies that analyzed ocean color satellite data have suggested that the primary mechanism of surface chlorophyll (Chl) response to the MJO is wind‐induced turbulent mixing and the corresponding mixed layer entrainment. In this study, this notion is examined with an ocean biophysical model in an ensemble framework, focusing on upper ocean processes (z &lt; 200 m). As a whole, the model's mean Chl state is lower than observations except in the tropical Pacific basin, but its seasonal variation is acceptable, particularly in the tropical Indian Ocean. In this basin, the model can simulate surface Chl responses to the MJO consistent with the observations in terms of the phase‐by‐phase anomaly evolution patterns. These Chl responses are mostly induced by surface wind forcing, which is consistent with previous studies. Further investigation of subsurface variations is performed at select grid points, and it is revealed that (1) entrained nutrients are the primary source of enhancement for surface Chl concentration and detrainment blooms are relatively less common; (2) in limited regions, Ekman pumping can effectively reduce Chl concentration; and (3) both entrainment/detrainment and Ekman pumping mechanisms rely on background states of nutrient availability, so the same forcing can result in completely different Chl responses depending on the background state.

Changes in the Tropical Pacific SST Trend from CMIP3 to CMIP5 and Its Implication of ENSO

This study assesses the changes in the tropical Pacific Ocean sea surface temperature (SST) trend and ENSO amplitude by comparing a historical run of the World Climate Research Programme Coupled Model Intercomparison Project (CMIP) phase-5 multimodel ensemble dataset (CMIP5) and the CMIP phase-3 dataset (CMIP3). The results indicate that the magnitude of the SST trend in the tropical Pacific basin has been significantly reduced from CMIP3 to CMIP5, which may be associated with the overestimation of the response to natural forcing and aerosols by including Earth system models in CMIP5. Moreover, the patterns of tropical warming over the second half of the twentieth century have changed from a La Niña–like structure in CMIP3 to an El Niño–like structure in CMIP5. Further analysis indicates that such changes in the background state of the tropical Pacific and an increase in the sensitivity of the atmospheric response to the SST changes in the eastern tropical Pacific have influenced the ENSO properties. In particular, the ratio of the SST anomaly variance in the eastern and western tropical Pacific increased from CMIP3 to CMIP5, indicating that a center of action associated with the ENSO amplitude has shifted to the east.

Interdecadal variability/long-term changes in global precipitation patterns during the past three decades: global warming and/or pacific decadal variability?

This study explores how global precipitation and tropospheric water vapor content vary on the interdecadal/long-term time scale during past three decades (1988–2010 for water vapor), in particular to what extent the spatial structures of their variations relate to changes in surface temperature. EOF analyses of satellite-based products indicate that the first two modes of global precipitation and columnar water vapor content anomalies are in general related to the El Nino-Southern oscillation. The spatial patterns of their third modes resemble the corresponding linear fits/trends estimated at each grid point, which roughly represent the interdecadal/long-term changes happening during the same time period. Global mean sea surface temperature (SST) and land surface temperature have increased during the past three decades. However, the water vapor and precipitation patterns of change do not reflect the pattern of warming, in particular in the tropical Pacific basin. Therefore, other mechanisms in addition to global warming likely exist to account for the spatial structures of global precipitation changes during this time period. An EOF analysis of longer-record (1949–2010) SST anomalies within the Pacific basin (60oN–60oS) indicates the existence of a strong climate regime shift around 1998/1999, which might be associated with the Pacific decadal variability (PDV) as suggested in past studies. Analyses indicate that the observed linear changes/trends in both precipitation and tropospheric water vapor during 1988–2010 seem to result from a combined impact of global mean surface warming and the PDV shift. In particular, in the tropical central-eastern Pacific, a band of increases along the equator in both precipitation and water vapor sandwiched by strong decreases south and north of it are likely caused by the opposite effects from global-mean surface warming and PDV-related, La Nina-like cooling in the tropical central-eastern Pacific. This narrow band of precipitation increase could also be considered an evidence for the influence of global mean surface warming.

East African long rains declined abruptly

In 2010–2011 the East African region suffered a severe drought that affected millions of people. The drought was a consequence of the failure of both the seasonal “short rains,” which typically run from October to December, and the following “long rains,” which typically run from March to May. The failure of the short rains was predictable given a well‐established connection with La Niña, but the failure of the long rains was not. In a new study, Lyon and DeWitt show that the most recent long rains failure is consistent with a recurrent large‐scale precipitation pattern that began with an abrupt shift around 1999. The authors used observations and climate model simulations to show that the abrupt decline in the East African long rains was linked to similarly abrupt changes in sea surface temperatures, mainly in the tropical Pacific basin. (Geophysical Research Letters, doi:10.1029/2011GL050337, 2012) —EB

A recent and abrupt decline in the East African long rains

The successive failure of the East African short rains (typically October‐December) and subsequent long rains (March‐May) in 2010–11 plunged much of the region into severe drought, impacting millions of people and triggering a humanitarian crisis. While poor short rains in 2010 were generally anticipated given linkages with La Niña, the subsequent long rains do not exhibit similar predictability. Here we show the long rains failure in boreal spring of 2011 is consistent with a recurrent large‐scale precipitation pattern that followed their abrupt decline around 1999. Using observations and climate model simulations, we show the abrupt decline in long rains precipitation is linked to similarly abrupt changes in sea surface temperatures, predominately in the tropical Pacific basin.

Developing a new sinuosity index for cyclone tracks in the tropical South Pacific

This paper introduces a new metric for tropical cyclone track shape within the tropical South Pacific (TSP) basin, based on measurements of track sinuosity. A sinuosity index (SI) is developed by applying a simple cube-root transformation to original track sinuosity values. Based on the resulting near-normal SI distribution, an ordinal four- category (quartile) naming system is then proposed for track-type classification. Track sinuosity patterns are also investigated over the last four decades (1970-2008). Analytical findings suggest that cyclone track sinuosity is an important parameter influencing the potential vulnerability of island archipelagoes to cyclone hazard. Principally, sinuously moving cyclones show some tendency for greater longevity and intensity than straighter- tracking storms and make up a larger proportion of systems forming in the western tropical South Pacific than those generated farther east. Although no long-term statistical trend can be established, track sinuosity is highly variable through time, implying that the TSP basin and the islands therein will continue to experience large but irregular inter-annual fluc- tuations in cyclone track morphology.

El Niño/Southern Oscillation behaviour since 1871 as diagnosed in an extended multivariate ENSO index (MEI.ext)

AbstractEl Niño/Southern Oscillation (ENSO) remains the most important coupled ocean–atmosphere phenomenon to cause global climate variability on seasonal to interannual time scales. This paper addresses the need for a reliable ENSO index that allows for the historical definition of ENSO events in the instrumental record back to 1871. The Multivariate ENSO Index (MEI) was originally defined as the first seasonally varying principal component of six atmosphere–ocean (COADS) variable fields in the tropical Pacific basin. It provides for a more complete and flexible description of the ENSO phenomenon than single variable ENSO indices such as the SOI or Niño 3.4 SST. Here we describe our effort to boil the MEI concept down to its most essential components (based on SLP, SST) to enable historical analyses that more than double its period of record to 1871–2005. The new MEI.ext confirms that ENSO activity went through a lull in the early‐ to mid‐20th century, but was just about as prevalent one century ago as in recent decades. We diagnose strong relationships between peak amplitudes of ENSO events and their duration, as well as between their peak amplitudes and their spacing (periodicity). Our effort is designed to help with the assessment of ENSO conditions through as long a record as possible to be able to differentiate between ‘natural’ ENSO behaviour in all its rich facets, and the ‘Brave New World’ of this phenomenon under evolving GHG‐related climate conditions. So far, none of the behaviour of recent ENSO events appears unprecedented, including duration, onset timing, and spacing in the last few decades compared to a full century before then. Copyright © 2011 Royal Meteorological Society

Potential predictability of sea surface temperature in a coupled ocean-atmosphere GCM

Using the Flexible Global Ocean-Atmosphere-Land System model (FGOALS) version g1.11, a group of seasonal hindcasting experiments were carried out. In order to investigate the potential predictability of sea surface temperature (SST), singular value decomposition (SVD) analyses were applied to extract dominant coupled modes between observed and predicated SST from the hindcasting experiments in this study. The fields discussed are sea surface temperature anomalies over the tropical Pacific basin (20°S-20°N, 120°E-80°W), respectively starting in four seasons from 1982 to 2005. On the basis of SVD analysis, the simulated pattern was replaced with the corresponding observed pattern to reconstruct SST anomaly fields to improve the ability of the simulation. The predictive skill, anomaly correlation coefficients (ACC), after systematic error correction using the first five modes was regarded as potential predictability. Results showed that: 1) the statistical postprocessing approach was effective for systematic error correction; 2) model error sources mainly arose from mode 2 extracted from the SVD analysis—that is, during the transition phase of ENSO, the model encountered the spring predictability barrier; and 3) potential predictability (upper limits of predictability) could be high over most of the tropical Pacific basin, including the tropical western Pacific and an extra 10-degrees region of the mid and eastern Pacific.