Equatorial Sea Surface Temperature Research Articles

Characteristics of the CMIP3 Models Simulating Realistic Response of Tropical Western Pacific Precipitation to Ninno3 SST Variability

The 20th century simulations from the World Climate Research Programme’s Coupled Model Intercomparison Project phase 3 (CMIP3) multi-model dataset are examined statistically using numerical skills to capture the characteristics of the models which realistically simulate responses of the tropical western Pacific (TWP) precipitation to sea surface temperature (SST) variability over the Nino3 region [150°-90°W, 5°S-5°N].The simultaneous correlation of precipitation anomaly over the TWP region of [90°-170°E, 20°S-20°N] with Nino3 SST variability is successfully reproduced with relatively high skills for June to August (JJA) as compared with those for December to February (DJF) and March to May. The high skill models have common characteristics of realistically simulating the observed largest precipitation response to Nino3 SST variability over the equatorial central Pacific east of the dateline. Furthermore, the realistic simulation of the climatological mean equatorial precipitation west of the dateline seems to be responsible for the realistic response of the TWP precipitation to Nino3 SST variability.A few of the models successfully simulate the delayed response of the JJA precipitation over the TWP region of [90°-170°E, 10°S-30°N] to the preceding DJF Nino3 SST variability with high skills. Those models reproduce the statistically observed features of subtropical northwestern Pacific SST anomaly, precipitation and SST anomalies over the Indian Ocean in JJA following DJF Nino3 SST variability. Another distinctive characteristic of those models is to reproduce almost null correlation of the equatorial central Pacific SST anomaly in JJA with the preceding DJF season Nino3 SST variability.

Physical Essence of the “Predictability Barrier”

Although the “predictability barrier” is a scientific problem brought forward during the middle 1990’s, it has yet to be clearly understood. In this paper, the authors used model results to study the “predictability barrier” of some atmospheric (climatic) systems, including the equatorial Pacific sea surface temperature (SST), East Asia trough (EAT), and South Asia wind field (monsoon). It is shown that the “predictability barrier” appeared not only in predicting the SST in the equatorial Pacific, but also in predicting the atmospheric circulation systems, such as the EAT and the monsoon. The “predictability barrier” for predicting the different climate systems appeared not only in spring, but also in different seasons. It appeared in spring for the equatorial Pacific SST, in summer for the EAT, and in fall and winter for the monsoon. Further analyses showed that the fundamental physical essence of the “predictability barrier” is the seasonal variability characteristics of the atmospheric (climate) system. The “predictability barrier” would occur when the system was at its weakest or worst continuous phase. Furthermore, the models and error patterns also had an important impact on the “predictability barrier” good models and special error patterns can minimize the “predictability barrier”.

Large‐scale controls on Ganges and Brahmaputra river discharge on intraseasonal and seasonal time‐scales

AbstractReliable water supply from the Ganges and Brahmaputra is of critical importance to the sustainability of the agricultural societies of India and Bangladesh. But, the flow in both basins is highly variable on time‐scales ranging from days to years, creating challenges for the optimization of agricultural practices, water resource management and disaster mitigation. The following questions are addressed. Is intraseasonal monsoon variability related to the subseasonal variability of river flow? Do variations in the large‐scale tropical sea‐surface temperature (SST) located both regionally and remotely promote seasonal and interannual variations of river discharge? And, if these relationships do exist, are they determinable with sufficient lead‐times to allow useful predictions for user communities in South Asia? We examine these questions using 50 years of daily river discharge data for both rivers calculated at the points where they enter Bangladesh, and with SST data in the Indo‐Pacific region. We also examine the question of determining the impact of man‐made dams, diversions and barrages on the data record, especially that of the Ganges. A comparison of discharge prior to 1974 (the time of the construction of the largest barrage) shows no statistical difference that cannot be explained by basin‐wide rainfall distributions. Changes that do occur are restricted to the dry‐season months.Subseasonal river discharge is found to be strongly tied to the monsoon intraseasonal cycle resulting in a near‐in‐phase timing of Ganges and Brahmaputra discharge. A basin isochrone analysis is used to couple stream‐flow variability and intraseasonal precipitation during the different phases of the intraseasonal cycle. On longer time‐scales, statistically significant correlations are found between mean monthly equatorial Pacific SST and the boreal summer Ganges discharge with lead times of 2–3 months. These relationships are tied to El Niño–Southern Oscillation (ENSO) oscillations in addition to SST variability in the southwest and northwest Pacific that also seems to be related to ENSO. The Brahmaputra discharge, on the other hand, shows somewhat weaker relationships with tropical SST. Strong lagged correlation relationships are found with SST in the Bay of Bengal but these are the result of very warm SSTs and exceptional Brahmaputra discharge during the summer of 1998. When this year is removed from the time series, relationships with SST anomalies weaken everywhere except in the northwest Pacific for the June discharge and in areas of the central Pacific straddling the Equator for the July discharge. In addition, the northwest Pacific relationship changes polarity for June and July discharges. Although the relationships are weaker than those found for the Ganges, they are persistent from month to month and suggest that two different and sequential factors influence Brahmaputra river flow. Copyright © 2009 Royal Meteorological Society

Natural variability of the Keetch - Byram Drought Index in the Hawaiian Islands

The Hawaiian Islands experience damaging wildfires on a yearly basis. Soil moisture or lack thereof influences the amount and flammability of vegetation. Incorporating daily maximum temperatures and daily rainfall amounts, the Keetch–Byram Drought Index (KBDI) estimates the amount of soil moisture by tracking daily maximum temperatures and rainfall. A previous study found a strong link between the KBDI and total area burned on the four main Hawaiian Islands. The present paper further examines the natural variability of the KBDI. The times of year at which the KBDI is highest, representing the highest fire danger, are found at each of the 27 stations on the island chain. Spectral analysis is applied to investigate the variability of the KBDI on longer time scales. Windward and leeward stations are shown to have different sensitivities to large-scale climatic fluctuations. An El Niño signal displays a strong relationship with leeward stations, when examined with a band-pass filter and with a composite of standardized anomalies. Departure patterns of atmospheric circulations and sea surface temperatures over the North Pacific are investigated for composites of extremely high KBDI values when fire risk is high. The winter, spring, and fall show anomalous surface anticyclonic circulations, surface divergence, and subsidence over the islands for the upper quartile of KBDI. The winter, spring, and fall composites of equatorial sea surface temperatures for the upper quartile of KBDI are investigated for possible links to atmospheric circulations. These analyses are an effort to allow fire managers some lead time in predicting future fire risks.

Southern Hemisphere forcing of Pliocene δ18O and the evolution of Indo‐Asian monsoons

The Milankovitch paradigm links the timing (phase) of ice volume minima to summer insolation maxima in the hemisphere where ice volume dominates; consistent application of this paradigm dictates that Pliocene ice volume minima should lag Southern Hemisphere summer insolation maxima. We infer the magnitude of this lag on the basis of the phase relationship between equatorial sea surface temperature and benthic δ18O. We infer that Pliocene δ18O minima should lag obliquity maxima by 19° (2.2 ka), broadly consistent with the current global marine δ18O chronology, and precession maxima by 32° (2 ka), a difference of 160° (10.2 ka) relative to the current global marine δ18O chronology. Only in the context of this revision are Pliocene summer and winter monsoon phase relationships consistent with direct orbital forcing across the entire Indo‐Asian region, including marine and terrestrial proxies from the Chinese Loess Plateau, the South China Sea, and the Arabian Sea. Strong Pliocene summer and winter monsoons were in phase with one another, strengthened at obliquity minima and precession minima; the summer monsoon was also strengthened at precession maxima, yielding a semiprecession spectral signal. Strong Pliocene monsoons at orbital extremes indicate a direct response to fast physics processes including sensible heating and cooling of the Asian landmass and, for the summer monsoon, the export of latent heat from the southern Indian Ocean. As Northern Hemisphere ice volume grew into the Pleistocene, the timing of strong winter and summer monsoons drifted apart becoming influenced by the combined effects of fast physics and slow physics (ice volume) variables. The phase of strong winter monsoons shifted toward ice maxima, and the phase of strong summer monsoons shifted toward ice minima.

Mid-Pliocene equatorial Pacific sea surface temperature reconstruction: a multi-proxy perspective

The Mid-Pliocene is the most recent interval of sustained global warmth, which can be used to examine conditions predicted for the near future. An accurate spatial representation of the low-latitude Mid-Pliocene Pacific surface ocean is necessary to understand past climate change in the light of forecasts of future change. Mid-Pliocene sea surface temperature (SST) anomalies show a strong contrast between the western equatorial Pacific (WEP) and eastern equatorial Pacific (EEP) regardless of proxy (faunal, alkenone and Mg/Ca). All WEP sites show small differences from modern mean annual temperature, but all EEP sites show significant positive deviation from present-day temperatures by as much as 4.4 degrees C. Our reconstruction reflects SSTs similar to modern in the WEP, warmer than modern in the EEP and eastward extension of the WEP warm pool. The east-west equatorial Pacific SST gradient is decreased, but the pole to equator gradient does not change appreciably. We find it improbable that increased greenhouse gases (GHG) alone would cause such a heterogeneous warming and more likely that the cause of Mid-Pliocene warmth is a combination of several forcings including both increased meridional heat transport and increased GHG.

Advance and prospectus of seasonal prediction: assessment of the APCC/CliPAS 14-model ensemble retrospective seasonal prediction (1980–2004)

We assessed current status of multi-model ensemble (MME) deterministic and probabilistic seasonal prediction based on 25-year (1980–2004) retrospective forecasts performed by 14 climate model systems (7 one-tier and 7 two-tier systems) that participate in the Climate Prediction and its Application to Society (CliPAS) project sponsored by the Asian-Pacific Economic Cooperation Climate Center (APCC). We also evaluated seven DEMETER models’ MME for the period of 1981–2001 for comparison. Based on the assessment, future direction for improvement of seasonal prediction is discussed. We found that two measures of probabilistic forecast skill, the Brier Skill Score (BSS) and Area under the Relative Operating Characteristic curve (AROC), display similar spatial patterns as those represented by temporal correlation coefficient (TCC) score of deterministic MME forecast. A TCC score of 0.6 corresponds approximately to a BSS of 0.1 and an AROC of 0.7 and beyond these critical threshold values, they are almost linearly correlated. The MME method is demonstrated to be a valuable approach for reducing errors and quantifying forecast uncertainty due to model formulation. The MME prediction skill is substantially better than the averaged skill of all individual models. For instance, the TCC score of CliPAS one-tier MME forecast of Niño 3.4 index at a 6-month lead initiated from 1 May is 0.77, which is significantly higher than the corresponding averaged skill of seven individual coupled models (0.63). The MME made by using 14 coupled models from both DEMETER and CliPAS shows an even higher TCC score of 0.87. Effectiveness of MME depends on the averaged skill of individual models and their mutual independency. For probabilistic forecast the CliPAS MME gains considerable skill from increased forecast reliability as the number of model being used increases; the forecast resolution also increases for 2 m temperature but slightly decreases for precipitation. Equatorial Sea Surface Temperature (SST) anomalies are primary sources of atmospheric climate variability worldwide. The MME 1-month lead hindcast can predict, with high fidelity, the spatial–temporal structures of the first two leading empirical orthogonal modes of the equatorial SST anomalies for both boreal summer (JJA) and winter (DJF), which account for about 80–90% of the total variance. The major bias is a westward shift of SST anomaly between the dateline and 120°E, which may potentially degrade global teleconnection associated with it. The TCC score for SST predictions over the equatorial eastern Indian Ocean reaches about 0.68 with a 6-month lead forecast. However, the TCC score for Indian Ocean Dipole (IOD) index drops below 0.40 at a 3-month lead for both the May and November initial conditions due to the prediction barriers across July, and January, respectively. The MME prediction skills are well correlated with the amplitude of Niño 3.4 SST variation. The forecasts for 2 m air temperature are better in El Niño years than in La Niña years. The precipitation and circulation are predicted better in ENSO-decaying JJA than in ENSO-developing JJA. There is virtually no skill in ENSO-neutral years. Continuing improvement of the one-tier climate model’s slow coupled dynamics in reproducing realistic amplitude, spatial patterns, and temporal evolution of ENSO cycle is a key for long-lead seasonal forecast. Forecast of monsoon precipitation remains a major challenge. The seasonal rainfall predictions over land and during local summer have little skill, especially over tropical Africa. The differences in forecast skills over land areas between the CliPAS and DEMETER MMEs indicate potentials for further improvement of prediction over land. There is an urgent need to assess impacts of land surface initialization on the skill of seasonal and monthly forecast using a multi-model framework.

Two Extra Components in the Brier Score Decomposition

Abstract The Brier score is widely used for the verification of probability forecasts. It also forms the basis of other frequently used probability scores such as the rank probability score. By conditioning (stratifying) on the issued forecast probabilities, the Brier score can be decomposed into the sum of three components: uncertainty, reliability, and resolution. This Brier score decomposition can provide useful information to the forecast provider about how the forecasts can be improved. Rather than stratify on all values of issued probability, it is common practice to calculate the Brier score components by first partitioning the issued probabilities into a small set of bins. This note shows that for such a procedure, an additional two within-bin components are needed in addition to the three traditional components of the Brier score. The two new components can be combined with the resolution component to make a generalized resolution component that is less sensitive to choice of bin width than is the traditional resolution component. The difference between the generalized resolution term and the conventional resolution term also quantifies how forecast skill is degraded when issuing categorized probabilities to users. The ideas are illustrated using an example of multimodel ensemble seasonal forecasts of equatorial sea surface temperatures.

Atlantic modulation of El Niño influence on summertime rainfall over southeastern South America

This study addresses the effect of Atlantic sea surface temperature (SST) anomalies on rainfall over southeastern South America during January–February, particularly during El Niño years, using observations as well as model simulations. It is found that the state of the equatorial Atlantic during El Niño years can modulate its influence on rainfall over southeastern South America, such that when the equatorial Atlantic is warm, the El Niño influence is weaker. This Atlantic influence is shown to occur through the response of the low level winds to equatorial SST anomalies: the convergence of westerly anomalies onto the warm anomaly decreases the equatorial trades and moisture flow into the Amazon and, moreover, reduces the northerly flow that brings moisture to southeastern South America. The total rainfall response in this region can thus be thought as the combination of rainfall anomalies from the equatorial Pacific and Atlantic oceans.

Effect of chlorophyll-a spatial distribution on upper ocean temperature in the central and eastern equatorial Pacific

Effect of the spatial distributions of chlorophyll-a concentration on upper ocean temperature and currents in the equatorial Pacific is investigated through a set of numerical experiments by using an ocean general circulation model. This study indicates that enhanced meridional gradient of chlorophyll-a between the equator and off-equatorial regions can strengthen zonal circulation and lead to a decrease in equatorial sea surface temperature (SST). However, the circulation changes by themselves are not effective enough to affect SST in the equatorial cold tongue (CT) region. The comparison between the experiments indicates that the CT SST are more sensitive to chlorophyll-a distribution away from the equator. The off-equatorial chlorophyll-a traps more solar radiation in the mixed layer, therefore, the temperature in the thermocline decreases. The cold water can then be transported to the equator by the meridional circulation within the mixed layer. Furthermore, the relation among CT SST, the surface heat flux, and the equatorial upwelling are discussed. The study implies the simulation biases of temperature on the equator are not only related to the local ocean dynamics but also related to some deficiency in simulating off-equatorial processes.

Pacific bidecadal climate variability regulated by tidal mixing around the Kuril Islands

18.6‐year period variability has been detected in various aspects of the climate, especially in and around the Pacific Ocean. Although it is believed to be caused by 18.6‐year period tidal cycle, no study has directly shown how the tidal cycle regulates such variability. Using a state‐of‐the‐art climate model, we show that the 18.6‐year cycle in strong tidal mixing localized around the Kuril Islands induces 18.6‐year periodicity in El Nino‐Southern Oscillation‐like Pacific climate variability. Influence of the tidal mixing propagates along the Pacific western rim as coastally trapped waves. Temperature anomaly is generated in the subsurface western equatorial Pacific, which propagates along the equatorial thermocline and eventually excites 18.6‐year periodicity in the equatorial sea surface temperature.

Interhemispheric thermal gradient and tropical Pacific climate

We explore the impact of interhemispheric thermal gradients forcing on the tropical Pacific ocean‐atmosphere climate in an intermediate coupled model. The equatorial zonal sea surface temperature (SST) gradient strengthens with an increased northward interhemispheric thermal gradient, the increase arising from earlier onset and later retreat of the seasonal cold tongue, and intensification during the peak cold season. When the mean interhemispheric thermal gradient is reversed, the central equatorial Pacific SST annual cycle abruptly reverses in phase, with its cold season in Mar–May rather than Sep–Nov. While startling, this response is consistent with a prevailing hypothesis that ties the cold tongue SST annual cycle phase to the hemispheric mean asymmetry of the Intertropical Convergence Zone. El Niño‐Southern Oscillation activity is also sensitive to the interhemispheric thermal gradient, with peak activity occurring when the mean gradient is small, reducing rapidly as the mean gradient increases in either direction.

Implications of seawater Mg/Ca variability for Plio-Pleistocene tropical climate reconstruction

Recent reconstructions of Mg and Ca concentrations of seawater indicate that seawater Mg/Ca changed significantly over the last 5 million years (Ma). Tropical sea surface temperature (SST) records for the last 5 Ma based on foraminiferal Mg/Ca paleothermometry assume constant seawater Mg/Ca. These SST records suggest that average equatorial Pacific SSTs remained thermally stable from 5 to 2 Ma, after which significant cooling occurred only in the eastern equatorial Pacific. This study examines the implications of adjusting available equatorial Pacific SST records based on Mg/Ca paleothermometry to account for the inferred past variations of seawater Mg/Ca. The results suggest that both the cold and the warm regions of the equatorial Pacific were much warmer during the early Pliocene (30–31 °C), and that both regions experienced a marked cooling from ∼ 4 Ma to ∼ 1 Ma. This new interpretation of foraminiferal Mg/Ca creates a discrepancy with alkenone unsaturation-based SST records from the eastern equatorial Pacific, which might be due to either overestimation of changes in past seawater Mg/Ca or to factors affecting the interpretation of the U K' 37 index. The adjusted SST records are consistent with the hypothesis that higher levels of greenhouse gases maintained the warmth of the early Pliocene.

Characteristics of North American Summertime Rainfall with Emphasis on the Monsoon

Abstract The core region of the North American summer monsoon is examined using spatially averaged daily rainfall observations obtained from gauges, with the objective of improving understanding of its climatology and variability. At most grid points, composite and interannual variations of the onset and end of the wet season are well defined, although, among individual stations that make up a grid average, variability is large. The trigger for monsoon onset in southern and eastern Mexico appears to be related to a change in vertical velocity, while for northwestern Mexico, Arizona, and New Mexico it is related to a reduction in stability, as indicated by a decrease in the lifted index. The wet-season rain rate is a combination of the wet-day rain rate, which decreases with distance from the coast, and the wet-day frequency, which is largest over the Sierra Madre Occidental. Thus the maximum total rate lies slightly to the west of the highest orography. As has been previously noted, onset is not always well correlated with total seasonal precipitation, so in these areas, variations of wet-day frequency and wet-day rain rate must be important. Correlations are small between the wet-day frequency and the wet-day rate, and the former is better correlated than the latter with the seasonal rain rate. Summer rainfall in central to southern Mexico exhibits moderate negative correlations with the leading pattern of sea surface temperature (SST) anomalies in the equatorial Pacific, which projects strongly onto El Niño. The influence of equatorial SSTs on southern Mexico rainfall seems to operate mainly through variability of the wet-day frequency, rather than through variations of the wet-day rain rate.

Atlantic forced component of the Indian monsoon interannual variability

The Indian monsoon interannual variability is modulated by the El Niño Southern Oscillation (ENSO), with a drier than normal monsoon season usually preceding peak El Niño conditions, and vice versa for La Niña phase. Pacific sea surface temperature (SST) anomalies, however, are not the only player. Building upon our recent discovery that atmospheric teleconnections between the tropical Atlantic and the Indian basin contributed to the weakening of the ENSO‐monsoon anticorrelation during the '80s and '90s, we investigate the role of south equatorial Atlantic SSTs in forcing the Indian monsoon rainfall (IMR). Using two observational data sets and two ensembles of simulations we show that the residual in the IMR time series for observed and modeled data, obtained by subtracting the ENSO‐forced component of the IMR that is linearly related to the NINO34 index, is significantly correlated with south equatorial Atlantic SSTs. Our results have important implications for seasonal forecast efforts.

The relationships between Pacific and Atlantic Ocean sea surface temperatures and Colombian streamflow variability

An evaluation of Pacific and Atlantic Ocean sea surface temperatures (SSTs) and Colombian streamflow was performed to identify coupled regions of SST and Colombian streamflow variability. Applying the Singular Value Decomposition (SVD) statistical method, SSTs (and Colombian streamflow for 10 stations) were evaluated for (a) the Pacific Ocean, (b) the Atlantic Ocean, and (c) the combined Pacific and Atlantic Ocean for a 41 year period of record (1960–2000). A lead-time approach was adopted such that spring–summer seasonal (April through September) SSTs were evaluated with streamflow for the following calendar-year (January through December). The El Niño-Southern Oscillation (ENSO) has been acknowledged in previous research efforts as an influence of Colombian streamflow. The ENSO signal identified that warmer (cooler) equatorial SSTs resulted in lesser (greater) streamflow. In general, the use of predefined published indices, such as ENSO, is appealing for water managers and forecasters. However, the utilization of SSTs for entire regions (Pacific and Atlantic Oceans) eliminates any spatial bias as to which oceanic SST region (or regions) impact hydrology. This will assist in the identification of regions that may not be represented in existing indices and could lead to improved forecasting. The SVD 1st temporal expansion series of Pacific (and the combined Pacific and Atlantic) Ocean SSTs and streamflow for several stations resulted in correlation values greater than that of well known climate indices (e.g., ENSO), which could result in improved streamflow predictability.

Influence of El Niño on the Upper-Ocean Circulation in the Tropical Atlantic Ocean

Abstract This study investigates the influence of El Niño on the upper-ocean circulation in the tropical Atlantic Ocean (via changes in the Atlantic trade winds) by analyzing observed sea surface temperature (SST) together with an ocean general circulation model integration forced by the NCEP–NCAR reanalysis. During periods with anomalously warm (cold) eastern equatorial Pacific SST, the southern Atlantic tropical cell is strengthened (weakened). The difference of the cell strength between El Niño and La Niña years is about 20% of the mean cell strength. However, the variability of the cell is not dominated by the remote forcing from the eastern equatorial Pacific but seems to be caused by intrinsic tropical Atlantic variability. A strengthening (weakening) for periods with anomalously warm (cold) eastern equatorial Pacific SST is also found for the zonal surface and subsurface currents. TOPEX/Poseidon altimetry data are used to validate the results based on the OGCM integration.

Mechanisms for the Interannual Variability in the Tropical Indian Ocean. Part I: The Role of Remote Forcing from the Tropical Pacific

Abstract A series of experiments are conducted using a coupled ocean–atmosphere general circulation model in regional coupled mode, which permits active air–sea interaction only within the Indian Ocean to the north of 30°S, with sea surface temperatures (SSTs) prescribed over the rest of the world oceans. In this paper, an ensemble of nine simulations has been analyzed with the observed SST anomalies for 1950–98 prescribed over the uncoupled region. The purpose of this study is to determine the major patterns of interannual variability in the tropical Indian Ocean that could be related to the global low-frequency fluctuations and to understand the physical links between the remote forcing and the regional coupled variations. The ensemble coupled simulations with prescribed SST outside the Indian Ocean are able to reproduce a considerable amount of observed variability in the tropical Indian Ocean during 1950–98. The first EOF modes of the simulated upper-ocean heat content and SST anomalies show structures that are quite consistent with those from the historical upper oceanic temperature and SST analyses. The dominant pattern of response is associated with an oceanic dynamical adjustment of the thermocline depth in the southwestern Indian Ocean. In general, a deepening of the thermocline in the southwest is usually accompanied by the enhanced upwelling and thermocline shoaling centered near the Sumatra coast. Further analysis shows that the leading external forcing is from the El Niño–Southern Oscillation (ENSO), which induces an anomalous fluctuation of the atmospheric anticyclones on both sides of the equator over the Indian Ocean, starting from the evolving stage of an El Niño event in boreal summer. Apart from weakening the Indian monsoon, the surface equatorial easterly anomalies associated with this circulation pattern first induce equatorial and coastal upwelling anomalies near the Sumatra coast from summer to fall, which enhance the equatorial zonal SST gradient and stimulate intense air–sea feedback in the equatorial ocean. Moreover, the persistent anticyclonic wind curl over the southern tropical Indian Ocean, reinforced by the equatorial air–sea coupling, forces substantial thermocline change centered at the thermocline ridge in the southwestern Indian Ocean for seasons. The significant thermocline change has profound and long-lasting influences on the SST fluctuations in the Indian Ocean. It should be noted that the ENSO forcing is not the only way that this kind of basinwide Indian Ocean fluctuations can be generated. As will be shown in the second part of this study, similar low-frequency fluctuations can also be generated by processes within the Indian and western Pacific region without ENSO influence. The unique feature of the ENSO influence is that, because of the high persistence of the atmospheric remote forcing from boreal summer to winter, the life span of the thermocline anomalies in the southwestern Indian Ocean is generally longer than that generated by regional coupled processes.

Relationship between the tropical Pacific and Indian Ocean sea‐surface temperature and monthly precipitation over the central highlands, Vietnam

AbstractThe relationship between monthly sea‐surface temperature (SST) in the tropical Pacific and the Indian Ocean and monthly precipitation over the Vietnamese central highlands (VCH) has been investigated by means of singular value decomposition. The seasonal variation of SST plays a critical role in the onset of the monsoon season and convective rain band movement in the inter‐tropical convergence zone.The relationships between precipitation and SST in both oceans vary significantly through the rainy season. In April, ENSO is strongly correlated with precipitation over the VCH, while the Indian Ocean SST only shows a significant correlation with precipitation in the northern VCH. In May, there is no significant relationship between precipitation and SST in either of the oceans. In June, precipitation over the VCH is negatively correlated with the northern Indian Ocean and the southern Pacific SST. Through July to September, no significant relationships were found between the Indian Ocean SST and precipitation patterns. The equatorial central to the eastern Pacific SST, in turn, is positively correlated with precipitation in a small area from the north to the south of the VCH. In October, precipitation over the VCH is strongly related to ENSO and positively correlated with the equatorial eastern Indian Ocean SST. For November, the north‐western Pacific as well as the equatorial eastern Indian Ocean SST is positively and strongly correlated with precipitation over the VCH.Lag‐time analyses demonstrate a potential link between the Pacific SST and monthly precipitation patterns through the rainy season from one to three months in advance, and between the Indian Ocean SST and monthly precipitation patterns in October and November from one to two months in advance. The analysed results provide a strong basis for a predictive scheme, but further analysis of skill levels needs to be developed. Copyright © 2007 Royal Meteorological Society

Satellite-Derived Surface Current Divergence in Relation to Tropical Atlantic SST and Wind

Abstract The relationships between tropical Atlantic Ocean surface currents and horizontal (mass) divergence, sea surface temperature (SST), and winds on monthly-to-annual time scales are described for the time period from 1993 through 2003. Surface horizontal mass divergence (upwelling) is calculated using surface currents estimated from satellite sea surface height, surface vector wind, and SST data with a quasi-linear, steady-state model. Geostrophic and Ekman dynamical contributions are considered. The satellite-derived surface currents match climatological drifter and ship-drift currents well, and divergence patterns are consistent with the annual north–south movement of the intertropical convergence zone (ITCZ) and equatorial cold tongue evolution. While the zonal velocity component is strongest, the meridional velocity component controls divergence along the equator and to the north beneath the ITCZ. Zonal velocity divergence is weaker but nonnegligible. Along the equator, a strong divergence (upwelling) season in the central/eastern equatorial Atlantic peaks in May while equatorial SST is cooling within the cold tongue. In addition, a secondary weaker and shorter equatorial divergence occurs in November also coincident with a slight SST cooling. The vertical transport at 30-m depth, averaged across the equatorial Atlantic Ocean between 2°S and 2°N for the record length, is 15(±6) × 106 m3 s−1. Results are consistent with what is known about equatorial upwelling and cold tongue evolution and establish a new method for observing the tropical upper ocean relative to geostrophic and Ekman dynamics at spatial and temporal coverage characteristic of satellite-based observations.