Local Sea Surface Temperature Anomalies Research Articles

Relationships between ENSO and the East Asian–western North Pacific monsoon: observations versus 18 CMIP5 models

The western North Pacific anomalous anticyclone (WNPAC) plays an essential role in linking El Nino and the East Asian–western North Pacific monsoon. The WNPAC and the corresponding WNP anomalous cyclone (WNPC) associated with La Nina are studied by examining the pre-industrial control runs of 18 Coupled Model Intercomparison Project Phase 5 models. The WNPAC is asymmetric with the WNPC during ENSO mature winters, which can be reproduced by seven out of the eighteen models. Based on the differences between the seven models and the other models, it is verified that the asymmetry results from the combined effects of the asymmetric remote forcing from the equatorial central-eastern Pacific and the asymmetric local sea surface temperature (SST) anomalies. It is further demonstrated that the asymmetric circulation anomalies over the tropical WNP contribute to the asymmetric decaying rates between El Nino and La Nina. Those models that can (cannot) simulate the asymmetry between the WNPAC and WNPC tend to reproduce ENSO with asymmetric (symmetric) decaying rates. In El Nino decaying summers, the WNPAC is still maintained due to the sequential effects of the local forcing of the underlying cold SST anomalies remaining from the previous winter and spring, and the remote forcing from the tropical Indian Ocean. Multi-model results show that the coupling of the local cold SST anomalies and the WNPAC forms a damping mode, which is only conducive to maintaining the WNPAC in the early summer, while the response of the WNPAC to the remote forcing from the tropical Indian Ocean intensifies from June to July, concurring with the establishment of the climatological WNP summer monsoon trough.

Remote forcing of Indian Ocean warming on Northwest Pacific during El Niño decaying years: a FOAM model approach

This paper attempts to analyze in detail the remote influence of the Indian Ocean Basin warming on the Northwest Pacific (NWP) during the year of decaying El Nino. Observation data and the Fast Ocean-Atmosphere coupled Model 1.5 were used to investigate the triggering conditions under which the remote influence is formed between the positive sea surface temperature (SST) anomaly in the North Indian Ocean and the Anomalous Northwest Pacific anticyclone (ANWPA). Our research show that it is only when there is a contributory background wind field over the Indian Ocean, i.e., when the Indian Summer Monsoon (ISM) reaches its peak, that the warmer SST anomaly in the North Indian Ocean incites significant easterly wind anomalies in the lower atmosphere of the Indo-West tropical Pacific. This then produces the remote influence on the ANWPA. Therefore, the SST anomaly in the North Indian Ocean might interfere with the prediction of the East Asia Summer Monsoon in the year of decaying El Nino. Both the sustaining effect of local negative SST anomalies in the NWP, and the remote effect of positive SST anomalies in the North Indian Ocean on the ANWPA, should be considered in further research.

Interannual variability of moisture source over southern Indian Ocean during boreal summer and its relationship with local SST

AbstractIn boreal summer, the southern Indian Ocean (SIO) features more evaporation than precipitation and thus it is a moisture source region for the Asian summer monsoon rainfall. In the present study, the interannual variability of moisture source over the SIO is analysed for boreal summer during 1979‐2008. The impacts of local sea surface temperature (SST) and the El Niño‐Southern Oscillation (ENSO) on the moisture source are compared and the possible mechanisms for the SST influence on the moisture source are investigated. The leading mode of the interannual variation of apparent moisture sink (<Q2>) over the SIO displays an obvious out‐of‐phase relationship over the western‐central SIO and the region to the northwest‐north of Australia. The interannual variations of < Q2> over the above two regions have significant positive correlation with local SST. An increase in local SST enhances convection and ascent, leading to more precipitation. Over the western‐central SIO, surface winds weaken corresponding to higher SST in the western‐central SIO and lower SST to the northwest‐north of Australia, leading to less oceanic evaporation. Thus, the variation of the moisture source over the western‐central SIO is contributed by both precipitation and evaporation. However, the variation of the moisture source to the northwest‐north of Australia is contributed mainly by precipitation. ENSO amplifies anomalous moisture source induced by local SST anomalies. ENSO and local SST work cooperatively to influence the moisture source over the SIO. Copyright © 2012 Royal Meteorological Society

Factors controlling January–April rainfall over southern India and Sri Lanka

Most of the annual rainfall over India occurs during the Southwest (June–September) and Northeast (October–December) monsoon periods. In March 2008, however, Southern peninsular India and Sri Lanka received the largest rainfall anomaly on record since 1979, with amplitude comparable to summer-monsoon interannual anomalies. This anomalous rainfall appeared to be modulated at intraseasonal timescale by the Madden Julian Oscillation, and was synchronous with a decaying La Niña event in the Pacific Ocean. Was this a coincidence or indicative of a teleconnection pattern? In this paper, we explore factors controlling rainfall over southern India and Sri Lanka between January and April, i.e. outside of the southwest and northeast monsoons. This period accounts for 20% of annual precipitation over Sri Lanka and 10% over the southern Indian states of Kerala and Tamil Nadu. Interannual variability is strong (about 40% of the January–April climatology). Intraseasonal rainfall anomalies over southern India and Sri Lanka are significantly associated with equatorial eastward propagation, characteristic of the Madden Julian Oscillation. At the interannual timescale, we find a clear connection with El Niño-Southern Oscillation (ENSO); with El Niños being associated with decreased rainfall (correlation of −0.46 significant at the 98% level). There is also a significant link with local SST anomalies over the Indian Ocean, and in particular with the inter-hemispheric sea surface temperature (SST) gradient over the Indian Ocean (with colder SST south of the equator being conducive to more rainfall, correlation of 0.55 significant at the 99% level). La Niñas/cold SSTs south of the equator tend to have a larger impact than El Niños. We discuss two possible mechanisms that could explain these statistical relationships: (1) subsidence over southern India remotely forced by Pacific SST anomalies; (2) impact of ENSO-forced regional Indian Ocean SST anomalies on convection. However, the length of the observational record does not allow distinguishing between these two mechanisms in a statistically significant manner.

The temporal and spatial characteristics of surrogate tropical cyclones from a multi-millenial simulation

Output from a simulation with the CSIRO Mark 2 climatic model has been used to investigate the secular variability of tropical cyclone formation over the globe using Gray’s Seasonal Genesis Parameter. This simulation differs from previous surrogate studies in using a coupled atmospheric-oceanic model, instead of specified sea surface temperatures, as well as being of multi-millenial duration, compared with decadal length simulations used elsewhere. Mean climatological values for each season for a 5,000-year period indicate that the model replicated the broad patterns of spatial and temporal variability. Results are presented in some detail for three regions, the southwest and northwest Pacific Oceans and the low latitude North Atlantic Ocean. A marked range of temporal variabilities of surrogate tropical cyclone numbers was obtained in the simulation, possibly indicating that the present, observed increase in these numbers may not be outside that attributable to natural variability. The component terms of the Seasonal Genesis Parameter permit the contribution of individual climatic terms to the generation of tropical cyclones to be identified. This approach highlighted the important role of relative vorticity and relative humidity, in addition to the governing influence of vertical wind shear. The remote influence of ENSO, versus that of local sea surface temperature anomalies, on surrogate tropical cyclone numbers was examined and revealed different outcomes depending on the region under consideration. The global total of surrogate tropical cyclone numbers exhibited noticeable interannual variability. The simulation reproduced most of the observed correlations between tropical cyclones and relevant climatic variables, but many of the correlations were not stable within the 5,000-year time series used. This suggests that observed correlations based on, typically, 100-years or less of data may not be representative of possible future outcomes. With minor exceptions all climatological time series associated with the Seasonal Genesis Parameter were found to be Gaussian.

Anomalous Rainfall over Southwest Western Australia Forced by Indian Ocean Sea Surface Temperatures

Abstract The potential impact of Indian Ocean sea surface temperature (SST) anomalies in modulating midlatitude precipitation across southern and western regions of Australia is assessed in a series of atmospheric general circulation model (AGCM) simulations. Two sets of AGCM integrations forced with a seasonally evolving characteristic dipole pattern in Indian Ocean SST consistent with observed “dry year” (PDRY) and “wet year” (PWET) signatures are shown to induce precipitation changes across western regions of Australia. Over Western Australia, a significant shift occurs in the winter and annual rainfall frequency with the distribution becoming skewed toward less (more) rainfall for the PDRY (PWET) SST pattern. For southwest Western Australia (SWWA), this shift primarily is due to the large-scale stable precipitation. Convective precipitation actually increases in the PDRY case over SWWA forced by local positive SST anomalies. A mechanism for the large-scale rainfall shifts is proposed, by which the SST anomalies induce a reorganization of the large-scale atmospheric circulation across the Indian Ocean basin. Thickness (1000–500 hPa) anomalies develop in the atmosphere mirroring the sign and position of the underlying SST anomalies. This leads to a weakening (strengthening) of the meridional thickness gradient and the subtropical jet during the austral winter in PDRY (PWET). The subsequent easterly offshore (westerly onshore) anomaly in the thermal wind over southern regions of Australia, along with a decrease (increase) in baroclinicity, results in the lower (higher) levels of large-scale stable precipitation. Variations in the vertical thermal structure of the atmosphere overlying the SST anomalies favor localized increased convective activity in PDRY because of differential temperature lapse rates. In contrast, enhanced widespread ascent of moist air masses associated with frontal movement in PWET accounts for a significant increase in rainfall in that ensemble set.

Possible Impact of the Indian Ocean SST on the Northern Hemisphere Circulation during El Niño*

Abstract Two atmospheric general circulation models (AGCMs), differing in numerics and physical parameterizations, are employed to test the hypothesis that El Niño–induced sea surface temperature (SST) anomalies in the tropical Indian Ocean impact considerably the Northern Hemisphere extratropical circulation anomalies during boreal winter [January–March +1 (JFM +1)] of El Niño years. The hypothesis grew out of recent findings that ocean dynamics influence SST variations over the southwest Indian Ocean (SWIO), and these in turn impact local precipitation. A set of ensemble simulations with the AGCMs was carried out to assess the combined and individual effects of tropical Pacific and Indian Ocean SST anomalies on the extratropical circulation. To elucidate the dynamics responsible for the teleconnection, solutions were sought from a linear version of one of the AGCMs. Both AGCMs demonstrate that the observed precipitation anomalies over the SWIO are determined by local SST anomalies. Analysis of the circulation response shows that over the Pacific–North American (PNA) region, the 500-hPa height anomalies, forced by Indian Ocean SST anomalies, oppose and destructively interfere with those forced by tropical Pacific SST anomalies. The model results validated with reanalysis data show that compared to the runs where only the tropical Pacific SST anomalies are specified, the root-mean-square error of the height anomalies over the PNA region is significantly reduced in runs in which the SST anomalies in the Indian Ocean are prescribed in addition to those in the tropical Pacific. Among the ensemble members, both precipitation anomalies over the SWIO and the 500-hPa height over the PNA region show high potential predictability. The solutions from the linear model indicate that the Rossby wave packets involved in setting up the teleconnection between the SWIO and the PNA region have a propagation path that is quite different from the classical El Niño–PNA linkage. The results of idealized experiments indicate that the Northern Hemisphere extratropical response to Indian Ocean SST anomalies is significant and the effect of this response needs to be considered in understanding the PNA pattern during El Niño years. The results presented herein suggest that the tropical Indian Ocean plays an active role in climate variability and that accurate observation of SST there is of urgent need.

Southwest Indian Ocean SST Variability: Its Local Effect and Remote Influence on Asian Monsoons*

Abstract An atmospheric general circulation model (AGCM) is used to examine the role of Indian Ocean sea surface temperature (SST) anomalies in regional climate variability. In particular, the authors focus on the effect of the basinwide warming that occurs during December through May after the mature phase of El Niño. To elucidate the relative importance of local and remote forcing, model solutions were sought for experiments where SST anomalies are inserted in the (i) tropical Indo-Pacific Oceans, (ii) tropical Pacific Ocean, and (iii) tropical Indian Ocean. A 10-member ensemble simulation is carried out for each of the three forcing scenarios. The model solutions demonstrate that precipitation variations over the southwest Indian Ocean are tied to local SST anomalies and are highly reproducible. Changes in the Indian Ocean–Walker circulation suppress precipitation over the tropical west Pacific–Maritime Continent, contributing to the development of a low-level anticyclone over the Philippine and South China Seas. Our model results indicate that more than 50% of the total precipitation anomalies over the tropical west Pacific–Maritime Continent is forced by remote Indian Ocean SST anomalies, offering an additional mechanism for the Philippine Sea anticyclone apart from Pacific SST. This anticyclone increases precipitation along the East Asian winter monsoon front from December to May. The anomalous subsidence over the Maritime Continent in conjunction with persistent anomalies of SST and precipitation over the Indian Ocean in spring prevent the northwestward migration of the ITCZ and the associated deep moist layer, causing a significant delay in the Indian summer monsoon onset in June by 6–7 days. At time scales of 5 days, however, the reproducibility of the northward progression of the ITCZ during the onset is low. Results indicate that Indian Ocean SST anomalies during December through May that develop in response to both atmospheric and oceanic processes to El Niño need to be considered for a complete understanding of regional climate variability, particularly around the Indian Ocean rim.

The relative roles of tropical and extratropical forcing on atmospheric variability

[1] A fast atmospheric General Circulation Model is used to generate three ensembles of atmospheric circulation. While the first ensemble is forced by global sea surface temperature (SST) anomalies, the forcing is confined to the tropics and extratropics, respectively, in the other two ensembles. The tropical SST anomalies have the largest impact on atmospheric variability. However, also the extratropical SST has a systematic effect on the atmosphere. In the Southern Hemisphere the extratropically forced signal reaches the strength of the tropically forced one on decadal time scales. The atmospheric response to global SST anomalies can be understood as a nearly linear superposition of the reactions to tropical and extratropical forcing, respectively, or, in a different view, as a superposition of a thermodynamic response to the global-mean SST anomaly and a dynamic response to local SST anomalies, the most important being El Nino.

Impact of Indian Ocean Sea Surface Temperature on Developing El Niño*

Abstract Prior to the 1976–77 climate shift (1950–76), sea surface temperature (SST) anomalies in the tropical Indian Ocean consisted of a basinwide warming during boreal fall of the developing phase of most El Niños, whereas after the shift (1977–99) they had an east–west asymmetry—a consequence of El Niño being associated with the Indian Ocean Dipole/Zonal mode. In this study, the possible impact of these contrasting SST patterns on the ongoing El Niño is investigated, using atmospheric reanalysis products and solutions to both an atmospheric general circulation model (AGCM) and a simple atmospheric model (LBM), with the latter used to identify basic processes. Specifically, analyses of reanalysis products during the El Niño onset indicate that after the climate shift a low-level anticyclone over the South China Sea was shifted into the Bay of Bengal and that equatorial westerly anomalies in the Pacific Ocean were considerably stronger. The present study focuses on determining influence of Indian Ocean SST on these changes. A suite of AGCM experiments, each consisting of a 10-member ensemble, is carried out to assess the relative importance of remote (Pacific) versus local (Indian Ocean) SST anomalies in determining precipitation anomalies over the equatorial Indian Ocean. Solutions indicate that both local and remote SST anomalies are necessary for realistic simulations, with convection in the tropical west Pacific and the subsequent development of the South China Sea anticyclone being particularly sensitive to Indian Ocean SST anomalies. Prior to the climate shift, the basinwide Indian Ocean SST anomalies generate an atmospheric Kelvin wave associated with easterly flow over the equatorial west-central Pacific, thereby weakening the westerly anomalies associated with the developing El Niño. In contrast, after the shift, the east–west contrast in Indian Ocean SST anomalies does not generate a significant Kelvin wave response, and there is little effect on the El Niño–induced westerlies. The Linear Baroclinic Model (LBM) solutions confirm the AGCM’s results.

Relation between SSTs and Predictability of Seasonal Mean Precipitation over the Western Tropical Pacific

This study investigates the difference of the predictability of seasonal mean precipitation with Japan Meteorological Agency (JMA) Atmospheric General Circulation Model (AGCM), using two types of prescribed sea surface temperature (SST). One set of seasonal prediction simulations is called (which stands for Seasonal prediction Model Intercomparison Project). In the SMIP, observed SSTs are prescribed. The other set is called HINDCAST. In the HINDCAST, SST anomalies at initial time are assumed to persist during the forecast period. The December-January-February (DJF) averaged precipitation predictability over the tropical Pacific in SMIP is higher than that in HINDCAST, as was expected. However, it was found that the June-July-August (JJA) averaged precipitation predictability over the western tropical Pacific in SMIP was lower than that in HJNDCAST. In the western tropical Pacific, there is a negative correlation between the observed precipitation anomalies and SST anomalies in JJA. The observed precipitation anomalies in early summer (May-June-July) are well correlated with the SST anomalies in spring (March-April-May). The simulated precipitation anomalies are strongly influenced by local SST anomalies in the same period. Because of this observed lag-correlation between precipitations and SSTs, and the property of simulated precipitation by the model, JJA averaged precipitation predictability over the western tropical Pacific in HINDCAST is higher than that in SMIP.

Coupling between Northward-Propagating, Intraseasonal Oscillations and Sea Surface Temperature in the Indian Ocean*

Using a hybrid atmosphere–ocean coupled model, it is shown that during the boreal summer northward-propagating, intraseasonal oscillations (NPISOs) are strongly coupled to the underlying sea surface temperature (SST) in the Indian Ocean sector. On the one hand, the intraseasonal atmospheric convection changes the SST through solar radiation, latent heat flux, and mixed-layer entrainment; on the other, the induced SST fluctuations feed back to affect the intraseasonal convection. The preferential northward, rather than southward, propagation of boreal summer ISOs in the Indian Ocean is partially explained by an interaction among the summer-mean climate state, the atmospheric disturbances, and the ocean surface temperature. A solution to an atmosphere-only model forced with daily SST produces much stronger NPISOs than a similar solution forced with monthly mean SST (AMIP-type run). The atmosphere-only model, however, even when it is forced by daily SST from the coupled model (with a small amount of noise in the initial and/or boundary conditions), is unable to reproduce the NPISOs in the coupled case. In the coupled system, intraseasonal SST anomalies are forced by intraseasonal atmospheric convection, and hence are in quadrature with the convection. In the stand-alone atmospheric model, however, SST acts only as a boundary forcing, and the resultant atmospheric convection has almost the same phase with the underlying SST. One consequence is that the intensity of the SST-forced intraseasonal convection in the stand-alone atmospheric model is considerably weaker than in the coupled model. Finally, solutions indicate that the northward movement of the off-equatorial convection in the northern Indian Ocean is more closely related to local intraseasonal SST anomalies than to the equatorial eastward-moving Madden–Julian oscillation: Positive (negative) SST anomalies in the northern Indian Ocean lead the active (break) phases of the intraseasonal convection by about 2 pentads (10 days). Therefore, intraseasonal SST anomalies in the northern Indian Ocean are potentially a useful index to forecast active (break) spells of the south Asian summer monsoon.

Influence of El Niño/southern oscillation, Pacific decadal oscillation, and local sea-surface temperature anomalies on peak season monsoon precipitation in India

CR Climate Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials CR 25:171-178 (2003) - doi:10.3354/cr025171 Influence of El Niño/southern oscillation, Pacific decadal oscillation, and local sea-surface temperature anomalies on peak season monsoon precipitation in India Shouraseni Sen Roy, Gregory B. Goodrich, Robert C. Balling Jr* Department of Geography, Arizona State University, Tempe, Arizona 85287, USA *Corresponding author. Email: robert.balling@asu.edu ABSTRACT: Many modeling and empirical studies have revealed that summer monsoon precipitation in India is significantly affected by El Niño/Southern Oscillation (ENSO) as well as by sea-surface temperatures (SSTs) in the Arabian Sea, Bay of Bengal, and Indian Ocean. Recently, the impact of the Pacific decadal oscillation (PDO) on Indian rainfall has been the focus of research. In this investigation, we collect monthly rainfall data for 18 grid cells covering India and develop statistical indices of ENSO, PDO, and local SSTs over the period 1925-1998. We find that ENSO reduces precipitation in southern India while having a small impact over most of the country. The PDO appears to amplify the ENSO signal in southern India, while local SSTs were directly related to monsoon precipitation totals in the southern peninsula region. While the associations are often statistically significant, the combination of ENSO, PDO, and SSTs explains less than 20% of the variance in monsoon rainfall throughout India. KEY WORDS: India monsoon rainfall · El Niño/Southern Oscillation · Pacific Decadal Oscillation · Sea surface temperatures Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in CR Vol. 25, No. 2. Online publication date: December 05, 2003 Print ISSN: 0936-577X; Online ISSN: 1616-1572 Copyright © 2003 Inter-Research.

Principal modes of interannual and decadal variability of summer rainfall over South America

AbstractUsing the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) product together with the Goddard Earth Observing System (GEOS) reanalysis and the National Center for Environmental Prediction (NCEP) sea‐surface temperature (SST) data, we have conducted a diagnostic study of the interannual and decadal scale variability of principal modes of summer rainfall over South America for the period 1979–1995. By filtering the annual and short (<12 months) time‐scale variations, results of empirical orthogonal function analysis show three leading modes of rainfall variation identified with interannual, decadal and long‐term variability. Together, these modes explain more than half the total variance of the filtered data.The first mode is highly correlated with El Niño–Southern Oscillation (ENSO), showing a regional rainfall anomaly pattern largely consistent with previous results. This mode captures the summer season interannual variability, not only the Northeast Brazil drought but also its connection with excessive rainfall over Southern Brazil and the Ecuador coast in El Niño years. Another distinctive feature is the strengthening of the low‐level flow along the eastern foothills of the eastern Andes, signifying an enhancement of the South American summer monsoon in response to an El Niño anomaly.The decadal variation displays a meridional shift of the Inter‐Tropical Convergence Zone (ITCZ), which is tied to the anomalous cross‐equatorial SST gradient over the Atlantic and the eastern Pacific. Associated with this mode is a large‐scale mass swing between polar regions and the mid‐latitudes. Over the South Atlantic and the South Pacific, the anomalous subtropical high and the associated anomalous surface wind are dynamically consistent with the distribution of local SST anomalies, suggesting the importance of atmospheric forcing at the decadal time scale.The long‐term variation shows that since 1980 there has been a decrease of rainfall from the northwest coast to the southeast subtropical region and a southwards shift of the Atlantic ITCZ, leading to increased rainfall over northern and eastern Brazil. Possible links of this mode to the observed SST warming trend over the subtropical South Atlantic and to the interdecadal SST variation over the extratropical North Atlantic are discussed. Copyright © 2001 Royal Meteorological Society

Tropical teleconnection and local response to SST anomalies during the 1997–1998 El Niño

The quasi‐equilibrium tropical circulation model (QTCM) is used to examine the response to various sea surface temperature (SST) anomalies in the tropical oceans during the 1997–1998 El Niño. Both local and remote responses are noted. The negative precipitation anomalies to the north and south of the El Niño‐Southern Oscillation (ENSO) enhanced precipitation region are largely a response to the warm SST anomalies in the central and eastern Pacific. However, in the western Pacific and maritime continent, reduction of rainfall is mainly caused by local cold SST anomalies. In the winter of the 1997–1998 El Niño, strong warm SST anomalies in the Indian Ocean contributed to the local enhanced rainfall. They affect precipitation anomalies in central, eastern, and southern Africa. The drought in northern South America is clearly a remote response to ENSO warm SST anomalies in the Pacific, while the SST anomalies in the Atlantic also impact the drought. The tropical Pacific cold SST anomalies surrounding the ENSO warm anomalies appear not to be caused by surface flux changes associated with atmospheric teleconnection (in simulations with specified SST in the ENSO warm region and a mixed‐layer ocean model elsewhere). Atmospheric circulation tends to spread the warm anomalies, but ocean dynamics appears also to be important for both cold and warm SST anomalies outside the equatorial upwelling region. In both the regional specified SST experiments and mixed‐layer experiments, relative subsidence tends to occur within convective zones, and it is generally localized. On the other hand, tropospheric temperature and wind anomalies spread much farther. The typical spatial scale of the remote response in temperature and wind fields tends to be larger than the dominant scale of the precipitation response. The remote response in precipitation anomalies does not appear to be related to temperature and wind anomalies in a simple manner.

Local and remote atmospheric response to tropical instability waves: A global view from space

A La Niña took place in the equatorial Pacific in 1999, and strong tropical instability waves (TIWs) developed, causing large meanders of a sea surface temperature (SST) front between the equator and 3°N. High‐resolution satellite measurements are used to describe the variability of SST, surface wind velocity, column‐integrated water vapor, cloud liquid water, and precipitation associated with these strong TIWs in 1999. Coherent ocean‐atmosphere patterns emerge in both the Pacific and Atlantic, revealing rich regional characteristics. In the far eastern Pacific, southeasterly trades strengthen (weaken) over positive (negative) SST anomalies, apparently due to enhanced (reduced) mixing with high‐speed winds aloft. In the central Pacific we find evidence that SST‐induced sea level pressure changes also contribute substantially to wind fluctuations. Similar SST‐wind covariability also exists in the Southern Hemisphere along 2°S, but the wind variability induced by unit SST anomaly is much larger than that north of the equator. In the central Pacific where the equatorial front is broad, the northern SST pattern has a large meridional scale and reaches as far north as 6°N. Further to the north in the Intertropical Convergence Zone (ITCZ) where local SST anomalies are small, significant variability is found in clouds and precipitation, which is further correlated with surface wind convergence. In the Atlantic, TIW signals in SST are strongly trapped near the equator, but they induce significant remote response in the ITCZ, which takes a more southern position than its Pacific counterpart and thus more susceptible to TIW influence.

Interannual Variability of the North American Warm Season Precipitation Regime

Interannual variability of the North American warm season-precipitation regime is examined in three regions of the United States and Mexico: Arizona-New Mexico, northwest Mexico, and southwest Mexico. Daily observed precipitation over the United States and Mexico for a 26-yr (1963-88) period and various fields from the National Centers for Environmental Prediction-National Center for Atmospheric Research Reanalysis are used to compare and contrast hydrologic conditions and atmospheric circulation features associated with early, late, wet, and dry monsoons in each region. Relationships between anomalous monsoon behavior and the El Nino-Southern Oscillation phenomenon are examined. Some factors associated with the atmosphere's lower boundary conditions that might influence the interannual variability of the warm season precipitation regime are discussed. The mean seasonal evolution of the North American monsoon system is characterized by the regular northward progression of heavy precipitation from southern Mexico by early June to the southwestern United States by early July. While the seasonal normal rainfall and its variability are largest in southwest Mexico, the mean seasonal percent departure from normal is largest in Arizona-New Mexico. Wet (dry) monsoons in southwest Mexico tend to occur during La Nina (El Nino). This association is attributed, in part, to the impact of local sea surface temperature anomalies on the land-sea thermal contrast, hence the strength of the monsoon. There is also a weak association between dry monsoons in Arizona-New Mexico (northwest Mexico) and La Nina (El Nino). Wet summer monsoons in Arizona-New Mexico tend to follow winters characterized by dry conditions in the southwestern United States and vice versa. Although the onset and duration of the monsoon are quite regular in each region, the precise date of onset in a given region is highly variable and likely to be unrelated to the date of onset in the other regions. Early monsoons in Arizona-New Mexico tend to have heavy seasonal rainfall while late monsoons in northwest Mexico tend to have deficient seasonal rainfall. The onset date in southwest Mexico is not related to seasonal rainfall. However, interannual fluctuations in rainfall over the entire monsoon region for the 2-month period after onset in southwest Mexico are highly correlated, suggesting that knowledge of the starting date in southwest Mexico may be useful for analyzing, understanding, and possibly predicting these fluctuations.

ENSO-like Interdecadal Variability: 1900–93

A number of recent studies have reported an ENSO-like EOF mode in the global sea surface temperature (SST) field, whose time variability is marked by an abrupt change toward a warmer tropical eastern Pacific and a colder extratropical central North Pacific in 1976‐77. The present study compares this pattern with the structure of the interannual variability associated with the ENSO cycle and documents its time history back to 1900. The analysis is primarily based on the leading EOFs of the SST anomaly and ‘‘anomaly deviation’’ fields in various domains and the associated expansion coefficient (or principal component) time series, which are used to construct global regression maps of SST, sea level pressure (SLP), and a number of related variables. The use of ‘‘anomaly deviations’’ (i.e., departures of local SST anomalies from the concurrent global-mean SST anomaly) reduces the influence of global-mean SST trends upon the structure of the EOFs and their expansion coefficient time series. An important auxiliary time series used in this study is a ‘‘Southern Oscillation index’’ based on marine surface observations. By means of several different analysis techniques, the time variability of the leading EOF of the global SST field is separated into two components: one identified with the ‘‘ENSO cycle-related’’ variability on the interannual timescale, and the other a linearly independent ‘‘residual’’ comprising all the interdecadal variability in the record. The two components exhibit rather similar spatial signatures in the global SST, SLP, and wind stress fields. The SST signature in the residual variability is less equatorially confined in the eastern Pacific and it is relatively more prominent over the extratropical North Pacific. The corresponding SLP signature is also stronger over the extratropical North Pacific, and its counterpart in the cold season 500-mb height field more closely resembles the PNA pattern. The amplitude time series of the ENSO-like pattern in the residual variability reflects the above-mentioned shift in 1976‐77, as well as a number of other prominent features, including a shift of opposite polarity during the 1940s.