Pacific Sea Surface Temperature Variability Research Articles

The Characteristic Variability of Boreal Wintertime Atmospheric Circulation in El Niño Events

Abstract The influence of the Pacific sea surface temperature (SST) on the characteristic variability of boreal wintertime atmospheric circulation among El Nino events is investigated by carrying out singular value decomposition (SVD) analysis and general circulation model (GCM) experiments. From the SVD analysis of SST and 300-hPa streamfunction in the El Nino winter, the first mode associated with the strength of the El Nino mode itself has a positive trend in its time series and the second mode associated with the North Pacific SST variation includes substantial event-to-event variability. The second mode is thus important for explaining the inter–El Nino variability in atmospheric circulation. Four El Ninos among 15 El Nino events from 1950 to 1998 have a mainly positively contribution from the first SVD mode, and the other four events are influenced by the second mode as well as the first mode. The atmospheric responses related to the latter four events show the weakening of the Aleutian low and an e...

Teleconnections between the tropical Pacific and the Sahel

AbstractA greater understanding of equatorial teleconnections is a key aspect of research into seasonal prediction and future climate change for tropical regions. Here the impact of Pacific sea surface temperature (SST) anomalies on rainfall fluctuations over the semi‐arid Sahel is explored, using a combination of observed and model data.The first key topic of this study is the identification of those aspects of anomalous Pacific SST variability that are most strongly linked to the Sahel. One of these, also illuminated by earlier studies, is similar to the classic El Niño Southern Oscillation pattern in the central and east Pacific, which in its El Niño phase increases the likelihood of Sahel drought. It is shown here that, although a part of this link is indirect (operating via Atlantic SSTs), its main effect appears to be through a direct atmospheric teleconnection. The other critical pattern, of equal importance and revealed here by a novel analysis technique, is the large‐scale zonal gradient of SSTs from the west Pacific to the east Indian Ocean. If weakened, this too enhances the likelihood of Sahel drought. Atmospheric general circulation model experiments, forced either by observed or idealized SSTs, are used to confirm these two influences on the Sahel. Crucially, their Sahelian impact is substantially reinforced when both are present and, additionally, further empirical analysis shows them to be largely independent.The second key topic is an investigation of the mechanisms for this Pacific‐Sahel teleconnection. These appear to involve anomalous stationary equatorial waves, with communication occurring in both the eastward and westward directions. In El Niño years (for example), a Kelvin wave emanates across the Atlantic from east Pacific convective heating anomalies, and an equatorial Rossby wave appears over the Indian Ocean in response to the anomalous west Pacific‐Indian Ocean SST gradients via convective heating anomalies over the Indian Ocean. These interact over Africa to enhance large‐scale subsidence over the Sahel, thus reducing seasonal rainfall totals. Interannual changes in propagating equatorial waves or in the residence of subseasonal regimes appear not to play a substantial role.

Surface flux response to interannual tropical Pacific sea surface temperature variability in AMIP models

A systematic comparison of observed and modeled atmospheric surface heat and momentum fluxes related to sea surface temperature (SST) variability on interannual time scales in the tropical Pacific is conducted. This is done to examine the ability of atmospheric general circulation models (AGCMs) in the Atmospheric Model Intercomparison Project (AMIP) to simulate the surface fluxes important for driving the ocean on interannual time scales. In order to estimate the model and observed response to such SST variability, various regression calculations are made between a time series representing observed ENSO SST variability in the tropical Pacific and the resulting surface flux anomalies. The models exhibit a range of differences from the observations. Overall the zonal wind stress anomalies are most accurately simulated while the solar radiation anomalies are the least accurately depicted. The deficiencies in the solar radiation are closely related to errors in cloudiness. The total heat flux shows some cancellation of the errors in its components particularly in the central Pacific. The performance of the GCMs in simulating the surface flux anomalies seems to be resolution dependent and low-resolution models tend to exhibit weaker flux responses. The simulated responses in the western Pacific are more variable than those of the central and eastern Pacific but in the west the observed estimates are less robust as well. Further improvements in atmospheric GCM flux simulation through better physical parametrization is clearly required if such models are to be used to their full potential in coupled modeling and climate forecasting.

Winter Monthly Mean Atmospheric Anomalies over the North Pacific and North America Associated with El Niño SSTs

The characteristics of winter monthly mean extratropical circulation associated with El Niño, including precipitation and surface temperature over the United States, are examined for nine El Niño events during 1950–94. Precipitation and surface temperature over the United States, also the 500-mb geopotential height and sea level pressure over the North Pacific and North America, are significantly different between early winter (November and December) and late winter (January to March). The typical El Niño-related U.S. precipitation and surface temperatures identified in many previous studies, as well as the Pacific–North American (PNA) circulation pattern, emerge in January and persist through February and March. The PNA patterns during these late winter months are coupled both with the tropical El Niño sea surface temperature (SST) variation and with the North Pacific SST variation. In contrast, the PNA patterns in the early winter months correlate only with the North Pacific SST. The tendency for the PNA pattern to occur during El Niño years is much less in early winter months than in late winter months. An ensemble analysis of 12 45-yr (1950–94) integrations of the National Center for Atmospheric Research Community Climate Model forced by the observed time-varying SST shows that the model 500-mb heights display a PNA-like pattern in both early and late winters of El Niño. The ensemble model response to the El Niño SST is thus unable to reproduce the observed differences in the extratropical atmospheric circulation between early and late winter months.

Covariabilities of Winter U.S. Precipitation and Pacific Sea Surface Temperatures

Abstract The variability of winter average U.S. precipitation displays strong geographical dependence with large variability in the southeastern and northwestern United States. The covariance of the U.S. winter mean precipitation with Pacific sea surface temperature (SST) is examined in this study using the singular value decomposition (SVD) method. The first SVD mode indicates the U.S. precipitation pattern that is associated with the tropical El Nino/La Nina SST variation, while the second and third SVD modes relate the precipitation variability in the Pacific Northwest and southeast that is associated with the North Pacific SST variation. About 45% of the U.S. precipitation variabilities is related to the Pacific SST anomalies, among which, 35% is related to the North Pacific SST and 10% is related to the tropical Pacific SST. Each SVD precipitation pattern is associated with well-organized 500-mb height and zonal mean zonal wind anomalies. It is shown that the North Pacific SST anomalies associated wi...

Climate impact of late quaternary equatorial pacific sea surface temperature variations

Magnesium/calcium data from planktonic foraminifera in equatorial Pacific sediment cores demonstrate that tropical Pacific sea surface temperatures (SSTs) were 2.8 degrees +/- 0.7 degrees C colder than the present at the last glacial maximum. Glacial-interglacial temperature differences as great as 5 degrees C are observed over the last 450 thousand years. Changes in SST coincide with changes in Antarctic air temperature and precede changes in continental ice volume by about 3 thousand years, suggesting that tropical cooling played a major role in driving ice-age climate. Comparison of SST estimates from eastern and western sites indicates that the equatorial Pacific zonal SST gradient was similar or somewhat larger during glacial episodes. Extraction of a salinity proxy from the magnesium/calcium and oxygen isotope data indicates that transport of water vapor into the western Pacific was enhanced during glacial episodes.

Tropical‐extratropical connection in the Atlantic atmosphere‐ocean variability

A question how sea surface temperature (SST) fluctuations in the tropical Atlantic affect the North Atlantic atmosphere‐ocean system is addressed using an atmospheric general circulation model coupled to a 50‐m‐deep slab ocean (MGCM), and a linear baroclinic model. A comparison of a 60‐yr integration, in which the tropical SSTs are prescribed to observations (TOGA‐ML), with the control MGCM run shows that the tropical SST variations selectively enhance variance of the North Atlantic Oscillation (NAO) by about 80%. This is partly due to extratropical height anomalies associated with two dominant modes, monopole and dipole, of tropical Atlantic SST variability because they both have a projection on to the NAO in the TOGA‐ML run. The simulated midlatitude height anomalies accompany characteristic SST anomaly patterns beneath, and they are consistent with observations. Analyses of steady linear responses to the heating in the tropical Atlantic and transient eddy feedback in midlatitude reveal that the substantial part of height anomalies is generated by the latter rather than the former. The anomalous eddy forcing is, however, organized not only by the heating in the tropical Atlantic but also by the tropical Pacific SST variations through the teleconnection over North America. It is suggested that the tropical Atlantic SST variability does affect the North Atlantic atmosphere and ocean, but the dynamical connection is tenuous in comparison with the Pacific.

Interannual Variability in Reconstructed Canadian Snow Cover, 1915–1992

Abstract Seasonal snow cover information over southern Canada was reconstructed from daily snowfall and maximum temperature data back to 1915 using a simple mass balance approach with snowmelt estimated via a calibrated temperature index method. The reconstruction method was able to account for 70%–80% of the variance in annual snow cover duration (SCD) over most of southern Canada for the 1955–1992 calibration period. The data were used to construct regional SCD anomaly series in four regions spanning the continent. The regional SCD series were characterized by high interannual variability, with most of the variance concentrated at periods less than 5 years. Spring (MAM) snow cover variability was characterized by a prominent spectral peak with a period of approximately 4 years, which appeared to be linked to tropical Pacific sea surface temperature variability. There was no evidence of statistically significant long-term trends in snow cover in any of the regions, but the data suggested that winter (DJF...

Satellite‐based identification of linked vegetation index and sea surface temperature Anomaly areas from 1982–1990 for Africa, Australia and South America

Meteorological satellite data from 1982 to 1990 were used to identify areas of significant association between tropical Pacific sea surface temperature (SST) and remotely sensed normalized difference vegetation index (NDVI) anomalies, here taken as a surrogate for rainfall anomalies. During this period, large areas of arid and semi‐arid Africa, Australia and South America experienced NDVI anomalies directly correlated to tropical Pacific SST anomalies. The results are limited by the relatively short time period of analysis. However, they confirm the disruptive effects of large‐scale tropical Pacific SST variations on arid and semiarid continental rainfall patterns in Africa, Australia, and South America, as reported previously.

Surface temperature trends and variations in New Zealand and the surrounding ocean, 1871–1993

AbstractWe compare homogenized series of maximum, minimum, and mean air temperature averaged over New Zealand, measured between 1871 and 1993, with rigorously quality controlled marine temperature data measured over the surrounding ocean surface. The marine data are those of sea‐surface temperature (SST) and air temperature measured at night (NMAT) on board ship, both corrected for time‐varying instrumental biases. There is mostly very good agreement between the variations in the three data sets on time‐scales down to a season. Some disagreements are related to short periods of sparsely observed marine data, particularly during the World Wars. Differences also exist between trends in the maximum and minimum New Zealand temperatures (NZT), particularly in winter. In addition, interannual variations in winter NZT tend not to track those of nearby marine temperatures quite as well as happens in other seasons.Although it is not the main purpose of the paper to discuss the causes of NZT change, New Zealand temperature is known to be influenced by the El Niño‐Southern Oscillation (ENSO) phenomena. Tropical East Pacific SST variations are strongly related to ENSO and so are well correlated with NZT on time‐scales of a few years to near a decade. We discuss these relationships, and also associations with Southern Hemisphere SST.We conclude that annual NZT and NMAT over the nearby ocean surface have both warmed by about 0.7°C since the beginning of the century, with a slightly smaller increase in SST. This confirms previous work on the magnitude of the warming of New Zealand climate this century. Warming in NMAT and NZT in each season varies in a similar way, with consistently slightly smaller increases in SST. We also conclude that the UK Meteorological Office seasonal historical marine temperature data set appears to be generally of very good quality in the New Zealand region.

On the Relationship between Tropical and North Pacific Sea Surface Temperature Variations

Abstract Empirical orthogonal function analysis of winter sea surface temperature (SST) anomalies over the Pacific domain (60°N–20°S) reveals an El Nino-Southern Oscillation (ENSO) mode that is linked to the eastern North Pacific, and a North Pacific mode that is linearly independent of ENSO. The North Pacific mode exhibits maximum amplitude and variance explained along ∼40°N, west of ∼170°W. SSTs in this region have decreased by ∼1.5°C from 1950 to 1987. The cooling in winter has been associated with a strengthening of the overlying westerly winds.

Spatial sampling requirements for tropical Pacific sea surface temperature variability

The Tropical Ocean and Global Atmosphere‐Tropical Atmosphere Ocean (TOGA‐TAO) array of moored buoys is a real‐time ocean‐observing system which measures wind and upper ocean temperature and currents in the equatorial Pacific Ocean. The mean square error for TOGA‐TAO sea surface temperature (SST) due to spatial sampling of 1500 km zonally and 200 km meridionaliy is expressed as the frequency‐wavenumber integral of a sampling design dependent filter multiplied by the space‐time spectral density of the SST field. Ten years of NOAA advanced very high resolution radiometer SST data, on a 1° by 1° by 1‐day grid, were used to obtain space‐time statistics. The zonal correlation length decreases with latitude. The zonal correlation length is 2000 km at the equator and 1500 km at 4°N. The temporal correlations are 90 days at all latitudes from 0°N to 5°N. Thé frequency‐wavenumber spectrum decreases as the inverse square of the wavenumber in the low‐wavenumber domain and as the inverse cube of the frequency in the low‐frequency domain. By using the sampling error formalism and the frequency‐wavenumber spectrum obtained from satellite‐derived SST, the sampling errors of the area‐ and time‐averaged SST over the averaging box of 15° zonally by 2° meridionally by 1 month were found to be about 12% of the variance of the area‐averaged SST for the TOGA‐TAO array spacing.

Tropical pacific sea surface temperature variations in a coupled atmosphere-ocean general circulation model

A Pacific Ocean general circulation model (horizontal resolution of 5° in longitude and 4° in latitude) is coupled with a global atmospheric general circulation model, both developed at the Meteorological Research Institute/Japan Meteorological Agency, to study the coupled atmosphere-ocean system in the tropical Pacific Ocean. Without any flux correction in the atmosphere-ocean interaction, seasonal variation of equatorial sea surface temperature ( SST) is reasonably simulated, though interannual and intraseasonal variations in the model are smaller than the observed variations. Although cooling trend of the equatorial SST is apparent in the initial 1–2 years, SST attains to a stable state with 1–2°C cooler than the observed SST in the western equatorial Pacific. The east-west difference of the model equatorial SST is smaller than the observed one mainly because the easterly wind in the eastern equatorial Pacific is not strong enough to upwell cold water there.

The Response of a Linear Model of the Tropical Pacific to Surface Winds from the NCAR General Circulation Model

Abstract An experiment in which surface wind stress data from the National Center for Atmospheric Research global circulation model (GCM) was used to drive a simple model of the tropical Pacific is described. First, a 15-year integration of the GCM was conducted, forcing the model with observed sea surface temperatures (SSTs) for the period 1958–72 (the FORCED case). A parallel integration was conducted using long-term monthly mean SSTs (the CONTROL case). All other boundary forcing in both GCM simulations was identical. Surface wind stress date from each GCM integration were then used to drive the Florida State University 1½ layer, reduced gravity model of the tropical Pacific. A separate integration of the ocean model was conducted using observed wind stress for the period 1961–72 (the OBSTRESS case). The goal of this research was to evaluate the response of a sophisticated GCM to tropical Pacific SST variability in terms of the surface wind stress field, and to investigate the sensitivity of a simple w...

A General Circulation Model Study of January Climate Anomaly Patterns Associated with Interannual Variation of Equatorial Pacific Sea Surface Temperatures

Abstract A general circulation model has been run in the perpetual January mode to produce several long-term simulations, each distinguished by a different imposed equatorial Pacific sea surface temperature. From each of them simulations we have extracted an eight-member ensemble of 90-day averaged fields. Ensemble-mean difference maps are presented in this paper, together with an estimate of the statistical significance of features which appear in thee maps. These results are compared with observational studies in the literature that present difference maps of Northern Hemisphere winter fields composited according to some index related to the two extremes of equatorial Pacific sea-surface temperature variation. The results show many anomaly patterns of high statistical significance that are also in good agreement with those observed. In the tropics, them include 990 mb wind, sea level pressure and rainfall anomalies constituting the Southern Oscillation, as well as a 200 mb height anomaly at all longitud...

Simulation of Seasonal Sea Surface Temperature Variations In the North Pacific

North Pacific sea surface temperature variations during the cooling season are simulated using climatological surface boat flux data in conjunction with a deepening mixed layer as determined by a convective adjustment model. Climatological oceanic initial conditions are constructed from the mechanical bathy-thermograph file of the National Oceanographic Data Center. The accuracy of the 90-day simulation depends on the accuracy of the initial and boundary conditions and on the ability to model the deepening of the mixed layer. The neglect of horizontal advection is most critical in the western boundary current region. The ability of such a model to accurately simulate SST changes over a large portion of the North Pacific indicates that the climatological surface heat fluxes computed by Clark (1967) are compatible with the decrease in the heat content of the upper, ocean during this period.

The 1968–69 Winter as an Outgrowth of Sea and Air Coupling During Antecedent Seasons

Abstract North Pacific sea surface temperature variations are analyzed as a result of and a controlling factor for the general atmospheric circulation during the period from May 1968 through the subsequent winter, when heavy rains in California, snows in Washington and Oregon, and disappearance of the Hawaiian trade winds occurred. The data indicate that a vast pool of abnormally warm water developed rapidly over southern portions of the North Pacific in June because of an abrupt May-to-June change in atmospheric circulation. This change favored strong subsidence in a deep Pacific anticyclone, dissipation of cloud, and greatly increased insolation. The climatologically stable Pacific anticyclone with its subsidence permitted the warm pool to remain through the subsequent fall. Penetration into the area of the pool by fronts and cold air masses during winter appears to have excited abnormal low-latitude cyclonic developments which transported deep moist air currents from the intertropical convergence zone ...