Winter Sea Surface Temperature Anomalies Research Articles

Interdecadal variations in the interannual relationship between winter tropical Pacific SST and subsequent summer Arctic sea ice in early 2000s

AbstractThe present study investigates the interdecadal change in the interannual relationship between winter sea surface temperature (SST) in the tropical central‐eastern Pacific and the subsequent summer Arctic sea ice concentration (SIC) and its possible mechanism using 1979–2023 monthly SST and SIC data from Hadley Center and atmospheric reanalysis data from NCEP/NCAR. It is found that the relationship between the winter tropical Pacific SST and the subsequent summer SIC of Beaufort Sea experienced an obvious interdecadal change from a significant negative correlation to a weak and insignificant correlation before and after the early 2000s, respectively. Before the early 2000s, when winter SST warming (cooling) anomalies occurred in the central and eastern tropical Pacific, it could sustain into summer and enhance a Rossby wave propagating from Tropic to high latitude, forming negative (positive) pressure anomalies in the North Pacific. Influenced by such anomalies, it is conducive to maintaining negative (positive) SST anomalies in the North Pacific from winter to subsequent summer. The summer SST cooling (warming) in the North Pacific could form a favourable atmospheric circulation anomaly for less (more) SIC. In contrast, after the early 21st century, the strong SST anomalies in winter over the eastern tropical Pacific would weaken with time and subsequently diminish in spring, leading to the SST anomalies in the North Pacific persist to spring, which can hardly affect summer SIC anomaly in the Beaufort Sea. Therefore, the relationship collapses after the the early 2000s due to the weakened persistence of the eastern tropical Pacific SST anomaly.

Impact of the Kuroshio large meander on local atmospheric circulation and precipitation in winter

The Kuroshio, which flows to the south of Japan, typically takes two paths on decadal timescales; the straight path and the large meander path, or the so-called Kuroshio large meander. This phenomenon is characterized by the presence of the cyclonic cold-core eddy located south of Japan, which leads to both negative and positive sea surface temperature (SST) anomalies along the southeastern coast of Japan. To clarify the atmospheric response to these SST anomalies in winter, we conducted a control experiment employing a regional atmospheric model with observed SSTs and two sensitivity experiments in which the SST boundary conditions were substituted with those corresponding to the periods of for the Kuroshio large meander and the straight path. The differences in these two sensitivity experiments showed that the surface wind response to the SST anomalies associated with the Kuroshio large meander was not only characterized by wind divergence over the cyclonic cold-core eddy, as reported previously but also wind convergence along the southeastern coast of Japan. Interestingly, this wind anomaly blew into the positive SST anomaly along the east coast of Japan at around 36°N. Similar wind anomalies along the east coast of Japan were observed in a reanalysis product. The results of the model simulation and the reanalysis product showed that during the Kuroshio large meander period, the number of rainy days increased significantly over the warm SST anomaly, while the precipitation and the number of rainy days decreased over the cyclonic cold-core eddy. Moisture budget analysis revealed that the observed decrease in precipitation was attributed to the disparity between the reduced evaporation and the anomalous horizontal moisture convergence in a region where the surface winds were divergent. This moisture convergence was mainly induced by a decrease in specific humidity, implying this change in specific humidity effectively mitigated the variation in precipitation.

The impact and mechanism analysis of preceding sea surface temperature anomalies on summer runoff in the Yangtze River basin and its southern region

AbstractThe Yangtze River basin (YRB) and its southern region in China (20°–34°N, 104°–123°E, YRBSC) are highly susceptible to climate change and experience extreme hydrological events. To understand the spatial and temporal distribution of summer runoff in these regions, a statistical diagnosis method was applied using monthly mean runoff grid data, global Sea Surface Temperature (SST) data and meteorological reanalysis data from 1980 to 2022. The analysis revealed that variations in the isotropic phase within the YRBSC and the north–south inverse phase with the Yangtze River as the boundary are the main modes of summer runoff. Furthermore, a strong correlation was observed between winter SST anomalies (SSTAs) and late summer runoff in the YRBSC, as determined through singular value decomposition (SVD). In the first type of positive SSTA years, the eastward advance of the South Asian high pressure (SAH) and westward shift of the subtropical high pressure (SH) result in sufficient water vapour, strong upward movement and increased summer runoff. The second type of positive SSTA years exhibits a westward retreat of the SAH, upward movement north of 28°N, and downward movement between 20°N and 28°N. These conditions, combined with water vapour intermixing and dispersion, lead to a northward increase and southward decrease of summer runoff in the YRBSC, with the boundary at 28°N. Additionally, the study analysed the extreme drought situation observed in the YRB during the summer of 2022. The findings of this research provide valuable insights for ecological environmental protection, water resource planning and management in the region.

Investigating Atmospheric Responses to and Mechanisms Governing North Atlantic Sea Surface Temperatures over 10-Year Periods

Abstract North Atlantic sea surface temperature (SST) variability plays a critical role in modulating the climate system. However, characterizing patterns of North Atlantic SST variability and diagnosing the associated mechanisms is challenging because they involve coupled atmosphere–ocean interactions with complex spatiotemporal relationships. Here we address these challenges by applying a time-evolving self-organizing map approach to a long preindustrial coupled control simulation and identify a variety of 10-yr spatiotemporal evolutions of winter SST anomalies, including but not limited to those associated with the North Atlantic Oscillation–Atlantic multidecadal variability (NAO–AMV)-like interactions. To assess mechanisms and atmospheric responses associated with various SST spatiotemporal evolutions, composites of atmospheric and oceanic variables associated with these evolutions are investigated. Results show that transient-eddy activities and atmospheric circulation responses exist in almost all the evolutions that are closely correlated to the details of the SST pattern. In terms of the mechanisms responsible for generating various SST evolutions, composites of ocean heat budget terms demonstrate that contributions to upper-ocean temperature tendency from resolved ocean advection and surface heat fluxes rarely oppose each other over 10-yr periods in the subpolar North Atlantic. We further explore the potential for predictability for some of these 10-yr SST evolutions that start with similar states but end with different states. However, we find that these are associated with abrupt changes in atmospheric variability and are unlikely to be predictable. In summary, this study broadly investigates the atmospheric responses to and the mechanisms governing the North Atlantic SST evolutions over 10-yr periods. Significance Statement Climate variability in the North Atlantic Ocean has wide-ranging impacts on global and regional climate. However, the processes involved include interactions between the ocean and atmosphere that vary across both space and time, making it challenging to characterize and predict. Using a novel machine learning approach, this study identifies various time evolutions of North Atlantic sea surface temperature patterns over 10-yr periods. This includes evolutions with similar start states but different trajectories, which have important implications for predictability. Furthermore, we investigate the mechanisms responsible for these evolutions and how different sea surface temperature patterns affect atmospheric circulation through small-scale atmospheric disturbances. These new insights into the complex ocean–atmosphere interactions over time are critical for improving decadal prediction skill.

Enhancement of the relationship between spring extreme precipitation over Southwest China and preceding winter sea surface temperature anomalies over the South Indian Ocean after the late 1980s

AbstractSpring extreme precipitation (SEP) significantly disturbs spring ploughing over Southwest China (SWC), bringing challenges for agricultural activities and production over the region. This study investigates the interannual relationship between SWC SEP and sea surface temperature (SST) in preceding winter, from a perspective of prediction. It is found that the dominate mode of SWC SEP interannual variability mainly reflects the variability over the eastern part. The variability of eastern SWC SEP has an enhanced connection with a dipole SST pattern over the South Indian Ocean in preceding winter after the late 1980s. Under the warmer SST background after the late 1980s, the South Indian Ocean dipole SST pattern can lead to significant convection activity over tropical central and western Indian Ocean. The tropical diabatic heating associated with enhanced convection can strengthen the western North Pacific anticyclone, which transports more water vapour to eastern SWC and induces stronger moisture convergence there, providing favourable moisture condition for the SEP occurrence. In addition, the dipole SST pattern can strengthen upward motion over eastern SWC through exiting a meridional circulation, providing a favourable dynamic condition for the SEP occurrence over there. In contrast, under the colder SST background before the late 1980s, the dipole SST pattern‐related convection and atmospheric circulation anomalies are weak over the Indo‐Pacific region. Therefore, the relationship of the dipole SST pattern with SEP over eastern SWC is weak before the late 1980s. The SST signals with one‐season leading could provide valuable source for seasonal prediction of eastern SWC SEP variability after the late 1980s.

Distinct evolutions of haze pollution from winter to the following spring over the North China Plain: role of the North Atlantic sea surface temperature anomalies

Abstract. This study reveals that haze pollution (HP) over the North China Plain (NCP) in winter can persist to the following spring during most years. The persistence of haze pollution over the NCP (HPNCP) is attributed to the maintenance of an anticyclonic anomaly (AA) over northeast Asia and southerly wind anomalies over the NCP. Southerly wind anomalies over the NCP reduce surface wind speed and increase relative humidity, which are conducive to above-normal HPNCP both in winter and spring. However, there exist several years when above-normal HPNCP in winter are followed by below-normal HPNCP in the following spring. The reversed HPNCP in winter and spring in these years is due to the inverted atmospheric anomalies over northeast Asia. In particular, AA over northeast Asia in winter is replaced by a cyclonic anomaly (CA) in the following spring. The resultant spring northerly wind anomalies over the NCP are conducive to below-normal HPNCP. These two distinctive evolutions of HPNCP and atmospheric anomalies over northeast Asia from winter to spring are attributed to the different evolutions of sea surface temperature anomalies (SSTAs) in the North Atlantic. In the persistent years, warm North Atlantic SSTAs in winter persist to the following spring via a positive air–sea interaction process and induce a negative spring North Atlantic Oscillation (NAO)-like pattern, which contributes to the AA over northeast Asia via an atmospheric wave train. By contrast, in the reverse years, cold SSTAs in the North Atlantic are maintained from winter to spring, which induce a positive spring NAO-like pattern and lead to a CA over northeast Asia via an atmospheric wave train. Hence, this study improves our understanding of the characteristics of haze pollution evolution from winter to the following spring and suggests the potential role of North Atlantic SSTAs to serve as an important preceding signal for haze pollution prediction one season ahead over the North China Plain.

Global Impact of ENSO on Dust Activities with Emphasis on the Key Region from the Arabian Peninsula to Central Asia

AbstractThe El Niño‐Southern Oscillation (ENSO) has been found to be closely related to dust aerosol activities in many parts of the world. However, previous studies have rarely explored the ENSO’s impacts in the global context. In this study, the responses of the interannual variability of global atmospheric dust content to the ENSO cycle (including the eastern Pacific and central Pacific types) are examined using the MERRA‐2 atmospheric aerosol reanalysis data. The ENSO‐related tropical eastern Pacific sea surface temperature (SST) anomalies in the antecedent winter (November to January) have the most significant effect on the airborne dust content in a region from the Arabian Peninsula to Central Asia during the following spring and summer (April to July), which is identified as the key region (KR) of the ENSO’s impacts. The positive (negative) tropical eastern Pacific SST anomalies in the preceding winter during the El Niño (La Niña) years correspond to significantly reduced (increased) dust column content in the following spring and summer over the KR, which was confirmed using singular value decomposition. With the above (below)‐normal winter SST anomalies in the tropical eastern Pacific, the Walker Circulation weakens (strengthens), and the divergence (convergence) over the western tropical Pacific and the convergence (divergence) over the southern tropical Indian Ocean in the lower troposphere are strengthened, which facilitate (restrain) the southward diffusion of airborne dust in the KR. Meanwhile, the increased (decreased) precipitation and soil moisture and the corresponding vegetation conditions help limit (promote) the dust emission in the KR during the El Niño (La Niña) years.

Recent Intensified Influence of the Winter North Pacific Sea Surface Temperature on the Mei-Yu Withdrawal Date

AbstractThe mei-yu withdrawal date (MWD) is a crucial indicator of flood/drought conditions over East Asia. It is characterized by a strong interannual variability, but its underlying mechanism remains unknown. We investigated the possible effects of the winter sea surface temperature (SST) in the North Pacific Ocean on the MWD on interannual to interdecadal time scales. Both our observations and model results suggest that the winter SST anomalies associated with the MWD are mainly contributed to by a combination of the first two leading modes of the winter SST in the North Pacific, which have a horseshoe shape (the NPSST). The statistical results indicate that the intimate linkage between the NPSST and the MWD has intensified since the early 1990s. During the time period 1990–2016, the NPSST-related SST anomalies persisted from winter to the following seasons and affected the SST over the tropical Pacific in July. Subsequently, the SST anomalies throughout the North Pacific strengthened the southward migration of the East Asian jet stream (EAJS) and the southward and westward displacement of the western North Pacific subtropical high (WPSH), leading to an increase in mei-yu rainfall from 1 to 20 July. More convincingly, the anomalous EAJS and WPSH induced by the SST anomalies can be reproduced well by numerical simulations. By contrast, the influence of the NPSST on the EASJ and WPSH were not clear between 1961 and 1985. This study further illustrates that the enhanced interannual variability of the NPSST may be attributed to the more persistent SST anomalies during the time period 1990–2016.

Role of Reemergence in the Central North Pacific Revealed by a Mixed Layer Heat Budget Analysis

AbstractWintertime sea surface temperature (SST) anomalies that disappear in summer and recur in the following winter are known as “reemergence.” The role of reemergence in recurrent SST anomalies in an area in the central North Pacific is investigated quantitatively for the first time by conducting an online mixed layer heat budget analysis with a realistic ocean model simulation. In contrast to past studies that have emphasized the importance of vertical entrainment of subsurface temperature anomalies, it is shown that several mechanisms are operating in recurrence of SST anomalies during 1977–1999. In particular, anomalous Ekman meridional advection of the mean meridional temperature gradient induced by zonal wind stress anomalies plays an important role in many years. Also, coincidence in the sign of SST anomalies in winter and surface heat flux anomalies in the following autumn leads to recurrence of SST anomalies.

SST Indexes in the Tropical South Atlantic for Forecasting Rainy Seasons in Northeast Brazil

May-to-July and February-to-April represent peak rainy seasons in two sub-regions of Northeast Brazil (NEB): Eastern NEB and Northern NEB respectively. In this paper, we identify key oceanic indexes in the tropical South Atlantic for driving these two rainy seasons. In Eastern NEB, the May-to-July rainfall anomalies present a positive relationship with the previous boreal winter sea surface temperature anomalies (SSTA) in the southeast tropical Atlantic (20°–10° S; 10° W–5° E). This positive relationship, which spread westward along the southern branch of the South Equatorial Current, is associated with northwesterly surface wind anomalies. A warmer sea surface temperature in the southwestern Atlantic warm pool increases the moisture flux convergence, as well as its ascending motion and, hence, the rainfall along the adjacent coastal region. For the Northern NEB, another positive relationship is observed between the February-to-April rainfall anomalies and the SSTA of the previous boreal summer in the Atlantic Niño region (3° S–3° N; 20° W–0°). The negative remote relationship noticeable between the Northern NEB rainfall and the concomitant Pacific Niño/Niña follows cold/warm events occurring during the previous boreal summer in the eastern equatorial Atlantic. The southeastern tropical Atlantic and Atlantic Niño SSTA indexes may, then, be useful to predict seasonal rainfall over the Eastern and Northern NEB, respectively, for about a 6 month leading period. The ability of both southeastern tropical Atlantic and Atlantic Niño SSTA indexes to forecast the Eastern and Northern NEB rainfall, with about a 6 month lead time, is improved when these indexes are respectively combined with the Niño3 (5° S–5° N; 150°–90° W) and the northeast subtropical Atlantic (20° N–35° N, 45° W–20° W), mainly from the 1970’s climate shift.

ENSO and Sea Surface Temperature Anomalies in Association with Canadian Wheat Yield Variability

ABSTRACTInterannual variations of spring wheat yields in Canadian agricultural regions are analyzed, together with the associated sea surface temperature (SST) anomalies in the northern hemisphere tropics and extratropics, from 1961 to 2015. The cubic trend is calculated and used to represent the trend related to advances in agricultural technology over this time period. The correlations between Canadian wheat yields at regional scales and the tropical El Niño–Southern Oscillation (ENSO) variability are not robust at any stage of the evolution of ENSO. Based on the power spectrum and cross-spectrum analysis, the most prominent yield variance is found in the Canadian Prairies, with a significant power peak of 4.5 years but does not co-vary significantly with interannual ENSO variability. ENSO weakly affects temperature and precipitation anomalies in the Canadian Prairie Region in summer—two important agroclimatic conditions for crop growth—and hence insignificantly impacts wheat yields. This indicates that there would be little benefit to including tropical ENSO indices in the operational wheat yield forecasting system. For Canadian wheat yield forecasting, attention should be paid to the preceding winter and spring SST anomalies in the northern extratropics. The SST anomalies associated with yields in the Canadian Prairie region and Central Region are generally stronger than those associated with yields in the Canadian Pacific Coast Region and eastern Maritime Region. In association with the Prairie Region and Central Region yields, SST shows pronounced anomalies in the mid-high latitudes of the North Pacific from winter to summer. The non-linearity of the SST anomalies associated with the Canadian yields is also clearly evident. Stronger (weaker) SST anomalies in the extratropical North Pacific correspond to low wheat yields in the Prairie (Central) Region, while weaker (stronger) SST anomalies correspond to high yields in the Prairie (Central) Region.

A Reconstruction of Subtropical Western North Pacific SST Variability Back to 1578, Based on a Porites Coral Sr/Ca Record from the Northern Ryukyus, Japan

AbstractWe present a seasonal reconstruction of sea surface temperature (SST) from 1578 to 2008, based on a Porites coral Sr/Ca record from the northern Ryukyus, within the Kuroshio southern recirculation gyre. Interannual SST anomalies are generally ~0.5°C, making Sr/Ca‐derived SST reconstructions a challenging task. Replicate measurements along adjacent coral growth axes, enabled by the laser ablation inductively coupled plasma mass spectrometry technique used here, give evidence of rather large uncertainties. Nonetheless, derived winter SST anomalies are significantly correlated with the Western Pacific atmospheric pattern which has a dominant influence on winter temperature in East Asia. Annual mean SSTs show interdecadal variations, notably cold intervals between 1670 and 1700 during the Maunder Minimum (MM) and between 1766 and 1788 characterized by a negative phase of the North Atlantic Oscillation. Cold summers in 1783 and 1784 coincide with the long‐lasting Laki eruption that had a profound impact on the Northern Hemisphere climate, including the severe “Tenmei” famine in Japan. The decades between 1855 and 1900 are significantly cooler than the first half of the twentieth century, while those between 1700 and 1765, following the MM, are warmer than average. SST variability in the Ryukyus is only marginally influenced by the Pacific Decadal Oscillation, so that external forcing remains the main driver of low‐frequency temperature changes. However, the close connection between the Kuroshio extension (KE) and its recirculation gyre suggests that decadal SST anomalies associated with the KE front also impact the Ryukyus, and there is a possible additional role for feedback of the Kuroshio‐Oyashio variability to the large‐scale atmosphere at decadal timescale.

Change in the relationship between the Australian summer monsoon circulation and boreal summer precipitation over Central China in the late 1990s

Recent study revealed a close connection between the Australian summer monsoon (ASM) and boreal summer precipitation over Central China (SPCC). This study further revealed a strengthening of the ASM–SPCC relationship around the late 1990s. It is found that the relationship between the ASM and the SPCC during 1979–1997 (1998–2014) relationship is statistically insignificant (significant). Further analysis indicated that during 1998–2014, the weakened ASM is concurrent with significant positive sea surface temperature (SST) in the Indian Ocean and South China Sea, which could persist into the following boreal summer and further lead to intensified East Asian summer monsoon, strengthened western North Pacific subtropical high, and anomalous ascending motion over Central China. Consequently, more moisture is transported from the western Pacific northward to Central China where significant anomalous convergence appears. Therefore, the ASM could potentially influence the SPCC during 1998–2014. By contrast, the ASM-related SST and atmospheric circulation anomalies in boreal winter are statistically insignificant during 1979–1997. Such an interdecadal change might be attributed to the interdecadal warming that occurred in the Indian Ocean and South China Sea around the late 1990s. This study might be useful for the prediction of the SPCC.

Contribution of the tropical western Atlantic thermal conditions during the preceding winter to summer temperature anomalies over the lower reaches of the Yangtze River basin–Jiangnan region

ABSTRACTUsing the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis and National Oceanic and Atmospheric Administration extended reconstructed sea surface temperature (SST) data sets, we examine the preceding factors in the Atlantic Ocean contributing to summer high temperature anomalies over the lower reaches of the Yangtze River basin–Jiangnan (LRYB–JN) region. We find that summer LRYB–JN surface air temperature (SAT) anomalies are highly correlated with, and physically connected to, the preceding winter SST and SAT anomalies in the tropical western Atlantic (TWA) region [(10°–30°N, 60°–35°W) and (0°–10°N, 50°–35°W)]. The effect of preceding winter thermal conditions in the TWA region can be explained by the following two mechanisms. Coupled with SST, positive TWA SAT anomalies can persist from the preceding winter to summer and excite a zonal wave train at middle latitudes extending from the Atlantic to Eurasia, with a deep and stable high pressure anomaly centred over the LRYB–JN region. As a result, high temperature anomalies occur over the LRYB–JN region. In addition to the above mechanism, the TWA SST–SAT coupled anomalies also exert an impact on summer LRYB–JN SAT by modulating the tropical Indian Ocean (TIO) SAT and SST variation, which causes summer LRYB–JN SAT anomalies. Given the contributions of TWA SAT anomalies, independent of and in concert with the TIO SAT variation, we define a preceding winter TWA–TIO SAT index, which measures the concurrent variation of TWA and TIO SATs. The preceding winter TWA–TIO index is more significantly correlated with the summer LRYB–JN SAT and therefore can reflect the latter better than the individual TWA or TIO SAT index.

Seasonal ENSO phase locking in the Kiel Climate Model: The importance of the equatorial cold sea surface temperature bias

The El Niño/Southern Oscillation (ENSO) is characterized by a seasonal phase locking, with strongest eastern and central equatorial Pacific sea surface temperature (SST) anomalies during boreal winter and weakest SST anomalies during boreal spring. In this study, key feedbacks controlling seasonal ENSO phase locking in the Kiel Climate Model (KCM) are identified by employing Bjerknes index stability analysis. A large ensemble of simulations with the KCM is analyzed, where the individual runs differ in either the number of vertical atmospheric levels or coefficients used in selected atmospheric parameterizations. All integrations use the identical ocean model. The ensemble-mean features realistic seasonal ENSO phase locking. ENSO phase locking is very sensitive to changes in the mean-state realized by the modifications described above. An excessive equatorial cold tongue leads to weak phase locking by reducing the Ekman feedback and thermocline feedback in late boreal fall and early boreal winter. Seasonal ENSO phase locking also is sensitive to the shortwave feedback as part of the thermal damping in early boreal spring, which strongly depends on eastern and central equatorial Pacific SST. The results obtained from the KCM are consistent with those from models participating in the Coupled Model Intercomparison Project phase 5 (CMIP5).

Cold Winter Over North America: The Influence of the East Atlantic (EA) and the Tropical/Northern Hemisphere (TNH) Teleconnection Patterns

Anomalous cold temperatures and strong cyclonic circulation were observed during winter 2013/14 over North America. In this article, we propose for the first time that positive East Atlantic (EA) and positive Tropical/Northern Hemisphere (TNH) patterns were dominant in the winter of 2013/14. The values of the EA and TNH indices for winter 2013/14 were the highest and the second highest for the period of record 1951-2014, respectively. The combined EA and TNH pattern is similar to the corresponding atmospheric circulation observed in the winter of 2013/14. The regression patterns of air temperatures on the EA and TNH index show negative anomalies over North America and the North Atlantic Ocean and positive anomalies over the North Pacific Ocean and the mid-latitude Atlantic Ocean. The regression pattern is similar to air temperature anomalies in winter 2013/14. In addition, the combined EA and TNH pattern correlates with sea surface temperature (SST) anomalies over the North Pacific and North Atlantic that are similar to the winter SST anomalies in winter 2013/14. The EA and TNH teleconnection patterns have contributed to the anomalous atmospheric circulation associated with the extreme cold winter over North America in 2013/14.

A simple model of ocean temperature re-emergence and variability

A simple stochastic one-dimensional model of interannual mid-latitude sea surface temperature (SST) variability that can be solved analytically is developed. A novel two-season approach is adopted, with the annual cycle divided into two seasons denoted summer and winter. Within each season the mixed layer depth is constant, and the transition of the mixed layer from summer to winter and vice versa is discontinuous. SST anomalies are forced by random atmospheric heat fluxes, assumed to be constant within each season for simplicity, with linear damping to represent atmospheric feedback. At the start of summer the initial SST anomaly is set equal to that at the end of the previous winter, and at the start of winter the initial temperature anomaly is found by instantaneously mixing the summer mixed layer with the heat stored below in the deeper winter mixed layer, thereby explicitly taking into account the ‘re-emergence mechanism’. Two simple auto-regressive equations for the summer and winter SST anomalies are obtained that can be easily solved. Model parameters include seasonal damping coefficients, mixed layer depths and standard deviations of the atmospheric forcing. Analytic expressions for season-to-season correlation and variability and power spectra are used to explore and illustrate the effects of the parameters quantitatively. Among the results it is found that, with regard to winter-to-winter temperature correlation, the re-emergence pathway is more influential than persistence via the summer mixed layer when the winter layer is more than twice the depth of the summer layer. With regard to winter temperature variability, the effect of a deeper winter mixed layer is to decrease the sensitivity to surface forcing and thus decrease variability, but also to increase persistence via re-emergence and thus increase variance at multidecadal scales.

Year-to-Year Reoutcropping of Eighteen Degree Water in an Eddy-Resolving Ocean Simulation

AbstractWinter outcropping of the Eighteen Degree Water (EDW) and its subsequent dispersion are studied using a ° eddy-resolving simulation of the Family of Linked Atlantic Modeling Experiments (FLAME). Outcropped EDW columns in the model simulations are detected in each winter from 1990 to 1999, and particles are deployed in the center of each outcropped EDW column. Subsequently, the trajectories of these particles are calculated for the following 5 yr. The particles slowly spread away from the outcropping region into the nonoutcropping/subducted EDW region south of ~30°N and eventually to the non-EDW region in the greater subtropical gyre. Approximately 30% of the particles are found in non-EDW waters 1 yr after deployment; after 5 yr, only 25% of the particles are found within EDW. The reoutcropping time is defined as the number of years between when a particle is originally deployed in an outcropping EDW column and when that particle is next found in an outcropping EDW column. Of the particles, 66% are found to reoutcrop as EDW in 1 yr, and less than 5% of the particles outcrop in each of the subsequent 4 yr. While the individual trajectories exhibit significant eddy-like motions, the time scale of reoutcropping is primarily set by the mean circulation. The dominance of reoutcropping in 1 yr suggests that EDW outcropping contributes considerably to the persistence of surface temperature anomalies from one winter to the next, that is, the reemergence of winter sea surface temperature anomalies.

Influence of the North Atlantic SST Variability on the Atmospheric Circulation during the Twentieth Century

Abstract The ocean–atmosphere coupling in the North Atlantic is investigated during the twentieth century using maximum covariance analysis of sea surface temperature (SST) and 500-hPa geopotential height analyses and performing regressions on dynamical diagnostics such as Eady growth rate, wave activity flux, and velocity potential. The North Atlantic Oscillation (NAO) generates the so-called SST anomaly tripole. A rather similar SST anomaly tripole, with the subpolar anomaly displaced to the east and a more contracted subtropical anomaly, which is referred to as the North Atlantic horseshoe pattern, in turn influences the atmosphere. In the fall and early winter, the response is NAO like and primarily results from subpolar forcing centered over the Labrador Sea and off Newfoundland. In summer, the largest atmospheric response to SST resembles the east Atlantic pattern and results from a combination of subpolar and tropical forcing. To emphasize the interannual to multidecadal variability, the same analysis is repeated after low-pass filtering. The SST influence is dominated by the Atlantic multidecadal oscillation (AMO), which also has a horseshoe shape, but with larger amplitude in the subpolar basin. A warm AMO phase leads to an atmospheric warming limited to the lower troposphere in summer, while it leads to a negative phase of the NAO in winter. The winter influence of the AMO is suggested to be primarily forced by the Atlantic SSTs in the northern subtropics. Such influence of the AMO is found in winter instead of early winter because the winter SST anomalies have a larger persistence, presumably because of SST reemergence.

Are Greenhouse Gases Changing ENSO Precursors in the Western North Pacific?*

Abstract Using multiple observational and model datasets, the authors document a strengthening relationship between boreal winter sea surface temperature anomalies (SSTAs) in the western North Pacific (WNP) and the development of the El Niño–Southern Oscillation (ENSO) in the following year. The increased WNP–ENSO association emerged in the mid-twentieth century and has grown through the present, reaching correlation coefficients as high as ~0.70 in recent decades. Fully coupled climate experiments with the Community Earth System Model, version 1 (CESM1), replicate the WNP–ENSO association and indicate that greenhouse gases (GHGs) are largely responsible for this observed increase. The authors speculate that shifts in the location of the largest positive SST trends between the subtropical and tropical western Pacific impact the low-level circulation in a manner that reinforces the link between the WNP and the development of ENSO. A strengthened GHG-driven relationship with the WNP provides an example of how anthropogenic climate change may directly influence one of the most prominent patterns of natural climate variability, ENSO, and potentially improve the skill of intraseasonal-to-interannual climate prediction.