Surface Heat Flux Anomalies Research Articles

Weakening of Northwest Pacific Anticyclone Anomalies during Post–El Niño Summers under Global Warming

The northwest Pacific anticyclone (NWPAC) anomalies during post–El Niño summers are a key predictor of the summer climate in East Asia and the northwestern Pacific (NWP). Understanding how this will change under global warming is crucial to project the changes in the variability of the northwest Pacific summer monsoon. Outputs from 18 selected coupled models from phase 5 of the Coupled Model Intercomparison Project show that the anomalous NWPAC response to El Niño will likely be weakened under global warming, which is attributed to the decreased zonal contrast between the tropical Indian Ocean (TIO) warming and the NWP cooling during post–El Niño summers. Under global warming, the NWPAC anomalies during the El Niño mature winter are weakened because of decreased atmospheric circulation in response to El Niño–Southern Oscillation (ENSO), which leads to the weakening of local air–sea interaction and then decreases the cold NWP SST anomalies. Furthermore, the decreased surface heat flux anomalies, the weakened anticyclone anomalies over the southeastern Indian Ocean, and the slackened anomalous easterlies over the north Indian Ocean weaken the warm TIO SST anomalies. However, the strengthened tropospheric temperature anomalies could enhance the anomalous TIO warming. Although the changes in TIO SST anomalies are indistinctive, the weakening of the SST anomaly gradient between the TIO and the NWP is robust to weaken the NWPAC anomalies during post–El Niño summers. Moreover, the positive feedback between the TIO–NWP SST anomalies and the NWPAC anomalies will enhance the weakening of NWPAC under global warming.

Linking the Tropical Northern Hemisphere Pattern to the Pacific Warm Blob and Atlantic Cold Blob

During 2013–15, prolonged near-surface warming in the northeastern Pacific was observed and has been referred to as the Pacific warm blob. Here, statistical analyses are conducted to show that the generation of the Pacific blob is closely related to the tropical Northern Hemisphere (TNH) pattern in the atmosphere. When the TNH pattern stays in its positive phase for extended periods of time, it generates prolonged blob events primarily through anomalies in surface heat fluxes and secondarily through anomalies in wind-induced ocean advection. Five prolonged (≥24 months) blob events are identified during the past six decades (1948–2015), and the TNH–blob relationship can be recognized in all of them. Although the Pacific decadal oscillation and El Niño can also induce an arc-shaped warming pattern near the Pacific blob region, they are not responsible for the generation of Pacific blob events. The essential feature of Pacific blob generation is the TNH-forced Gulf of Alaska warming pattern. This study also finds that the atmospheric circulation anomalies associated with the TNH pattern in the North Atlantic can induce SST variability akin to the so-called Atlantic cold blob, also through anomalies in surface heat fluxes and wind-induced ocean advection. As a result, the TNH pattern serves as an atmospheric conducting pattern that connects some of the Pacific warm blob and Atlantic cold blob events. This conducting mechanism has not previously been explored.

Simulations of three natural decadal climate variability phenomena in CMIP5 experiments with the UKMO HadCM3, GFDL-CM2.1, NCAR-CCSM4, and MIROC5 global earth system models

The ability of four Earth System Models (ESMs) to simulate major attributes of sea-surface temperature (SST) manifestations of the Pacific Decadal Oscillation (PDO), the tropical Atlantic SST gradient (TAG) variability, and the West Pacific Warm Pool (WPWP) SST variability was assessed. Data from simulation experiments conducted under the CMIP5 with the CM2.1, HadCM3, MIROC5, and CCSM4 ESMs from 1861 to 2005 were used. A Multi-Model Ensemble was also formed by combining data from the four ESMs. Aerosol optical depths and solar radiation were specified in these experiments. The simulation of PDO spatial pattern, annual cycle, and preferred timescales is the closest to observed attributes in these ESMs. The simulated and observed TAG SST patterns are generally similar, but their average annual cycles are very different and simulated TAG variability does not have a preferred decadal timescale unlike observed variability. No one of the ESMs is able to simulate the WPWP SST pattern. Simulated and observed average annual cycles of the WPWP SST are similar, and the dominant timescale of WPWP variability is close to 8 years in all ESMs and observations. Simulation skills of the three phenomena fluctuate from decade to decade in all ESMs. Major volcanic eruptions substantially influence observed and simulated indices and simulation skills of the three phenomena. The mixing of net surface heat flux anomalies to 200 m depth in the tropical and mid-latitude Pacific, and the WPWP region causes a delayed and multiyear response of the PDO and WPWP indices to volcanic eruptions.

Variability of the subtropical mode water in the Southwest Pacific

AbstractThe variability of Subtropical Mode Water (STMW) in the Southwest Pacific is investigated using a 28 year‐long time series (1986–2014) of high‐resolution expendable bathythermograph data north of New Zealand (PX06) and a shorter time series, the Roemmich‐Gilson monthly Argo optimal interpolation for the 2004–2014 period. The variability in STMW inventories is compared to the variability in air‐sea heat fluxes, mixed layer depths and transport of the East Auckland Current (EAUC) to assess both the atmospheric and oceanic roles influencing the formation and decay of STMW. The STMW north of New Zealand has a short lifespan with little persistence of the water mass from 1 year to the next one. Deeper mixed layers and negative anomalies in surface heat fluxes are correlated with increased formation of STMW. The heat content of the STMW layer is anticorrelated with inventories, particularly during the El Niño years. This suggests that large volumes of STMW are coincident with cooler conditions in the prior winter and less oceanic heat storage. There is significant seasonal and interannual variability in STMW inventories, however there are no trends in STMW properties, including its core layer temperature over the last decade. The variability of the winter EAUC transport is highly correlated with the STMW inventories and thermocline depth in the following spring, suggesting ocean dynamics deepen the thermocline and precondition for deeper mixed layers.

On the persistence and coherence of subpolar sea surface temperature and salinity anomalies associated with the Atlantic multidecadal variability

AbstractThis study identifies key features associated with the Atlantic multidecadal variability (AMV) in both observations and a fully coupled climate model, e.g., decadal persistence of monthly mean subpolar North Atlantic (NA) sea surface temperature (SST) and salinity (SSS) anomalies, and high coherence at low frequency among subpolar NA SST/SSS, upper ocean heat/salt content, and the Atlantic Meridional Overturning Circulation (AMOC) fingerprint. These key AMV features, which can be used to distinguish the AMV mechanism, cannot be explained by the slab ocean model results or the red noise process but are consistent with the ocean dynamics mechanism. This study also shows that at low frequency, the correlation and regression between net surface heat flux and SST anomalies are key indicators of the relative roles of oceanic versus atmospheric forcing in SST anomalies. The oceanic forcing plays a dominant role in the subpolar NA SST anomalies associated with the AMV.

An observational study of the North Pacific storm‐track impact on the midlatitude oceanic front

AbstractA lagged Maximum Covariance Analysis is used to examine the impact of North Pacific storm‐track activity on midlatitude oceanic frontal intensity in this study. It is found that an enhanced storm track tends to intensify the oceanic frontal intensity with a lag of 1–2 months. The forcing effect of storm‐track anomalies on oceanic frontal intensity is strongest in autumn, followed by that in summer and winter, and it is weakest in spring. Moreover, the mixed layer heat budget analysis suggests that sea surface temperature anomalies (SSTAs) related to oceanic fronts are primarily attributed to the storm‐track‐induced net surface heat flux and Ekman advection anomalies, while contributions of geostrophic advection and entrainment are relatively small. In summer and autumn, the impact of net surface heat flux anomalies on SSTAs plays a more important role than that of Ekman heat transport anomalies. Whereas in winter, Ekman heat transport anomaly forcing is comparable to the net surface heat flux forcing. Anomalous turbulent heat fluxes contribute to generating net surface heat flux anomalies in those three seasons, while the shortwave radiative fluxes make a strong contribution in summer but have little impact in winter. The anomalies of both net surface heat flux and Ekman heat transport are presumed to be associated with storm‐track‐induced surface wind anomalies. Results of the present study provide observational evidences for the positive feedback between the North Pacific storm‐track activity and midlatitude oceanic frontal intensity.

Near‐surface salinity and temperature structure observed with dual‐sensor drifters in the subtropical South Pacific

AbstractThree surface drifters equipped with temperature and salinity sensors at 0.2 and 5 m depths were deployed in April/May 2015 in the subtropical South Pacific with the objective of measuring near‐surface salinity differences seen by satellite and in situ sensors and examining the causes of these differences. Measurements from these drifters indicate that water at a depth of 0.2 m is about 0.013 psu fresher than at 5 m and about 0.024°C warmer. Events with large temperature and salinity differences between the two depths are caused by anomalies in surface freshwater and heat fluxes, modulated by wind. While surface freshening and cooling occurs during rainfall events, surface salinification is generally observed under weak wind conditions (≤4 m/s). Further examination of the drifter measurements demonstrates that (i) the amount of surface freshening and strength of the vertical salinity gradient heavily depend on wind speed during rain events, (ii) salinity differences between 0.2 and 5 m are positively correlated with the corresponding temperature differences for cases with surface salinification, and (iii) temperature exhibits a diurnal cycle at both depths, whereas the diurnal cycle of salinity is observed only at 0.2 m when the wind speed is less than 6 m/s. The amplitudes of the diurnal cycles of temperature at both depths decrease with increasing wind speed. The mean diurnal cycle of surface salinity is dominated by events with winds less than 2 m/s.

The Central Role of Ocean Dynamics in Connecting the North Atlantic Oscillation to the Extratropical Component of the Atlantic Multidecadal Oscillation

Abstract The relationship between the North Atlantic Oscillation (NAO) and Atlantic sea surface temperature (SST) variability is investigated using models and observations. Coupled climate models are used in which the ocean component is either a fully dynamic ocean or a slab ocean with no resolved ocean heat transport. On time scales less than 10 yr, NAO variations drive a tripole pattern of SST anomalies in both observations and models. This SST pattern is a direct response of the ocean mixed layer to turbulent surface heat flux anomalies associated with the NAO. On time scales longer than 10 yr, a similar relationship exists between the NAO and the tripole pattern of SST anomalies in models with a slab ocean. A different relationship exists both for the observations and for models with a dynamic ocean. In these models, a positive (negative) NAO anomaly leads, after a decadal-scale lag, to a monopole pattern of warming (cooling) that resembles the Atlantic multidecadal oscillation (AMO), although with smaller-than-observed amplitudes of tropical SST anomalies. Ocean dynamics are critical to this decadal-scale response in the models. The simulated Atlantic meridional overturning circulation (AMOC) strengthens (weakens) in response to a prolonged positive (negative) phase of the NAO, thereby enhancing (decreasing) poleward heat transport, leading to broad-scale warming (cooling). Additional simulations are used in which heat flux anomalies derived from observed NAO variations from 1901 to 2014 are applied to the ocean component of coupled models. It is shown that ocean dynamics allow models to reproduce important aspects of the observed AMO, mainly in the Subpolar Gyre.

Ocean Heat Uptake and Interbasin Transport of the Passive and Redistributive Components of Surface Heating

Abstract Global warming induces ocean circulation changes that not only can redistribute ocean reservoir temperature stratification but also change the total heat content anomaly of the ocean. Here all consequences of this process are referred to collectively as “redistribution.” Previous model studies of redistributive effects could not measure the net global contribution to the amount of ocean heat uptake by redistribution. In this study, a global ocean model experiment with abrupt increase in surface temperature is conducted with a new passive tracer formulation. This separates ocean heat uptake into contributions due to redistribution temperature and surface heat flux anomalies and those due to the passive advection and mixing of surface heat flux anomalies forced in the atmosphere. For a decline in the Atlantic meridional overturning circulation of about 40%, redistribution nearly doubles the Atlantic passive anomalous surface heat input and depth penetration of temperature anomalies. However, smaller increases in the Indian and Pacific Oceans cause the net global redistributive contribution to be only 25% of the passive contribution. Despite the much larger anomalous surface heat input in the Atlantic, the Pacific gains heat content anomaly similar to that in the Atlantic because of export from the Atlantic and Indian Oceans via the global conveyor belt. Of this interbasin heat transport, most of the passive component comes from the Indian Ocean and the redistributive component comes from the Atlantic.

An Equatorial–Extratropical Dipole Structure of the Atlantic Niño

Abstract Equatorial Atlantic variability is dominated by the Atlantic Niño peaking during the boreal summer. Studies have shown robust links of the Atlantic Niño to fluctuations of the St. Helena subtropical anticyclone and Benguela Niño events. Furthermore, the occurrence of opposite sea surface temperature (SST) anomalies in the eastern equatorial and southwestern extratropical South Atlantic Ocean (SAO), also peaking in boreal summer, has recently been identified and termed the SAO dipole (SAOD). However, the extent to which and how the Atlantic Niño and SAOD are related remain unclear. Here, an analysis of historical observations reveals the Atlantic Niño as a possible intrinsic equatorial arm of the SAOD. Specifically, the observed sporadic equatorial warming characteristic of the Atlantic Niño (~0.4 K) is consistently linked to southwestern cooling (~−0.4 K) of the Atlantic Ocean during the boreal summer. Heat budget calculations show that the SAOD is largely driven by the surface net heat flux anomalies while ocean dynamics may be of secondary importance. Perturbations of the St. Helena anticyclone appear to be the dominant mechanism triggering the surface heat flux anomalies. A weakening of the anticyclone will tend to weaken the prevailing northeasterlies and enhance evaporative cooling over the southwestern Atlantic Ocean. In the equatorial region, the southeast trade winds weaken, thereby suppressing evaporation and leading to net surface warming. Thus, it is hypothesized that the wind–evaporation–SST feedback may be responsible for the growth of the SAOD events linking southern extratropics and equatorial Atlantic variability via surface net heat flux anomalies.

Multiannual Ocean–Atmosphere Adjustments to Radiative Forcing

Abstract In radiative forcing and climate feedback frameworks, the initial stratospheric and tropospheric adjustments to a forcing agent can be treated as part of the forcing and not as a feedback, as long as the average global surface temperature response is negligible. Here, a very large initial condition ensemble of the Community Earth System Model is used to analyze how the ocean shapes the fast response to radiative forcing. It is shown that not only the stratosphere and troposphere but also the ocean adjusts. This oceanic adjustment includes meridional ocean heat transport convergence anomalies, which are locally as large as the surface heat flux anomalies, and an increase of the Atlantic meridional overturning circulation. These oceanic adjustments set the lower boundary condition for the atmospheric response of the first few years, in particular, the shortwave cloud radiative effect. This cloud adjustment causes a nonlinear relationship between global energy imbalance and temperature. It proceeds with a characteristic time scale of a few years in response to the forcing rather than scaling nonlinearly with global mean temperature anomaly. It is proposed that even very short time scales are treated as a fully coupled problem and encourage other modeling groups to investigate whether our description also suits their models’ behavior. A definition of the forcing term (“virtual forcing”) including oceanic adjustment processes is introduced and serves as an interpretive idea for longer time scales.

Relationship of boreal summer 10–20-day and 30–60-day intraseasonal oscillation intensity over the tropical western North Pacific to tropical Indo-Pacific SST

The present study contrasts interannual variations in the intensity of boreal summer 10–20-day and 30–60-day intraseasonal oscillations (ISOs) over the tropical western North Pacific and their factors. A pronounced difference is found in the relationship of the two ISOs to El Nino-Southern Oscillation. The 10–20-day ISO intensity is enhanced during El Nino developing summer, whereas the 30–60-day ISO intensity is enhanced during La Nina decaying summer. The above different relationship is interpreted as follows. The equatorial central and eastern Pacific SST anomalies modify vertical wind shear, lower-level moisture, and vertical motion in a southeast-northwest oriented band from the equatorial western Pacific to the tropical western North Pacific where the 10–20-day ISOs originate and propagate. These background field changes modulate the amplitude of 10–20-day ISOs. Preceding equatorial central and eastern Pacific SST anomalies induce SST anomalies in the North Indian Ocean in summer, which in turn modify vertical wind shear and vertical motion over the tropical western North Pacific. The modified background fields influence the amplitude of the 30–60-day ISOs when they reach the tropical western North Pacific from the equatorial region. A feedback of ISO intensity on local SST change is identified in the tropical western North Pacific likely due to a net effect of ISOs on surface heat flux anomalies. This feedback is more prominent from the 10–20-day than the 30–60-day ISO intensity change.

A multi-model ensemble view of winter heat flux dynamics and the dipole mode in the Mediterranean Sea

Changes in surface heat fluxes affect several climate processes controlling the Mediterranean climate. These include the winter formation of deep waters, which is the primary driver of the Mediterranean Sea overturning circulation. Previous studies that characterize the spatial and temporal variability of surface heat flux anomalies over the basin reveal the existence of two statistically dominant patterns of variability: a monopole of uniform sign and an east–west dipole of opposite signs. In this work, we use the 12 regional climate model ensemble from the EU-FP6 ENSEMBLES project to diagnose the large-scale atmospheric processes that control the variability of heat fluxes over the Mediterranean Sea from interannual to decadal timescales (here defined as timescales > 6 year). Our findings suggest that while the monopole structure captures variability in the winter-to-winter domain-average net heat flux, the dipole pattern tracks changes in the Mediterranean climate that are connected to the East Atlantic/Western Russia (EA/WR) atmospheric teleconnection pattern. Furthermore, while the monopole exhibits significant differences in the spatial structure across the multi-model ensemble, the dipole pattern is very robust and more clearly identifiable in the anomaly maps of individual years. A heat budget analysis of the dipole pattern reveals that changes in winds associated with the EA/WR pattern exert dominant control through both a direct effect on the latent heat flux (i.e., wind speed) and an indirect effect through specific humidity (e.g., wind advection). A simple reconstruction of the heat flux variability over the deep-water formation regions of the Gulf of Lion and the Aegean Sea reveals that the combination of the monopole and dipole time series explains over 90 % of the heat flux variance in these regions. Given the important role that surface heat flux anomalies play in deep-water formation and the regional climate, improving our knowledge on the dynamics controlling the leading modes of heat flux variability may enhance our predictability of the climate of the Mediterranean area.

Dominant Modes of Interannual Variability in Eurasian Surface Air Temperature during Boreal Spring

Abstract This study investigates interannual variations of surface air temperature (SAT) over mid- and high latitudes of Eurasia during boreal spring and their association with snow, atmospheric circulation, and sea surface temperature (SST) changes. The leading mode of spring SAT variations is featured by same-sign anomalies over most regions. The second mode features a tripole anomaly pattern with anomalies over the central part opposite to those over the eastern and western parts of Eurasia. A diagnosis of surface heat flux anomalies suggests that snow change contributes partly to SAT anomalies in several regions mainly by modulating surface shortwave radiation but cannot explain SAT changes in other regions. Atmospheric circulation anomalies play an important role in spring SAT variability via wind-induced heat advection and cloud-induced surface radiation changes. Positive SAT anomalies are associated with anomalous westerly winds from the North Atlantic Ocean or with anomalous anticyclone and southerly winds. Negative SAT anomalies occur in regions of anomalous cyclone and northerly winds. Atmospheric circulation anomalies associated with the first mode have a close relationship to spring Arctic Oscillation (AO), indicating the impact of the AO on continental-scale spring SAT variations over the mid- and high latitudes of Eurasia. The atmospheric circulation anomalies associated with the second mode feature a wave pattern over the North Atlantic and Eurasia. Such a wave pattern is related to a tripole SST anomaly pattern in the North Atlantic Ocean, signifying the contribution of the North Atlantic Ocean state to the formation of a tripole SAT anomaly pattern over the mid- and high latitudes of Eurasia.

Regional change in snow water equivalent–surface air temperature relationship over Eurasia during boreal spring

Present study investigates local relationship between surface air temperature and snow water equivalent (SWE) change over mid- and high-latitudes of Eurasia during boreal spring. Positive correlation is generally observed around the periphery of snow covered region, indicative of an effect of snow on surface temperature change. In contrast, negative correlation is usually found over large snow amount area, implying a response of snow change to wind-induced surface temperature anomalies. With the seasonal retreat of snow covered region, region of positive correlation between SWE and surface air temperature shifts northeastward from March to May. A diagnosis of surface heat flux anomalies in April suggests that the snow impact on surface air temperature is dominant in east Europe and west Siberia through modulating surface shortwave radiation. In contrast, atmospheric effect on SWE is important in Siberia and Russia Far East through wind-induced surface sensible heat flux change. Further analysis reveals that atmospheric circulation anomalies in association with snowmelt over east Siberia may be partly attributed to sea surface temperature anomalies in the North Atlantic and the atmospheric circulation anomaly pattern associated with snowmelt over Russia Far East has a close association with the Arctic Oscillation.

Thermally Forced Transients in the Thermohaline Circulation

AbstractThe response of a convective ocean basin to variations in atmospheric temperature is explored using numerical models and theory. The results indicate that the general behavior depends strongly on the frequency at which the atmosphere changes relative to the local response time to air–sea heat flux. For high-frequency forcing, the convective region in the basin interior is essentially one-dimensional and responds to the integrated local surface heat flux anomalies. For low-frequency forcing, eddy fluxes from the boundary current into the basin interior become important and act to suppress variability forced by the atmosphere. A theory is developed to quantify this time-dependent response and its influence on various oceanic quantities. The amplitude and phase of the temperature and salinity of the convective water mass, the meridional overturning circulation, the meridional heat flux, and the air–sea heat flux predicted by the theory compare well with that diagnosed from a series of numerical model calculations in both strongly eddying and weakly eddying regimes. Linearized analytic solutions provide direct estimates of each of these quantities and demonstrate their dependence on the nondimensional numbers that characterize the domain and atmospheric forcing. These results highlight the importance of mesoscale eddies in modulating the mean and time-dependent ocean response to atmospheric variability and provide a dynamical framework with which to connect ocean observations with changes in the atmosphere and surface heat flux.

Air–Sea Relationship Associated with Precipitation Anomaly Changes and Mean Precipitation Anomaly over the South China Sea and the Arabian Sea during the Spring to Summer Transition

Abstract The period from April to June is the time of transition from spring to summer over the north Indian Ocean and the South China Sea. Analysis shows that precipitation anomaly changes from April to June may indicate summer (June–August) mean precipitation anomalies over the South China Sea and the Arabian Sea. This study documents and compares the evolution of precipitation, surface wind, and sea surface temperature (SST) anomalies during the spring to summer transition corresponding to April–June precipitation anomaly changes and April–June mean precipitation anomalies over the South China Sea and the Arabian Sea. Over the South China Sea, a clear signal of local air–sea interaction is identified corresponding to the precipitation anomaly change, as indicated by a sequence of less precipitation, higher SST, more precipitation, and lower SST. In contrast, the mean precipitation anomaly features a response to remote SST forcing and a local forcing of atmosphere on the ocean. The evolution of surface heat flux anomalies supports the air–sea interaction over the South China Sea during the transition season. Over the Arabian Sea, local SST forcing contributes to both precipitation anomaly changes and mean precipitation anomalies through modulating atmospheric stability. A local negative feedback of atmosphere on SST is observed in the Arabian Sea as in the South China Sea. The surface heat fluxes make a large contribution to local SST change before May in the South China Sea but a small one in the Arabian Sea. Surface heat fluxes are important for local SST change after May in both the South China Sea and the Arabian Sea.

The Initiation and Developing Mechanisms of Central Pacific El Niños

Abstract The initiation and developing mechanisms of four major central Pacific (CP) El Niño events in 1994, 2002, 2004, and 2009 were investigated by analyzing oceanic and atmospheric reanalysis data. A mixed layer heat budget analysis was conducted and the result shows that the initiation mechanism of the 1994 CP El Niño is very different from other CP El Niños in 2000s, while the developing mechanisms are similar among these events. The initial sea surface temperature (SST) warming of the 1994 El Niño was caused by enhanced solar radiation, which was related to atmospheric meridional overturning circulation in association with positive SST anomaly forcing in the subtropical Pacific. The subtropical SST anomalies also induced anticyclonic surface wind stress curl anomalies, which caused the formation of subsurface warmer waters in the off-equatorial regions. The off-equatorial subsurface warmer waters were transported farther equatorward by the mean subsurface ocean currents, leading to the subsurface warming in the central equatorial Pacific. The deepened thermocline anomaly at the equator further promoted a positive advective and thermocline feedback so that the SST anomaly grew. During the initiation phase of the 2000s El Niños, ocean dynamics played a dominant role, while the effect of surface heat flux anomalies was minor. Preexisting subsurface warmer waters appeared in the equatorial region during their initiation phases. Such subsurface anomalies can cause the SST warming in the central Pacific through induced anomalous eastward zonal currents that advect high mean SST eastward. This positive zonal advective feedback, along with a positive thermocline feedback, continued to warm the local SST throughout the developing phase of the 2000s El Niño events.

Relative contribution of ENSO and East Asian winter monsoon to the South China Sea SST anomalies during ENSO decaying years

AbstractPresent study investigates respective contributions of El Niño‐Southern Oscillation (ENSO) and East Asian winter monsoon (EAWM) to two periods of sea surface temperature (SST) anomalies in the South China Sea (SCS), one subsequent to the ENSO mature phase and the other in the following summer, during 1982–2010. It is found that the SST anomalies in the first period are related to the EAWM and the ENSO impact independent of EAWM is weak, which differs from previous studies. The SST anomalies in the second period are mainly due to the ENSO effect. A heat budget analysis shows that changes in shortwave radiation and wind‐related surface latent heat flux as well as horizontal advections are the processes leading to the EAWM‐related SST anomalies in the first period. The ENSO‐related SST anomalies in the second period are contributed by shortwave radiation and latent heat flux anomalies as well as meridional advection and vertical upwelling processes near the southeast coast of Indochina Peninsula. The ENSO contribution to the SCS SST anomalies in the first period is larger before than after the late 1970s. This is attributed to an earlier development of ENSO events that is followed by larger equatorial central and eastern Pacific SST anomalies in boreal summer and, subsequently, larger North Indian Ocean SST anomalies around October. Consequently, anomalous surface anticyclone over the SCS is stronger, leading to larger surface heat flux anomalies and thus a larger SST increase around October before than after the late 1970s.

Daily scale wintertime sea surface temperature and IPC-Navidad variability in the southern Bay of Biscay from 1981 to 2010

Abstract. The combination of remotely sensed gappy Sea surface temperature (SST) images with the missing data filling DINEOF (data interpolating empirical orthogonal functions) technique, followed by a principal component analysis of the reconstructed data, has been used to identify the time evolution and the daily scale variability of the wintertime surface signal of the Iberian Poleward Current (IPC), or Navidad, during the 1981–2010 period. An exhaustive comparison with the existing bibliography, and the vertical temperature and salinity profiles related to its extremes over the Bay of Biscay area, show that the obtained time series accurately reflect the IPC-Navidad variability. Once a time series for the evolution of the SST signal of the current over the last decades is well established, this time series is used to propose a physical mechanism in relation to the variability of the IPC-Navidad, involving both atmospheric and oceanic variables. According to the proposed mechanism, an atmospheric circulation anomaly observed in both the 500 hPa and the surface levels generates atmospheric surface level pressure, wind-stress and heat-flux anomalies. In turn, those surface level atmospheric anomalies induce mutually coherent SST and sea level anomalies over the North Atlantic area, and locally, in the Bay of Biscay area. These anomalies, both locally over the Bay of Biscay area and over the North Atlantic, are in agreement with several mechanisms that have separately been related to the variability of the IPC-Navidad, i.e. the south-westerly winds, the joint effect of baroclinicity and relief (JEBAR) effect, the topographic β effect and a weakened North Atlantic gyre.