Annual Cycle Of Sea Surface Temperature Research Articles

Arabian Sea SST evolution during spring to summer transition period and the associated processes in coupled climate models

ABSTRACTMany climate models have problems in simulating the sea surface temperature (SST) in the tropical Indian Ocean (TIO). The Coupled Model Inter‐comparison Project Phase 5 (CMIP5) models, in general, underestimate SST over the entire TIO region. This study examines the SST evolution during spring to summer transition months (May and June) over the Arabian Sea (AS) region in the historical simulations of 13 CMIP5 models and the Climate Forecasting System coupled models CFSv1 and CFSv2. The annual cycle of SST shows that the summer monsoon cooling is not adequately captured by many models. Based on the state of June SST tendency, models have been divided in to three groups, the first group (G1) consists of models having stronger than observed cooling, second group (G2) considers models having closer to observed cooling and the third group (G3) includes models having lesser than observed cooling. Mixed layer heat budget analysis revealed that atmospheric flux is mainly responsible for unrealistic SST warming in most of the G3 models during June. The vertical mixing and horizontal advection contribute considerably to the SST cooling in summer (June) especially for G1 and G2 models. On the other hand, spring warming in all the models is consistently forced by the surface heat flux. It is also found that the monsoon low‐level jet (LLJ) is not accurately represented in most of the models. The misrepresentation of LLJ causes bias in the oceanic processes leading to unrealistic SST evolution in many models. One way of LLJ affecting the oceanic processes is by modulating mixed layer depth (MLD). It is observed in general that the models with deeper MLD display strong SST cooling. The model deficiency in representing AS SST is speculated to be a major limiting factor in capturing the monsoon rainfall in the current coupled models. The proper simulation of AS SST is therefore very crucial for the accurate representation of Indian summer monsoon precipitation.

ENSO Variability Simulated by a Coupled General Circulation Model: ECHAM5/MPI-OM

AbstractThe accurate simulation of the equatorial sea surface temperature (SST) variability is crucial for a proper representation or prediction of the El Nino-Southern Oscillation (ENSO). This paper describes the tropical variability simulated by the Max Planck Institute (MPI) for meteorology coupled atmosphere-ocean general circulation model (CGCM). A control simulation with pre-industrial greenhouse gases is analyzed, and the simulation of key oceanic features, such as SST, is compared with observations. Results from the 400-yr control simulation show that the model's ENSO variability is quite realistic in terms of structure, strength, and period. Also, two related features (the annual cycle of SST and the-phase locking of ENSO events), which are significant in determining the model's performance of realistic ENSO prediction, are further validated to be well reproduced by the MPI climate model, which is an atmospheric model ECHAM5 (which fuses the EC for European Center and HAM for Hamburg) coupled to ...

Role of upper ocean processes in the seasonal SST evolution over tropical Indian Ocean in climate forecasting system

In this study role of upper ocean processes in the evolution of sea surface temperature (SST) seasonal variations over the tropical Indian Ocean (TIO) is investigated in climate forecast system version1 (CFSv1) and version2 (CFSv2). Analysis reveals that CFSv2 could capture seasonal evolution of SST, wind speed and mixed layer depth better than CFSv1 with some biases. Discrepancy in reproducing the evolution of seasonal SST in coupled models leads to bias in the spatial and temporal distribution of precipitation. This has motivated to carry out mixed layer heat budget analysis in determining seasonal evolution of TIO SST. Spatial pattern of mixed layer heat budget from observations and models suggest that the processes responsible for SST tendency differ from region to region over the TIO. Further it is found that models underestimated SST tendency compared to the observations. Misrepresentation of advective processes and heat flux (HF) over the TIO is mainly responsible for the distortion of seasonal SST change in the coupled models. Sub-regional heat budget analysis reveals that CFSv1 is unable to reproduce the annual cycle of mixed layer temperature (MLT) tendency over the Arabian Sea, while CFSv2 captured the annual cycle of SST with systematic cold bias. Misrepresentation of the annual cycles of net HF and horizontal advection (Hadv) are accountable for the low rate of change of MLT during most of the year. Hadv during summer season is underestimated by 50 and 25 % respectively in CFSv1 and CFSv2. Further, CFSv1 fails to simulate MLT tendency due to improper evolution of HF annual cycle over the Bay of Bengal. Though annual cycle of HF in CFSv2 is well represented over the Bay of Bengal, its contribution to MLT change is underestimated compared to observations. Over the southern TIO region, MLT tendency is dominated by HF and Hadv terms in both observations and models. Contribution of HF to the annual cycle of MLT tendency is underestimated in CFSv1 whereas it is overestimated in CFSv2. Contribution of Hadv to MLT change is underestimated by about 50 % in CFSv1 and 10–20 % in CFSv2 over southern TIO. These errors in HF and Hadv are associated with biases in HF components and surface wind representation. Evolution of lead–lag relationship between HF and MLT/SST in both the observations and models suggest the importance of HF in SST evolution over the TIO region. Over all, CFSv2 produced better SST seasonal/annual cycle in spite of having cold bias. This improvement in CFSv2 may be attributed to better cloud–aerosol–radiation physics, which reduces radiation biases. Updated land-surface, ocean and sea ice processes and ocean component may be responsible for improved circulation and annual cycle of ocean–atmospheric components (winds and ocean circulation). However, there is a requirement for improved parameterization of turbulent HF and radiation estimates in CFSv2 to reduce the cold SST bias.

Evaluation of the eastern equatorial Pacific SST seasonal cycle in CMIP5 models

Abstract. The annual cycle of sea surface temperature (SST) in the eastern equatorial Pacific (EEP) has the largest amplitude in the tropical oceans, but it is poorly represented in the coupled general circulation models (CGCMs) of the Coupled Model Intercomparison Project Phase 3 (CMIP3). In this study, 18 models from CMIP5 are evaluated in terms of their capability of simulating the SST annual cycle in the EEP. Fourteen models are able to simulate the annual cycle fairly well, which suggests that the performances of CGCMs have been improved. The results of multi-model ensemble (MME) mean show that CMIP5 CGCMs can capture the annual cycle signal in the EEP with a correlation coefficient up to 0.9. Moreover, the CMIP5 models can simulate the westward propagation character of the EEP SST – in particular, EEP region 1 (EP1) near the eastern coast leading EEP region 2 (EP2) near the central equatorial Pacific by 1 to 2 months in spring. However, the models fail to reproduce the in-phase SST relationship between EP1 and EP2 in August and September. For amplitude simulations, the model SST in EP1 shows weaker seasonal variation than the observations due to the large warm SST biases from the southeastern tropical Pacific in the boreal autumn. In EP2, the simulated SST amplitudes are nearly the same as the observations while there is the presence of a quasi-constant cold bias associated with poor cold tongue simulation in the CGCMs. To improve CGCM simulation of a realistic SST seasonal cycle, local and remote SST biases that exist in both CMIP3 and CMIP5 CGCMs must be resolved at least for simulating the SSTs in the central equatorial Pacific and the southeastern tropical Pacific.

Spatial and temporal variability of satellite-derived sea surface temperature in the Barents Sea

The Barents Sea (BS) is an important region for studying climate change. This sea is located on the main pathway of the heat transported from low to high latitudes. Since oceanic conditions in the BS may influence vast areas of the Arctic Ocean, it is important to continue to monitor this region and analyse the available oceanographic data sets. One of the important quantities that can be used to track climate change is the sea surface temperature (SST). In this study, we have analysed the 32 years, (1982–2013) National Oceanic and Atmospheric Administration (NOAA) Optimum Interpolation SST Version 2 data for the BS. Our results indicate that the regionally averaged SST trend in the BS (about 0.03°C year–1) is greater than the global trend. This trend varies spatially with the lowest values north from 76° N and the highest values (about 0.06°C year–1) in proximity of Svalbard and in coastal regions near the White Sea. The SST and 2 m air temperature (AT) trends are high in winter months in the open BS region located west from Novaya Zemlya. Such trends can be linked to a significant retreat of sea ice in this area in recent years. In this article, we also documented spatial patterns in the annual cycle of SST in the BS. We have shown that the interannual variability of SST is similar in different regions of the BS and well correlated with the interannual patterns in AT variability.

The Effect of Greenhouse Gas–Induced Changes in SST on the Annual Cycle of Zonal Mean Tropical Precipitation

Abstract Models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) project changes to the seasonality of both tropical sea surface temperature (SST) and precipitation when forced by an increase in greenhouse gases. Nearly all models project an amplification and a phase delay of the annual cycle for both quantities, indicating a greater annual range and extrema reached later in the year. The authors investigate the nature of the seasonal precipitation changes in AGCM experiments forced by SST perturbations, which represent idealizations of the changes in annual mean, amplitude, and phase as simulated by CMIP5 models. A uniform SST warming is sufficient to force both amplification and a delay of the annual cycle of precipitation. The amplification is due to an increase in the annual mean vertical water vapor gradient, while the delay is affected by changes in the seasonality of the circulation. A budget analysis of this simulation reveals a large degree of similarity with the CMIP5 results. In the second experiment, only the seasonal characteristics of SST are changed. In response to an amplified annual cycle of SST, the annual cycle of precipitation is amplified, while for a delayed SST, the annual cycle of precipitation is delayed. Assuming that SST changes can entirely explain the seasonal precipitation changes, the AGCM simulations herein suggest that the annual mean warming explains most of the amplitude increase and much of the phase delay in the CMIP5 models. However, imperfect agreement between the changes in the SST-forced AGCM simulations and the CMIP5 coupled simulations suggests that coupled effects may play a significant role.

Semi-annual cycle of sea-surface temperature in the East/Japan Sea and cooling process

Semi-annual cycles of the sea-surface temperature (SST) in the East/Japan Sea (EJS) and their forcings were examined by analysing National Oceanic and Atmospheric Administration/Advanced Very-High-Resolution Radiometer data, scatterometer wind vectors, and heat flux data. The semi-annual cycle contributed to the total variance of the SST by 8% and amounted to 25% of the amplitudes of the annual SST cycle, particularly in the Tatarskiy Strait and along the continental shelf off Russia. The lowest phase, corresponding to the minimum SST, occurred during early November and 6 months earlier in May or June depending on the position. The forcings of the semi-annual cycle were not semi-annual but substantially annual with a lag of 6 months, which gave rise to SST cooling in spring and autumn. Our analyses illustrated that SST cooling in autumn was caused by direct, local atmospheric wind forcings, whereas the cooling with large amplitudes of the semi-annual cycle in spring was caused by the non-local, remotely forced cold water advection of the Liman Current associated with sea-ice melting in the Tatarskiy Strait. The phase lag of 1–2 months between the complete melting of the sea ice in the Tatarskiy Strait and the surface cooling along the Russian continental shelf was related to the advection of cold water from sea ice in the form of the Liman Cold Current. The semi-annual cycle also resulted from asymmetry of the time series of the SST due to a long cold winter and a short warm summer. To understand how SST curves are distorted and asymmetric, we suggested two mathematical parameters of kurtosis and skewness. In addition, we suggest that the ratio of the semi-annual amplitude to the annual amplitude of SST harmonics can be used as a typical indicator of the asymmetry in year-to-year SST variations.

Mid-Holocene tropical Pacific climate state, annual cycle, and ENSO in PMIP2 and PMIP3

Using the Paleoclimate Modeling Inter-comparison Project Phase 2 and 3 (PMIP2 and PMIP3), we investigated the tropical Pacific climate state, annual cycle, and El Nino-Southern Oscillation (ENSO) during the mid-Holocene period (6,000 years before present; 6 ka run). When the 6 ka run was compared to the control run (0 ka run), the reduced sea surface temperature (SST) and the reduced precipitation due to the basin-wide cooling, and the intensified cross-equatorial surface winds due to the hemispheric discrepancy of the surface cooling over the tropical Pacific were commonly observed in both the PMIP2 and PMIP3, but changes were more dominant in the PMIP3. The annual cycle of SST was weaker over the equatorial eastern Pacific, because of the orbital forcing change and the deepening mixed layer, while it was stronger over the equatorial western pacific in both the PMIP2 and PMIP3. The stronger annual cycle of the equatorial western Pacific SST was accompanied by the intensified annual cycle of the zonal surface wind, which dominated in the PMIP3 in particular. The ENSO activity in the 6 ka run was significantly suppressed in the PMIP2, but marginally reduced in the PMIP3. In general, the weakened air-sea coupling associated with basin-wide cooling, reduced precipitation, and a hemispheric contrast in the climate state led to the suppression of ENSO activity, and the weakening of the annual cycle over the tropical eastern Pacific might lead to the intensification of ENSO through the frequency entrainment. Therefore, the two opposite effects are slightly compensated for by each other, which results in a small reduction in the ENSO activity during the 6 ka in the PMIP3. On the whole, in PMIP2/PMIP3, the variability of canonical (or conventional) El Nino tends to be reduced during 6 ka, while that of CP/Modoki El Nino tends to be intensified.

Reconstructing El Niño-Southern Oscillation activity and ocean temperature seasonality from short-lived marine mollusk shells from Peru

A critical need exists for quantitative reconstructions of long-term El Niño Southern Oscillation (ENSO) variability in the eastern tropical Pacific. Presented here is a method to quantitatively estimate past changes 1) in the seasonal amplitude of sea surface temperature (SST) in the Peruvian coastal upwelling system and 2) in the amplitude of ENSO-related interannual variability in the eastern tropical Pacific. The seasonal amplitude of SST (ΔT) along the length of the Peruvian coast is strongly correlated with the Niño1+2 index. We show that the frequency distribution of ΔT values provided by a modern sample of 13 Mesodesma donacium shells faithfully reflects modern ENSO variability at the regional scale, including the range of anomalies from La Niña to moderate El Niño events, but excludes extreme warm anomalies because of high shell mortality. We propose to use the frequency distribution of ENSO anomalies in paleoclimate studies for comparisons between shell records, coral records, and GCM simulations. Reconstruction uncertainties can be quantified using Monte Carlo simulations. The method presented here opens new perspectives for quantitative paleo-ENSO reconstructions in the Eastern Pacific since it may be applied with any mollusk species from Peru provided at least one annual cycle of SST is faithfully recorded by shell δ18O.

Atmospheric correction for sea surface temperature retrieval from single thermal channel radiometer data onboard Kalpana satellite

An atmospheric correction method has been applied on sea surface temperature (SST) retrieval algorithm using Very High Resolution Radiometer (VHRR) single window channel radiance data onboard Kalpana satellite (K-SAT). The technique makes use of concurrent water vapour fields available from Microwave Imager onboard Tropical Rainfall Measuring Mission (TRMM/TMI) satellite. Total water vapour content and satellite zenith angle dependent SST retrieval algorithm has been developed using Radiative Transfer Model [MODTRAN ver3.0] simulations for Kalpana 10.5–12.5 μm thermal window channel. Retrieval of Kalpana SST (K-SST) has been carried out for every half-hourly acquisition of Kalpana data for the year 2008 to cover whole annual cycle of SST over Indian Ocean (IO). Validation of the retrieved corrected SST has been carried out using near-simultaneous observations of ship and buoys datasets covering Arabian Sea, Bay of Bengal and IO regions. A significant improvement in Root Mean Square Deviation (RMSD) of K-SST with respect to buoy (1.50–1.02 K) and to ship datasets (1.41–1.19 K) is seen with the use of near real-time water vapour fields of TMI. Furthermore, comparison of the retrieved SST has also been carried out using near simultaneous observations of TRMM/TMI SST over IO regions. The analysis shows that K-SST has overall cold bias of 1.17 K and an RMSD of 1.09 K after bias correction.

Influence of the annual cycle of sea surface temperature on the monsoon onset

[1] Differing from the traditional focus on land‐sea thermal contrasts, this paper examines the influence of the annual cycle of sea surface temperature (SST) on the monsoon onset. It is found that SST is a major driver of tropical circulations in the atmospheric boundary layer. In the trade wind regions, the annual cycle of SST involves a shift in the warmest SST axis (WSSTA) between two local maxima on either sides of the equator or slight movement of WSSTA north of the equator; consequently, WSSTA has little effect on the regional meridional SST gradient and wind direction. However, in the monsoon regions, the annual cycle of SST is characterized by an abrupt northward jump of WSSTA, resulting in a marked change in regional meridional SST gradient and consequent onset of winds. The onset of the southwesterlies lags behind the abrupt northward jump of WSSTA. As an example, the climatological onset of the monsoon in the Bay of Bengal (BOB) shows that the abrupt northward jump of WSSTA is caused by an increase in SST in the BOB, leading to the monsoon onset after about 2 pentads. SST shows a maximum before the monsoon onset, contributing to destabilization of the atmosphere. A strong meridional SST gradient occurs between BOB and south of the equator after the abrupt northward jump of WSSTA, associated with cross‐equatorial flow in the eastern Indian Ocean. Case studies indicate that an abrupt northward jump in WSSTA from the equator to north of 10°N is a good omen for the monsoon onset. Citation: Jiang, X., and J. Li (2011), Influence of the annual cycle of sea surface temperature on the monsoon onset, J. Geophys. Res., 116, D10105, doi:10.1029/2010JD015236.

Interannual climate variability in the Miocene: High resolution trace element and stable isotope ratios in giant clams

High resolution stable isotope and trace elemental ratios of a recent Tridacna squamosa from Vietnam and a Middle to Late Miocene (10–13Ma) Tridacna gigas from Indonesia are presented. The seasonal pattern of modern sea surface temperature (SST) variability offshore Vietnam is faithfully recorded in the δ18O of the T. squamosa shell carbonate, confirming the potential of Tridacna shells as sub-annual resolution climate archives. Cultivation of the T. squamosa specimen in controlled conditions after recovery from the natural environment facilitated a quantitative calibration of the δ18O signal to ambient water temperatures. An age model for the Miocene T. gigas shell from Indonesia was therefore constructed on the basis of its δ18O profile, assuming a single-peak annual SST cycle. The magnitude of these oscillations was 5–7°C. Mg/Ca and the growth-banding pattern in the Miocene T. gigas correlates well with shell δ18O during the later part of the organism's lifespan. Ba/Ca is negatively correlated to Mg/Ca, with a lag of several months, suggesting a different phasing of the annual primary productivity cycle from that of SST. Furthermore, δ18O and Mg/Ca show prominent deviations to warmer conditions with a periodicity of ~3years. These shifts demonstrate the existence of substantial interannual sea surface temperature variability in the Miocene, a period with elevated global temperatures compared to the present day.

The correctness to the spuriously simulated semi-annual cycle of the sea surface temperature in the equatorial eastern Pacific

One of the challenges faced by the climate model of the Community Climate System Model version 3 (CCSM3) is the spuriously simulated semi-annual cycle of the sea surface temperature (SST) in the equatorial eastern Pacific. This model bias has limited the performance of the climate simulation and prediction. Based on the surface wave-circulation coupled theory, an atmosphere-wave-ocean coupled model was developed, which incorporates the MASNUM (key laboratory of Marine Sciences and Numerical Modeling) wave number spectral model into CCSM3. The new coupled atmosphere-wave-ocean model successfully removes the spurious semi-annual cycle simulated by the original CCSM3 and reasonably produces an SST annual cycle with warm and cold phases in April and August, respectively. The correlation between the simulated and observed SST in the equatorial eastern Pacific is improved from 0.66 to 0.93. The ocean surface layer heat budget analysis indicates that the wave-induced vertical mixing is responsible for improving the simulation of the SST seasonal cycle in the equatorial eastern Pacific.

Contrasting seasonal fluxes of planktonic foraminifera and impacts on paleothermometry in the Mozambique Channel upstream of the Agulhas Current

[1] We assess d 18 O‐ and Mg/Ca–based paleothermometry in relation to seasonal variations in the shell fluxes of the surface‐dwelling planktonic foraminifera G. ruber and G. trilobus. Using deep‐moored sediment traps in the Mozambique Channel, upstream of the Agulhas Current, we find a distinct antiphase response to the annual cycle in sea surface temperature (SST) in G. ruber and G. trilobus. Maximum fluxes of G. ruber occur in late austral summer (February–March) when SST ranges between 28.7°C and 30.3°C. By contrast, G. trilobus maxima appear in early winter (June–July) at a lower SST between 25.3°C and 27.0°C. Cross‐correlation of the 2.5 year time series data confirmed that the G. ruber/G. trilobus ratio (R/T ratio) closely followed the annual cycle in SST, as did their paired d 18 O and Mg/Ca. In all proxies, G. ruber showed a 3 week phase lag against SST and G. trilobus of 6 weeks consistent with (semi‐) lunar population turnover rates in combination with a 1 week settling time to the 2500 m deep ocean floor. After correcting for these lags, we derived five independent equations for improved paleothermometry in the southwestern Indian Ocean that specify flux‐ weighted annual mean SST. The offset between flux‐weighted SST of G. ruber and G. trilobus and annual mean instrumental SST is about 0.8°C despite their strong seasonality in shell export. This is far less than the mean difference in instrumental SST between summer and winter of 3.2°C. Both species therefore closely reflect annual mean SST in sediments within a margin of +0.5°C for G. ruber and –0.3°C for G. trilobus .W e also find that multiple linear regression of the five proxies (R/T ratio and the d 18 O, Mg/Ca of G. ruber and G. trilobus) strongly reduces the effects of fast rotating eddies and improves the correlation coefficient from an r 2 of 0.4 to an r 2 of 0.8.

Variações do ciclo anual da temperatura da superfície do mar no pacífico tropical

Variações do ciclo anual da Temperatura da Superfície do Mar (TSM) do Pacífico Tropical entre 20ºN e 20ºS, para o período de 1854 a 2006, foram estudadas usando análises de ondaletas. O estudo mostrou que o ciclo anual, considerando-se a variância para em torno de 1 ano, de TSM possui variabilidade interanual e interdecenal. Existem indicações de que a maior parte das variações interanuais no ciclo anual de TSM das áreas do hemisfério Norte (HN) e do hemisfério Sul (HS), está relacionada ao ciclo do El Niño-Oscilação Sul (ENOS). A maioria dos picos da Média por Escalas da Potência localizada da ondaleta (MEPO) para a escala entre 0,7 e 1,2 anos, para as áreas de ambos os hemisférios, coincide com extremos do ENOS. Os Espectros da Potência Global (EPGs) mostraram variância máxima do ciclo anual de TSM entre 120ºE e 140ºE na faixa do HN (entre equador e 20ºN), e variância crescente da linha da data para leste com o máximo entre 100ºW e 80ºW na faixa do HS (entre equador e 20ºS). Portanto, o ciclo anual de TSM apresenta variações entre os HN e HS com a máxima variância localizando-se no hemisfério leste para o HN e no hemisfério oeste para o HS.

The Inverse Effect of Annual-Mean State and Annual-Cycle Changes on ENSO

Abstract The influence of the tropical Pacific annual-mean state on the annual-cycle amplitude and El Niño–Southern Oscillation (ENSO) variability is studied using the Max Planck Institute for Meteorology coupled general circulation model (CGCM) ECHAM5/Max Planck Institute Ocean Model (MPI-OM1). In a greenhouse warming experiment, an intensified annual cycle of sea surface temperature (SST) in the eastern tropical Pacific is associated with reduced ENSO variability, and vice versa. Analysis showed that the annual-mean states, especially the surface warming in the western Pacific and the thermocline deepening in the central Pacific, which is concurrent with the strong annual cycle, act to suppress ENSO amplitude and to intensify the annual-cycle amplitude, and vice versa. The western Pacific warming acts to reduce air–sea coupling strength and to shorten the ocean adjustment time scale, and the deepening of central Pacific thermocline acts to diminish vertical advection of the anomalous ocean temperature by the annual-mean upwelling. Consequently, ENSO activity is suppressed by the annual-mean states during the strong annual-cycle decades, and the opposite case associated with the weak annual-cycle decades is also true. Furthermore, the time integration of an intermediate ENSO model forced with different background state configurations, and a stability analysis of its linearized version, show that annual-mean background states during the weak (strong) annual-cycle decades are characterized by an enhanced (reduced) linear growth rate of ENSO or similarly large (small) variability of ENSO. However, the annual-cycle component of the background state changes cannot significantly modify ENSO variability. Using a hybrid coupled model, it is demonstrated that diagnosed annual-mean background states corresponding to a reduced (enhanced) annual cycle suppress (enhance) the development of the annual cycle of SST in the eastern equatorial Pacific, mainly through the weakening (intensifying) of zonal temperature advection of annual-mean SST by the annual-cycle zonal current. The above results support the idea that climate background state changes control both ENSO and the annual-cycle amplitude in opposing ways.

A Ten-year Period of Daily Sea Surface Temperature at a Coastal Station in Reunion Island, Indian Ocean (July 1993 – April 2004): Patterns of Variability and Biological Responses

Sea surface temperature (SST) was recorded hourly by an automatic data recorder in Reunion Island, at Pointe des Galets (21°55 S, 55°17 E) during 1993-2004. The data logger was installed on a beacon located at the port entrance exposed to the open sea. The SST measurements associated with auxiliary environmental data, such as wind stress, depict the main patterns of variability at various timescales for the marine climate of Reunion Island. The ’ten-year’ annual mean temperature is 25.7°C and the annual amplitude is 4.6°C. The highest monthly means are observed in February and March (28.0°C) and the lowest occur in September (23.4°C). The daily variation ranges from 0.25 to 0.74°C according to the season. In summer, the tropical cyclones are the major cause of short term variability, with sudden drops of SST than can exceed 2°C within a few hours. The annual cycle of SST is closely associated with that of wind stress, with a lagged response of about 2 months of SST to wind forcing. Throughout the ‘ten-year period’ covered by our dataset, the coldest years were 1993 and 2000, and the warmest were 2003 and 2004. A trend of increasing SST is suggested for the two major seasons, with a magnitude of 0.088°C/yr in summer and 0.052°C/yr in winter. Finally, the SST trend and variability depicted at our sampling site is shown to reflect the SST patterns of the whole south tropical Indian Ocean. Biological responses to SST variability are shown by coral bleaching events and the local tuna fishery. The major coral bleaching events recorded in Reunion in 1998, 2001, 2003 and 2004 occurred during episodes of intense and sustained anomalous high temperatures. The catch per unit effort (CPUE) of the local pelagic fishery was negatively correlated to SST anomalies with a decreasing trend of CPUE observed over the years, accompanied by an overall increase of SST. The SST observations made in Reunion Island, in a largely unsampled region of the Indian Ocean, show their relevance at a larger regional scale and their usefulness in monitoring changes of some biological components of the marine ecosystem. These examples highlight the need to maintain networks of automatic loggers worldwide at coastal stations.

Bimodal variation of SST and related physical processes over the North Indian Ocean: special emphasis on satellite observations

Using sea surface temperature (SST) and wind speed retrieved by the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), for the period of 1998–2003, we have studied the annual cycle of SST and confirmed the bimodal distribution of SST over the north Indian Ocean. Detailed analysis of SST revealed that the summer monsoon cooling (winter cooling) over the eastern Arabian Sea (Bay of Bengal) is more prominent than winter cooling (summer monsoon cooling). A sudden drop in surface short wave radiation by 57 W m−2 (74 W m−2) and rise in kinetic energy per unit mass by 24 J kg−1 (26 J kg−1) over the eastern Arabian Sea (Bay of Bengal) is observed in summer monsoon cooling period. The subsurface profiles of temperature and density for the spring warming and summer monsoon cooling phases are studied using the Arabian Sea Monsoon Experiment (ARMEX) data. These data indicate a shallow mixed layer during the spring warming and a deeper mixed layer during the summer monsoon cooling. Deepening of the mixed layer by 30 to 40 m with corresponding cooling of 2°C is found from warming to summer monsoon cooling in the eastern Arabian Sea. The depth of the 28°C isotherm in the eastern Arabian Sea during the spring warming is 80 m and during summer monsoon cooling it is about 60 m, while over the Bay of Bengal the 28°C isotherm is very shallow (35 m), even during the summer monsoon cooling. The time series of the isothermal layer depth and mixed layer depth during the warming phase revealed that the formation of the barrier layer in the spring warming phase and the absence of such layers during the summer cooling over the Arabian Sea. However, the barrier layer does exist over the Bay of Bengal with significant magnitude (20–25 m). The drop in the heat content with in first 50 m of the ocean from warming to the cooling phase is about 2.15 × 108 J m−2 over the Arabian Sea.

Interhemispheric thermal gradient and tropical Pacific climate

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

Diagnosing the Annual Cycle Modes in the Tropical Atlantic Ocean Using a Directly Coupled Atmosphere–Ocean GCM

Abstract The annual cycle of sea surface temperature (SST) in the tropical Atlantic of a directly coupled atmosphere–ocean general circulation model (CGCM) is decomposed into the parts forced by different surface fluxes (denoted as modes) for the two extreme months of March and August using forced ocean experiments. Almost all previous diagnostic work of the forcing of the SST annual cycle in the Atlantic has concentrated on the near-equatorial region. Here, the annual cycle is examined within the latitude range of 25°S–25°N to facilitate comparison with the interannual variability. The structure of the response to the different surface flux forcings bears some resemblance to the interannual SST modes in the tropical Atlantic, which are diagnosed using rotated empirical orthogonal function (REOF) analysis. Diagnosis of the forcing of the annual cycle modes and the interannual modes shows that they do not always have a common cause. Hence, the simple interpretation that the leading interannual modes are perturbations to the annual cycle is not always valid. In particular, the equatorial SST annual cycle mode is primarily driven by variations in vertical velocity while the equatorial interannual mode is associated with eastward-propagating thermocline anomalies and is forced by both thermocline anomalies and vertical velocity anomalies. As for the interannual modes, there exist off-equatorial annual cycle modes in both the Northern and Southern Hemispheres. The annual cycle off-equatorial mode in both hemispheres is shown to be primarily driven by heat flux variations. The Southern Hemisphere interannual mode is primarily driven by heat flux variations while the Northern Hemisphere interannual mode shows a strong influence of thermocline depth anomalies. In addition, the Southern Hemisphere interannual mode is centered about 10° south of the annual cycle mode. An interannual mode that has maximum variability along the South American coast south of the equator is shown to be associated with thermocline depth anomalies. This interannual mode has no analog in the annual cycle modes. The coupled model simulation of the annual cycle is found to be fairly realistic so that the results presented here could have applicability to the observed Atlantic.