Summertime Sea Surface Temperature Research Articles

Interannual Fluctuations and Their Low-Frequency Modulation of Summertime Heavy Daily Rainfall Potential in Western Japan

Heavy rainfall under the conditions of the changing climate has recently garnered considerable attention. The statistics on heavy daily rainfall offer vital information for assessing present and future extreme events and for clarifying the impacts of global climate variability and change, working to form a favorable background. By analyzing a set of large-ensemble simulations using a global atmospheric model, this study demonstrated that two different physical processes in global climate variability control the interannual fluctuations in the 99th- and 90th-percentile values of summertime daily rainfall (i.e., the potential amounts) on Kyushu Island in western Japan. The 90th-percentile values were closely related to large-scale horizontal moisture transport anomalies due to changes in the subtropical high in the northwestern Pacific, which was usually accompanied by basin-scale warming in the Indian Ocean subsequent to the wintertime El Niño events. The contributions of the sea surface temperatures over the northern Indian Ocean and the eastern tropical Pacific Ocean showed low-frequency modulations, mainly due to the influences of the global warming tendency and the interdecadal variability in the climate system, respectively. In contrast, tropical cyclone activity played a major role in changing the 99th-percentile value. The potentials of both the tropical cyclone intensity and the existence density fluctuated, largely owing to the summertime sea surface temperature over the tropical Pacific, which can be modulated by the El Niño diversity on interdecadal timescales.

Low Cloud–SST Feedback over the Subtropical Northeast Pacific and the Remote Effect on ENSO Variability

Abstract Low clouds frequent the subtropical northeastern Pacific Ocean (NEP) and interact with the local sea surface temperature (SST) to form positive feedback. Wind fluctuations drive SST variability through wind–evaporation–SST (WES) feedback, and surface evaporation also acts to damp SST. This study investigates the relative contributions of these feedbacks to NEP SST variability. Over the summer NEP, the low cloud–SST feedback is so large that it exceeds the evaporative damping and amplifies summertime SST variations. The WES feedback causes the locally enhanced SST variability to propagate southwestward from the NEP low cloud deck, modulating El Niño–Southern Oscillation (ENSO) occurrence upon reaching the equator. As a result, a second-year El Niño tends to occur when there are significant warm SST anomalies over the subtropical NEP in summer following an antecedent El Niño event and a second-year La Niña tends to occur when there are significant cold SST anomalies over the subtropical NEP in summer following an antecedent La Niña event The mediating role of the NEP low cloud–SST feedback is confirmed in a cloud-locking experiment with the Community Earth System Model, version 1 (CESM1). When the cloud–ocean coupling is disabled, SST variability over the NEP weakens and the modulating effect on ENSO vanishes. The nonlocal effect of the NEP low cloud–SST feedback on ENSO has important implications for climate prediction.

Mechanisms for Severe Drought Occurrence in the Balsas River Basin (Mexico)

This work provides an assessment of the two most intense seasonal droughts that occurred over the Balsas River Basin (BRB) in the period 1980–2017. The detection of the drought events was performed using the 6 month scale standardized precipitation–evapotranspiration index (SPEI-6) and the 6 month standardized precipitation index (SPI-6) in October. Both indices were quite similar during the studied period, highlighting the larger contribution of precipitation deficits vs. temperature excess to the drought occurrence in the basin. The origin of the atmospheric water arriving to the BRB (1 May 1980–31 October 2017) was investigated by using a Lagrangian diagnosis method. The BRB receives moisture from the Caribbean Sea and the rest of the tropical Atlantic, the Gulf of Mexico, the eastern north Pacific and from three terrestrial evaporative sources: the region north of BRB, the south of BRB and the BRB itself. The terrestrial evaporative source of the BRB itself is by far the main moisture source. The two most intense drought events that occurred in the studied period were selected for further analysis. During the severe drought of 2005, the summertime sea surface temperature (SST) soared over the Caribbean Sea, extending eastward into a large swathe of tropical North Atlantic, which was accompanied by the record to date of hurricane activity. This heating generated a Rossby wave response with westward propagating anticyclonic/cyclonic gyres in the upper/lower troposphere. A cyclonic low-level circulation developed over the Gulf of Mexico and prevented the moisture from arriving to the BRB, with a consequent deficit in precipitation. Additionally, subsidence also prevented convection in most of the months of this drought period. During the extreme drought event of 1982, the Inter Tropical Convergence Zone (ITCZ) remained southern and stronger than the climatological mean over the eastern tropical Pacific, producing an intense regional Hadley circulation. The descent branch of this cell inhibited the development of convection over the BRB, although the moisture sources increased their contributions; however, these were bounded to the lower levels by a strong trade wind inversion.

Marine Heatwave of Sea Surface Temperature of the Oyashio Region in Summer in 2010–2016

The sea surface temperature (SST) of the Oyashio region in boreal summer abruptly increased in 2010 and high summertime SST repeated every year until 2016. Observations and an ocean reanalysis show that this marine heatwave occurred not only at the surface but also at deeper depths down to 200 m. Furthermore, salinity in summer also increased in parallel with the temperature. The rises in temperature and salinity indicate the strengthening of the Kuroshio water influence. The sea surface height and velocity show that the southward intrusion of the Oyashio near the coast in summer weakened from 2010 accompanied by an increase in anticyclonic eddies from the Kuroshio Extension. The much more frequent existence of anticyclonic eddies to the east of the first intrusion of the Oyashio in summer is closely associated with the weakening of the first intrusion and the strengthening of the second intrusion. It is suggested that the rise in the water temperature could increase a catch of yellowtail (Seriola quinqueradiata) in northern Japan.

Southern Ocean Heat Storage, Reemergence, and Winter Sea Ice Decline Induced by Summertime Winds

AbstractThe observational record shows a substantial 40-yr upward trend in summertime westerly winds over the Southern Ocean, as characterized by the southern annular mode (SAM) index. Enhanced summertime westerly winds have been linked to cold summertime sea surface temperature (SST) anomalies. Previous studies have suggested that Ekman transport or upwelling is responsible for this seasonal cooling. Here, another process is presented in which enhanced vertical mixing, driven by summertime wind anomalies, moves heat downward, cooling the sea surface and simultaneously warming the subsurface waters. The anomalously cold SSTs draw heat from the atmosphere into the ocean, leading to increased depth-integrated ocean heat content. The subsurface heat is returned to the surface mixed layer during the autumn and winter as the mixed layer deepens, leading to anomalously warm SSTs and potentially reducing sea ice cover. Observational analyses and numerical experiments support our proposed mechanism, showing that enhanced vertical mixing produces subsurface warming and cools the surface mixed layer. Nevertheless, the dominant driver of surface cooling remains uncertain; the relative importance of advective and mixing contributions to the surface cooling is model dependent. Modeling results suggest that sea ice volume is more sensitive to summertime winds than sea ice extent, implying that enhanced summertime westerly winds may lead to thinner sea ice in the following winter, if not lesser ice extent. Thus, strong summertime winds could precondition the sea ice cover for a rapid retreat in the following melt season.

Implications of acute temperature and salinity tolerance thresholds for the persistence of intertidal invertebrate populations experiencing climate change.

To predict whether populations of marine animals will persist in the face of changing climate conditions, it is informative to understand how past climate conditions have shaped present‐day tolerance thresholds. We examined 4 species of intertidal invertebrates (Nucella lamellosa, Littorina scutulata, Littorina sitkana, and Balanus glandula) inhabiting the coasts of Vancouver Island, Canada, where the east coast experiences historically warmer sea surface temperature (SST), warmer low tide (i.e., emersion) rock surface temperature (RST), and lower sea surface salinity (SSS) than the west coast. To determine if east coast populations have higher tolerance thresholds to acute stress than west coast populations, animals from 3 sites per coast were exposed to stressful temperatures and salinities in common garden experiments. Emersion temperature tolerance differed between populations only in N. lamellosa and B. glandula, tolerance thresholds being 1.4–1.5°C higher on the east coast. Water temperature tolerance differed between populations only in B. glandula and L. scutulata but was highest on the west coast. No differences in salinity tolerance were observed within any species. Thus, there is limited evidence of divergence among east and west coast populations in tolerance of acute stress despite the substantial historical differences in extreme temperature and salinity conditions between coasts. However, based on present‐day summertime SST and RST and known rates of change in these parameters, we predict present‐day tolerance thresholds would be sufficient to allow adults of these populations to tolerate extreme temperatures predicted for the next several hundred years, and that even a slow rate of change in acute tolerance thresholds might suffice to keep up with future temperature extremes.

An intensified East Asian winter monsoon in the Japan Sea between 7.9 and 6.6 Ma

Abstract The Japan Sea was a semi-closed marginal sea mainly connected to the subarctic northwestern Pacific via shallow seaways during the late Miocene. We use a multiple regression analysis with common extant radiolarian species groups to estimate the sea-surface temperature (SST) for the period between 9.1 and 5.3 Ma. Our results show a cooling of 8 °C between 7.9 and 6.6 Ma, when the SST dropped from 24 °C to 16 °C. We infer that this cooling dominantly reflects wintertime cooling related to an intensified East Asian winter monsoon. On the other hand, cooling of the summertime SST occurred from 6.6 to 5.8 Ma, suggesting that the late Miocene global cooling is composed of a wintertime cooling phase from 7.9 to 6.6 Ma and summertime cooling phase from 6.6 to 5.8 Ma.

Regional Structure in the Marine Heat Wave of Summer 2015 Off the Western United States

One of the largest warm water anomalies (marine heat waves [MHWs]) ever recorded occurred in the northeast Pacific during 2014--2016. This MHW was caused by large-scale atmospheric ridging and affected fisheries and ecosystems from Alaska through California, including a bloom of toxic algae spanning the entire coastline. Regional variations in MHW severity are common along coastlines worldwide but are generally unexplained. During the 2014--16 MHW, the summertime sea-surface temperature (SST) anomalies were often stronger along the southern half of the coastline off the western continental United States. The reason for this north-south difference in severity of the MHW within the California Current System (CCS) has remained unclear. The scientific community's lack of understanding of regional variations within MHWs prevents accurate prediction of SST anomalies and resulting ecological and economic impacts. We show the north-south difference in SST anomalies was due to a known wind pattern determined by the coastline shape. The wind anomalies in summer have a quasi-dipole structure: the northern lobe extends southwest from Washington/Oregon, and the southern lobe has opposite sign and extends south from Cape Mendocino in a triangle due to a hydraulic expansion fan in the marine boundary layer. The alternating wind intensifications and relaxations typically last several days. However, the large-scale ridging during the MHW was associated with unusual persistence in this pattern: in summer 2015 a single wind relaxation in the southern lobe lasted two weeks. These wind anomalies induce changes in SST, likely via changes in wind-driven vertical entrainment of cold water from below the mixed layer, and mixed-layer shoaling; the net air-sea heat flux anomaly is small. The July 2015 wind relaxation persisted so long that the changes in SST exceeded pre-existing SST variations. The resulting SST anomalies have a dipole pattern similar to the wind anomalies. These dipole SST anomalies explain the north-south asymmetry in the CCS MHW. We suggest that during future persistent ridging events, the SST anomalies off the western continental U.S. will develop a north-south split structure similar to July 2015.

Long-term trends and regional variability in extreme temperature and salinity conditions experienced by coastal marine organisms on Vancouver Island, Canada

Studies of ocean temperature and salinity typically focus on global or large-scale trends that may not accurately reflect the conditions experienced locally by marine organisms. When assessing impacts of climate change, it is important to document local conditions, and also to focus on the most physiologically stressful time of year. Our goals were therefore to (1) characterize long-term trends in sea surface temperature (SST) and salinity (SSS) experienced by coastal marine organisms during the most stressful time of year around Vancouver Island; and (2) document variation between east and west coasts of Vancouver Island in terms of SST, SSS, and daytime rock surface temperatures in the intertidal zone. Over an 82-yr period, from 1935 to 2016, summertime SST on both coasts increased by 0.67–0.78 °C (i.e., 0.82–0.97 °C per century). Trends in salinity differed between coasts: east coast salinity increased by 3.9 while west coast salinity decreased by 0.64. Although long-term SST trends are the same on both coasts, during the most stressful time of year, east coast waters are on average 4.3 °C warmer and salinity is 7.8 lower than on the west coast. Rock temperature in the mid and upper intertidal zone during daytime low tides is 3.9–4.2 °C warmer on the east coast. Populations of marine organisms inhabiting the coasts of Vancouver Island have therefore been experiencing long-term changes in abiotic stress, as well as persistent spatial variation in climate-related conditions during the most stressful months of the year.

Does Pacific Variability Influence the Northwest Atlantic Shelf Temperature?

AbstractThe relationship between North Pacific variability and sea surface temperature (SST) of the Northwest Atlantic continental shelf is examined over interannual time scale in 1982–2014. Statistically significant negative correlations exist between Pacific Decadal Oscillation (PDO) index and SST in the Gulf of Maine (GoM) in spring and summer. Cross‐correlation analysis further suggests significant negative lead‐lag correlations, with the spring PDO leading the GoM SST by 0–3 months while the summer PDO lags by 1–3 months. These correlations are dominated by the interannual component of the PDO. Statistical relationships are placed in context by further investigating the physical processes controlling the upper ocean mixed layer temperature budget in the GoM. The results reveal contrasting roles between the atmosphere and the ocean in spring and summer, respectively. Local atmospheric forcings, in particular the radiative air‐sea fluxes, are the dominant driver for the interannual variability of springtime SST over the Northwest Atlantic shelf. In contrast, oceanic terms are important in controlling the interannual variability of summertime SST. As a result, reconstructed SST using atmospheric forcings successfully reproduces the statistical relationship with PDO in spring, but not in summer. Furthermore, it is shown that the SST anomalies in the central and eastern North Pacific play a key role in these relationships.

Coupling between marine boundary layer clouds and summer-to-summer sea surface temperature variability over the North Atlantic and Pacific

Climate modes of variability over the Atlantic and Pacific may be amplified by a positive feedback between sea-surface temperature (SST) and marine boundary layer clouds. However, it is well known that climate models poorly simulate this feedback. Does this deficiency contribute to model-to-model differences in the representation of climate modes of variability? Over both the North Atlantic and Pacific, typical summertime interannual to interdecadal SST variability exhibits horseshoe-like patterns of co-located anomalies of shortwave cloud radiative effect (CRE), low-level cloud fraction, SST, and estimated inversion strength over the subtropics and midlatitudes that are consistent with a positive cloud feedback. During winter over the midlatitudes, this feedback appears to be diminished. Models participating in the Coupled Model Intercomparison Project phase 5 that simulate a weak feedback between subtropical SST and shortwave CRE produce smaller and less realistic amplitudes of summertime SST and CRE variability over the northern oceans compared to models with a stronger feedback. The change in SST amplitude per unit change in CRE amplitude among the models and observations may be understood as the temperature response of the ocean mixed layer to a unit change in radiative flux over the course of a season. These results highlight the importance of boundary layer clouds in interannual to interdecadal atmosphere–ocean variability over the northern oceans during summer. The results also suggest that deficiencies in the simulation of these clouds in coupled climate models contribute to underestimation in their simulation of summer-to-summer SST variability.

The phenology of Arctic Ocean surface warming.

In this work, we explore the seasonal relationships (i.e., the phenology) between sea ice retreat, sea surface temperature (SST), and atmospheric heat fluxes in the Pacific Sector of the Arctic Ocean, using satellite and reanalysis data. We find that where ice retreats early in most years, maximum summertime SSTs are usually warmer, relative to areas with later retreat. For any particular year, we find that anomalously early ice retreat generally leads to anomalously warm SSTs. However, this relationship is weak in the Chukchi Sea, where ocean advection plays a large role. It is also weak where retreat in a particular year happens earlier than usual, but still relatively late in the season, primarily because atmospheric heat fluxes are weak at that time. This result helps to explain the very different ocean warming responses found in two recent years with extreme ice retreat, 2007 and 2012. We also find that the timing of ice retreat impacts the date of maximum SST, owing to a change in the ocean surface buoyancy and momentum forcing that occurs in early August that we term the Late Summer Transition (LST). After the LST, enhanced mixing of the upper ocean leads to cooling of the ocean surface even while atmospheric heat fluxes are still weakly downward. Our results indicate that in the near‐term, earlier ice retreat is likely to cause enhanced ocean surface warming in much of the Arctic Ocean, although not where ice retreat still occurs late in the season.

Antarctic Ocean and Sea Ice Response to Ozone Depletion: A Two-Time-Scale Problem

Abstract The response of the Southern Ocean to a repeating seasonal cycle of ozone loss is studied in two coupled climate models and is found to comprise both fast and slow processes. The fast response is similar to the interannual signature of the southern annular mode (SAM) on sea surface temperature (SST), onto which the ozone hole forcing projects in the summer. It comprises enhanced northward Ekman drift, inducing negative summertime SST anomalies around Antarctica, earlier sea ice freeze-up the following winter, and northward expansion of the sea ice edge year-round. The enhanced northward Ekman drift, however, results in upwelling of warm waters from below the mixed layer in the region of seasonal sea ice. With sustained bursts of westerly winds induced by ozone hole depletion, this warming from below eventually dominates over the cooling from anomalous Ekman drift. The resulting slow time-scale response (years to decades) leads to warming of SSTs around Antarctica and ultimately a reduction in sea ice cover year-round. This two-time-scale behavior—rapid cooling followed by slow but persistent warming—is found in the two coupled models analyzed: one with an idealized geometry and the other with a complex global climate model with realistic geometry. Processes that control the time scale of the transition from cooling to warming and their uncertainties are described. Finally the implications of these results are discussed for rationalizing previous studies of the effect of the ozone hole on SST and sea ice extent.

Radiosonde observational evidence of the impact of an extremely cold SST spot on a mesoscale anticyclone

AbstractRadiosondes were launched in August 2006 from a research vessel above a spot around the Kuril Islands between the North Pacific and the Okhotsk Sea characterized by an extremely cold sea surface temperature (SST) and a large horizontal temperature gradient caused by strong tidal mixing. Because this cold SST was caused solely by this oceanic process, the observed data chiefly reflect the oceanic influence on the atmosphere. In this area, where the summertime SST is typically lower than in other seas in the same latitude belt, a minimum SST of 2°C and a horizontal gradient of 7°C/10 km were observed in August 2006. During the cruise, the area was covered by a layer of fog up to about 1000 m thick. The following observations reveal the presence of an atmospheric mesoscale anticyclone and a cold dome caused by ocean cooling of the atmosphere. The wind direction over the cold spot and the strong gradient region was northeasterly below the altitude of about 500 m, in contrast to the southwesterly geostrophic wind, and the area of northeasterly wind was characterized by a cold air mass and a high‐pressure anomaly. This ageostrophic wind distribution indicates mesoscale anticyclonic circulation below 500 m. The fog layer was also thinner in the northeasterly wind area than in other areas, suggesting vertical dry air advection (i.e., a subsidence) associated with the mesoscale anticyclone. In addition, our observational results suggest that the pressure adjustment mechanism can explain this mesoscale circulation.

Summertime sea surface temperature and salinity fronts in the southern Taiwan Strait

Based on hydrographic survey and satellite data, three fronts were observed in the southern Taiwan Strait in summer. They are the Taiwan Bank Front (TBF), the Southwest Coastal Upwelling Front (SCUF), and the Pearl River Plume Extension Front (PRPEF). In addition to the temperature aspect of TBF and SCUF as indicated in previous studies, we find that TBF and SCUF could also be identified according to salinity. The TBF is closely related to the Taiwan Bank upwelling, tidal mixing, and the Pearl River Plume Extension. Different hydrographic conditions on the southern and northern sides of this front lead to a south–north asymmetric structure of the TBF. The relatively small Simpson–Hunter number around the Taiwan Bank indicates that the TBF may be a tidal front. The SCUF separates the wind-driven cold, saline coastal upwelling water from the warm, less saline offshore water. Owing to frontal instability, SCUF exhibits both short temporal (several days) and small spatial (several kilometres) scales, indicative of intense sub-mesoscale processes. Nonetheless, the weak summertime SCUF was revealed in the log-transformed satellite frontal map. Finally, apart from the commonly observed dominant PRPEF in summer, a bifurcation from the PRPEF was identified in most hydrographic sections. Once generated, this bifurcation is carried by the topography-following current away from the PRPEF.

The impact of the Indonesian Throughflow and tidal mixing on the summertime sea surface temperature in the western Indonesian Seas

A numerical model is used to investigate how the Indonesian Throughflow and tidal mixing are affecting the seasonal cycle of the sea surface temperature (SST) in the Indonesian Seas. The SST in these seas is considered to play a major role on the development of the Australian Summer Monsoon. Based on a quantitative assessment of the heat budget, the Indonesian Throughflow is found to affect the SST in the western Indonesian Seas primarily during Austral summer. The Throughflow advects the warm water from the Pacific and maintains the warm SST when the Northwestern Monsoonal wind induces coastal upwelling along the northern side of the Nusa Tenggara and cools the SST. Such balance is supported by observations. The hydrographic sections show the isotherms tilting upward toward the northern coast of the Nusa Tenggara when satellite observations show slight decrease of the SST in the region. Tidal mixing is found to cool the SST during summer the most. This is because the Northwest Monsoonal wind induces coastal upwelling near where strong tidal mixing above seamount occurs and brings the tidally well‐mixed upper thermocline water to the surface. The surface Ekman flow also spreads this cool water around the Banda Sea where tidal mixing does not occur. The impact of tidal mixing on the SST is also found to come largely from that occurring above seamounts. The impact of tidal mixing on the continental shelves is limited to shelf‐breaks because cold subsurface water is necessary for enhanced vertical mixing to cool the SST.

Characterization of coral bleaching environments and their variation along the Bahia state coast, Brazil

The relation between climate variability and coral bleaching in the Bahia reefs was investigated in an attempt to characterize the bleaching environments. The following 13-year time series were derived from the remote-sensing, analysis and reanalysis data: maximum summertime sea surface temperature (SST), maximum sea surface temperature (MaxSST) accumulated in 5 days (SSTAc5day), diffuse attenuation coefficient for downward irradiance at 490 nm (K 490), rainfall and magnitude of surface wind fields, including the zonal (U) and meridional components. Principal component analysis, non-metric multidimensional scaling (MDS) and cluster and similarity analyses indicate the complex nature of the bleaching patterns and the influence of the strong 1997–1998 El Niño. A significant (global R-value = 0.65; p < 0.01) compounding effect of the reef location and bleaching intensity on the differentiation of bleaching environments was detected. A combination of high SSTAc5day and low K 490 may cause coral bleaching in the northernmost reefs. Evidence clearly points to a scenario where the influence of reef location, bleaching year and intensity may produce a compounded effect that determines the bleaching environments in Bahia.

Dichloromethane in the Indian Ocean: Evidence for in-situ production in seawater

We measured partial pressures of CH 2Cl 2 (dichloromethane) and CCl 3F (CFC-11) in air and surface seawater of the Indian and Southern Oceans from November 2009 to January 2010. The excess CH 2Cl 2 saturation anomaly (ΔSA CH2Cl2 > 0) in surface seawater between 10°S and 40°S, which is the corrected value of saturation anomaly after subtracting the effects of summertime sea-surface temperature increase and of summertime decrease of atmospheric CH 2Cl 2 level, provides evidence for in-situ production of CH 2Cl 2. Depth profiles from the South Indian Ocean at 5°S, 20°S and 30°S showed concentration maxima of dihalomethanes (CH 2I 2, CH 2ClI and CH 2Cl 2) and chlorophyll-a in the subsurface layer (20–150 m). Our data suggest that phytoplankton production of dihalomethanes leads to concentration maxima in the subsurface layer and ΔSA CH2Cl2 > 0 in surface seawater between 10°S and 40°S. The average oceanic emission of CH 2Cl 2 derived from the region between 10°S and 40°S in the South Indian Ocean was calculated to be 0.29–0.43 μg m −2 d −1.

Indian Ocean intraseasonal sea surface temperature variability during boreal summer: Madden‐Julian Oscillation versus submonthly forcing and processes

Intraseasonal sea surface temperature (SST) variability in the Indian Ocean during boreal summer is investigated with a series of experiments using the Hybrid Coordinate Ocean Model (HYCOM). QuickSCAT winds and satellite‐observed outgoing longwave radiation (OLR) are used to identify the wind and convection patterns associated with atmospheric intraseasonal oscillations (ISOs). Effects of the Madden‐Julian Oscillation (MJO; 30–90 days) and submonthly ISOs are separately examined. Similar to winter, MJO forcing dominates summertime SST variability, even though submonthly forcing is stronger. Wind plays a much larger role in altering SSTs than either shortwave fluxes or precipitation. Different from winter cases, the maximum summertime SST variability shifts to the Arabian Sea (AS) and the Bay of Bengal (BOB), when ISOs also shift to the Northern Hemisphere. In the BOB, surface heat fluxes due to changes in wind speed have a stronger influence on SST than upwelling and advection induced by wind stress, whereas in winter, the effects of surface heat fluxes and oceanic upwelling and advection are comparable. This difference arises from the barrier layer and thin surface mixed layer in the BOB, which reduce the effects of upwelling and amplify the effects of surface heat fluxes. In the AS, surface heat fluxes and entrainment cooling due to changes in wind speed have a larger effect on MJO‐scale SST than upwelling induced by wind stress, while the two have comparable effects on submonthly SST. In the equatorial region, wind speed and stress are equally important.

Summertime sea surface temperature fronts associated with upwelling around the Taiwan Bank

It is well known that upwelling of subsurface water is dominant around the Taiwan Bank (TB) and the Penghu (PH) Islands in the southern Taiwan Strait in summertime. Sea surface temperature (SST) frontal features and related phenomena around the TB upwelling and the PH upwelling were investigated using long-term AVHRR (1996–2005) and SeaWiFS (1998–2005) data received at the station of National Taiwan Ocean University. SST and chlorophyll-a (Chl-a) images with a spatial resolution of 0.01° were generated and used for the monthly SST and Chl-a maps. SST fronts were extracted from each SST images and gradient magnitudes (GMs); the orientations were derived for the SST fronts. Monthly maps of cold fronts where the cooler SSTs were over a shallower bottom were produced from the orientation. Areas with high GMs (0.1–0.2 °C/km) with characteristic shapes appeared at geographically fixed positions around the TB/PH upwelling region where SSTs were lower than the surrounding waters. The well-shaped high GMs corresponded to cold fronts. Two areas with high Chl-a were found around the TB and PH Islands. The southern border of the high-Chl-a area in the TB upwelling area was outlined by the high-GM area. Shipboard measurements of snapshot vertical sections of temperature (T) and salinity (S) along the PH Channel showed a dome structure east of PH Islands, over which low SST and high GM in the maps of the corresponding month were present. Clear evidence of upwelling (vertically uniform distributions of T and S) was indicated at the TB edge in the T and S sections close to TB upwelling. This case of upwelling may be caused by bottom currents ascending the TB slope as pointed out by previous studies. The position of low SSTs in the monthly maps matched the upwelling area, and the high GMs corresponded to the area of eastern surface fronts in the T/S sections.