Sea Surface Temperature Warming Trend Research Articles

Role of Japan Sea Throughflow in the spatial variability of the long-term sea surface temperature trend

AbstractThe Japan Sea shows a much stronger warming of long-term sea surface temperature (SST) than surrounding oceans. The warming trend possesses a meridionally alternating zonal band pattern, with weak trends along the paths of the Japan Sea Throughflow and strong trends in the remaining interior region. Using idealized models of the Japan Sea Throughflow and atmospheric heating, this study examines the process behind the formation of such spatial patterns in the SST trend. We find that zonal band structures form in a flat rectangular coastline model, and heat budget analysis shows that horizontal heat transport, due to throughflow, reduces the warming effect created by the surface heat flux. A weak SST trend appears around the jet, while a strong SST trend appears elsewhere. Bathymetric effects are also examined using a model with realistic coastline settings. The location of the western boundary current stabilizes, and the coastal branch begins to disconnect from the Japanese coastline toward the north, allowing a more stable SST warming region to form in the southern interior region. Lagrangian particle tracking experiments confirm that a weak (strong) SST trend corresponds to a short (long) residence time, and eddies in the Japan Sea prolong the residence time in interior regions. The model results suggest that the accumulation time of surface heating is essential to the spatial distribution of the long-term SST warming trend.

Simulated and projected SST of Asian marginal seas based on CMIP6 models

Sea surface temperature (SST) is an important element in studying the global ocean-atmospheric system, as well as its simulation and projection in climate models. In this study, we evaluate the simulation skill of the Coupled Model Intercomparison Project Phase 6 (CMIP6) models in simulating the climatological SST in the Asian Marginal Seas (AMS), known as the most rapidly warming region over the global ocean. The results show that the spatial patterns and seasonal variability of Asian Marginal Seas (AMS) climatological SST simulated by the CMIP6 models are generally in good agreement with the observations, but there are simulation biases in the values. In boreal winter, the simulated climatological SST tends to be overestimated in the Japan/East Sea and the East China Seas (ECSs) by up to 2°C, while being underestimated in the Sea of Okhotsk by up to 2°C. In boreal summer, the simulated climatological SSTs are overestimated in the Indonesian seas and western Arabian Sea, while being underestimated in the Sea of Okhotsk and the northern ECSs by 1.2–1.5 and 2°C, respectively. Furthermore, we calculate the projected sea surface warming trends in the AMS under different future scenarios in the CMIP6 models. The results show warming trends of 0.8–1.8, 1.7–3.4, and 3.8–6.5°C/century for the Shared Socio-Economic Pathway (SSP) of low- (global radiative forcing of 2.6 W/m² by the year 2100), medium- (global radiative forcing of 4.5 W/m² by 2100) and high-end (8.5 W/m² by 2100) pathways, respectively. In addition, the middle and high latitudes of the AMS are found to have faster warming trends than the low latitudes, with the most rapidly warming occurring in the Sea of Okhotsk, which is around 2 times larger than the global mean SST warming trend. The SST warming trends are relatively slow in the South China Sea and the Indonesian seas, roughly equal to the global mean SST warming trend.

What Causes the Arabian Gulf Significant Summer Sea Surface Temperature Warming Trend?

The present study investigated the significant sea surface temperature (SST) warming trend during the summer season over the Arabian Gulf (AG) and its links with the large-scale atmospheric driver, namely, the Atlantic multidecadal oscillation (AMO), from 1900 to 2021. The link between the AMO and the AGs oceanic circulations has received little scientific attention. It has been found that there is a significant spatial positive trend, with a maximum of up to 0.6 °C per decade over the far northern end, while the time series trend shows a significant shift after 1995, with an average value of about 0.36 °C per decade. The spatial trend in the AG is eight times and four times higher than the global value from 1980 to 2005 using HadISST and OISST, respectively. The AMOs significant role in the AGs SST significant warming trend has been confirmed by the spatial and temporal correlation coefficient, which is above 0.50 and 0.48, respectively, with statistical significance at the 99% level. The underlying mechanisms that explained the AMO-related AGs SST decadal variability can be explained as follows: when the AMO is in a positive phase, the surface northwesterly wind weakens, leading to (1) less advection of the relatively cold air masses from Southern Europe to the AG and surrounding areas, (2) less evaporation, and thus less surface cooling (3); thus, this enhances the water masses stratification and decreases the upwelling process, and vice versa occurs for the negative phase. For the air temperature, the positive AMO phase coincides with the occurrence of warm air masses covering all of the Arabian Peninsula, North Africa, and Southern Europe. These processes prove that the AMO is a possible candidate for the AGs SST decadal variability, hence enabling a better evaluation of future climate scenarios for this important region. Our results provide initial insights into the AMO-driven spatiotemporal variability in the SST over the AG and prove that the relation is nonstationary over time. Further analyses are required to explore whether the impacts of the AMO are extended to other oceanic variables such as evaporation rate, heat transport, etc.

An increase in marine heatwaves without significant changes in surface ocean temperature variability

Marine heatwaves (MHWs)—extremely warm, persistent sea surface temperature (SST) anomalies causing substantial ecological and economic consequences—have increased worldwide in recent decades. Concurrent increases in global temperatures suggest that climate change impacted MHW occurrences, beyond random changes arising from natural internal variability. Moreover, the long-term SST warming trend was not constant but instead had more rapid warming in recent decades. Here we show that this nonlinear trend can—on its own—appear to increase SST variance and hence MHW frequency. Using a Linear Inverse Model to separate climate change contributions to SST means and internal variability, both in observations and CMIP6 historical simulations, we find that most MHW increases resulted from regional mean climate trends that alone increased the probability of SSTs exceeding a MHW threshold. Our results suggest the need to carefully attribute global warming-induced changes in climate extremes, which may not always reflect underlying changes in variability.

Sea Surface Temperature Changes Over the Twentieth and Twenty-First Centuries in East Vietnam Sea Simulated by Multi CMIP5 Models

The East ​​Vietnam Sea plays important roles in the Pacific Northwest region. The projection of changes in sea surface temperature (SST) in these regions is an important research topic in marine science. However, this is a very difficult problem due to the lack of available long-term projection data. Recently, with the development of numerical modeling technology, it has become an important way to help us understand climate change. This paper focuses on studying the SST changes in the East Vietnam Sea during the history of the 20th century and the change under 3 emission scenarios in the 21st century based on a combination of 20 global models (GCM) from Phase 5 of the the Climate Model Intercomparison Project (CMIP5) and together with the observed data set. Compared with the observed data, most of the global GCMs models can simulate well the spatial and seasonal changes of the SST over the East Vietnam Sea regions. The spatial and annual SST trends over the the 20th century based on both observations and multimodel ensemble averages show that the warming trend of SST over most of the East Vietnam Sea with the largest warming trend occurred in the center and southern regions of the East Vietnam Sea. However, compared with the observation, CMIP5 underestimated SST trends over most regions of East Vietnam Sea. In addition, there is a consistency between the CMIP5 and the spatial and seasonal observations of the SST trend in the East Vietnam Sea areas. The future SST projections for East Vietnam Sea indicate that RCP 4.5 and RCP 8.5 exhibit a gradual increase in annual SST during the 21st century at a rate of 0.1 °C and 0.3 °C per 10 years respectively. The lowest emission mitigation scenario, RCP 2.6, produces the lowest rate of warming. By the end of the 21st century, the annual SST is projected to increase by 0.5-2.0 °C in 3 emission scenarios of typical representative concentration pathways (RCP) 8.5, 4.5 and 2.6.

2021 Texas cold snap: Manifestation of natural variability and a recent warming trend

In this paper, we demonstrate that the extreme cold snap in Texas in February 2021 resulted from a large-scale background circulation anomaly over the North Pacific and North America that was a manifestation of both natural variability (namely, a negative Pacific–North American pattern, PNA) and a warming trend that began in the late 1990s. Numerical experiments revealed that the negative PNA pattern could be driven by negative (La Niña-like) sea surface temperature (SST) anomalies in the equatorial eastern Pacific on an interannual–decadal timescale and the SST warming trend in the subtropical North Atlantic on a longer timescale. We propose that the excitation of the most unstable mode of the Northern Hemisphere winter, which exhibits a combined negative PNA and positive East Atlantic pattern resulting from SST anomalies and trends, leads to the dominance of the large-scale circulation anomaly observed in February 2021 and over the last few decades.Our study demonstrated the key role of extratropical wave activity and the intrinsic mode in causing the extreme cold in North America. The arguments favoring polar or tropical influences may partially explain the source of variability, considering that extratropical weather and climate variability could be modulated by not only polar or tropical perturbations but also the enhanced tropical–extratropical interaction and mid-latitude instability. A systematic approach involving both empirical diagnostics and a suite of carefully designed numerical experiments is required to determine the relative influence of various sources of variability.Regarding future changes, models of the Coupled Model Intercomparison Project Phase 6 project a weakening, rather than an enhancement, of the negative PNA pattern that favors the occurrence of cold snaps despite failing to simulate the observed enhanced trend since the late 1990s.

Paleoclimate Records of the Middle Okinawa Trough Since the Middle Holocene: Modulation of the Low-Latitude Climate

The ubiquity of glycerol dibiphytanyl glycerol tetraethers (GDGTs) and their temperature sensitivity make them one of the most effective tools for paleoclimate reconstruction. High- and low-latitude climates influence the Okinawa Trough (OT). It receives diverse inputs from the East China Sea, the western Pacific, and the Kuroshio Current, providing good conditions for paleoclimate studies. Here, isoprenoid GDGTs (isoGDGTs), branched GDGTs, and hydroxylated GDGTs (OH-GDGTs) were studied to reconstruct the sea surface temperature (SST) of the central OT for the past 8.2 kyr using the tetraether index of 86 carbon atoms at low latitudes (TEXH86) and the ring index of OH-GDGTs (RI-OH). The GDGT-0/crenarchaeol ratios ranged from 0.39 to 0.98. The branched and isoprenoid tetraether index and the methane index values were lower than 0.1 and 0.5, respectively, indicating that the isoGDGTs were mainly derived from marine Thaumarchaeota and that TEXH86 could be used to reconstruct the paleotemperatures. The TEXH86 SSTs ranged from 21.6 to 27.2°C during 8.2 kyr. The overall range of TEXH86 SSTs is close to the UK’37 SST of the middle OT and reflects the mean annual SST. In contrast, RI-OH temperatures varied from 17.4 to 26.0°C, showing a lower trend than TEXH86 SSTs. The core top RI-OH temperature is 24.1°C, in line with the mean annual seawater temperature at 40 m (24.2°C) in the study area, which likely reflects the subsurface temperature in this case. The small overall warming trend of TEXH86 SSTs agrees with the increasing intensity of the Kuroshio Current during the last 8.2 kyr, indicating that the SST evolution is governed by the Kuroshio Current that transports heat from the western tropical Pacific. The decreasing temperature differences between TEXH86 and RI-OH and between UK’37 and RI-OH showed increased mixing of the upper water column, which was in good accordance with the increasing low-latitude winter insolation decoupling from the East Asian summer monsoon. The cold event that occurred at 7.4–6.6 kyr was magnified (∼5°C) at the TEXH86 and RI-OH temperatures and possibly caused by tephra’s significant input (∼7.3 kyr).

Atmosphere-Ocean Coupled Variability in the Arabian/Persian Gulf

The Arabian Gulf comprises one of the world's most unique and fragile marine ecosystems; it is susceptible to the adverse effects of climate change due to its shallow depth and its location within an arid region that witnesses frequent severe atmospheric events. To reproduce these effects in numerical models, it is important to obtain a better understanding of the region's sea surface temperature (SST) variability patterns, as SST is a major driver of circulation in shallow environments. To this end, here, empirical orthogonal function (EOF) decomposition analysis was conducted to investigate interannual to multi-decadal SST variability in the Gulf from 1982 to 2020, using daily Level 4 Group for High Resolution SST (GHRSST) data. In this way, three dominant EOF modes were identified to contribute the Gulf's SST variability. Significant spatial and temporal correlations were found suggesting that throughout the 39-year study period, SST variability could be attributed to atmospheric changes driven by the El Nio-Southern Oscillation (ENSO), Atlantic Multi-decadal Oscillation (AMO), and Indian Ocean Dipole (IOD) climate modes. Spatial and temporal analyses of the dataset revealed that the average SST was 26.7°C, and that the warming rate from 1982 to 2020 reached up to 0.59°C/decade. A detailed examination of SST changes associated with heat exchange at the air-sea interface was conducted using surface heat fluxes from fifth generation (ERA5) European Centre for Medium-Range Weather Forecasts (ECMWF). Despite the SST warming trend, the accumulation of heat during the study period is suggesting that there was an overall loss of heat (cooling). This cooling reverted into heating in 2003 and has since been increasing.

Increasing coral calcification in Orbicella faveolata and Pseudodiploria strigosa at Flower Garden Banks, Gulf of Mexico

Coral reefs are globally in decline and western Atlantic reefs have experienced the greatest losses in live coral cover of any region. The Flower Garden Banks (FGB) in the Gulf of Mexico are high-latitude, remote reefs that are an outlier to this trend, as they have maintained coral cover ≥ 50% since at least 1989. Quantifying the long-term trends in coral growth of key reef-building coral species, and the underlying environmental drivers, leads to a better understanding of local sensitivities to past changes that will ultimately allow us to better predict the future of reef growth at FGB. We obtained coral cores and constructed growth records for two of the most abundant hermatypic coral species at FGB, Pseudodiploria strigosa and Orbicella faveolata. Our records cover 57 yrs of growth for P. strigosa (1957–2013) and 45 yrs for O. faveolata (1970–2014). Linear extension and calcification rates of both species have increased significantly, but skeletal density did not change over the respective time periods. Extension and calcification data of both species combined were negatively correlated with the discharge from the Atchafalaya River, but positively correlated with maximum sea surface temperatures (SST). These data provide evidence that runoff from the Atchafalaya River impacts FGB corals and is a major control on coral growth at FGB. The increase in growth at FGB can be attributed to the significant warming trend in maximum monthly SSTs. Given the warming trend and recent increase in severity of bleaching at FGB, the prognosis is that bleaching events will become more deleterious with time, which will lead to a breakdown in the positive relationship between coral growth and maximum SST. This study provides further evidence that some high-latitude, cooler reef sites have experienced a stimulation in coral growth with ocean warming.

Long-term variability of Sea Surface Temperature in the Tropical Indian Ocean in relation to climate change and variability

Abstract The long-term change in the sea surface temperature (SST) of the Tropical Indian Ocean (TIO) and the underlying mechanisms are examined in detail by analysing multiple reanalysis datasets and historical simulations of a global climate model. A robust basin-wide warming signal was found in SST, with the highest long-term warming trend (~0.09 ± 0.015 °C decade−1) seen over the north-western and equatorial Indian Ocean regions. The robust warming trend of TIO SST is a dominant warming signal of the global tropical oceans and is largely in phase with similar warming in western Pacific and Atlantic Oceans. The central equatorial Indian Ocean and south TIO regions went through significant warming during recent decades. The weakened cross-equatorial flow and the associated cyclonic winds over the Arabian Sea region have resulted in equatorial westerlies with an enhanced loss of latent heat over the central and eastern equatorial Indian Ocean. The anomalous cyclonic wind stress curl, in turn, caused mixed layer stratifications in the western Indian Ocean. The advective heat transfer from the south-eastern Indian Ocean to the western Indian Ocean and reduced oceanic cooling by vertical processes, overcome the cooling by the net loss of surface heat fluxes, and favor surface warming the TIO.

What did determine the warming trend in the Indonesian sea?

The Indonesian sea is the only low-latitude pathway connecting two tropical oceans, which plays an important role in the coupled ocean-atmosphere mode in the Indo-Pacific sector. A small change in the sea surface temperature (SST) in the Indonesian sea has a significant influence on the precipitation and air-sea heat flux. During the past 33 years, the SST in the Indonesian sea has indicated a warming trend on the average of 0.19 ± 0.04 decade−1, which is larger than global SST warming trend. Moreover, the warming trend indicates seasonal variations, in which maximum trend occurred during boreal summer season. Investigation on the potential driver for this warming trend, namely, the net surface heat flux, resulted in an opposite trend (cooling trend), while the Ekman pumping and the wind mixing only play a minor role on the SST warming. Here, we proposed the upper layer process associated with an increasing trend in precipitation and decreasing trend in mixed layer depth (MLD) for the SST warming within the Indonesian seas. Shoaling of MLD gives a favorable condition for the surface heat flux to warm the surface ocean. However, shoaling of MLD could not solely explain the total SST warming within the Indonesian seas. The seasonal dependence in the warming trend, highest during boreal summer, was significantly related to the Indo-Pacific climate modes, namely the negative Indian Ocean Dipole (IOD) and La Niña events. Higher warming trend observed in the south of Makassar Strait and in the eastern Indonesian seas, in the vicinity of the Maluku Sea and the northeastern part of the Banda Sea, was significantly associated with the La Niña event. Meanwhile, strong warming trend observed in the Karimata Strait and Java Sea, and in the Flores Sea south of Sulawesi Island seems to be enhanced by the negative IOD event. Our rough quantitative estimate of the possible mechanism leading to the SST warming suggests that other mechanism might be at work in warming the SST within the Indonesian seas. Horizontal heat advection associated with an increasing trend of the heat transport from the Pacific into the Indian Ocean by the Indonesian Throughflow (ITF) might play a role in causing the warming trend within the Indonesian seas. However, to what extend this heat advection could modulate the SST warming is still unresolved in the present study. Further study based on realistic model output as well as long-term observational records is necessary to describe the dynamics underlying the warming trend within the Indonesian seas.

Seasonal Sea Surface Temperature Variability over Tropical Indo-Pacific Region Simulated by FIO-ESM Model Under Global Warming

The long-term trend and seasonal variability of sea surface temperature (SST) over tropical Indo-Pacific region under the global warming projection simulated by FIO-ESM model are analyzed. At seasonal time scale, significant warming trend over Indo-Pacific area is well captured by FIO-ESM model over the warm pool region under RCP8.5 scenario. The La Nina-like warming pattern is dominant in the tropical Pacific and the negative Indian Ocean Dipole warming pattern takes place in the Indian Ocean in global warming projection separately. The strength of SST warming trend is found to be seasonal dependent over Indo-Pacific region. The spatial distribution of calendar month where fastest/slowest SST warming trend take place in tropical Indo-Pacific has been assessed in FIO-ESM simulation, which is corresponded with the distribution of SST climatology closely in Pacific but not in Indian Ocean.

Long‐Term SST Variability on the Northwest Atlantic Continental Shelf and Slope

AbstractThe meridional coherence, connectivity, and regional inhomogeneity in long‐term sea surface temperature (SST) variability over the Northwest Atlantic continental shelf and slope from 1982–2018 are investigated using observational data sets. A meridionally concurrent large SST warming trend is identified as the dominant signal over the length of the continental shelf and slope between Cape Hatteras in North Carolina and Cape Chidley, Newfoundland and Labrador, Canada. The linear trends are 0.37 ± 0.06 and 0.39 ± 0.06 °C/decade for the shelf and slope regions, respectively. These meridionally averaged SST time series over the shelf and slope are consistent with each other and across multiple longer observational data sets with records dating back to 1900. The coherence between the long‐term meridionally averaged time series over the shelf and slope and basin‐wide averaged SST in the North Atlantic implies approximately two thirds of the warming trend during 1982–2018 may be attributed to natural climate variability and the rest to externally forced change including anthropogenic warming.

Tropical Indian Ocean warming contributions to China winter climate trends since 1960

This study investigates observed and modeled contributions of global sea surface temperature (SST) to China winter climate trends in 1960–2014, including increased precipitation, warming through about 1997, and cooling since then. Observations and Atmospheric Model Intercomparison Project (AMIP) simulations with prescribed historical SST and sea ice show that tropical Indian Ocean (TIO) warming and increasing rainfall causes diabatic heating that generates a tropospheric wave train with anticyclonic 500-hPa height anomaly centers in the TIO or equatorial western Pacific (TIWP) and northeastern Eurasia (EA) and a cyclonic anomaly over China, referred to as the TIWP-EA wave train. The cyclonic anomaly causes Indochina moisture convergence and southwesterly moist flow that enhances South China precipitation, while the northern anticyclone enhances cold surges, sometimes causing severe ice storms. AMIP simulations show a 1960–1997 China cooling trend by simulating increasing instead of decreasing Arctic 500-hPa heights that move the northern anticyclone into Siberia, but enlarge the cyclonic anomaly so it still simulates realistic China precipitation trend patterns. A separate idealized TIO SST warming simulation simulates the TIWP-EA feature more realistically with correct precipitation patterns and supports the TIWP-EA teleconnection as the primary mechanism for long-term increasing precipitation in South China since 1960. Coupled Model Intercomparison Project (CMIP) experiments simulate a reduced TIO SST warming trend and weak precipitation trends, so the TIWP-EA feature is absent and strong drying is simulated in South China for 1960–1997. These simulations highlight the need for accurately modeled SST to correctly attribute regional climate trends.

Interpreting the sea surface temperature warming trend in the Yellow Sea and East China Sea

Previous studies have demonstrated that the low-frequency sea surface temperature (SST) variability in the Yellow Sea and East China Sea (YECS) is linked to large-scale climate variability, but explanations on the mechanisms vary. This study examines the low-frequency variability and trends of some atmospheric and oceanic variables to discuss their different effects on the YECS warming. The increasing temperature trend is also observed at a hydrographic section transecting the Kuroshio. The increasing rate of ocean temperature decreases with depth, which might result in an increase in vertical stratification and a decrease in vertical mixing, and thus plays a positive role on the YECS warming. The surface net heat flux (downward positive) displays a decreasing trend, which is possibly a result of the YECS warming, and, in turn, inhibits it. Wind speeds show different trends in different datasets, such that its role in the YECS warming is uncertain. The trends in wind stress divergence and curl have large uncertainties, so their effects on SST warming are still unclear. The Kuroshio heat transport calculated in this study, displays no significantly increasing trend, so is an unlikely explanation for the SST warming. Limited by sparse ocean observations, sophisticated assimilative climate models are still needed to unravel the mechanisms behind the YECS warming.

Changes in tropical cyclone activity offset the ocean surface warming in northwest Pacific: 1981–2014

AbstractTropical cyclones (TCs) leave a cold wake in the sea surface temperature (SST). In the northwest Pacific, TC activity and SST have both increased since the 1980s, but the extent to which ocean surface warming is affected by the changing TC activity is unknown. Analysis of the 1981–2014 period indicates that the intensified effect of TC cold wakes has offset the SST warming trend by 37% during the typhoon season, implying that the observed SST warming might be underestimated. This factor could affect long‐term climate simulations that are forced with prescribed SST.

Climate variability and socio-environmental changes in the northern Aegean (NE Mediterranean) during the last 1500 years

We provide new evidence on sea surface temperature (SST) variations and paleoceanographic/paleoenvironmental changes over the past 1500 years for the north Aegean Sea (NE Mediterranean). The reconstructions are based on multiproxy analyses, obtained from the high resolution (decadal to multi-decadal) marine record M2 retrieved from the Athos basin. Reconstructed SSTs show an increase from ca. 850 to 950 AD and from ca. 1100 to 1300 AD. A cooling phase of almost 1.5 °C is observed from ca. 1600 AD to 1700 AD. This seems to have been the starting point of a continuous SST warming trend until the end of the reconstructed period, interrupted by two prominent cooling events at 1832 ± 15 AD and 1995 ± 1 AD. Application of an adaptive Kernel smoothing suggests that the current warming in the reconstructed SSTs of the north Aegean might be unprecedented in the context of the past 1500 years. Internal variability in atmospheric/oceanic circulations systems as well as external forcing as solar radiation and volcanic activity could have affected temperature variations in the north Aegean Sea over the past 1500 years. The marked temperature drop of approximately ∼2 °C at 1832 ± 15 yr AD could be related to the 1809 ΑD ‘unknown’ and the 1815 AD Tambora volcanic eruptions. Paleoenvironmental proxy-indices of the M2 record show enhanced riverine/continental inputs in the northern Aegean after ca. 1450 AD.The paleoclimatic evidence derived from the M2 record is combined with a socio-environmental study of the history of the north Aegean region. We show that the cultivation of temperature-sensitive crops, i.e. walnut, vine and olive, co-occurred with stable and warmer temperatures, while its end coincided with a significant episode of cooler temperatures. Periods of agricultural growth in Macedonia coincide with periods of warmer and more stable SSTs, but further exploration is required in order to identify the causal links behind the observed phenomena. The Black Death likely caused major changes in agricultural activity in the north Aegean region, as reflected in the pollen data from land sites of Macedonia and the M2 proxy-reconstructions. Finally, we conclude that the early modern peaks in mountain vegetation in the Rhodope and Macedonia highlands, visible also in the M2 record, were very likely climate-driven.

Comparison of long-term variability of Sea Surface Temperature in the Arabian Sea and Bay of Bengal

Long-term variability of Sea Surface Temperature (SST) in the Arabian Sea (AS) and Bay of Bengal (BoB) during 1880–2010 is examined using all the available data sets of monthly SST, wind speed, incoming solar radiation, cloud cover, long wave radiation, latent heat flux, net surface heat flux and surface currents. SST time series reveal an upward (increasing) trend (warming) after 1940 in both the basins. Wavelet analyses of the SST time series reveal that AS SST is controlled by semi-annual and annual forcing while the BoB SST is influenced predominantly by annual forcing. The SST responses in both the basins are in phase to the climatic events such as El Niño, La Niña and Indian Ocean Dipole, but a stronger impact is noticed in the AS SST. The climatic events affected the seasonal peaks of SST warming (April–May) and cooling (August–September) in both the basins. Time series of SST anomaly also showed in phase responses to the climatic events and an upward trend from 1960 to 2010 in both the basins. Long-term decreasing trends observed in surface wind speed, latent heat flux and advective process via the weakened surface currents, together with the long-term increasing trend in P − E (Precipitation, P minus Evaporation, E) contributed to the SST warming trend in both the basins. In association with the upward trend in P − E, the Simple Ocean Data Assimilation (SODA) Reanalysis Sea Surface Salinity time series exhibited freshening (a decreasing trend) and enhanced the salinity stratification in the BoB, thus modulated the BoB SST warming. These cumulative processes led to the warmer SSTs in the BoB compared to that in AS.

Indian Ocean and monsoon coupled interactions in a warming environment

Several studies have drawn attention to the steady warming of the equatorial and tropical Indian Ocean (IO) sea surface temperature (SST) observed during recent decades. An intriguing aspect of the IO SST warming trend is that it is has been accompanied by a pronounced weakening of the large-scale boreal summer monsoon circulation. Based on a detailed diagnostic analysis of observed datasets, reanalysis products and IPCC AR4 coupled model output, this study examines how the observed changes in the summer monsoon circulation could have contributed to this SST warming trend. The present results reveal that the weakening trend of the summer monsoon cross-equatorial flow has favored a reorientation of surface westerlies towards the equatorial IO during recent decades, relative to summer monsoons of earlier decades, which were dominated by stronger cross-equatorial flow. Our analysis suggests that the weakening of the summer monsoon cross-equatorial flow has in turn significantly accelerated the SST warming in the central equatorial IO. While the trend in the equatorial westerlies has promoted downwelling and thermocline deepening in the eastern equatorial IO, the central equatorial IO warming is attributed to reduced upwelling in response to a weakening trend of the wind-stress curl. The observed trends in Indian monsoon rainfall and the near-equatorial SST warming are shown to be closely related to variations in the meridional gradient of the monsoon zonal winds. An examination of the twentieth century simulations from 22 IPCC AR4 models, suggests that some models capture the recent equatorial IO warming associated with the weakened summer monsoon circulation reasonably well. The individual member models, however, exhibit significant inter-model variations in representing the observed response of the IO and monsoon coupled system.

Remote Influence of the Tropical Atlantic on the Variability and Trend in North West Australia Summer Rainfall

Abstract Rainfall in North West Australia (NWA) has been increasing over the past decades, occurring mainly in the austral summer season (December–March). A range of factors such as decreased land albedo in Australia and increasing anthropogenic aerosols in the Northern Hemisphere, identified using simulations from climate models, have been implicated in this wetting trend. However, the impact of land albedo and aerosols on Australian rainfall remains unclear. In addition, previous studies showed that dominant sea surface temperature (SST) signals in the Pacific–Indian Ocean including El Niño–Southern Oscillation (ENSO), ENSO Modoki, and the Indian Ocean dipole mode have no significant impact on the NWA rainfall trend. The present study proposes another viewpoint on the remote influence of tropical Atlantic atmospheric vertical motion on the observed rainfall variability and trend in NWA. It is found that, with the atmospheric ascent instigated by the warming of SST over the tropical Atlantic, a Rossby wave train is emanating southeastward from off the west coast of subtropical South America to the midlatitudes of the South Atlantic Ocean. It then travels eastward embedded in the westerly jet waveguide over the South Atlantic and South Indian Oceans. The eastward-propagated Rossby wave induces an anticyclonic anomaly in the upper troposphere over Australia, which is at the exit of the westerly jet waveguide. This leads to an in situ upper-tropospheric divergence, ascending motion and a lower-tropospheric convergence, and the associated increase in rainfall in NWA. Thus, the increasing trend in atmospheric upward motion induced by the warming trend of SST in the tropical Atlantic may partially explain the observed rainfall trend in NWA.