Variability of surface layer hydrography in the tropical Pacific Ocean

Subtropical South Indian Ocean salinity maxima region plays an important role in transporting temperature anomalies towards north along the isopycnals following geostrophic pathways. In this study, interannual and decadal changes in temperature and salinity at the base of mixed layer during austral winters are investigated for the Argo era. Winter time deep mixed layer allows these Temperature/Salinity (T/S) changes to penetrate to the permanent pycnocline. Interannual changes in the mixed layer depth (MLD) are mostly driven by convective buoyancy and wind forcing. Contribution of different atmospheric and oceanic forcing to the changes in mixed layer temperature and salinity are shown using mixed layer budget calculation. It is observed that net heat flux term dominates the temperature changes whereas meridional advection plays a important role in driving salinity changes in the mixed layer. Mixed layer T/S changes are subducted to the permanent pycnocline mainly by lateral induction process because of large meridional MLD gradient. Density compensated anomalies also contribute to the T/S changes at the bottom of the mixed layer. Interannual temperature anomalies due to spiciness and heaving are further explored.

Understanding decadal changes in monsoon rainfall is essential for mitigation policies in the next few decades, but comparatively less is known about the Southern Hemisphere monsoon. We find that decadal variations of the Southern Hemisphere land monsoon (SHLM) have a zonal dipole pattern with a significant periodicity of 10–15 years. This spatial pattern is characterized by wet southern African (SAF) monsoon and Australian monsoon (AUM) but a dry southeastern South American monsoon (SESAM). Moisture budget analysis shows that changes in monsoon circulation explain over 70% of the decadal variation. Tropical dynamics dominate the monsoon circulation change, and extratropical Rossby waves also have a contribution. The wet SAF and AUM but dry SESAM pattern is mainly driven by a tropical sea surface temperature (SST) gradient, specifically warm SST over the western Pacific and cold SST over the tropical Indian, central Pacific, and eastern Pacific Oceans. This gradient enhances the Walker circulation and subtropical highs over the South Indian and South Pacific Oceans, increasing moisture convergence over the SAF and AUM. The resultant Walker circulation change decreases ascending motion over the SESAM. The enhancement of AUM rainfall triggers the Pacific–South American pattern of the Rossby wave train, which propagates eastward along the SH subpolar jet, generating descending motion over the SESAM. The tropical and extratropical mechanisms are verified in a coupled climate model with nudged SST and a simplified model with artificial atmospheric heating imposed over eastern Australia, respectively. Our findings help unify the understanding of decadal variations of the regional monsoons and may add confidence for decadal prediction of SHLM changes in the following decades.

<p>Sea surface temperatures in the eastern equatorial Pacific exert powerful influence on the climate beyond the tropics through strong atmosphere-ocean coupling. Records of eastern Pacific sea surface temperatures are of vital importance for identifying the linkages between short-term climate variability and long-term climate trends. Here we reconstruct eastern equatorial Pacific sea surface temperature and salinity from paired trace metal and stable isotope analyses in foraminifera from a sediment core near the Galápagos Islands. Sea surface temperatures are correlated with reconstructed Northern and Southern hemisphere temperature records suggesting a common origin. We propose that this temperature signal originates in the extra-tropics and is transmitted to the eastern Pacific surface via its source waters. We find exceptions to this cooling during the Little Ice Age and during the last century, where notable sea surface temperature increases are observed. We calculate δ<sup>18</sup>O<sub>sw </sub>from paired stable isotope and trace element analyses and derive salinity, which reveals a significant trend toward fresher surface waters in the eastern equatorial Pacific. The overall trend toward cooler and fresher sea surface conditions is consistent with longer-term trends from both the Eastern and Western Pacific.</p>

The effect of errors and biases in the atmospheric forcing for oceanic mixed layer model predictions is studied using data sensitivity techniques. First the bulk model of Garwood is used to predict 17 years of mixed layer evolution and temperature structure at Ocean Station Papa using forcing derived from the 3 h atmospheric observations. The model is then integrated again varying, one at a time, each atmospheric forcing variable by a Gaussian error whose spread is proportional to the standard deviations of that variable during late winter or midsummer. The results of those integrations are then compared with the control run to assess the effects of the added random errors or biases. A positive or negative bias in the atmospheric forcing is much more detrimental to the ocean prediction than is a random error with zero mean. The predicted mixed layer depths are more sensitive to errors introduced in the forcing in winter than in summer. Conversely, the mixed layer temperature is more sensitive to ...

Formation and evolution of barrier layers (BLs) and associated temperature inversions (TIs) were investigated using a 1-yr time series of oceanic and air–sea surface observations from three moorings deployed in the eastern Pacific fresh pool. BL thickness and TI amplitude showed a seasonality with maxima in boreal summer and autumn when BLs were persistently present. Mixed layer salinity (MLS) and mixed layer temperature (MLT) budgets were constructed to investigate the formation mechanism of BLs and TIs. The MLS budget showed that BLs were initially formed in response to horizontal advection of freshwater in boreal summer and then primarily maintained by precipitation. The MLT budget revealed that penetration of shortwave radiation through the mixed layer base is the dominant contributor to TI formation through subsurface warming. Geostrophic advection is a secondary contributor to TI formation through surface cooling. When the BL exists, the cooling effect from entrainment and the warming effect from detrainment are both significantly reduced. In addition, when the BL is associated with the presence of a TI, entrainment works to warm the mixed layer. The presence of BLs makes the shallower mixed layer more sensitive to surface heat and freshwater fluxes, acting to enhance the formation of TIs that increase the subsurface warming via shortwave penetration.

Moored wind measurements at near-equatorial locations along 110°W, 125°W, 140°W, 170°W, and 165°E are used to investigate the space-time variability of the tropical Pacific wind field. These measurements complement previous studies that relied on island winds in the central Pacific or a few moored measurements in the eastern Pacific. Results indicate that the energetic portion of the zonal and meridional wind is significantly coherent over meridional scales of about 200 km and zonal scales of 1500 km. Even at these separations the estimated coherence often accounts for less than 50% of the variance. Temporal subsampling indicated (in agreement with previous studies) that at least ten samples per month were required to resolve monthly wind speed to within 1 m s−1 in the eastern equatorial Pacific. West of the date line and in the intertropical convergence zone (ITCZ), nearly daily sampling was required. Investigation showed that little error in the daily average of derived quantities such as wind speed and stress was associated with computing these variables from daily vector averages of the wind components rather than from hourly values of the components that were subsequently averaged.

A global ocean circulation model is used to examine the dynamical response of the tropical Pacific Ocean to forcing associated with the Madden‐Julian Oscillation (MJO). Model sensitivity experiments first reveal that MJO‐related changes in sea level are caused primarily by changes in wind stress. Further, the MJO‐related changes in sea surface temperature (SST) are mainly caused by buoyancy (heat) flux in the Indian Ocean, by wind stress in the central tropical Pacific, and by both buoyancy flux and wind stress in the eastern tropical Pacific. Additional model sensitivity studies quantify the tropical Pacific Ocean response to MJO wind forcing. The simulations reveal that the subsurface temperature variations associated with the MJO propagate eastward along the thermocline and rise to the surface in the eastern Pacific. Zonal advection plays a dominant role in SST variation in the central Pacific; vertical advection plays an important role in the evolution of subsurface and surface temperatures in the eastern Pacific. Finally, it is shown that MJO wind forcing can rectify SST variations through nonlinear interactions of the intraseasonal variations of the zonal currents and the zonal SST gradient.

The oxygen minimum zone (OMZ) in the eastern tropical South Pacific Ocean is tightly connected to the coastal upwelling system off Peru. The high biological productivity off Peru is therefore, driven by the complex interplay between the amount of nutrients recycled by remineralisation processes in the OMZ and the upwelling which brings these nutrients to the surface layer. However, surprisingly little is known about organic matter cycling and its effects on biogeochemical processes in the OMZ off Peru. To this end we conducted a first comprehensive study on the role of organic matter for the biogeochemical processes and the maintenance of the OMZ off Peru. M138 combined measurements of marine biogeochemistry, microbiology, physical oceanography and air chemistry with foci on (i) the efficiency of the biological pump, (ii) the nitrogen cycle processes in the OMZ, (iii) the ventilation of the OMZ as well as (iv) the air/sea gas exchange across the ocean/atmosphere interface and (v) aerosol deposition. The METEOR cruise M138 was performed as part of the third phase of the SFB754 'Climate-Biogeochemistry Interactions in the Tropical Ocean' (www.sfb754.de).

<p>Based on SODA reanalysis data set from 1980 to 2016, this paper combined with a variety of mathematical statistical methods to study the intraseasonal variability characteristics of barrier layer thickness and its physical correlation with climate modes in the Bay of Bengal, and quantitatively explored the dynamic mechanism of intraseasonal variability of barrier layer in different sea areas in the Bay of Bengal by means of Marine dynamic diagnosis method. The relative contributions of different physical processes, such as oceanic advection, Kelvin waves, Rossby waves and freshwater fluxes (rainfall and river runoff), to the barrier layer were evaluated. The physical relationship between the seasonal variation of barrier layer thickness and the Indian Ocean dipole (IOD) is also discussed. The results show that the thickness of the barrier layer varies most obviously in the northern coast of the bay of Bengal and the western coast of Sumatra, and the maximum value of the barrier layer occurs in November ~ December every year, while the variation of the barrier layer in the northern coast is more regular than that in the southern coast. Horizontal advance and entrainment affect the thickness of barrier layer by affecting the salinity of the mixed layer. However, the thickness of barrier layer is mainly caused by the change of isothermal layer due to the obvious stratification of sea surface salinity in the Bay of Bengal. In the southern part of the Bay of Bengal near the equator, during the positive IOD events, the isothermal layer shallowness was caused by the negative anomaly of equatorial zonal wind stress from October to December. In negative IOD events, the equatorial zonal wind stress appears positive abnormality after June, which leads to the increase of isothermal layer in this period. As a result, the thickness of barrier layer In positive IOD years is smaller than that in normal years from October to December, and that in negative IOD years is greater than that in normal years from June to September. However, in the northern Bay of Bengal, the seasonal variability of barrier layer caused by different IOD events was not obvious. At the same time, the net heat flux upward at the air-sea interface will lead to instability and deepen the local mixed layer.</p>

We investigated whether the maximum intensity of tropical cyclones (TC) in the North Pacific Ocean depends on sea surface temperature (SST) and tropical cyclone heat potential (TCHP). The study used reanalysis data sets for both the oceans and atmosphere: daily, 10‐day, and monthly oceanic data sets; six‐hour and monthly atmospheric data sets; and a daily satellite SST data set, for the July‐to‐October season from 2002 to 2005. For each TC, we summed TCHP from the time of genesis to the time of first reaching a minimum central pressure (MCP), to obtain an accumulated TCHP. In a linear regression analysis, the relationship between maximum TC intensity and accumulated TCHP differed between the eastern and western Pacific: high values of accumulated TCHP were needed before a TC attained a certain MCP in the western Pacific. In addition, the background convective available potential energy (CAPE) value was nearly four times larger in the western Pacific than in the eastern Pacific. The static stability was also 6.5% lower, the inertial stability 29.7% higher, and the size of tropical cyclones 38.2% larger in the western Pacific than in the eastern Pacific. The result indicated a deeper Rossby penetration depth and stronger TC in the western Pacific. Finally, we validated the TCHP values derived from three oceanic reanalysis data sets by using Argo profiling float observations. We found that use of only the daily data can reproduce the cooling effect of a passage of a TC, which caused a decrease in the TCHP values.

In this paper the amplitude-phase characteristics of the annual adjustment of the thermocline in the entire tropical Pacific Ocean are described and numerical experiments with a tropical ocean model are performed to assess the roles of the major wind systems in controlling the annual variation of the thermocline. In the region between about 8°N and 10°S, the annual adjustment of the thermocline is controlled by both the Ekman pumping and equatorial wave propagation. The local wind stress forcing plays a dominant role in the central North Pacific (3°–8°N, 170°–120°W) where the thermocline exhibits the largest amplitude due to the prominent annual variation of the wind stress curl south of the ITCZ. In the equatorial central Pacific (2°N–5°S, 170°–120°W), the annual cycle also exhibits a pronounced unimodal seasonal variation (deepest in December and shallowest in May–June). This distinctive annual cycle results primarily from the adjustment of the waves, which are excited around 4°N, 110°W by the annual march of the ITCZ and excited in the equatorial western Pacific by the monsoon flows. The December maximum and May–June minimum then propagate westward in the off-equatorial waveguides along 5°N (3°–7°N) and 6°S (3°–9°S) to the western boundary. These annual Rossby waves are reflected at the western ocean boundary. The bimodal annual variation in the equatorial western Pacific is caused by the combined effects of the annual Rossby wave reflection and the monsoon westerly forcing during transitional seasons. The bimodal variations in the equatorial far eastern Pacific are determined by the remote forcing through the eastward propagation of Kelvin waves. The thermocline variations in the North Pacific poleward of 8°N and in the South Pacific poleward of 10°S form approximately an annual seesaw oscillation with maximum depth occurring in May–June (October–November) and minimum in December (April–May) in the North (South) Pacific. These regions are characterized by an Ekman regime.

<sec><p indent="0mm">The impacts of anthropogenic climate change may be felt through changes in modes of natural climatic variability. The El Niño-Southern Oscillation (ENSO) is the most important year-to-year fluctuation of the climate system, varying between anomalously cold (La Niña) and warm (El Niño) conditions. Underpinning occurrences of ENSO events is the positive feedback between trade wind intensity and zonal contrasts in sea surface temperature (SST), referred to as the Bjerknes feedback. The trade winds normally pile up warm surface water in the western Pacific while upwelling colder subsurface water in the east along the equator and off the west coast of South America. The resulting east-west surface temperature contrast reinforces an east-west air pressure difference across the basin that in turn drives the trade winds. During La Niña, the system strengthens; but during El Niño, the trade winds weaken as atmospheric pressure rises in the western Pacific and falls in the eastern Pacific. The Bjerknes feedback now operates in reverse, with weakened trade winds and SST warming tendencies along the Equator reinforcing one another. These swings in temperature are accompanied by changes in the structure of the subsurface ocean, the position of atmospheric convection and associated global teleconnection patterns. </sec><sec> ENSO is spawned in the tropical Pacific, but its reach is global. It alters the general circulation of the atmosphere from one season to the next through far-field teleconnections, leading to droughts, floods, heat waves, and other extreme weather events across our planet. These natural hazards have widespread impacts on human and natural systems, including agriculture, public health, power generation and consumption, financial markets, transportation, tourism, freshwater resources, civil conflict, wildfires, fisheries, marine and terrestrial ecosystems, and biodiversity. The character of ENSO depends on the long-term average background climatic conditions on which it develops. However, because of human activities since the start of the Industrial Revolution in the mid-18th century, these background conditions have been changing at a pace that has greatly accelerated in recent decades. We have come to appreciate just how profound these changes are, thanks to five Intergovernmental Panel on Climate Change assessments since 1990. </sec><sec> In this review, we provide an update from the community since 2021, on the progress of changing ENSO and its impacts under greenhouse warming. Latest climate models suggest that a warming pattern featuring faster warming in the eastern Pacific than in the surrounding regions is possible as greenhouse warming progressively overwhelm other factors including decadal climate variability; that ENSO variability post-1960s has already increased from that pre-1960 characterized by an increase in strong El Niño, strong La Niña, and multiyear La Niña events, consistent with the projected increase pre-2100; and that the long-term change in ENSO SST variability is likely to be detected by 2030 in the eastern equatorial Pacific. Further, the changing mean state and increase in ENSO variability have intensified two-way interactions between the tropical and extratropical North Pacific, increased variability of the Western Pacific Subtropical High, increased SST variability over the north tropical Atlantic, hastened warming over the Antarctic shelf ocean, and exacerbated the economic cost of global warming. Under persistent greenhouse warming into 2300, the increased ENSO pre-2100 reverses to a subsequent reduction as the faster warming in the equatorial eastern Pacific develops into a permanent El Niño-like condition, highlighting a strong nonlinear ENSO response. </sec>

The barrier layer (BL) — a salinity stratification embedded in the upper warm layer — is a common feature of the tropical oceans. In the northern Indian Ocean, it has the potential to significantly alter the air–sea interactions. In the present paper, we investigate the spatio-temporal structure of BL in the Arabian Sea during summer monsoon. This season is indeed a key component of the Asian climate. Based on a comprehensive dataset of Conductivity–Temperature–Depth (CTD) and Argo in situ hydrographic profiles, we find that a BL exists in the central Arabian Sea during summer. However, it is highly heterogeneous in space, and intermittent, with scales of about ∼100 km or less and a couple of weeks. The BL patterns appear to be closely associated to the salinity front separating two water masses (Arabian Sea High Salinity Water in the Northern and Eastern part of the basin, fresher Bay of Bengal Water to the south and to the west). An ocean general circulation model is used to infer the formation mechanism of the BL. It appears that thick (more than 40 m) BL patterns are formed at the salinity front by subduction of the saltier water mass under the fresher one in an area of relatively uniform temperature. Those thick BL events, with variable position and timing, result in a broader envelope of thinner BL in climatological conditions. However, the individual patterns of BL are probably too much short-lived to significantly affect the monsoonal air–sea interactions.

Previous studies have hypothesized that climatologically thick salinity-stratified Barrier Layers (BL) in the North Indian Ocean (NIO) could influence the upper ocean heat budget, sea surface temperature (SST) and monsoon. Here, we investigate the performance of state-of-the-art climate models from the Coupled Model Intercomparison Project phase 6 (CMIP6) in simulating the barrier layer thickness (BLT) in the NIO. CMIP6 models generally reproduce the main features of the BLT seasonal cycle and spatial distribution, but with a shallow November-February (NDJF) BLT bias in regions with thick observed BLT (eastern equatorial Indian Ocean [EEIO], Bay of Bengal [BoB] and southeastern Arabian Sea [SEAS]). CMIP6 models display an easterly equatorial zonal surface wind bias linked to dry rainfall and cold SST biases in the southern BoB, through the Bjerknes feedback loop. The easterly equatorial bias is also responsible for the shallow isothermal layer depth (ILD) and BLT bias in the EEIO. The underestimated rainfall over the BoB leads to higher sea surface salinity (SSS) and too deep mixed layer depth (MLD), resulting in the BoB BLT bias. The intensity of the easterly equatorial bias also contributes to the inter-model spread in BoB BLT bias, through the propagation of EEIO ILD signals into the coastal waveguide. Finally, the SEAS BLT bias is due to a too deep MLD, which is predominantly controlled by the high SSS related to attenuated monsoonal currents around India and a reduced inflow of BoB low-salinity water. The BL effect on the mixed layer entrainment cooling does not seem to operate in CMIP6 simulations. Rather, deep salinity-related MLD biases in the BoB result in a diminished cooling rate in response to winter negative surface heat fluxes, and hence alleviate cold BoB SST biases. This suggests that salinity effects alleviate the biases that develop through the positive Bejrknes feedback loop between BoB SST, BoB rainfall and equatorial wind stresses in CMIP6.

The stability of the barrier layers of bilayer passive films that form on metal and alloy surfaces, when in contact with oxidizing aqueous environments, is explored within the framework of the Point Defect Model (PDM) using phase-space analysis (PSA), in which the rate of growth of the barrier layer into the metal, (dL{sup +}/dt), and the barrier layer dissolution rate, (dL{sup -}/dt), are plotted simultaneously against the barrier layer thickness. A point of intersection of dL{sup -}/dt with dL{sup +}/dt indicates the existence of a metastable barrier layer with a steady state thickness greater than zero. If dL{sup -}/dt > (dL{sup +}/dt){sub L=0}, where the latter quantity is the barrier layer growth rate at zero barrier layer thickness, the barrier layer cannot exist, even as a metastable phase, as the resulting thickness would be negative. Under these conditions, the surface is depassivated and the metal may corrode at a rapid rate. Depassivation may result from a change in the oxidation state of the cation upon dissolution of the barrier layer, such that the dissolution rate becomes highly potential dependent (as in the case of transpassive dissolution of chromium-containing alloys, for example, in which the reaction Cr{sub 2}O{sub 3} + 5H{sub 2}O {yields} 2CrO{sub 4}{sup 2-} + 10H {sup +} + 6e{sup -} results in the destruction of the film), or by the action of some solution-phase species (e.g., H{sup +}, Cl{sup -}) that enhances the dissolution rate to the extent that dL{sup -}/dt > (dL{sup +}/dt){sub L=0}. The boundaries for depassivation may be plotted in potential-pH space to develop Kinetic Stability Diagrams (KSDs) as alternatives to the classical Pourbaix diagrams for describing the conditions under which metals or alloys exist in contact with an aqueous environment. The advantage of KSDs is that they provide kinetic descriptions of the state of a metal or alloy that is in much closer concert with the kinetic phenomenon of passivity and depassivation than are equilibrium thermodynamic diagrams. Thus, KSDs more accurately account for the limits of passivity in highly acidic systems, where acid depassivation occurs, and at high potentials, where transition to the transpassive state may occur in some systems. In any event, phase space analysis of the PDM permits specification of the conditions over which reactive metals will remain passive in contact with aqueous systems and hence of the conditions that must be met for the existence of our metals-based civilization.