Meridional Heat Advection Research Articles

Potential impact of wintertime Arctic forcing on the subsequent sea surface temperature anomalies in the tropical eastern Pacific

Despite extratropical forcing being recognized as an important factor that can modulate El Niño-Southern oscillation (ENSO) properties on the interannual time scale, little is known about whether and how Arctic forcing changes the tropical sea surface temperature (SST). This current study reveals a significant link between the net surface sensible heat flux (SHF) in the Arctic and the SST anomalies in the tropical eastern Pacific (TEP). Specifically, anomalous upward SHF into the Arctic atmosphere in February leads to a warmer TEP in the subsequent spring and summer. A northeast-southwest-tilted North Pacific Oscillation-like atmospheric pattern associated with the upward Arctic SHF anomaly induces SST cooling in the subtropical North Pacific via positive Wind-Evaporation-SST feedback, which further promotes TEP SST warming via meridional heat advection, thermocline feedback, and nonlinear processes. The spring-to-summer TEP SST anomalies driven by the preceding anomalous Arctic SHF can potentially modulate the seasonal evolution of ENSO. Our findings imply that we should take into account the Arctic-tropics linkages when comprehensively understanding the ENSO variability and improving ENSO projection skills.

Arctic glaciers record wavier circumpolar winds

Glaciers in the Arctic respond sensitively to climate change, recording the polar amplification of global warming with increasing mass loss. Here, we use glacier mass balances in Svalbard and northern Arctic Canada to categorize tropospheric variability and the associated summer circulation over the Arctic. We establish a link between annual glacier mass balances and their respective atmospheric forcings since 1950 using GRACE/GRACE-FO satellite data (2002–2021), as well as regional climate models and reanalysis data (1950–2019). We find that asynchronous behaviour of mass balance between the regions has become very likely since the early 2000s, exceeding the range of previous decadal variability. Related tropospheric circulation exhibits more meridional patterns, a greater influence of meridional heat advection and a wavier summer circulation. The traceable impact on glacier mass balances emphasizes the importance of dynamic next to thermodynamic climate changes for the future of glacier mass loss, Arctic ecology and societal impacts.

Comparison of ocean heat content estimated using two eddy-resolving hindcast simulations based on OFES1 and OFES2

Abstract. In this study, we have compared the ocean heat content (OHC), estimated using two eddy-resolving hindcast simulations based on Ocean General Circulation Model for the Earth Simulator version 1 (OFES1) and version 2 (OFES2). Results from a global objective analysis of subsurface temperature (EN4) were taken as a reference. Both EN4 and OFES1 suggest that OHC has increased in most regions of the top 2000 m during 1960–2016, which is mainly associated with the deepening of neutral density surfaces and variations along the neutral density surfaces of regional importance. Upon comparing the results obtained from the two OFES hindcasts, we found substantial differences in the temporal and spatial distributions of the OHC, especially in the Atlantic Ocean. A basin-wide heat budget analysis showed that there was less surface heating for the major basins in OFES2. The horizontal heat advection was mostly similar; however, OFES2 had a significantly stronger meridional heat advection associated with the Indonesian Throughflow (ITF) above 300 m. Additionally, large discrepancies in the vertical heat advection were also evinced when the two OFES results were compared, especially at a depth of 300 m in the Indian Ocean. We inferred that there are large discrepancies in the vertical heat diffusion (those that cannot be directly evaluated in this study due to data unavailability), which, along with the different magnitudes of sea surface heat flux and vertical heat advection, were the major factors responsible for the examined differences in OHC. This work suggests that OFES1 provides a reasonable multi-decadal estimate of global and basin-integrated warming trends above 700 m, except for the top 300 m for the Pacific Ocean and between 300–700 m for the Indian Ocean. Although the estimates of the global OHC during 1960–2016 are consistent with observations between 700–2000 m, caution is warranted while examining the basin-wide multi-decadal OHC variations using OFES1. The seemingly suboptimal OHC estimate based on OFES2 suggests that any conclusions on long-term climate variations derived from OFES2 might suffer from large drifts, necessitating audits.

The 2019 Benguela Niño

High interannual sea surface temperature anomalies of more than 2°C were recorded along the coasts of Angola and Namibia between October 2019 and January 2020. This extreme coastal warm event that has been classified as a Benguela Niño, reached its peak amplitude in November 2019 in the Angola Benguela front region. In contrast to classical Benguela Niños, the 2019 Benguela Niño was generated by a combination of local and remote forcing. In September 2019, a local warming was triggered by positive anomalies of near coastal wind-stress curl leading to downwelling anomalies through Ekman dynamics off Southern Angola and by anomalously weak winds reducing the latent heat loss by the ocean south of 15°S. In addition, downwelling coastal trapped waves were observed along the African coast between mid-October 2019 and early January 2020. Those coastal trapped waves might have partly emanated from the equatorial Atlantic as westerly wind anomalies were observed in the central and eastern equatorial Atlantic between end of September to early December 2019. Additional forcing for the downwelling coastal trapped waves likely resulted from an observed weakening of the prevailing coastal southerly winds along the Angolan coast north of 15°S between October 2019 and mid-February 2020. During the peak of the event, latent heat flux damped the sea surface temperature anomalies mostly in the Angola Benguela front region. In the eastern equatorial Atlantic, relaxation of cross-equatorial southerly winds might have contributed to the equatorial warming in November 2019 during the peak of the 2019 Benguela Niño. Moreover, for the first time, moored velocities off Angola (11°S) revealed a coherent poleward flow in the upper 100 m in October and November 2019 suggesting a contribution of meridional heat advection to the near-surface warming during the early stages of the Benguela Niño. During the Benguela Niño, a reduction of net primary production in the Southern Angola and Angola Benguela front regions was observed.

The thermodynamic and dynamic control of the sensible heat polynya in the western Cosmonaut Sea

A prominent feature in the west Cosmonaut Sea is the reoccurrence of a sensible heat polynya — the western Cosmonaut Sea Polynya (wCSP), which normally exists in late austral autumn and early winter. The Southern Ocean Sate Estimate (SOSE) reanalysis product is employed to investigate the thermodynamic and dynamic processes controlling the formation and evolution of a real wCSP event in 2009, when the polynya properties revealed from satellite observations are reasonably simulated by SOSE. An oceanic heat budget analysis was conducted for the surface layer above the thermocline, and the results reveal that heat advection is the major term contributing to the surface heat content variation in this area. The precursor of wCSP — an embayment — was formed during cyclonic atmospheric circulations. The negative wind stress curl from a cyclone induced upwelling that brought the warm circumpolar deep water to the surface and presumably melted ice. Meanwhile, the location of the cyclone relative to the wCSP created weaker easterlies over the northern boundary of polynya and stronger easterlies over the southern boundary of polynya. This leads to difference in the meridional oceanic heat transports across the northern and southern boundaries, resulting in net positive meridional oceanic heat transport into the polynya. Both the vertical heat advection and the meridional heat advection contribute significantly to increased heat content in the surface layer of wCSP, causing sea-ice melt and formation of the embayment. The cyclonic wind field promotes convergence of sea ice over the northern area of the embayment and results in the polynya. The formation of wCSP is likely related to the strength and position of Southern Annular Mode on interannual scale, and may potentially affect the biological productivity by controlling the availability of light in the mixed layer.

The Importance of Central Pacific Meridional Heat Advection to the Development of ENSO

AbstractThe relationship between the Warm Water Volume (WWV) ENSO precursor and ENSO SST weakened substantially after ~2000, coinciding with a degradation in dynamical model ENSO prediction skill. It is important to understand the drivers of the equatorial thermocline temperature variations and their linkage to ENSO onsets. In this study, a set of ocean reanalyses is employed to assess factors responsible for the variation of the equatorial Pacific Ocean thermocline during 1982-2019. Off-equatorial thermocline temperature anomalies carried equatorward by the mean meridional currents associated with Pacific Tropical Cells are shown to play an important role in modulating the central equatorial thermocline variations, which is rarely discussed in the literature. Further, ENSO events are delineated into two groups based on precursor mechanisms: the western equatorial type (WEP) ENSO, when the central equatorial thermocline is mainly influenced by the zonal propagation of anomalies from the western Pacific, and the off-equatorial central Pacific (OCP) ENSO, when off-equatorial central thermocline anomalies play the primary role. WWV is found to precede all WEP ENSO by 6-9 months, while the correlation is substantially lower for OCP ENSO events. In contrast, the central tropical Pacific (CTP) precursor, which includes off-equatorial thermocline signals, has a very robust lead correlation with the OCP ENSO. Most OCP ENSO events are found to follow the same ENSO conditions, and the number of OCP ENSO increases substantially since the 21st century. These results highlight the importance of monitoring off-equatorial subsurface preconditions for ENSO prediction and to understand multi-year ENSO.

CMIP5 model biases in the climatological mean state of the western Pacific warm pool

Biases in coupled general circulation models represent important limits for climate prediction. Based on historical runs of 26 coupled climate models of the Fifth Phase of the Coupled Model Intercomparison Project, the error of the climatological mean of the western Pacific warm pool (WPWP) is investigated. The results show that simulation of the morphology of the WPWP is significantly influenced by ocean currents. The upper-ocean heat budget analysis also indicates that heat advection plays a key role in determining the shape of the WPWP during the simulation. For the shrinkage of the tropical region and south section of smaller WPWPs, both the zonal heat advection bias caused by zonal ocean currents bias and meridional heat advection bias caused by the meridional sea temperature gradient bias are the crucial factors, while for the extension of larger WPWPs, the zonal heat advection bias is more important. For the northern section of WPWPs, the horizontal heat advection biases are still responsible for the shrinkage and extension in the region from 4°N to 7°N, while in the north of 7°N, the biases of WPWP are related to that of short wave radiation. In addition, in the equator and south section of the WPWP, the advective feedback plays a key role in the development of biases.

Meridional Heat Advection due to Mixed Rossby Gravity Waves in the Equatorial Indian Ocean

Abstract This study examines heat advection due to mixed Rossby gravity waves in the equatorial Indian Ocean using moored buoy observations at (0°, 80.5°E) and (0°, 90°E) and an ocean general circulation model (OGCM) output. Variability associated with mixed Rossby gravity waves is defined as that at periods of 10–30 days, where both observations and the OGCM results show high energy in meridional velocity and meridional gradient of temperature. The 10–30-day variability in meridional velocity causes convergence of heat flux onto the equator, the net effect of which amounts to 2.5°C month−1 warming at the depth of the thermocline. Detailed analysis shows that the wave structure manifested in temperature and velocity is tilted in the x–z plane, which causes the phase lag between meridional velocity and meridional temperature gradient to be a half cycle on the equator and results in sizable thermocline warming. An experiment with a linear continuously stratified model shows that the contributions of many baroclinic modes, and the right zonal wavelength of wind forcing, are essential in generating the correct wave structure. It is also shown that contributions of mixed Rossby gravity waves to cross-equatorial heat transport are negligible, as temperature variability associated with this wave mode has a node on the equator.

Eastern Indian Ocean warming associated with the negative Indian Ocean dipole: A case study of the 2010 event

Warm sea surface temperature (SST) anomalies of more than 1 °C occurred in the southeastern tropical Indian Ocean and peaked during August to October 2010. The anomalous SST warming was associated with the negative phase of the Indian Ocean dipole (IOD) phenomenon. In this study, observational data from a moored buoy were used together with satellite and atmospheric reanalysis data sets to clarify the processes that produced the anomalously warm SST in 2010. We focused on the location 5°S, 95°E where in situ measurements of more than 10 years by a moored buoy were available. The buoy observations captured the oceanic conditions related to the anomalous warming event of 2010. Heat balance analysis demonstrated that air‐sea heat fluxes and horizontal heat advections mainly account for the mixed layer temperature variation. Reduced latent heat loss had a major role in producing the warm SST anomalies. Meridional heat advection also contributed to the warm SST anomalies where the southeastward surface current brought warmer water to the southeastern tropical Indian Ocean. The present results from the observations suggest that air‐sea heat exchanges play an active role in the SST anomalies in the southeastern tropical Indian Ocean during the negative IOD. In contrast, in the case of cold SST anomalies in the eastern Indian Ocean during the positive IOD, ocean heat advections mainly control the mixed layer temperature variation. These results suggest that the IOD includes different feedback mechanisms in its positive and negative phases.

Eddy-advective effects on the temperature and wind speed of the sea surface in the Northwest Pacific Subtropical Countercurrent area from satellite observations

In this study, satellite microwave and altimeter data from 1998 to 2007 are used to quantify the eddy-induced meridional heat advection (EMHA) in the Northwest Pacific Subtropical Countercurrent area. Generally, from March to May, the robust EMHA is formed at the point where meridional currents of eddies cross a zonal front of climatological background sea surface temperature (SST). The EMHA shifts westwards with eddies and varies seasonally with the SST front. It warms (cools) the sea surface west of anticyclonic (cyclonic) eddies, inducing noticeable SST anomalies (SSTAs), which are westwardly phase shifted from the eddy-induced sea surface height anomalies by about 90°. Surface wind subsequently varies with the induced SSTAs: it blows faster (slower) over the warm (cold) SST regions than the surroundings. The spatial variations of SST and sea surface wind due to the EMHA shift westwards with eddy motion. These findings from satellite observations give us the possibility of studying the role of oceanic eddies in ocean–atmosphere interaction at the timescale of weather systems in an open ocean.

Interannual Variability of the Upper Ocean in the Southeast Pacific Stratus Cloud Region

Abstract Persistent stratus/stratocumulus cloud decks in the southeast Pacific near the coasts of Peru and northern Chile play an important role in regional and global climate variability. Interannual variability of the upper ocean under stratus cloud decks in the southeast Pacific is investigated using ocean general circulation model (OGCM) experiments. The model was first forced with daily surface fluxes based on the NCEP–NCAR reanalysis and satellite-derived surface shortwave and longwave radiation for the period of 1979–2004. Gridded surface heat flux estimates used in the model integration agree well with those based on Woods Hole Oceanographic Institution (WHOI) Improved Meteorology (IMET) buoy measurements at 20°S, 85°W. Also, the OGCM is able to reproduce well the observed interannual SST and sea surface height variations in this region. The results suggest that the interannual variation of the upper ocean north of 20°S is mostly associated with ENSO variability. Additional model experiments were conducted to examine the relative importance of ocean dynamics and surface heat fluxes in determining the interannual variation in SST. The results of these experiments indicate that upper-ocean dynamics play a dominant role in controlling the interannual variation of SST north of 20°S in the stratus cloud region. The upper-ocean heat budget analysis shows that meridional heat advection associated with ENSO events primarily controls the interannual SST variation in the stratus cloud region north of 20°S.

Coupled Ocean–Atmosphere–Ice Response to Variations in the Southern Annular Mode

Abstract The coupled ocean–atmosphere–ice response to variations in the Southern Annular Mode (SAM) is examined in the National Center for Atmospheric Research (NCAR) Community Coupled Climate Model (version 2). The model shows considerable skill in capturing the predominantly zonally symmetric SAM while regional deviations between model and observation SAM winds go a long way in explaining the generally small differences between simulated and observed SAM responses in the ocean and sea ice systems. Vacillations in the position and strength of the circumpolar winds and the ensuing variations in advection of heat and moisture result in a dynamic and thermodynamic forcing of the ocean and sea ice. Both meridional and zonal components of ocean circulation are modified through Ekman transport, which in turn leads to anomalous surface convergences and divergences that strongly affect the meridional overturning circulation and potentially the pathways of intermediate water ventilation. A heat budget analysis demonstrates a conspiring of oceanic meridional heat advection, surface heat fluxes, and changes in mixed layer depth, which acts in phase to imprint a strong circumpolar SAM signature onto sea surface temperatures (SSTs), while other oceanic processes, including vertical advection, are shown to play only a minor role in contrast to previous suggestions. Lagged correlations show that although the SAM is mainly controlled by internal atmospheric mechanisms, the thermal inertia of the ocean reimprints the SAM signature back onto surface air temperatures (SATs) on time scales longer than the initial atmospheric signal. Sea ice variability is well explained by a combination of atmospheric and oceanic dynamic and thermodynamic forcing, and by an albedo feedback mechanism that allows ice extent anomalies to persist for many months. Nonzonally symmetric components of the SAM winds, particularly in the region surrounding the Antarctic Peninsula, have important effects for other climate variables.

A simple coupled model of tropical Atlantic decadal climate variability

A linear, zonally averaged model of the interaction between the tropical Atlantic (TA) atmosphere and ocean is presented. A balance between evaporation and meridional heat advection in the mixed layer determines the sea surface temperature tendency. The atmosphere is a fixed‐depth, sub‐cloud layer in which the specific humidity anomaly is determined by a steady‐state balance between evaporation, meridional advection, and a parameterized humidity exchange with the free atmosphere. When the model is integrated, forced with observed surface wind anomalies from 1965 to the present, its simulation of the observed sea surface temperature (SST) is realistic and comparable to a simulation with a full ocean GCM. A statistical representation of surface winds and their relationship to the SST gradient across the equator is used to formulate and test a coupled model of their regional variability. Forced on both sides of the equator, in the trade‐wind regions, with “white‐noise” windspeed perturbations, the SST‐wind relationship in the near‐equatorial region feeds back positively on existing SST anomalies and gives rise to decadal variability.

Variability of the sea surface temperature in the eastern equatorial Pacific during 1986–1988

Time series of surface wind and upper ocean temperature and velocity, obtained from equatorial moorings along 110°W, are used to assess the importance of various oceanic and atmospheric processes in the variation of the mixed layer temperature for the period January 1986 to June 1988. This period coincides with the onset and development of the 1986–1987 El Niño‐Southern Oscillation warm event and a subsequent cold event in 1988. Results of the temperature equation analyses indicate that seasonal and interannual variability of sea surface temperature (SST) in the eastern Pacific cannot be accounted for by observed surface heat flux; oceanic processes play an important role in the heating of the surface water. Although no single process dominated SST change, the most important processes in the mean balance were the net incoming surface heat flux, the penetrative solar radiation, and the vertical turbulent flux out the bottom of the mixed layer. The mean vertical entrainment could not be estimated with the available data. On the seasonal time scales, both the vertical turbulent heat flux and the vertical entrainment variations were well correlated with SST change. Zonal advection was a significant contribution to the heat flux variability, but its fluctuations were poorly correlated with the mixed layer heating. In particular, it was found that zonal advective heat flux tended to be out of phase with the spring warming. At higher frequencies, little zonal advective heat was found to be associated with the passage of a Kelvin event in January 1987. Surprisingly, meridional heat advection appeared more important than the zonal heat advection in modifying the local SST as this event passed the mooring location.