Mixed Layer Temperature Balance Research Articles

The Role of Nonlocal Processes in Upper Layer Heat Budget in the North Atlantic

The physical processes that determine spatio-temporal variability of the upper mixed layer (UML) temperature in the North Atlantic have been investigated. The study of physical processes is based on the closed UML heat budget. The components for the mixed layer temperature balance are computed using the monthly data from ORA-S3 ocean reanalysis over the period 1959 — 2011. This paper deals with the relative contribution of heat budget terms into generating the both intra-annual and interannual-to-decadal variations of the UML temperature. The result of the study shows that the local heat flux on the ocean surface plays a major role in the intra-annual cycle of the UML temperature, while the nonlocal processes, such as advection and diffusion of heat, affect the interannual-decadal variability of the UML temperature. The obtained results are vital for studying the physics of ocean-atmosphere interactions.

Seasonal Mixed Layer Temperature Balance in the Southeastern Tropical Atlantic

AbstractMost climate models simulate sea surface temperatures (SSTs) that are consistently 1–4 °C warmer than observed in eastern tropical Atlantic. These biases undermine seasonal prediction efforts and the credibility of climate change projections in this region. To understand what drives the seasonal cycle in upper ocean temperature near the eastern boundary of the tropical Atlantic, we use a 5‐year moored buoy data set from the Prediction and Research Moored Array in the Atlantic at 6°S, 8°E. The buoy is located along the southeastern edge of the Atlantic cold tongue where the seasonal cycle in SST, which is maximum in March and minimum in August, is influenced by the meridional movement of the Intertropical Convergence Zone (ITCZ) and formation of low‐level marine stratocumulus clouds. Associated with these seasonal changes in atmospheric conditions, surface heat fluxes on seasonal timescales are most strongly controlled by shortwave radiation and latent heat flux. The seasonal mixed layer shoals, warms, and freshens in the boreal spring coincident with a southward migration of the ITCZ. The shallow mixed layer amplifies heating from solar radiation on mixed layer temperature at this time. Conversely, during the boreal summer, upwelling leads to entrainment of cold and salty water into the surface layer. From this analysis, we discuss the relative importance of the different components of the seasonal mixed layer heat balance at 6°S, 8°E and how they can be used to better understand the sources of climate model SST biases.

Influence of the Reflected Rossby Waves on the Western Arabian Sea Upwelling Region

Abstract The sea surface temperature (SST) in the western Arabian Sea upwelling region is known to influence the amount of precipitation associated with the Indian summer monsoon. Thus, understanding what determines the SST in this region is an important issue. Using outputs from an ocean general circulation model with and without strong damping in the eastern equatorial Indian Ocean, this study examines how the reflection of semiannual Kelvin waves at the eastern boundary of the Indian Ocean may influence the western Arabian Sea upwelling region. The downwelling Kelvin waves generated in boreal spring are reflected at the eastern boundary and reach the western equatorial Indian Ocean as reflected Rossby waves about 6 months later. The resulting westward current along the equator in the western equatorial Indian Ocean transports warmer water to the western Arabian Sea upwelling region. Thus, the SST in this region becomes colder especially in boreal fall without the reflected Rossby waves. These results are further supported by the analysis of the mixed layer temperature balance. Surprisingly, vertical processes do not contribute to the SST difference, even though the thermocline becomes shallower without the downwelling Rossby waves. This is because the mixed layer is shoaling rapidly from September to November, and there is basically no entrainment of water from below. In contrast, the reflected Rossby waves do not have large impacts on the SST in other seasons mainly because the zonal SST gradient is not as strong and/or the amplitude of Rossby waves is weaker.

Formation and erosion of the seasonal thermocline in the Kuroshio Extension Recirculation Gyre

Data from the Kuroshio Extension Observatory (KEO) surface mooring are used to analyze the balance of processes affecting the upper ocean heat content and surface mixed layer temperature variations in the Recirculation Gyre (RG) south of the Kuroshio Extension (KE). Cold and dry air blowing across the KE and its warm RG during winter cause very large heat fluxes out of the ocean that result in the erosion of the seasonal thermocline in the RG. Some of this heat is replenished through horizontal heat advection, which may enable the seasonal thermocline to begin restratifying while the net surface heat flux is still acting to cool the upper ocean. Once the surface heat flux begins warming the ocean, restratification occurs rapidly due to the low thermal inertia of the shallow mixed layer depth. Enhanced diffusive mixing below the mixed layer tends to transfer some of the mixed layer heat downward, eroding and potentially modifying sequestered subtropical mode water and even the deeper waters of the main thermocline during winter. Diffusivity at the base of the mixed layer, estimated from the residual of the mixed layer temperature balance, is roughly 3×10−4m2/s during the summer and up to two orders of magnitude larger during winter. The enhanced diffusivities appear to be due to large inertial shear generated by wind events associated with winter storms and summer tropical cyclones. The diffusivity's seasonality is likely due to seasonal variations in stratification just below the mixed layer depth, which is large during the summer when the seasonal thermocline is fully developed and low during the winter when the mixed layer extends to the top of the thermocline.

Mixed layer temperature balance in the eastern Indian Ocean during the 2006 Indian Ocean dipole

An anomalous climate mode, the positive Indian Ocean dipole (IOD), occurred in 2006 with the anomalous sea surface temperature (SST) distribution in the tropical Indian Ocean. Using various types of observational data, we investigated the temperature variation in the surface mixed layer in the eastern Indian Ocean to clarify the processes that produced the anomalous SST variation in 2006. Analysis was conducted at an intraseasonal time scale and focused on a location (5°S, 95°E) where in situ measurements by the Triangle Trans‐Ocean Buoy Network were available. Temporal changes in the mixed layer temperature were obtained from the buoy data. Air‐sea heat fluxes and horizontal heat advection were estimated from the buoy data, satellite‐based data, and reanalysis products. Heat balance analysis demonstrated that air‐sea heat fluxes and horizontal heat advection mainly accounted for the mixed layer temperature variation. The results indicate that the relative importance of the heat fluxes and horizontal heat advections changed remarkably with the onset of the IOD. During January to mid‐May 2006, before the onset of the IOD, the temperature variation was mainly explained by the net surface heat flux at an intraseasonal time scale. During the IOD in late August to November 2006, the northwestward horizontal temperature gradient and the surface current produced large horizontal heat advection that exceeded the contribution of surface heat fluxes. These results confirm the importance of oceanic processes in the evolution of the IOD, and the heat balance analysis would be a fundamental example in validating model outputs for the Indian Ocean.

Meridional Structure of the Seasonally Varying Mixed Layer Temperature Balance in the Eastern Tropical Pacific

Abstract The eastern tropical Pacific Ocean is important climatically because of its influence on the El Niño–Southern Oscillation (ENSO) cycle and the American monsoon. Accurate prediction of these phenomena requires a better understanding of the background climatological conditions on which seasonal-to-interannual time-scale anomalies develop in the region. This study addresses the processes responsible for the seasonal cycle of sea surface temperature (SST) in the eastern tropical Pacific using 3 yr (April 2000–March 2003) of moored buoy and satellite data between 8°S and 12°N along 95°W. Results indicate that at all latitudes, surface heat fluxes are important in the mixed layer temperature balance. At 8°S, in a region of relatively deep mean thermocline and mixed layer, local storage of heat crossing the air–sea interface accounts for much of the seasonal cycle in SST. In the equatorial cold tongue and the intertropical convergence zone, where mean upwelling leads to relatively thin mixed layers, vertical turbulent mixing with the upper thermocline is a major contributor to SST change. Lateral temperature advection by seasonally varying large-scale currents is most significant near the equator but is generally of secondary importance. There is a hemispheric asymmetry in seasonal SST variations, with larger amplitudes in the Southern Hemisphere than in the Northern Hemisphere. This asymmetry is mainly due to forcing from the southerly component of the trade winds, which shifts the axis of equatorial upwelling south of the equator while creating an oceanic convergence zone to the north that limits the northward spread of cold upwelled water. In general, results support the Mitchell and Wallace hypothesis about the importance of southerly winds and ocean–atmosphere feedbacks in establishing seasonally varying climatological conditions in the eastern tropical Pacific.

A model study of the seasonal mixed layer heat budget in the equatorial Atlantic

In the present study, the physical processes that control the seasonal cycle of sea surface temperature in the tropical Atlantic Ocean are investigated. A high‐resolution ocean general circulation model is used to diagnose the various contributions to the mixed layer heat budget. The simulation reproduces the main features of the circulation and thermal structure of the tropical Atlantic. A close examination of the mixed layer heat budget is then undertaken. At a first order, the mixed layer temperature balance in the equatorial band results from cooling by vertical processes and heating by atmospheric heat fluxes and eddies (mainly tropical instability waves). Cooling by subsurface processes is the strongest in June–August, when easterlies are strong, with a second maximum in December. Heating by the atmosphere is maximum in February–March and September–October, whereas eddies are most active in boreal summer. Unlike previous observational studies, horizontal advection by low‐frequency currents plays here only a minor role in the heat budget. Off equator, the sea surface temperature variability is mainly governed by atmospheric forcing all year long, except in the northeastern part of the basin where strong eddies generated at the location of the thermal front significantly contribute to the heat budget in boreal summer. Finally, comparisons with previously published heat budgets calculated from observations show good qualitative agreement, except that subsurface processes dominate the cooling over zonal advection in the present study.

Mixed Layer Temperature Balance on Intraseasonal Timescales in the Equatorial Pacific Ocean*

Abstract The purpose of this study is to document the zonal evolution of processes affecting sea surface temperature (SST) variability on intraseasonal timescales in the equatorial Pacific Ocean. Data primarily from the Tropical Atmosphere Ocean (TAO) array of moored buoys are used, focusing on four sites along the equator with decade-long time series. These sites are located in the western Pacific warm pool (165°E), the eastern Pacific equatorial cold tongue (110° and 140°W), and the transition zone between these two regions (170°W). Results indicate that SST variability on intraseasonal timescales is most significantly influenced by local surface heat fluxes in the western Pacific (165°E), zonal advection in the central Pacific (170°W), and vertical advection and entrainment in the eastern Pacific (110° and 140°W). East of the date line, oceanic equatorial Kelvin waves strongly mediate dynamical processes controlling intraseasonal SSTs variations, while surface fluxes tend to damp these dynamically gene...