Diagnostic of the diurnal cycle of turbulence of the Equatorial Atlantic Ocean upper boundary layer

This work is an attempt to diagnose the turbu-lence field of the equatorial Atlantic Ocean dur-ing the dry period when the mixed layer is more highly developed using the General Ocean Turbulence Model (GOTM). A relaxation scheme assimilates the vertical profiles of in situ ob-servations (current velocity, sea temperature and salinity) during simulations. In the absence of direct turbulence observations and modeling studies of the equatorial Atlantic Ocean, the results are compared qualitatively to observed and simulated results for the equatorial Pacific Ocean. Similarities are noted between the At-lantic simulation and previous studies per-formed in the Pacific Ocean. The mechanism of nocturnal turbulence production, namely deep- cycle turbulence, is well captured by GOTM si-mulations. This deep nocturnal turbulence ap-pears rather suddenly during the night in the simulations and consequently seems to be un-related to surface wind and radiation forcing.

Tropical Atlantic Climate Response to Different Freshwater Input in High Latitudes with an Ocean-Only General Circulation Model

Tropical Atlantic climate change is relevant to the variation of Atlantic meridional overturning circulation(AMOC) through different physical processes. Previous coupled climate model simulation suggested a dipole-like SST structure cooling over the North Atlantic and warming over the South Tropical Atlantic in response to the slowdown of the AMOC. Using an ocean-only global ocean model here, an attempt was made to separate the total influence of various AMOC change scenarios into an oceanicinduced component and an atmospheric-induced component. In contrast with previous freshwater-hosing experiments with coupled climate models, the ocean-only modeling presented here shows a surface warming in the whole tropical Atlantic region and the oceanic-induced processes may play an important role in the SST change in the equatorial south Atlantic. Our result shows that the warming is partly governed by oceanic process through the mechanism of oceanic gateway change, which operates in the regime where freshwater forcing is strong, exceeding 0.3 Sv. Strong AMOC change is required for the gateway mechanism to work in our model because only when the AMOC is sufficiently weak, the North Brazil Undercurrent can flow equatorward, carrying warm and salty north Atlantic subtropical gyre water into the equatorial zone. This threshold is likely to be model-dependent. An improved understanding of these issues may have help with abrupt climate change prediction later.

Interdecadal Variations of the South Asian Summer Monsoon Circulation Variability and the Associated Sea Surface Temperatures on Interannual Scales

We investigate the interannual variability of the South Asian summer monsoon（SASM） circulation, which has experienced a significant interdecadal change since 2000. This change is primarily influenced by sea surface temperatures（SSTs）in the tropical Pacific and North Atlantic oceans. During the pre-2000 period examined in this study（1979-99）, the SASM is negatively correlated with eastern Pacific SSTs（the canonical ENSO mode） and positively correlated with the negative phase of the North Atlantic SST tripole（NAT）. During the post-2000 period（2000-14）, the SASM is negatively correlated with central Pacific SSTs and positively correlated with the positive phase of the NAT pattern. The associated Pacific SSTs change from the eastern to central region, leading to the rising（subsiding） branch of the Walker circulation moving westwards to the Maritime Continent in the latter period, which can impact the interannual variability of the SASM through modulating the wind field in the troposphere. In addition to Pacific SSTs, the NAT SSTs can propagate energy from the North Atlantic to the South Asian High（SAH） region through the wave activity flux, and then further impact the SASM via the SAH.Because the SASM is intimately related with precipitation over the Asian region, we briefly discuss the features of the precipitation patterns associated with the SASM during the two periods. The westward shifting Walker circulation leads to the shrinking and weakened anomalous westerlies of the SASM in the lower level, inducing the Maritime Continent rainfall location to move westwards and more moisture to arrive in southern China from the Pacific Ocean in the latter period.