M. Dolores Pérez-Hernández

Two Modes of Gulf Stream Variability Revealed in the Last Two Decades of Satellite Altimeter Data

Monthly mapped sea level anomalies (MSLAs) of the NW Atlantic in the region immediately downstream oftheGulfStream(GS)separationpointrevealaleadingmodeinwhichthepathshiftsapproximately 100km meridionally about a nominal latitude of 398N, producing coherent sea level anomaly (SLA) variability from 728 to 508W. This mode can be captured by use of a simple 16-point index based on SLA data taken along the maximum of the observed variability in the region 338–468N and 458–758W. The GS shifts between 2010 and 2012 are the largest of the last decade and equal to the largest of the entire record. The second group of EOF modesofvariabilitydescribesGSmeanders,whichpropagatemainlywestwardinterruptedbybriefperiodsof eastwardorstationarymeanders.Thesemeandershavewavelengthsofapproximately400kmandcanbeseen in standard EOFs by spatial phase shifting of a standing meander pattern in the SLA data. The spectral properties of these modes indicate strong variability at interannual and longer periods for the first mode and periods of a few to several months for the meanders. While the former is quite similar to a previous use of the altimeter for GS path, the simple index is a useful measure of the large-scale shifts in the GS path that is quickly estimatedand updatedwithoutchangesin previous estimates. Thetime-scaleseparationallows alowpass filtered 16-point index to be reflective of large-scale, coherent shifts in the GS path.

Wind-driven cross-equatorial flow in the Indian Ocean

AbstractMeridional velocity, mass, and heat transport in the equatorial oceans are difficult to estimate because of the nonapplicability of the geostrophic balance. For this purpose a steady-state model is utilized in the equatorial Indian Ocean using NCEP wind stress and temperature and salinity data from the World Ocean Atlas 2005 (WOA05) and Argo. The results show a Somali Current flowing to the south during the winter monsoon carrying −11.5 ± 1.3 Sv (1 Sv ≡ 106 m3 s−1) and −12.3 ± 0.3 Sv from WOA05 and Argo, respectively. In the summer monsoon the Somali Current reverses to the north transporting 16.8 ± 1.2 Sv and 19.8 ± 0.6 Sv in the WOA05 and Argo results. Transitional periods are considered together and in consequence, there is not a clear Somali Current present in this period. Model results fit with in situ measurements made around the region, although Argo data results are quite more realistic than WOA05 data results.

The Atlantic Water boundary current north of Svalbard in late summer

Data from a shipboard hydrographic/velocity survey carried out in September 2013 of the region north of Svalbard in the Nansen Basin are analyzed to characterize the Atlantic Water (AW) boundary current as it flows eastward along the continental slope. Eight meridional transects across the current, spanning an alongstream distance of 180 km, allow for a detailed description of the current and the regional water masses. During the survey the winds were light and there was no pack-ice. The mean section reveals that the boundary current was O(40 km) wide, surface-intensified, with a maximum velocity of 20 cm/s. Its mean transport during the survey was 3.11 ± 0.33 Sv, of which 2.31 ± 0.29 Sv was AW. This suggests that the two branches of AW entering the Arctic Ocean via Fram Strait—the Yermak Plateau branch and the Svalbard branch—have largely combined into a single current by 30°E. At this location the boundary current meanders with a systematic change in its kinematic structure during offshore excursions. A potential vorticity analysis indicates that the flow is baroclinically unstable, consistent with previous observations of AW anticyclones offshore of the current as well as the presence of a near-field cyclone in this data set. Our survey indicates that only a small portion of the boundary current is diverted into the Kvitoya Trough (0.17 ± 0.08 Sv) and that the AW temperature/salinity signal is quickly eroded within the trough.

On the seasonal variability of the Canary Current and the Atlantic Meridional Overturning Circulation

The Atlantic meridional overturning circulation (AMOC) is continually monitored along 26°N by the RAPID-MOCHA array. Measurements from this array show a 6.7 Sv seasonal cycle for the AMOC, with a 5.9 Sv contribution from the upper-mid-ocean. Recent studies argue that the dynamics of the eastern Atlantic is the main driver for this seasonal cycle; specifically, Rossby waves excited south of the Canary Islands. Using inverse modelling, hydrographic, mooring and altimetry data, we describe the seasonal cycle of the ocean mass transport around the Canary Islands and at the eastern boundary, under the influence of the African slope, where eastern component of the RAPID-MOCHA array is situated. We find a seasonal cycle of -4.1±0.5 Sv for the oceanic region of the Canary Current, and +3.7±0.4 Sv at the eastern boundary. This seasonal cycle along the eastern boundary is in agreement with the seasonal cycle of the AMOC, that requires the lowest contribution to the transport in the upper-mid-ocean to occur in fall. However, we demonstrate that the linear Rossby wave model used previously to explain the seasonal cycle of the AMOC is not robust, since it is extremely sensitive to the choice of the zonal range of the wind stress curl and produces the same results with a Rossby wave speed of zero. We demonstrate that the seasonal cycle of the eastern boundary is due to the recirculation of the Canary Current and to the seasonal cycle of the poleward flow that characterizes the eastern boundaries of the oceans.